Question

Designate whether each of the following compounds is aromatic or not aromatic.

• A. e, g aromatic and a, b, c, d, f, h not aromatic
• B. b, e, f, g aromatic and a, c, d, h not aromatic
• C. a, b, c, d aromatic and e, f, g, h not aromatic
• D. a, c, d, e, h aromatic and b, f, g not aromatic

Hint:

\textbf{Huckel's Rule for Aromaticity:} \begin{itemize} \item Cyclic, planar molecule \item Fully conjugated $\pi$-system \item Contains 4n+2 $\pi$ electrons (n=0,1,2...) \end{itemize}

Answer 1

The Correct Option is D

Aromatic compounds (follow Huckel's rule):
(a) Cyclic, planar, conjugated with 6$\pi$ electrons (4n+2 where n=1)
(c) Cyclic, planar, conjugated with 6$\pi$ electrons
(d) Cyclic, planar, conjugated with 6$\pi$ electrons
(e) Cyclic, planar, conjugated with 6$\pi$ electrons
(h) Cyclic, planar, conjugated with 6$\pi$ electrons

Non-aromatic compounds:
(b) Not fully conjugated (sp$^3$ hybridized carbon breaks conjugation)
(f) Not planar (twisted structure prevents conjugation)
(g) Has 4$\pi$ electrons (doesn't satisfy 4n+2 rule)

Answer 2

To determine whether each compound is aromatic or not aromatic, we must apply Hückel's rules for aromaticity.

Concept Used:

For a compound to be considered aromatic, it must satisfy all of the following criteria:

1. The molecule must be cyclic.
2. The molecule must be planar.
3. The molecule must be fully conjugated (i.e., every atom in the ring must have a p-orbital that can participate in resonance).
4. The molecule must have (4n + 2) π electrons, where n is a non-negative integer (n = 0, 1, 2, ...). This is known as Hückel's rule.

A compound that is cyclic, planar, and fully conjugated but has 4n π electrons is considered anti-aromatic and is highly unstable.

A compound that fails any of the first three criteria (cyclic, planar, or conjugated) is considered non-aromatic.

The question asks to designate compounds as "aromatic" or "not aromatic". The "not aromatic" category includes both anti-aromatic and non-aromatic compounds.

Step-by-Step Solution:

Step 1: Analysis of Compound (a) - Cyclopentadienyl anion

• It is cyclic, planar, and fully conjugated.
• Number of π electrons = 4 (from two double bonds) + 2 (from the lone pair of the carbanion) = 6.
• Since 6 fits the (4n + 2) rule (for n=1), the compound is aromatic.

Step 2: Analysis of Compound (b) - Cyclopentadienyl cation

• It is cyclic, planar, and fully conjugated (the carbocation has an empty p-orbital).
• Number of π electrons = 4 (from two double bonds).
• Since 4 fits the 4n rule (for n=1), the compound is anti-aromatic, and thus not aromatic.

Step 3: Analysis of Compound (c) - Cyclobutadienyl dication

• It is cyclic, planar, and fully conjugated.
• Number of π electrons = 2 (from one double bond).
• Since 2 fits the (4n + 2) rule (for n=0), the compound is aromatic.

Step 4: Analysis of Compound (d) - Cyclobutadienyl dianion

• It is cyclic, planar, and fully conjugated.
• Number of π electrons = 2 (from one double bond) + 4 (from two lone pairs) = 6.
• Since 6 fits the (4n + 2) rule (for n=1), the compound is aromatic.

Step 5: Analysis of Compound (e) - Tropylium (cycloheptatrienyl) cation

• It is cyclic, planar, and fully conjugated.
• Number of π electrons = 6 (from three double bonds).
• Since 6 fits the (4n + 2) rule (for n=1), the compound is aromatic.

Step 6: Analysis of Compound (f) - Cyclooctatetraene

• It is cyclic and has 8 π electrons (which would make it anti-aromatic if planar).
• However, the molecule is not planar; it adopts a tub-like conformation to avoid the instability of anti-aromaticity.
• Since it is not planar, it is not aromatic (it is non-aromatic).

Step 7: Analysis of Compound (g) - Cyclobutadiene

• It is cyclic, planar, and fully conjugated.
• Number of π electrons = 4 (from two double bonds).
• Since 4 fits the 4n rule (for n=1), the compound is anti-aromatic, and thus not aromatic.

Step 8: Analysis of Compound (h) - Cyclopropenyl cation

• It is cyclic, planar, and fully conjugated.
• Number of π electrons = 2 (from one double bond).
• Since 2 fits the (4n + 2) rule (for n=0), the compound is aromatic.

Final Result:

Based on the analysis:

• Aromatic compounds: (a), (c), (d), (e), (h)
• Not Aromatic compounds: (b), (f), (g)

Therefore, the correct option is: a, c, d, e, h aromatic and b, f, g not aromatic.