[brunettt]

I think I understand if a reaction will proceed as unimolecular or bimolecular within the context of each mechanism.  For example, if someone were to tell me a reaction would undergo substitution I could determine if it was SN1 or SN2.  However, I know substitution reactions compete with elimimation reactions.  My problem arises in determining which mechanism, elimination or substitution, will be favored within a reaction.  Are there any guidelines I need to follow or rules and trends I need to look at when determining this?  Any help would be greatly appreciated, I've tried looking at this and understanding it, but I still can't seem to get it.

[BonkersBow]

This is explained quite well by Peter Sykes in his book "A guidebook to mechanism in Organic Chemistry". E2 is generally accompanied by Sn2 as E1 is accompanied by Sn1. You want to know how to manipulate the ratios of the products. One thing sykes doesnt explain is why for a given substrate the E2/Sn2 ratio is substantially higher than the E1/Sn1 ratio-(I think this is due to steric factors, all eliminations involve attack of hydrogen which juts out of the molecule. Sn2 involves the attack of carbon surrounded by four other groups hindering attack. For Sn1 a flat sp2 hybrid carbonium ion. So steric hinderence is worse for Sn2 making the reaction more greatly favour E2 when the reaction goes bimolecular. Not shure though!).
In summary: bulky groups round the carbon favour E1 over Sn1 and E2 over Sn2; large bulky bases favour elimination; the proportion of elimination rises going primary<secondary<tertiary carbon, elimination forms an alkene which is stablised by the greates hyperconjugation from tertiary structure; strong bases with low nucleophlicity(e.g. pyridine) favour elimination (removal of proton); Higher temperature favours elimination, more particles formed by elim greater entropy;type of leaving group effects rate of E2, involved in rate limiting step e.g.
tosylate<Br
if you cant get the book do get in touch

[BonkersBow]

Check this out http://www.personal.kent.edu/~cearley/Organic1/SNvsE/pathways.htm