L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
Chpt. 33 p.1
HALOGENO—COMPOUNDS 鹵代化合物
I.
Introduction
Organic halogen compounds can be conveniently be divided into three categories
halogenoalkanes 鹵烷,
halogenoalkenes 鹵烯
and
halogenoarenes
(A)
Halogenoalkanes 鹵烷
Halogenoalkanes are also called haloalkanes or alkyl halides. In such compounds, the halogen
atom is bonded to a saturated sp3 carbon atom and have the following functional group
—C—X
where X = F, Cl, Br or I
Depending on how many alkyl groups are attached to the carbon which is bonded to this
halogen atom. the alkyl halides can be divided into three types
一級鹵烷
Examples
Chloroethane
(B)
二級鹵烷
2—bromobutane
三級鹵烷
2-iodo-2-methylpropane
Halogenoalkenes 鹵烯
The halogeno atom is bonded to an sp2 carbon which forms a double bond.
Examples
1.
chloroethene 氯乙烯
2.
trichloroethene 三氯乙烯
(C)
Halogenoarenes
Halogenoarenes are also called halo-arenes or aryl halides. In such compounds, the halogen
atom is bonded to an aromatic ring.
Examples
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
II.
Chpt. 33 p.2
Reactions of Halogenoalkanes 鹵烷的化學反應
The major reactions of halogenoalkanes fall into two general types :
1.
Nucleophilic substitution 親核取代反應 (SN ) and
2.
Elimination 消去反應 (E).
In halogenoalkanes, the electronegativity values C=2.5. 1=2.5. Br=2.8. Cl=3.0. F=4.0 show that,
except for C—I bond. the C—X bond is polarized:
In both substitution and elimination reactions, the carbon-halogen bond breaks heterolytically to
give the very stable halide ions Cl- , Br- or IIn nucleophilic substitution, this halogen is replaced by a nucleophile 親核體 which attacks
the electropositive carbon atom by donating an electron pair to it.
Nu:-
C
Nu
X
+
C
X
In elimination, a base removes a proton on the  carbon so that a double bond is formed.
B
+
H
H
C
C
H
+
X
H
HB +
X
H
Note :
<1> The two types of reactions may compete with one another when the reagents can act as either a
nucleophile or a base.
<2>
For a given aqueous bass (e.g. OH-). the reactions which takes place are
Primary haloalkanes : __________________________
Secondary haloalkanes : _________________________
Tertiary haloalkanes : ___________________________
(A)
Nucleophilic substitution 親核取代反應
This is the most important type of organic reactions because it provide useful methods for
preparing a number of different functional groups and chain lengths.
The nucleophile may be a molecule or anion which must have an unshared pair of electrons and
be able to attack the electropositive carbon.

R—C+X
The nucleophilic substitutions may proceed by quite different mechanisms —
Bimolecular Nucleophilic Substitution 雙分子取代機理 (SN2)
or
Unimolecular Nucleophilic Substitution 單分子取代機理 (SN1)
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
Chpt. 33 p.3
Which mechanism is operated depends on
<1> structure of alkyl halide 鹵烷的結構:例一級鹵烷、二級或三級鹵烷,
<2> nature of the solvent 溶劑的性質, and
<3> the nucleophilc strength (or nucleophilicity) of the attacking agent 親核能力大小,
(1)
Bimolecular Nucleophilic Substitution (SN2)
SN2 mechanism applies to the methyl and primary halides.
Example :
The alkaline hydrolysis of a bromoethane (a primary halogenoalkane)
The rate of reaction is found to depend on the concentrations of both bromoethane and
hydroxide. .i.e.
Rate  [CH3CH2Br] [OH-]
If the concentration of either is double, the rate doubles:
Mechanism:
The reaction involves both starting materials in the rate determining step which is said to
be bimolecular. Such SN2 reaction can be explain by one step mechanism:
H
H
C
OH-
Br
H
HO
CH3
1.
2.
3.
