Question

Which of these statements are correct?
(A) $ 1,3,5 $ -cycloheptatriene is not acidic while $ 1,3 $ -cyclopentadiene is acidic in nature
(B) P-chlorophenol is less acidic than p-fluorophenol
(C) Dipole moment of azulene is greater than naphthalene
(D) Mesomeric effect is temporary effect

Answer 1

Hint: The compounds which can release a proton easily can be known as more acidic. $ 1,3 $ -cyclopentadiene on losing a proton from $ 1,3 $ -cyclopentadienyl anion which is aromatic and stable. Thus, $ 1,3 $ -cyclopentadiene is more acidic compared to $ 1,3,5 $ -cycloheptatriene. Due to the presence of d-orbitals in P-chlorophenol is more acidic compared to p-fluorophenol. Naphthalene is a symmetric molecule and does not have a dipole moment. Mesomeric effect is a permanent effect.

Complete answer:
 $ 1,3,5 $ -cycloheptatriene is an aromatic compound and stable but $ 1,3 $ -cyclopentadiene upon losing a proton from $ 1,3 $ -cyclopentadienyl anion which is an aromatic compound and stable. Thus, the given statement is correct.
p-chlorophenol has a chlorine atom and d-orbital extent is possible which makes it more acidic than p-fluorophenol. Thus, the given statement is not correct.
Azulene is the fused system of cycloheptatriene and cyclopentadiene. It has a dipole moment of $ 1.08\delta $ whereas naphthalene is a symmetric molecule and does not have a dipole moment. Thus, the given statement is correct.
Mesomeric effect is the permanent delocalization of pi-electrons. The two pi-bonds were converted into a new pi-bond. Thus, the given statement is not correct.

Note:
Aromatic compounds are stable compounds that are cyclic, planar, conjugation of pi-electrons, obeying Huckel’s rule. Huckel’s rule is given by $ \left( {4n + 2} \right)\pi $ electron, where n is any whole number. Benzene and its derivatives are aromatic compounds.