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Organic Chemistry, 7th Edition
L. G. Wade, Jr.
Alkyl Halides: Nucleophilic
Substitution and Elimination
Dr. Manal Fawzy Abou Taleb
Alkyl Halides
Introduction
Nomenclature of Alkyl Halides
Physical Properties of Alkyl Halides
Preparation of Alkyl Halides
Reactions of Alkyl Halides
Nucleophilic Substitution Reactions
Elimination Reactions
Uses of Alkyl Halides
Introduction
What Is an Alkyl Halide
• Alkyl Halides are organic compounds having one
or more halogen atoms bonded to a carbon atom.
IUPAC Nomenclature
• Name as haloalkane.
• Identify the longest continuous carbon chain
– It must contain any double or triple bond if present
– Number from end nearest any substituent (alkyl or halogen)
– If any multiple bonds are present, number from end closest
to these.
– The halogens are written as prefixes: fluoro- (F), chloro(Cl), bromo- (Br) and iodo- (I)
Naming with Multiple Halides
• If more than one of
the same kind of
halogen is present,
use prefix di, tri,
tetra
• If there are several
different halogens,
number them and list
them in alphabetical
order
Naming if Two Halides or Alkyl Are Equally
Distant from Ends of Chain
• Begin at the end nearer the substituent whose name comes
first in the alphabet
In case of halobenzenes, the benzene ring is numbered so as to
give the lowest possible numbers to the substituents e.g.
Systematic Common Names

Common names are often used for simple alkyl halides. To assign a
common name:
Name all the carbon atoms of the molecule as a single alkyl group.
Name the halogen bonded to the alkyl group.
Combine the names of the alkyl group and halide, separating the
words with a space.
iso-butyl bromide
sec-butyl bromide
7
Many Alkyl Halides That Are Widely Used
Have Common Names
•
•
•
•
•
Chloroform
Carbon tetrachloride
Methylene chloride
Methyl iodide
Trichloroethylene
• A halogen attached to a carbon next to a doubly bonded
carbon is an Allylic Halide
 A halogen is attached directly to a doubly bonded carbon is
called: Vinylic halides
• A halogen one carbon away from an aromatic ring is
Benzylic Halide
 A halogen attached directly to a benzene ring is an Aryl
halide
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
Classes of alkyl halides
• 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
PHYSICAL PROPERTIES OF ORGANIC
HALIDES
Haloalkanes have higher b.p. and m.p. than alkanes
∵ dipole-dipole interactions are present between haloalkane
molecules
• The boiling points increases with increasing in molecular
weights.
m.p. and b.p. increase in the order:
RCH2F < RCH2Cl < RCH2Br < RCH2I
∵ larger, more polarizable halogen atoms increase the
dipole-dipole interactions between the molecules
No. of carbon   m.p. and b.p. 
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
Explain Why
Alkyl halides have higher melting point than the corresponding
alkanes, alkenes, and alkynes
because:
1. Polarity
2. Molecular weight
Preparation of Halogeno-compounds
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 alcohols:
3° alcohol > 2° alcohol > 1° alcohol > CH3OH
• This is related to the stability of the reaction
intermediate (i.e. stability of carbocations)
Preparation of Halogeno-compounds
Reaction with Hydrogen Halides (HX)
• Dry HCl is bubbled through alcohols in the presence
of ZnCl2 catalyst
• For the preparation of bromo- and iodoalkanes, no
catalyst is required
Preparation of Halogeno-compounds
• The reactivity of hydrogen halides: HI > HBr > HCl
• e.g.
Preparation of Halogeno-compounds
Reaction with Phosphorus Halides
Haloalkanes can be prepared from the vigorous reaction
between cold alcohols and phosphorus(III) halides
Reaction with Phosphorus Halides
R-OH
+
PX5 or PCl5
CH3CH2OH +
PCl5
R-X
2 CH3CH2-Cl + POCl3 + H2O
3 CH3CH2OH + PBr3
3 CH3CH2Br + H3PO3
Addition of Thionyl Chloride to Alcohols
Pyridine
R-OH + SOCl2
CH3CH2OH +
SOCl2
Pyridine
R-Cl +
SO2 + HCl
CH3CH2Cl + SO2 + HCl
Preparing Alkyl Halides from Alkanes: Radical
Halogenation
• Alkane + Cl2 or Br2, heat or light replaces C-H with C-X
but gives mixtures
– Hard to control
– Via free radical mechanism
• It is usually not a good idea to plan a synthesis that uses
this method—multiple products
20
Preparation of Halogeno-compounds
Addition of Alkenes and Alkynes
•Alkyl dihalides are prepared from anti addition of bromine (Br2) or
chlorine (Cl2) (addition of halogen)
Addition of Alkenes and Alkynes
The most effective means of preparing an alkyl halide is from
addition of HCl, HBr, HI to alkenes or alkyne to give Markovnikov
product Anti-Markinikoff’s rule.
