haloalkanes and haloarenes - science...

HALOALKANES AND HALOARENES

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Chapter Outline:

HALOALKANESAND

HALOARENES

Prerequisites•LearningObjectives•Introduction•Classification•Nomenclature•NatureofC-XBond•Methodsofpreparationsarylhalides•Usesofsomepolyhalogencompounds•Summary•


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PREREQUISITES

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Organic compounds i.e., hydrocarbons are classified into different types based on the type of functional group present in it.If the hydrogen in hydrocarbon is replaced by halogen then they are called halo compounds.Organic compound it named according to IUPAC system.Prefix + Root word + 10 suffix + 20 suffix.Depending on the no.of carbon to which the carbon is attached it is classified as 10, 20, 30 and 40. Primary if carbon attached to 1 carbon, 20 – if carbon attached to 2 carbons and 30 – if carbon attached to 3 carbons and 40 – if carbon attached to four carbons.Carbon adjacent to functional group called α- carbon, next called β next γ and so on.Carbon with +ve charge called carbocation and –ve charge called carbanion.Carbon attached to four different atoms or groups called chiral carbon or stereo center or asymmetric carbon.The light which travels only in one direction called plane polarized light.The compound which rotate plane polarized light is called optically active compound, if its rotates to right called dextro- rotatory or (+) isomer and to left called laevo rotatory or (-) isomers.A pair of non super imposable mirror images are called enantiomers.If the configuration of the compounds remains same even after the reaction then it is called retension in configuration.If 50 : 50 mixture of two configurations obtained after the reaction then is called racemization.

Inthischapterwearegoingtodiscussabout.

Classification of halo alkanes and arenes

Nomenclature of these compounds

Methods of preparation of mono halo alkanes, tri haloalkanes and halo arenes.

Physical and chemical properties of these compounds.

About the nature of carbon halogen bond.

Uses of poly halogen compounds.

LEARNING OBJECTIVES

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INTRODUCTION

CLASSIFICATION

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Compounds formed by the replacement of one or more hydrogen atoms of the aliphatic and aromatic hydrocarbon by same no.of halogen atoms are called aliphatichalogen derivatives, which are commonly called alkylhalidesandarylhalides respectively.

These are classified in to three types based on the carbon to which the halogen is attached. Primary alkyl halide if halogen is attached to 10 carbon. 20 alkyl halide if halogen is attached to 20 carbon. 30 alkyl halide if halogen is attached to 30 carbon.

Eg: Ethyl bromide where bromine is attached to10carbon.

CH3 C BrCH3

CH3

CH3 C Br

H

H

CH3 C Br

CH3

H

Ethyl bromide (10 Alkyl halide)

2- Bromo propane (20 Alkyl halide)

2- Bromo - 2- Methyl propane (30 Alkyl halide)

Depending on the no.of halogens these may be classified as mono, di, tri, or poly halogen compounds.

C2H5X

CH2X

CH2X

CH2X

CHX

CH2X

Monohalo Alkane Monohalo arene

Dihalo arene

Dihalo Alkane

Trihalo Alkane

X

XX

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Mono halo compounds

The general formula of mono halo alkanes is CnH2n+1 X (saturated)

These can be classified as 10, 20 and 30 halo alkanes. Depending upon the hybridization of the carbon atom to which halogen is attached these are of different types.

H C Br

H

H

H C C Br

H H

H H

Methyl bromide

Ethyl bromide

(b)Allylichalides: In which the halogen is attached to sp3 carbon which in turn bonded to a double bonded carbon.

Eg: CH2=CH–CH2–Cl→3– Chloro-1- propene. (or) allylic chloride

H

CH2=CH C Cl

H3 – Chloro-1- propene

(c)Benzylichalide: In which the halogen is attached to sp3 carbon which in turn attached to aromatic ring.

CH2 Cl

Chloro phenyl methane

(a)Alkylhalides: In which the halogen attached carbon undergo sp3 hybridization Eg: CH3 – Br, C2 H5Br

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NOMENCLATURE

Chain isomerism

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Di halo compounds

Isomerism

General formula of these compounds is Cn H2n X2 (Saturated).

Vicinaldihalide:If the two halogens are attached to adjacent carbon.

CH2Cl CH2Cl 1,2- Dinchloro ethane

Alkylhalidesexhibit

Chain isomerism

Positional isomerism

Alkyl halide with minimum 4 carbon atom exhibit chain isomerism.

Eg: n-butyl chloride and isobutyl chloride are chain isomers.

CH3 CH2 CH2 CH2 Cln-butyl chloride

(d)Vinylichalide: In which the halogen is attached to sp2 carbon i.e., a double bonded carbon.

(e)Arylhalide: In which the halogen is attached to sp2 carbon in an aromatic ring.

Vicinal dihalide

Bromo benzene

CH2=C Br

H

Br

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Formula CommanName IUPACName

CH3ClCH3 CH2 CH2Cl

Methyl chloriden-propyl chloride

Iso propyl chloride

t-pentyl iodide

neo-pentyl bromide

Vinyl chlorideCH2 = CHCl

CH2 = CH - CH2Cl

CH3 - CHCl2

CH2Cl - CH2Cl

Chloro ethene

3-Bromo propeneChloro phenylmethane

1-chloro-2-methyl benzene(or)2-chloro toluene.

