Halogeno-compounds and Nucleophilic Substitution

1
Halogeno-compounds and Nucleophilic Substitution
2
Classification

1. Halogenoalkanes (alkyl halides)
2. Unsaturated halides

3
Halogenoalkanes

• Alkanes with one or more hydrogen atoms replaced
  by halogen atoms.

4
Unsaturated halides
5
Unsaturated halides
halobenzene
5
6
Physical Properties of Halogeno-compounds
7
Boiling Point and Melting Point

• C?X bond is polar in nature
• Molecules are held by dispersion forces and
  dipole-dipole attractions.
• Haloalkanes gt alkanes of comparable
  relative molecular masses
• All RX(XCl, Br, I) are liquids or solids at room
  conditions except
• CH3Cl, CH3Br, C2H5Cl

8
Boiling Point and Melting Point
CH3X lt C2H5X lt C3H7X lt C4H9X lt RCH2F lt RCH2Cl lt
RCH2Br lt RCH2I ? Dispersion forces ? as molecular
size ?
9
Variation of boiling points with the number of
carbon atoms of straight-chain haloalkanes
10
Density
CH3X gt C2H5X gt C3H7X gt C4H9X gt ? packing
efficiency ? as molecular size ? packing
efficency is the dominant factor
11
Density
RCH2F RCH2Cl ? less dense than water RCH2Br,
RCH2I and ArX ? denser than water
11
12
Solubility

• All are immiscible with water but miscible with
  organic solvents

13
Preparation of Halogeno-compounds
14
A. Preparation of Halobenzenes (refer to
pp.50-51)
B. Preparation of Haloalkanes
15
Preparation of Haloalkanes
1. Free radical halogenation of alkanes
Not suitable for preparing mono-substituted
haloalkanes as further substitution may occur.
15
16
Q.28(b)
17
Q.28(b)
Lower reactivity but higher selectivity
18
2. SN reactions of ROH with HX
Reactivity HI gt HBr gt HCl 3o gt 2o
gt 1o
19
Reaction conditions -

• Chlorination
• - heating under reflux
• - HCl(g) or HCl(aq)
• - ? least reactive
• ? ZnCl2 is always used as catalyst

20
Lucas test - To distinguish different types of
R-OH with not more than 6 carbon atoms
1? R-OH ? no apparent reaction in 5 minutes 2?
R-OH ? reaction mixture turns cloudy in 5
minutes 3? R-OH ? reaction mixture turns cloudy
almost immediately.
21
Lucas test - To distinguish different types of
R-OH with not more than 6 carbon atoms
conc. HCl
R-OH R-Cl
ZnCl2
soluble in H2O if R has no more than 6 C atoms
22
