Transcript Organic Chemistry Fifth Edition

Chapter 4
Alcohols and Alkyl Halides
Overview of Chapter
This chapter introduces chemical reactions and
their mechanisms by focusing on two reactions
that yield alkyl halides.
(1) alcohol + hydrogen halide
ROH + HX  RX + H2O
(2) alkane + halogen
RH + X2  RX + HX
Both are substitution reactions.
4.1
Functional Groups
Functional Group
A structural unit in a molecule responsible for its
characteristic behavior under a particular set of
reaction conditions
Families of Organic Compounds
and their Functional Groups
Alcohol
Alkyl halide
ROH
RX (X = F, Cl, Br, I)
Amine
primary amine: RNH2
secondary amine: R2NH
tertiary amine: R3N
Families of Organic Compounds
and their Functional Groups
Epoxide
C
C
O
Ether
Nitrile
ROR'
RCN
Nitroalkane
Sulfide
Thiol
RNO2
RSR'
RSH
Many Classes of Organic Compounds
Contain a Carbonyl Group
O
O
C
C
Carbonyl group
R
Acyl group
Many Classes of Organic Compounds
Contain a Carbonyl Group
O
O
C
C
Carbonyl group
R
H
Aldehyde
Many Classes of Organic Compounds
Contain a Carbonyl Group
O
O
C
C
Carbonyl group
R
Ketone
R'
Many Classes of Organic Compounds
Contain a Carbonyl Group
O
O
C
C
Carbonyl group
R
OH
Carboxylic acid
Many Classes of Organic Compounds
Contain a Carbonyl Group
O
O
C
C
Carbonyl group
R
Ester
OR'
Many Classes of Organic Compounds
Contain a Carbonyl Group
O
O
C
C
Carbonyl group
R
Amide
NH2
4.2
IUPAC Nomenclature
of Alkyl Halides
IUPAC Nomenclature
There are several kinds of IUPAC nomenclature.
The two that are most widely used are:
functional class nomenclature
substitutive nomenclature
Both types can be applied to alcohols and
alkyl halides.
Functional Class Nomenclature of Alkyl Halides
Name the alkyl group and the halogen as
separate words (alkyl + halide).
CH3F
CH3CH2CHCH2CH2CH3
Br
CH3CH2CH2CH2CH2Cl
H
I
Functional Class Nomenclature of Alkyl Halides
Name the alkyl group and the halogen as
separate words (alkyl + halide).
CH3F
CH3CH2CH2CH2CH2Cl
Methyl fluoride
Pentyl chloride
CH3CH2CHCH2CH2CH3
Br
1-Ethylhexyl bromide
H
I
Cyclohexyl iodide
Substitutive Nomenclature of Alkyl Halides
Name as halo-substituted alkanes.
Number the longest chain containing the
halogen in the direction that gives the lowest
number to the substituted carbon.
CH3CH2CH2CH2CH2F
CH3CHCH2CH2CH3
Br
CH3CH2CHCH2CH3
I
Substitutive Nomenclature of Alkyl Halides
Name as halo-substituted alkanes.
Number the longest chain containing the
halogen in the direction that gives the lowest
number to the substituted carbon.
CH3CH2CH2CH2CH2F
1-Fluoropentane
CH3CH2CHCH2CH3
I
3-Iodopentane
CH3CHCH2CH2CH3
Br
2-Bromopentane
Substitutive Nomenclature of Alkyl Halides
Cl
CH3
CH3
Cl
Halogen and alkyl groups
are of equal rank when
it comes to numbering
the chain.
Number the chain in the
direction that gives the
lowest number to the
group (halogen or alkyl)
that appears first.
Substitutive Nomenclature of Alkyl Halides
Cl
5-Chloro-2-methylheptane
CH3
CH3
2-Chloro-5-methylheptane
Cl
4.3
IUPAC Nomenclature
of Alcohols
Functional Class Nomenclature of Alcohols
Name the alkyl group and add "alcohol" as a
separate word.
CH3CH2OH
CH3
CH3CCH2CH2CH3
CH3CHCH2CH2CH2CH3
OH
OH
Functional Class Nomenclature of Alcohols
Name the alkyl group and add "alcohol" as a
separate word.
CH3CH2OH
Ethyl alcohol
CH3CHCH2CH2CH2CH3
OH
1-Methylpentyl alcohol
CH3
CH3CCH2CH2CH3
OH
1,1-Dimethylbutyl
alcohol
Substitutive Nomenclature of Alcohols
Name as "alkanols." Replace -e ending of alkane
name with -ol.
