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24/09/07

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…

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Mechanism of the reaction between Ethanal and HCN

Conditions: K+CN­ = catalyst
Reagents: + HCN(g)




Ethanal
Intermediate




Mechanism of the reaction between Propanone and HCN

Conditions: K+CN­ = catalyst
Reagents:




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1. 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.
Q. How can you distinguish between an Aldehyd and a Ketone? (4 marks)

Dissolve in 2,4…

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2. 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…

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