Reaction intermediate

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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

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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

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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.

Chromic acid

From Wikipedia, the free encyclopedia

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

Chromic acid is a chemical compound with the chemical formula H₂CrO₄. It is also a jargon for a solution formed by the addition of sulfuric acid to aqueous solutions of dichromate. It consists at least in part of chromium trioxide.

The term "chromic acid" is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the molecular species, H₂CrO₄ of which the trioxide is the anhydride. Chromic acid features chromium in an oxidation state of +6 (and a valence of VI or 6). It is a strong and corrosive oxidizing agent and a moderate carcinogen.

Molecular chromic acid

Partial predominance diagram for chromate

Molecular chromic acid, H₂CrO₄, in principle, resembles sulfuric acid, H₂SO₄. It would ionize accordingly:

H₂CrO₄ ⇌ [HCrO₄]⁻ + H⁺

The pK_a for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6. The structure of the mono anion has been determined by X-ray crystallography. In this tetrahedral oxyanion, three Cr-O bond lengths are 156 pm and the Cr-OH bond is 201 pm

[HCrO₄]⁻ condenses to form dichromate:

2 [HCrO₄]⁻ ⇌ [Cr₂O₇]²⁻ + H₂O, logK_D = 2.05.

Furthermore, the dichromate can be protonated:

[HCr₂O₇]⁻ ⇌ [Cr₂O₇]²⁻ + H⁺, pK_a = 1.8

Loss of the second proton occurs in the pH range 4–8, making the ion [HCrO₄]⁻ a weak acid.

Molecular chromic acid could in principle be made by adding chromium trioxide to water (cf. manufacture of sulfuric acid).

CrO₃ + H₂O ⇌ H₂CrO₄

In practice, the reverse reaction occurs: molecular chromic acid dehydrates. Some insights can be gleaned from observations on the reaction of dichromate solutions with sulfuric acid. The first colour change from orange to red signals the conversion of dichromate to chromic acid. Under these conditions deep red crystals of chromium trioxide precipitate from the mixture, without further colour change.

Chromium trioxide is the anhydride of molecular chromic acid. It is a Lewis acid and can react with a Lewis base, such as pyridine in a non-aqueous medium such as dichloromethane (Collins reagent).

Structure of tetrachromic acid H₂Cr₄O₁₃·2H₂O, one component of concentrated "chromic acid". The H-atom positions are calculated, not observed. Color code: red = O, white = H, blue = Cr.

Higher chromic acids with the formula H₂Cr_nO_(3n+1) are probable components of concentrated solutions of chromic acid.

Uses

Chromic acid is an intermediate in chromium plating, and is also used in ceramic glazes, and colored glass. Because a solution of chromic acid in sulfuric acid (also known as a sulfochromic mixture or chromosulfuric acid) is a powerful oxidizing agent, it can be used to clean laboratory glassware, particularly of otherwise insoluble organic residues. This application has declined due to environmental concerns. Furthermore, the acid leaves trace amounts of paramagnetic chromic ions (Cr³⁺) that can interfere with certain applications, such as NMR spectroscopy. This is especially the case for NMR tubes. Piranha solution can be used for the same task, without leaving metallic residues behind.

Chromic acid was widely used in the musical instrument repair industry, due to its ability to "brighten" raw brass. A chromic acid dip leaves behind a bright yellow patina on the brass. Due to growing health and environmental concerns, many have discontinued use of this chemical in their repair shops.

It was used in hair dye in the 1940s, under the name Melereon.

It is used as a bleach in processing black and white photographic reversal film.

Reactions

Chromic acid is capable of oxidizing many kinds of organic compounds and many variations on this reagent have been developed:

• Chromic acid in aqueous sulfuric acid and acetone is known as the Jones reagent, which will oxidize primary and secondary alcohols to carboxylic acids and ketones respectively, while rarely affecting unsaturated bonds.
• Pyridinium chlorochromate is generated from chromium trioxide and pyridinium chloride. This reagent converts primary alcohols to the corresponding aldehydes (R–CHO).
• Collins reagent is an adduct of chromium trioxide and pyridine used for diverse oxidations.
• Chromyl chloride, CrO₂Cl₂ is a well-defined molecular compound that is generated from chromic acid.

Illustrative transformations

• Oxidation of methylbenzenes to benzoic acids.
• Oxidative scission of indene to homophthalic acid.
• Oxidation of secondary alcohol to ketone (cyclooctanone) and nortricyclanone.

Use in qualitative organic analysis

In organic chemistry, dilute solutions of chromic acid can be used to oxidize primary or secondary alcohols to the corresponding aldehydes and ketones. Similarly, it can also be used to oxidize an aldehyde to its corresponding carboxylic acid. Tertiary alcohols and ketones are unaffected. Because the oxidation is signaled by a color change from orange to brownish green (indicating chromium being reduced from oxidation state +6 to +3), chromic acid is commonly used as a lab reagent in high school or undergraduate college chemistry as a qualitative analytical test for the presence of primary or secondary alcohols, or aldehydes.

