How do you determine if an E2 reaction is regiochemical or stereochemical?

Question

I more or less understand E2 reactions once I know they're regiochemical and stereochemical, but I'm having trouble identifying them. Is it really as simple as regiochemical ones having numerous beta positions (with protons) and stereochemical ones having only one or two beta positions?

Answer 1

E2 reactions are regioselective. The carbon in an bromo alkane is labeled the alpha carbon. The carbon next to the alpha carbons are the beta carbons. There can be more than 1 constitutional isomer. The beta carbon to lose a proton is the carbon with the fewest hydrogens. This leads to the most stable produce.The most stable of the products will be the major product. In otherwords if one beta carbon is primary and one is secondary the secondary carbon will lose the proton.

E2 (and E1) reactions are steroselective. Anti elimination is faster and more common in E2. Anti elimination occurs when constituents are removed from opposite side of the double bond. More than one steroisomer can be formed. The isomer with the bulky groups on opposite are more stable.