In alkyl groups, the size of the inductive effect has the following order:

>

>

>

This is to be expected from our previous discussion of the inductive effect. However, the order is not the same when these groups are attached to an unsaturated system (e.g. double bond, or benzene).  The reason for this disturbance of the order is that there is another effect at work, which allows the alkyl groups to donate electrons.  This is known as hyperconjugation.

This is similar to the mesomeric effect.  Obviously, the proton which is ejected in the scheme above should not be considered as being a loose proton.  It is still loosely associated with the parent molecule.  A reversal of the inductive electron donating order can be explained as being due to the presence of hydrogen atoms attached to the carbon adjacent to the unsaturated system.  e.g.:

CH3	>	MeCH2	>	Me2CH	>	Me3C

Clearly, if this is the case, the greatest effect will be with CH₃, and the least (i.e. none) with CMe₃.