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24/04/2016 CBSE Cl ass 12 Chemistry Notes : Haloalkanes and Haloarenes

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CBSE Class 12 Chemistry Notes :Haloalkanes and HaloarenesOctober 22, 2014 by Anuj William Leave a Comment

The replacement of hydrogen atom(s) in hydrocarbon, aliphatic or aromatic, by halogen atom(s) results in

the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively.

Classication of Halogen Derivatives

On the basis of number of halogen atoms present, halogen derivatives are classied as mono, di, tri, tetra,

etc., halogen derivatives, e.g.,

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On the basis of the nature of the carbon to which halogen atom is attached, halogen derivatives are

classied as 1, 2, 3, allylic, benzylic, vinylic and aryl derivatives, e.g.,

General Methods of Preparation of Haloalkanes

1. From Alcohols


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In Grooves method, ZnC1 is used to weaken the C-OH bond. In case of 3 alcohols, ZnC1 is not required.

The reactivity order of halogen acids is HI > HBr > HCl.

Darzen procedure is the best method for preparing alkyl halides from alcohols since both the by products

(SO and HCl) are gaseous and escape easily.

2. Free Radical Halogenation of Alkanes

Addition of Hydrogen Halides on Alkenes

1. Finkelstein Reaction

2. Swarts Reaction

H C Br + AgF H C F + AgBr

Hg F , COF and SbF can also be used as a reagent for Swarts reaction.

3. Hunsdiecker Reaction

2 2

2

3 3

2 2 2 3


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Physical Properties of Haloalkanes

1. Boiling point orders

1. R I > R Br > R CI > R F

2. CH (CH ) CH Br > (CH ) CHCH Br > (CH ) CBr

3. CH CH CH > CH CH X > CH X

2. Bond strength of haloalkanes decreases as the size of the halogen atom increases. Thus, the order of

bond strength is

CH F > CR Cl > CR Br > CH I

3. Dipole moment decreases as the electronegativity of the halogen decreases.

4. Haloalkanes though polar but are insoluble in water as they do not form hydrogen bonding with water.

5. Density order is

RI > RBr > RCl > RF (For the same alkyl group)

CH I > C H I > C H I

Chemical Reactions of Haloalkanes

1. Nucleophilic Substitution Reactions S reactions)

3 2 2 2 3 2 2 3 3

3 2 2 3 2 3

3 3 3 3

3 2 5 3 7

N


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kCN is predominantly ionic and provides cyanide ions in solution, which is ambident nucleophile and bind

with carbon side to form as the major product, while AgCN is covalent and form isocyanide as the major

product.

Like KCN, KNO form R-ONO while AgNO produces R-NO as product. Vinyl chloride is less reactive

towards nucleophilic substitution reactions due to resonance.

Nucleophilic substitution reactions are of two types

a) S 1 type (Unimolecular nucleophilic reactions proceed in two steps:

2 2 2

N


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Rate, r = k [RX). It is a rst order reaction.

Reactivity order of alkyl halide towards S 1 mechanism

3 > 2 > 1

Polar solvents, low concentration of nucleophiles and weak nucleophiles favour S 1 mechanism.

In S 1 reactions, partial racemisation occurs due to the possibility of frontal as well as backside attack on

planar carbocation.

(b) S 2 type (Bimolecular nucleophilic substitution) These reactions proceed in one step and is a second

order reaction with r = k[RX] [Nu].

During S 2 reaction, inversion of conguration occurs (Walden inversion) i.e., starting with dextrorotatory

halide a laevo product is obtained and vice-versa, e.g.,

N

N

N

N

N


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Reactivity of halides towards S 2 mechanism is

1 > 2 > 3

Rate of reaction in S 2 mechanism depends on the strength of the attacking nucleophile. Strength of some

common nucleophiles is

:CN > : I > : OR > : OH > CH COO: > H O > F

Non-polar solvents, strong nucleophiles and high concentration of nucleophiles favour S 2 mechanism.

Relative rates of some alkyl halides in S 1 and S 2 reactions are in the order

Resonating structure of benzyl carbocations are

Relative reactivity of alkyl halides for same alkyl group is

N

N

3

2

N

N N


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RI > RBr > RCI > RF

2. Elimination Reactions

Dehydrohalogenation is a elimination reaction in which halogen is from -carbon atom and the

hydrogen from the -carbon according to Saytzeff rule, e.g.,

Ease of dehydrohalogenation among halides

3 > 2 > 1

3. Reduction

4. Reaction with Metals


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Grignard reagent is never isolated in the solid state as it explodes in dry state. So it is used as ethereal

solution.