Number chain in direction that gives lowest number
to the carbon that bears the —OH group.
CH3CH2OH
CH3
CH3CCH2CH2CH3
CH3CHCH2CH2CH2CH3
OH
OH
Substitutive Nomenclature of Alcohols
Name as "alkanols." Replace -e ending of alkane
name with -ol.
Number chain in direction that gives lowest number
to the carbon that bears the —OH group.
CH3CH2OH
Ethanol
CH3CHCH2CH2CH2CH3
OH
2-Hexanol or Hexan-2-ol
CH3
CH3CCH2CH2CH3
OH
2-Methyl-2-pentanol
or 2-Methylpentan-2-ol
Substitutive Nomenclature of Alcohols
OH
CH3
CH3
OH
Hydroxyl groups outrank
alkyl groups when
it comes to numbering
the chain.
Number the chain in the
direction that gives the
lowest number to the
carbon that bears the
OH group
Substitutive Nomenclature of Alcohols
OH
6-Methyl-3-heptanol
CH3
CH3
5-Methyl-2-heptanol
OH
4.4
Classes of Alcohols
and Alkyl Halides
Classification
Alcohols and alkyl halides are classified as
primary
secondary
tertiary
according to their "degree of substitution."
Degree of substitution is determined by counting
the number of carbon atoms directly attached to
the carbon that bears the halogen or hydroxyl group.
Classification
H
CH3CH2CH2CH2CH2F
OH
primary alkyl halide
secondary alcohol
CH3
CH3CHCH2CH2CH3
Br
secondary alkyl halide
CH3CCH2CH2CH3
OH
tertiary alcohol
4.5
Bonding in Alcohols
and Alkyl Halides
Dipole Moments
alcohols and alkyl halides are polar
H
H
H
+
C
+
H
H
+
O –
H
C
Cl
H
 = 1.7 D
 = 1.9 D
–
Dipole Moments
alcohols and alkyl halides are polar
 = 1.7 D
 = 1.9 D
Dipole-Dipole Attractive Forces
+
–
+
–
–
+
+
–
+
–
Dipole-Dipole Attractive Forces
+
–
+
–
–
+
+
–
+
–
4.6
Physical Properties of
Alcohols and Alkyl Halides:
Intermolecular Forces
Boiling point
Solubility in water
Density
Effect of Structure on Boiling Point
CH3CH2CH3
CH3CH2F
CH3CH2OH
Molecular
weight
44
48
46
Boiling
point, °C
-42
-32
+78
Dipole
moment, D
0
1.9
1.7
Effect of Structure on Boiling Point
CH3CH2CH3
Molecular
weight
44
Boiling
point, °C
-42
Dipole
moment, D
0
Intermolecular forces
are weak.
Only intermolecular
forces are induced
dipole-induced dipole
attractions.
Effect of Structure on Boiling Point
CH3CH2F
Molecular
weight
48
Boiling
point, °C
-32
Dipole
moment, D
1.9
A polar molecule;
therefore dipole-dipole
and dipole-induced
dipole forces contribute
to intermolecular
attractions.
Effect of Structure on Boiling Point
CH3CH2OH
Molecular
weight
46
Boiling
point, °C
+78
Dipole
moment, D
1.7
Highest boiling point;
strongest intermolecular
attractive forces.
Hydrogen bonding is
stronger than other
dipole-dipole attractions.
Figure 4.4 Hydrogen bonding in ethanol
–
–
+
+
Figure 4.4 Hydrogen bonding in ethanol
Boiling point increases with increasing
number of halogens
Compound
CH3Cl
CH2Cl2
CHCl3
CCl4
Boiling Point
-24°C
40°C
61°C
77°C
Even though CCl4 is the only compound in this list without
a dipole moment, it has the highest boiling point.
Induced dipole-induced dipole forces are greatest in CCl4
because it has the greatest number of Cl atoms. Cl is more
polarizable than H.
But trend is not followed when halogen
is fluorine
Compound
CH3CH2F
CH3CHF2
CH3CF3
CF3CF3
Boiling Point
-32°C
-25°C
-47°C
-78°C
But trend is not followed when halogen
is fluorine
Compound
CH3CH2F
CH3CHF2
CH3CF3
CF3CF3
Boiling Point
-32°C
-25°C
-47°C
-78°C
Fluorine is not very polarizable and induced dipoleinduced dipole forces decrease with increasing
fluorine substitution.
