Haloalkanes

Haloalkanes

Mono halogen derivatives of alkanes are called haloalkanes. Haloalkanes are represented by general formula R – X, Where, R is an alkyl group (C_nH_2n+1) – and X is a halogen atom (X=F, Cl, Br or I). Haloalkanes are further classified into primary, secondary, tertiary haloalkane on the basis of type of carbon atom to which the halogen is attached.

Primary haloalkane

Examples:

Secondary haloalkane

Tertiary haloalkane

Nomenclature

Common system

In the common system, haloalkanes are named as alkyl halides. It is derived by naming the alkyl group followed by the halide.

IUPAC system

Let us write the IUPAC name for the below mentioned haloalkanes by applying the general rules of nomeclature that are already discussed in Unit no : 11

Nature of C – X bond in haloalkane

Carbon halogen bond is a polar bond as halogens are more electro negative than carbon. The carbon atom exhibits a partial positive charge (б+) and halogen atom a partial negative charge (б-)

The C –X bond is formed by overlap of sp³ orbital of carbon atom with half filled p- orbital of the halogen atom. The atomic size of halogen increases from fluorine to iodine, which increases the C – X bond length. Larger the size, greater is the bond length, and weaker is the bond formed. The bond strength of C – X decreases from C – F to C – I in CH₃X. The changes in the value of bond length, bond enthalpy and bond polarity, as we more from C –F to C –I, is given in the table.

Table showing carbon – halogen bond length, bond enthalpy and polarity of bond.

Methods of preparation

Haloalkanes are prepared by the following methods

1) From alcohols

Alcohols can be converted into halo alkenes by reacting it with any one of the following reagent 1. hydrogen halide 2. Phosphorous halides 3. Thionyl chloride

a) Reaction with hydrogen halide

Mixture of con.HCl and anhydrous ZnCl₂ is called Lucas reagent.

The order of reactivity of halo acids with alcohol is in the order HI > HBr > HCl. The order of reactivity of alcohols with halo acid is tertiary > secondary > primary.

b) Reaction with phosphorous halides

Alcohols react with PX₅ or PX₃ to form haloalkane. PBr₃ and PI ₃ are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine, respectively.

Example

c) Reaction with thionyl chloride (Sulphonyl chloride)

Example

This reaction is known as Darzen's halogenation

2) From alkenes

Alkenes react with halogen acids (HCl, HBr, HI) to give haloalkane. The mode of addition follows Markovnikov’s rule.

3) From alkanes

Alkanes react with halogens (Cl₂ or Br₂) in the presence of ultra violet light to form haloalkane. This reaction is a free radical substitution reaction and gives a mixture of mono, di or poly substituted haloalkane.

Example

Chlorination of methane gives different products which have differences in the boiling points. Hence, these can be separated by fractional distillation.

4) Halogen exchange reactions

a) Finkelstein reaction

Chloro or bromoalkane on heating with a concentrated solution of sodium iodide in dry acetone gives iodo alkanes. This reaction is called Finkelstein reaction, (S_N2 reaction).

b) Swarts reaction

Chloro or bromo alkanes on heating with metallic fluorides like AgF, SbF₃ or Hg₂F₂ gives fluoro alkanes. This reactions is called Swarts reaction.

Example

5) From silver salts of fatty acids (Hunsdiccker reaction)

Silver salts of fatty acids when refluxed with bromine in CCl₄ gives bromo alkane

Physical Properties

1. Pure haloalkanes are colourless. Bromo and iodo alkanes are coloured in the presence of light.

2. Haloalkanes having one, two or three carbon atoms are in the gaseous state at normal temperature. Haloalkanes having more than three carbon atoms are liquids or solids.

3). Boiling point and Melting point

i) Haloalkanes have higher boiling point and melting point than the parent alkanes having the same number of carbons because the intermolecular forces of attraction (dipole – dipole interaction and vander Waals forces) are stronger in haloalkane.

ii) The boiling point and melting point of haloalkanes decreases with respect to the helogen in the following order.

Example

CH₃I > CH₃Br > CH₃Cl > CH₃F

iii) The boiling points of chloro, bromo and iodo alkanes increase with the increase in the number of halogen atoms.

For Example:

CCl₄  > CHCl₃  > CH₂Cl₂  > CH₃Cl

iv) The boiling point and melting point of mono haloalkane increase with the increase in the number of carbon atoms.

