Aluminium oxide

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Aluminium_oxide

 

Aluminium(III) oxide
(Aluminium oxide)

Names
IUPAC name Aluminium oxide
Systematic IUPAC name Aluminium(III) oxide
Other names Dialuminium trioxide
Identifiers
CAS Number	1344-28-1
3D model (JSmol)	Interactive image Interactive image
ChEMBL	ChEMBL3707210
ChemSpider	8164808
DrugBank	DB11342
ECHA InfoCard	100.014.265
EC Number	215-691-6
PubChem CID	9989226
RTECS number	BD120000
UNII	LMI26O6933
CompTox Dashboard (EPA)	DTXSID1052791

InChI

SMILES
Properties
Chemical formula	Al2O3
Molar mass	101.960 g·mol−1
Appearance	white solid
Odor	odorless
Density	3.987 g/cm3
Melting point	2,072 °C (3,762 °F; 2,345 K)
Boiling point	2,977 °C (5,391 °F; 3,250 K)
Solubility in water	insoluble
Solubility	insoluble in all solvents
log P	0.31860
Magnetic susceptibility (χ)	−37.0×10−6 cm3/mol
Thermal conductivity	30 W·m−1·K−1
Refractive index (nD)	nω = 1.768–1.772 nε = 1.760–1.763 Birefringence 0.008
Structure
Crystal structure	Trigonal, hR30
Space group	R3c (No. 167)
Lattice constant	a = 478.5 pm, c = 1299.1 pm
Coordination geometry	octahedral
Thermochemistry
Std molar entropy (S⦵298)	50.92 J·mol−1·K−1
Std enthalpy of formation (ΔfH⦵298)	−1675.7 kJ/mol
Pharmacology
ATC code	D10AX04 (WHO)
Hazards
GHS labelling:
Pictograms
NFPA 704 (fire diamond)	0 0 0
Flash point	Non-flammable
NIOSH (US health exposure limits):
PEL (Permissible)	OSHA 15 mg/m3 (total dust) OSHA 5 mg/m3 (respirable fraction) ACGIH/TLV 10 mg/m3
REL (Recommended)	none
IDLH (Immediate danger)	N.D.
Related compounds
Other anions	aluminium hydroxide aluminium sulfide aluminium selenide
Other cations	boron trioxide gallium(III) oxide indium oxide thallium(III) oxide
Supplementary data page
Aluminium oxide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).  verify (what is  ?) Infobox references

Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula Al₂O₃. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly called alumina and may also be called aloxide, aloxite, or alundum in various forms and applications. It occurs naturally in its crystalline polymorphic phase α-Al₂O₃ as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire. Al₂O₃ is used as feedstock to produce aluminium metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point.

Natural occurrence

Corundum is the most common naturally occurring crystalline form of aluminium oxide. Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colours to trace impurities. Rubies are given their characteristic deep red colour and their laser qualities by traces of chromium. Sapphires come in different colours given by various other impurities, such as iron and titanium. An extremely rare δ form occurs as the mineral deltalumite.

History

The field of aluminium oxide ceramics has a long history. Aluminium salts were widely used in ancient and medieval alchemy. Several vintage textbooks cover the history of the field. A 2019 textbook by Andrew Ruys contains a detailed timeline on the history of aluminium oxide from ancient times to the 21st century.

Properties

Aluminium oxide in its powdered form

Al₂O₃ is an electrical insulator but has a relatively high thermal conductivity (30 Wm⁻¹K⁻¹) for a ceramic material. Aluminium oxide is insoluble in water. In its most commonly occurring crystalline form, called corundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.

Aluminium oxide is responsible for the resistance of metallic aluminium to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of aluminium oxide (4 nm thickness) forms on any exposed aluminium surface in a matter of hundreds of picoseconds. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising. A number of alloys, such as aluminium bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. The aluminium oxide generated by anodising is typically amorphous, but discharge-assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline aluminium oxide in the coating, enhancing its hardness.

Aluminium oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988. Aluminium oxide is on the EPA's Toxics Release Inventory list if it is a fibrous form.

Amphoteric nature

Aluminium oxide is an amphoteric substance, meaning it can react with both acids and bases, such as hydrofluoric acid and sodium hydroxide, acting as an acid with a base and a base with an acid, neutralising the other and producing a salt.

Al₂O₃ + 6 HF → 2 AlF₃ + 3 H₂O

Al₂O₃ + 2 NaOH + 3 H₂O → 2 NaAl(OH)₄ (sodium aluminate)

Structure

Corundum from Brazil, size about 2×3 cm.

The most common form of crystalline aluminium oxide is known as corundum, which is the thermodynamically stable form. The oxygen ions form a nearly hexagonal close-packed structure with the aluminium ions filling two-thirds of the octahedral interstices. Each Al³⁺ center is octahedral. In terms of its crystallography, corundum adopts a trigonal Bravais lattice with a space group of R3c (number 167 in the International Tables). The primitive cell contains two formula units of aluminium oxide.

Aluminium oxide also exists in other metastable phases, including the cubic γ and η phases, the monoclinic θ phase, the hexagonal χ phase, the orthorhombic κ phase and the δ phase that can be tetragonal or orthorhombic. Each has a unique crystal structure and properties. Cubic γ-Al₂O₃ has important technical applications. The so-called β-Al₂O₃ proved to be NaAl₁₁O₁₇.

