chapter 4 alcohols and alkyl halides
DESCRIPTION
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 + H 2 O (2) alkane + halogen - PowerPoint PPT PresentationTRANSCRIPT
Chapter 4Alcohols and Alkyl
Halides
(1) alcohol + hydrogen halide
ROH + HX RX + H2O
(2) alkane + halogen
RH + X2 RX + HX
Both are substitution reactions.
Overview of Chapter
This chapter introduces chemical reactions and their mechanisms by focusing on two reactionsthat yield alkyl halides.
4.1Functional Groups
A structural unit in a molecule responsible for itscharacteristic behavior under a particular set ofreaction conditions
Functional Group
Alcohol ROH
Alkyl halide RX (X = F, Cl, Br, I)
Amine primary amine: RNH2
secondary amine: R2NH
tertiary amine: R3N
Families of Organic Compoundsand their Functional Groups
Ether ROR'
Nitrile RCN
Nitroalkane RNO2
Sulfide RSR'
Thiol RSH
Epoxide C C
O
Families of Organic Compoundsand their Functional Groups
O
C
Carbonyl group
O
C
Acyl group
R
Many Classes of Organic Compounds Contain a Carbonyl Group
O
C
Carbonyl group
O
C
Aldehyde
R H
Many Classes of Organic Compounds Contain a Carbonyl Group
O
C
Carbonyl group
O
C
Ketone
R R'
Many Classes of Organic Compounds Contain a Carbonyl Group
O
C
Carbonyl group
O
C
Carboxylic acid
R OH
Many Classes of Organic Compounds Contain a Carbonyl Group
O
C
Carbonyl group
O
C
Ester
R OR'
Many Classes of Organic Compounds Contain a Carbonyl Group
O
C
Carbonyl group
O
C
Amide
R NH2
Many Classes of Organic Compounds Contain a Carbonyl Group
4.2IUPAC Nomenclature
of Alkyl Halides
The two that are most widely used are:functional class nomenclaturesubstitutive nomenclature
Both types can be applied to alcohols andalkyl halides.
IUPAC Nomenclature
There are several kinds of IUPAC nomenclature.
Name the alkyl group and the halogen asseparate words (alkyl + halide).
Functional Class Nomenclature of Alkyl Halides
CH3F CH3CH2CH2CH2CH2Cl
CH3CH2CHCH2CH2CH3
Br
H
I
Name the alkyl group and the halogen asseparate words (alkyl + halide).
Functional Class Nomenclature of Alkyl Halides
CH3F CH3CH2CH2CH2CH2Cl
CH3CH2CHCH2CH2CH3
Br
Methyl fluoride Pentyl chloride
1-Ethylhexyl bromide Cyclohexyl iodide
H
I
Name as halo-substituted alkanes.
Number the longest chain containing thehalogen in the direction that gives the lowestnumber to the substituted carbon.
Substitutive Nomenclature of Alkyl Halides
CH3CH2CH2CH2CH2F CH3CHCH2CH2CH3
Br
CH3CH2CHCH2CH3
I
Name as halo-substituted alkanes.
Number the longest chain containing thehalogen in the direction that gives the lowestnumber to the substituted carbon.
Substitutive Nomenclature of Alkyl Halides
CH3CH2CH2CH2CH2F CH3CHCH2CH2CH3
Br1-Fluoropentane
3-Iodopentane
2-BromopentaneCH3CH2CHCH2CH3
I
Substitutive Nomenclature of Alkyl Halides
Halogen and alkyl groupsare of equal rank when it comes to numberingthe chain.
Number the chain in thedirection that gives the lowest number to thegroup (halogen or alkyl)that appears first.
CH3
Cl Cl
CH3
Substitutive Nomenclature of Alkyl Halides
5-Chloro-2-methylheptane
2-Chloro-5-methylheptane
CH3
Cl Cl
CH3
4.3IUPAC Nomenclature
of Alcohols
Name the alkyl group and add "alcohol" as aseparate word.
