Question

 

f.) Predict the products of oxidation and dehydration reactions of alcohol

g.) Describe the oxidation and reduction reactions of thiols to form or break disulfides

h.) Predict the products of oxidation and reduction reactions of aldehydes and ketones

i.) Describe the formation and hydrolysis of hemiacetals and acetals from alcohols and aldehydes or ketones

Answer 1

f) The Oxidation of Alcohols. This page looks at the oxidation of alcohols using acidified sodium or potassium dichromate(VI) solution. This reaction is used to make aldehydes, ketones and carboxylic acids, and as a way of distinguishing between primary, secondary and tertiary alcohols.

• 1' alcohol → Carboxylic acid
• 2' alcohol → Ketone
• 3' alcohol → No reaction

Dehydration of Alcohols:

Alcohol upon reaction with protic acids tends to lose a molecule of water to form alkenes. These reactions are known as dehydration of alcohols.

Dehydration of alcoholsfollows a three step mechanism:

1. Formation of protonated alcohol
2. Formation of carbocation
3. Formation of alkenes

g)

Disulfide (sulfur-sulfur) linkages between two cysteine residues are an integral component of the three-dimensional structure of many proteins. The interconversion between thiols and disulfide groups is a redox reaction: the thiol is the reduced state, and the disulfide is the oxidized state.

Disulfide bonds are usually formed from the oxidation of sulfhydryl (–SH) groups, especially in biological contexts. The transformation is depicted as follows:

2 RSH ⇌ RS–SR + 2 H⁺ + 2 e⁻

Although the hydrogenation or reduction of disulfides is usually not practical, the equilibrium constant for the reaction provides a measure of the standard redox potential for disulfides:

RSSR + H₂ → 2 RSH

h)It depends on whether the reaction is done under acidic or alkaline conditions. Under acidic conditions, the aldehyde is oxidised to a carboxylic acid. Under alkaline conditions, this couldn't form because it would react with the alkali. A salt is formed instead. Oxidation of ketones involves cleavage of bond between carbonyl carbon and a-carbon on either side of keto group giving a mixture of carboxylic acids.

In general terms, reduction of an aldehyde leads to a primary alcohol. Reduction of a ketone leads to a secondary alcohol.

i) Alcohols add reversibly to aldehydes and ketones to form hemiacetals. and acetal is formed by reaction with two equivalents (or an excess amount) of an alcohol and elimination of water. Once pure hemiacetal or acetals are obtained they may be hydrolyzed back to their starting components by treatment with aqueous acid and an excess of water.