CHAPTER HALOALKANES15

FJ / Chemistry Unit, KMPk / Mac 2006

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

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

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

15.1.1 : Classification of HaloalkanesHaloalkanes are classified according to the nature of carbon atom bonded to the halogen.

Example :Classify the following haloalkanes :

15.1.2 : IUPAC NomenclatureHaloalkanes 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.

Example :i.

ii.2-bromobutane1-bromo-3-chloro-4-methylhexane

Example :iii.

iv.4-(2-flouroethyl)heptane2-chloro-1,1-dimethylcyclopentane

Example :v.

vi.vii.4-bromocyclohexene(chloromethyl)benzene2-chlorotoluene

15.1.3 : Structure of HaloalkaneThe 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. +-electrophilic site

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 (+) carbon atom bonded to the halogen (-).

General reaction :

(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 :

(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 :

(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)

15.2.2 :Mechanisms of Nucleophilic Substitution ReactionThey are 2 important mechanisms for the substitution reaction:

(A).Unimolecular Nucleophilic Substitution Reaction (SN1)

(B).Bimolecular Nucleophilic Substitution Reaction (SN2)

(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

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

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

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

SN1 mechanism :

Step 1 : Formation of a carbocation

Step 2 : Nucleophilic attack on the carbocation

Loss of proton, H+ to solvent

Example 2 :Write the mechanism for the following reaction.

SN1 Mechanism :

Step 1 : Formation of carbocation

Rearrangement :

Step 2 : Nucleophilic attack on the carbocation

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).

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.

(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.

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

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 :transition stateH

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.

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

SN2 Mechanism :

transition state

Comparison of SN1 and SN2 Reactions

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 :

alkenebasehaloalkane

Example :

i.

ii.majorminormajorminor

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.

Grignard Reagent ( alkylmagnesium halide)

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.

Example :

i.

ii.

iii.

(ii).Synthesis of 1o alcoholMethanal reacts with the Grignard reagent, followed by the hydrolysis produces primary alcohol.

Example :

i.

ii.

(iii).Synthesis of 2o alcoholGrignard reagent reacts with aldehydes to produce secondary alcohol.

Example :

i.

ii.

(iv).Synthesis of 3o alcoholGrignard reagent reacts with ketons to produce the tertiary alcohol.

Example :

i.

ii.

(v).Synthesis of carboxylic acidGrignard reagent reacts with carbon dioxide (CO2) followed by hydrolysis to form carboxylic acid.

Example :

15.2.5 : Wurtz ReactionReaction of haloalkane (RX) with an alkali metal (usually Na) to synthesise longer alkane. i.To prepare an even number of carbon atoms alkane 2RX + 2Na RR + 2NaX

Example:

2CH3CH2Br + 2Na CH3CH2CH2CH3 + 2NaBrdry ether

ii.To prepare a odd number of carbon atoms alkane

RX + RX + 6Na RR + RR + RR + 6NaX

Example:

CH3CH2Br + CH3Br + 6Na CH3CH2CH2CH3 + CH3CH2CH3 + CH3CH3 + 6NaBr

15.2.6 : Importance of Haloalkanes as Inert Substance