Functional Class Nomenclature of Alcohols
CH3CH2OH
CH3CHCH2CH2CH2CH3
OH
CH3CCH2CH2CH3
OH
CH3
Name the alkyl group and add "alcohol" as aseparate word.
Functional Class Nomenclature of Alcohols
CH3CH2OH
CH3CHCH2CH2CH2CH3
OH
CH3CCH2CH2CH3
OH
CH3
Ethyl alcohol
1-Methylpentyl alcohol
1,1-Dimethylbutylalcohol
Name as "alkanols." Replace -e ending of alkanename with -ol.
Number chain in direction that gives lowest numberto the carbon that bears the —OH group.
Substitutive Nomenclature of Alcohols
CH3CH2OH
CH3CHCH2CH2CH2CH3
OH
CH3CCH2CH2CH3
OH
CH3
Name as "alkanols." Replace -e ending of alkanename with -ol.
Number chain in direction that gives lowest numberto the carbon that bears the —OH group.
Substitutive Nomenclature of Alcohols
CH3CH2OH
CH3CHCH2CH2CH2CH3
OH
CH3CCH2CH2CH3
OH
CH3
Ethanol
2-Hexanol or Hexan-2-ol
2-Methyl-2-pentanolor 2-Methylpentan-2-ol
OH
CH3
Substitutive Nomenclature of Alcohols
Hydroxyl groups outrank alkyl groups when it comes to numberingthe chain.
Number the chain in thedirection that gives the lowest number to thecarbon that bears theOH group
CH3
OH
Substitutive Nomenclature of Alcohols
6-Methyl-3-heptanol
5-Methyl-2-heptanol
OH
CH3
CH3
OH
4.4Classes of Alcohols
and Alkyl Halides
Alcohols and alkyl halides are classified asprimarysecondarytertiary
according to their "degree of substitution."
Degree of substitution is determined by countingthe number of carbon atoms directly attached tothe carbon that bears the halogen or hydroxyl group.
Classification
CH3CH2CH2CH2CH2F
CH3CHCH2CH2CH3
Br
primary alkyl halide
secondary alkyl halide
Classification
CH3CCH2CH2CH3
OH
CH3
tertiary alcohol
H
OH
secondary alcohol
4.54.5
Bonding in AlcoholsBonding in Alcohols
and Alkyl Halidesand Alkyl Halides
H
HH
H
alcohols and alkyl halides are polar
= 1.7 D = 1.9 D
H
H
C O
H
H
C Cl++
––––++
++
Dipole Moments
alcohols and alkyl halides are polar
= 1.7 D = 1.9 D
Dipole Moments
+ –
– +
+ – + – + –
Dipole-Dipole Attractive Forces
+ –
– +
+ – + – + –
Dipole-Dipole Attractive Forces
Boiling point
Solubility in water
Density
4.6
Physical Properties of
Alcohols and Alkyl Halides:
Intermolecular Forces
44 48 46
-42 -32 +78
0 1.9 1.7
CH3CH2CH3 CH3CH2F CH3CH2OH
Molecularweight
Boilingpoint, °C
Dipolemoment, D
Effect of Structure on Boiling Point
CH3CH2CH3
Molecular weight 44
Boilingpoint, °C -42
Dipolemoment, D 0
Effect of Structure on Boiling Point
Intermolecular forcesare weak.
Only intermolecularforces are induceddipole-induced dipoleattractions.
48
-32
1.9
CH3CH2F
Molecularweight
Boilingpoint, °C
Dipolemoment, D
A polar molecule;therefore dipole-dipoleand dipole-induceddipole forces contributeto intermolecular attractions.
Effect of Structure on Boiling Point
46
+78
1.7
CH3CH2OH
Molecularweight
Boilingpoint, °C
Dipolemoment, D
Highest boiling point;strongest intermolecularattractive forces.
Hydrogen bonding isstronger than otherdipole-dipole attractions.
