[Hikaru]

S_Ni is intramolecular substitution reaction.
How is it different from S_Ni'?

[azmanam]

' usually refers to allylic substitution.  in general:

H2C=CH-CH2-LG  +  Nuc:  -->  Nuc-CH2-CH=CH2

where the nucleophile attacks the carbon of the double bond, the electrons move to form a new double bond on the adjacent carbon, and the leaving group leaves.

This might be what it refers to here.

[Mitch]

I think this has more to do with nucleophillic substitution that occurs within a molecule with itself.

[macman104]

I think this has more to do with nucleophillic substitution that occurs within a molecule with itself.

But if S_Ni is already intramolecular, then what designation would the ' have?

[azmanam]

Here's the SNi wikipedia page.

http://en.wikipedia.org/wiki/SNi

Seems more like substitution resulting from tight ion pair, thus proceeding with retention.  So maybe this general mechanism, but with allylic substitution?

[Hikaru]

Thanks a lot.
I think it is like azmanam said - that it is allylic substitution.
Can it be both allylic substitution and intermolecular?

Actually this question comes from a problem:
Which one reacts with SOCl₂the fastest?
Ph-CH₂OH 
CH₂=CHCH₂OH
CH₃CH=CHOH
CH₃CH₂CH₂OH
I am told they occur through S_Ni' mechanisms.

[azmanam]

careful with your prefixes.  SN2' reactions are the intermolecular ones.  The whole point here is that we're dealing with SNi' reactions, which are intramolecular.

Let's assume that SNi' refers to intramolecular allylic substitution.  Which compound appears capable of allylic substitution under this mechanism?  What is the mechanism for this SNi' reaction?

[azmanam]

Better late than never, right?

Here's a page talking specifically about SNi' that actually refers to one of the choices in the multiple choice question:

http://www.cem.msu.edu/~reusch/VirtTxtJml/special2.htm#top6