What is the correct order of decreasing stability of the following cations.

Correct answer: 1)

Concept:

• Carbocation is the intermediate of carbon-containing positive charge.
• It has six electrons in the valence shell.
• There are three factors contributing to the stability of carbocations: (a) Inductive Effect (b) Hyperconjugation (c) Resonance.
• Inductive effect: When two, unlike atoms, form a covalent bond, the electron-pair forming the sigma bond is never shared equally between the two atoms but is shifted slightly towards the more electronegative species. 
• Inductive effect may be due to a single atom or a group of atoms. Relative inductive effects are measured with reference to hydrogen.
• Those which donate electrons to the carbon chain are called electron-donating groups (EDG) or electron-releasing groups (ERG) and are said to exert a +I effect.
• Those which withdraw electrons from the carbon chain are called electron-withdrawing groups (EWG) and are said to exert –I effect.
• Hyperconjugation is the ability of the σ  bond electrons of an σ  bond to undergo conjugation with the adjacent σ  electrons. It is also known as Baker-Nathan Effect, No-Bond Resonance. 
• Resonance/Mesomeric Effect is a π -π interaction and acts through weak pi bonds. 

Explanation:

• Carbocation I has six α-hydrogen.
• So, it is stabilized by hyperconjugation (+H effect) using these alpha hydrogens.
• Also, this carbocation involves stabilization through the +I effect of two methyl groups.
• Carbocation II is stabilized by a positive mesomeric effect (i.e,+M effect). 
• This carbocation will be most stable because mesomeric effects are more stabilizing as compared to hyperconjugation and inductive effect. 
• Carbocation III has five α-hydrogen.
• So, it is stabilized by hyperconjugation (via +H effect) using these five alpha hydrogens. but the −I effect of the OCH₃ group will destabilize the carbocation.
• Hence, less alpha hydrogens and electron-withdrawing inductive effect will make this carbocation less stable than the other two carbocations.

• Hence, the order of stability of carbocation is: II>>I>III

Conclusion:

Hence, the order of stability of carbocation is II>>I>III.

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