SolutionsSteric Effects
1. Order by SN2 rate
Order from fastest to slowest towards iodide: neopentyl bromide, bromoethane, 2-bromopropane, bromomethane and 2-bromo-2-methylpropane.
Bromomethane (145) > bromoethane (1) > 2-bromopropane (7.8·10−3). 2-Bromo-2-methylpropane (< 5·10−4) and neopentyl bromide (1.3·10−5) practically do not react by SN2.
2. Linear chains
Why does 1-bromopropane (0.80) react at almost the same rate as bromoethane (1)?
The added methyl group is far from the reacting carbon and can rotate out of the way of the incoming nucleophile.
3. Neopentyl bromide
Neopentyl bromide, (CH3)3CCH2Br, is a primary halide. Why does it react so slowly by SN2?
Branching at the adjacent carbon (a tert-butyl group) blocks the back face of the reacting carbon and prevents the approach of the nucleophile.
4. Synthesis of MTBE
To prepare methyl tert-butyl ether, (CH3)3COCH3, by an SN2 reaction, which combination would you choose: CH3Br + (CH3)3CO− or (CH3)3CBr + CH3O−?
CH3Br + (CH3)3CO−: bromomethane is unhindered. tert-Butyl bromide does not undergo SN2 and, with a strong base such as methoxide, would give mainly elimination (2-methylpropene).