Halogeno-compounds

1
Chapter 32
Halogeno-compounds
32.1 Introduction 32.2 Nomenclature of
Halogeno-compounds 32.3 Physical Properties of
Halogeno-compounds 32.4 Preparation of
Halogeno-compounds 32.5 Reactions of
Halogeno-compounds 32.6 Nucleophilic Substitution
Reactions 32.7 Elimination Reactions 32.8 Uses of
Halogeno-compounds
2
32.1 Introduction (SB p.169)

• Haloalkanes are organic compounds having one or
  more halogen atoms replacing hydrogen atoms in
  alkanes

• Haloalkanes are classified into primary,
  secondary and tertiary, based on the number of
  alkyl groups attached to the carbon atom which is
  bonded to the halogen atom

3
32.1 Introduction (SB p.169)
Halobenzenes are organic compounds in which the
halogen atom is directly attached to a benzene
ring e.g.
? not a halobenzene, because the chlorine atom is
not directly attached to the benzene ring
4
32.2 Nomenclature of Halogeno-compounds (SB
p.170)

• Naming haloalkanes are similar to those for
  naming alkanes
• The halogens are written as prefixes fluoro-
  (F), chloro- (Cl), bromo- (Br) and iodo- (I)
• e.g.

5
32.2 Nomenclature of Halogeno-compounds (SB
p.170)
When the parent chain has both a halogen and an
alkyl substituent, the chain is numbered from the
end nearer the first substituent regardless of
what substituents are e.g.
6
32.2 Nomenclature of Halogeno-compounds (SB
p.171)
In case of halobenzenes, the benzene ring is
numbered so as to give the lowest possible
numbers to the substituents e.g.
7
32.2 Nomenclature of Halogeno-compounds (SB
p.171)
Example 32-1 Draw the structural formulae and
give the IUPAC names of all isomers with the
following molecular formula. (a) C4H9Br
Answer
8
32.2 Nomenclature of Halogeno-compounds (SB
p.171)
Example 32-1 Draw the structural formulae and
give the IUPAC names of all isomers with the
following molecular formula. (b) C4H8Br2
Answer
9
32.2 Nomenclature of Halogeno-compounds (SB
p.172)
Check Point 32-1 Draw the structural formulae
and give the IUPAC names for all the structural
isomers of C5H11Br.
Answer
10
32.2 Nomenclature of Halogeno-compounds (SB
p.172)

11
32.3 Physical Properties of Halogeno-compounds
(SB p.173)
Name Formula Melting point (C) Boiling point (C) Density at 20C (g cm3)
Chloro-derivatives Chloromethane Chloroethane 1-Chloropropane 1-Chlorobutane 1-Chloropentane 1-Chlorohexane (Chloromethyl)benzene Chlorobenzene CH3Cl CH3CH2Cl CH3(CH2)2Cl CH3(CH2)3Cl CH3(CH2)4Cl CH3(CH2)5Cl C6H5CH2Cl C6H5Cl 97.7 136 123 123 99 83 39 45.2 23.8 12.5 46.6 78.5 108 133 179 132 0.889 0.886 0.883 0.878 1.100 1.106
12
32.3 Physical Properties of Halogeno-compounds
(SB p.173)
Name Formula Melting point (C) Boiling point (C) Density at 20C (g cm3)
Bromo-derivatives Bromomethane Bromoethane 1-Bromopropane 1-Bromobutane 1-Bromopentane 1-Bromohexane (Bromomethyl)benzene Bromobenzene CH3Br CH3CH2Br CH3(CH2)2Br CH3(CH2)3Br CH3(CH2)4Br CH3(CH2)5Br C6H5CH2Br C6H5Br 93.7 119 109 113 95 85 3.9 30.6 3.6 38.4 70.8 101 129 156 201 156 1.460 1.354 1.279 1.218 1.176 1.438 1.494
13
32.3 Physical Properties of Halogeno-compounds
(SB p.173)
Name Formula Melting point (C) Boiling point (C) Density at 20C (g cm3)
Iodo-derivatives Iodomethane Iodoethane 1-Iodopropane 1-Iodobutane 1-Iodopentane 1-Iodohexane (Iodomethyl)benzene CH3I CH3CH2I CH3(CH2)2I CH3(CH2)3I CH3(CH2)4I CH3(CH2)5I C6H5CH2I 66.5 108 101 103 85.6 24.5 42.5 72.4 102 130 155 181 decompose 2.279 1.940 1.745 1.617 1.517 1.437 1.734
14
32.3 Physical Properties of Halogeno-compounds
(SB p.174)
Boiling Point and Melting Point
15
32.3 Physical Properties of Halogeno-compounds
(SB p.174)
Haloalkanes have higher b.p. and m.p. than
alkanes? dipole-dipole interactions are present
between haloalkane molecules
m.p. and b.p. increase in the order RCH2F lt
RCH2Cl lt RCH2Br lt RCH2I ? larger, more
polarizable halogen atoms increase the
dipole-dipole interactions between the
molecules No. of carbon ? ? m.p. and b.p. ?
16
32.3 Physical Properties of Halogeno-compounds
(SB p.174)
Density