Molten aluminium oxide near the melting temperature is roughly 2/3 tetrahedral (i.e. 2/3 of the Al are surrounded by 4 oxygen neighbors), and 1/3 5-coordinated, with very little (<5%) octahedral Al-O present. Around 80% of the oxygen atoms are shared among three or more Al-O polyhedra, and the majority of inter-polyhedral connections are corner-sharing, with the remaining 10–20% being edge-sharing. The breakdown of octahedra upon melting is accompanied by a relatively large volume increase (~33%), the density of the liquid close to its melting point is 2.93 g/cm³. The structure of molten alumina is temperature dependent and the fraction of 5- and 6-fold aluminium increases during cooling (and supercooling), at the expense of tetrahedral AlO₄ units, approaching the local structural arrangements found in amorphous alumina.

Production

See also: List of countries by aluminium oxide production

Aluminium hydroxide minerals are the main component of bauxite, the principal ore of aluminium. A mixture of the minerals comprise bauxite ore, including gibbsite (Al(OH)₃), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH)), along with impurities of iron oxides and hydroxides, quartz and clay minerals. Bauxites are found in laterites. Bauxite is typically purified using the Bayer process:

Al₂O₃ + H₂O + NaOH → NaAl(OH)₄

Al(OH)₃ + NaOH → NaAl(OH)₄

Except for SiO₂, the other components of bauxite do not dissolve in base. Upon filtering the basic mixture, Fe₂O₃ is removed. When the Bayer liquor is cooled, Al(OH)₃ precipitates, leaving the silicates in solution.

NaAl(OH)₄ → NaOH + Al(OH)₃

The solid Al(OH)₃ Gibbsite is then calcined (heated to over 1100 °C) to give aluminium oxide:

2 Al(OH)₃ → Al₂O₃ + 3 H₂O

The product aluminium oxide tends to be multi-phase, i.e., consisting of several phases of aluminium oxide rather than solely corundum. The production process can therefore be optimized to produce a tailored product. The type of phases present affects, for example, the solubility and pore structure of the aluminium oxide product which, in turn, affects the cost of aluminium production and pollution control.

Sintering Process

The Sintering Process is a high-temperature method primarily used when the Bayer Process is not suitable, especially for ores with high silica content or when a more controlled product morphology is required. Firstly, Bauxite is mixed with additives like limestone and soda ash, then heating the mixture at high temperatures (1200 °C to 1500 °C) to form sodium aluminate and calcium silicate.^[24] After sintering, the material is leached with water to dissolve the sodium aluminate, leaving behind impurities. Sodium aluminate is then precipitated from the solution and calcined at around 1000 °C to produce alumina. This method is useful for the production of complex shapes and can be used to create porous or dense materials.

Applications

Aluminium oxide output in 2005

Known as alpha alumina in materials science, and as alundum (in fused form) or aloxite in mining and ceramic communities, aluminium oxide finds wide use. Annual global production of aluminium oxide in 2015 was approximately 115 million tonnes, over 90% of which was used in the manufacture of aluminium metal. The major uses of speciality aluminium oxides are in refractories, ceramics, polishing and abrasive applications. Large tonnages of aluminium hydroxide, from which alumina is derived, are used in the manufacture of zeolites, coating titania pigments, and as a fire retardant/smoke suppressant.

Over 90% of aluminium oxide, termed smelter grade alumina (SGA), is consumed for the production of aluminium, usually by the Hall–Héroult process. The remainder, termed specialty alumina, is used in a wide variety of applications which take advantage of its inertness, temperature resistance and electrical resistance.

Fillers

Being fairly chemically inert and white, aluminium oxide is commonly used as a filler for plastics. Aluminium oxide is a common ingredient in sunscreen and is often also present in cosmetics such as blush, lipstick, and nail polish.

Glass

Many formulations of glass have aluminium oxide as an ingredient. Aluminosilicate glass is a commonly used type of glass that often contains 5% to 10% alumina.

Catalysis

Aluminium oxide catalyses a variety of reactions that are useful industrially. In its largest scale application, aluminium oxide is the catalyst in the Claus process for converting hydrogen sulfide waste gases into elemental sulfur in refineries. It is also useful for dehydration of alcohols to alkenes.

Aluminium oxide serves as a catalyst support for many industrial catalysts, such as those used in hydrodesulfurization and some Ziegler–Natta polymerizations.

Gas purification

Aluminium oxide is widely used to remove water from gas streams.

Abrasion

Aluminium oxide is used for its hardness and strength. Its naturally occurring form, corundum, is a 9 on the Mohs scale of mineral hardness (just below diamond). It is widely used as an abrasive, including as a much less expensive substitute for industrial diamond. Many types of sandpaper use aluminium oxide crystals. In addition, its low heat retention and low specific heat make it widely used in grinding operations, particularly cutoff tools. As the powdery abrasive mineral aloxite, it is a major component, along with silica, of the cue tip "chalk" used in billiards. Aluminium oxide powder is used in some CD/DVD polishing and scratch-repair kits. Its polishing qualities are also behind its use in toothpaste. It is also used in microdermabrasion, both in the machine process available through dermatologists and estheticians, and as a manual dermal abrasive used according to manufacturer directions.

Paint

Main article: Alumina effect pigment

Aluminium oxide flakes are used in paint for reflective decorative effects, such as in the automotive or cosmetic industries.

