[cardene]

I'm confused from two problems in a problem set where the goal is to identify which reaction mechanism takes place (Sn1/Sn2/E1/E2/no rxn).

The framework I've been using doesn't explain the following two reactions.  I keyed in the right answer.

Framework: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx3.htm#hal9

Reaction 1:


Isn't this a 1° substituent?  Aren't those favored for Sn2?

Reaction 2:


According to the framework this is a "Secondary R₂CH-", which should be Sn2/E2.  My only guess why this is no reaction is that the Alkyl Group is too "bulky"...but I honestly have difficulty using a generic term like "bulky" as a predictive model for when a reaction fails to go to completion.  If anyone has a framework for "bulky" substituents...I would greatly appreciate it.

Can someone please help fill the knowledge gap that I have?  Thanks!

[Dan]

Isn't this a 1° substituent?  Aren't those favored for Sn2?

(CH₃)₃COH is tert-butanol:

According to the framework this is a "Secondary R₂CH-", which should be Sn2/E2.  My only guess why this is no reaction is that the Alkyl Group is too "bulky"...

A good guess. In this example the reactive centre is flanked by two (bulky) quaternary centres - these flanking quaternary centres greatly hinder the approach of nucleophiles. Look up the substitution chemistry of neopentyl haloalkanes.

[TheHitokiri]

Reaction 1:

As Dan has said,it's a tert-butanol so you should know tert-butanol would undergo which kind of mechanism pathway.

Consider the following factors:
1. High T favours elimination. Is T high?
2. Strong base favour SN2/E2. Is there a strong base?
3. Bulky base favour elimination. Is there a bulky base?
4. Weak nucleophile favour SN1/E1 while strong nucleophile favour SN2/E2. Is the nucleophile in this case strong or weak?
5. Polar aprotic solvent favours SN2/E2 while polar protic solvent favours SN1/E1. Is the solvent polar aprotic or polar protic?

Yup, that should lead you to your answer.

Reaction 2:

If you think E2 is possible then take a look at the carbon adjacent to the C-Br. For E2 to occur, the C-H adjacent to the C-Br must be anti-periplanar or syn-coplanar in rigid molecules.

But is there any H attached to the carbon adjacent to the C-Br? That would help reduce your choices.

If you want to understand better, try drawing the envelope-conformation of the cyclopentane, that would help in determining whether nucleophilic substitution is feasible if its not clear.