2
$\begingroup$

Problem

The products formed in the reaction

$$\ce{CH2=CHOCH2CH3 + HI ->}$$

are

(a) $\ce{CH2=CHI}$ and $\ce{CH3CH2I}$
(b) $\ce{CH2=CHI}$ and $\ce{CH3CH2OH}$
(c) $\ce{CH3CHO}$ and $\ce{CH3CH2I}$
(d) $\ce{CH3CH(I)OCH2CH3}$

Answer

(c) $\ce{CH3CHO}$ and $\ce{CH3CH2I}$

Question

Does ether cleavage occur or electrophilic addition of $\ce{HI}$ to the double bond take place or both? I'm equally inclined towards both (c) and (d).

$\endgroup$
4
  • 4
    $\begingroup$ d is formed reversibly, then CH3CH(+)OCH2CH3 is cleaved by iodide. $\endgroup$
    – user55119
    Apr 29 at 17:56
  • $\begingroup$ The substrate is an enol ether, which is very sensitive to acid. And HI is a very strong acid, which can cleave usually stable normal ether linkages. $\endgroup$ Apr 30 at 5:34
  • $\begingroup$ @MathewMahindaratne Why is the cleavage reaction favored over the electrophilic addition reaction? The cleavage reaction will proceed via breaking of a sigma bond whereas E+ addition proceeds via breaking a pi bond, so shouldn't E+ addition give a thermodynamically more stable product? $\endgroup$
    – Siddhant
    Apr 30 at 5:48
  • $\begingroup$ $$\ce{H2C=CH-O-CH2CH3 ->[H+] H3C-CH=O^+\!-CH2CH3 ->[I-] H3C-CHO + I-CH2CH3}$$ The intermediate is more stable by resonance as shown. $\endgroup$ Apr 30 at 5:50
2
$\begingroup$

Here is a mechanism for the cleavage. After proton transfer, the carbocation has a resonance contributor. Nucleophilic attack can occur on the ethyl group because there is a good leaving group.

enter image description here

The electrophilic addition can happen, with iodide attacking the carbocation. However, it is reversible. Iodide is a good leaving group, and the carbocation is resonance stabilized. The cleavage reaction is much less reversible.

enter image description here

$\endgroup$
1
  • $\begingroup$ Thanks Ben, your answer made things absolutely clear $\endgroup$
    – Siddhant
    May 7 at 13:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.