Chemistry · General Principles Of Organic Chemistry · NEET
It is a reaction in which the bonds inside a single molecule break and reform so that the atoms end up in a different arrangement. The molecular formula does not change, only the skeleton or the position of a group changes. In NEET's basic-principles chapter it is one of the four fundamental reaction types (along with substitution, addition and elimination), and it almost always happens through a carbocation.
A carbocation is electron-poor and unstable. If a nearby atom can shift over to make a more stable carbocation, the molecule will do it because more stable means lower energy. Stability order is 3 degree greater than 2 degree greater than 1 degree (also benzylic and allylic are very stable). So a primary or secondary carbocation next to a more substituted carbon rearranges to become secondary or tertiary.
Both are 1,2-shifts, meaning the group moves from the carbon right next to the positive carbon (the C+). In a hydride shift, an H moves across with its bonding electron pair (it moves as H- minus, a hydride). In a methyl shift, a whole CH3 group moves across with its electron pair. The molecule always chooses the shift that gives the more stable carbocation; a hydride shift is generally preferred if it already gives the more stable ion.
No. Rearrangement only moves atoms around inside the same molecule, so the molecular formula stays identical. What changes is the connectivity, so you get an isomer (usually a more branched or more stable skeleton). This is why the products of a reaction with rearrangement can look surprising.
Look for a step that makes a carbocation (acidic dehydration, SN1, addition of HX to an alkene, Friedel-Crafts alkylation). Then check the carbon next door: if shifting one H or one CH3 would upgrade the carbocation from 1 degree or 2 degree to 2 degree or 3 degree, expect a rearranged product. If the carbocation is already tertiary, no rearrangement is needed.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. NCERT introduces rearrangement as one of the four fundamental reaction types in Organic Chemistry Some Basic Principles and Techniques, and the 1,2-hydride shift is shown there. It also underlies carbocation stability questions that NEET asks every year.
A 3 degree (tertiary) carbocation is more stable than a 2 degree (secondary) one because more alkyl groups donate electron density through the +I effect and hyperconjugation. A carbocation rearranges precisely to reach this more stable state.
Only one shift happens per step, and the molecule takes the shift that gives the more stable carbocation. If both a neighbouring H and a neighbouring CH3 are available, the shift chosen is the one leading to the lower-energy cation; usually the hydride shift is enough.
No. Rearrangement occurs only when a more stable carbocation can be reached. A carbocation that is already tertiary, benzylic or allylic is stable and normally does not rearrange.