Biomedical applications

Aluminium oxide is a representative of bioinert ceramics. Due to its excellent biocompatibility, high strength, and wear resistance, alumina ceramics are used in medical applications to manufacture artificial bones and joints. In this case, aluminium oxide is used to coat the surfaces of medical implants to give biocompatibility and corrosion resistance. It is also used for manufacturing dental implants, joint replacements, and other medical devices.

Composite fiber

Aluminium oxide has been used in a few experimental and commercial fiber materials for high-performance applications (e.g., Fiber FP, Nextel 610, Nextel 720). Alumina nanofibers in particular have become a research field of interest.

Armor

Some body armors utilize alumina ceramic plates, usually in combination with aramid or UHMWPE backing to achieve effectiveness against most rifle threats. Alumina ceramic armor is readily available to most civilians in jurisdictions where it is legal, but is not considered military grade.

Abrasion protection

An aluminium oxide layer can be grown as a protective coating on aluminium by anodizing or by plasma electrolytic oxidation (see the "Properties" above). Both the hardness and abrasion-resistant characteristics of the coating originate from the high strength of aluminium oxide, yet the porous coating layer produced with conventional direct current anodizing procedures is within a 60–70 Rockwell hardness C range which is comparable only to hardened carbon steel alloys, but considerably inferior to the hardness of natural and synthetic corundum. Instead, with plasma electrolytic oxidation, the coating is porous only on the surface oxide layer while the lower oxide layers are much more compact than with standard DC anodizing procedures and present a higher crystallinity due to the oxide layers being remelted and densified to obtain α-Al2O3 clusters with much higher coating hardness values circa 2000 Vickers hardness.

Alumina is used to manufacture tiles which are attached inside pulverized fuel lines and flue gas ducting on coal fired power stations to protect high wear areas. They are not suitable for areas with high impact forces as these tiles are brittle and susceptible to breakage.

Electrical insulation

Aluminium oxide is an electrical insulator used as a substrate (silicon on sapphire) for integrated circuits, but also as a tunnel barrier for the fabrication of superconducting devices such as single-electron transistors, superconducting quantum interference devices (SQUIDs) and superconducting qubits.

For its application as an electrical insulator in integrated circuits, where the conformal growth of a thin film is a prerequisite and the preferred growth mode is atomic layer deposition, Al₂O₃ films can be prepared by the chemical exchange between trimethylaluminium (Al(CH₃)₃) and H₂O:

2 Al(CH₃)₃ + 3 H₂O → Al₂O₃ + 6 CH₄

H₂O in the above reaction can be replaced by ozone (O₃) as the active oxidant and the following reaction then takes place:

2 Al(CH₃)₃ + O₃ → Al₂O₃ + 3 C₂H₆

The Al₂O₃ films prepared using O₃ show 10–100 times lower leakage current density compared with those prepared by H₂O.

Aluminium oxide, being a dielectric with relatively large band gap, is used as an insulating barrier in capacitors.

Other

Aluminium residue from a '50s vintage ice cream scoop.

Before the advent of domestic plastics aluminium ice cream scoops would with wear and tear leave aluminium residue.

In lighting, translucent aluminium oxide is used in some sodium vapor lamps. Aluminium oxide is also used in preparation of coating suspensions in compact fluorescent lamps.

In chemistry laboratories, aluminium oxide is a medium for chromatography, available in basic (pH 9.5), acidic (pH 4.5 when in water) and neutral formulations. Additionally, small pieces of aluminium oxide are often used as boiling chips.

Health and medical applications include it as a material in hip replacements and birth control pills.

It is used as a scintillator and dosimeter for radiation protection and therapy applications for its optically stimulated luminescence properties.

Insulation for high-temperature furnaces is often manufactured from aluminium oxide. Sometimes the insulation contains a percentage of silica depending on the temperature rating of the material. The insulation can be made in blanket, board, brick and loose fiber forms for various application requirements.

It is also used to make spark plug insulators.

Using a plasma spray process and mixed with titania, it is coated onto the braking surface of some bicycle rims to provide abrasion and wear resistance.

Most ceramic eyes on fishing rods are circular rings made from aluminium oxide.

In its finest powdered (white) form, called Diamantine, aluminium oxide is used as a superior polishing abrasive in watchmaking and clockmaking.

Aluminium oxide is also used in the coating of stanchions in the motocross and mountain bike industries. This coating is combined with molybdenum disulfide to provide long term lubrication of the surface.

Reaction intermediate

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Reaction_intermediate 

In chemistry, a reaction intermediate, or intermediate, is a molecular entity arising within the sequence of a stepwise chemical reaction. It is formed as the reaction product of an elementary step, from the reactants and/or preceding intermediates, but is consumed in a later step. It does not appear in the chemical equation for the overall reaction.

For example, consider this hypothetical reaction:

A + B → C + D

If this overall reaction comprises two elementary steps thus:

A + B → X

X → C + D

then X is a reaction intermediate.

The phrase reaction intermediate is often abbreviated to the single word intermediate, and this is IUPAC's preferred form of the term. But this shorter form has other uses. It often refers to reactive intermediates. It is also used more widely for chemicals such as cumene which are traded within the chemical industry but are not generally of value outside it.

IUPAC definition

The IUPAC Gold Book defines an intermediate as a compound that has a lifetime greater than a molecular vibration, is formed (directly or indirectly) from the reactants, and reacts further to give (either directly or indirectly) the products of a chemical reaction. The lifetime condition distinguishes true, chemically distinct intermediates, both from vibrational states and from transition states (which, by definition, have lifetimes close to that of molecular vibration).

