Chemistry
CHE Organic 275
Chapter 4
Alcohols and
Alkyl Halides
Overview
In this section chemical reactions and their mechanisms are
introduced 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 of these reactions are substitution reactions
Families of Organic Compounds
Organic compounds can be grouped into families
Families can be defined by their functional groups
A functional group is a collection of atoms at a site within a
molecule with a common bonding pattern
The functional group reacts in a typical way, generally
independent of the rest of the molecule
The functional groups also affect the reactions, structure
and physical properties of every compound where they
occur
Alkenes are a Functional Group
Alkenes have a carbon-carbon double bond
Double bonds in simple and complex alkenes react the same
way
H
H
CC
H
CH3
H2C
H2C
Br2
H
H
CH2
CH
CH3
Br2
Br
H
H
Br
H
H
Br
H3C
Br
H
H2C
CH2
H2C
CH
CH3
Functional Groups With
Multiple Carbon-Carbon Bonds
Alkenes have a carbon-carbon double bond
Alkynes have a carbon-carbon triple bond
Arenes have special bonds represented as alternating single
and double C-C bonds in a 6-membered ring
C
C
C
C
C
C
C
C
Alkene
Alkyne
C
C
Arene
(aromatic ring)
Functional Groups With Single
Bonds to Oxygen
Alcohol: C bonded to O of a hydroxyl group (C-OH)
Ether: 2 C s bonded to the same O (C-O-C)
C
OH
Alcohol
C
C
O
Ether
OH
ethanol
O
methyl tert-butyl ether
(MTBE)
Constitutional Isomerism in
Functional Groups
CH3CH2OCH2CH3
CH3CH2CH2CH2OH
diethyl ether
butanol
Constitutional isomerism is not limited to alkanes
Different functional groups may have the same formulae, and
therefore can be constitutional isomers
Example: butyl alcohol and diethyl ether (C4H10O)
Different Positions of
Functional Groups
H3C
CH3CH2CH2NH2
propylamine
CH NH2
H3C
isopropylamine
Changing the position of functional group along the carbon
chain will also lead to different isomers
Example: isopropyl amine and propylamine (C3H9N)
Functional Groups With Single
Bonds to Nitrogen
Amine: C bonded to N (C-N)
Three types of amines (primary, secondary and tertiary)
Primary amines have one C-N bond and two N-H bonds
Secondary amines have 2 C-N bonds and one N-H bond
Tertiary amines have 3 C-N bonds
C
C
NH2
Primary
Amine
C
N
H
C
Secondary
Amine
N
C
C
Tertiary
Amine
Other Functional Groups With
Single Bonds to Carbon
C
Cl
Alkyl
Chloride
C
Br
Alkyl
Bromide
C
Alkyl
Iodide
I
C
SH
Thiol
C
S
Sulfide
Alkyl Halide: C bonded to halogen (C-X) (X = Cl, Br or I)
Thiol: C bonded to SH group
Sulfide: Two C s bonded to the same S (C-S-C)
C
Dipoles in Functional Groups
Functional groups with carbon singly bonded to an
electronegative atom are polar
A partial positive charge is on C (+), and a partial negative
charge (-) is on the electronegative atom
Functional Groups With Double
Bonds to Oxygen
O
O
H
Aldehyde
Ketone
Aldehydes have one hydrogen bonded to C=O
Ketones have two carbons bonded to C=O
C=O is also referred to as a carbonyl
More Functional Groups With
Double Bonds to Oxygen
O
O
OH
Carboxylic
Acid
O
O
Ester
O
NH2
Amide
Cl
Acid
Chloride
Carboxylic Acids have an OH bound to the C=O
Esters have an O-C bound to the C=O to the C=O
Amides have a N bound to the C=O
Acid Chlorides have a Cl bound to the C=O
Dipole Moments in
Double Bonds to Oxygen
Carbonyl C has a partial positive charge (+)
Carbonyl O has a partial negative charge (-)
IUPAC Nomenclature of
Alcohols and Alkyl Halides
-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.
Nomenclature of Alkyl Halides
Name the alkyl group and the halogen as
separate words (alkyl + halide)
C H 3F
CH3CH2CH2CH2CH2Cl
H
I
Nomenclature of Alkyl Halides
- Name the alkyl group and the halogen as
separate words (alkyl + halide)
C H 3F
CH3CH2CH2CH2CH2Cl
Methyl fluoride
Pentyl chloride
H
I
Cyclohexyl iodide
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
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
C H3
C H3
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
C H3
C H3
2-Chloro-5-methylheptane
Cl
Functional Class Nomenclature
of Alkyl Halides
Name the alkyl group and add "alcohol" as a
separate word.