5. lsomerisation

General Methods of Preparation of Aryl Halides

1. By Halogenation of Aromatic Hydrocarbons

It is an electrophilic substitution reaction.

2. By Side Chain Halogenation

(It involves free radical mechanism.)

3. From Benzene Diazonium Salt


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4. From Phenol

Physical Properties of Aryl Halides

1. Aryl halides are colourless liquids or colourless solids with characteristic odour.

2. Boiling point generally increases with increase in the size of aryl group or halogen atom. Boiling point

order

Ar I > Ar Br > Ar Cl > Ar F

3. The melting point of p -isomer is more than 0- and m-isomer.

This is because of more symmetrical nature of p-isomer.

4. Due to resonance in chlorobenzene, C-CI bond is shorter and hence, its dipole moment is less than that

ofcyclohexylchloride.

Chemical Properties of Aryl Halides

1. Nucleophilic Substitution Reaction

Aryl halides are less reactive towards nucleophilic substitution reaction. Their low reactivity is attributed due


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to the following reasons:

1. Due to resonance, C-X bond has partial double bond character.

2. Stabilisation of the molecule by delocalisation of electrons.

3. (Instability of phenyl carbocation.

However, aryl halides having electron withdrawing groups (like NO , -SO H, etc.) at ortho and parapositions undergo nucleophilic substitution reaction easily.

Presence of electron withdrawing group (-NO ) increases the reactivity.

2. Electrophilic Substitution Reactions

Halogens are deactivating but O, p-directing. Thus, chlorination, nitration, sulphonation and Friedel Crafts

reaction give a mixture of o- and P- chloro substituted derivatives.

i) Halogenation

2 3

2


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ii) Nitration

iii) Sulphonation

iv) Friedel-Crafts reaction


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3. Reaction with Metals

(i) Wurtz Fittig reaction

ii) Fitting reaction

(iii) Ullmann reaction


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Dlhalogen Derivatives

Dichloromethane (CH Cl ) is widely used as a solvent, as a propellant in aerosols. Direct contact of

dichloromethane in humans causes intense burning and milk redness of the skin.

Trihalogen Derivatives

1. Chloroform [Trichloromethane, CHCl ]

Methods of preparation

Properties

1. Oxidation of CHCl gives poisonous gas phosgene (carbonyl chloride).

To avoid this oxidation CHCl iI .toreci in dark brown bottles and lled to the brim. 1% ethanol is added to

chloroform which converts harmful phosgene gas into diethyl carbonate.

2. CHCl is widely used in the production of freon refrigerant R-22.

2 2

3

3

3

3


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3. On nitration, it gives tear producing insecticide substance chloropicrin.

2. Iodoform tri-iodornethane, CHl )

Iodoform is prepared by iodoform reaction.

Compounds containing either CH CO- or CH CH(OH) group form yellow colour iodoform with I and NaOH.

Iodoform when comes in contact with organic matter, decomposes easily to free iodine, an antiseptic. Dueto its objectionable smell, it has been replaced by other formulations containing iodine.

Polyhalogen Derivatives

1. Tetrachloromethane Carbon Tetrachloride, CCl )

Preparation

CCI is a colourless, non-inammable, poisonous liquid, soluble in alcohol and ether.

Uses

Carbon tetrachloride is used

3

3 3 2

4

4


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1. as a solvent for oils, fats, resins

2. in dry cleaning

3. as re extinguisher under the name pyrene.

2. Freons

The chlorouorocarbon compounds of methane and ethane are collectively known as freons. These are

usually produced for aerosol propellants, refrigeration and air conditioning purposes. Carbon tetra chloride

when reacts with antimony triuoride in the presence of SbCl as catalyst, dichlorouromethane (freon) is

obtained.

3. DDT(p, p-Dichlorodiphenyltrichloroethane)

DDT is the rst chlorinated organic insecticide. Its stability and fat solubilityis a great problem.

It is prepared from chloral and chlorobenzene in the presence of conc. H SO

4. Perchloroethane(C Cl )

It is used as moth repellant and is also known as articial camphor.

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5 Comments Sort by

Monty Tembhurne

It is an excellent information

Like Reply Mar 31, 2016 11:56pm

Shubham Dubey STUDENT OF CLASS 11 SCIENCE~MATH ~COMP at M.J.R.P. Public School.Ghazipur

Good it's helpful


Vishal Jain

it sucks it's almost similar to the notes summerised in u-like .....


Vishal Kumar Nachiketan Public School

grt

Like Reply 1 Dec 2, 2015 9:48am

Siddhant Mishra

i like it...

Like Reply Nov 17, 2015 2:22am

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