Lucas test - To distinguish different types of
R-OH with not more than 6 carbon atoms
conc. HCl
R-OH R-Cl
ZnCl2
insoluble in H2O
23
Reaction conditions -
(ii) Bromination - heating under reflux -
HBr(aq) is generated in situ
NaBr H2SO4 ? NaHSO4 HBr
NaBr H3PO4 ? NaH2PO4 HBr
R-OH HBr ? R-Br H2O
23
24
(ii) Bromination
NaBr(s)
R-OH R-Br H2O
conc. H2SO4
The functions of conc. H2SO4 include -
1. generating HBr for the reaction 2.
increasing the yield of the reaction by
shifting the equilibrium position to the right
(by removing H2O) 3. increasing the rate of the
reaction by protonating the OH group.
24
25
Protonating the OH group weakens the C O bond.
It causes the H2O to leave more easily than the
HO?
26
Reaction conditions -
(iii) Iodination - heating under reflux -
HI(aq) is generated in situ
NaI H3PO4 ? NaH2PO4 HI
R-OH HI ? R-I H2O
26
27
Reaction conditions -
(iii) Iodination
Conc. H3PO4 is preferred to H2SO4 as the latter
may oxidize HI to give I2
NaI H2SO4 ? NaHSO4 HI
HI H2SO4 ? I2
27
28
3. Reactions with PX3, PCl5 or SOCl2
Red P Br2 / I2 ? PBr3 / PI3 (in situ
preparation)
Substitution reactions but NOT SN reactions
Less side products(arise from rearrangements)
than SN reactions of ROH with HX
29
4. Electrophilic Addition of Alkenes and Alkynes
(pp.41-46)
CH2 CH2 HBr ? CH3CH2Br
CH2 CH2 Br2 ? CH2BrCH2Br
CH ? CH HBr ? CH2 CHBr
CH2 CHBr HBr ? CH3CHBr2
CH ? CH 2Br2 ? CHBr2CHBr2
30
Reactions of Halogeno-compounds
31
Reactions of Halogeno-compounds
Two types -
1. Nucleophilic substitution reaction (SN
reaction)
Nucleophiles OH?, CN?, RO? (anions)
NH3, H2O (molecules
with lone pairs)
31
32
C?X bond is polar
? partial ve charge(?) on carbon atom
? susceptible to nucleophilic attack
leaving group
substrate
32
33
(alcohol)
(ether)
(nitrile)
(amine)
34
(alcohol)
(ether)
(nitrile)
C2H5OH as a common solvent for RX and the
nucleophiles(OH?, CN?)
(amine)
35
Reactions of Halogeno-compounds
Two types -
2. Elimination reaction
HX is eliminated
heat
Bases OH?, NH3
35
36
Reactions of Halogeno-compounds
The attacking species (OH?, NH3) can act as
nucleophiles or bases depending on the reaction
conditions.
Elimination reactions always competes with SN
reactions
36
37
As a base
As a nucleophile
38
Mechanisms of SN reactions