Solubility in water
Alkyl halides are insoluble in water.
Methanol, ethanol, isopropyl alcohol are
completely miscible with water.
The solubility of an alcohol in water
decreases with increasing number of
carbons (compound becomes
more hydrocarbon-like).
Figure 4.5 Hydrogen Bonding
Between Ethanol and Water
Density
Alkyl fluorides and alkyl chlorides are
less dense than water.
Alkyl bromides and alkyl iodides are
more dense than water.
All liquid alcohols have densities of
about 0.8 g/mL.
4.7
Preparation of Alkyl Halides from
Alcohols and Hydrogen Halides
ROH + HX  RX + H2O
Reaction of Alcohols with Hydrogen Halides
ROH +
HX  RX + HOH
Hydrogen halide reactivity
HF
least reactive
HCl
HBr
HI
most reactive
Reaction of Alcohols with Hydrogen Halides
ROH +
HX  RX + HOH
Alcohol reactivity
CH3OH
Methanol
RCH2OH R2CHOH
Primary Secondary
least reactive
R3COH
Tertiary
most reactive
Preparation of Alkyl Halides
25°C
(CH3)3COH + HCl
(CH3)3CCl + H2O
78-88%
OH + HBr
80-100°C
Br + H2O
73%
CH3(CH2)5CH2OH + HBr
120°C
CH3(CH2)5CH2Br + H2O
87-90%
Preparation of Alkyl Halides
A mixture of sodium bromide and sulfuric
acid may be used in place of HBr.
CH3CH2CH2CH2OH
NaBr
H2SO4
heat
CH3CH2CH2CH2Br
70-83%
4.8
Mechanism of the Reaction of
Alcohols with Hydrogen Halides:
Hammond’s Postulate
About mechanisms
A mechanism describes how reactants are
converted to products.
Mechanisms are often written as a series of
chemical equations showing the elementary steps.
An elementary step is a reaction that proceeds
by way of a single transition state.
Mechanisms can be shown likely to be correct,
but cannot be proven correct.
About mechanisms
For the reaction:
(CH3)3COH + HCl
tert-Butyl alcohol
25°C
(CH3)3CCl + H2O
tert-Butyl chloride
the generally accepted mechanism involves
three elementary steps.
Step 1 is a Brønsted acid-base reaction.
Step 1: Proton Transfer
(CH3)3C
..
O: + H
..
:
Cl
..
H
Like proton transfer
from a strong acid to
water, proton transfer
from a strong acid to
an alcohol is normally
very fast.
fast, bimolecular
H
+
(CH3)3C O :
+
H
tert-Butyloxonium ion
.. –
: Cl:
..
Step 1: Proton Transfer
(CH3)3C
..
O: + H
..
:
Cl
..
H
Two molecules react
in this elementary
step; therefore it is
bimolecular.
fast, bimolecular
H
+
(CH3)3C O :
+
H
tert-Butyloxonium ion
.. –
: Cl:
..
Step 1: Proton Transfer
(CH3)3C
..
O: + H
..
:
Cl
..
H
fast, bimolecular
H
+
(CH3)3C O :
+
H
tert-Butyloxonium ion
.. –
: Cl:
..
Species formed in
this step (tertbutyloxonium ion) is
an intermediate in
the overall reaction.
Potential Energy Diagram for Step 1

(CH3)3CO
Potential
energy
H
Cl
H
(CH3)3COH + H—Cl
Reaction coordinate

+
(CH3)3CO
H
H + Cl –
Hammond's Postulate
If two succeeding states (such as a
transition state and an unstable intermediate)
are similar in energy, they are similar in structure.
Hammond's postulate permits us to infer the
structure of something we can't study (transition
state) from something we can study
(reactive intermediate).
Step 2: Carbocation Formation
Dissociation of the
alkyloxonium ion involves
H
bond-breaking, without
+
any bond-making to
(CH3)3C O :
compensate for it. It
has a high activation
H
energy and is slow.
slow, unimolecular
H
+
+
:O:
tert-Butyl cation
H
(CH3)3C
Step 2: Carbocation Formation
H
A single molecule
reacts in this step;
therefore, it is
unimolecular.
