7.7 Carbocation Rearrangements: 1,2-Hydride Shifts and 1,2-Alkyl Shifts

Carbocations typically appear as intermediates in multistep mechanisms. They are usually too unstable to exist for a prolonged time because they are extremely electron deficient due to (1) the carbon atom’s +1 formal charge and (2) its lack of an octet. As we saw in Section 7.3, carbocations commonly behave as Lewis acids to form a bond with an electron-rich Lewis base. Carbocations can also eliminate H+ (Section 7.6) to yield an alkene or alkyne. Given a chance, however, carbocations can also undergo a rearrangement before taking part in one of these steps with another species.

Equations 7-25 and 7-26 show two carbocation rearrangements. Equation 7-25 is a 1,2-hydride shift. A hydride anion (H–) is said to shift because a hydrogen atom migrates along with the pair of electrons initially making up the C—H bond. The numbering system denotes the number of atoms over which the hydride anion migrates; the atom to which the hydrogen is initially bonded is designated as the number 1 atom and any adjacent atom can be designated as a number 2 atom. Thus, a “1,2” shift refers to the hydrogen migrating to an adjacent atom.

Two types of carbocation rearrangements, including 1,2-hydride shift and 1,2-methyl shift are illustrated. The first chemical reaction titled, A 1, 2-hydride shift, shows a condensed structural formula of a compound showing two carbon atoms linked by a single bond, with the left carbon atom carrying a positive charge. It is linked to a hydrogen atom and a methyl group by a single bond each while the right carbon atom is linked two methyl groups and a hydrogen atom by a single bond each. The hydrogen atom is labeled hydrogen migrates with two electrons. A curved arrow points from the hydrogen atom toward the carbon atom carrying a positive charge. The resultant shows an exchange of positive charge and hydrogen between the two carbon atoms. The second reaction titled, A 1, 2-methyl shift, shows a condensed structural formula of a compound showing two carbon atoms linked by a single bond, with the left carbon atom carrying positive charge linked to a hydrogen atom and a methyl group by a single bond each while the right carbon atom is linked to three methyl groups by a single bond each. The first methyl group is labeled CH 3 migrates with two electrons. A curved arrow points from a methyl group toward a carbon atom with a positive charge. The resultant shows an exchange of positive charge and methyl group between the two carbon atoms.

Equation 7-26 shows a 1,2-alkyl shift; more specifically, a methyl group is transferred, so this rearrangement is called a 1,2-methyl shift. The numbering system is no different from that of the hydride shift because the migrating group—the methyl group—is transferred to an adjacent atom.

Both Equations 7-25 and 7-26 share identical curved arrow notation. In both cases, a single curved arrow indicates that the initial bond between the C atom and the migrating group is broken, and those electrons are used to form a bond to the adjacent C. Meanwhile, the +1 formal charge is also shifted over one atom to the atom that was initially bonded to the migrating group.

Carbocation rearrangements are important to consider whenever carbocations are formed in a particular reaction. These reactions include SN1 and E1 reactions (Chapters 8 and 9) and electrophilic addition reactions (Chapter 11).

In a carbocation rearrangement, the positively charged carbon atom of the carbocation is very electron poor because it carries a full positive charge and it has less than an octet of electrons. On the other hand, a single bond to hydrogen or carbon on an adjacent atom is relatively electron rich because two electrons are localized in the bonding region. Therefore, the single curved arrow that is used to depict a carbocation rearrangement in Equation 7-27 represents the flow of electrons from an electron-rich site to an electron-poor site.

A chemical reaction shows the flow of electrons denoted by curved arrows. It shows two carbon atoms linked by a single bond, each containing two vacant single bonds. The carbon atom on the left is marked with a positive charge, and labeled electron-poor. The carbon atom on the right carries a third single bond connecting it to an R group and labeled electron-rich. The resultant shows an exchange of positive charge and R group between the two carbon atoms.

“Watching” a Bond Break

An elementary step can be extremely fast—on the order of picoseconds (1 ps = 10–12 s) or femtoseconds (1 fs = 10–15 s). Is it possible, then, to actually observe one taking place? A few decades ago, the answer would have been no, but now we have the ability to do so, at least with some types of reactions. How is it done? In essence, snapshots of a reaction are taken using a really fast camera. But not a camera in the traditional sense. In this case, the “camera” is constructed from lasers capable of producing light pulses lasting < 1000 fs. Such a femtosecond laser is shown here.

A photo shows a man wearing shades while operating an ultrafast optic laser in a dark room.

The decomposition of gaseous ICN into I + CN, carried out in 1988 by Stewart O. Williams and Dan G. Imre of the University of Washington, has been studied using this technology. Two lasers were used—one to provide a ~125-fs pulse of 306-nm light to break the I—C bond and a second to provide a ~125-fs pulse of ~389-nm to 433-nm light at various delay times after the first pulse. The second pulse excited the newly forming CN species, causing it to emit fluorescence that could be detected and correlated with the I—C distance over time. They found that the I—C bond was nearly completely broken at ~60 fs.

Even though studies such as this one focus on specific chemical reactions in the gas phase, the knowledge we gain can be used to develop a deeper understanding of reaction dynamics in general.

YOUR TURN 7.13
SHOW ANSWERS

Supply the curved arrow notation for the carbocation rearrangement shown here.

A chemical reaction represents carbocation rearrangement to mark the curved arrow notation. The reaction shows a closed six-ring structure with a hydrogen atom linked to carbon-2 by a single bond and a four carbon side chain emerging from carbon-2 of the ring. The carbon-2 on the side chain is marked with a positive charge. The resultant shows an exchange of a positive charge and a hydrogen atom at carbon-2 of the side chain.

The single curved arrow shows a C—H bond breaking from the tertiary carbon and simultaneously forming to the secondary carbon.

A chemical reaction represents carbocation rearrangement to mark the curved arrow notation. The reaction shows a closed six-ring structure with a hydrogen atom linked to carbon-2 by a single bond and a four carbon side chain emerging from carbon-2 of the ring. The single bond is interacting with a positive charge and the carbon-2 on the side chain is marked with a positive charge. The resultant shows an exchange of a positive charge and a hydrogen atom at carbon-2 of the side chain.

problem 7.18 Supply the appropriate curved arrows and draw the product for this carbocation undergoing (a) a 1,2-hydride shift and (b) a 1,2-methyl shift.

A condensed structural formula shows a closed six-ring structure, with two vacant single bonds emerging at carbon-1 and 3 and a positive charge marked at carbon-2.