Alternative reagents

In oxidations of alcohols or aldehydes into carboxylic acids, chromic acid is one of several reagents, including several that are catalytic. For example, nickel(II) salts catalyze oxidations by bleach (hypochlorite). Aldehydes are relatively easily oxidized to carboxylic acids, and mild oxidizing agents are sufficient. Silver(I) compounds have been used for this purpose. Each oxidant offers advantages and disadvantages. Instead of using chemical oxidants, electrochemical oxidation is often possible.

Safety

Hexavalent chromium compounds (including chromium trioxide, chromic acids, chromates, chlorochromates) are toxic and carcinogenic. Chromium trioxide and chromic acids are strong oxidizers and may react violently if mixed with easily oxidizable organic substances.

Chromic acid burns are treated with a dilute sodium thiosulfate solution.

Aqua regia

From Wikipedia, the free encyclopedia

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

 

Aqua regia 

Aqua regia (/ˈreɪɡiə, ˈriːdʒiə/; from Latin, "regal water" or "royal water") is a mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3. Aqua regia is a fuming liquid. Freshly prepared aqua regia is colorless, but it turns yellow, orange, or red within seconds from the formation of nitrosyl chloride and nitrogen dioxide. It was so named by alchemists because it can dissolve noble metals such as gold and platinum, though not all metals.

Preparation and decomposition

Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine gas:

HNO₃ + 3 HCl → NOCl + Cl₂ + 2 H₂O

as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, aqua regia loses its potency. Nitrosyl chloride (NOCl) can further decompose into nitric oxide (NO) and elemental chlorine (Cl₂):

2 NOCl → 2 NO + Cl₂

This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia also contain nitric oxide (NO). Because nitric oxide readily reacts with atmospheric oxygen, the gases produced also contain nitrogen dioxide, NO₂ (red fume):

2 NO + O₂ → 2 NO₂

Applications

Aqua regia is primarily used to produce chloroauric acid, the electrolyte in the Wohlwill process for refining the highest purity (99.999%) gold.

Aqua regia is also used in etching and in specific analytic procedures. It is also used in some laboratories to clean glassware of organic compounds and metal particles. This method is preferred among most over the more traditional chromic acid bath for cleaning NMR tubes, because no traces of paramagnetic chromium can remain to spoil spectra. While chromic acid baths are discouraged because of the high toxicity of chromium and the potential for explosions, aqua regia is itself very corrosive and has been implicated in several explosions due to mishandling.

Because its components react quickly, resulting in its decomposition, aqua regia quickly loses its effectiveness (yet remains a strong acid), so its components are usually only mixed immediately before use.

Chemistry

Dissolving gold

Pure gold precipitate produced by the aqua regia chemical refining process

Aqua regia dissolves gold, although neither constituent acid will do so alone. Nitric acid is a powerful oxidizer, which will dissolve a very small quantity of gold, forming gold(III) ions (Au³⁺). The hydrochloric acid provides a ready supply of chloride ions (Cl⁻), which react with the gold ions to produce tetrachloroaurate(III) anions ([AuCl₄]⁻), also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors formation of tetrachloroaurate(III) anions. This results in a removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become chloroauric acid. In addition, gold may be dissolved by the chlorine present in aqua regia. Appropriate equations are:

Au + 3 HNO
₃ + 4 HCl ${\displaystyle \stackrel{-\rightharpoonup }{\leftharpoondown }}$ [AuCl
₄]⁻
+ 3 NO
₂ + H
₃O⁺
+ 2 H
₂O

or

Au + HNO
₃ + 4 HCl ${\displaystyle \stackrel{-\rightharpoonup }{\leftharpoondown }}$ [AuCl
₄]⁻
+ NO + H
₃O⁺
+ H
₂O.

Solid tetrachloroauric acid may be isolated by evaporating the excess aqua regia, and decomposing the residual nitric acid by repeatedly heating the solution with additional hydrochloric acid. That step reduces nitric acid (see decomposition of aqua regia). If elemental gold is desired, it may be selectively reduced with reducing agents such as sulfur dioxide, hydrazine, oxalic acid, etc. The equation for the reduction of oxidized gold (Au³⁺) by sulfur dioxide (SO₂) is the following:

2 [AuCl₄]⁻(aq) + 3 SO₂(g) + 6 H₂O(l) → 2 Au(s) + 12 H⁺(aq) + 3 SO2−4(aq) + 8 Cl⁻(aq)

Dissolution of gold by aqua regia.

Initial state of the transformation.

 

Intermediate state of the transformation.

 

Final state of the transformation.

