Fun with ketones

Chemistry Level 2

Consider the compound butan-2-one

Do the following steps

  1. Add KCN/H2O - A

  2. Add H3O+/ H2O - B

  3. Heat - C

Find number of cyclic diesters C


The answer is 3.

Aniswar S K by Aniswar S K , 20, India

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1 solution

Aniswar S K
May 25, 2017

After 1st step you get 2 cyano butan-2-ol (both enantiomers)

Then hydrolyze the cyanide to carboxylic acid

Then cyclic diester formation

You have 2 chiral centres but only 3 products as you have to carefully observe the centre of symmetry in one of the compounds!!!

1 Helpful 0 Interesting 0 Brilliant 0 Confused

Yes we get a cyclic six membered diester.I first marked 2 for the race mic mixture formed when both the ethyl are syn but when they are anti there exists a Centre of symmetry making the answer 2+1=3

Spandan Senapati - 4 years ago

A silly doubt:

Please tell why aldol condensation will not occur?

KCN can react with water to form OH-.

How do we decide that nucleophillic addintion vs aldol condensation?

I think former dominates coz aldol condensation is very slow and here we don't have a strong base.

Harsh Shrivastava - 4 years ago

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Ya for aldol,Haloform Reactions you need strong bases coz alpha hydrogen is weakly acidic and so strong bases would be efficient in extracting it.

Spandan Senapati - 4 years ago

And for the reaction KCN+OH(-)=KOH+CN(-).KOH produced is a strong Base and readily dissociates so the eq shifts in the backward direction.

Spandan Senapati - 4 years ago

Yup cyanide is a weak base but good nucleophile

Pka hcn = 10

Pka h20 = 15.7

& Pka alpha hydrogen = 20

Even oh- cant extract proton from ketone completely. it extracts some and the equilibrium slowly shifts forward

Aniswar S K - 4 years ago

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