+
(CH3)3C O :
H
slow, unimolecular
H
+
+
:O:
tert-Butyl cation
H
(CH3)3C
Step 2: Carbocation Formation
H
+
(CH3)3C O :
H
The product of this
step is a carbocation.
It is an intermediate
in the overall process.
slow, unimolecular
H
+
+
:O:
tert-Butyl cation
H
(CH3)3C
Potential Energy Diagram for Step 2

(CH3)3C
H
O 
H
Potential
energy
(CH3)3C
+
(CH3)3CO
H
H
Reaction coordinate
+
+ H 2O
Transition State for Carbocation Formation
Note the “carbocation character” in the transition
state:

(CH3)3C
H
O 
H
Whatever stabilizes carbocations will stabilize
the transition state leading to them.
Carbocation
R
+
C
R
R
The key intermediate in reaction of secondary
and tertiary alcohols with hydrogen halides is
a carbocation.
A carbocation is a cation in which carbon has
6 valence electrons and a positive charge.
Figure 4.8 Structure of tert-Butyl Cation
CH3
+
H3C
C
CH3
Positively charged carbon is sp2 hybridized.
All four carbons lie in same plane.
Unhybridized p orbital is perpendicular to
plane of four carbons.
Step 3: Carbocation Capture
Bond formation between
the positively charged
..
–
+
carbocation and the
(CH3)3C
+ : Cl:
..
negatively charged
chloride ion is fast.
fast, bimolecular
(CH3)3C
..
Cl
.. :
tert-Butyl chloride
Two species are
involved in this step.
Therefore, this step
is bimolecular.
Step 3: Carbocation Capture
(CH3)3C
+
+
.. –
: Cl:
..
This is a Lewis acidLewis base reaction.
The carbocation is the
Lewis acid; chloride
ion is the Lewis base.
fast, bimolecular
(CH3)3C
..
Cl
.. :
tert-Butyl chloride
The carbocation is
an electrophile.
Chloride ion is a
nucleophile.
Step 3: Carbocation Capture
+
H3C CH3
C+
+
Cl
–
CH3
Lewis acid
Lewis base
Electrophile
Nucleophile
H3C CH3
C
H3C
Cl
Potential Energy Diagram for Step 3

(CH3)3C
Cl

Potential
energy
+
(CH3)3C + Cl–
(CH3)3C—Cl
Reaction coordinate
4.9
Potential Energy Diagrams for
Multistep Reactions:
The SN1 Mechanism
Potential Energy Diagram - Overall
The potential energy diagram for a
multistep mechanism is simply a collection of the
potential energy diagrams for the individual
steps.
Consider the three-step mechanism for the
reaction of tert-butyl alcohol with HCl.
(CH3)3COH + HCl
25°C
(CH3)3CCl + H2O
carbocation
formation
carbocation
capture
R+
proton
transfer
ROH
+
ROH2
RX
carbocation
formation
–
+
(CH3)3C
O
H
Cl
carbocation
capture
‡
R+
H
proton ‡
transfer
ROH
+
ROH2
RX
carbocation
formation
H
+
(CH3)3C
O
‡
carbocation
capture
‡
+
R+
H
proton
transfer
ROH
+
ROH2
RX
carbocation
formation
‡
carbocation
capture
+
(CH3)3C
R+
proton
transfer
ROH
+
ROH2
RX
–
Cl
‡
Mechanistic Notation
The mechanism just described is an
example of an SN1 process.
SN1 stands for substitution-nucleophilicunimolecular.
The molecularity of the rate-determining
step defines the molecularity of the
overall reaction.
Mechanistic Notation
The molecularity of the rate-determining step
defines the molecularity of the overall reaction.
+
(CH3)3C
H
O
+
H
Rate-determining step is unimolecular
dissociation of alkyloxonium ion.
4.10
Structure, Bonding, and Stability
of Carbocations
Carbocations
R
+
C
R
R
Most carbocations are too unstable to be
isolated, but occur as reactive intermediates in
a number of reactions.
When R is an alkyl group, the carbocation is
stabilized compared to R = H.
Carbocations
H
+
C
H
H
Methyl cation
least stable
Carbocations
H3C
+
C
H
H
Ethyl cation
(a primary carbocation)
is more stable than CH3+
Carbocations
H3C
+
C
CH3
H
Isopropyl cation
(a secondary carbocation)
is more stable than CH3CH2+
Carbocations
H3C
+
C
CH3
CH3
tert-Butyl cation
(a tertiary carbocation)
is more stable than (CH3)2CH+
Figure 4.14 Stabilization of Carbocations
via the Inductive Effect
+
Positively charged
carbon pulls
electrons in  bonds
closer to itself.