The different steps of a multi-step reaction often differ widely in their reaction rates. Where the difference is significant, an intermediate consumed more quickly than another may be described as a relative intermediate. A reactive intermediate is one which due to its short lifetime does not remain in the product mixture. Reactive intermediates are usually high-energy, are unstable and are seldom isolated.

Common reaction intermediates

Carbocations

Cations, often carbocations, serve as intermediates in various types of reactions to synthesize new compounds.

Carbocation intermediates in alkene addition

Carbocations are formed in two major alkene addition reactions. In an HX addition reaction, the pi bond of an alkene acts as a nucleophile and bonds with the proton of an HX molecule, where the X is a halogen atom. This forms a carbocation intermediate, and the X then bonds to the positive carbon that is available, as in the following two-step reaction.

CH₂CH₂ + HX → CH₃CH+2 + X⁻

CH₃CH+2 + X⁻ → CH₃CH₂X

Similarly, in an H₂O addition reaction, the pi bond of an alkene acts as a nucleophile and bonds with the proton of an [H₃O]⁺ molecule. This forms a carbocation intermediate (and an H₂O atom); the oxygen atom of H₂O then bonds with the positive carbon of the intermediate. The oxygen finally deprotonates to form a final alcohol product, as follows.

CH₂CH₂ + [H₃O]⁺ → CH₃CH+2 + H₂O

CH₃CH+2 + H₂O → [CH₃CH₂OH₂]⁺

[CH₃CH₃OH₂]⁺ + H₂O → CH₃CH₂OH + [H₃O]⁺

Carbocation intermediates in nucleophilic substitution

Nucleophilic substitution reactions occur when a nucleophilic molecule attacks a positive or partially positive electrophilic center by breaking and creating a new bond. S_N1 and S_N2 are two different mechanisms for nucleophilic substitution, and S_N1 involves a carbocation intermediate. In S_N1, a leaving group is broken off to create a carbocation reaction intermediate. Then, a nucleophile attacks and forms a new bond with the carbocation intermediate to form the final, substituted product, as shown in the reaction of 2-bromo-2-methylpropane to form 2-methyl-2-propanol.

(CH₃)₃CBr → (CH₃)₃C⁺

(CH₃)₃C⁺ + H₂O → (CH₃)₃COH+2

(CH₃)₃COH+2 → (CH₃)₃COH + H⁺

In this reaction, (CH₃)₃C⁺ is the formed carbocation intermediate to form the alcohol product.

Carbocation intermediates in elimination reactions

β-elimination or elimination reactions occur through the loss of a substituent leaving group and loss of a proton to form a pi bond. E1 and E2 are two different mechanisms for elimination reactions, and E1 involves a carbocation intermediate. In E1, a leaving group detaches from a carbon to form a carbocation reaction intermediate. Then, a solvent removes a proton, but the electrons used to form the proton bond form a pi bond, as shown in the pictured reaction on the right.

Carboanions

A carboanion is an organic molecule where a carbon atom is not electron deficient but contain an overall negative charge. Carboanions are strong nucleophiles, which can be used to extend an alkene's carbon backbone in the synthesis reaction shown below.

C₂H₂ with NaNH₂ in NH₃(l) → CHC⁻

CHC⁻ + BrCH₂CH₃ → CHC−CH₂CH₃

The alkyne carbanion, CHC⁻, is a reaction intermediate in this reaction.

Radicals

Radicals are highly reactive and short-lived, as they have an unpaired electron which makes them extremely unstable. Radicals often react with hydrogens attached to carbon molecules, effectively making the carbon a radical while stabilizing the former radical in a process called propagation. The formed product, a carbon radical, can react with non-radical molecule to continue propagation or react with another radical to form a new stable molecule such as a longer carbon chain or an alkyl halide.

The example below of methane chlorination shows a multi-step reaction involving radicals.

Methane chlorination

Methane chlorination is a chain reaction. If only the products and reactants are analyzed, the result is:

CH₄ + 4 Cl₂ → CCl₄ + 4 HCl

However, this reaction has 3 intermediate reactants which are formed during a sequence of 4 irreversible second order reactions until we arrive at the final product. This is why it is called a chain reaction. Following only the carbon containing species in series:

CH₄ → CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄

Reactants: CH₄ + 4 Cl₂

Products: CCl₄ + 4 HCl

The other species are reaction intermediates: CH₃Cl, CH₂Cl₂, CHCl₃

These are the set of irreversible second-order reactions:

CH₄ + Cl₂ → CH₃Cl + HCl

CH₃Cl + Cl₂ → CH₂Cl₂ + HCl

CH₂Cl₂ + Cl₂ → CHCl₃ + HCl

CHCl₃ + Cl₂ → CCl₄ + HCl

These intermediate species' concentrations can be calculated by integrating the system of kinetic equations. The full reaction is a free radical propagation reaction which is filled out in detail below.

Initiation: This reaction can occur by thermolysis (heating) or photolysis (absorption of light) leading to the breakage of a molecular chlorine bond.

Cl−Cl hν→ Cl• + Cl•

When the bond is broken it produces two highly reactive chlorine atoms.

Propagation: This stage has two distinct reaction classes. The first is the stripping of a hydrogen from the carbon species by the chlorine radicals. This occurs because chlorine atoms alone are unstable, and these chlorine atoms react with one the carbon species' hydrogens. The result is the formation of hydrochloric acid and a new radical methyl group.

