Sn1 or sn2?!!

Okay so I'm doing second year uni course and the past papers have no solutions!! Any help would be greatly appreciated, thanks!

Would you expect the following reaction to be SN1 or SN2? Give reasons for your answer.

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Inserted a pic of the question but it looks really tiny so attached it too!

Reply 1

Plantagenet Crown

Original post by Engclasshelp

Inserted a pic of the question but it looks really tiny so attached it too!

It's a secondary haloalkane so I'd say possibly sn2 pathway because of aprotic solvent which correlates with ethoxide's nucleophilicity...

(edited 7 years ago)

Reply 2

Engclasshelp

Original post by Plantagenet Crown

It's a secondary haloalkane so it proceeds via an Sn1 pathway.

Thanks so much! Could be really great an explain the rules a little bit to me, like just the principles? :s-smilie:

Reply 3

KombatWombat

A secondary haloalkane can go by both SN1 and SN2, depending on solvent, nucleophile and the substrate.

NaOEt is a pretty good nucleophile, and DMSO is an aprotic solvent which would lead me to guess SN2.

This is a pretty good series of articles that explains what you should be thinking about: http://www.masterorganicchemistry.com/2012/11/21/deciding-sn1sn2e1e2-1-the-substrate/

Reply 4

Treen98

Original post by Plantagenet Crown

It's a secondary haloalkane so I'd say it's more likely to proceed via an Sn1 pathway.

Original post by KombatWombat

Woah sorry to jump in here guys but I'd seen this thread and was wondering also. So is it SN1 or SN2?

Reply 5

Plantagenet Crown

Original post by Engclasshelp

Thanks so much! Could be really great an explain the rules a little bit to me, like just the principles? :s-smilie:

Tertiary haloalkanes always proceed by sn1 because they're too sterically crowded to proceed via sn2 and the resulting carbonation is stabilised.

Primary haloalkanes aren't sterically crowded so the approaching nucleophile can line up 180 degrees to the C-X antibonding and proceed in a single step via sn2.

Secondary ones can sometimes do sn1 and other times sn2 depending on the conditions, sometimes it can be quite hard to know which. Most of the secondary haloalkane substitutions I've done at Uni have proceeded via the sn1 pathway. I guess that as the solvent is DMSO (an aprotic solvent), sn2 could be argued although my lecturer did this exact same example last year and said it was sn1 so it can be confusing :lol:

(edited 7 years ago)

Reply 6

KombatWombat

Original post by Treen98

Woah sorry to jump in here guys but I'd seen this thread and was wondering also. So is it SN1 or SN2?

I think in an exam you could pick up a fair few marks by arguing either SN1 or SN2 as long as you used the appropriate arguments.

I’m pretty sure it’s SN2 though; sterics does disfavour it but solvent (how well the nucleophile and carbocation/SN2 transition state are solvated) and the strength of the nucleophile are pretty crucial.

I had a quick google, this seems to confirm what I’m saying for these exact conditions:
http://www.masterorganicchemistry.com/2012/09/12/two-types-of-elimination-reactions/
Poor nucleophile, polar protic solvent = SN1
Good nucleophile, polar aprotic solvent = SN2
(Though because NaOEt is an even better base, E2 is the major pathway. Not what the question’s about though!)