• SN1 reactions
• First Order Unimolecular Nucleophilic
  Substitution Reactions

B. SN2 reactions Second Order Bimolecular
Nucleophilic Substitution Reactions
39
SN1 reactions
slow
fast
39
40
2-step, 1st order, unimolecular
slow
fast
40
41
Optically active
Optically inactive
Racemic mixture
42
SN2 reactions
1-step, 2nd order, bimolecular
43
SN2 reactions
5-coordinated (Trigonal bipyramidal)
44
SN2 reactions

• To minimize the repulsion between OH? and the
  leaving group, Br?
• backside attack is preferred
• inversion of configuration

45
SN2 reactions
inversion
()
(-)
46
?9.9?
??34.6?
??9.9?
47
Q.44(a)
?H lt 0 ? E(C O) gt E(C Cl)
48
Q.44(b)
?H lt 0 ? E(C O) gt E(C Cl)
49
Factors Affecting the Rates of SN Reactions at a
saturated (sp3) carbon

• A. The structure of the substrate
• The concentration of the nucleophile
• The reactivity of the nuecleophile
• The nature of the leaving group
• The nature of solvent

50
The Structure of the Substrate
1. SN1 Reactivity depends on the stability of
the carbocaion formed in the r.d.s.
Stability of carbocations - Benzylic gt allylic
gt 3? gt 2? gt 1? gt CH3
50
51
Alkyl groups stabilize the positively charged
carbocation by positive inductive effect
51
52
Allylic
Allylic carbocation is stabilized by resonance
effect.
52
New Way Chemistry for Hong Kong A-Level 3A
53
benzylic
The ve charge is shared by the benzene ring by
resonance effect.
Resonance effect gt inductive effect
Stability of carbocations - Benzylic gt allylic
gt 3? gt 2? gt 1? gt CH3
53
New Way Chemistry for Hong Kong A-Level 3A
54
Types of halides Relative rate of SN1 reaction
CH3X 1.0
CH3CH2X (1?) 1.6
(CH3)2CHX (2?) 32
(CH3)3CX (3?) 1?107
SN1 reactions are much more significant for 3?
alkyl halides
55
2. SN2 Reactivity depends on how easily the
nucleophile can approach the reaction site.
Steric effect - More alkyl groups attached to
the site ? stronger van der Waals repulsions
experenced by the nucleophile when approaching
the site ? higher activation ? lower reactivity
56
2. SN2 Reactivity depends on how easily the
nucleophile can approach the reaction site.
Steric effect - CH3X lt 1? lt 2? lt 3?
Reactivity - CH3X gt 1? gt 2? gt 3?
57
Steric hindrance hindrance to reaction caused
by bulky groups of atoms around the reaction site
58
Not favours SN2 though 1?
59
Relative rate of SN2 reaction
Relative rate of SN1 reaction
Types of halides
1.0
CH3X
1.6
CH3CH2X (1?)
32
(CH3)2CHX (2?)
1?107
(CH3)3CX (3?)
60
Overall reactivity (SN1 SN2)
61
1? benzylic and 1? allylic halides favour BOTH
SN1 SN2 mechanisms. However, they proceed
faster via SN1 routes.
62
Reactivity of SN reactions depends on

