This tutorial introduces the rearrangement reactions that stabilize carbocations, and explains the mechanisms of 1,2-hydride and 1,2-alkyl shifts.
Carbocations are intermediates in several reactions covered in intro organic chemistry (e.g., SN1 and E1 reactions). They contain a positively-charged carbon atom that has three bonds, typically to alkyl groups and hydrogen atoms.
The stability of a carbocation depends on the number of alkyl substituents that are bonded to the positively-charged carbon. The more alkyl substituents, the more stable the carbocation:
Reactions involving carbocations sometimes give unexpected products. In an SN1 (unimolecular nucleophilic substitution) reaction, the nucleophile might add to a carbon other than the one with the leaving group. Similarly, in an E1 (unimolecular elimination) reaction, the double bond in the product might be in an unexpected position. These observations can often be explained by rearrangement reactions, where the carbocation intermediate rearranges, forming a different carbocation.
Rearrangements reactions usually occur to increase the stability of a carbocation. So, a less stable carbocation (e.g., 1° or 2°) might undergo a rearrangement reaction to form a more stable carbocation (2° or 3°). The most common rearrangement reactions are 1,2-hydride shifts and 1,2-alkyl shifts.
A 1,2-hydride shift is a rearrangement reaction that can occur if there is a hydrogen atom on the carbon adjacent to the positively-charged carbon of a carbocation. In this reaction, the hydrogen atom (on carbon 1) shifts to the positively-charged carbon (carbon 2); hence, it is a 1,2-shift. As carbon 1 now only has three bonds, it has gained a positive charge.
As this reaction usually occurs to form a more stable carbocation, the positive charge will generally be more stable on carbon 1 than it was on carbon 2 (so, carbon 1 will usually have more alkyl substituents than carbon 2).
The term hydride describes an electron-rich hydrogen atom. In a 1,2-hydride shift, the moving hydrogen atom takes both electrons in the carbon-hydrogen bond along with it.
The mechanism for a 1,2-hydride shift is shown by drawing a curved arrow from the carbon-hydrogen bond to the positively-charged carbon:
A 1,2-alkyl shift is a rearrangement reaction that can occur if there is an alkyl group (methyl, ethyl, etc) on the carbon next to the positively-charged carbon of a carbocation. In this reaction, the alkyl group (on carbon 1) shifts to the positively-charged carbon (carbon 2), taking the electrons from the carbon-carbon bond with it, and leaving a positive charge on carbon 1.
The mechanism for a 1,2-alkyl shift is shown by drawing a curved arrow from the carbon-carbon bond (of the migrating alkyl group) to the positively-charged carbon:
While this example shows a shifting methyl group, note that other alkyl groups can also shift.
If a reaction has a carbocation intermediate, the formation of unexpected products suggests that the carbocation underwent a rearrangement reaction. In these reactions, a less stable carbocation may rearrange to form a more stable carbocation (where carbocation stability is typically based on the number of alkyl substituents on the positively-charged carbon). The most common rearrangement reactions are 1,2-hydride shifts (in which a hydrogen atom and the positive charge trade places) and 1,2-alkyl shifts (in which an alkyl group and the positive charge trade places).
Carbocation rearrangements can occur in reactions involving carbocation intermediates, such as the alkene hydration reaction.