FJ / Chemistry Unit, KMPk / Mac 2006 1

CHAPTER

HALOALKANES

15

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15.1 : Introduction

Haloalkanes or alkyl halides - compounds that contains halogen atom bonded to an sp3 hybridized carbon atom.

General formula : R-X or CnH2n+1 X (acyclic)

or CnH2n-1 X (cyclic)

where X : halogen atom (F, Cl, Br or I)

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15.1.1 : Classification of Haloalkanes Haloalkanes are classified according to

the nature of carbon atom bonded to the halogen. General Formula Classification

methyl halide- halogen is bonded to

methyl group

Primary (10) halide- halogen is bonded to

10 carbon atom

Secondary (20) halide- halogen is bonded to

20 carbon atom

R CH2 X

R CH X

R

CH3X

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R C X

R

R

General Formula Classification

Tertiary (30) halide- halogen is bonded to 30

carbon atom

Aryl halide- halogen is bonded to

aromatic ring

** Not a aryl halide

X

CH2X

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Example :

Classify the following haloalkanes :

No. Haloalkanes Classification

i. 10

ii. 20

iii. 30

iv. 30

CH3CH2Br

CH3CH(Cl)CH3

(CH3)3C(Br)

ClH3C

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15.1.2 : IUPAC Nomenclature

Haloalkanes are named as alkanes with halogen as substituents.

Locate and number the parent chain from the direction that gives the substituent encountered first the lower number.

Show halogen substituents by the

prefixes flouro-, chloro-, bromo- and iodo-, and list them in alphabetical order along with other substituents.

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Example :

i.

ii.

CH3CHCH2CH3

Br

BrCH2CH2CHCHCH2CH3

CH3

Cl

2-bromobutane

1-bromo-3-chloro-4-methylhexane

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Example :

iii.

iv.

CH3CH2CH2CHCH2CH2CH3

CH2CH2F

4-(2-flouroethyl)heptane

H3C CH3Cl

2-chloro-1,1-dimethylcyclopentane

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Example :

v.

vi. vii.

4-bromocyclohexene

(chloromethyl)benzene

CH2Cl

Br

ClCH3

2-chlorotoluene

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15.1.3 : Structure of Haloalkane The carbon – halogen bond in haloalkene is polar

because halogens is more electronegative than carbon.

The polar C – X bond causes the carbon bearing the halogen is susceptible to nucleophilic attack.

Haloalkanes are reactive and undergo nucleophilic

substitution and elimination reaction.

δ+ δ- electrophilic siteC X

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15.2 : Chemical Properties15.2.1 : Nucleophilic Substitution

Reaction

Haloalkanes undergo nucleophilic substitution reactions in which the halogen atom is replaced by a nucleophile.

In this reaction, the nucleophile attacks the partially positive charge (δ+) carbon atom bonded to the halogen (δ-).

General reaction :

R X + Nu: R Nu + X:_ __ _

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(a) : Hydrolysis of Haloalkane with Aqueous Solution of NaOH (H2O/NaOH)

Alkaline hydrolysis is carried out by boiling R-X with NaOH(aq) to form alcohol.

Example :

H2OR X + NaOH R OH + NaX__

H2OCH3

_CH3 C Br + NaOH_

CH3 CH3

_CH3 C OH + NaBr_CH3

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(b) : Reaction of Haloalkane with Potassium Cyanide (KCN)

When R-X is refluxed with KCN in alcohol, the halogen atom is substituted by the CN- to produce a nitrile compound.

Example :

refluxalcoholR X + CN R CN + X_ __ _

CH3CH2Br + KCN CH3CH2CN + KBralcoholreflux

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(c) :Reaction of Haloalkane with Ammonia (NH3)

When R-X is heated with excess concentrated NH3, the halogen atom is replaced by the amino group, NH2

-.

Example :

(amine)

CH3CH2Cl + excess NH3 CH3CH2NH2 + NH4+Cl_

NH3R X RNH3+X_ R NH 2 + NH4+X

___ NH3

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15.2.2 :Mechanisms of Nucleophilic Substitution Reaction

They are 2 important mechanisms for the substitution reaction:

(A). Unimolecular Nucleophilic Substitution Reaction (SN1)

(B). Bimolecular Nucleophilic Substitution Reaction (SN2)

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(A) : Unimolecular Nucleophilic Substitution Reaction (SN1)

The term unimolecular means there is only one molecule involved in the transition state of the rate-limiting step.

SN1 reactions are governed mainly by the relative stabilities of carbocations.

Relative reactivities of haloalkanes in an SN1 reaction :

R-X < R-X < R-X 1o 2o 3o

increasing reactivity

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The rate of SN1 reaction does not depend on the concentration of nucleophile.