Example

CH₃CH₂CH₂Cl > CH₃CH₂Cl > CH₃Cl

v) Among isomeric alkyl halides the boiling point decreases with the increase in branching in the alkyl group; with increase in branching, the molecule attains spherical shape with less surface area. As a result the inter molecular forces become weak, resulting in lower boiling points.

Example

4. Solubility

Haloalkanes are polar covalent compounds soluble in organic solvents, but insoluble in water because they cannot form hydrogen bonds with water molecules

5. Density

The density of liquid alkyl halides are higher than these of hydrocarbons of comparable molecular weight.

Chemical properties

Haloalkanes are one of the most reactive classes of organic compounds. Their reactivity is due to the presence of polar carbon – halogen bond in their molecules. The reactions of haloalkane may be divided into the following types

1. Nucleophilic substitution reactions

2. Elimination reactions

3. Reaction with metals

4. Reduction

1) Nucleophilic substitution reactions

We know that the C^δ+ - X^δ- present in halo alkane is polar and hence the nucleophilic reagents are attracted by partially positively charged carbon atoms resulting in substitution reactions.

Reaction with aqueous alkali or moist silver oxide.(Hydrolysis)

Haloalkane reacts with aqueous solution of KOH or moist silver oxide (Ag₂O/H₂O) to form alcohols.

Example

i) Reaction with alcoholic ammonia (Ammonolysis)

Haloalkanes react with alcoholic ammonia solution to form alkyl amines.

Example

However, with excess of halo alkane, secondary and tertiary amines along with quartenary ammonium salts are obtained

Ambident Nucleophiles

Nucleophiles such as cyanide and nitrite ion which can attack nucleophilic centre from two sides are called ambident nucleophiles. These nucleophiles can attack with either of the two sides depending upon the reaction conditions and the reagent used.

ii) Reaction with alcoholic KCN

Haloalkanes react with alcoholic KCN solution to form alkyl cyanides.

Example

iii) Reaction with alcoholic AgCN

Haloalkanes react with alcoholic AgCN solution to form alkyl isocyanide.

Example

iv) Reaction with sodium or potassi-um nitrite

Haloalkanes react with alcoholic solution of NaNO₂ or KNO₂ to form alkyl nitrites.

Example

v) Reaction with silver nitrite

Haloalkanes  react  with  alcoholic solution of AgNO₂ to form nitro alkanes. Example

vi) Reaction with sodium or potassi-um hydrogen sulphide

Haloalkanes react with sodium or potassium hydrogen sulphide to form thio alcohols.

Example

vii) Williamson ether synthesis

Haloalkane, when boiled with sodium alkoxide gives corresponding ethers.

Example

This method can be used to prepare mixed (unsymmetrical) ethers also.

Mechanism of Nucleophilic substitu-tion reaction

The mechanism of nucleophilic substitution reaction is classified as

a) Bimolecular Nucleophilic substitution reaction (SN₂)

b) Unimolecular Nucleophilic substitution reaction (SN₁)

S_N2 Mechanism

The rate of S_N2 reaction depends upon the concentration of both alkyl halide and the nucleophile.

Rate of reaction = k₂ [alkylhalide] [nucleophile]

It follows second order kinetics and occurs in one step.

This reaction involves the formation of a transition state in which both the reactant molecules are partially bonded to each other. The attack of nucleophile occurs from the back side (i.e opposite to the side in which the halogen is attacked). The carbon at which substitution occurs has inverted configuration during the course of reaction just as an umbrella has tendency to invert in a wind storm. This inversion of configuration is called Walden inversion; after paul walden who first discovered the inversion of configuration of a compound in S_N2 reaction.

S_N2 reaction of an optically active haloalkane is always accompanied by inversion of configuration at the asymmetric centre. Let us consider the following example

When 2 - Bromooctane is heated with sodium hydroxide, 2 – octanol is formed with invesion of configuration. (-) – 2 – Bromo octane is heated with sodium hydroxide (+) – 2 – Octanol is formed in which – OH group occupies a position opposite to what bromine had occupied,

a) (-) 2 – Bromo octane

b) Transition State

c) (+) 2 – Octanol (product)

S_N1 Mechanism

S_N1 stands for unimolecular nucleophilic substitution

‘S’ stands for substitution

‘N’ stands for nucleophilic

‘1’ stands for unimolecular (one molecule is involved in the rate determining step)

The rate of the following S_N1 reaction depends upon the concentration of alkyl halide (RX) and is independent of the concentration of the nucleophile (OH−).