H
Br
CH3
H
H
+
HO
Br
CH3
The nucleophile OH- approaches the carbon atom attached to Br on the opposite side of
the molecule in order to minimize repulsion between RX and OH-.
A bond begins to form between O and C whereas the C—Br bond weakens, and a
transition state is reached in which C is partially bonded to both O and Br.
Once the transition has been formed, it is rapidly converted into the products – ethanol
and bromide (the leaving group).
The energy profile for the reaction is shown as follow:
Note: <1>
<2>
The bimolecular nucleophilic substitution is a second order reaction.
Other primary halides undergo substitution reactions by this SN2 pathway:
HO:- + CH3—Br 
CH3CH2O- + CH3CH2—Br 
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
<3>
Chpt. 33 p.4
Stereochemistry of SN2 reactions
One consequence of the backside attack (從後攻擊) of nucleophile on the electropositive
carbon by a one step mechanism as SN2 is that the configuration of groups around the carbon
subjected to his attack becomes inverted ( complete inversion 完全立體倒置 )
Example : Nucleophilic attack of CN- on CH3CH2Br
The configuration inversion can be easily demonstrated by the following cyclic molecule.
(2)
Unimolecular Nucleophilic Substitution (SN1)
SN1 mechanism applies mostly to tertiary alkyl halide
Example:
The hydrolysis of 1,1-dimethyibromoethane in aqueous ethanol .
(CH3)3CBr + H2O  (CH3)3CBr + HBr
Experimental results show that the reaction is first order with respect to the
halogenoalkane.
Rate  (CH3)3CBr
Mechanism
The mechanism is usually made up of two steps.
1.
The tertiary halide first ionizes spontaneously in solution. forming a carbocation/carbonium ion
and a halide ion.
CH3
H3C
C
rds
Br
CH3
The rate of this ionization is fairly small, and the equilibrium lies well over to the side of
undissociated alkyl halide.
2.
The carbocation formed reacts rapidly with a nucleophile.
(i)
If the solvent is water. an alcohol is formed.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
Chpt. 33 p.5
(ii) If the solvent is an alcohol. an ether is formed.
The energy profile of an reaction is shown in the following figure.
Note :
<1> The slow and rate determining step is the breaking of the C+—Br- bond to form the
carbocation.
<2>
The carbocation usually not recombine with the bromide ion due to the high concentration of
the solvent (water).
<3>
The overall reaction is a SN1 reaction because there is only one molecule involved in the
transition state of the rate determining step.
<4>
Stereochemistry of SN1 Reaction:
SN1 reaction produce racemic mixture from optically pure starting material. It is because the
intermediate, in which carbon is sp2 hybridized and planar, could be attacked from either side
with equal chance to produce equimolar amounts of enantomeric products.
<5>
Tertiary halides usually undergo reaction because they can form a more stable carbocation as
the intermediate due
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
Chpt. 33 p.6
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
III.
Chpt. 33 p.7
Factors affecting the rate of SN1 and SN2 Reactions
(A)
Effect of Structure
1.
SN1 Reactivity — The order of reactivity is the order of carbocation stability.
The allylic and benzylic_halides are more reactive because the carbocations
can be stabilized by resonance delocalization.
formed
RCH=CH-CH2—X
Allyic halide
CH2 X
Benzylic halide
The order of SN1 reactivity:
CH2 X
R
> RCH=CH-CH2—X >
R
C
R
X
Note:
<1> Only the above types of halides can undergo SN1 reaction because they can form relatively
stable carbocation.
<2>
Relative rates of SN1 reactivity in alkyl halides
2.
SN2 Reactivity —
The order of reactivity decreases with the increase in steric hindrance for
approaching nucleophile due to the branching of the carbon atom going
to be attacked.
Note : <1>
some halides even though they are primary in nature are still very unreactive.
e.g.
