Aldehydes and Ketones

Distinguishing Aldehydes and Ketones

There are two simple tests that can be done to tell aldehydes and ketones apart.

Fehling's solution

This has two parts to it. Start off with a few cm's depth of Fehlings 1 in a test tube, then add Fehlings 2 dropwise. A gelatenous precipitate forms to begin with, then a deep blue solution will form. This can then be warmed with the sample. An aldehyde will give a brick red precipitate upon warming, whereas there will be no reaction with a ketone.

Tollens Reagent

This is an Ammoniacal sivler nitrate solution. In a test tube, start with a few cm depth of silver nitrate (AgNO₃). then add a few drops of Sodium Hydroxide and a brown precipitate will form. Next add Ammonia until a colourless solution forms and the precipitate has just disappeared. a silver mirror forms upon warming with an aldehyde whereas there is no reaction with ketones

They both test positive for aldehydes, as they rely on the fact that aldehydes can be oxidised further, to carboxylic acids. this can be repreasented by the following equation:

RCHO + [O] → RCOOH

Reactions

Aldehydes and Ketones undergo nucleophilic addition to the carbonyl carbon. The general mechanism is shown below, where Nu^- repreasents a nucleophile.

The general mechanism for the nucleophilic addition to aldehydes and ketones.

In these reactions, the nucleophile has a negative charge. This means it readily attacks areas of positive charge. Oxygen is a very electronegative atom. This means it will try to withdraw any electrons that are covalently bonded to it, closer to the atom. This means it will pull the carbonyl carbon atom's electrons away from it, leaving the carbon atom partially positively charged. This is given the symbol d+. This area of relative positive charge is then easily attacked by nucleophiles

At this level there are two reactions which use this mechanism.

• Addition of Hydride (H^-) ion.
• addition of the Cyanide (CN^-) ion.

In the addition of the Hydride ion, the ion is provided by NaBH₄.The mechanism for this reaction is shown below.

The mechanism for the nucleophilic addition of the Hydride ion to aldehydes and ketones.

In the addition of the Cyanide ion, the ion is provided by HCN, which is made by mixing dilute acid with sodium cyanide. The mechanism for this reaction is shown below.

The mechanism for the nucleophilic addition of the Cyanide ion to aldehydes and ketones.

In the above reaction, you may notice that the product has a 3D structure, with four different functional groups attached. This is important because it means the molecule is Optically Active, i.e. when you look at the molecule and its mirror image, they are not superimposable, as shown below.

Optical isomers are mirror images of each other and are non-superimposable

The molecules above are essentially the same but the bonds have a different arrangement in space. This can only happen when there are four different groups attached to one carbon atom, and the carbon atom this occurs on, is said the chiral carbon.

The two different molecules are formed due to the carbonyl group being planar. This means that the nucleophile, (in this case the cyanide ion) can attack from either above or below the plane of the carbonyl group. As there is no preferance in which side the attack occurs, the products form in a 50:50 mixture.

The nucleophilic attack of the cyanide ion is also important as a way of lengthening the carbon chain in the molecule. It can also be used to make 2-hydoxypropanenitrile. This is very important in synthesis, and is used to make both 2-hydroxypropanoic acid and 1-aminopropan-2-ol.

Important reactions of 2-hydroxypropanenitrile.

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