Which Major Pathway Predominates for This Organic Reaction?

[Ian_Brooks]

Homework Statement

[PLAIN]http://sphotos.ak.fbcdn.net/hphotos-ak-snc3/hs527.snc3/29952_406506367415_501337415_4229454_854152_n.jpg

Homework Equations

Alkyl halides can undergo substitution reactions (via SN1 or SN2 mechanisms) and/or elimination reactions (via E1 or E2 mechanisms). Illustrate your understanding of these pathways in the following by:
i) indicating the reaction mechanism(s) favoured by the particular type of: leaving group,
substrate, nucleophile/base and solvent employed,
ii) predicting the major pathway(s) that will occur under these conditions, and
iii) providing the structure(s) of the major product(s) of the reaction.

The Attempt at a Solution

Leaving Group: TsO(-) is a good LG
Substrate: Secondary, which is favoured by all
Nucleophile/base:I(-) is a weak base, and not a great nucleophile, which promotes E1
Solvent: Polar aprotic, which promotes both SN2 and E2

I'm not sure if it's supposed to be SN2, E2 or E1! I know E2 is promoted with strong bases, but does it really need it? The other two questions in this section were E2 and E1, so maybe, to test broadness of capability, it whould be an SN@? But I don't see how the variables would support this, besides MAYBE the Nucleophile I(-) would support substitution rather than either E1 or E2?

 

[Trave11er]

Hi Ian_Brooks!

Your "atempts" are all correct. However, it is hardly true that iodide ion is a bad nucleophile (e.g. http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch08/ch8-5.html). It will definitely substitute TsO- which is as you said a good leaving group! You were right that in acetonitrile both Sn2 and E2 are favourable and a small amount of the elimination product may form, but as the reaction is carried without heating and iodide ion is a very poor base (pKa of HI acid is -10) the main product will come from Sn2 reaction.

 

[Ian_Brooks]

Thanks for the clarification! Nucleophilicity increases down the group, I have to remember that