• Relative molecular mass ?
• ? density ?
• ? closer packing of the smaller molecules in the
  liquid phase
• Bromo and iodoalkanes are all denser than water
  at 20C

17
32.3 Physical Properties of Halogeno-compounds
(SB p.174)
Solubility
Although C X bond is polar, it is not polar
enough to have a significant effect on the
solubility of haloalkanes and halobenzenes ?
Immiscible with water ? Soluble in organic
solvents
18
32.4 Preparation of Halogeno-compounds (SB p.175)
Preparation of Haloalkanes
Substitution of Alcohols

• Prepared by substituting OH group of alcohols
  with halogen atoms
• Common reagents used HCl, HBr, HI, PCl3 or PBr3
• The ease of substitution of alcohols3 alcohol
  gt 2 alcohol gt 1 alcohol gt CH3OH
• This is related to the stability of the reaction
  intermediate (i.e. stability of carbocations)

19
32.4 Preparation of Halogeno-compounds (SB p.175)
Reaction with Hydrogen Halides

• Dry HCl is bubbled through alcohols in the
  presence of ZnCl2 catalyst

• For the preparation of bromo- and iodoalkanes, no
  catalyst is required

20
32.4 Preparation of Halogeno-compounds (SB p.176)

• The reactivity of hydrogen halides HI gt HBr gt
  HCl
• e.g.

21
32.4 Preparation of Halogeno-compounds (SB p.176)
Reaction with Phosphorus Halides
Haloalkanes can be prepared from the vigorous
reaction between cold alcohols and
phosphorus(III) halides
22
32.4 Preparation of Halogeno-compounds (SB p.177)
Addition of Alkenes and Alkynes
Addition of halogens or hydrogen halides to an
alkene or alkyne can form a haloalkane e.g.
23
32.4 Preparation of Halogeno-compounds (SB p.177)
Preparation of Halobenzenes
Halogenation of Benzene
Benzene reacts readily with chlorine and bromine
in the presence of catalysts (e.g. FeCl3, FeBr3,
AlCl3)
24
32.4 Preparation of Halogeno-compounds (SB p.177)
From Benzenediazonium Salts
25
32.4 Preparation of Halogeno-compounds (SB p.178)
Check Point 32-2 State the major products of the
following reactions (a) CH3CHOHCH2CH3 PBr3
?? (b) CH3CH CH2 HBr ?? (c) CH3C ? CH 2HBr
?? (d)
Answer
26
32.5 Reactions of Halogeno-compounds (SB p.178)

• Carbon-halogen bond is polar
• Carbon atom bears a partial positive charge
• Halogen atom bears a partial negative charge

27
32.5 Reactions of Halogeno-compounds (SB p.178)

• Characteristic reaction
• Nucleophilic substitution reaction

• Alcohols, ethers, esters, nitriles and amines can
  be formed by substituting OH, OR, RCOO ,
  CN and NH2 groups respectively