• Electronic effect
• (a) Inductive effect ( or -)

polarization of ? electron cloud
(b) Mesomeric effect ( or -)
polarization of ? electron cloud
(c) Resonance effect
delocalization of ? electron cloud
2. Steric effect
63
The Concentration of the Nucleophile
SN1 no effect ? the nucleophile is NOT
involved in the rate determining step
SN2 nucleophile ? ? rate of reaction ?
64
The Reactivity of the Nucleophile
(Nucleophilicity)
SN1 no effect ? the nucleophile is NOT
involved in the rate determining step
SN2 Nucleophilicity? ? rate of reaction ?
65
Trends of Nucleophilicity -
(i) Charged nucleophiles gt neutral ones
65
66
Trends of Nucleophilicity -

• For some species,
• the order of nucleophilicity follows the order
  of basicity

gtgt
66
67
(iii) For halide ions, the order of
nucleophilicity follows the order of
polarisability but not basicity
Nucleophilicity / polarisability - I? gt Br?
gt Cl? gt F?
Basicity - I? lt Br? lt Cl? lt F?
Acidity - HI gt HBr gt HCl gt HF
68
68
69
R Br I? Na R I
NaBr(s)

• I? is a stronger nucleophile than Br?
• ? the forward reaction is preferred
• even though E(C Br) gt E(C I)

SN2 mechanism is favoured in non-polar or
slightly polar solvents (e.g. propanone) (Refer
to effect of solvent on p.72)
70
The Nature of the Leaving Group
1. For halogens,
Bond Bond enthalpy (kJ mol-1)
C ? F 484
C ? Cl 338
C ? Br 276
C ? I 238
Bond strength C I lt C Br lt C Cl lt C F
71
The Nature of the Leaving Group
1. For halogens,
Bond strength C I lt C Br lt C Cl lt C F
The ease of leaving I? gt Br? gt Cl? gt F?
Since the r.d.s. of both SN1 and SN2 reactions
involves breaking of C X bonds
Reactivity for both SN1 and SN2 reactions -R
I gt R Br gt R Cl gt R F
71
72
The Nature of the Leaving Group
I? is both a good leaving group and a good
nucleophile ? Useful in organic synthesis
Good leaving group
Strong nucleophile
72
73
The Nature of the Leaving Group
2. Neutral molecules are better leaving groups
than anions
Ease of leaving Br? gt OH? (C-O gt C-Br)
Nucleophilicity/basicity OH? gt Br?
74
protonation
Better leaving group
Protonating the OH group weakens the C O bond.
It causes the H2O to leave more easily than the
HO?
75
Is the forward reaction preferred ? Explain
No. Basicity CH3 O? gt H O? Nucleophilicity
CH3 O? gt H O? Ease of leaving CH3 O?
lt H O?
76
Suggest a route for the following conversion.
CH3 OH ? CH3 CN
Ease of leaving CN? gt HO? Nucleophilicity/basici
ty CN? lt HO?
77
Suggest a route for the following conversion.
CH3 OH ? CH3 CN
Acidity HCN lt HI
basicity CN? gt I?
78
The Nature of solvents (optional)
Highly polar solvents favour SN1 reactions ?
carbocation can be stabilized by polar solvent
via ion-dipole interaction
Non-polar or slightly polar solvents favour SN2
reactions
79
Unreactivity of Vinylic and Aryl Halides(at sp2 C)
Vinylic halide
Aryl halide
No significant SN1 SN2 reactions
79
80
Interpretation -
1. The carbocation intermediates are highly
unstable ? SN1 not favoured
unstable
stabilized by resonance effect
81
Q.45
Delocalization of ? e? among 7 C atoms
81
82
Q.45
83
Q.45
84
Interpretation -
2. C X bond strength -
85
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86
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87
Both SN1 SN2 not favored
88
Stronger bond
Both SN1 SN2 not favored
89
89
90
Interpretation -
1. The carbocation intermediates are highly
unstable ? SN1 not favoured
2. C X bond strength - SN1 SN2 not favored
3. The ? electron cloud of benzene tends to
repel the approaching nucleophiles
91
Vinylic and aryl halides are unreactive towards
SN1 and SN2 reactions as
both mechanisms involve the breaking of C X
bond in the r.d.s.
92
Non-SN substitution only happens at drastic
conditions
93
Tests for halogeno-compounds
Test 1 Shake RX with AgNO3(aq), and observe
the precipitation of AgX (Ethanol is always
added as co-solvent)
R X H2O ? R OH H X?
X? Ag ? AgX(s)
94
Tests for halogeno-compounds
Test 2
X? Ag ? AgX(s)
95
Positive results -
96
Reactivity - R I gt R Br gt R Cl
97
Reactivity - R I gt R Br gt R Cl
Aryl and vinylic halides give ve results
97
98
?