The rate depends only on the concentration of the substrate, alkyl halide.

∴ rate = k [R3C-X]

* SN1 is a first order reaction

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The mechanism of SN1 reaction involves 2 steps.

Step 1 : Formation of a carbocation (rate determining step)

3o alkyl halide carbocation halide ion

Step 2 : Nucleophilic attack on the carbocation

R

_R C X_R R

_R C + X_

R

+slow

R

_R C Nu_R

fast+

R

_R C + Nu:_

R

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Example 1 : Reaction of 2-bromo-2-methylpropane with H2O.

SN1 mechanism :

Step 1 : Formation of a carbocation

CH3

_CH3 C Br + H2O_CH3 CH3

_CH3 C OH + HBr_

CH3

+slowCH3

_CH3 C Br_

CH3 CH3

_CH3 C + Br_

CH3

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Step 2 : Nucleophilic attack on the carbocation

Loss of proton, H+ to solvent

fastCH3

_CH3 C + H2O

CH3

+

CH3

_CH3 C O_CH3 H

H+

CH3

_CH3 C OH + H3O+_CH3

H

HCH3

_CH3 C O + H2O_

CH3+

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Example 2 :Write the mechanism for the following reaction.

SN1 Mechanism :

Step 1 : Formation of carbocation

CH3

_CH3 C CH2Br + NaOH(aq)_CH3 CH3

_CH3 C CH2CH3 + NaBr_

OH

slowCH3

_CH3 C CH2 Br_

CH3

_ _

CH3

_CH3 C CH2 + Br_CH3

+

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Rearrangement :

Step 2 : Nucleophilic attack on the carbocation

1,2-methyl shift+

CH3_CH3 C CH2

_

CH3 CH3

_CH3 C CH2 _CH3

+

_+

CH3_CH3 C CH2 + OH

_

CH3

_CH3 C CH2CH3_CH3

OH

fast

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Exercise 1 : (Feb 2003)

Write a reasonable structures of products formed when 1-iodobutane reacts with

i. KCNii. NaOH/H2Oiii. excess NH3

Write the mechanism for the reaction in (ii).

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Exercise 2 : (Mac 2002)

The structure of compound A is as follows:

i. Give IUPAC name for A

ii.Compound A react with OH- forming an alcohol. Write the mechanism for the formation of this alcohol and name the reaction.

CH3 C Br

CH3

CH3

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(B) :Bimolecular Nucleophilic Substitution Reaction (SN2)

The term bimolecular means that the transition state of the rate limiting step involves the collision of two molecules.

SN2 reactions are governed mainly by steric factors (steric effect).

Steric effect- is an effect on relative rates caused by the space-filling properties of those parts of a molecule attached at or near to the reacting site.

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The reactivity on SN2 reaction depends on the size of atoms or groups attached to a C – X.

The presence of bulky alkyl groups will prevent the nucleophilic attack and slow the reaction rate.

Relative reactivities of haloalkanes in an SN2 reaction :

R-X < R-X < CH3-X

2o 1o

increasing reactivity

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The rate of reaction depends on the concentration of the haloalkane and the concentration of nucleophile.

∴ rate = k [R-X] [Nu:-]

* SN2 is a second order reaction.

The mechanism of SN2 occurs in a single step.

General Mechanism :

slow fast

H

R

C X Nu C X Nu:- H

H H

R

Nu C + X-R

H

transition state

H

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In SN2 reaction, the nucleophile attacks from the back side of the electrophilic carbon, that is, from the side directly opposite bonded to the halogen.

The transition state involves partial bonding between the attacking nucleophile and the haloalkane.

Back-side attack causes the product formed has inverse configuration from the original configuration.

* turns the tetrahedron of the carbon atom inside out, like umbrella caught by the wind.

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Example 3 :Reaction of ethyl bromide with aqueous sodium hydroxide.

SN2 Mechanism :

CH3CH2Br + NaOH(aq) CH3CH2OH + NaBr

slow fast

HH

:OH- C Br OH C BrHH

CH3

OH C + Br- CH3

HH

CH3

transition state

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Comparison of SN1 and SN2 Reactions

SN1 SN2

A two-step mechanism A one-step mechanism

A unimolecular rate-determining step

A bimolecular rate-determining step

Second order : rate = k [RX] [Nu]

First order : rate = k [RX]

Strong nucleophile Weak nucleophile

Carbocation rearrangement

No carbocation rearrangement

Reactivity order :3o > 2o > 1o

Reactivity order : methyl > 1o > 2o

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15.2.3 : Elimination Reaction (dehydrohalogenation of haloalkanes)

Halogen can be removed from one carbon of a haloalkane and hydrogen from an adjacent carbon to form a carbon-carbon double bond in the presence of a strong base.

