3.3.8- Aldehydes and ketones (PAPER 2)
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What is the difference between aldehydes and ketones?
The position of the carbonyl (C=O) group Aldehydes have the carbonyl group on an end carbon- have the ending -al Ketones have the carbonyl group on the inner carbon- have the ending -one
Draw the structure of propanone and propanal
Propanone CH3COCH3 Propanal CH3CH2CHO
Difference in oxidation between aldehydes and ketones- why is one not easily oxidised.
Aldehydes can be oxidised to carboxylic acids in the presence of acidified potassium dichromate whereas ketones cannot be as they do not have a H atom attached to their carbonyl.
What can be used to distinguish between aldehydes and ketones?
Tollen’s reagent
How to make tollen’s reagent?
Add few drops of NaOH to colourless silver nitrate solution to form a pale brown precipitate. Add few drops of dilute ammonia until precipitate dissolves.
How to test for aldehyde or ketone using tollen’s reagent- results?
Add aldehyde or ketone to tollen’s reagent and place in hot water bath- not bunsen burner as aldehydes and ketones are flammable. Aldehyde reduces tollen’s reagent to silver producing a silver mirror. Tollen’s is oxidising agent so oxidises aldehyde, ketones cannot be oxidised. No silver precipitate formed with ketone.
What is another method of distinguishing between aldehydes and ketones?
Fehling’s solution It is also an oxidising agent but is blue as it contains Cu2+ ions: warm aldehyde or ketone with fehling’s solution in water bath Cu²⁺ ions oxidise the aldehyde to the respective carboxylic acid, forming brick red precipitate of copper (I) oxide Ketones are not oxidised by fehling’s, solution remains blue
How are oxidising and reducing agents represented?
Oxidising agent = [O] Reducing agent = [H]
Difference in reduction between aldehydes and ketones?
Aldehydes are reduced to primary alcohols, ketones are reduced to secondary alcohols.
Draw the mechanism for nucleophilic addition of propanal using H- from NaBH4
Negative H- ion with lone pair attacks d+ C of propanal- curly arrow from lone pair of H to C, and from double bond to O Intermediate structure, no more double bond- C on end attached to 2 H and O with lone pair Curly arrow from lone pair on O to H+ forming primary alcohol with OH on end
How does potassium cyanide react with carbonyl compounds? How is potassium cyanide prepared?
Produces hydroxynitriles Nucleophile (CN-) ion attacks carbonyl (C=O) group and adds on to make hydroxynitrile (substance containing OH and Cn group) Potassium cyanide must be dissolved in acidic solution to dissociate to form K+ and CN- ions
Draw the mechanism for nucleophilic addition of propanal using CN- nucleophile
Curly arrow from lone pair on CN- to delta positive C Curly arrow from double bond to O, double bond breaks, e- go to O C on end joined to H, CN and O with lone pair Curly arrow from lone pair on O to H+ from acid OH group joins on end
What is generic equation between aldehyde and KCN?
RCHO (aq) + KCN (aq) + H+ (aq) —->RCH(OH)CN(aq) + K+ (aq)
What is general equation between ketone and KCN?
RCOR’ (aq) + KCN (aq) + H+ (aq) —->RCR’(OH)CN(aq) + K+ (aq)
What happens if we use an unsymmetrical ketone or aldehyde?
A mixture of enantiomers is produces- optical isomerism.
What are the risks when using KCN?
It is an irritant and very dangerous if ingested or inhaled When it reacts with moisture it can produce the toxic gas HCN
How to overcome the risks when using KCN?
Use a fume cupboard to prevent exposure to toxic fumes Wear safety goggles Wear gloves when handling Lab coat to prevent clothing contamination
Why is KCN favoured over HCN?
HCN is hard to store as it is a gas and it is a very poisonous gas.