[Sis290025]

1. Predict the relative reaction times from fastest to slowest for the following compounds with NaI in acetone: 1-chlorobutane, 1-bromobutane, 2-chlorobutane.

I am assuming that this is under SN2 reaction conditions with the solvent and compound given.


2-chlorobutane = secondary substrate = slowest
1-bromobutane = primary substrate = fastest (Br is larger than Cl)
1-chlorobutane = primary substrate = moderate rate

2. Indicate whether the following statements refer to SN1 or SN2 reactions.

tertiary alcohols - SN1??
sp2 planar like transition state -  SN1?? I am unsure of this one.
reaction rate independent of the concentration and nature of nucleophile = SN1??
inversion and retention of stereochemistry - SN2??
primary and secondary alcohols - SN2???
carbocation intermediate - SN1 ??

Thanks.

[Dan]

inversion and retention of stereochemistry - SN2??

Can you explain why SN2 would give both inversion and retention?

[Sis290025]

There would be an inversion of sterochemistry in SN2 because the nucleophile attacks the carbon from the back side? It also happens if the substrate is chiral? Retention results in a frontside attack? If there are two inversions, then wouldn't a net retention of configuration occur?

[Dan]

There would be an inversion of sterochemistry in SN2 because the nucleophile attacks the carbon from the back side? It also happens if the substrate is chiral?

Correct, the term "inversion" is only applicable at a chiral centre, because if the centre is not chiral then there is no stereochemistry to invert.

Retention results in a frontside attack?

No, frontside attack would result in retention, but I assume that's what you meant to say. Is it possible for an SN2 reaction to occur in this manner? Why?

If there are two inversions, then wouldn't a net retention of configuration occur?

Yes, but an SN2 reaction still proceeds with inversion - to say that an SN2 reaction can proceed with retention because two consecutive SN2 reactions will give net retention is like saying that -1 is a positive number beause -1 x -1 = 1 (if you see what I mean...).
The question refers to a single SN1 or SN2 reaction.

Are SN1 reactions stereospecific? Why?