CH₃−H + Cl• → CH₃• + H−Cl

CH₂Cl−H + Cl• → CH₂Cl• + H−Cl

CHCl₂−H + Cl• → CHCl₂• + H−Cl

CCl₃−H + Cl• → CCl₃• + H−Cl

These new radical carbon containing species now react with a second CHCCl₂ molecule. This regenerates the chlorine radical and the cycle continues. This reaction occurs because while the radical methyl species are more stable than the radical chlorines, the overall stability of the newly formed chloromethane species more than makes up the energy difference.

CH₃• + Cl−Cl → CH₃Cl + Cl•

CH₂Cl• + Cl−Cl → CH₂Cl₂ + Cl•

CHCl₂• + Cl−Cl → CHCl₃ + Cl•

CCl₃• + Cl−Cl → CCl₄ + Cl•

During the propagation of the reaction, there are several highly reactive species that will be removed and stabilized at the termination step.

Termination: This kind of reaction takes place when the radical species interact directly. The products of the termination reactions are typically very low yield in comparison to the main products or intermediates as the highly reactive radical species are in relatively low concentration in relation to the rest of the mixture. This kind of reaction produces stable side products, reactants, or intermediates and slows the propagation reaction by lowering the number of radicals available to propagate the chain reaction.

There are many different termination combinations, some examples are:

Union of methyl radicals from a C-C bond leading to ethane (a side product).

CH₃• + CH₃• → CH₃−CH₃

Union of one methyl radical to a Cl radical forming chloromethane (another reaction forming an intermediate).

CH₃• + Cl• → CH₃Cl

Union of two Cl radicals to reform chlorine gas (a reaction reforming a reactant).

Cl• + Cl• → Cl₂

Applications

Biological intermediates

Reaction intermediates serve purposes in a variety of biological settings. An example of this is demonstrated with the enzyme reaction intermediate of metallo-β-lactamase, which bacteria can use to acquire resistance to commonly used antibiotics such as penicillin. Metallo-β-lactamase can catalyze β-lactams, a family of common antibiotics. Spectroscopy techniques have found that the reaction intermediate of metallo-β-lactamase uses zinc in the resistance pathway.

Another example of the importance of reaction intermediates is seen with AAA-ATPase p97, a protein that used in a variety of cellular metabolic processes. p97 is also linked to degenerative disease and cancer. In a study looking at reaction intermediates of the AAA-ATPase p97 function found an important ADP.P_i nucleotide intermediate is important in the p97 molecular operation.

An additional example of biologically relevant reaction intermediates can be found with the RCL enzymes, which catalyzes glycosidic bonds. When studied using methanolysis, it was found that the reaction required the formation of a reaction intermediate.

Chemical processing industry

In the chemical industry, the term intermediate may also refer to the (stable) product of a reaction that is itself valuable only as a precursor chemical for other industries. A common example is cumene which is made from benzene and propylene and used to make acetone and phenol in the cumene process. The cumene itself is of relatively little value in and of itself, and is typically only bought and sold by chemical companies.

Activated complex

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Activated_complex 

Reaction coordinate diagram showing the activated complex in the region with highest potential energy.

In chemistry, an activated complex represents a collection of intermediate structures in a chemical reaction when bonds are breaking and forming. The activated complex is an arrangement of atoms in an arbitrary region near the saddle point of a potential energy surface. The region represents not one defined state, but a range of unstable configurations that a collection of atoms pass through between the reactants and products of a reaction. Activated complexes have partial reactant and product character, which can significantly impact their behaviour in chemical reactions.

The terms activated complex and transition state are often used interchangeably, but they represent different concepts. Transition states only represent the highest potential energy configuration of the atoms during the reaction, while activated complex refers to a range of configurations near the transition state. In a reaction coordinate, the transition state is the configuration at the maximum of the diagram while the activated complex can refer to any point near the maximum.

Transition state theory (also known as activated complex theory) studies the kinetics of reactions that pass through a defined intermediate state with standard Gibbs energy of activation ΔG°^‡. The transition state, represented by the double dagger symbol represents the exact configuration of atoms that has an equal probability of forming either the reactants or products of the given reaction.

The activation energy is the minimum amount of energy to initiate a chemical reaction and form the activated complex. The energy serves as a threshold that reactant molecules must surpass to overcome the energy barrier and transition into the activated complex. Endothermic reactions absorb energy from the surroundings, while exothermic reactions release energy. Some reactions occur spontaneously, while others necessitate an external energy input. The reaction can be visualized using a reaction coordinate diagram to show the activation energy and potential energy throughout the reaction.

Activated complexes were first discussed in transition state theory (also called activated complex theory), which was first developed by Eyring, Evans, and Polanyi in 1935.

Reaction rate

Transition state theory

Transition state theory explains the dynamics of reactions. The theory is based on the idea that there is an equilibrium between the activated complex and reactant molecules. The theory incorporates concepts from collision theory, which states that for a reaction to occur, reacting molecules must collide with a minimum energy and correct orientation. The reactants are first transformed into the activated complex before breaking into the products. From the properties of the activated complex and reactants, the reaction rate constant is

$${\displaystyle k=K\frac{k_{\text{B}}T}{h}}$$

where K is the equilibrium constant, $k_{\text{B}}$ is the Boltzmann constant, T is the thermodynamic temperature, and h is the Planck constant. Transition state theory is based on classical mechanics, as it assumes that as the reaction proceeds, the molecules will never return to the transition state.

