Vivian Marlowe
Chromium is a metallic element that is known to form a variety of compounds with alkane, alkene, and alcohol. Chromium exhibits a wide range of reactivity with these compounds, which can be harnessed for various synthetic purposes. One of the most common forms of chromium used in these reactions is chromic acid (H2CrO4), which serves as a strong reagent for oxidizing alcohols to ketones and carboxylic acids. This reaction is commonly employed in undergraduate laboratories for educational purposes. However, chromic acid is not frequently used in advanced organic chemistry research due to its high toxicity and the availability of more useful reagents. Beyond chromic acid, chromium(VI) complexes, such as chromium trioxide, are also employed in the oxidation of alcohols, particularly primary and secondary alcohols, to form carbonyl compounds. These reactions showcase the diverse reactivity of chromium with alkane, alkene, and alcohol, highlighting its importance in synthetic chemistry.
| Characteristics | Values |
|---|---|
| Chromium's reaction with alkanes | Oxidation of alkanes gives carboxylic acid except oxidation with CrO2Cl2 which gives an aldehyde |
| Chromium's reaction with alkenes | N/A |
| Chromium's reaction with alcohols | Chromic acid (H2CrO4) is a strong acid and a reagent for oxidizing alcohols to ketones and carboxylic acids |
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What You'll Learn
- Chromium (VI) reagents are more selective than potassium permanganate (KMnO4) in oxidising alcohols to aldehydes and ketones
- Chromic acid (H2CrO4) is formed by combining chromium compounds with aqueous acid
- Chromic acid oxidises primary alcohols to aldehydes, which can be further oxidised to carboxylic acids
- Secondary alcohols are oxidised to ketones, which are not susceptible to further oxidation by dichromate
- The oxidation of allylic or benzylic alcohols requires either a vast excess of oxidant or prolonged reaction times
Chromium (VI) reagents are more selective than potassium permanganate (KMnO4) in oxidising alcohols to aldehydes and ketones
Chromium (VI) reagents are highly selective reagents for the oxidation of alcohols to aldehydes and ketones. They are more selective than potassium permanganate (KMnO4). One such chromium (VI) reagent is chromic acid (H2CrO4), formed from chromium trioxide (CrO3) or sodium dichromate (Na2Cr2O7) in the presence of sulfuric acid. This is also known as the Jones reagent.
Chromic acid is a strong acid and a reagent for oxidizing primary alcohols to carboxylic acids and secondary alcohols to ketones. The oxidation of primary alcohols by chromic acid occurs through the addition of the alcohol oxygen to chromium, which makes it a good leaving group. A base, usually water, can then remove a proton from the carbon, forming a new π bond and breaking the O-Cr bond.
The oxidation of secondary alcohols to ketones by chromic acid is also possible. The chromic ester formed by the nucleophilic attack of the alcohol's oxygen atom on the chromium atom undergoes an α-elimination reaction. This oxidation step forms a carbon-oxygen double bond.
Chromium (VI) reagents are more selective than potassium permanganate (KMnO4) because they can differentiate between isomeric alcohols. For example, cis-4-tert-butyl-cyclohexane reacts faster than the trans isomer. The formation of a chromic ester is sensitive to the steric environment of the oxygen atom, which acts as a nucleophile in forming the Cr—O bond.
Potassium permanganate (KMnO4) is a strong oxidizing agent that oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones. However, it is less selective than chromium (VI) reagents and may lead to overoxidation, cleaving carbon-carbon bonds if the temperature and concentrations are not precisely controlled.
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Chromic acid (H2CrO4) is formed by combining chromium compounds with aqueous acid
Chromic acid (H2CrO4) is a naturally occurring oxide and a very strong acid. It is formed by combining chromium compounds with aqueous acid. It is also formed 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 is an intermediate in chromium plating and is also used in ceramic glazes and coloured glass. It is a strong oxidizing agent and can be used to clean laboratory glassware, particularly otherwise insoluble organic residues.
Chromic acid is also used as a bleach in processing black-and-white photographic reversal film. It is capable of oxidizing many kinds of organic compounds and several variations of this reagent have been developed. Chromic acid in aqueous sulfuric acid and acetone is known as the Jones reagent, which oxidizes primary and secondary alcohols to carboxylic acids and ketones, respectively, while rarely affecting unsaturated bonds.
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 colour change from orange to brownish-green, 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.
Chromium trioxide and pyridinium chloride produce pyridinium chlorochromate. This reagent converts primary alcohols to the corresponding aldehydes. Hexavalent chromium compounds such as chromium trioxide, chromates, chromic acids, and chlorochromate are toxic and carcinogenic. Therefore, chromic acid oxidation is used only in the aerospace industry and not on any other industrial scale. Chromic acids are strong oxidizers and can react violently if mixed with some easily oxidizable organic substances, which can cause explosions or fires.