1-halo-2.2-dimethylprooane (neopentyl halide)
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
<2>
Chpt. 33 p.8
Benzvlic and aclylic halide are also quite reactive in SN2 reactions. In fact, they are even more
reactive than primary and methyl halide. They can react by either an S N1 or SN2 mechanism. Besides
being able to form relatively stable carbocations, simple allyl and benzyl substrate are sterically
unhindered as the primary alkyl halides.
<3> Relative rates of nucleophilic substitution in different alkyl bromides:
(i)
Tertiary bromide :
_________ reaction with _______ rate
(ii)
methyl bromide :
_________ reaction with _______ rate
(iii)
primary & secondary halide : ___________ reaction with _________ rate.
(B) Effect of concentration and reactivity of the nucleophile
1.
SN1 reaction : The rate is not affected by the concentration of nucleophile because the
nucleophile does not take part in the rate determining step.
2.
SN2 reaction: The rate depends on the concentration of the nucleophile which takes part in
the rate determining step.
Also, the relative strength of the nucleophiles affects the rate.
(a)
(b)
Negatively charged nucleophiles (e.g OH- ) are stronger than uncharged nucleophiles (e.g.
H2O )
In some cases the order of nucleophilicity follows the order of basicity:
RO- > OH- >> ROH > H2O
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
Chpt. 33 p.9
(C) Nature of leaving group
The best leaving groups are the most stable species after they depart.
1. For halogens, the ease of leaving is
I-
>
Br-
> Cl- >
F-
This order should be in agreement with the bond length and bond strength
e.g.
C—I bond is the __________ in bond length and hence the _____________ bond strength.
2.
Uncharged or neutral molecules are better leaving groups the charged ones. The ease of leaving
is:
H2O >> OH- > -OR (stronger base)
Predict which of the following reactions would occur:
(1) An alcohol is attacked by a nucleophile.
X:- + R—OH 
(2) An alcohol is dissolved in acid and then attacked by a nucleophile. The alcohol is
protonated.
X:- + R—OH2+

Exercise :
For each set of reactions below arrange them in order of decreasing rate. Give brief reasons for the
order.
250C
1. (a) (CH3)3CBr + H2O
(CH3)3 COH + HBr
250C
(b) (CH3)3CI + H2O
(CH3)3 COH + HI
250C
(c) (CH3)3CBr + CH3OH
(CH3)3 COCH3 + HBr
250C
2. (a) (CH3)3CI + H2O
(CH3)3 COH + HI
250C
(b) (CH3)2CHI + H2O
(CH3)2 CHOH + HI
(c)
250C
I
(CH3)2C
OH
+
H2O
(CH3)2C
+
HI
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
IV.
Chpt. 33 p.10
Unreactivity of Vinylic and Aryl halide
Compounds with a halogen attached directly to an sp carbon (in the case of vinylic halide and
aryl halide )are generally unreactive to both and SN2 reactions.
Reason :
<1> Vinylic cations and benzene cations ( SN1 ) are highly unstable
<2> The C—X bond of a vinylic halide or aryl halide is stronger than that of an alkyl halide
due to delocalization.
Exercise:
Each of the following compounds A. B, C and D is treated with excess aqueous sodium
hydroxide solution, under identical conditions:
O
Cl
CH2Br
A
CH2Cl
C
Cl
B
C
D
(a)
Arrange them in order of increasing rate1of reactions, stating briefly the reason for your
choice.
(b)
Suggest an experimental procedure to compare the rates of A and D with sodium
hydroxide.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
V.
Elimination
(A)
Elimination of Monohalide
Chpt. 33 p.11
Halogenoalkanes can undergo-reaction in which hydrogen halide is eliminated and an alkene is
formed.
Example ：2-chloropropane can undergo elimination as well as substitution when refluxing with
a solution of sodium hydroxide and ethanol.
substitution
CH3 CH CH3 +
CH3CH2ONa
Cl
elimination
Mechanism for the elimination
C2H5OH
CH2 CH CH3
CH2=CHCH3
+
Cl
Cl
At the transition state, the C—H bond and C—Cl bond are partially broken, while the C=C
bond and O—H bonds are partially formed.
