Aldehyes + Ketones - OCR

Revision notes on aldyhydes and ketones (Chains, Rings and Spec. module) for AS Chemistry.

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Chemical properties of Aldehydes and Ketones
Reactions common to both Aldehydes and Ketones
The difference between an Aldehyde and a Ketone is the position of the C=O (Carbonyl group).
Aldehyde ­ always on a carbon at the end of the chain
Ketone ­ Always on a carbon at middle carbon of the chain
1. BOTH Aldehydes and Ketones can be reduced
Aldehyde reduced to Primary Alcohol
Reducing agent = Sodium tetrahydroborate III (NaBH4)
Ketone reduced to Secondary Alcohol
Reduction Oxidisation Gain of Electrons Loss of Electrons
True for all secondary alcohols
Carbonyl + 2[H] (NaBH4) Alcohol
Propanal + NaBH4 Propan-1-ol
Propanone + NaBH4 Propan-2-ol
2. BOTH Aldehydes and Ketones undergo nucleophilic substitution using HCN
A nuclophile is an electron pair donor
E.g. NH3 ­ 1 lone pair
H2O ­ 2 lone pairs
OH­ ­ 3 lone pairs
Cl­ ­ 4 lone pairs
Therefore nucleophiles attack
E.g. C+
=O ­
Due to a difference in electronegativity the O is
electronegative than the C so attracts the electron
pair more strongly
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Mechanism of the reaction between Ethanal and HCN
Conditions: K+CN­ = catalyst
Reagents: + HCN(g)
Mechanism of the reaction between Propanone and HCN
Conditions: K+CN­ = catalyst
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BOTH react with 2,4, Dinitrophenylhydrazine (2,4, DNPH)
2,4,DNPH is an orange solution
The reaction is used as a test for the C=O double bond
Reacts to give an orange/yellow precipitate
Example exam Q.
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ONLY Aldehydes can react in the Silver mirror test (Tollens Test)
Reagents: Silver nitrate dissolved in Amonnia (Ag+ )
Conditions: Hot water bath
If an Aldehyde is present the Silver ion (Ag+) changes to a silver atom (Ag)
Ag+ + 1e­ Ag(s)
ion ­ 46e­ atom ­ 47e­
As the Aldehyde was oxidised the Ag+ gained an electron and was reduced
Aldehydes Carboxylic Acid Ketones No further reaction
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