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Chromic acid oxidises primary alcohols to aldehydes, which can be further oxidised to carboxylic acids
Chromic acid (H2CrO4) is a strong acid and a reagent for oxidizing primary alcohols to aldehydes and carboxylic acids. It is formed by adding acid to a source of chromium. The oxidation of an alcohol to form an aldehyde or ketone is an important process in synthesis. The hydroxy hydrogen of the alcohol is replaced by a leaving group, and a base then abstracts the proton bound to the alcohol carbon. This results in the elimination of the leaving group and the formation of a new carbon-oxygen double bond.
Chromic acid oxidises primary alcohols to aldehydes. This reaction occurs when the alcohol oxygen atom bridges the carbon and chromium atoms, forming a chromic ester. This is followed by an α-elimination reaction, where a carbon-oxygen double bond forms. The aldehyde can then react with water to form a gem-diol.
The aldehydes produced in the first reaction can be further oxidised to carboxylic acids. This is achieved through the addition of the alcohol oxygen to chromium, which makes it a good leaving group. A base, most likely water, can then remove a proton from the carbon, forming a new π bond and breaking the O-Cr bond.
Chromium trioxide (CrO3) is a common oxidising agent used by organic chemists to oxidise secondary alcohols to ketones. During this reaction, CrO3 is reduced to form H2CrO3. A common method for this oxidation uses chromic acid (H2CrO4) as the oxidising agent. This reaction is known as the Jones oxidation, which uses acetone as a co-solvent to prevent over-oxidation of the organic product.
Pyridinium chlorochromate (PCC) is a milder version of chromic acid that can convert primary alcohols into aldehydes without oxidising them further to carboxylic acids. However, this reagent is being replaced in laboratories by Dess-Martin periodinane (DMP), which offers higher yields and less rigorous reaction conditions.
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Secondary alcohols are oxidised to ketones, which are not susceptible to further oxidation by dichromate
Chromium (VI) reagents are selective oxidizing agents for alcohols. Chromic acid (H2CrO4) is a strong acid and a reagent for oxidizing primary alcohols to carboxylic acids and secondary alcohols to ketones. The oxidation of a secondary alcohol involves the loss of hydrogen atoms and electrons from the alcohol functional group (-OH), resulting in the formation of a ketone functional group (-CO-). The reaction conditions and the oxidizing agent used can determine the extent of the oxidation.
Mild oxidizing agents such as pyridinium chlorochromate (PCC) or Collins reagent can be used to oxidize secondary alcohols to ketones. For example, if you heat the secondary alcohol propan-2-ol with sodium or potassium dichromate(VI) solution acidified with dilute sulfuric acid, propanone is formed. However, stronger oxidizing agents such as potassium permanganate (KMnO4) or sodium dichromate (Na2Cr2O7) can also be used, but they are more likely to cause over-oxidation, leading to the formation of carboxylic acids.
The oxidation of a secondary alcohol typically leads to the formation of a ketone rather than an aldehyde or a carboxylic acid. This is because, in the case of secondary alcohols, there is no hydrogen atom attached to the carbon atom that can be removed to form an aldehyde or carboxylic acid. The oxidizing agent is removing the hydrogen from the -OH group, and a hydrogen from the carbon atom is attached to the hydroxyl group.
The products of oxidation depend on whether the alcohol is primary, secondary, or tertiary. Primary alcohols can be oxidized to form an aldehyde, which can then be further oxidized to form a carboxylic acid. Tertiary alcohols, on the other hand, cannot be oxidized by acidified sodium or potassium dichromate(VI) solution.
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The oxidation of allylic or benzylic alcohols requires either a vast excess of oxidant or prolonged reaction times
Chromium (VI) reagents are selective reagents for oxidizing alcohols to aldehydes and ketones. Chromic acid (H2CrO4), a strong acid, is a commonly used reagent for oxidizing primary alcohols to carboxylic acids and secondary alcohols to ketones. However, chromic acid is highly toxic and, therefore, finds limited use.
The oxidation of allylic or benzylic alcohols, in particular, requires either a vast excess of oxidant or prolonged reaction times. This is because the oxidation of these alcohols is often very slow and requires large excesses of the reagent. For instance, MnO2, a commonly used reagent for the oxidation of allylic or benzylic alcohols, requires proper activation to obtain acceptable yields, and the reactions are typically slow.
To overcome the challenges associated with the oxidation of allylic or benzylic alcohols, researchers have explored alternative reagents and conditions. Floreancig reported the use of catalytic amounts of DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) in the presence of excess MnO2 for oxidative transformations. While DDQ provided excellent yields in a short duration, its high cost limits its large-scale use.
Another approach to improve the oxidation of allylic or benzylic alcohols is by using chromium(III)-impregnated Nafion 511 (NAFK) as a catalytic oxidant in the presence of t-butyl hydroperoxide. This method offers good yields of ketones (80-100%), but it also leads to the formation of complex mixtures during the oxidation of primary alcohols.
Overall, while chromium reagents are effective for oxidizing alcohols, the oxidation of allylic or benzylic alcohols specifically requires special considerations due to the slow reaction kinetics and the need for excess reagents.
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