28
32.5 Reactions of Halogeno-compounds (SB p.179)

• Another characteristic reaction
• Elimination reaction

• Bases and nucleophiles are the same kind of
  reagents
• Nucleophilic substitution and elimination
  reactions always occur together and compete each
  other

29
32.6 Nucleophilic Substitution Reactions (SB
p.179)
Reaction with Sodium Hydroxide
The reactions proceed in 2 different reaction
mechanismsbimolecular nucleophilic substitution
(SN2) unimolecular nucleophilic substitution (SN1)
30
32.6 Nucleophilic Substitution Reactions (SB
p.180)
Bimolecular Nucleophilic Substitution (SN2)
Example CH3 Cl OH ?? CH3OH Cl
Rate kCH3ClOH Order of reaction 2? both
species are involved in rate determining step
31
32.6 Nucleophilic Substitution Reactions (SB
p.181)
Reaction mechanism of the SN2 reaction

• The nucleophile attacks from the backside of the
  electropositive carbon centre
• In the transition state, the bond between C and O
  is partially formed, while the bond between C and
  Cl is partially broken

32
32.6 Nucleophilic Substitution Reactions (SB
p.181)
Transition state involve both the nucleophile and
substrate? second order kinetics of the reaction
33
32.6 Nucleophilic Substitution Reactions (SB
p.182)
Stereochemistry of SN2 Reactions

• The nucleophile attacks from the backside of the
  electropositive carbon centre
• The configuration of the carbon atom under attack
  inverts

34
32.6 Nucleophilic Substitution Reactions (SB
p.182)
Unimolecular Nucleophilic Substitution (SN1)
Example

• Kinetic study shows that
• Rate k(CH3)3CCl
• The rate is independent of OH
• Order of reaction 1 ? only 1 species is
  involved in the rate determining step

35
32.6 Nucleophilic Substitution Reactions (SB
p.183)
Reaction mechanism of SN1 reaction involves 2
steps and 1 intermediate formed

• Step 1
• Slowest step (i.e. rate determining step)
• Formation of carbocation and halide ion

36
32.6 Nucleophilic Substitution Reactions (SB
p.183)

• Step 2
• Fast step
• Attacked by a nucleophile to form the product

37
32.6 Nucleophilic Substitution Reactions (SB
p.183)

• Rate determining step involves the breaking of
  the C Cl bond to form carbocation
• Only 1 molecule is involved in the rate
  determining step ? first order kinetics of the
  reaction

38
32.6 Nucleophilic Substitution Reactions (SB
p.184)
Stereochemistry of SN1 Reactions

• The carbocation formed has a trigonal planar
  structure
• The nucleophile may either attack from the
  frontside or the backside

39
32.6 Nucleophilic Substitution Reactions (SB
p.184)
For some cations, different products may be
formed by either mode of attack
e.g.
The reaction is called racemization
40
32.6 Nucleophilic Substitution Reactions (SB
p.184)
The above SN1 reaction leads to racemization ?
formation of trigonal planar carbocation
intermediate
41
32.6 Nucleophilic Substitution Reactions (SB
p.185)
The attack of the nucleophile from either side of
the planar carbocation occurs at equal rates and
results in the formation of the enantiomers of
butan-2-ol in equal amounts
42
32.6 Nucleophilic Substitution Reactions (SB
p.185)
Factors Affecting the Rates of SN1 and SN2
Reactions
Most important factors affecting the relative
rates of SN1 and SN2 reactions 1. The structure
of the substrate 2. The concentration and
strength of the nucleophile (for SN2 reactions
only) 3. The nature of the leaving group
43
32.6 Nucleophilic Substitution Reactions (SB
p.186)
The Structure of the Substrate

• 1. SN2 reactions
• The reactivity of haloalkanes in SN2
  reactions CH3X gt 1 haloalkane gt 2 haloalkane
  gt 3 haloalkane
• Steric hindrance affects the reactivity ? bulky
  alkyl groups will inhibit the approach of
  nucleophile to the electropositive carbon
  centre ? energy of transition state? ?
  activation energy ?