Effect of Structure on Boiling Point
+–
–+
Figure 4.4 Hydrogen bonding in ethanol
Figure 4.4 Hydrogen bonding in ethanol
Boiling point increases with increasingnumber of halogens
CH3Cl -24°C
CH2Cl2 40°C
CHCl3 61°C
CCl4 77°C
Compound Boiling Point
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 halogenis fluorine
CH3CH2F -32°C
CH3CHF2 -25°C
CH3CF3 -47°C
CF3CF3 -78°C
Compound Boiling Point
But trend is not followed when halogenis fluorine
CH3CH2F -32°C
CH3CHF2 -25°C
CH3CF3 -47°C
CF3CF3 -78°C
Compound Boiling Point
Fluorine is not very polarizable and induced dipole-induced dipole forces decrease with increasing fluorine substitution.
Solubility in water
Alkyl halides are insoluble in water.
Methanol, ethanol, isopropyl alcohol arecompletely miscible with water.
The solubility of an alcohol in waterdecreases with increasing number of carbons (compound becomesmore hydrocarbon-like).
Figure 4.5 Hydrogen Bonding Between Ethanol and Water
Density
Alkyl fluorides and alkyl chlorides areless dense than water.
Alkyl bromides and alkyl iodides are more dense than water.
All liquid alcohols have densities of about 0.8 g/mL.
4.74.7
Preparation of Alkyl Halides fromPreparation of Alkyl Halides from
Alcohols and Hydrogen HalidesAlcohols and Hydrogen Halides
ROH + HX RX + H2O
ROH + HX RX + HOH
Hydrogen halide reactivity
HF HCl HBr HI
Reaction of Alcohols with Hydrogen Halides
least reactive most reactive
Alcohol reactivity
CH3OH RCH2OH R2CHOH R3COHMethanol Primary Secondary Tertiary
ROH + HX RX + HOH
Reaction of Alcohols with Hydrogen Halides
least reactive most reactive
(CH3)3COH + HCl (CH3)3CCl + H2O
78-88%
25°C
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O
87-90%
120°C
Preparation of Alkyl Halides
73%
80-100°C
OH + HBr
Br + H2O
CH3CH2CH2CH2OH CH3CH2CH2CH2Br
NaBrH2SO4
70-83%heat
A mixture of sodium bromide and sulfuric acid may be used in place of HBr.
Preparation of Alkyl Halides
4.84.8Mechanism of the Reaction of Mechanism of the Reaction of
Alcohols with Hydrogen Halides: Alcohols with Hydrogen Halides: Hammond’s PostulateHammond’s Postulate
A mechanism describes how reactants areconverted to products.
Mechanisms are often written as a series ofchemical equations showing the elementary steps.
An elementary step is a reaction that proceedsby way of a single transition state.
Mechanisms can be shown likely to be correct,but cannot be proven correct.
About mechanisms
the generally accepted mechanism involves three elementary steps.
Step 1 is a Brønsted acid-base reaction.
About mechanisms
(CH3)3CCl + H2O(CH3)3COH + HCl25°C
tert-Butyl alcohol tert-Butyl chloride
For the reaction:
Step 1: Proton Transfer ..H Cl:..
+
fast, bimolecular
..
H
O: +(CH3)3C
H
Cl:..
:–+
tert-Butyloxonium ion
H
O:..
(CH3)3C
Like proton transferfrom a strong acid towater, proton transferfrom a strong acid toan alcohol is normallyvery fast.
..H Cl:..
+
fast, bimolecular
..
H
O: +(CH3)3C
H
Cl:..
:–+
tert-Butyloxonium ion
H
O:..
(CH3)3C
Two molecules reactin this elementarystep; therefore it is bimolecular.
Step 1: Proton Transfer
..H Cl:..
+
fast, bimolecular
..
H
O: +(CH3)3C
H
Cl:..
:–+
tert-Butyloxonium ion
H
O:..
(CH3)3C
Step 1: Proton Transfer
Species formed inthis step (tert-butyloxonium ion) isan intermediate inthe overall reaction.
