Presentation on theme: "Preparation and identification of an aldehyde. Primary alcohols are oxidised to form aldehydes and then carboxylic acids. Propan-1-olPropanal Propanoic."— Presentation transcript:
Primary alcohols are oxidised to form aldehydes and then carboxylic acids. Propan-1-olPropanal 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.
H + /Cr 2 O 7 2– Propan-1-ol Cold water in Cold water out Small beaker in ice-water The apparatus
A small flame is used to warm the alcohol, then the acidified dichromate is added. The colour changes from orange to green.
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.
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) Tertiary alcohols are not oxidised by oxidising agents such as dichromate or permanganate. Ketones do not react with Tollen’s reagent.