Question

Consider the following compounds/species:
i)
ii)
iii)
iv)
v)
vi)
vii)
The number of compounds/species which obey Huckel's rule is _____

• A. 5
• B. 4
• C. 6
• D. 2

Answer 1

The Correct Option is B

What is Hückel's Rule?
Hückel's rule tells us if a compound is aromatic (special and stable). It must: 
- Be a ring (cyclic). 
- Have alternating double bonds (conjugated). 
- Be flat (planar). 
- Have 4n + 2 π electrons (like 2, 6, 10, etc., where n = 0, 1, 2...).

Checking Each Option
i) Two fused benzene rings: This is naphthalene. It has 10 π electrons (4n + 2, n = 2) and is flat. Obeys Hückel's rule. - 
ii) Five-membered ring with O: This is furan. It has 6 π electrons (4 from the ring + 2 from oxygen’s lone pair, n = 1) and is flat. Obeys Hückel's rule. 
iii) Two parallel lines: This is not a ring, so it can’t be aromatic. Does not obey Hückel's rule. 
iv) Triangle with a + charge: This is the cyclopropenyl cation. It has 2 π electrons (n = 0) and is flat. Obeys Hückel's rule. 
v) Triangle with a - charge: This is the cyclopropenyl anion. It has 4 π electrons (doesn’t fit 4n + 2). Does not obey Hückel's rule. 
vi) Seven-membered ring: This is cycloheptatriene. It has 6 π electrons (n = 1), but it’s not flat because of the CH₂ group. Does not obey Hückel's rule. 
vii) Three fused benzene-like rings: This is anthracene. It has 14 π electrons (4n + 2, n = 3) and is flat. Obeys Hückel's rule.

Final Count
The compounds that obey Hückel's rule are i, ii, iv, and vii. That’s 4 compounds.