Potentialenergy
Reaction coordinate
Potential Energy Diagram for Step 1
H Cl
(CH3)3CO
H
(CH3)3COH + H—Cl+ Cl –
+(CH3)3CO
H
H
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).
Hammond's Postulate
+(CH3)3C O
H
:
H
+
slow, unimolecular
(CH3)3C O
H
:
H
:
tert-Butyl cation
+
Step 2: Carbocation FormationDissociation of the alkyloxonium ion involvesbond-breaking, withoutany bond-making to compensate for it. Ithas a high activationenergy and is slow.
+(CH3)3C O
H
:
H
+
slow, unimolecular
(CH3)3C O
H
:
H
:
tert-Butyl cation
+
A single moleculereacts in this step;therefore, it isunimolecular.
Step 2: Carbocation Formation
+(CH3)3C O
H
:
H
+
slow, unimolecular
(CH3)3C O
H
:
H
:
tert-Butyl cation
+
Step 2: Carbocation Formation
The product of thisstep is a carbocation.It is an intermediatein the overall process.
Potentialenergy
Reaction coordinate
Potential Energy Diagram for Step 2
+(CH3)3CO
H
H
(CH3)3C + H2O+
(CH3)3C O
H
H
Note the “carbocation character” in the transition state:
Transition State for Carbocation Formation
(CH3)3C O
H
H
Whatever stabilizes carbocations will stabilize the transition state leading to them.
The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation.
A carbocation is a cation in which carbon has6 valence electrons and a positive charge.
CR R
R
+
Carbocation
Figure 4.8 Structure of tert-Butyl Cation
Positively charged carbon is sp2 hybridized.
All four carbons lie in same plane.
Unhybridized p orbital is perpendicular to plane of four carbons.
CC
CHCH33
HH33CC
CHCH33
++
fast, bimolecular
+(CH3)3C+
tert-Butyl chloride
..(CH3)3C Cl:
..
..Cl::
.. –
Step 3: Carbocation Capture
Bond formation betweenthe positively chargedcarbocation and the negatively charged chloride ion is fast.
Two species are involved in this step.Therefore, this stepis bimolecular.
fast, bimolecular
The carbocation is an electrophile.Chloride ion is anucleophile.
This is a Lewis acid-Lewis base reaction.The carbocation is theLewis acid; chlorideion is the Lewis base.
Step 3: Carbocation Capture
+(CH3)3C+
tert-Butyl chloride
..(CH3)3C Cl:
..
..Cl::
.. –
Step 3: Carbocation Capture
+
Cl–
Lewis acid Lewis base
Electrophile Nucleophile
C
CH3
CH3H3C
+ C
H3C
CH3H3C
Cl+
Potentialenergy
Reaction coordinate
Potential Energy Diagram for Step 3
(CH3)3C + Cl–+
(CH3)3C Cl
(CH3)3C—Cl
4.94.9Potential Energy Diagrams for Potential Energy Diagrams for
Multistep Reactions:Multistep Reactions:The SThe SNN1 Mechanism1 Mechanism
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.
Potential Energy Diagram - Overall
(CH3)3COH + HCl (CH3)3CCl + H2O25°C
proton transfer
ROHROHROHROH22
++
carbocation formation
RR++
carbocation capture
RXRX
proton transfer
ROHROHROHROH22
++
carbocation formation
RR++
carbocation capture
RXRX
(CH3)3C(CH3)3C OO
HH
HH ClCl++ ––
‡‡
‡‡
proton transfer
ROHROHROHROH22
++
carbocation formation
RR++
carbocation capture
RXRX
(CH3)3C(CH3)3C++
OO
HH
HH++
‡‡
‡‡
proton transfer
ROHROHROHROH22
++
carbocation formation
RR++
carbocation capture
RXRX
(CH3)3C(CH3)3C++
ClCl––
‡‡‡‡
The mechanism just described is an example of an SN1 process.
