Aldehydes & Ketones
- Both aldehydes and ketones both contain carbonyl groups, which is a C=O bond.
- The carbonyl group is formed by the overlap of p-orbitals above and below the C and O atoms.
- This forms a pi-bond between the C and O.
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-The C atom of the carbonyl group is joined to at least 1 H.
-The suffex "-al" dictates the parent chain, which is the largest unbranched chain.
-The carbonyl C is always C1.
-The C atom of the carbonyl group is attrached to 2 other C's.
-Ketones are named with the suffix "-one" on the end to show the largest parent chain.
- Aromatic carbonyls:
-These contain both a benzene ring and a carbonyl group.
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- A suitable oxidising agent is a solution contain H+/Cr2O4(2-), which is usually contained in acidified potasium dichormate - K2Cr2O7 mixed with H2SO4.
- Primary alchols:
-These oxidise first into aldehydes and then carboxylic acid.
CH3CH2OH + [O] -> CH3CHO + H2O (This is under distillation to prevent further reaction).
-To produce carboxylic acids reflux is required:
CH3CH2OH + 2[O] -> CH3COOH + H2O
- Secondary alcohols:
-These are oxidised into ketones.
CH3CHOHCH3 + [O] -> CH3COCH3 + H2O
- Aldehydes thus are oxidised into carboxylic acids even without ever being an alcohol, and when doing so the orange colour of potassium dichromate turns green.
(Tertiary alchols obviously do not oxidise)
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- An example of a primary alcohol -> aldehyde -> carboxylic acid.
- An example of a secondary alcohol oxidised to a ketone.
(Remember that a molecule of H2O is produced!)
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- Reducing aldehydes and ketones:
-By warming these compounds in a water solvent with NaBH4 they will be reduced.
- Reduction of aldehydes:
-These are reduced to primary alcohols by NaBH4.
CH3CH2CHO + 2[H] -. CH3CH2CHOH
- Reduction of ketones:
-Reduced to secondary alcohols.
CH3COCH2CH2CH3 + 2[H] -> CH3CHOHCH2CH2CH3
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- Aldehydes and ketones react with NaBH4 in a nucleophilic addition reaction.
-Essentially the NaBH4 acts as a source of H- ions via BH4-, acting as the nucleophile.
(Quick recap: a nucleophile is an atom or group that is attracted to an electron-deficient centre where it donates a pair of electrons to form a new covalent bond).
- So the H- ion attacks the Cδ+ and forms a bond, from which an intermediate forms.
- This intermediate donates an electron pair to a H atom of a H2O molecule, forming a dative covalent bond and a OH-.
- This produces an alcohol.
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- Detecting a carbonyl group:
-These can be detected with 2,4-DNP, and when mixed with methanol and sulfuric acid is known as Brady's solution.
-When it is added to a carbonyl a orange/yellow precipitate is formed for positive.
-The test is ONLY positive for aldehydes and ketones, no other C=O bond.
- Once you know that the compound is a carbonyl then it can be found to be either an aldehyde or a ketone:
-Adding Tollens' reagent is a weak oxidising agent and will only oxidise aldehydes to carboxylic acids.
-This produces a "silver mirror" effect for positive.
-This occurs when the initial brown precipitate is mixed with Ammonia(aq).
-Ketones will NOT be oxidised.
- To identify the actual carbonyl compound it can be mixed with the 2,4-DNP and then the deritive formed is purified.
-After this the pure crystals are melted and then the melting point measured.
-This is compared to a database and the correct compound is identified, the clear difference in melting point of the derivative compared to the carbonyl is much larger.
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