[ibenami]

I have a question:

If I have a primary alkyl halide which can theoretically undergo either E2 or SN2 reactions with -OH, generally SN2 will prevail because E2 prefers tertiary alkyl halides. however, if the E2 reaction creates a conjugated diene which adds to its stability, will it prefer the E2, or the SN2? or will both occur?

Cl-CH=CH-CH2-CH2-Cl   +  -OH   -->  ?

[Mandeep Deka]

See, it all depends on the reaction conditions you use.
In your problem, E2 will prevail over SN2, because OH- is a very strong base, and the hydrogen atom which it will abstract to bring about the elimination is very acidic because of the presence of a double bond next to it. So in this problem the elimination product shall predominate even though both SN2 and E2 will occur simultaneously!!