CH3CH2OH
C H3
CH3CCH2CH2CH3
CH3CHCH2CH2CH2CH3
OH
OH
Functional Nomenclature
of Alkyl Halides
Name the alkyl group and add "alcohol" as a
separate word.
CH3CH2OH
Ethyl alcohol
CH3CHCH2CH2CH2CH3
OH
1-Methylpentyl alcohol
C H3
CH3CCH2CH2CH3
OH
1,1-Dimethylbutyl
alcohol
Substitutive Nomenclature
of Alkyl Halides
Name as "alkanols." Replace -e ending of alkane
name by -ol.
Number chain in direction that gives lowest number
to the carbon that bears the OH group.
CH3CH2OH
C H3
CH3CCH2CH2CH3
CH3CHCH2CH2CH2CH3
OH
OH
Substitutive Nomenclature
of Alkyl Halides
Name as "alkanols." Replace -e ending of alkane
name by -ol.
Number chain in direction that gives lowest number
to the carbon that bears the OH group.
CH3CH2OH
Ethanol
CH3CHCH2CH2CH2CH3
OH
2-Hexanol
C H3
CH3CCH2CH2CH3
OH
2-Methyl-2-pentanol
Substitutive Nomenclature
of Alkyl Halides
OH
C H3
C H3
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 Alkyl Halides
OH
6-Methyl-3-heptanol
C H3
C H3
5-Methyl-2-heptanol
OH
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
C H3
CH3CHCH2CH2CH3
Br
secondary alkyl halide
CH3CCH2CH2CH3
OH
tertiary alcohol
Dipole Moments in
Functional Groups
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
+
+
+
+
+
Effect of Structure on
Boiling Point
CH3CH2CH3
CH3CH2F
Molecular
weight
44
48
46
Boiling
point, C
-42
-32
+78
0
1.9
1.7
Dipole
moment, D
CH3CH2OH
Effect of Structure on
Boiling Point
CH3CH2CH3
Molecular
weight
Boiling
point, C
Dipole
moment, D
44
-42
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.
Hydrogen Bonding in Ethanol
+
+
Hydrogen Bonding in Ethanol
Boiling Point Increases
with the Number of Halogens
Compound
CH3Cl
CH2Cl2
CHCl3
CCl4
Boiling Point
-24C
40C
61C
77C
- 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.
Fluorine Does Not Follow
This Trend
Compound
CH3CH2F
CH3CHF2
CH3CF3
CF3CF3
Boiling Point
-32C
-25C
-47C
-78C
Fluorine Does Not Follow
This Trend
Compound
CH3CH2F
CH3CHF2
CH3CF3
CF3CF3
Boiling Point
-32C
-25C
-47C
-78C
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).
Hydrogen Bonding Between
Water and Ethanol
Density
Alkyl fluorides and alkyl chlorides are
less dense than water.
Di, tri and tetrachloro alkanes are often
more dense than water
Alkyl bromides and alkyl iodides are
more dense than water.
All liquid alcohols have densities of
about 0.8 g/mL.
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
HCl
least reactive
HBr
HI
most reactive
Reaction of Alcohols
with Hydrogen Halides
ROH +
HX RX + HOH
Alcohol reactivity
CH3OH RCH2OH R2CHOH R3COH
Methanol Primary Secondary Tertiary
least reactive
most reactive
Preparation of Alkyl Halides
OH
+
+
HCl
HBr
25C
80-100C
Br
OH
OH +
HBr
+
HOH
+
Cl
HOH
120C
Br
+
HOH
Preparation of Alkyl Halides
A mixture of sodium bromide and sulfuric
acid may be used in place of HBr.
OH
NaBr
H2SO4
heat
Br
Mechanism of the Formation
of Alkyl Halides
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:
OH
tert-Butyl alcohol
+
HCl
25C
Cl
+ HOH
tert-Butyl chloride
The generally accepted mechanism involves
three elementary steps.
Step 1 is a Brnsted acid-base reaction.
Step 1: Proton Transfer
H
O
H
Cl
fast, bimolecular
+
OH
H
tert-Butyloxonium ion
Cl
-
Like proton transfer
from a strong acid to
water, proton transfer
from a strong acid to
an alcohol is normally
very fast.
Step 1: Proton Transfer
H
O
H
Cl
Two molecules react
in this elementary
step; therefore it is
bimolecular.
+
OH
H
tert-Butyloxonium ion
Cl
-
Step 1: Proton Transfer
H
O
H
Cl
fast, bimolecular
+
OH
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
Cl
H
(CH3)3COH + HCl
Reaction coordinate
H
-
+
(CH3)3CO
H
H + Cl
Step 2: Carbocation Formation
Dissociation of the
alkyloxonium ion involves
bond-breaking, without
any bond-making to
compensate for it. It
has a high activation
energy and is slow.
