General Organic Chemistry-II
2.1
ANSWERS
Section A—Answers to Subjective Questions
1. (a) F2CHCO2H is a stronger acid than FCH2COOH because two electronegative F atoms present, each
exerts -I effect and decreases the e-density around H of COOH more. Hence H as H+ ion can easily
be removed.
(b) CH3OCH2CO2H is a stronger acid because the methoxy group contains more electronegative oxygen
atom which exerts -I effect.
(c) CH3COCH2C N has more acidic a-hydrogen as CN group is strongly electron withdrawing and
stabilises CH3 COCH C
N via resonance.
(d)
is more acidic due to –I and –R effect of CH3CO group at para position.
(e)
is more acidic. In meta isomer, CH3CO group has only –I effect.
(f) CH2 = NH is more acidic as carbon is sp2 hybridised. N is also sp2 hybridised, As s-character
increases, electronegativity of an atom increases and electron density around H atom decreases,
acidic strength then increases.
(g)
(h)
2. (i) B < A < D < C
(ii) A < C < D < B
(iii) D < C < A < B
(iv) A < D < B < C
(v) A < B < C < D
(vi) A ª C < B < D
(vii) D < C < B < A
(viii) C < A < D < B
(ix) A < B < C < D
(x) B < C < D < A
3. Protonation occurs at an oxygen atom because oxygen is more electronegative than N atom. Moreover,
the lone pair of nitrogen participates into resonance with C = O group making oxygen atom more electron
rich and more susceptible for protonation.
2.2
Organic Chemistry
4. (a) B is favoured over A because B is more stable than A due to conjugation.
(b)
5. (a)
(–I, –R effect of NO2 group)
(b)
(more stabilization due to extended conjugation).
(c)
(more stabilization via resonance with the benzene ring and with NO2 group).
(d)
(e)
(f)
6. (a)
(CH3)3 P
(large size and less electronegativity of P)
ClCH2COO–
(strong –I effect of Cl)
CH3NH–
(less electronegativity of N atom)
In aniline, the lone pair on N participates into resonance with the benzene ring. As a result, the e–
density on N decreases. The tendency to accept H+ ion decreases.
In cyclohexyl amine, cyclohexyl group exerts +I effect and makes the N aotm more e– rich and more
susceptible to accept H+ ion. Hence, cyclohexylamine is more basic and has a lower pKa value than
aniline.
2.3
General Organic Chemistry-II
(b) p-nitroaniline would be less basic than aniline because NO2 group exerts very strong –I and –R
effect from para position.
7. (a) Isopropyl group exerts +I effect and increases the electron density around H of COOH group making
the H atom more difficult to leave as H+ ion.
Hence, m-isopropyl benzoic acid is a weaker acid than benzoic acid.
(b) Acidic strength µ
1
pK a
3, 5-dinitrobenzoic acid contains two NO2 group, each exerts –I effect from 3 and 5 - position while
m-nitrobenzoic acid has only one NO2 group. 3, 5-dinitrobenzoic acid is a stronger acid and has a
lower pKa value than m-nitrobenzoic acid.
8. o-hydroxybenzoic acid has a lower pKa value and is stronger acid than o-methoxy benzoic acid because
the anion from o-hydroxy benzoic acid gets stabilised via intramolecular H-bonding
9. (a) CH 3 CH = CH
(e) Cl CH2 CH2 CO2–
(b) CH3 CO2–
(c) Cl CH2 CO2–
(d) (CH3)3CO–
10.
In (B), the N atom is sp2 hybridised whereas in (A), N atom is sp3. More the s-character, less the basicity
and higher pKb and lower the pKa value.
11. (i) D < C < A < B
(ii) D < C < A < B
(iii) B < A < C < D
(iv) D < B < A < C
(v) D < C < B < A
12. N, N-dimethyl aniline is only slightly more basic than aniline due to crowding so that there is less
participation of lone pair of N into resonance with benzene ring. Therefore, the availability of lone pair is
more on N in N, N-dimethyl aniline than in aniline. In 2, 6-dimethyl-N N-dimethyl aniline, steric hindrance
is so much (crowding) that lone pair does not participate into resonance with benzene. Hence, it is much
more basic than 2, 6-dimethyl aniline.
2.4
Organic Chemistry
13. The correct order of increasing base strength is
14. With any dibasic acid, the first hydrogen ion is removed more easily than the second. Thus, K1 is larger
than K2 and the value of pK1 is smaller than pK2.
pK1 of o-phthalic acid is smaller than pK2 of terephthalic acid because of crowding at ortho position, ophthalic acid gives away H+ ion easily.
But pK2 of o-phthalic acid is larger than pK2 of terephthalic acid because the anion of o-phthalic acid gets
stabilised via H-bonding (Chelation)
15. The oxygen atom in
is electron-withdrawing, making the nitrogen more positive. The
e-density on N-atom decreases, basicity decreases. Moreover, the
than
cation is less stabilised
.
16. Nitrogen labeled (c) is the most basic. This is due to the presence of electron donating methyl group
while nitrogen (a) and nitrogen (b) both participates into resonance and acquires positive charge.
The order of increasing basicity of three nitrogen atom is
a<b<c
17. The correct order of increasing acidity is
2.5
General Organic Chemistry-II
18. Tetra methyl ammonium hydroxide is more basic because it gets ionised in water to give (CH3)4N+ and
OH– ions.
19. (a)
(b)
(d)
20. Cation (b) is more stable than (a) because of delocalisation of +ve charge with the benzene ring.
21. The correct order of increasing stability of radicals is
22. The order of increasing stability is
23. Most stable carbonium is
Least stable carbonium ion is
24. The correct order of increasing stability is
2.6
Organic Chemistry
25. (a) H3C——Br
(b) H3C——OH
(c) H3C——OH
26. In bond between C2—C3, the inductive effect is the least.
27. (i)
(II) is more stable than (I) because each and every atom has a complete octet.
(ii) (a)
(b)
28. (I) is not a major contributor because the positively charge carbon has only sextet.
(II) can not be the major contributor because the more electronegative oxygen carries a positive charge
on it.
29. (i) Benzylic free radical is more stable due to extended resonance.
(ii) O2NCH2—CH2O is more stable than CH3CH2O because NO2 group is electron withdrawing and
helps in the dispersal of negative charge
30. (a), (b), (d) and (e) are nucleophile due to the presence of lone pair of electron.
(b) is nucleophile due to the presence of p electron cloud.
(c) is an electrophile because Cl has incomplete octet.
31. (a) Addition
(b) Substitution
(c) Substitution and acid - base
(d) a-elimination
(e) Addition
(f) b-elimination
(g) Rearrangement
(h) Substitution
(i) g-elimination
(j) Rearrangement
32. (a) Nucleophilic substitution
(b) Electrophilic addition reaction
(c) Nucleophilic substitution reaction
(d) Elimination reaction
33. Reaction
species I
species II
(a)
(b)
(c)
(d)
Nu
E
Nu
E
E
Nu
E
Nu