Question

Which of the following is correct with respect to $\left( { - {\text{I}}} \right)$ effect of the substituents? (R = alkyl)?
A.$\;-N{H_2} > {\text{ }} - OR{\text{ }} > {\text{ }} - F$
B.$-N{H_2} < {\text{ }} - OR{\text{ }} < {\text{ }} - F$
C.$-N{R_2} > {\text{ }} - OR{\text{ }} > {\text{ }} - F$
D.$-N{R_2} < {\text{ }} - OR{\text{ }} < {\text{ }} - F$

Answer 1

This question can be answered from the knowledge of the inductive effect of the substituents in the aromatic compounds. For elements with electronegativity lower than that of carbon, the shared pair of electrons is drawn closer towards the carbon atom, while in case of elements with electronegativity higher than carbon, like $N,O,F$, etc. the shared pair of electron is drawn away from the carbon atom. This availability of electrons is called the “Inductive Effect”.

Complete step by step answer:
Among the given substituents, one has got an ‘N’ atom, one has an ‘O’ atom and the other is ‘F’. Among all these atoms, F has the highest electronegativity.
So when these elements are bonded to the carbon atoms in an organic compound, -F will show the highest negative inductive effect, followed by $- OR$and then$- N{R_2}$, where R is any alkyl group.
Alkyl groups themselves show $\left( { + {\text{I}}} \right)$ effect (order of $\left( { + {\text{I}}} \right)$ effect, $R > H$).
Hence, the O and N atoms bonded to any alkyl group are bound to show less electron withdrawing ability as compared to $- F$.

So, the answer (D): $-N{R_2} < {\text{ }} - OR{\text{ }} < {\text{ }} - F$

Note:
The inductive effects are electronic effects that arise due to the difference in the bond polarities. The carbon atom in the organic compounds can be seen to form covalent bonds with H, O, N, F, S, P, etc. There exists a difference in the electronegativities of these elements with carbon due to which influence the availability of electrons in their bonds with carbon.
Those organic substituent groups that increase the availability of electrons are said have $\left( { + {\text{I}}} \right)$ effect, while those that withdraw the shared pair of electrons are said to have $\left( { - {\text{I}}} \right)$ effect.
The groups with $\left( { + {\text{I}}} \right)$ effect are in order of decreasing $\left( { + {\text{I}}} \right)$ effect$- {O^ - },{\text{ }} - CO{O^ - },{\text{ }} - C{R_3},{\text{ }} - CH{R_2},{\text{ }} - C{H_2}R,{\text{ }} - C{H_3},{\text{ }} - H$
The Groups with $\left( { - {\text{I}}} \right)$ effect in the decreasing $\left( { - {\text{I}}} \right)$ effect: $- F,{\text{ }} - Cl,{\text{ }} - Br,{\text{ }} - I,{\text{ }} - OH,{\text{ }} - OR,{\text{ }} - OAr,{\text{ }} - N{H_2},{\text{ }} - N{R_2},{\text{ }} - OH,{\text{ }} - OR,{\text{ }} - COOH,{\text{ }}$
$- CHO,{\text{ }} - COOR,{\text{ }} - CON{H_2},{\text{ }} - COCl,{\text{ }} - Ph,{\text{ - }}N{O_2}$