SN1 stands for substitution-nucleophilic-
unimolecular.
The molecularity of the rate-determining step defines the molecularity of theoverall reaction.
Mechanistic Notation
The molecularity of the rate-determining step defines the molecularity of the overall reaction.
(CH3)3C+
O
H
H+
Rate-determining step is unimoleculardissociation of alkyloxonium ion.
Mechanistic Notation
4.104.10
Structure, Bonding, and StabilityStructure, Bonding, and Stability
of Carbocationsof Carbocations
Carbocations
Most carbocations are too unstable to beisolated, but occur as reactive intermediates ina number of reactions.
When R is an alkyl group, the carbocation isstabilized compared to R = H.
CR R
R
+
CH H
H
+
Methyl cation
least stable
Carbocations
CH3C H
H
+
Ethyl cation(a primary carbocation)
is more stable than CH3+
Carbocations
CH3C CH3
H
+
Isopropyl cation(a secondary carbocation)
is more stable than CH3CH2+
Carbocations
CH3C CH3
CH3
+
tert-Butyl cation(a tertiary carbocation)
is more stable than (CH3)2CH+
Carbocations
Positively chargedcarbon pullselectrons in bondscloser to itself.
+
Figure 4.14 Stabilization of Carbocations via the Inductive Effect
Positive charge is"dispersed ", i.e., sharedby carbon and thethree atoms attachedto it.
Figure 4.14 Stabilization of Carbocations via the Inductive Effect
Electrons in C—Cbonds are more polarizable than thosein C—H bonds; therefore, alkyl groupsstabilize carbocationsbetter than H.
Figure 4.14 Stabilization of Carbocations via the Inductive Effect
Electronic effects transmitted through bonds are called "inductive effects."
Electrons in this bond can be sharedby positively chargedcarbon because the orbital can overlap with the empty 2porbital of positivelycharged carbon
+
Figure 4.15 Stabilization of Carbocations via Hyperconjugation
Electrons in this bond can be sharedby positively chargedcarbon because the orbital can overlap with the empty 2porbital of positivelycharged carbon
Figure 4.15 Stabilization of Carbocations via Hyperconjugation
Notice that an occupiedorbital of this type isavailable when sp3
hybridized carbon is attached to C+, but is not available when His attached to C+. Therefore, alkyl groupsstabilize carbocationsbetter than H does.
Figure 4.15 Stabilization of Carbocations via Hyperconjugation
4.114.11Effect of Alcohol Structure on Effect of Alcohol Structure on
Reaction RateReaction Rate
The more stable the carbocation, the fasterit is formed.
Tertiary carbocations are more stable thansecondary, 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 fasterthan methanol.
Slow step is: R O
H
H
+ •• R+
+ O
H
H
••••
High activation energy for formation of methyl cation.
CHCH33 OO
HH
HH
++ ••••
CHCH33++
++ OO
HH
HH
••••••••Eact
CHCH33++
OO
HH
HH
••••++
RCHRCH22 OO
HH
HH
++ ••••
RCHRCH22++
++ OO
HH
HH
••••••••
Smaller activation energy for formation ofSmaller activation energy for formation ofprimaryprimary carbocation. carbocation.
RCHRCH22++
OO
HH
HH
••••++
Eact
RR22CHCH OO
HH
HH
++ ••••
RR22CHCH++
++ OO
HH
HH
••••••••
Activation energy for formation of Activation energy for formation of secondarysecondary carbocation is less than that for formation ofcarbocation is less than that for formation ofprimaryprimary carbocation. carbocation.
RR22CHCH++
OO
HH
HH
••••++
Eact
Eact
RR33CC OO
HH
HH
++ ••••
RR33CC++
++ OO
HH
HH
••••••••
Activation energy for formation of Activation energy for formation of tertiarytertiary carbocation is less than that for formation ofcarbocation is less than that for formation ofsecondarysecondary carbocation. carbocation.