Symmetry

An activated complex with high symmetry can decrease the accuracy of rate expressions. Error can arise from introducing symmetry numbers into the rotational partition functions for the reactants and activated complexes. To reduce errors, symmetry numbers can by omitted by multiplying the rate expression by a statistical factor:

$${\displaystyle k=l^{‡}\frac{k_{\text{B}}T}{h}\frac{Q_{‡}}{Q_A{Q}_B}{e}^{-\frac{\epsilon }{k_{\text{B}}T}}}$$

where the statistical factor $l^{‡}$ is the number of equivalent activated complexes that can be formed, and the Q are the partition functions from the symmetry numbers that have been omitted.

The activated complex is a collection of molecules that forms and then explodes along a particular internal normal coordinate. Ordinary molecules have three translational degrees of freedom, and their properties are similar to activated complexes. However, activated complexed have an extra degree of translation associated with their approach to the energy barrier, crossing it, and then dissociating.

Acetone

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Acetone

 

Acetone

Names
IUPAC name Acetone
Preferred IUPAC name Propan-2-one
Systematic IUPAC name 2-Propanone
Other names Acetonum (Latin pronunciation: [aˈkeːtonum]) Dimethyl ketone Dimethyl carbonyl Ketone propane β-Ketopropane Propanone 2-Propanone Pyroacetic spirit (archaic) Spirit of Saturn (archaic)

Acetone (2-propanone or dimethyl ketone) is an organic compound with the formula (CH₃)₂CO. It is the simplest and smallest ketone (R−C(=O)−R'). It is a colorless, highly volatile, and flammable liquid with a characteristic pungent odor.

Acetone is miscible with water and serves as an important organic solvent in industry, home, and laboratory. About 6.7 million tonnes were produced worldwide in 2010, mainly for use as a solvent and for production of methyl methacrylate and bisphenol A, which are precursors to widely used plastics. It is a common building block in organic chemistry. It serves as a solvent in household products such as nail polish remover and paint thinner. It has volatile organic compound (VOC)-exempt status in the United States.

Acetone is produced and disposed of in the human body through normal metabolic processes. Small quantities of it are present naturally in blood and urine. People with diabetic ketoacidosis produce it in larger amounts. Ketogenic diets that increase ketone bodies (acetone, β-hydroxybutyric acid and acetoacetic acid) in the blood are used to counter epileptic attacks in children who suffer from refractory epilepsy.

Name

From the 17th century, and before modern developments in organic chemistry nomenclature, acetone was given many different names. They included "spirit of Saturn", which was given when it was thought to be a compound of lead and, later, "pyro-acetic spirit" and "pyro-acetic ester".

Prior to the name "acetone" being coined by French chemists (see below), it was named "mesit" (from the Greek μεσίτης, meaning mediator) by Carl Reichenbach, who also claimed that methyl alcohol consisted of mesit and ethyl alcohol. Names derived from mesit include mesitylene and mesityl oxide which were first synthesised from acetone.

Unlike many compounds with the acet- prefix which have a 2-carbon chain, acetone has a 3-carbon chain. That has caused confusion because there cannot be a ketone with 2 carbons. The prefix refers to acetone's relation to vinegar (acetum in Latin, also the source of the words "acid" and "acetic"), rather than its chemical structure.

History

Acetone was first produced by Andreas Libavius in 1606 by distillation of lead(II) acetate.

In 1832, French chemist Jean-Baptiste Dumas and German chemist Justus von Liebig determined the empirical formula for acetone. In 1833, French chemists Antoine Bussy and Michel Chevreul decided to name acetone by adding the suffix -one to the stem of the corresponding acid (viz, acetic acid) just as a similarly prepared product of what was then confused with margaric acid was named margarone. By 1852, English chemist Alexander William Williamson realized that acetone was methyl acetyl; the following year, the French chemist Charles Frédéric Gerhardt concurred. In 1865, the German chemist August Kekulé published the modern structural formula for acetone. Johann Josef Loschmidt had presented the structure of acetone in 1861, but his privately published booklet received little attention. During World War I, Chaim Weizmann developed the process for industrial production of acetone (Weizmann Process).

Production

In 2010, the worldwide production capacity for acetone was estimated at 6.7 million tonnes per year. With 1.56 million tonnes per year, the United States had the highest production capacity, followed by Taiwan and China. The largest producer of acetone is INEOS Phenol, owning 17% of the world's capacity, with also significant capacity (7–8%) by Mitsui, Sunoco and Shell in 2010. INEOS Phenol also owns the world's largest production site (420,000 tonnes/annum) in Beveren (Belgium). Spot price of acetone in summer 2011 was 1100–1250 USD/tonne in the United States.

Current method

Acetone is produced directly or indirectly from propene. Approximately 83% of acetone is produced via the cumene process; as a result, acetone production is tied to phenol production. In the cumene process, benzene is alkylated with propylene to produce cumene, which is oxidized by air to produce phenol and acetone:

Other processes involve the direct oxidation of propylene (Wacker-Hoechst process), or the hydration of propylene to give 2-propanol, which is oxidized (dehydrogenated) to acetone.

Older methods

Previously, acetone was produced by the dry distillation of acetates, for example calcium acetate in ketonic decarboxylation.

