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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
Structure and Properties
F
F
F
+
B
–
Or
F
F
BF
Filled orbital
Back bonding
Vacant orbital
F
F
F
B
F
F
F
B F
F
F
B
F
F
F
B
N
N
CH
3
CH
3
Pyridine 2-Methylpyridine
(2-picoline)
4-Methylpyridine
(4-picoline)
N
structures may be represented as shown below.
81
3 decreases.
pp33 and
bbr3p orbital of boron to overlap effectively with the 3p orbital
p
their energy levels.
Steric effects of the substituents
Steric effects are very important in determining the strength of Lewis acids and bases. In the reactions
3.
82
2-MethylpyridinePyridine 4-Methylpyridine
CH
3
CH
3
CH
3
B
N
CH
3
CH
3
CH
3
CH
3
B
N
CH
3
CH
3
N
B
H
H
H
H
H
H
C
C
Steric hindrance
R
R
R
RR′
R′
R′
CH
3
CH
3
CH
3
B
B
N
109.5°
120°
R
R
sp
2
Hybridization sp3 Hybridization Mesitylene
3)3
Pyridine a
3
3 3)3
front strain or F strain
formed.
back strain or B strain
3
boron is in sp2 hybridization and changes to sp33
33
triangular (sp2 sp3
Structure and Properties
R
R
R
N
R = CH
3
Bulky substituents
Nitrogen is forced to
planar (sp + p hybridization)
weak donors, e.g. (SiH
3
) 3N
Small substituents
(no strain), good
donors, e.g. Me
3
N
R = SiH
3
N
R
R
R
B strain in Et3N
B strain in quinuclidine
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
2
H2C
H
2
C
N
B
CH
3
CH
3
CH
3
NB
(having sp3
s character to
be used in these bonds and more p
3
sp
hybridization) with a lone pair in a pure p
suited for donation to an
83
3
3
333
33
3
1
) than et3−1) showing its higher stability.
84
REVISION QUESTIONS
1. Ionization constants Ka for formic acid and acetic acid are 17.7 ×
–5
–5
. Which acid is
K
a
Ka3
3
−−
−
+3−− is a stronger
base.
+
3
4
Lewis concept.
2
4
−
3
−
3+− 2
4
2−
4
+
2
−
2
2
−
2
3
−
4
−
3
3
2
−
2
4
−
3
−
.
3
+ 3−
3
3+3−3.
3
23 3 2(g)
3
(g) ag+ − 2 2
4
3
−
−
4
3
2
3
3
2
3
(g) ag
+
Q
Structure and Properties
(h)
2
24
4
(l)
−
3
85
232
3
22
3
32 2
3
3
(a) arrhenius acids (b) arrhenius bases
(c) bronsted acids (d) bronsted bases
(b)
13. Write three methods for determining the strength of acids.
(b)
−
3 +2
3+2 3++
+
+
4
−
3
(b)
444
4
432
33
33)3
3
3
(c)
3
86
3
3
3
+
(d) 2 is an acid.
3322
17.
(b)
(c)
explain the resonance effect of substituent on the strengths of Lewis acids and Lewis bases.
(d)
3
3
3
3)3 as Lewis acid)
18.
(b)
(c)
3
(d)
acids.
3.
33.
(d) br is Lewis as well as bronsted base.
3 is a Lewis acid but not a bronsted acid.
2 is acidic in nature according to Lewis concept.
3)33
87
Structure and Properties
(a)
3)3
than pyridine.
(b)
3)3
3)2
(c)
4 a
3
(d)
3
3
(e)
2
2
(f) every arrhenious base is also bronsted base.
MULTIPLE CHOICE QUESTIONS
1. +
−
ions in
(a) arrhenius (b) Lewis
(c) bronsted (d) Lowry
2.
(a)
+
− ions do not exist in water
(b)
(c)
ions
(d) all of these
3.
(a) accepts a proton (b) eleases a proton
(c) accepts an electron pair (d) Is amphoteric
(a) an acid (b) a base
(c) a neutral substance (d) amphoteric
5.
2
+3+ +
−
2
3
+
−
C
88
2−+2−
(a) 2− 2
(c)
7. + b
(a)
−
−
+ a
−
b−
(c) a−
8.
base.
S W
(a)
S W
(c)
(a)
(c) amphiprotic substances (d) Polyprotic bases
−
is an example of
3
(a)
amphiprotic ion (d) amphoteric ion
(c)
11.
(a) ain (b) onate
(c) eact with (d) one of these
12.
(a) Proton (b) base
(c) electron (d) electron pair
13.
(a) Lewis acids (b) Lewis bases
(c) bronsted acid (d) bronsted bases
14.
(a) a − ion from base to acid
(c) electron pair from acid to base (d) electron pair from base to acid
Structure and Properties
2 + +− 2++ 3+ +
2+2+ 3+ + − 2+a 3+ + a
+ ions in aqueous solution
17. all nucleophiles are
(a) arrhenius acid (b) bronsted base
(c) Lewis acids (d) Lewis bases
ANSWERS
89
−
−
1. (a) 2. (d) 3. (b) 4. (b) 5. (a) 6. (b) 7. (c) 8. (a) (c)
(c) 11. (b) 12. (d) 13. (a) 14. (d) 15. (c) 16. (a) 17. (d)
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