Organic A2 Chemistry

Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid.

The esterification reaction is both slow and reversible. The equation for the reaction between an acid RCOOH and an alcohol R'OH (where R and R' can be the same or different) is:

So, for example, if you were making ethyl ethanoate from ethanoic acid and ethanol, the equation would be:

The top chain comes from the carboxylic acid and the bottom chain comes from the alcohol.

This method will work for alcohols and phenols.

If you add an acyl chloride to an alcohol, you get a vigorous (even violent) reaction at room temperature producing an ester and clouds of steamy acidic fumes of hydrogen chloride.

For example, if you add the liquid ethanoyl chloride to ethanol, you get a burst of hydrogen chloride produced together with the liquid ester ethyl ethanoate.

This is an easy way of producing an ester from an alcohol because it happens at room temperature, and is irreversible. Making an ester from an alcohol and a carboxylic acid (the usual alternative method) needs heat, a catalyst and is reversible - so that it is difficult to get a 100% conversion.

You get exactly the same organic product whether you use lithium tetrahydridoaluminate or sodium tetrahydridoborate.

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

For example, with propanone you get propan-2-ol:

Reduction of a ketone leads to a secondary alcohol.

Brady's reagent is 2,4 - DNP. A positive result is orange/yellow crystalline dirivative or precipitate. Tests that carbonyl group is present. Doesn't specify whether it is an aldehyde or ketone.

Add ammonium solution of silver nitrate (tollen's reagent). If there is a silver mirror on test tube, it is an aldehyde, if there is not reaction it is a ketone.

Add acidified potassium dichromate. If there is a colour change from orange to green it is an aldehyde. If there is no reaction it is a ketone.

Add iodine in a solution of NaOH. If a yellow precipitate forms there is a CH3 group attached to the carbonyl group.

When nitriles are hydrolysed you can think of them reacting with water in two stages - first to produce an amide, and then the ammonium salt of a carboxylic acid.

For example, ethanenitrile would end up as ammonium ethanoate going via ethanamide.

In practice, the reaction between nitriles and water would be so slow as to be completely negligible. The nitrile is instead heated with either a dilute acid such as dilute hydrochloric acid, or with an alkali such as sodium hydroxide solution.

The nitrile is heated under reflux with dilute hydrochloric acid. Instead of getting an ammonium salt as you would do if the reaction only involved water, you produce the free carboxylic acid.

For example, with ethanenitrile and hydrochloric acid you would get ethanoic acid and ammonium chloride.

In solution in water, a hydrogen ion is transferred from the -COOH group to a water molecule.

As the cyanide ion approaches the slightly positive carbon atom, the lone pair of electrons is attracted towards the carbon and forms a bond with it. At the same time the two electrons in one of the bonds joining the carbon to the oxygen are repelled until they end up entirely on the oxygen - giving it a negative charge.

The negative ion formed then picks up a hydrogen ion from somewhere - for example, from a hydrogen cyanide molecule.

The reaction with pure water is so slow that it is never used. The reaction is catalysed by dilute acid, and so the ester is heated under reflux with a dilute acid like dilute hydrochloric acid or dilute sulphuric acid.

Here are two simple examples of hydrolysis using an acid catalyst. First, hydrolysing ethyl ethanoate:

. . . and then hydrolysing methyl propanoate: