Question

The most stable carbocation among the following is:
1)$C{{H}_{3}}-C{{H}_{2}}-C{{H}^{\oplus }}_{2}$
2) ${{(C{{H}_{3}})}_{3}}-C{{H}^{\oplus }}-C{{H}_{3}}$
3) $C{{H}_{3}}-C{{H}_{2}}-C{{H}^{\oplus }}-C{{H}_{2}}-C{{H}_{3}}$
4) $C{{H}_{3}}-C{{H}^{\oplus }}-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{3}}$

Answer 1

Hint: The answer to this question includes the fact that the stability of carbocation increases from primary to tertiary carbons that is more stable will be the tertiary carbocation. This fact leads to the correct answer.

Complete step by step answer:
In the lower classes of chemistry we have studied about the trends in stability, inductive effect and various other factors.
Let us now see the stability of carbocation and its variation.
The stability of carbocation is dependent on three facts:
- Neighbouring number of carbon atoms.
- Carbon multiple bonds present
- The adjacent atoms with lone pair of electrons on it

Now, in the above question it is only the hydrocarbons present and no multiple bond nor atoms with lone pair of electrons present on it. Thus, the stability will depend on the number of carbon atoms present adjacent to carbocation.
Since in option 2), the carbocation is a secondary carbocation but attached to only one methyl group. Therefore, the stability of this carbocation is less.
In option number 1), the carbocation present is the primary carbocation and is attached to ethyl group whereas in option 3) and 4) the carbocations present are the secondary carbocations but in case of option 4), the electrons cannot move as freely as that of carbocation in given option 3).
The main reason for this is the hyperconjugation which is nothing but the interaction of the electrons in a sigma bond with an adjacent antibonding or partially filled p orbital which gives an extended bonding.
Therefore, based on this concept, we can say that the carbocation in option 3) is more stable as the electrons can move freely in this molecule.
So, the correct answer is “Option C”.

Note: Note that the stability of carbocation also depends upon the inductive effect, resonance, isomeric effect and also electronegativity. Therefore, observe the question carefully otherwise the answers may go wrong.