• The carbonyl C is highly positive due to
• ve inductive effective of O
• ve inductive effective of Cl
• ve mesomeric effective of O

99
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100
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101
acetylide ion
102
Products have one more C atom than RX
? lengthening of carbon chain
103
Lengthening of carbon chain
104
Q.46
OH? is a better nucleophile than CN? The backward
reaction is favoured
In acidic medium, CN? loses its nucleophilicity
105
Q.46
106
Q.46
107
Q.46
108
Q.46
109
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110
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111
Nucleophilicity -
Further substitution reactions occur
CH3NH2 CH3Br ? (CH3)2NH HBr
(CH3)2NH CH3Br ? (CH3)3N HBr
(CH3)3N CH3Br ? (CH3)4NBr?
112
A mixture of products is obtained ? Not suitable
for preparing 1? amine
113
NH3 CH3Br ? CH3NH2 HBr
To prepare 1? amino, NH3 must be used in large
excess
114
For strong bases,
elimination competes with SN
115

• Conditions in favour of Elimination
• higher temperature
• the presence of a strong base

116
In a strongly basic medium,
elimination
In a strongly acidic medium,
addition
117
Mechanisms of Elimination Reactions(Optional)
E2
1-step, 2nd order, bimolecular
dehydrohalogenation
118
E1
2-step, 1st order, unimolecular
OH?
119
Q.47
120
Elimination of Dihalides
alkyne
121
Elimination of Dihalides
alkyne
122
Stronger bases can be used to convert dihalides
to alkynes directly
NH2? from NaNH2 (sodamide) C2H5O? from C2H5ONa
(sodium ethoxide)
123
Suggest a route for the each of the following
conversions.
124
Elimination vs Substitution
E1 vs SN1
When the same reactants are used, all factors
(except temperature) affect E1 and SN1 reactions
equally.
Temperature is the ONLY factor for distinguishing
between E1 and SN1
125
E1 involves the breaking of strong CH bond ?
Only happens at higher temperatures
At higher temperatures, Vibrational motions of
the groups around the reaction site prevent the
nucleophile from approaching the reaction site. ?
SN1 is not favoured.
126
E1 vs SN1 Example
No need to distinguish between E1 E2
127
E2 vs SN2
Effect of structure on reactivity
SN2 CH3X gt 1? gt 2? gt 3?
E2 3? gt 2? gt 1?
128
E2 3? gt 2? gt 1?
OH?
No. of reaction site 3? gt 2? gt 1?
129
SN2 predominates
CH3X, 1?
1? favours E2 if a bulky and strong base is used
E2 competes with SN2
2?
E2 predominates in strongly basic medium
3?
130
Bulky and strong base
(1?)
(1) SN reactions are suppressed due to great
steric hindrance
(2) Strong base favours E2 reactions
131
Effect of strength of base/nucleophile on
reactivity
Strong bases favour E2 more than E1 Strong
nucleophiles favour SN2 more than SN1 However,
the attacking species tends to behave as a strong
base rather than a strong nucleophile.
132
Effect of strength of base/nucleophile on
reactivity
Reasons -
E2 reactions involve the breaking of the strong
CH bond ? high activation energy ? E2 is more
favoured than SN2 when approached by a strong
attacking species
133
Effect of temperature on reactivity
Same as above for E1 vs SN1
The effect of temperature is more important for
comparing E2/SN2 reactions than E1/SN1 reactions
since rate determining steps are considered in
the former.
134
Reaction conditions Reaction conditions Structure Structure Structure Structure
Strength of Base Temperature CH3X 1o 2o 3o
Strong High SN2 SN2 E2 E2
Strong Low SN2 SN2 E2/SN2 E2
Weak High SN2 SN2 SN2/E2 E1
Weak Low SN2 SN2 SN2 SN1
135
Q.49
strong
(a) (CH3)2CHONa CH3Br
Major(100)
?
136
Q.49
strong
2?
?
(b) (CH3)2CHBr CH3ONa
side-product
137
Q.50
strong
1?
(a) CH3CH2ONa CH3CH2Br
C2H5OH
55?C
138
Q.50
strong
3?
(b) CH3CH2ONa
C2H5OH
55?C
139
Q.50
strong
3?
CH3CH2ONa
25?C
C2H5OH
140
Q.48(a)
1?

• E2 is preferred to SN2 since
• strong base and high temperature favour
  elimination
• the product is stabilized by extensive
  delocalization of ? electrons