Figure 4.14 Stabilization of Carbocations
via the Inductive Effect




Positive charge is
"dispersed ", i.e., shared
by carbon and the
three atoms attached
to it.
Figure 4.14 Stabilization of Carbocations
via the Inductive Effect


Electrons in C—C
bonds are more
polarizable than those
in C—H bonds;
therefore, alkyl groups
stabilize carbocations
better than H.
Electronic effects transmitted through bonds
are called "inductive effects."
Figure 4.15 Stabilization of Carbocations
via Hyperconjugation
+
Electrons in this 
bond can be shared
by positively charged
carbon because the
 orbital can overlap
with the empty 2p
orbital of positively
charged carbon
Figure 4.15 Stabilization of Carbocations
via Hyperconjugation
Electrons in this 
bond can be shared
by positively charged
carbon because the
 orbital can overlap
with the empty 2p
orbital of positively
charged carbon
Figure 4.15 Stabilization of Carbocations
via Hyperconjugation


Notice that an occupied
orbital of this type is
available when sp3
hybridized carbon is
attached to C+, but is
not available when H
is attached to C+.
Therefore, alkyl groups
stabilize carbocations
better than H does.
4.11
Effect of Alcohol Structure on
Reaction Rate
H
Slow step is:
+ •
R O•
H
+
R
+ •• O ••
H
H
The more stable the carbocation, the faster
it is formed.
Tertiary carbocations are more stable than
secondary, which are more stable than primary,
which are more stable than methyl.
Tertiary alcohols react faster than secondary,
which react faster than primary, which react faster
than methanol.
High activation energy for formation of methyl cation.
+
CH3
+
H
O ••
H
H
Eact
+
CH3 + •• O ••
H
H
+ •
CH3 O •
H
Smaller activation energy for formation of
primary carbocation.
+
+
H
O ••
RCH2
H
Eact
+
RCH2 + •• O ••
H
H
H
+ •
RCH2 O •
H
Activation energy for formation of secondary
carbocation is less than that for formation of
primary carbocation.
+
R2CH
+
H
O ••
H
Eact
H
+ •
R2CH O •
H
+
R2CH + •• O ••
H
H
Activation energy for formation of tertiary
carbocation is less than that for formation of
secondary carbocation.
+
R3C
+
H
O ••
H
H
+ •
R3C O •
H
Eact
+
R3C + •• O ••
H
H
4.12
Reaction of Methyl and Primary
Alcohols with Hydrogen Halides.
The SN2 Mechanism
Preparation of Alkyl Halides
25°C
(CH3)3COH + HCl
(CH3)3CCl + H2O
78-88%
OH + HBr
80-100°C
Br + H2O
73%
CH3(CH2)5CH2OH + HBr
120°C
CH3(CH2)5CH2Br + H2O
87-90%
Preparation of Alkyl Halides
Primary carbocations are too high in energy to
allow SN1 mechanism. Yet, primary alcohols
are converted to alkyl halides.
Primary alcohols react by a mechanism called
SN2 (substitution-nucleophilic-bimolecular).
CH3(CH2)5CH2OH + HBr
120°C
CH3(CH2)5CH2Br + H2O
87-90%
The SN2 Mechanism
Two-step mechanism for conversion of alcohols
to alkyl halides:
(1) proton transfer to alcohol to form
alkyloxonium ion
(2) bimolecular displacement of water
from alkyloxonium ion by halide
Example
CH3(CH2)5CH2OH + HBr
120°C
CH3(CH2)5CH2Br + H2O
Mechanism
Step 1:
Proton transfer from HBr to 1-heptanol
CH3(CH2)5CH2
..
O: + H
..
:
Br
..
H
fast, bimolecular
H
+
CH3(CH2)5CH2 O :
H
Heptyloxonium ion
+
.. –
: Br:
..
Mechanism
Step 2:
Reaction of alkyloxonium ion with bromide
ion.
H
.. –
+
: Br: + CH3(CH2)5CH2 O :
..
H
slow, bimolecular
H
CH3(CH2)5CH2
..
