This page was last updated on 04-Mar-15
Material for lectures on 18th, 22nd & 25th February
Note that a mechanism for the hydrogenation of ethene on a catalyst surface was given in the lab schedule. It is taken from Catalytic hydrogenation of alkenes. It applies to all akenes. Study and understand it.
In the meantime, study this website, which contains most of the material of the scheduled lecture:
In addition to the addition reactions of akenes and their mechanisms, there are two types of rearrangement of carbonations that I plan/planned to deal with: the hydride shift (which they don't mention) and the methyl (or methide) shift which they do. The hydride shift (transfer of H- to the next carbon atom) accounts for the fact that treatment of, for example, 3-methylbut-1-ene, , with hydrogen bromide, forms in greater amount than the expected . See if you can come up with the mechanism after reading about the reactions of alkenes. Don't bother with the reaction of α-pinene, it's unnecessarily complicated.
N.B. Carbocation rearrangements are not useful preparatively, since they don't normally give good yields. They are important by virtue of the fact that they often mess up reactions that we hope to carry out!
A term that will not be familiar to you is "α" (alpha) as in "α-pinene" or "α-halo-alcohol or ether". This refers to a carbon next to the carbon in a functional group. 2-chloroethanol is an α-halo-alcohol, since the chlorine is attached to the carbon atom next to that bearing the -OH group. Another term is "vic-dihalide". "vic" is short for "vicinal" (explained in the text) which means neighbouring. 1,2-dichloroethane is a vic-dihalide, since the chlorines are on neighbouring carbon atoms.
The Hammond postulate is explained better than I explained it in class.
The terms "regioselective" and "stereoselective" are introduced and explained. This material connects with what was done in the Friday practical.
We are not studying hydroboration nor oxymercuration. Suffice it so say that hydroboration results in the anti-Markovnikov addition of H-OH and oxymercuration gives the Markovnikov addition of H-OH, but with no complications from the rearrangement of carbocations.
Don't bother with "epoxidation" (formation of "epoxides" such as ).
If you like videos, here are some very good ones from http://leah4sci.com/:
You may say there are three contributing structures, putting the positive charge on the bromine as shown above, as well as on each of the two neighbouring carbons, but these are not structures in their own right. In short, they do not exist, despite what the video implies! They just illustrate that the positive charge is distributed between the three atoms (with more on a secondary carbon than on a primary one).