Hi,

The number of carbons stemming from a carbocation will determine if it is primary, secondary, tertiary or quaternary.

Recall that carbon forms 4 bonds.

If 1 of those bonds is to carbon --> Primary

2 --> Secondary
3 --> Tertiary
4 --> Quaternary

Carbon groups are more stabilizing to carbocations than hydrogens as they are more electron donating.

Therefore as the number of carbon groups increases, the stability of carbocations increases.

Thus for more carbocation rearrangements, you are almost always going from:

secondary --> tertiary
tertiary --> quaternary

(Recall primary is unstable)

I have attached a picture here: https://ibb.co/6ZzJ1Kj


You will not go tend to go in the other direction, unless this is an enzyme question -- enzymes do what they want, so you could get given prompts about exceptions to reactivity.


Please let me know if this is unclear.