Dissolving platinum

Similar equations can be written for platinum. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product:

Pt(s) + 4 NO−3(aq) + 8 H⁺(aq) → Pt⁴⁺(aq) + 4 NO₂(g) + 4 H₂O(l)

3 Pt(s) + 4 NO−3(aq) + 16 H⁺(aq) → 3 Pt⁴⁺(aq) + 4 NO(g) + 8 H₂O(l)

The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion:

Pt⁴⁺(aq) + 6 Cl⁻(aq) → [PtCl₆]²⁻(aq)

Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of chloroplatinous acid (H₂[PtCl₄]) and nitrosoplatinic chloride ([NO]₂[PtCl₄]). The nitrosoplatinic chloride is a solid product. If full dissolution of the platinum is desired, repeated extractions of the residual solids with concentrated hydrochloric acid must be performed:

2 Pt(s) + 2 HNO₃(aq) + 8 HCl(aq) → [NO]₂[PtCl₄](s) + H₂[PtCl₄](aq) + 4 H₂O(l)

and

[NO]₂[PtCl₄](s) + 2 HCl(aq) ⇌ H₂[PtCl₄](aq) + 2 NOCl(g)

The chloroplatinous acid can be oxidized to chloroplatinic acid by saturating the solution with molecular chlorine (Cl₂) while heating:

H₂[PtCl₄](aq) + Cl₂(g) → H₂[PtCl₆](aq)

Dissolving platinum solids in aqua regia was the mode of discovery for the densest metals, iridium and osmium, both of which are found in platinum ores and are not dissolved by aqua regia, instead collecting as insoluble metallic powder (elemental Ir, Os) on the base of the vessel.

Dissolution of platinum by aqua regia.

 

Initial state of the transformation.

 

Intermediate state of the transformation.

 

Final state of the transformation (four days later).

Precipitating dissolved platinum

As a practical matter, when platinum group metals are purified through dissolution in aqua regia, gold (commonly associated with PGMs) is precipitated by treatment with iron(II) chloride. Platinum in the filtrate, as hexachloroplatinate(IV), is converted to ammonium hexachloroplatinate by the addition of ammonium chloride. This ammonium salt is extremely insoluble, and it can be filtered off. Ignition (strong heating) converts it to platinum metal:

3 [NH₄]₂[PtCl₆] → 3 Pt + 2 N₂ + 2 [NH₄]Cl + 16 HCl

Unprecipitated hexachloroplatinate(IV) is reduced with elemental zinc, and a similar method is suitable for small scale recovery of platinum from laboratory residues.

Reaction with tin

Aqua regia reacts with tin to form tin(IV) chloride, containing tin in its highest oxidation state:

4 HCl + 2 HNO₃ + Sn → SnCl₄ + NO₂ + NO + 3 H₂O

Reaction with other substances

It can react with iron pyrite to form Iron(III) chloride:

FeS₂ + 5 HNO₃ + 3 HCl → FeCl₃ + 2 H₂SO₄ + 5 NO + 2 H₂O

History

Aqua regia first appeared in the De inventione veritatis ("On the Discovery of Truth") by pseudo-Geber (after c. 1300), who produced it by adding sal ammoniac (ammonium chloride) to nitric acid. The preparation of aqua regia by directly mixing hydrochloric acid with nitric acid only became possible after the discovery in the late sixteenth century of the process by which free hydrochloric acid can be produced.

The fox in Basil Valentine's Third Key represents aqua regia, Musaeum Hermeticum, 1678

The third of Basil Valentine's keys (c. 1600) shows a dragon in the foreground and a fox eating a rooster in the background. The rooster symbolizes gold (from its association with sunrise and the sun's association with gold), and the fox represents aqua regia. The repetitive dissolving, heating, and redissolving (the rooster eating the fox eating the rooster) leads to the buildup of chlorine gas in the flask. The gold then crystallizes in the form of gold(III) chloride, whose red crystals Basil called "the rose of our masters" and "the red dragon's blood". The reaction was not reported again in the chemical literature until 1895.

Antoine Lavoisier called aqua regia nitro-muriatic acid in 1789.

When Germany invaded Denmark in World War II, Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of German physicists Max von Laue (1914) and James Franck (1925) in aqua regia to prevent the Nazis from confiscating them. The German government had prohibited Germans from accepting or keeping any Nobel Prize after jailed peace activist Carl von Ossietzky had received the Nobel Peace Prize in 1935. De Hevesy placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute. It was subsequently ignored by the Nazis who thought the jar—one of perhaps hundreds on the shelving—contained common chemicals. After the war, de Hevesy returned to find the solution undisturbed and precipitated the gold out of the acid. The gold was returned to the Royal Swedish Academy of Sciences and the Nobel Foundation. They re-cast the medals and again presented them to Laue and Franck.