Engineering Chemistry S Datta

MECHANISM OF ORGANIC REACTIONS 173
Q. 8. 2-Methylbutanol on dehydration gives 2-methyl but-2-ene—explain.
CH3
CH3
⏐
⏐
Ans. CH 3
CH 2
⎯CH⎯ CH OH + H + ⎯⎯→ CH 3
CH 2
CH⎯CH
2
2
H O + H
CH3
2
⏐ +
CH 3
CH 2
CH CH
–H O
⎯⎯→
The formed carbocation is 2°. This rearranges to the most stable 3° carbocation by 1,2
hydride shift.
2
CH 3
CH 3
CH 3 CH 2 C CH 2 CH3CH2
C CH 3
H
+
+
3° carbocation
This species then eliminates a proton to give 2-methyl but-2-ene.
H
CH 3
CH 3
CH 3 C C CH 3 CH3
CH C CH 3
H
Q. 9. Give an example of nucleophilic addition.
Ans. First step of nucleophilic addition HCN to ethanal:
2-methyl but-2-ene
–
CN +
H
C
CH 3
H
O NC C O:
CH 3
: :
–
To complete the reaction, the negatively charged oxygen acts as a base and accepts an
H + from HCN.
Second step of nucleophilic addition of HCN to ethanal:
H
NC C O : + HCN
: :
–
NC
H
C OH+CN –
CH 3
CH 3
Q. 10. Benzophenone does not form bisulphite compound. Explain.
OH
⏐
C 6
H 5
COC 6
H 5
+ NaHSO 3
⎯⎯→ C 6
H 5
⎯C⎯SO3 Na
⏐
CH
6 5