Question

Among the following, weakest activating group is-
A.$ - N{H_2}$
B.$ - NHCOC{H_3}$
C.$ - N{R_2}$
D.$ - C{H_3}$

Answer 1

Hint:The groups attached to aromatic rings can be of two types-activating and deactivating groups. The activating and deactivating group vary the rate of electrophilic substitution reaction. When electron activating groups are present on the ring, they direct the incoming electrophile to the ortho and para position.

Complete answer:
-In electrophilic aromatic substitution reactions, various substituents can be attached on the benzene ring. In these reactions, the group that is attached to the aromatic system is replaced by an electrophile. The substituents on the ring can be classified into two types- electron donating and electron withdrawing.
-Electron withdrawing substituents withdraw the electron density towards them making the aromatic ring electron-deficient. As the incoming electrophile needs an electron-rich site, the ring being electron-deficient decreases the rate of reaction. Some examples include $ - N{O_2}, - CN, - COR, - COOR, - COC{H_3}, - S{O_3}H$
-Electron donating substituents increase the electron density on the aromatic ring. Hence the ring becomes electron-rich. Electrophiles can easily attack such an electron-rich ring. Thus electron donating groups increase the rate of electrophilic aromatic substitution reaction. Some examples include $ - N{R_2}, - C{H_3}, - C{l^ - },B{r^ - }, - OC{H_3}$
$ - N{H_2}, - NHCOC{H_3}, - N{R_2}, - C{H_3}$ are electron releasing groups. The groups containing nitrogen are strong activating groups because nitrogen atoms have a free or unshared pair of electrons. -This unshared pair of electrons can resonate in the ring making it electron-rich. Hence such groups increase the rate of reaction and are also called as activating groups. The $ - C{H_3}$ group has no such lone pair. It has only positive inductive effect but the nitrogen containing groups have both positive inductive and resonance effects.
-In inductive effect , permanent dipole arises in a molecule because of the unequal sharing of electrons in the molecule. This effect can be seen only in the case of sigma bonds. Electron donating groups show the tendency to donate the electrons to the carbon to which that group is attached. Due to this effect carbon gets partial negative charge and this effect is called as positive inductive effect.
In resonance effect, there is delocalization of electrons in the molecule. It involves pi bond.
-Some groups release electrons through such delocalization of electrons. Such an effect is called the positive resonance effect. For example, groups like $ - N{H_2}, - N{R_2}$ when attached to an aromatic ring delocalize their lone pair over the ring. Such an effect is $ + R$ or positive resonance effect.
-Resonance effect is stronger than inductive effect. Hence $ - C{H_3}$ is the weakest activating group among all.
So the correct option is D.

Note:
Groups showing positive inductive and positive resonance effect increase the rate of electrophilic aromatic substitution reactions. In inductive effect the electrons are only displayed for some extent. But in resonance there is complete transfer of electrons. Both are permanent types of effect.