C
+
OH
H
slow, unimolecular
C+
tert-Butyl cation
OH
H
Step 2: Carbocation Formation
C
A single molecule
reacts in this step;
therefore, it is
unimolecular.
+
OH
H
slow, unimolecular
C+
tert-Butyl cation
OH
H
Step 2: Carbocation Formation
C
The product of this
step is a carbocation.
It is an intermediate
in the overall process.
+
OH
H
slow, unimolecular
C+
tert-Butyl cation
OH
H
Step 2 Potential Energy Diagram
H
+
(CH3)3C
O +
H
Potential
energy
(CH3)3C
+
(CH3)3CO
H
H
Reaction coordinate
+
+ H 2O
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.
Carbocation Structure
C H3
+
H 3C
C
C H3
Positively charged carbon is sp2 hybridized.
All four carbons lie in same plane.
Unhybridized p orbital is perpendicular to
plane of four carbons.
Carbocation Capture
Cl
C+
-
fast,
bimolecular
Cl
tert-Butyl chloride
Bond formation between
the positively charged
carbocation and the
negatively charged
chloride ion is fast.
Two species in are
involved this step.
Therefore, this step
is bimolecular.
Carbocation Capture
Cl
C+
-
fast,
bimolecular
Cl
tert-Butyl chloride
This is a Lewis acidLewis base reaction.
The carbocation is the
Lewis acid; chloride
ion is the Lewis base.
The carbocation is an
electrophile.
Chloride ion is a
nucleophile.
Carbocation Capture
(CH3)3C
+
Cl
Lewis acid
Lewis base
Electrophile
Nucleophile
(CH3)3CCl
Step 3 Potential Energy Diagram
+
(CH3)3C
Cl
-
Potential
energy
+
(CH3)3C + Cl
(CH3)3CCl
Reaction coordinate
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
25C
(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
carbocation
capture
H
+
(CH3)3C
O
+
H
R+
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.
H
+
(CH3)3C
O
+
H
Rate-determining step is unimolecular
dissociation of alkyloxonium ion.
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
H 3C
+
C
H
H
Ethyl cation
(a primary carbocation)
is more stable than CH3+
Carbocations
H 3C
+
C
C H3
H
Isopropyl cation
(a secondary carbocation)
is more stable than CH3CH2+
Carbocations
H 3C
+
C
C H3
C H3
tert-Butyl cation
(a tertiary carbocation)
is more stable than (CH3)2CH+
Inductive Stabilization of
Cations
+
+
positively charged
carbon pulls
electrons in bonds
closer to itself
Inductive Stabilization
+
+
+
+
positive charge is
"dispersed ", i.e., shared
by carbon and the
three atoms attached
to it
Inductive Stabilization
+
+
+
+
electrons in CC
bonds are more
polarizable than those
in CH bonds;
therefore, alkyl groups
stabilize carbocations
better than H.
Electronic effects transmitted through
bonds are called "inductive effects."
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
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
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.
Slow step is:
+
R O
H
+
R
H
+ 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.
+
C H3
+
H
O
H
Eact
+
H
C H3 + O
H
+
C H3 O
H
H
Smaller activation energy for formation of
primary carbocation.
+
RCH2
+
H
O
H
Eact
+
RCH2 + O
H
H
+
RCH2 O
H
H
Activation energy for formation of secondary
carbocation is less than that for formation of
primary carbocation.
+
R2CH
+
H
O
H
H
+
R2CH O
H
Eact
+
R2CH + O
H
H
Activation energy for formation of tertiary
carbocation is less than that for formation of
secondary carbocation.
+
R 3C
+
H
O
H
H
+
R 3C O
H
Eact
+
R 3C + O
H
H
Preparation of Alkyl Halides
OH
+
+
HCl
HBr
25C
80-100C
Br
OH
OH +
HBr
+
HOH
+
Cl
HOH
120C
Br
+
HOH
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).
OH +
HBr
120C
Br
+
HOH
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
OH +
HBr
120C
Br
+
HOH
Mechanism of SN2 Reaction
Step 1:
Proton transfer from HBr to 1-heptanol
H Br
O
H
Fast, bimolecular
+
OH
H
heptyloxonium ion
-
Br
Mechanism of SN2 Reaction
Step 2:
Reaction of alkyloxonium ion with bromide
ion.
Br-
O+ H
H
Slow, bimolecular
Br +
1-bromoheptane
O
H
H
+
Br
C H2
OH2
CH3(CH2)4CH2
proton
transfer
ROH
+
ROH2
RX
Other Methods of Converting
Alcohols to Alkyl Halides
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%)
Halogenation of Alkanes
Energetics:
RH + X2 RX + HX
explosive for F2
exothermic for Cl2 and Br2
endothermic for I2
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
This is an example of a radical reaction
Free Radicals
A free radical contains unpaired electrons
Examples: O2
NO
Cl
..
