How would one judge whether a secondary carbon would react via SN1 or SN2? (Or both?)

  • Thread starter soggybread
  • Start date
  • #1
20
0
For example, if we have 2-butanol, a compound with a secondary [tex]\alpha[/tex] carbon and
I reacted this with HCl and NH4Br

I would get 2-bromobutane (via nucleophillic substitution), but how would I determine whether this goes via SN1 or SN2 to achieve this?

Now, if this was 1-bromobutane, I know it would be via an SN2 reaction (steric hindrance)
And if it was t-bromobutane, it would be via an SN1 reaction (carbocation)

But since 2-bromobutane rests right in the middle, where do I find this balance? It would be a little bit of both the carbocation and steric hinderance involved, wouldn't it?

My question:
I've found some SN1/SN2 tables online (i.e. http://www.cem.msu.edu/~reusch/VirtualText/alhalrx3.htm#hal9) , which seem to point that 2-bromobutane should undergo SN2, because Br- is a weak base.


But does that mean literally 100% of this entire reaction goes through SN2 then?
Or is there a small ratio, like 95% SN2, and 5% SN1, but small enough to assume that SN2 is the entire reaction mechanism?

Thanks for clearing this up.
 
Last edited by a moderator:

Answers and Replies

  • #2
445
5


For example, if we have 2-butanol, a compound with a secondary [tex]\alpha[/tex] carbon and
I reacted this with HCl and NH4Br

I would get 2-bromobutane (via nucleophillic substitution), but how would I determine whether this goes via SN1 or SN2 to achieve this?

Consider what would happen here if you had, for instance (R)-butan-2-ol. What would be the product by SN1, or by SN2, and how do they differ?
 
  • #3
chemisttree
Science Advisor
Homework Helper
Gold Member
3,617
638


Protonating the alcohol makes it a wonderful leaving group. Excess acid converts the water into H3O+ and prevents it from adding back. Seems like you would get a little of SN1 and SN2. Maybe a couple of percent of the SN1 product. Solvent has an effect as well. How might a less polar solvent affect the outcome?
 

Related Threads on How would one judge whether a secondary carbon would react via SN1 or SN2? (Or both?)

  • Last Post
Replies
5
Views
9K
  • Last Post
Replies
2
Views
22K
Replies
3
Views
1K
Replies
2
Views
4K
  • Last Post
Replies
6
Views
1K
  • Last Post
Replies
5
Views
2K
Replies
2
Views
9K
  • Last Post
Replies
2
Views
9K
Top