Explain with mechanism why halogenation of ketones under basic
condition gives a tri-halogenated product while under acidic conditions,
the reaction stops at mono-halogenated product.
Answers
Answer:
The acidity of α-H of carbonyl compound is high, therefore halogenation takes place at α position. Also, the halogen attack follows S
N
2
mechanism via enolization of ketone. In basic medium, polyhalogenation can take place at the same αC due to the higher acidity of its proton. Thus, reaction of 2-bromo-3-pentanone with bromine in alkaline medium will give 2,2-dibromo-3-pentanone.
CH
3
CH
2
BrCOC
2
H
5
+Br
2
→CH
3
CHBr
2
COC
2
H
5
In acid catalysed reaction, relative rates of formation of the enols determines the product distribution, In ethyl methyl ketone (2-butanone), since but-2-en-2-ol is more stable than but-1-en-2-ol (more substituted alkene is more stable). Therefore will give halogenation product as 3-chloro-2-butanone.
CH
3
CH
2
COCH
3
+H
+
→CH
3
CH=C(OH)CH
3
⟶
Br
2
CH
3
CHBrCOCH
3
During polyhalogenation, the stability of CCl
3
−
over CH
3
−
favours reaction (i) over (ii).
Thus option C and D are correct.
Explanation: