Preparation and identification of an aldehyde Primary alcohols

Preparation and identification of an aldehyde

Primary alcohols are oxidised to form aldehydes and then carboxylic acids. Propan-1 -ol Propanal Propanoic acid

Although both propan-1 -ol and propanoic acid form hydrogen bonds (due to the presence of the –OH group), propanal does not form hydrogen bonds with itself. This causes propanal to have a much lower boiling point than the alcohol or acid (48 °C, compared with 97 °C for the alcohol and 141 °C for the acid). To make a sample of propanal from propan-1 -ol, we must distill off the aldehyde.

The apparatus H+/Cr 2 O 72– Cold water out Cold water in Propan-1 -ol Small beaker in ice-water

A small flame is used to warm the alcohol, then the acidified dichromate is added. The colour changes from orange to green.

Liquid starts to condense in the condenser.

It drains down the tube…. and drips into the beaker.

The Tollen’s test for aldehydes Tollen’s reagent is [Ag(NH 3)2]+. It is a mild oxidising agent which can oxidise an aldehyde to a carboxylic acid, but which is not strong enough to oxidise alcohols. When Tollen’s reagent is reduced, it forms silver metal, which sometimes appears as a black precipitate or suspension, and sometimes coats the test tube to make a ‘silver mirror’. Tollen’s reagent must be freshly-prepared.

Take about 1 m. L of silver nitrate solution.

Add a drop of sodium hydroxide solution to form the milky-brown precipitate of silver oxide.

Add ammonia solution to complex the precipitate.

Collect a few drops of the aldehyde and add them to the Tollen’s reagent.

Gently heat the mixture.

The positive test for aldehyde is either a black precipitate (left), or a silver mirror (right). You are most likely to get a silver mirror when using a new test tube — one that has no scratches on the inside of the glass.

To make propanoic acid instead of propanal, we would oxidise propan-1 -ol under reflux. As the aldehyde is formed it boils off, condenses in the cold condenser, and drips back down into the reaction flask where it can be further oxidised to the acid.

Secondary alcohols are oxidised to ketones: propan-2 -ol propanone (acetone) Ketones do not react with Tollen’s reagent. Tertiary alcohols are not oxidised by oxidising agents such as dichromate or permanganate.