Haloalkenes – Definition, Classification, Uses
Mono halogen derivatives of alkanes are called haloalkanes. Haloalkanes are represented by general formula R – X, Where, R is an alkyl group (CnH2n+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.
In the common system, haloalkanes are named as alkyl halides. It is derived by naming the alkyl group followed by the halide.
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 (δ+) and halogen atom a partial negative charge (δ-)
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 move from C – F to C – I, is given in the table.
Methods of Preparation
Haloalkanes are prepared by the following methods
1. From Alcohols
Alcohols can be converted into halo alkanes by reacting it with any one of the following reagent
- Hydrogen halide
- Phosphorous halides
- 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 PI3 are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine, respectively.
(c) Reaction with thionyl chloride (Sulphonyl chloride)
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.
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
Chloro or bromoalkane on heating with a concentrated solution of sodium iodide in dry acetone gives iodo alkanes. This reaction is called Finkelstein reaction, (SN2 reaction).
Chloro or bromo alkanes on heating with metallic fluorides like AgF, SbF3 or Hg2F2 gives fluoro alkanes. This reactions is called Swarts reaction.
From silver salts of fatty acids (Hunsdiccker reaction)
Silver salts of fatty acids when refluxed with bromine in CCl4 gives bromo alkane.
- Pure haloalkanes are colourless. Bromo and iodo alkanes are coloured in the presence of light.
- 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.
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.
CH3I > CH3Br > CH3Cl > CH3F
(iii) The boiling points of chloro, bromo and iodo alkanes increase with the increase in the number of halogen atoms.
CCl4 > CHCl3 > CH2Cl2 > CH3Cl
(iv) The boiling point and melting point of mono haloalkane increase with the increase in the number of carbon atoms.
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.
Haloalkanes are polar covalent compounds soluble in organic solvents, but insoluble in water because they cannot form hydrogen bonds with water molecules.
The density of liquid alkyl halides are higher than these of hydrocarbons of comparable molecular weight.
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
- Nucleophilic Substitution Reactions
- Elimination Reactions
- Reaction with Metals
Nucleophilic Substitution Reactions
We know that the Cδ+ – Xδ- 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.
(i) Reaction with alcoholic ammonia (Ammonolysis)
Haloalkanes react with alcoholic ammonia solution to form alkyl amines.
However, with excess of halo alkane, secondary and tertiary amines along with quartenary ammonium salts are obtained.
Nucleophiles such as cyanide and nitrite ion which can attack nucleophilic centre from two sides of the nucleophile 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.
(iii) Reaction with alcoholic AgCN
Haloalkanes react with alcoholic AgCN solution to form alkyl isocyanide.
(iv) Reaction with sodium or potassium nitrite
Haloalkanes react with alcoholic solution of NaNO2 or KNO2 to form alkyl nitrites.
(v) Reaction with silver nitrite
Haloalkanes react with alcoholic solution of AgNO2 to form nitro alkanes.
(vi) Reaction with sodium or potassium hydrogen sulphide
Haloalkanes react with sodium or potassium hydrogen sulphide to form thio alcohols.
(vii) Williamson ether synthesis
Haloalkane, when boiled with sodium alkoxide gives corresponding ethers.
This method can be used to prepare mixed (unsymmetrical) ethers also.
Mechanism of Nucleophilic substitution reaction
The mechanism of nucleophilic substitution reaction is classified as
(a) Bimolecular Nucleophilic substitution reaction (SN2)
(b) Unimolecular Nucleophilic substitution reaction (SN1)
The rate of SN 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; aftr paul walden who first discovered the inversion of confiuration of a compound in SN2 reaction.
SN2 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)
SN1 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 SN1 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 SN1 reaction follows first order kinetics and occurs in two steps.
We understand SN1 reaction mechanism by taking a reaction between tertiary butyl bromide with aqueous KOH.
This reaction takes place in two steps as shown below
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
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.
Hydrolysis of optically active 2 – bromo butane gives racemic mixture of ± butan-2-ol
The order of reactivity of haloalkanes towards SN1 and SN2 reaction is given below.
When a haloalkane 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 Saytzef ’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)
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.
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 carbocation
Step – 2
Elimination of a proton from the β – carbon to produce an alkene
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.
(b) Reaction with active metals like sodium, lead etc
Haloalkane reacts with active metals like sodium, lead etc in the presence of dry ether to form organo metallic compounds.
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.
Uses of haloalkane
- Is used as a solvent in pharmaceutical industry
- Is used for producing pesticides and drugs
- Is used as an anaesthetic
- Used as a preservative for anatomical specimens
- Is used as an antiseptic for dressing wounds
- Is used as dry cleaning agent
- 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.
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