44
32.6 Nucleophilic Substitution Reactions (SB
p.186)
Steric effects in the SN2 reaction
45
32.6 Nucleophilic Substitution Reactions (SB
p.187)
2. SN1 reactions

• Critical factor the relative stability of the
  carbocation formed
• Tertiary carbocations are the most stable? 3
  electron-releasing alkyl groups stabilize the
  carbocation by releasing electrons
• Methyl, 1, 2 carbocation have much higher
  energy? activation energies for SN1 reactions
  are very large and rate of reaction become very
  small

46
32.6 Nucleophilic Substitution Reactions (SB
p.187)
The Concentration and Strength of the Nucleophile

• Only affect SN2 reactions
• Concentration of nucleophile ? ? rate ?

47
32.6 Nucleophilic Substitution Reactions (SB
p.187)

• Relative strength of nucleophiles can be
  correlated with two structural features
• (I) A negatively charged nucleophile (e.g. OH)
  is always a stronger nucleophile than a neutral
  nucleophile (e.g. H2O)
• (II) In a group of nucleophiles in which the
  nucleophilic atom is the same, the order of
  nucleophilicity roughly follows the order of
  basicity
• e.g. RO gt OH gtgt ROH gt H2O
• Strength ? ? rate ?

48
32.6 Nucleophilic Substitution Reactions (SB
p.188)
The Nature of Leaving Group

• Halide ion departs as a leaving group
• For the halide ion, the ease of leaving I gt
  Br gt Cl gt F
• This is in agreement with the order of bond
  enthalpies of carbon-halogen bonds

Bond Bond enthalpy (kJ mol1)
C F 484
C Cl 338
C Br 276
C I 238
C I bond is weakest ? I is the best leaving
group
49
32.6 Nucleophilic Substitution Reactions (SB
p.188)

• Uncharged or neutral compounds are better leaving
  groupse.g. The ease of leaving of oxygen
  compounds
• H2O gtgt OH gt RO
• Strongly basic ions rarely act as leaving
  groupe.g.

50
32.6 Nucleophilic Substitution Reactions (SB
p.188)
When an alcohol is dissolved in a strong acid, it
can react with a halide ion ? the acid
protonates the OH group, and the leaving group
becomes a neutral water molecule e.g.
51
32.6 Nucleophilic Substitution Reactions (SB
p.188)
Comparision of Rates of Hydrolysis of Haloalkanes
and Halobenzene
1. Experiment 1 Comparison of the rates of
hydrolysis of 1-chlorobutane, 1-bromobutane and
1-iodobutane (a) Objective To study the effect
of the nature of the halogen leaving group on the
rate of hydrolysis of haloalkanes
52
32.6 Nucleophilic Substitution Reactions (SB
p.189)
(b) Procedure

• Put 2 cm3 of ethanol and 1 cm3 of 0.1 M aqueous
  silver nitrate into each of three test tubes
• Place them in a water bath at 60C
• After 5 mins, add 5 drops of 1-chlorobutane the
  test tube A, 5 drops of 1-bromobutane to B and 5
  drops of 1-iodobutane to C
• Shake each test tube and observe for 10 mins

53
32.6 Nucleophilic Substitution Reactions (SB
p.189)
(c) Result and Observation A precipitate of
silver halide is formed in each of the three test
tubes
54
32.6 Nucleophilic Substitution Reactions (SB
p.190)
(d) Discussion

• Water molecule is the nucleophile of the reaction
• Haloalkanes react with water by nucleophilic
  substitutions
• The halide ion departs as the leaving group
• The ease of leaving of halide ions decreases I
  gt Br gt Cl
• The order of precipitates appeared tends to
  follow the order of ease of leaving of the halide
  ions, which subsequently form precipitates with
  Ag ions from AgNO3
• Ag(aq) X(aq) ?? AgX(s)