RR33CC++
OO
HH
HH
••••++
4.124.12Reaction of Methyl and Primary Reaction of Methyl and Primary
Alcohols with Hydrogen Halides.Alcohols with Hydrogen Halides.The SThe SNN2 Mechanism2 Mechanism
Preparation of Alkyl Halides
(CH3)3COH + HCl (CH3)3CCl + H2O
78-88%
73%
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O
87-90%
25°C
80-100°C
120°C
OH + HBr
Br + H2O
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O
87-90%
120°C
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).
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
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O
120°C
Example
O
CH3(CH2)5CH2
Step 1: Proton transfer from HBr to 1-heptanol
H
..: H Br:
..
..+
+
H
O: +
H
..Br:
..:
–
fast, bimolecular
Heptyloxonium ion
CH3(CH2)5CH2
Mechanism
+
H
O:
H
CH3(CH2)5CH2
Step 2: Reaction of alkyloxonium ion with bromide ion.
Br:: ..
.. –+
1-Bromoheptane
slow, bimolecular
..Br:
..CH3(CH2)5CH2 + O
H
:
H
:
Mechanism
CH2CH2 OH2OH2
++
BrBr––
CH3(CH2)4CH2CH3(CH2)4CH2
proton transfer
ROHROHROHROH22
++
RXRX
‡‡
‡‡
4.134.13More on Activation EnergyMore on Activation Energy
Arrhenius equation
A quantitative relationship between the activation energy, the rate constant, and the temperature:
/actE RTk Ae
A is the preexponential, or frequency factor (related to the collision frequency and geometry), R=8.314 x 10-3 kJ/Kmol, T is in K.
The true activation barrier is known as G‡ and takes into account the enthalpy and entropy of activation.
4.144.14Other Methods for Converting Other Methods for Converting
Alcohols to Alkyl HalidesAlcohols to Alkyl Halides
Reagents for ROH to RX
Thionyl chloride
SOCl2 + ROH RCl + HCl + SO2
Phosphorus tribromide
PBr3 + 3ROH 3RBr + H3PO3
SOCl2CH3CH(CH2)5CH3
OH
K2CO3
CH3CH(CH2)5CH3
Cl
(81%)
(Pyridine often used instead of K2CO3)
(CH3)2CHCH2OH
(55-60%)
(CH3)2CHCH2BrPBr3
Examples
4.154.15
Halogenation of AlkanesHalogenation of Alkanes
RH + XRH + X22 RX + HX RX + HX
Energetics
RH + X2 RX + HX
explosive for F2
exothermic for Cl2 and Br2
endothermic for I2
4.164.16
Chlorination of MethaneChlorination 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
4.174.17Structure and Stability of Free Structure and Stability of Free
RadicalsRadicals
Free Radicals
contain unpaired electrons.. ..
. .O O::Examples: O2
NO N O:..
:.
Cl Cl ...:
..
Alkyl Radicals
Most free radicals in which carbon bearsthe unpaired electron are too unstable to beisolated.
Alkyl radicals are classified as primary,secondary, or tertiary in the same way thatcarbocations are.
CR R
R
.
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.
CR R
R
.
Alkyl Radicals
The order of stability of free radicals is the same as for carbocations.
CH H
H
.
Methyl radical
less stablethan
CH3C H
H
.
Ethyl radical(primary)
CH3C CH3
H
.
Isopropyl radical(secondary)
less stablethan
less stablethan
CH3C CH3
CH3
.
tert-Butyl radical(tertiary)
Alkyl Radicals
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.
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.
– +
Homolytic
Heterolytic
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.
Homolytic
Measures of Free Radical Stability
Bond-dissociation enthalpy measurements tell us that isopropyl radical is 13 kJ/mol more stable than propyl.
410 397
CH3CH2CH3
CH3CH2CH2
+
H
.
.CH3CHCH3
+
H
.
.
Measures of Free Radical Stability
Bond-dissociation enthalpy measurements tell us that tert-butyl radical is 30 kJ/mol more stable than isobutyl.
