Return to Contents Chapter 18: Enols and Enolates Ch 18 contents
Enol and Enolate Answers
 
Qu1: How many α-hydrogens are there in each of the following aldehydes and ketones : 
 
(a) (CH3)2C=O  =  6 (2 x CH3
(b) CH3CHO  = 3 (1 x CH3
(c) H2C=O  =  0 (the 2H are attached directly to the carbonyl, not the adjacent a-position)
(d) PhCHO = 0 (there are no H on the adjacent a-C which is a substituted aromatic carbon)
(e) 2-pentanone = CH3C=OCH2CH2CH3  =  5 (1 x CH3 plus 1 x CH2)
(f)  3-pentanone = CH3CH2C=OCH2CH3 4 (2 x CH2)
Qu 2:  
 
(a)
(b)
(c)
   
Qu 3:  
 
MECHANISM OF ACID CATALYSED TAUTOMERISATION
Step 1:
First, an acid-base reaction. The Lewis basic O atom of the carbonyl is protonated by the acid catalyst giving an oxonium ion.
mechanism of acid catalysed tautomerisation
Step 2:
Another acid-base reaction.  Removal of an α-hydrogen by a water molecule functioning as a base allows formation of the C=C and neutralises the positive charge on the O giving the enol and regenerates the catalyst.
MECHANISM OF BASE CATALYSED TAUTOMERISATION
Step 1:
First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen giving the reactive enolate.
mechanism of base catalysed tautomerisation
Step 2:
The negative charge is resonance stabilised to the more electronegative O atom.
Step 3:
An acid-base reaction. The alkoxide deprotonates a water molecule reforming the catalyst hydroxide and the enol
   
Qu 4: Acetophenone is a typical ketone.....
 
 
Qu 5: As we have an α,β-unsaturated ketone, then there could be both direct (1,2-) or conjugate (1,4-) addition reactions: