[soggybread]

For example, if we have 2-butanol, a compound with a secondary alpha  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.

[Oxygen]

For example, if we have 2-butanol, a compound with a secondary alpha  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.

The alcohol itself is a poor leaving group, and thus if you were to go through an SN1 reaction it would take a ton of heat and extreme basic conditions to get it to leave.  Thus, it's almost necessary to have it go through SN2 especially because it is in acidic conditions.  A product mixture of SN1 and SN2 products is always possible, but I would think it would be rather unlikely under such acidic conditions. 

[soggybread]

Hmm, that makes good sense. Thanks for confirming and explaining what I had in mind!

[Oxygen]

you're welcome Glad I could *delete me*

[irciniastatin J]

It's been a while since I looked at some old school SN1/SN2 but it seems the acid is present to protonate the -OH and facilitate the departure of water, generating a secondary carbocation that is then attacked by a weak source of bromide ion.

It is also possible that the protonated -OH is directly displaced by bromide via SN2. If I remember correctly most substrates for SN2 don't require preactivation.

Tough one.

[orgopete]

That is a real problem. While conditions, reagents, and reactants can be used to predict which it will be, but as you have pointed out, this is a reaction in which it biased away from one of those definitive reactions. If someone actually had real data, that would be useful. For example, if rather than just 2-butanol, it was (R)-2-butanol and the product of this reaction were given, such as (S)- or racemic 2-bromobutane. 

So, let me draw from reactions I know. I agree with 1-bromobutane. That reaction is also heated, so just mixing them together probably isn't working. If it were a simple SN2 reaction, the rate might be slow, but it should still go. The higher the reaction is heated, the more likely the activation energy of other reactions might also be overcome. If cyclohexanol is heated with phosphoric acid, there is a very fast protonation reaction that takes place and then supposedly loss of water to the carbocation. This loses a proton to give cyclohexene. Because it is heated, I am going to guess that butene is also going to form. Since these are not good E2 reaction conditions, elimination is probably an E1 reaction. Therefore I am going to go with SN1 as the reaction mechanism.

[sjb]

I'm getting a strange feeling of deja lu - see e.g. http://www.physicsforums.com/showthread.php?t=359383