General reaction :

alkene

basehaloalkane

H_ C C + :B C C + X_ _ _ __ _

X

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Example :

i.

ii.

BrCH3CH2OH

CH3CH2ONa

CH3

CH3+

CH2

major minor

CH3CHCHCH3 CH3C CHCH3

Br

CH3 CH3CH3CH2OHCH3CH2ONa

+

CH3CHC CH2

CH3

major

minor

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15.2.4 : Synthesis of Organomagnesium Compound ( Grignard Reagent ) Prepared by the reaction of haloalkanes with

magnesium metal in anhydrous ether as a solvent.

Example :i.

ii.

etherR-X + Mg R-MgXGrignard Reagent

( alkylmagnesium

halide)etherCH3CH2CH2Br + Mg CH3CH2CH2MgBr

etherCl + Mg MgCl

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15.2.4.1 : Synthesis of Alkanes, Alcohols and Carboxylic Acids from Grignard Reagents.

The Grignard reagents undergo many reactions that make them useful as a starting materials in the synthesis of other organic compounds.

(i). Synthesis of alkane

The Grignard reagent is hydrolyzed to an alkane when warmed with H2O.

RMgX + H2O R-H + Mg(OH)XH+

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Example :

i.

ii.

iii.

H+CH3CH2MgBr + H2O CH3CH3 + Mg(OH)Br

H2O/H+CH2MgCl CH3

+ Mg(OH)Cl

H+CH3CH-MgBr + H2O CH3CH2CH3 + Mg(OH)Br

CH3

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(ii). Synthesis of 1o alcohol

Methanal reacts with the Grignard reagent, followed by the hydrolysis produces primary alcohol.

R-MgX + H-C-H R-C-OMgX

O H

H

H2O,H+

R-C-OH + Mg(OH)X

H

H

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Example :

i.

ii.

CH3MgBr + H-C-H

OH3O+

CH3-C-OH + Mg(OH)Br

H

H

OH3O+MgBr

+ H-C-H + Mg(OH)BrCH2OH

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(iii). Synthesis of 2o alcohol

Grignard reagent reacts with aldehydes to produce secondary alcohol.

R-MgX + H-C-R' R-C-OMgX

O R'

H

H2O,H+

R-C-OH + Mg(OH)X

R'

H

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Example :

i.

ii.

OH2O/H+MgCl

+ CH3-C-H + Mg(OH)ClC-OH

CH3

H

O

H2O/H+

H

CH3CH2MgBr + CH3CH2-C-H

CH3CH2-C-CH2CH3 + Mg(OH)Br

OH

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(iv). Synthesis of 3o alcohol

Grignard reagent reacts with ketons to produce the tertiary alcohol.

R-MgX + R'-C-R" R-C-OMgX

O R'

R"

H2O,H+

R-C-OH + Mg(OH)X

R'

R"

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Example :

i.

ii.

O

H2O/H+

CH3

CH3CH2MgBr + CH3-C-CH3

CH3CH2-C-CH3 + Mg(OH)Br

OH

MgCl+ CH3-C-CH3 + Mg(OH)ClH3O+ C-OH

CH3

CH3

O

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(v). Synthesis of carboxylic acid

Grignard reagent reacts with carbon dioxide (CO2) followed by hydrolysis to form carboxylic acid.

RMgX + O C O R-C-O-MgX

O

O

R-C-O-MgX + H2O R-C-OH + Mg(OH)X

OH+

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Example :

CH3CH2MgI + CO2 CH3CH2-C-O-MgI

O

CH3CH2-C-O-MgI + H2O CH3CH2COH + Mg(OH)I

OH+ O

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15.2.5 : Wurtz Reaction

Reaction of haloalkane (RX) with an alkali metal (usually Na) to synthesise longer alkane.

i. To prepare an even number of carbon atoms alkane

2R−X + 2Na → R−R + 2NaX

Example:

2CH3CH2Br + 2Na → CH3CH2CH2CH3 + 2NaBr

dry ether

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ii.To prepare a odd number of carbon atoms alkane

R−X + R’−X + 6Na → R−R + R−R’ + R’−R’

+ 6NaX

Example:

CH3CH2Br + CH3Br + 6Na → CH3CH2CH2CH3 + CH3CH2CH3

+ CH3CH3 + 6NaBr

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15.2.6 : Importance of Haloalkanes as Inert Substance

Haloalkanes Uses

CCl4

(carbon tetrachloride)

solvent for dry cleaning, spot

removing

CHCl3

(chloroform)

solvent for cleaning and

degreasing work

CF2Cl2 , Freon-12

(dichlorodifluoromethane)

propellants in aerosol sprays

CFC(chloroflourocarbons)

refrigerant gas

DDT(DichloroDiphenylTrichloroet

hane)

insecticide protects