:O
.
.
:N
..
O:
.
..
O:
..
: Cl .
..
Free 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.
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.
3 > 2 > 1 > methyl
Alkyl Radicals
H
.
C
H 3C
H
less stable
than
H
Methyl radical
H 3C
less stable
than
.
C
C H3
H
Isopropyl radical
(secondary)
.
C
H
H
Ethyl radical
(primary)
H 3C
less stable
than
.
C
C H3
C H3
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 waysheterolytically 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 energy 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 Radical
Stability
Bond-dissociation energy measurements tell
us that isopropyl radical is 13 kJ/mol more
stable than propyl.
.
CH3CH2CH2
.
+
CH3CHCH3
H.
+
410
397
H.
CH3CH2CH3
Measures of Radical Stability
Bond-dissociation energy measurements tell
us that tert-butyl radical is 30 kJ/mol more
stable than isobutyl.
.
(CH3)2CHCH2
+
H.
410
(CH3)3CH
380
(CH3)3C .
+
H.
Mechanism of the
Chlorination of Methane
Free-radical chain mechanism.
Initiation step:
.. ..
: Cl: Cl:
.. ..
..
..
: Cl . + . Cl :
..
..
The initiation step "gets the reaction going"
by producing free radicalschlorine 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 the
Chlorination of Methane
First propagation step:
H 3C : H
+
..
. C l:
..
Second propagation step:
.. ..
H3C . + : Cl: Cl:
.. ..
H 3C .
+
..
H : Cl:
..
..
..
H3C: Cl: + . Cl:
..
..
Mechanism of the
Chlorination of Methane
First propagation step:
H 3C : H
+
..
. C l:
..
Second propagation step:
.. ..
H3C . + : Cl: Cl:
.. ..
.. ..
H3C : H + : Cl: Cl:
.. ..
H 3C .
+
..
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:
H 3C : H
+
..
. C l:
..
Second propagation step:
.. ..
H3C . + : Cl: Cl:
.. ..
.. ..
H3C : H + : Cl: Cl:
.. ..
H 3C .
+
..
H : Cl:
..
..
..
H3C: Cl: + . Cl:
..
..
..
..
H3C: Cl: + H : Cl:
..
..
Termination Steps
stop chain reaction by consuming free radicals
..
H 3C . + . C l :
..
..
H3C: Cl:
..
hardly any product is formed by termination step
because concentration of free radicals at any
instant is extremely low
Chlorination Other Alkanes
can be used to prepare alkyl chlorides
from alkanes in which all of the hydrogens
are equivalent to one another
CH3CH3 + Cl2
420C
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.
Chlorination of Alkanes
Chlorination of butane gives a mixture of
1-chlorobutane and 2-chlorobutane.
(28%)
CH3CH2CH2CH2Cl
CH3CH2CH2CH3
C l2
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 Hydride
Abstraction
divide by 4.6
4.6
4.6
18
4.6
18%
4.6%
=1
= 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%)
C H3
CH3CCH2Cl
C H3
CH3CCH3
H
H
Cl2
h
C H3
(37%)
CH3CCH3
Cl
Percentage of product that results from
replacement of indicated hydrogen
7.0%
37%
Chlorination of Alkanes
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
Hologenation
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 Applications
Cl
C l2
h
(64%)
Chlorination is useful for synthesis only when
all of the hydrogens in a molecule are equivalent.
Synthetic Applications
H
CH3CCH2CH2CH3
C H3
Br2
h
Br
CH3CCH2CH2CH3
C H3
(76%)
Bromination is highly selective for substitution
of tertiary hydrogens.
Major synthetic application is in synthesis of
tertiary alkyl bromides.
Summary
Summary: Chapter 4
4.1
Functional Groups
4.2
IUPAC Nomenclature of Alkyl Halides
4.3
IUPAC Nomenclature of Alcohols
4.4
Classes of Alcohols and Alkyl Halides
4.5
Bonding in Alcohols and Alkyl Halides
4.6
Physical Properties of Alcohols and Alkyl Halides
4.7
Preparation of Alcohols and Alkyl Halides
4.8
Mechanism of the reaction of Alcohols with HX
4.9
Potential Energy Diagrams for the SN1 Reaction
4.10
Structure of Carbocations
Summary
Summary: Chapter 4
4.11
Effect of Alcohol Structure on Reaction Rate
4.12
The SN2 Mechanism
4.13
Other Methods of Converting Alcohols to Alkyl Halides
4.14
Halogenation of Alkanes
4.15
Chlorination of Methane
4.16
Structure and Stability of Free Radicals
4.17
Mechanism of Methane Chlorination
4.18
Halogenation of Higher Alkanes