Hence Rate of the reaction = k[alkyl halide]

R−Cl + OH⁻  → R – OH + Cl⁻

This S_N1 reaction follows first order kinetics and occurs in two steps.

We understand S_N1 reaction mechanism by taking a reaction between tertiary butyl bromide with aqueous KOH.

This reaction takes place in two steps as shown below

Step - 1 Formation of carbocation

The polar C - Br bond breaks forming a carbocation and bromide ion. This step is slow and hence it is the rate determining step.

The carbocation has 2 equivalent lobes of the vacant 2p orbital, so it can react equally rapidly from either face

Step - 2

The nucleophile immediately reacts with the carbocation. This step is fast and hence does not affect the rate of the reactions.

As shown above, the nucleophilic reagent OH- can attack carbocation from both the sides.

In the above example the substrate tert-butyl bromide is not optically active, hence the obtained product is optically inactive. If halo alkane substrate is optically active then, the product obtained will be optically inactive racemic mixture. As nucleophilic reagent OH- can attack carbocation from both the sides, to form equal proportion of dextro and levorotatory optically active isomers which results in optically inactive racemic mixture.

Example

Hydrolysis of optically active 2 - bromo butane gives racemic mixture of ± butan-2-ol

The order of reactivity of haloalkanes towards S_N1 and S_N2 reaction is given below. S_N2 reaction

2) Elimination reactions

When a haloalkene containing a hydrogen on β carbon is treated with an ethanolic solution of potassium hydroxide, an alkene is formed. In this reaction a double bond between α and β carbon is formed by releasing a halogen attached to a α carbon and a hydrogen to a β carbon of halo alkane. This reaction is called β elimination reaction. (dehydrohalogenation).

Some haloalkanes yield a mixture of olefins in different amounts. It is explained by Saytzeff’s Rule, which states that ‘In a dehydrohalogenation reaction, the preferred product is that alkene which has more number of alkyl groups attached to the doubly bonded carbon (more substituted double bond is formed)

Example

Elimination reactions may proceed through two different mechanisms namely E₁ and E₂

E₂ reaction mechanism

The rate of E₂ reaction depends on the concentration of alkyl halide and base

Rate = k [alkyl halide] [base]

It is therefore, a second order reaction. Generally primary alkyl halide undergoes this reaction in the presence of alcoholic KOH. It is a one step process in which the abstraction of the proton from the β carbon and expulsion of halide from the ∝ carbon occur simultaneously. The mechanism is shown below.

E₁ reaction mechanism

Generally, tertiary alkyl halide which undergoes elimination reaction by this mechanism in the presence of alcoholic KOH. It follows first order kinetics. Let us consider the following elimination reaction.

Step - 1 Heterolytic fission to yield a carbo-cation

Step - 2 Elimination of a proton from the - carbon to produce an alkene

3) Reaction with metals

Haloalkane reacts with metals, to form a compound containing carbon - metal bond known as organometallic compounds.

a) Grignard reaction

When a solution of halo alkane in ether is treated with magnesium, we get alkyl magnesium halide known as Grignard reagent.

Example

b) Reaction with active metals like so-dium, lead etc

Haloalkane reacts with active metals like sodium, lead etc in the presence of dry ether to form organo metallic compounds.

4) Reduction reactions

Haloalkanes are reduced to alkanes by treating with H₂ in the presence of metal catalyst like nickel, palladium etc or with hydroiodic acid in the presence of red phosphorous.

The chemistry of haloalkane can be well understood by the following flowchart.

Uses of haloalkane

Chloroform: As a solvent in pharmaceutical industry and for producing pesticides and drugs As an anaesthetic.

As a preservative for anatomical specimens.

Iodoform: Iodoform is used as an antiseptic for dressing wounds.

Carbon tetrachloride: Carbon tetrachloride is used as dry cleaning agent

It is used as a solvent for oils, fats and waxes

As the vapour of CCl₄ is non – combustible, it is used under the name pyrene for extinguishing the fire in oil or petrol.