Q.Write structures of various carbocations that can be obtained from 2-methylbutane. Arrange these carbocations in order of increasing stability.

Carbocations from 2-Methylbutane

Concept: Carbocation stability increases with substitution (3° > 2° > 1°) due to hyperconjugation and inductive effects.

2-Methylbutane has the structure $\ce{CH3-CH(CH3)-CH2-CH3}$. Removing a hydrogen atom and an electron from different carbons generates distinct carbocations:

1. Primary carbocations (two possible):

   • From C-1: $\ce{CH2^+-CH(CH3)-CH2-CH3}$
   • From C-4: $\ce{CH3-CH(CH3)-CH2-CH2^+}$

2. Secondary carbocation (one):

   • From C-3: $\ce{CH3-CH(CH3)-CH^+-CH3}$

3. Tertiary carbocation (one):

   • From C-2: $\ce{CH3-C^+(CH3)-CH2-CH3}$ …

Understanding Carbocation Stability

When 2-methylbutane loses a hydrogen atom along with its bonding electrons, the carbon that held that hydrogen becomes electron-deficient and carries a positive charge. The stability of the resulting carbocation depends on how well the surrounding structure can donate electron density to that empty orbital.

Two effects govern this:

Hyperconjugation – neighbouring C–H or C–C bonds donate electron density through orbital overlap with the empty p-orbital on the positively charged carbon. More alkyl groups attached to the carbocation centre means more hyperconjugative structures, hence greater stability.

Inductive effect – alkyl groups are electron-donating relative to hydrogen, so they push electron density toward the positive centre through σ-bonds, partially neutralising the charge.

Both effects lead to the same stability order: tertiary > secondary > primary > methyl.

Generating All Possible Carbocations from 2-Methylbutane

The structure of 2-methylbutane is:

Label the carbons for clarity:

• C-1: the terminal methyl on the left ($\ce{CH3}$–)
• C-2: the branching carbon bearing the methyl group ($\ce{-CH(CH3)-}$)
• C-3: the methylene next to C-2 ($\ce{-CH2-}$)
• C-4: the terminal methyl on the right ($\ce{-CH3}$)

Now remove one hydrogen from each distinct position:

1. Carbocation at C-1 (primary)

Removing H from the left terminal methyl:

This carbon is attached to one alkyl group (the rest of the chain), making it a 1° carbocation.

2. Carbocation at C-2 (tertiary)

Removing H from the branching carbon:

This carbon is attached to three alkyl groups (two methyls and an ethyl), making it a 3° carbocation.

3. Carbocation at C-3 (secondary)

Removing H from the methylene group:

This carbon is attached to two alkyl groups (an isopropyl and a methyl), making it a 2° carbocation.

4. Carbocation at C-4 (primary)

Removing H from the right terminal methyl:

This carbon is attached to one alkyl group, making it another 1° carbocation.