chapter 4 alcohols and alkyl halides

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


O

C

Carbonyl group

O

C

Ketone

R R'


O

C

Carbonyl group

O

C

Carboxylic acid

R OH


O

C

Carbonyl group

O

C

Ester

R OR'


O

C

Carbonyl group

O

C

Amide

R NH2


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.


CH3CH2CH2CH2CH2F CH3CHCH2CH2CH3

Br1-Fluoropentane

3-Iodopentane

2-BromopentaneCH3CH2CHCH2CH3

I


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


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.


CH3CH2OH

CH3CHCH2CH2CH2CH3

OH

CH3CCH2CH2CH3

OH

CH3

Ethanol

2-Hexanol or Hexan-2-ol

2-Methyl-2-pentanolor 2-Methylpentan-2-ol

OH

CH3


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


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

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


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.


46

+78

1.7

CH3CH2OH

Molecularweight

Boilingpoint, °C

Dipolemoment, D

Highest boiling point;strongest intermolecularattractive forces.

Hydrogen bonding isstronger than otherdipole-dipole attractions.


+–

–+

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


But trend is not followed when halogenis fluorine

CH3CH2F -32°C

CH3CHF2 -25°C

CH3CF3 -47°C

CF3CF3 -78°C


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.


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:..

:–+


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:..

:–+


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


+(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.


+(CH3)3C+

tert-Butyl chloride

..(CH3)3C Cl:

..

..Cl::

.. –


+

Cl–

Lewis acid Lewis base

Electrophile Nucleophile

C

CH3

CH3H3C

+ C

H3C

CH3H3C

Cl+

Potentialenergy

Reaction coordinate


(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

++


RR++

carbocation capture

RXRX

(CH3)3C(CH3)3C OO

HH

HH ClCl++ ––

‡‡

‡‡

proton transfer

ROHROHROHROH22

++


RR++

carbocation capture

RXRX

(CH3)3C(CH3)3C++

OO

HH

HH++

‡‡

‡‡

proton transfer

ROHROHROHROH22

++


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.


Electrons in C—Cbonds are more polarizable than thosein C—H bonds; therefore, alkyl groupsstabilize carbocationsbetter than H.


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


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.


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


(CH3)3COH + HCl (CH3)3CCl + H2O

78-88%

73%



87-90%

25°C

80-100°C

120°C

OH + HBr

Br + H2O



87-90%

120°C


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



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:


Cl:..

...H3C : H H : Cl:

..

..H3C .+ +


+ +

+ +


Cl:..

...H3C . :

..Cl

..: Cl

..: ..

H3C : H H : Cl:..

..:..Cl..

: Cl..

: ..H3C

..

..: Cl:

H3C..

..: Cl:


Almost all of the product is formed by repetitivecycles of the two propagation steps.

Cl:..

...H3C : H H : Cl:

..

..H3C .+ +


+ +

+ +


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

chapter 4 alcohols and alkyl halides

Documents

alkyl groups

number chain

lowest number

separate words alkyl

group halogen

kinds of iupac nomenclature

longest chain

carbonyl group4