410
(CH3)3CH
(CH3)2CHCH2
+
H
.
.(CH3)3C
+H
.
.380
4.184.18Mechanism of Methane ChlorinationMechanism of Methane Chlorination
Mechanism of Chlorination of Methane
The initiation step "gets the reaction going"by producing free radicals—chlorine atomsfrom chlorine molecules in this case.
Initiation step is followed by propagationsteps. Each propagation step consumes onefree radical but generates another one.
:..Cl..
: Cl..
: .. ..Cl..
: . :Cl..
...+
Free-radical chain mechanism.
Initiation step:
Cl:..
...H3C : H H : Cl:
..
..H3C .+ +
First propagation step:
Second propagation step:
+ + Cl:..
...H3C . :
..Cl
..: Cl
..: ..
H3C..
..: Cl:
Mechanism of Chlorination of Methane
Cl:..
...H3C : H H : Cl:
..
..H3C .+ +
Second propagation step:
+ +
+ +
First propagation step:
Cl:..
...H3C . :
..Cl
..: Cl
..: ..
H3C : H H : Cl:..
..:..Cl..
: Cl..
: ..H3C
..
..: Cl:
H3C..
..: Cl:
Mechanism of Chlorination of Methane
Almost all of the product is formed by repetitivecycles of the two propagation steps.
Cl:..
...H3C : H H : Cl:
..
..H3C .+ +
Second propagation step:
+ +
+ +
First propagation step:
Cl:..
...H3C . :
..Cl
..: Cl
..: ..
H3C : H H : Cl:..
..:..Cl..
: Cl..
: ..H3C
..
..: Cl:
H3C..
..: Cl:
Termination Steps
stop chain reaction by consuming free radicals
Hardly any product is formed by termination stepbecause concentration of free radicals at anyinstant is extremely low.
+H3C . Cl:..
... H3C
..
..: Cl:
4.194.19
Halogenation of Higher AlkanesHalogenation of Higher Alkanes
CH3CH3 + Cl2 CH3CH2Cl + HCl420°C
(78%)
Cl + Cl2 + HClh
(73%)
Chlorination of Alkanes
can be used to prepare alkyl chlorides from alkanes in which all of the hydrogens are equivalent to one another
Major limitation:
Chlorination gives every possible monochloride derived from original carbonskeleton.
Not much difference in reactivity ofdifferent hydrogens in molecule.
Chlorination of Alkanes
CH3CH2CH2CH3 Cl2
h
Example
Chlorination of butane gives a mixture of1-chlorobutane and 2-chlorobutane.
CH3CH2CH2CH2Cl
CH3CHCH2CH3
Cl
(28%)
(72%)
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
4.6%
4.6%4.6%
18%
18%18%
4.6%
4.6%4.6%
18%
Relative Rates of Hydrogen Atom Abstraction
divide by 4.6
4.6
4.6= 1
4.6
18= 3.9
A secondary hydrogen is abstracted 3.9 times faster than a primary hydrogen by a chlorine
atom.
4.6%18%
Similarly, chlorination of 2-methylbutane gives a mixture of isobutyl chloride and tert-butyl chloride.
H
CH3CCH3
CH3
CH3CCH2Cl
CH3
H
Cl
CH3CCH3
CH3
h
Cl2
(63%)
(37%)
Percentage of Product that Results from Replacement of Indicated Hydrogen
7.0%
37%
377.0
Relative Rates of Hydrogen Atom Abstraction
divide by 7
7= 1
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 skeletonas 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
Chlorination is useful for synthesis only when all of the hydrogens in a molecule are equivalent.
(64%)
Cl2
h
Cl
Synthetic Application of Bromination of an Alkane
Bromination is highly selective for substitution of tertiary hydrogens.
Major synthetic application is in synthesis of tertiary alkyl bromides.
(76%)
Br2
h
H
CH3CCH2CH2CH3
CH3
Br
CH3CCH2CH2CH3
CH3