Question

Which of the following functional groups shows +R effect?
A. $- CHO$
B. $- N{O_2}$
C. $- CN$
D. $- N{R_2}$

Answer 1

:Hint: +R effect is shown by the groups which are capable of donating electrons. This donation of electrons withdraws the electrons from the rest of the molecule which causes the decrease in electron density of the molecule. It is also known as +M effect.

Complete answer:
+R effect means the positive resonance effect. This effect is shown by the groups which have lone pairs so that they donate the electrons. This donation of electrons decreases the electron density of the whole molecule. Now, in the given option:
In the case of option A, there is no lone pair present in the group so it cannot donate electrons and hence does not show a positive resonance effect. It shows a negative resonance effect.
In the case of option B, there is also absence of lone pairs of electrons in the group so it cannot give it away and does not show a positive resonance effect. So it will show a negative resonance effect.
In the case of option C, there is also absence of lone pairs of electrons in the group so it cannot give it away and does not show a positive resonance effect. So it will show a negative resonance effect.
In the case of option D, $- N{R_2}$ contains a lone pair of electrons so it will give away its electron and show a positive resonance effect.

Hence, option D is correct.

Note:
In other words, when interaction between a lone pair of electrons and a pi bond takes place then polarity is produced in a molecule. This polarity is called the resonance effect. It is also produced due to interaction of two pi bonds between two adjacent atoms. Resonance effect is shown by molecules which contain conjugated double bonds or in molecules having at least one lone pair of electrons and one double bond.