55
32.6 Nucleophilic Substitution Reactions (SB
p.190)
2. Experiment 2 Comparison of the rates of
hydrolysis of primary, secondary and tertiary
haloalkanes and halobenzene
(a) Objective To study the effect of the
structure of haloalkanes on the rate of
hydrolysis of them and to compare the rates of
hydrolysis of haloalkanes and halobenzene
56
32.6 Nucleophilic Substitution Reactions (SB
p.190)
(b) Procedure

• Put 2 cm3 of ethanol and 1 cm3 of 0.1 M aqueous
  silver nitrate into each of four test tubes
• Add 5 drops of 1-chlorobutane the test tube D, 5
  drops of 2-chlorobutane to E, 5 drops of
  2-chloro-2-methylpropane to F and 5 drops of
  chlorobenzene to G
• Shake each test tube well and observe for 10 mins

57
32.6 Nucleophilic Substitution Reactions (SB
p.190)
(c) Result and Observation Except test tube G,
a white precipitate of silver chloride was
formed in each of test tubes D, E and F.
58
32.6 Nucleophilic Substitution Reactions (SB
p.191)

• (d) Discussion
• The halogen-compounds used in the experiment are
  of different classes

• The rate of formation of the white precipitate of
  silver chloride decreases in the order
• 2-chloro-2-methylpropane gt 2-chlorobutane ?
  1-chlorobutane gtgt chlorobenzene

59
32.6 Nucleophilic Substitution Reactions (SB
p.191)

• The rate of hydrolysis of halogeno-compounds is
  related to the structure of the substrate around
  the carbon which is being attacked

• The experimental condition favours SN1 reactions
• ? tertiary haloalkane reacts at the fastest rate
  while primary haloalkane proceeds at a slower
  rate
• Chlorobenzene can be hydrolyzed to phenol under
  severe conditions (cannot be carried out in
  school laboratory)

60
32.6 Nucleophilic Substitution Reactions (SB
p.192)
Unreactivity of Halobenzene

• Halobenzenes are comparatively unreactive to
  nucleophilic substitution reactions
• ? the p orbital on the carbon atom of the
  benzene ring and that on the halogen atom
  overlap side-by-side to form a delocalized ?
  bonding system

61
32.6 Nucleophilic Substitution Reactions (SB
p.192)

• ? Delocalization of ? electrons throughout the
  ring and halogen atom
• ? The C X bond has partial double bond
  character ? stronger than that of haloalkane
• ? larger amount of energy is required to break
  the bond
• ? substitution reactions become more difficult
  to occur

• ? Delocalization of ? electrons makes the
  polarity of C X bond ? ? electropositive
  carbon center is less susceptible to
  nucleophilic attack

62
32.6 Nucleophilic Substitution Reactions (SB
p.192)

• Delocalized electrons repel any approaching
  nucleophiles? unreactive towards SN2 reactions
• Benzene cations are highly unstable because of
  loss of aromaticity? unreactive towards SN1
  reactions

63
32.6 Nucleophilic Substitution Reactions (SB
p.192)
Example 32-2 The reactions between three
bromine-containing compounds and aqueous silver
nitrate at room conditions are summarized in the
following table (a) What is the pale
yellow precipitate produced in the reaction
between silver nitrate and sodium bromide?
Compound Reaction with aqueous silver nitrate
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
Solution (a) Silver bromide
Answer
64
32.6 Nucleophilic Substitution Reactions (SB
p.192)
Example 32-2 (b) Write an ionic equation for the
reaction.
Answer
Solution (b) Ag(aq) Br(aq) ?? AgBr(s)
65
32.6 Nucleophilic Substitution Reactions (SB
p.192)
Example 32-2 (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?
Answer
Solution (c) The hydrolysis of 1-bromobutane
takes time. Precipitation of AgBr occurs only
after OH from water has replaced Br from
1-bromobutane.
66
32.6 Nucleophilic Substitution Reactions (SB
p.192)
Example 32-2 (d) Briefly explain why bromobenzene
does not give any precipitate with aqueous
silver nitrate.
Answer
Solution (d) The C Br bond of bromobenzene is
strengthened due to the delocalization of ?
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
aqueous silver nitrate.
67
32.6 Nucleophilic Substitution Reactions (SB
p.193)
Example 32-3 Which is the stronger nucleophile in
each of the following pairs? Explain your choice
briefly. (a) OH and H2O (b) OH and CH3CH2O
Answer
Solution (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, this increases the electron density on the
oxygen atom. This makes CH3CH2O to be a stronger
nucleophile than OH.
68
32.6 Nucleophilic Substitution Reactions (SB
p.194)
Check Point 32-3 Predict whether the following
substitution reaction follows mainly SN1 or SN2
pathway. Briefly explain your answer. (a) CH3I
OH ?? CH3OH I
Answer
69
32.6 Nucleophilic Substitution Reactions (SB
p.194)
Check Point 32-3 Predict whether the following
substitution reaction follow mainly SN1 or SN2
pathway. Briefly explain your answer. (b)
Answer
70
32.6 Nucleophilic Substitution Reactions (SB
p.194)
Reaction with Potassium Cyanide
A nitrile is formed when a haloalkane is heated
under reflux with an aqueous alcoholic solution
of potassium cyanide
e.g.
71
32.6 Nucleophilic Substitution Reactions (SB
p.194)

