Reactions of Alcohol

Reactions of Alcohols

Reactions of alcohols

Alcohols contain an –OH group covalently bonded to a carbon atom.

However, this –OH group does not behave in the same way as the hydroxide ion OH– because NaOH is a base and CH3OH is not.

Alcohols, when dissolved in water, do not alter the pH of the water.

Although the hydrogen atom is connected to an oxygen atom, alcohols do not readily donate the proton (they are weaker acids than water).

Reactions of Alcohol

Like the alkanes and alkenes, alcohols undergo complete combustion in a plentiful supply of oxygen gas, producing only carbon dioxide and water as products.

You may have used alcohol burners in an investigation in chapter 6.

The complete combustion of ethanol is as follows:

When balancing an equation for the combustion of an alcohol it is important to remember that there is an oxygen atom in the alcohol, unlike alkanes and alkenes

Oxidation of ethanol

The process of oxidation was defined in chapter 10 as the loss of electrons.

In organic chemistry oxidation is easily recognized as the gain of oxygen or the loss of hydrogen from a compound.

The oxidation reactions of alcohols vary, depending upon the type of alcohol involved.

Primary, secondary and tertiary alcohols all give different reactions with strong oxidizing agents such as acidified potassium dichromate(VI) solution or acidified potassium manganate(VII) solution.

Alcohol oxidizes to carboxylic acid In the laboratory, when an aqueous solution

of a primary alcohol such as ethanol is mixed with potassium dichromate (VI) and sulfuric acid, and the mixture heated under reflux, the alcohol is fully oxidized to a carboxylic acid.

During the process, the alcohol is initially oxidized to an aldehyde; however, by heating under reflux the aldehyde is further oxidized to a carboxylic acid.

When the reaction is ‘complete’, the condenser is turned around and the reaction mixture is distilled to collect an aqueous solution of the carboxylic acid.

If the aldehyde is the desired product during this reaction, then the reaction can be carried out at room temperature and the aldehyde can be distilled off from the mixture.

Some distillation apparatus is shown at the beginning of this chapter (p. 341).

The oxidation reaction of the primary alcohol (e.g. ethanol) to a carboxylic acid may be represented simply by an equation in which the symbol [O] represents the oxygen supplied by the oxidizing agent:

Alternatively, we may write half-equations and a complete equation to represent the redox nature of the reaction:

A secondary alcohol has the hydroxyl group on a carbon that is bonded to two other carbons. Propan-2-ol and butan-2-ol are examples of secondary alcohols.

Secondary alcohol oxidation

When secondary alcohols are oxidized, ketones are formed.

This reaction is very similar to the one in which aldehydes are produced, but the placement of the hydroxyl group results in the production of a ketone rather than an aldehyde and, ultimately, a carboxylic acid.

The oxidation reaction of a secondary alcohol such as propan-2-ol to a ketone may be represented simply as:

Alternatively, we may write half-equations and a complete equation to represent the redox nature of the reaction:

When these oxidation reactions are performed in a laboratory investigation, the change in colour of the oxidizing agent indicates that the reaction has proceeded.

Potassium dichromate, K2Cr2O7, changes colour from orange (Cr2O72–) to green (Cr3+) during this reaction. If potassium manganate(VII), KMnO4, is used instead, it changes colour from purple to colourless.

Tertiary alcohols, those with the hydroxyl group bonded to a carbon atom that is bonded to three other carbon atoms, are not easily oxidized.

An example of a tertiary alcohol is 2-methylpropan-2-ol.