Question

Identify the species from following that exhibits no bond resonance.

• A. $\mathrm{CH_3CH_2Br}$
• B. $\mathrm{\overset{(+)}{C}H_2CH_3}$
• C. $\mathrm{CH_3CH_2NO_2}$
• D. $\mathrm{C_6H_6}$

Hint:

Always associate the phrase "no bond resonance" instantly with hyperconjugation. Look for carbocations, alkenes, or free radicals possessing alpha-hydrogens, as they are classical structural examples that display this electronic behavior.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the species from the given list that undergoes or displays "no bond resonance".

Step 2: Key Formula or Approach:
"No bond resonance" is the historical synonym and alternative name for the concept of

hyperconjugation (also known as the Baker-Nathan effect). It involves the delocalization of $\sigma$-electrons belonging to a $\mathrm{C-H}$ bond of an alkyl group directly into an adjacent unhybridized p-orbital or $\pi$-system. This phenomenon requires at least one $\alpha$-hydrogen attached to an $\mathrm{sp^3}$ carbon adjacent to an $\mathrm{sp^2}$ carbon (carbocation, free radical, or alkene).

Step 3: Detailed Explanation:
Let's evaluate each option to check for hyperconjugation eligibility:

• $\mathrm{CH_3CH_2Br}$: A neutral saturated alkyl halide with no empty p-orbitals or adjacent $\pi$-systems for conjugation.

• $\mathrm{\overset{(+)}{C}H_2CH_3}$ (Ethyl Carbocation): The carbocationic carbon ($\mathrm{- \overset{(+)}{C}H_2}$) is $\mathrm{sp^2}$ hybridized with a vacant p-orbital. The adjacent $\alpha$-carbon ($\mathrm{-CH_3}$) contains three $\alpha$-hydrogens. The $\sigma$-electrons of these $\mathrm{C-H}$ bonds actively delocalize into the vacant p-orbital, illustrating structural hyperconjugation structures (no bond resonance).

• $\mathrm{CH_3CH_2NO_2}$: Saturated compound containing a nitro group, does not typically exhibit hyperconjugation relative to a vacant p-orbital or alkene.

• $\mathrm{C_6H_6}$ (Benzene): Exhibits traditional $\pi$-$\pi$ resonance, not no bond resonance.

Step 4: Final Answer:
The species that displays no bond resonance (hyperconjugation) is the ethyl carbocation $\mathrm{\overset{(+)}{C}H_2CH_3}$, matching option (B).