[sweetdaisy186]

Hey guys!

I am having a lot of trouble with the E1-E2 and SN1-SN2 reactions. I can tell the difference between first and second order. But I can't tell the difference between an E2 and SN2 reaction. I understand that first order reactions can occur with tertiay carbons and that strong bases and primary carbons deal with second order reactions. E1 reactions usually occur with SN1 reactions. Is there a checklist I can find somewhere to help me determine which reaction is going to occur? This would help me out soo much! Whenever I try to predict the product of a compound, I always end up getting the wrong answer. I have been working problem after problem and it just not sinkng in.

Thanks for any type of help that anyone can give me! I greatly appreicate it!

[movies]

I think the easiest way to differentiate them is to think about what has to happen first.  You have two options.

1) Direct displacement (or elimination) of your leaving group.  In this case you form a bond and break a bond all at once.  The steps are concerted.

2) Step-wise substitution or elimination.  In this case, the leaving group falls of spontaneously forming a carbocation.  After that happens, there is either a nucleophilic attack on the cation or an elimination.

Choice 1 is either an S_N2 or E2 reaction because it involves two molecules coming together to cause the reaction to proceed.  Therefore when you look at the kinetics of the reaction you find that the rate depends on the concentration of both the nucleophile (or base) and the substrate with the leaving group.  Overall, the reaction is second order (one from each component).

Choice 2 is either an S_N1 or E1 reaction because the rate is determined by a unimolecular process (the spontaneous loss of a leaving group).    Once the leaving group falls off, the second step (nuceophilic attack or elimination) occurs very rapidly.  When you work out the kinetics for a process like this, you will find that the concentration of the nucleophile or base has essentially no effect on the rate of the overall reaction; only the concentration of the substrate with the leaving group matters.  This is due to the fact that the loss of the leaving group to form the carbocation is much, much slower than the attack or elimination.  All in all, the rate law only depends on the concentration of one molecule and therefore the overall process is first order.

So, when I approach a problem like this I try to figure out whether or not the leaving group is likely to be displaced by the nucleophile directly (S_N2) or a carbocation is likely to form first, followed by nucleophilic attack (S_N1).

I hope this helps.

[sweetdaisy186]

yes, that was very helpful! Thank you very much! My only problem is that I can't tell if the leaving group is going to be displaced first or if the carbocation is going to form first. But that does make a lot of sense! Thanks again!

[movies]

For that, think about leaving group ability and carbocation stability.  If you have a good leaving group and an unhindered carbon (usually that also means a high energy carbocation) then S_N2 will be more likely because the carbocation is hard to access and the nucleophile isn't prevented from attacking by sterics.

If you have a poor leaving group (like OH) and a sterically bulky carbon (e.g., tert-butanol) then S_N2 will be very difficult.  However, S_N1 would be relatively easy with an acid because you could protonate the alcohol, lose water and make a stabilized tertiary carbocation.  S_N2 reaction is blocked in such cases because of the difficulty in attack tertiary carbons.