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:

{ 6 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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Mike Harkins May 5, 2014 at 2:53 am

How would a benzene ring come into play in sn1 e1 mechanisms? More specifically 1-(1-chloroethly)-3-methly benzene? Is it possible for the ring to expand? Will allylic resonance help with rearranging the carbocation and ultimately produce more products? Thank you

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James May 5, 2014 at 4:47 pm

Not at all in terms of ring expansion/contraction. Sometimes the benzene ring can perform what is known as “neighbouring group participation” where it helps to remove the leaving group, but that’s only covered in rare cases. Would need to see the exact molecule you’re talking about.

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