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The tertiary butyl group – often written t-Bu, or tertbutyl, or –C(CH3)3  has one particular distinction in organic chemistry. It takes up a lot of space!

Imagine a ceiling fan with three blades rotating constantly. Well that’s a pretty accurate description of a methyl group (CH3) in miniature.  Now imagine three ceiling fans, all attached to the same point, all constantly in motion! That gives you an idea of what a tert-butyl group is like.

Another way of saying this is that it is “bulky”. Bulky groups participate in steric interactions.

Train yourself to have the following Pavlovian response. Whenever you see “t-Bu” or “tert-butyl” or “-C(CH3)3”, a little bell should go off, and you should immediately think: STERIC INTERACTIONS (“sterics” for short).

In the near term, here’s how this property of the tert-butyl group will play out. If you see –C(CH3)3 attached to a cyclohexane ring, it is a near-100% bet that it will occupy the equatorial position, since in the axial position it will have unfavorable steric interactions with other axial groups. Remember: the chair form that is preferred will have the fewest steric interactions. 

In the medium term, steric hindrance is going to affect some types of reactions, like substitution and elimination. More on that soon.

Thanks for reading! James

PS Practice:

tert-butyl: (ding!) STERIC INTERACTIONS.


t-butyl : (ding!) STERIC INTERACTIONS.