• Cyanide ion (CN) acts as a nucleophile

• Halobenzenes do not react with potassium cyanide
• The reaction is very useful because the nitrile
  can be hydrolyzed to carboxylic acids which can
  be reduced to alcohols

• A useful way of introducing a carbon atom into an
  organic molecule, so that the length of the
  carbon chain can be increased

72
32.6 Nucleophilic Substitution Reactions (SB
p.195)
Reaction with Ammonia
When a haloalkane is heated with an aqueous
alcoholic solution of ammonia under a high
pressure, an amine is formed
e.g.
73
32.6 Nucleophilic Substitution Reactions (SB
p.195)

• Ammonia is a nucleophile because the presence of
  a lone pair of electrons on the nitrogen atom

• As the lone pair electrons on nitrogen atom in
  ethylamine are still available, the ethylamine
  will compete with ammonia as the nucleophile.
• A series of further substitutions take place
• A mixture of products is formed

74
32.6 Nucleophilic Substitution Reactions (SB
p.195)

• The reaction stops at the formation of a
  quaternary ammonium salt
• The competing reactions can be minimized by using
  an excess of ammonia

75
32.6 Nucleophilic Substitution Reactions (SB
p.195)
Example 32-4 Give the reagents and reaction
conditions needed for each of the following
conversions (a) (CH3)3CBr ?? (CH3)3COH (b) CH3I
?? CH3OC2H5 (c) CH3I ?? (CH3)4NI
Answer
Solution (a) Dilute NaOH (b) C2H5ONa or Na in
C2H5OH (c) NH3 in excess CH3I
76
32.6 Nucleophilic Substitution Reactions (SB
p.196)
Check Point 32-4 Give the name(s) and structural
formula(e) of the major organic product(s) formed
in each of the following reactions. (a)
(b) (c)
Answer
77
32.7 Elimination Reactions (SB p.196)
Formation of Alkenes
The elimination of HX from adjacent atoms of a
haloalkane is widely used for synthesizing
alkenes e.g.
78
32.7 Elimination Reactions (SB p.196)
The elements of a hydrogen halide are eliminated
from a haloalkane in this way, the reaction is
called dehydrohalogenation
79
32.7 Elimination Reactions (SB p.196)
Dehydrohalogenation of most haloalkanes yields
more than one product e.g.
80
32.7 Elimination Reactions (SB p.197)

• The major product will be the more stable alkene

• The more stable alkene has the more highly
  substituted double bond
• Elimination follows the Saytzeffs rule when the
  elimination occurs to give the more highly
  substituted alkene as the major product
• The stabilities of alkenes

81
32.7 Elimination Reactions (SB p.197)
Elimination Versus Substitution

• Nucleophiles are potential bases
• Bases are potential nucleophiles
• In SN2 pathway, elimination and nucleophilic
  substitution compete each other

82
32.7 Elimination Reactions (SB p.198)