Note : <1>
The reaction is an E2 reaction ((i.e. bimolecular elimination ) because ____ molecules
are involved in the transition.
<2>
The competition between the SN2 reaction and reactions occurs most primary and
secondary halides.
The relative importance of either type of reaction is dependent on the base
( nucleophile ), the temperature and the structure of the halide.
<3>
Elimination E2 is more favored to SN2 when the following conditions occur.
(a)
The nucleophile is a more basic one:
e.g. RO- is __________ basic than OH- , so _________ would lead to ______
rather SN2 .
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
(b)
The reaction temperature is higher.
(c)
More alkyl groups are attached to the —C—X group.
Chpt. 33 p.12
The steric hindrance will be increased for the nucleophile to attack the carbon
in —C—X to lead substitution.
<4> Tertiary halides may give different eliminated products
(B)
Elimination versus Substitution
In most unimolecular reactions the SN1 reaction is favored over the E reaction. However. E
reaction will be more favourable when
<1> the temperature is increased and/or
<2> a strong base is used.
Example :
The reactions between sodium ethoxide (a strong base) and several bromoalkanes
Note : A high concentration of base in a non-aqueous solvent (e.g. potassium hydroxide in ethanol) at
a reflux temperature favours elimination.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
Chpt. 33 p.13
Summary of important reaction pathways in different substrates:
CH3X
methyl
SN2
reactions
only
20
RCH2X 1°
R2CHX
Mainly SN2 except
with a hindered
strong base and
then
E2 mainly
Mainly SN2 with
weak bases and E2
mainly with strong
bases
R3C-X
30
No SN2 reaction.
In hydrolysis gives SNl / E1, and
at lower temperatures SN1
predominates. When a strong
base is used, E2 predominates.
Exercise
1.
(a)
(b)
Route (1) gives a higher yield of X. Give the mechanism of this route.
Why does route (ii) no give X but a different product instead? State the product and type of the
reaction involved.
2.
(a) A student attempts to prepare an ether via the following nucleophilic substitution:
However, he does not get the desired product but finds that a mixture of other products are
formed instead.
Give two reasons for the failure and suggest what products could have formed instead.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
(C)
Chpt. 33 p.14
Elimination of Dihalides
Structure of a dihalide ：
Dihalide are dehydrohalogenated by similar mechanism but here the dehydrohalogenation occur
in two steps.
R
H
H
C
C
H
R' +
:B-
R
C
C
:B-
R'
Br
Br Br
R
C
C
R'
Note : These two steps may be carried out as separate reactions or they may be carried out
consecutively in a single mixture.
sodium amide is a stronger base that can affect both dehydrohalogenation in a single
reaction mixture. Examples
Br2
CH3CH=CHCH3
CCl4
CH3 CH2 CH CH2
Br Br
H
H
CH3 C
C
C2H5ONa
KOH
CH3 CH2 C
Ethanol / reflux
C
CH3
Ethanol / reflux
Br Br
CH
Br H
CH3 CH2 C
H
VI.
CH3 C
CH3
NaNH2
H3C
CH2 C
CH
mineral oil
CH
Br
Synthetic Applications of Elimination reactions
Elimination reactions are very useful in a number of synthetic conventions.
Examples
By means of simple chemical equations only, outline how the the following conventions can be
carried out.. indicate the necessary reagents in each step of conversion.
a)
CH2CH2Br
CHCH3
OH
Br
b)
Br
Br
c)
CH3CHBrCH2Br
CH3CHBrCH3
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
(C)
Chpt. 33 p.15
Formation of C—N Bonds - Forming amines 胺的形成
Methods
1.
By heating the halide with concentrated ammonia in a sealed tube, a mixture of amines is
formed.
CH3CH2-I
+
CH3CH2-I
+
CH3CH2-NH2
CH3CH2-I
+
(CH3CH2)2NH
CH3CH2-I
+
(CH3CH2)3N
:NH3
CH3CH2NH2
+
HI
Note: Addition of iodide slowly to a large of ammonia should minimize the second and
subsequent reactions.
