3.3.8 - Aldehydes and ketones

What is the functional group of aldehydes and ketones?

Aldehydes and ketones contain the carbonyl functional group ( C=O)

What is the difference between aldehydes and ketones?

Aldehyde - carbonyl group is at end of the chain.
Ketone - Carbonyl group is in the middle of the chain.

How can aldehydes and ketones be prepared?

By oxidation of alcohols (primary alcohol = aldehyde, secondary alcohol = ketone).

What is the oxidising agent?

acidified pottasium dichromate

What would need to be set up for further oxidation of an aldehyde?

Further oxidation can be done, the apparatus must be set up to distill off the aldehyde.
Aldehydes can be further oxidised to carboxylic acids. The reaction mixture would need to be heated under reflux for this. The aldehyde would condense and drop back into the ration mixture to be further oxidised to the carbxylid acid.

Why can't ketones be further oxidised?

Ketones are resistant to being oxidised as they don't have a readily available hydrogen atom present like aldehydes. Therefore no further oxidation can take place with secondary alcohols.

Sum up the oxidation of alcohol

Primary alcohol → aldehyde -(further oxidation)--> carboxylic acid
Secondary alcohol → ketone (further oxidation not possible).

What is tollens reagent?

Tollens reagent → formed when aqueous ammonia is added to silver nitrate

Tollens reagent with aldehyde?

When gently warmed with tollens reagent, an aldehyde becomes oxidised. When the aldehyde is oxidsed, the solution becomes reduced and the silver ammonia ions become reduced to solid silver, forming a 'silver mirror'.

Tollens reagent with ketone?

When tollens reagent is gently warmed with a ketone, no silver mirror is seen, as ketones can't be oxidised by tollens reagent.

What is Fehling's reagent?

Fehlings reagent → Alkaline solution containing copper(II) ions which act as the oxidising agent.

Fehlings with aldehyde?

When an aldehyde is warmed with fehlings solution, the aldehyde is oxidised, and the blue solutions (due to copper (II) ions present) will turn brick red as the Cu2+ ions are reduced.

Fehlings with ketone?

When a ketone is warmed with fehlings solution, no reaction takes place as ketones can't be oxidised.

Pottasium dichromate with aldehyde and ketone?

When heated with acidified potassium dichromate, aldehydes would be oxidised and the colour would change from orange to green. The ketone will not be oxidised so there will be no colour change.

sum up the different ways to differentiate between aldehyde and ketone?

Tollens → aldehyde- silver mirror, ketone - no result.
Fehlings → Aldehyde - blue to red, ketone - no result
Acidified potassium dichromate → aldehyde - orange to green, ketone - no result.

What is the most common reducing agent?

The most common reducing agent (from aldehyde to alcohol) is sodium tetrahydroborate (NaBH4).

What nucleophile does NaBH4 produce? Which part of the aldehyde and ketone does it reduce? Why?

This usually produced the hydride ion nucleophile (H-). This hydride ion can reduce a carbonyl group in an aldehyde or a ketone but it isn't strong enough to reduce a C=C bond as the hydride is attracted to the C in the C=O bond but is repelled by the high electron density of the C=C bond.

How many hydrogens are needed to reduce a carboxylic acid to a primary alcohol?

Carboxylic acid to primary alcohol (4H)

How many hydrogens are needed to reduce a aldehyde or ketones to a primary alcohol?

Aldehyde and ketone to alcohol (2H)

What are the different ways of reduction?

Reduction is :
Loss of O
Gain of hydrogen
Gain in electrons

What reactions do carbonyl compounds usually undergo?

Many of the reactions which carbolyl compounds undergo are nucleophillic addition reactions.

Why do carbonyl compounds usually undergo nucelophillic addition reactions?

This is because the carbonyl group is polarised, as the oxygen atom is more electronegative than the carbon atom. This leaves the carbon atom slightly positive charged, so it is more suceptible to attack by a nucleophile like the cyanide ion.

What are products of addition reacts with cyanide called?

hydroxynitriles.

What type of isomerism so hydroxynitriles show?

Optical isomerism

Why do hydroxynitriles show optical isomerism?

he products display optical isomerism as the CN- nucleophile can attack from the bottom or the top of the carbonyl. The reaction mixtur produced is a racemic mixture.