Preparation of Alkenes from Haloalkanes and Alkyl Sulfonates Bimolecular Elimination Revisited

PREPARATION OF ALKENES FROM HALOALKANES AND ALKYL SULFONATES BIMOLECULAR ELIMINATION REVISITED 

With the physical aspects of alkene structure and stability as a backgronnd, let ns now return to the various ways in which alkenes can be made. The most general approach is by elimination, in which two adjacent groups on a carbon framework are removed. The E2 

CHAPTER 11 Alkenes Infrared Spectroscopy and Mass Spectrometry 

Reminder -HBr under the reaction arrow indicates the species removed from the starting material in the elimination reaction. 

In Chapter 7 we discussed how haloalkanes (or alkyl sulfonates) in the presence of strong base can nndergo elimination of the elements of HX with simultaneons formation of a carbon-carbon donble bond. With many substrates, removal of a hydrogen can take place from more than one carbon atom in a molecule, giving rise to constitutional (donble-bond) isomers. In snch cases, can we control which hydrogen is removed—that is, the regio-selectivity of the reaction (Section 9-9) The answer is yes, to a limited extent. A simple example is the elimination of hydrogen bromide from 2-bromo-2-methylbutane. Reaction with sodinm ethoxide in hot ethanol fnmishes mainly 2-methyl-2-butene, but also some 2-methyl-1 -butene. 

---