[fab]

Can someone please help understand why the attached reaction is an SN1 reaction? I understand that that its a tertiary halide..but there is no temperature given for me to say that its an SN1 reaction or E1. I initially thought this reaction would proceed through an E2 mechanism....

Thanks in advance

[Dan]

I initially thought this reaction would proceed through an E2 mechanism....

Can you explain why you thought this? I think you have some misunderstandings here - E2 and S_N2 can be ruled out immediately as they require a strong base or good nucleophile, respectively; tertiary alkyl halides are also generally too hindered for S_N2.

E1 and S_N1 both occur by the same stabilised tertiary carbocation intermediate. This carbocation can either be trapped by MeOH in an S_N1 reaction, or MeOH may abstract a proton in an E1 reaction. Both pathways are viable, but also reversible, ie.

Alkene + MeOH + H⁺ <> carbocation + MeOH <> Ether + H⁺

As MeOH is the solvent, and therefore in vast excess I expect the eqilibrium to lie on the side of the ether (the S_N1 product).

[orgopete]

Since the SN1 and E1 reactions proceed via the same carbocation intermediate, the product ratios are difficult to control and both substitution and elimination usually take place.

http://www2.chemistry.msu.edu:80/faculty/reusch/VirtTxtJml/alhalrx3.htm#hal8