Question

Species in which hyperconjugation does not occur is

• A. ${\mathrm{CH}}_3\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$
• B. ${\left({\mathrm{CH}}_3\right)}_2\stackrel{+}{\mathrm{C}}\mathrm{H}$
• C. ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2$
• D. $({\mathrm{CH}}_3{)}_3\mathrm{C}-\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$

Answer 1

The correct option is D

$({\mathrm{CH}}_3{)}_3\mathrm{C}-\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$

The explanation for the incorrect option:

Option (D): $({\mathrm{CH}}_3{)}_3\mathrm{C}-\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$

• The hyperconjugation effect is a permanent effect in which the sigma electrons of an alkyl group's $\mathrm{C}-\mathrm{H}$ bond are localized to an unsaturated system atom or an atom with an unshared p orbital.
• Since it helps in the dispersion of positive charges, hyperconjugation stabilizes the carbocation. As a result, the more alkyl groups connected to a positively charged carbon atom, the stronger the hyperconjugation interaction and carbonation stability.
• H-atoms play an important part in the phenomenon of hyperconjugation. As a result, as the number of sigma-hydrogen atoms increases, the number of hyper conjugative structures increases, and the inductive effect increases.
• Compounds with no hydrogen linked to the neighboring ${\mathrm{sp}}^2$ carbon do not exhibit hyperconjugation.
• ${\mathrm{CH}}_3-\stackrel{\stackrel{{\mathrm{CH}}_3}{|}}{\underset{\underset{{\mathrm{CH}}_3}{|}}{\mathrm{C}}}-\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$ has no hydrogen attached to ${\mathrm{sp}}^2$ carbon, hence it will not show any hyperconjugation.

The explanation for the correct options:

Option (A): ${\mathrm{CH}}_3\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$

1. A permanent effect known as hyperconjugation occurs when the C-H bond electrons of an alkyl group are localized to an atom in an unsaturated system or to an atom with an unshared p orbital.
2. Hyperconjugation occurs due to the alpha hydrogen, i.e. the hydrogen attached to alpha carbon, and alpha carbon is that carbon to which the functional group is directly attached.
3. ${\mathrm{CH}}_3-\underset{\underset{\mathrm{H}}{|}}{\overset{+}{\mathrm{C}}}-\mathrm{H}$ As shown in the structure, ${\mathrm{CH}}_3\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$ contains alpha hydrogen, therefore, it will show hyperconjugation.
4. Hence, it is the correct option

Option (B): ${\left({\mathrm{CH}}_3\right)}_2\stackrel{+}{\mathrm{C}}\mathrm{H}$

1. Hyperconjugation occurs due to the alpha hydrogen, i.e. the hydrogen attached to alpha carbon, and alpha carbon is that carbon to which the functional group is directly attached.
2. ${\mathrm{CH}}_3-\underset{\underset{{\mathrm{CH}}_3}{|}}{\stackrel{+}{\mathrm{C}}}-\mathrm{H}$ as shown in the structure, ${\left({\mathrm{CH}}_3\right)}_2\stackrel{+}{\mathrm{C}}\mathrm{H}$ contains alpha hydrogen, therefore, it will show hyperconjugation.
3. Hence, it is the correct option

Option (C): ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2$

1. Hyperconjugation occurs due to the alpha hydrogen, i.e. the hydrogen attached to alpha carbon, and alpha carbon is that carbon to which the functional group is directly attached.
2. Propene contains alpha hydrogen, hyperconjugation will occur
3. Hence, it is the correct option.

Therefore, option (D) is correct.