Br
.. :
1-Bromoheptane
+
:O:
H
+
–
Br
CH2
OH2
CH3(CH2)4CH2
‡
proton
transfer
ROH
+
ROH2
RX
‡
4.13
More on Activation Energy
Arrhenius equation
A quantitative relationship between the activation
energy, the rate constant, and the temperature:
k  Ae Eact / RT
A is the preexponential, or frequency factor
(related to the collision frequency and geometry),
R=8.314 x 10-3 kJ/Kmol, T is in K.
The true activation barrier is known as DG‡ and
takes into account the enthalpy and entropy of
activation.
4.14
Other Methods for Converting
Alcohols to Alkyl Halides
Reagents for ROH to RX
Thionyl chloride
SOCl2 + ROH  RCl + HCl + SO2
Phosphorus tribromide
PBr3 + 3ROH  3RBr + H3PO3
Examples
CH3CH(CH2)5CH3
SOCl2
K2CO3
CH3CH(CH2)5CH3
Cl
OH
(81%)
(Pyridine often used instead of K2CO3)
(CH3)2CHCH2OH
PBr3
(CH3)2CHCH2Br
(55-60%)
4.15
Halogenation of Alkanes
RH + X2  RX + HX
Energetics
RH + X2  RX + HX
explosive for F2
exothermic for Cl2 and Br2
endothermic for I2
4.16
Chlorination of Methane
Chlorination of Methane
carried out at high temperature (400 °C)
CH4 + Cl2  CH3Cl + HCl
CH3Cl + Cl2  CH2Cl2 + HCl
CH2Cl2 + Cl2  CHCl3 + HCl
CHCl3 + Cl2  CCl4 + HCl
4.17
Structure and Stability of Free
Radicals
Free Radicals
contain unpaired electrons
.. ..
: O O:
Examples: O2
.
.
..
.
: N O:
NO
Cl
..
: Cl .
..
Alkyl Radicals
R
.
C
R
R
Most free radicals in which carbon bears
the unpaired electron are too unstable to be
isolated.
Alkyl radicals are classified as primary,
secondary, or tertiary in the same way that
carbocations are.
Figure 4.17 Structure of Methyl Radical
Methyl radical is planar, which suggests that
carbon is sp2 hybridized and that the unpaired
electron is in a p orbital.
Alkyl Radicals
R
.
C
R
R
The order of stability of free radicals is the
same as for carbocations.
Alkyl Radicals
H
.
C
H3C
H
less stable
than
H
Methyl radical
H3C
less stable
than
.
C
CH3
H
Isopropyl radical
(secondary)
.
C
H
H
Ethyl radical
(primary)
H3C
less stable
than
.
C
CH3
CH3
tert-Butyl radical
(tertiary)
Alkyl Radicals
The order of stability of free radicals can be
determined by measuring bond strengths.
By "bond strength" we mean the energy
required to break a covalent bond.
A chemical bond can be broken in two
different ways—heterolytically or homolytically.
Homolytic
In a homolytic bond cleavage, the two electrons in
the bond are divided equally between the two atoms.
One electron goes with one atom, the second with
the other atom.
In a heterolytic cleavage, one atom retains both
electrons.
Heterolytic
–
+
Homolytic
The species formed by a homolytic bond cleavage
of a neutral molecule are free radicals. Therefore,
measure enthalpy cost of homolytic bond cleavage to
gain information about stability of free radicals.
The more stable the free-radical products, the weaker
the bond, and the lower the bond-dissociation energy.
Measures of Free Radical Stability
Bond-dissociation enthalpy measurements tell
us that isopropyl radical is 13 kJ/mol more
stable than propyl.
.
CH3CH2CH2
.
+
CH3CHCH3
H.
+
410
397
H.
CH3CH2CH3
Measures of Free Radical Stability
Bond-dissociation enthalpy measurements tell
us that tert-butyl radical is 30 kJ/mol more
stable than isobutyl.
.
(CH3)2CHCH2
+
(CH3)3C .
H.
+
410
380
H.
(CH3)3CH
4.18
Mechanism of Methane Chlorination
Mechanism of Chlorination of Methane
Free-radical chain mechanism.
Initiation step:
..
.. ..
..
. + . Cl:
: Cl: Cl
:
:
Cl
..
.. ..
..
The initiation step "gets the reaction going"
by producing free radicals—chlorine atoms
from chlorine molecules in this case.
Initiation step is followed by propagation
steps. Each propagation step consumes one
free radical but generates another one.