Ca(CH₃COO)₂ → CaO_(s) + CO_2(g) + (CH₃)₂CO

After that time, during World War I, acetone was produced using acetone-butanol-ethanol fermentation with Clostridium acetobutylicum bacteria, which was developed by Chaim Weizmann (later the first president of Israel) in order to help the British war effort, in the preparation of Cordite. This acetone-butanol-ethanol fermentation was eventually abandoned when newer methods with better yields were found.

Chemical properties

Acetone is reluctant to form a hydrate:

(CH₃)₂C=O + H₂O ⇌ (CH₃)₂C(OH)₂ K = 10⁻³ M⁻¹

Like most ketones, acetone exhibits the keto–enol tautomerism in which the nominal keto structure (CH₃)₂C=O of acetone itself is in equilibrium with the enol isomer (CH₃)C(OH)=(CH₂) (prop-1-en-2-ol). In acetone vapor at ambient temperature, only 2.4×10⁻⁷% of the molecules are in the enol form.

In the presence of suitable catalysts, two acetone molecules also combine to form the compound diacetone alcohol (CH₃)C=O(CH₂)C(OH)(CH₃)₂, which on dehydration gives mesityl oxide (CH₃)C=O(CH)=C(CH₃)₂. This product can further combine with another acetone molecule, with loss of another molecule of water, yielding phorone and other compounds.

Acetone is a weak Lewis base that forms adducts with soft acids like I₂ and hard acids like phenol. Acetone also forms complexes with divalent metals.

Under ultraviolet light, acetone fluoresces.

The flame temperature of pure acetone is 1980 °C.

Polymerisation

At its melting point (−96 °C) is claimed to polymerize to give a white elastic solid, soluble in acetone, stable for several hours at room temperature. To do so, a vapor of acetone is co-condensed with magnesium as a catalyst onto a very cold surface.

Natural occurrence

Humans exhale several milligrams of acetone per day. It arises from decarboxylation of acetoacetate. Small amounts of acetone are produced in the body by the decarboxylation of ketone bodies. Certain dietary patterns, including prolonged fasting and high-fat low-carbohydrate dieting, can produce ketosis, in which acetone is formed in body tissue. Certain health conditions, such as alcoholism and diabetes, can produce ketoacidosis, uncontrollable ketosis that leads to a sharp, and potentially fatal, increase in the acidity of the blood. Since it is a byproduct of fermentation, acetone is a byproduct of the distillery industry.

Metabolism

Acetone can then be metabolized either by CYP2E1 via methylglyoxal to D-lactate and pyruvate, and ultimately glucose/energy, or by a different pathway via propylene glycol to pyruvate, lactate, acetate (usable for energy) and propionaldehyde.

Uses

About a third of the world's acetone is used as a solvent, and a quarter is consumed as acetone cyanohydrin, a precursor to methyl methacrylate.

Chemical intermediate

Acetone is used to synthesize methyl methacrylate. It begins with the initial conversion of acetone to acetone cyanohydrin via reaction with hydrogen cyanide (HCN):

(CH₃)₂CO + HCN → (CH₃)₂C(OH)CN

In a subsequent step, the nitrile is hydrolyzed to the unsaturated amide, which is esterified:

(CH₃)₂C(OH)CN + CH₃OH → CH₂C(CH₃)CO₂CH₃ + NH₃

The third major use of acetone (about 20%) is synthesizing bisphenol A. Bisphenol A is a component of many polymers such as polycarbonates, polyurethanes, and epoxy resins. The synthesis involves the condensation of acetone with phenol:

(CH₃)₂CO + 2 C₆H₅OH → (CH₃)₂C(C₆H₄OH)₂ + H₂O

Many millions of kilograms of acetone are consumed in the production of the solvents methyl isobutyl alcohol and methyl isobutyl ketone. These products arise via an initial aldol condensation to give diacetone alcohol.

2 (CH₃)₂CO → (CH₃)₂C(OH)CH₂C(O)CH₃

Condensation with acetylene gives 2-methylbut-3-yn-2-ol, precursor to synthetic terpenes and terpenoids.

Solvent

Acetone is a good solvent for many plastics and some synthetic fibers. It is used for thinning polyester resin, cleaning tools used with it, and dissolving two-part epoxies and superglue before they harden. It is used as one of the volatile components of some paints and varnishes. As a heavy-duty degreaser, it is useful in the preparation of metal prior to painting or soldering, and to remove rosin flux after soldering (to prevent adhesion of dirt and electrical leakage and perhaps corrosion or for cosmetic reasons), although it may attack some electronic components, such as polystyrene capacitors.

Although itself flammable, acetone is used extensively as a solvent for the safe transportation and storage of acetylene, which cannot be safely pressurized as a pure compound. Vessels containing a porous material are first filled with acetone followed by acetylene, which dissolves into the acetone. One litre of acetone can dissolve around 250 litres of acetylene at a pressure of 10 bars (1.0 MPa).

Acetone is used as a solvent by the pharmaceutical industry and as a denaturant in denatured alcohol. Acetone is also present as an excipient in some pharmaceutical drugs.

Lab and domestic solvent

A variety of organic reactions employ acetone as a polar, aprotic solvent, e.g. the Jones oxidation.

