How will you convert benzene into (a) p-nitrobromobenzene (b) m-nitrobromobenzene
Halogens attached to benzene ring is ortho and para directing where as nitro group is meta directing.
Arrange the following set of compounds in the order of their decreasing relative reactivity with an electrophile. Give reason.
The methoxy group $\left(-\mathrm{OCH}_3\right)$ is electron releasing group. It increases the electron density in benzene nucleus due to resonance effect (+ $R$-effect). Hence, it makes anisole more reactive than benzene towards electrophile.
In case of alkyl halides, halogens are moderately deactivating because of their strong $-I$ effect. Thus, overall electron density on benzene ring decreases. It makes further substitution difficult.
$-\mathrm{NO}_2$ group is electron withdrawing group. It decreases the electron density in benzene nucleus due to its strong - $R$ - effect and strong $-I$-effect. Hence, it makes nitrobenzene less reactive. Therefore, overall reactivity of these three compounds towards electrophiles decreases in the following order
Despite their $-I$ effect, halogens are $o$ - and $p$-directing in haloarenes. Explain.
Halogens have $(-I)$ and $(+R)$ effect, these groups are deactivating due to their $(-I)$ effect and they are ortho, para directing due to $(+R)$ effect.
Why does presence of a nitro group make the benzene ring less reactive in comparison to the unsubstituted benzene ring. Explain.
The meta - directing substituents (like $-\mathrm{NO}_2$ group) withdraw electrons from the benzene ring and thus, deactivate the benzene ring for further substitution and make the benzene ring less reactive in comparison to the unsubstituted benzene ring.
Suggest a route for the preparation of nitrobenzene starting from acetylene?
Acetylene when passed through red hot iron tube at 873 K , undergoes cyclic polymerisation benzene which upon subsequent nitration gives nitrobenzene.