Mechanism of Chlorination of Methane
First propagation step:
H3C : H
+
..
. Cl:
..
Second propagation step:
.. ..
:
H3C . + : Cl: Cl
..
..
H3C .
+
..
H : Cl:
..
..
..
H3C: Cl: + . Cl:
..
..
Mechanism of Chlorination of Methane
First propagation step:
H3C : H
+
..
. Cl:
..
Second propagation step:
.. ..
:
H3C . + : Cl: Cl
..
..
.. ..
: Cl
:
H3C : H + : Cl
..
..
H3C .
+
..
H : Cl:
..
..
..
H3C: Cl: + . Cl:
..
..
..
..
H3C: Cl: + H : Cl:
..
..
Almost all of the product is formed by repetitive
cycles of the two propagation steps.
First propagation step:
H3C : H
+
..
. Cl:
..
Second propagation step:
.. ..
:
H3C . + : Cl: Cl
..
..
.. ..
: Cl
:
H3C : H + : Cl
..
..
H3C .
+
..
H : Cl:
..
..
..
H3C: Cl: + . Cl:
..
..
..
..
H3C: Cl: + H : Cl:
..
..
Termination Steps
stop chain reaction by consuming free radicals
..
H3C . + . Cl:
..
..
H3C: Cl:
..
Hardly any product is formed by termination step
because concentration of free radicals at any
instant is extremely low.
4.19
Halogenation of Higher Alkanes
Chlorination of Alkanes
can be used to prepare alkyl chlorides
from alkanes in which all of the hydrogens
are equivalent to one another
CH3CH3 + Cl2
420°C
CH3CH2Cl + HCl
(78%)
+ Cl2
h
Cl + HCl
(73%)
Chlorination of Alkanes
Major limitation:
Chlorination gives every possible
monochloride derived from original carbon
skeleton.
Not much difference in reactivity of
different hydrogens in molecule.
Example
Chlorination of butane gives a mixture of
1-chlorobutane and 2-chlorobutane.
(28%)
CH3CH2CH2CH2Cl
CH3CH2CH2CH3
Cl2
h
CH3CHCH2CH3
(72%)
Cl
Percentage of Product that Results from
Substitution of Indicated Hydrogen if
Every Collision with Chlorine Atoms is
Productive
10%
10%
10%
10%
10%
10%
10%
10%
10%
10%
Percentage of Product that Actually Results
from Replacement of Indicated Hydrogen
18%
18%
4.6%
4.6%
4.6%
4.6%
4.6%
4.6%
18%
18%
Relative Rates of Hydrogen Atom Abstraction
divide by 4.6
18%
4.6%
4.6
=1
4.6
18
4.6
= 3.9
A secondary hydrogen is abstracted 3.9 times
faster than a primary hydrogen by a chlorine
atom.
Similarly, chlorination of 2-methylbutane
gives a mixture of isobutyl chloride and
tert-butyl chloride.
(63%)
CH3
CH3CCH2Cl
CH3
CH3CCH3
H
H
Cl2
h
CH3
CH3CCH3
(37%)
Cl
Percentage of Product that Results from
Replacement of Indicated Hydrogen
7.0%
37%
Relative Rates of Hydrogen Atom Abstraction
divide by 7
7.0
7
=1
37
7
= 5.3
A tertiary hydrogen is abstracted 5.3 times faster
than a primary hydrogen by a chlorine atom.
Selectivity of Free-radical Halogenation
R3CH
> R2CH2 >
RCH3
chlorination:
5
4
1
bromination:
1640
82
1
Chlorination of an alkane gives a mixture of
every possible isomer having the same skeleton
as the starting alkane. Useful for synthesis only
when all hydrogens in a molecule are equivalent.
Bromination is highly regioselective for
substitution of tertiary hydrogens. Major synthetic
application is in synthesis of tertiary alkyl
bromides.
Synthetic Application of Chlorination of an Alkane
Cl
Cl2
h
(64%)
Chlorination is useful for synthesis only when
all of the hydrogens in a molecule are equivalent.
Synthetic Application of Bromination of an Alkane
Br
H
CH3CCH2CH2CH3
CH3
Br2
h
CH3CCH2CH2CH3
CH3
(76%)
Bromination is highly selective for substitution
of tertiary hydrogens.
Major synthetic application is in synthesis of
tertiary alkyl bromides.