141
Q.48(b)
142
Q.48(a)
weaker base
Markovnikovs addition
SN2
143
Q.48(c)
E2
(2?)
trans-addition
144
Q.48(d)
CH2CHCH2Br
145
Q.48(d)
CH2CHCH2Br
CH3CHCHBr
146
Q.48(d)
CH2CHCH2Br
CH3CHCHBr
CH3CBrCH2
No recommended since a mixture of products is
formed
147
Q.48(d)
Markovnikovs addition
148
Q.51(a)
strong base
(2?)
149
Q.51(b)
Strong base
(2?)
?OC2H5
I?
minor
150
?OH
I?
weaker base
151
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152
30.2 Nomenclature of Halogeno-compounds (SB
p.210)
Example 30-2
Draw the structural formulae and give the IUPAC
names of all isomers with the following molecular
formulae. (a) C4H9Br
Answer
153
30.2 Nomenclature of Halogeno-compounds (SB
p.210)
Example 30-2
154
30.2 Nomenclature of Halogeno-compounds (SB
p.210)
Example 30-2
Draw the structural formulae and give the IUPAC
names of all isomers with the following molecular
formulae. (b) C4H8Br2
Answer
155
30.2 Nomenclature of Halogeno-compounds (SB
p.210)
Example 30-2
Back
156
30.2 Nomenclature of Halogeno-compounds (SB
p.211)
Check Point 30-2
Draw the structural formulae and give the IUPAC
names for all the structural isomers of C5H11Br.
Answer
157
30.2 Nomenclature of Halogeno-compounds (SB
p.211)
Check Point 30-2
158
30.2 Nomenclature of Halogeno-compounds (SB
p.211)
Back
Check Point 30-2
159
30.4 Preparation of Halogeno-compounds (SB p.218)
Back
Check Point 30-4
State the major products of the following
reactions. (a) CH3CHOHCH2CH3 PBr3
?? (b) CH3CHCH2 HBr ?? (c) CH3C?CH 2HBr
?? (d)
Answer
160
30.6 Nucleophilic Substitution Reactions (SB
p.230)
Let's Think 1
Answer
161
30.6 Nucleophilic Substitution Reactions (SB
p.230)
Let's Think 1
Back
162
30.6 Nucleophilic Substitution Reactions (SB
p.232)
Example 30-6A
The reactions between three bromine-containing
compounds and silver nitrate solution at room
conditions are summarized in the following
table
Compound Reaction with silver nitrate solution
Sodium bromide A pale yellow precipitate appears immediately
1-Bromobutane No reaction at first a pale yellow precipitate appears after several minutes
Bromobenzene No reaction even after several hours
163
30.6 Nucleophilic Substitution Reactions (SB
p.232)
Example 30-6A
(a) What is the pale yellow precipitate produced
in the reaction between silver nitrate and sodium
bromide? (b) Write an ionic equation for the
reaction. (c) Why does silver nitrate produce no
immediate precipitate with 1-bromobutane, even
though it contains bromine? Why is there the
formation of the pale yellow precipitate after
several minutes? (d) Briefly explain why
bromobenzene does not give any precipitate with
silver nitrate solution.
Answer
164
30.6 Nucleophilic Substitution Reactions (SB
p.232)
Example 30-6A
Back
(a) Silver bromide (b) Ag(aq) Br(aq) ??
AgBr(s) (c) The hydrolysis of 1-bromobutane takes
time. Precipitation of AgBr occurs only after OH
from water has replaced Br from
1-bromobutane. (d) The C ?Br bond of
bromobenzene is strengthened due to the
delocalization of p electrons throughout the
benzene ring and the halogen atom. As the
breaking of the C ? Br bond of bromobenzene
requires a larger amount of energy than
1-bromobutane, the substitution reaction becomes
more difficult to occur. Thus, bromobenzene does
not give any precipitate with silver nitrate
solution.
165
30.6 Nucleophilic Substitution Reactions (SB
p.232)
Back
Example 30-6B
Which is the stronger nucleophile in each of the
following pairs? Explain your choice briefly. (a)
OH and H2O (b) OH and CH3CH2O
Answer
(a) OH is a stronger nucleophile than H2O
because it carries a negative charge while H2O is
electrically neutral. (b) CH3CH2O is a stronger
nucleophile than OH. It is because the ethyl
group (CH3CH2 ?) is an electron-releasing group.
It increases the electron density on the oxygen
atom. This makes CH3CH2O a stronger nucleophile
than OH.
166
30.6 Nucleophilic Substitution Reactions (SB
p.233)
Check Point 30-6A
Predict whether the following substitution
reactions follow mainly SN1 or SN2 pathway.
Briefly explain your answer. (a) CH3I OH ??
CH3OH I
Answer
167
30.6 Nucleophilic Substitution Reactions (SB
p.233)
Check Point 30-6A
(a) The reaction follows mainly the SN2
mechanism. It is because the haloalkane (CH3I) is
a methyl halide. There is little steric hindrance
for the nucleophile to attack the carbon atom of
the molecule. On the other hand, if the reaction
follows the SN1 mechanism, the carbocation (CH3)
formed is not stabilized by the inductive effects
of alkyl groups. Thus, the SN1 mechanism for this
reaction is unfavourable.
168
30.6 Nucleophilic Substitution Reactions (SB
p.233)
Check Point 30-6A

• Predict whether the following substitution
  reactions follow mainly SN1 or SN2 pathway.
  Briefly explain your answer.
• (b)

