Elimination (E1) with 1,2-alkyl shift

by James

Description: When secondary (or primary) carbocations are formed adjacent to a quaternary carbon, 1,2-alkyl shifts can occur.

Notes: The acids in these reactions is often sulfuric acid (H2SO4) or tosic acid (TsOH) because the conjugate base of these acids are very poor nucleophiles.

Examples:

Notes: In the first and second cases a methyl group shifts over by one carbon (in the second example, silver nitrate  AgNO3 (-) is a Lewis acid that makes halides such as Br better leaving groups).

In the third example the cyclobutane ring expands to give the five-membered ring, which has less ring strain.

In the fourth example the ring expands to form a seven-membered ring from a six-membered ring (recall that primary carbocations are unstable relative to secondary carbocations).

Mechanism: Formation of a carbocation is achieved here through protonation of the alcohol with strong acid (Step 1, arrows A and B) followed by loss of water to give the carbocation (Step 2, arrow C) . Next, migration of a methyl group from the adjacent quaternary carbon through a transition state such as that pictured (Step 3, arrow D) gives a tertiary carbocation which undergoes deprotonation (Step 4, arrows E and F) to form the most substituted alkene (between C-2 and C-3)

Notes:

{ 4 comments… read them below or add one }

gurjit April 18, 2013 at 7:15 pm

Hi its very good site where I have sort out my many problems.but in 1,2 alkyl shift I just want to confirm is alkyl shift takes place for for stability of carbocation.pls clear me.

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Andrew December 13, 2013 at 12:51 am

you are cleared.

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Pankaj Vidh April 19, 2013 at 12:57 pm

About the last example…..why would a six-membered ring expand to form a seven-memebered ring? Wouldn’t a seven-mebered ring have more angle-strain?

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james April 22, 2013 at 6:28 pm

There isn’t a strong driving force for the formation of the seven membered ring, but it’s not completely inaccessible. Seven membered rings don’t have significantly prohibitive angle strain.

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