Because acetone is cheap, volatile, and dissolves or decomposes with most laboratory chemicals, an acetone rinse is the standard technique to remove solid residues from laboratory glassware before a final wash. Despite common desiccatory use, acetone dries only via bulk displacement and dilution. It forms no azeotropes with water (see azeotrope tables). Acetone also removes certain stains from microscope slides.

Acetone freezes well below −78 °C. An acetone/dry ice mixture cools many low-temperature reactions. Make-up artists use acetone to remove skin adhesive from the netting of wigs and mustaches by immersing the item in an acetone bath, then removing the softened glue residue with a stiff brush. Acetone is a main ingredient in many nail polish removers because it breaks down nail polish. It is used for all types of nail polish removal, like gel nail polish, dip powder and acrylic nails.

Biology

Proteins precipitate in acetone. The chemical modifies peptides, both at α- or ε-amino groups, and in a poorly understood but rapid modification of certain glycine residues.

In pathology, acetone helps find lymph nodes in fatty tissues (such as the mesentery) for tumor staging. The liquid dissolves the fat and hardens the nodes, making them easier to find.

Medical

Dermatologists use acetone with alcohol for acne treatments to chemically peel dry skin. Common agents used today for chemical peeling are salicylic acid, glycolic acid, azelaic acid, 30% salicylic acid in ethanol, and trichloroacetic acid (TCA). Prior to chemexfoliation, the skin is cleaned and excess fat removed in a process called defatting. Acetone, hexachlorophene, or a combination of these agents was used in this process.

Acetone has been shown to have anticonvulsant effects in animal models of epilepsy, in the absence of toxicity, when administered in millimolar concentrations. It has been hypothesized that the high-fat low-carbohydrate ketogenic diet used clinically to control drug-resistant epilepsy in children works by elevating acetone in the brain. Because of their higher energy requirements, children have higher acetone production than most adults – and the younger the child, the higher the expected production. This indicates that children are not uniquely susceptible to acetone exposure. External exposures are small compared to the exposures associated with the ketogenic diet.

Safety

Acetone's most hazardous property is its extreme flammability. In small amounts, acetone burns with a dull blue flame; in larger amounts, fuel evaporation causes incomplete combustion and a bright yellow flame. When hotter than acetone's flash point of −20 °C (−4 °F), air mixtures of 2.5‑12.8% acetone (by volume) may explode or cause a flash fire. Vapors can flow along surfaces to distant ignition sources and flash back.

Static discharge may also ignite acetone vapors, though acetone has a very high ignition initiation energy and accidental ignition is rare. Acetone's auto-ignition temperature is the relatively high 465 °C (869 °F); moreover, auto-ignition temperature depends upon experimental conditions, such as exposure time, and has been quoted as high as 535 °C. Even pouring or spraying acetone over red-glowing coal will not ignite it, due to the high vapour concentration and the cooling effect of evaporation.

Acetone should be stored away from strong oxidizers, such as concentrated nitric and sulfuric acid mixtures. It may also explode when mixed with chloroform in the presence of a base. When oxidized without combustion, for example with hydrogen peroxide, acetone may form acetone peroxide, a highly unstable primary explosive. Acetone peroxide may be formed accidentally, e.g. when waste peroxide is poured into waste solvents.

Toxicity

Acetone occurs naturally as part of certain metabolic processes in the human body, and has been studied extensively and is believed to exhibit only slight toxicity in normal use. There is no strong evidence of chronic health effects if basic precautions are followed. It is generally recognized to have low acute and chronic toxicity if ingested and/or inhaled. Acetone is not currently regarded as a carcinogen, a mutagen, or a concern for chronic neurotoxicity effects.

Acetone can be found as an ingredient in a variety of consumer products ranging from cosmetics to processed and unprocessed foods. Acetone has been rated as a generally recognized as safe (GRAS) substance when present in drinks, baked foods, desserts, and preserves at concentrations ranging from 5 to 8 mg/L.

Acetone is however an irritant, causing mild skin and moderate-to-severe eye irritation. At high vapor concentrations, it may depress the central nervous system like many other solvents. Acute toxicity for mice by ingestion (LD₅₀) is 3 g/kg, and by inhalation (LC₅₀) is 44 g/m³ over 4 hours.

Environmental effects

Although acetone occurs naturally in the environment in plants, trees, volcanic gases, forest fires, and as a product of the breakdown of body fat, the majority of the acetone released into the environment is of industrial origin. Acetone evaporates rapidly, even from water and soil. Once in the atmosphere, it has a 22-day half-life and is degraded by UV light via photolysis (primarily into methane and ethane.) Consumption by microorganisms contributes to the dissipation of acetone in soil, animals, or waterways.

EPA classification

In 1995, the United States Environmental Protection Agency (EPA) removed acetone from the list of volatile organic compounds. The companies requesting the removal argued that it would "contribute to the achievement of several important environmental goals and would support EPA's pollution prevention efforts", and that acetone could be used as a substitute for several compounds that are listed as hazardous air pollutants (HAP) under section 112 of the Clean Air Act. In making its decision EPA conducted an extensive review of the available toxicity data on acetone, which was continued through the 2000s. It found that the evaluable "data are inadequate for an assessment of the human carcinogenic potential of acetone".

Extraterrestrial occurrence

On 30 July 2015, scientists reported that upon the first touchdown of the Philae lander on comet 67P's surface, measurements by the COSAC and Ptolemy instruments revealed sixteen organic compounds, four of which were seen for the first time on a comet, including acetamide, acetone, methyl isocyanate, and propionaldehyde.