Answer
169
30.6 Nucleophilic Substitution Reactions (SB
p.233)
Check Point 30-6A
(b) The reaction follows mainly the SN1
mechanism. It is because the haloalkane is a
secondary haloalkane with a bulky phenyl group
attached directly to the carbon atom bearing the
halogen atom. The bulky phenyl group exerts a
dramatic steric hindrance to the approaching
nucleophile. Therefore, the SN2 mechanism for
this reaction is not favoured. On the other hand,
the carbocation formed in the SN1 reaction is
stabilized by both the inductive effect of the
electron-releasing ethyl group and the resonance
effect of the phenyl group.
Back
170
30.6 Nucleophilic Substitution Reactions (SB
p.234)
Let's Think 2
Can you tell why haloalkanes undergo substitution
reactions more readily than alkanes, but the
reaction takes place fairly slowly?
Answer
It is because haloalkanes possess a polar C ? X
bond inviting the attack of nucleophiles or
bases. However, the polarity of the C ? X bond is
not so high.
Back
171
30.6 Nucleophilic Substitution Reactions (SB
p.235)
Example 30-6C
Give the reagents and reaction conditions needed
for each of the following conversions (a) (CH3)3C
Br ?? (CH3)3COH (b) CH3I ?? CH3OC2H5 (c) CH3I ??
(CH3)4NI
Answer
(a) dilute NaOH (b) CH3CH2ONa or Na in
CH3CH2OH (c) NH3 in excess CH3I
Back
172
30.6 Nucleophilic Substitution Reactions (SB
p.235)
Check Point 30-6B

• Give the name(s) and structural formula(e) of the
  major organic product(s) formed in each of the
  following reactions.
• (a)

Answer
173
30.6 Nucleophilic Substitution Reactions (SB
p.235)
Check Point 30-6B
Give the name(s) and structural formula(e) of the
major organic product(s) formed in each of the
following reactions. (b)
Answer
174
30.6 Nucleophilic Substitution Reactions (SB
p.235)
Back
Check Point 30-6B
Give the name(s) and structural formula(e) of the
major organic product(s) formed in each of the
following reactions. (c)
Answer
175
30.7 Elimination Reactions (SB p.239)
Let's Think 3
How can the following compounds be made from
bromopropane? (a) Propanol (b) Propene (c)
Butanenitrile
Answer
(a) By heating in NaOH (b) By heating in
alcoholic KOH (c) By reacting with aqueous
alcoholic KCN
Back
176
30.7 Elimination Reactions (SB p.240)
Example 30-7
(a) Hot and concentrated alcoholic potassium
hydroxide can eliminate hydrogen iodide from the
compound CH3CH2CHICH3. Suggest and name two
possible products.
Answer
177
30.7 Elimination Reactions (SB p.240)
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Example 30-7
(b) Draw the structural formulae and give the
names of all possible products formed by
elimination of hydrogen bromide from the
dibromoalkane, CH3CHBrCHBrCH3.
Answer
178
30.7 Elimination Reactions (SB p.240)
Check Point 30-7

1. Notice how the hydrogen and halogen atoms come
   off from adjacent carbon atoms in an elimination
   reaction. Could (iodomethyl)benzene undergo an
   elimination to give a HI molecule? Why?

Answer
(a) No, because there is no hydrogen atom
available on the carbon atom adjacent to the
carbon atom that is directly bonded to the iodine
atom.
179
30.7 Elimination Reactions (SB p.240)
Check Point 30-7
(b) 2-Iodo-2-methylbutane gives two elimination
products one is 2-methylbut-2-ene, what is the
other one?
Answer
(b) 2-Methylbut-1-ene
180
30.7 Elimination Reactions (SB p.240)
Check Point 30-7
(c) Arrange the following compounds in order of
increasing tendency towards elimination
reactions A 2-bromo-2-methylbutane, B
1-bromopentane and C 2-bromopentane Explain
your answer.
Answer
181
30.7 Elimination Reactions (SB p.240)
Check Point 30-7
(c) B lt C lt A The rate of elimination depends on
the stability of the alkene formed. The condensed
structural formulae of the alkenes formed from
the elimination reactions is A CH3CH
CCH3CH3 B CH3CH2CH2CH CH2 C CH3CH2CH
CHCH3 A more highly substituted alkene is more
stable and is formed more readily.
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