X
R-CH CH-R
H2 C CH2
+
R-CH CH2-R
HI
+
CH3-CH CH2
HX
+
CH3CH2I
H3 C
HBr
CH
CH3
(Markinikoff rule)
Br
CH3-CH CH2
+
peroxide
HBr
H3 C CH2-CH2-Br
(Anti-Markinikoff rule)
Preparation of Halogeno-compounds
Preparation of Halobenzenes
Halogenation of Benzene
Benzene reacts readily with chlorine and bromine in the
presence of catalysts (e.g. FeCl3, FeBr3, AlCl3)
Preparation of Halogeno-compounds
Check Point 32-2
State the major products of the following reactions:
(a) CH3CHOHCH2CH3 + PBr3 
(b) CH3CH = CH2 + HBr 
(c) CH3C  CH + 2HBr 
(a) CH3CHBrCH2CH3
(b) CH3CHBrCH3
(c) CH3CBr2CH3
Answer
Reactions of Halogeno-compounds
• Carbon-halogen bond is polar
• Carbon atom bears a partial positive charge
• Halogen atom bears a partial negative charge
Reactions of Halogeno-compounds
• Characteristic reaction:
Nucleophilic substitution reaction
• Alcohols, ethers, esters, nitriles and amines can be
formed by substituting – OH, – OR, RCOO –, – CN
and – NH2 groups respectively
Nu= OH, OR, OCOR, NH2, RNH, SH, SR, RC=C, CN, X’
Reactions of Halogeno-compounds
• Another characteristic reaction:
Elimination reaction
Haloalkane
Base
Alkene
• Bases and nucleophiles are the same kind of reagents
• Nucleophilic substitution and elimination reactions
always occur together and compete each other
Reaction of Grignared reagent
a- Formation of Grignard reagent
R
X
X
Ar
+
+
Mg
Mg
Dry ether
R
Dry ether
Ar
MgX
MgX
(X=Cl, Br, I)
(X=Cl, Br, I)
b- Reaction of Grignard reagent
H2O
R
R'OH
MgX
HC
R
R
H
H
CH
R
H
+ Mg(OH)X
+
Mg(OR')X
+
Mg(HCC)X
Nucleophilic Substitution Reactions
Reaction with Sodium Hydroxide
The reactions proceed in 2 different reaction mechanisms:
bimolecular nucleophilic substitution (SN2)
unimolecular nucleophilic substitution (SN1)
Nucleophilic Substitution Reactions
Bimolecular Nucleophilic Substitution (SN2)
Example: CH3 – Cl + OH–  CH3OH + Cl–
Experiment
number
1
2
3
4
Initial
Initial
Initial rate
[CH3Cl]
[OH–]
(mol dm–3 s–1)
–3
–3
(mol dm ) (mol dm )
0.001
0.002
0.001
0.002
1.0
1.0
2.0
2.0
4.9  10–7
9.8  10–7
9.8  10–7
19.6  10–7
Results of kinetic study of reaction of CH3Cl with OH–
Rate = k[CH3Cl][OH–]
Order of reaction = 2
 both species are involved in rate determining step
Nucleophilic Substitution Reactions
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
Nucleophilic Substitution Reactions
Transition state involve both the
nucleophile and substrate
 second order kinetics of the
reaction
Energy profile of the reaction of CH3Cl and OH- by SN2
mechanism
Nucleophilic Substitution Reactions
Stereochemistry of SN2 Reactions
• The nucleophile attacks from the backside of the
electropositive carbon centre
• The configuration of the carbon atom under attack
inverts
Nucleophilic Substitution Reactions
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
Nucleophilic Substitution Reactions
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
Nucleophilic Substitution Reactions
Step 2:
• Fast step
• Attacked by a nucleophile to form the product
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
Energy profile of the
reaction of (CH3)3CCl
and OH- by SN1
mechanism
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
Nucleophilic Substitution Reactions
For some cations, different products may be formed by
either mode of attack
e.g.
The reaction is called racemization
Nucleophilic Substitution Reactions
The above SN1 reaction leads to racemization
∵ formation of trigonal planar carbocation intermediate
Nucleophilic Substitution Reactions
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
Nucleophilic Substitution Reactions
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
Nucleophilic Substitution Reactions
•
Delocalized electrons repel any approaching
nucleophiles
 unreactive towards SN2 reactions
•
Benzene cations are highly unstable because of loss
of aromaticity
 unreactive towards SN1 reactions
Nucleophilic Substitution Reactions
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.
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
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.
• Ammonia is a nucleophile because the presence of a
lone pair of electrons on the nitrogen atom
The END