According to $H$ückel's rule, a cyclic $\pi$-molecular orbital system must contain which of the following to be aromatic?

The number of $-CH_2-$ (methylene) groups in the product formed from the following reaction sequence is. . . . . . .

Among the following compounds,the order of decreasing reactivity towards electrophilic substitution is:

Identify the correct statements regarding the presence of a $-NO_{2}$ group in a benzene ring:
$A$. It increases the electron density of the benzene ring.
$B$. It deactivates the benzene ring towards electrophilic aromatic substitution.
$C$. It activates the benzene ring towards nucleophilic aromatic substitution.
$D$. It directs the incoming electrophile to the ortho and para positions.

Explain the reaction of benzene with $n$-propyl chloride.

The order of reactivity of the following compounds towards electrophilic substitution will be - [where $\phi = C_6H_5$]:
$I. \phi-CH_3$
$II. \phi-CH_2-CH_3$
$III. \phi-CH(CH_3)_2$
$IV. \phi-C(CH_3)_3$