Chemistry - GOC Question with Solution | TestHub
Place the partial charges on the following molecules. In how many of the molecules (d), (f), (g), (h) and (i) does the ring carbon para to the substituent (para to N in (d)) carry a partial positive charge (δ+) ?
(a) 
(b) 
(c) 
(d) 
(e) 
(f) 
(g) 
(h) 
(i) 
Options:
Answer:
Solution:
The electronegative atom pulls electron density toward itself (δ-), and the polarity alternates along the conjugated path. Para to the substituent (or to N), the carbon is δ+ in pyridine (d), benzaldehyde (h), and benzonitrile (i); it is δ- in phenol (f) and bromobenzene (g). Ethylbenzene (e) is shown but not counted. Therefore, there are 3 molecules, making the answer (C).
(a) 
(b) 
(c) 
(d) Pyridine: N is δ-, C2 and C6 are δ+, C3 and C5 are δ-, and C4 (para to N) is δ+.
(e) Ethylbenzene: The CH2 carbon is polarized by the ring, and the sign alternates around the ring.
(f) Phenol: The lone pair of O is donated into the ring, so the ortho and para carbons are δ- (para δ-).
(g) Bromobenzene: The lone pairs on Br are donated into the ring in the same way, so the para carbon is δ-.
(h) Benzaldehyde: The C=O group withdraws electrons (-M), so the polarization is opposite to that in phenol; ortho and para carbons are δ+ (para δ+).
(i) Benzonitrile: The C≡N group withdraws electrons in the same way, so the para carbon is δ+.