Mechanism: Ammonia acts the nucleophile in the above reaction:
H3N:
CH2 I
CH3
2.
By reducing the nitrile formed from nucleophilic substitution:
SN2
CH3CH2CH2-Br
+
CN -
Exercises
By means of simple chemical equations only, outline how the following conventions can be carried out.
Indicate the necessary reagents in each step of conversion.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
VIII.
Applications of Halogeno-compounds
(A)
Solvents
Chpt. 33 p.16
Halogenoalkanes dissolve in oil and grease. They are therefore widely used as solvents in the
laboratory, in industry and commercially.
(1)
Trichloromethane (Chloroform.) is used as a solvent for organic synthesis and for
solvent extraction.
(2)
(B)
Trichloethene (tetrachloroethene) is used as a dry cleaning agent and for removing oil
and greases from metal surfaces.
In Fire Extinguishers
Fully halogenated alkanes, being non-inflammable, volatile and dense, are used in fire
extinguishers.
Examples :
<1> Dibromochlorofluoromethane, BCF
<2> Bromotrifluoromethane BTM
(used in Mass Transit Railway)
(C)
Freons (Chlorofluorocarbons : CFC)
1.
Halogenoalkanes which have boiling temperatures just below room temperature can be easily
liquefied by a slight increase in pressure. Thus, they are suitable for use as the liquid in
refrigerators. Such compounds are chlorofluorocarbons (CFC) which are commonly called
freons.
Examples :
Dichiorodifluoromethane
1,2-dichloro-1,1,2,2-tetrafluroethane
Chlorofluorohydrocarbons can also vaporize easily when the pressure and can be used as
propellant liquid in aerosol sprays.
2.
The Environmental problems associated with CFC
Because of the lack of reactivity and insolubility in water, there is no natural process for
removing CFC. In fact they drift up into the stratosphere (the upper atmosphere), where they
receive sufficient ultraviolet Light to cause photolysis. The chlorine radicals formed in
photolysis take part in reactions that convert ozone into dioxyen.
(a)
(b)
(c)
Cl + O3  ClO + O2
ClO + O  Cl + O2
ClO + O3  ClO2 + O2
(a) and (b) form a chain reaction. Thus, one chlorine molecule radical can destroy thousand of
ozone molecules.
Problems
<1>
The fall in ozone concentration is serious because it decreases the absorption of UV light by
atmosphere. One result could be increase in skin cancer among humans.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 33: Haloalkane 鹵代化合物
<2>
Chpt. 33 p.17
Due to the strong absorption of infrared light by the CFCs. they can contribute to the
“greenhouse effect” which warms the earth by absorbing infrared radiation given by the earth.
(CFCs are 10000 times more effective than carbon dioxide in absorbing infrared radiation.)
Improvement
“Hydrochlorofluorocarbons”( HCFCs) and “hydrofluorocarbons” (HFCs) are used instead of CFCs.
The presence of the hydrogen atoms make these compounds less stable than CFCs. They do not reach
the stratosphere in such large concentrations.
A programme of tests, including toxicity tests, is being carried out on these compounds.
(D)
Chlorinated Aromatic Compounds — used as insecticide
In the presence of concentrated sulphuric acid, chlorobenzene and condenses with trichloroethanal to
give a compound called DDT.
DDT is a powerful insecticide. Its fat solubility causes it to be concentrated in animals up the food
chain, eventually accumulating in amounts that reach a toxic level to the animals. It may also lead to
disturbance of ecological balance leading to replacement of one pest by another.
(E)
Anaesthetics
2-bromo-2-chloro-1,1,1-trifluoroethane can be used as anaesthetic agent in surgery.
Chloroethane can be used as a local anaesthetic by spraying on the skin. Its volatility causes it
to evaporate readily and the cooing results in temporary localised anesthesia.