Nucleophiles React Faster with Methyl Bromide than with tert-Butyl Bromide

We teach students that bromide reacts faster with methyl bromide than with tert-butyl bromide, due to increased crowding in the transition state of the tert-butyl system. This is not completely true.

Open "Sn2 bromide+methyl bromide...". Transition states for two SN2 reactions, bromide+methyl bromide and bromide+tert-butyl bromide, will appear as space-filling models.
Neither transition state is crowded.
Switch to a ball-and-spoke model (Model menu). Click on (Measure Distance) and measure the C-Br distance in each of the two transition states (click on a CBr "bond" in each and read the distance at the bottom right of the screen).  

The carbon-bromine distance in the transition state in the tert-butyl system is longer than that in the methyl system, (2.9Å compared to 2.5Å). This leads to an increase in charge separation.

For each of the transition states, select Surfaces from the Display menu and double click on the line "density potential..." in the dialog which results.

Electrostatic potential maps show greater charge separation (a greater range of colors) in transition state for addition to tert-butyl bromide than in the transition state for addition to methyl bromide. Electrostatics and not steric crowding is the cause of the decrease in reaction rate.