• Substitution is favoured when the substrate is
  primary alcohol and the base is hydroxide ion

• Elimination is favoured when the substrate is
  secondary alcohol

83
32.7 Elimination Reactions (SB p.198)

• With tertiary haloalkanes, SN2 reactions cannot
  take place ? Elimination is highly favoured
  especially at high temperatures ? Substitution
  occurs through SN1 mechanism only

84
32.7 Elimination Reactions (SB p.198)
Eliminations will be favoured when
using 1. higher temperatures 2. strong
sterically hindered bases (e.g. (CH3)3CO)
85
32.7 Elimination Reactions (SB p.199)
CH3X Methyl RCH2X 1 R2CHX 2 R3CX 3
Gives SN2 reactions only Gives mainly SN2 and gives mainly E with a strong sterically hindered base (e.g. (CH3)3CO) Gives mainly SN2 with a weak base (e.g. I, CN, RCO2) and gives mainly E with a strong base (e.g. RO) No SN2 reaction. In hydrolysis, gives SN1 or E. At low temperatures, SN1 is favoured. When a strong base (e.g. RO) is used or at high temperatures, E predominates.
Summary of the reaction pathways for the
substitution and elimination reactions of simple
haloalkanes
86
32.7 Elimination Reactions (SB p.199)
Formation of Alkynes

• Alkynes can be produced by dehydrohalogenation of
  dihaloalkanes
• Two molecules of hydrogen halides are eliminated
• e.g.

87
32.7 Elimination Reactions (SB p.199)
Example 32-5 (a) Hot and concentrated alcoholic
potassium hydroxide can eliminate hydrogen iodide
from the compound CH3CH2CHICH3. Suggest and name
two possible products.
Answer
88
32.7 Elimination Reactions (SB p.199)
Example 32-5 (b) Draw the structural formulae and
give the names of all possible products formed
by elimination of hydrogen bromide from the
dibromoalkane, CH3CHBrCHBrCH3.
Answer
89
32.7 Elimination Reactions (SB p.200)
Check Point 32-5 (a) 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
90
32.7 Elimination Reactions (SB p.200)
Check Point 32-5 (b) 2-Iodo-2-methylbutane gives
two elimination products one is
2-methylbut-2-ene, what is the other one?
Answer
91
32.7 Elimination Reactions (SB p.200)
Check Point 32-5 (c) Arrange the following
compounds in order of increasing tendency
towards elimination reactions 2-bromo-2-methylbu
tane, 1-bromopentane and 2-bromopentane
Answer
92
32.8 Uses of Halogeno-compounds (SB p.200)
As Solvents in Dry-cleaning

• Chlorinated hydrocarbons are good solvents for
  oil and greases? widely used in the dry-cleaning
  industry
• e.g. trichloroethene, CCl2 CHCl
  tetrachloroethene, CCl2 CCl2
• Properties that favour the use1. Relatively
  non-flammable
• 2. Volatile
• 3. Little or no structural effect on fabrics

93
32.8 Uses of Halogeno-compounds (SB p.201)
As Raw Materials for Making Addition Polymers

• Poly(chloroethene) (also known as PVC)
• Produced by means of the addition polymerization
  of the chloroethene monomers in the presence of a
  peroxide catalyst

94
32.8 Uses of Halogeno-compounds (SB p.201)

• Polar C Cl bond results in dipole-dipole
  interactions between polymer chains, making PVC
  hard and brittle and used to make pipes and
  bottles

95
32.8 Uses of Halogeno-compounds (SB p.201)

• PVC becomes flexible when plasticizer is added

• Used to make shower curtains, raincoats,
  artificial leather, insulating coating of
  electrical wires

96
32.8 Uses of Halogeno-compounds (SB p.201)

• Poly(tetrafluoroethene) (PTFE, Teflon)
• Produced through addition polymerization of the
  tetrafluoroethene monomers under high pressure
  and in the presence of catalyst

97
32.8 Uses of Halogeno-compounds (SB p.201)

• Teflon has a high melting point and is
  chemically inert
• Used to make non-stick frying pans

98
The END