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Dear all,

Please could you help me by explaining how does t-butyl alcohol (2-methylpropan-2-ol) react with sulphuric acid to generate a carbocation.

Thanks in advance.

[Yggdrasil]

Carbocations are formed when a leaving group separates from an organic molecule.  In order for this to occur, there are two major requirements:

1)  The leaving group must be a very good leaving group.  This means that the leaving group forms a stable compound.  Good leaving groups include groups with relatively low charge density (preferably, a leaving group should be positive while on the molecule, and neutral or at least more neutral when off) and groups which gain resonance stabilization when they separate from the molecule.

2)  The carbocation must be stable.  Since carbocations are electron defficient (they have an empty pi orbital) any effect which donates electron density to the carbon (inductive effect from neighboring alkyl groups, neighboring lone pairs or pi orbitals to stabilize through resonance/conjugation) will stabilize the carbocation.

Keeping these two criteria in mind, let us examine t-butyl alcohol.  If the OH group leaves from t-butyl alcohol will form a t-butyl carbocation, what is known as a tertiary carbocation (because it has three alkyl substituents).  Since alkyl groups are electron-donating, they will stabilize the carbocation.  In general, many secondary- and most tertiary- carbocations are stable enough to allow their formation.  Therefore, t-butyl alcohol fits the second criteria for carbocation formation.

However, OH^- is a very poor leaving group.  It has a formal negative charge and a relatively small atomic radius, giving it a relatively high charge density.  This high charge density strongly disfavors its formation.  Therefore, on its own t-butyl alcohol is unlikely to form a carbocation.

However, in the presence of sulfuric acid, the OH group will be protonated, forming OH₂⁺.  When this group leaves, it forms water a neutral and extremely stable molecule.   Therefore, in the presence of sulfuric acid, t-butyl alcohol fits both criteria for carbocation formation:  the OH₂⁺ is a very good leaving group and the tertiary carbocation is very stable (for a carbocation).