1, 1-dichloro ethane.

1, 2-dichloro ethane

1, 2-dichloro benzene



Allyl chlorideBenzyl chloride

O-Chloro toluene

ethylidene chloride

ethylene dichloride

O-dichloro benzene

m-dichloro benzene

p-dichloro benzene

Chloro methane

1-chloro propane

2-chloro propane

2-iodo-2-methyl butane

1-Bromo -2,2- dimethyl propane(CH3)3 C CH2 Br

CH3 CH CH3

Cl

CH3 C CH2 CH3

CH3

l

CH2Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

CH3

Positional isomerism

Alkyl halides with minimum 3 carbon atoms (mono halides) and 2 carbon atoms (dihalides) exhibit positional isomers.

Eg: a) 1-chloro propane and 2-chloro propane

CH3 CH2 CH2 Cl

CH3 CHCl CH3

1-chloro propane

2-chloro propane

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CH3 CHCl2

CH2Cl CH2Cl

1,1- dichloro ethane

1,2- dichloro ethane

b) 1,1-dichloro ethane and 1, 2-dichloro ethane.

If there is any double bond in the molecule then may exhibit geometrical isomerism, and if there is any chiral carbon in the molecule they may also exhibit optical isomerism.

The bond between carbon and halogen in alkyl halide is polar in natural since halogens are more electronegative than carbon. So carbon bears a partial positive charge and halogen partial negative charge.

NATURE OF C-X BOND

From alcohols

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H C X X=F,Cl,Br,I

H

H

Carbon halogen bond length increases as we move from C - F to C - I

CH3 F<CH3 Cl<CH3 Br<CH3 I

CH3 F>CH3 Cl>CH3 Br>CH3 I

Bond energy increases from C-F to C-I

Methods of preparation of mono halogen compounds

Alkyl halides are best prepared from alcohols. The reaction follow either SN1 or SN2 mechanism.

1) When alcohol is treated with dry HCl and anhydrous ZnCl2 corresponding alkyl halide is formed. Mixture of (1:1) dry HCl and anhydrous ZnCl2 is called Lucas reagent.

R OH+dryHCl RCl2+H2OZnCl2Alcohol

anhydrous

R OH+HX RX+H2O

The reactivity of alcohols towards HX is allyl, benzyl > 30 > 20 >10 and the reactivity of halogen acids is HI > HBr > HCl > HF.

(Reaction of 10 and 20 alcohols with HCl requires a catalyst like ZnCl2 but 30 react with HCl at room temperature).

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3ROH+PX3 3RX+H3PO3

ROH+PX5 RX+POX3+HX

Alcohols react with PX3 and PX5 to give corresponding alkyl halide. PBr5 and PI5 are highly unstable due to steric hinderence, so only chlorides are prepared by this method.

Alcohols react with thionyl chloride to give pure alkyl halide because the other two products are escapable gases. This process is called Darzens procedure. Bromide and iodide are not prepared because SOBr2 and SOI2 does not exist.

ROH+SOCI2 RCl +HCl+SO2

From hydrocarbon

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Alkyl bromides and iodines can be prepared by treating alcohol with red phosphorous with Bromine or iodine.

Alkyl bromide can be obtained by the action of sodium bromide on alcohol in presence of H2SO4

Alkyl iodide can be obtained by heating alcohol with sodium or potassium iodide in 95% phosphoric acid

ROH+NaBr+H2SO4 RBr+NaHSO4+H2O

ROH RXRedP/X2

X2=Br2orI2

ROH RI95%H3PO4

NaI

Alcohol

Alcohol

Alkylbromide

Alkyliodide

This reaction takes place in presence of sunlight or heat and follows free radical mechanism. Reaction of F2 with alkanes is explosive therefore they are prepared by halogen exchange methods. Iodination occurs only in presence of an oxidizing agent such as HgO, HIO3 HNO3 because direct reaction is a reversible reaction.

Alkane AlkylhalideRH+X2 RX+HX

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Addition of halogen acid to alkane follow Markownikoff rule and the mechanism is electrophilic addition. At high temperature addition of halogen becomes reversible and hence does not occur.

Propane2-BromoPropane

R CH=CH2+HBr R CH CH3

Br

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Alkyl chlorides and bromides are converted into their iodides or fluorides by treating them with NaI/acetone or inorganic fluorides like Hg2F2 respectively. Conversion of alkyl halides to iodides or fluorides is called Finkelstein reaction.

Ethylene

Ethylhalide

C=C+HX C CH X

Halogen exchange method

From silver salts of fatty acid

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RCl

RCl

or RI+NaCl

or RF+Hg2Cl2

or

or

RBr

RBr

NaBr

Hg2Br2

Alkyllodide

Alkylfluoride

NaI/acetone

NaI/acetone

Alkyl fluorides which cannot be prepared by Finkelstein reaction can be prepared by treating chlorides or bromides with mercurous fluoride or antimony fluoride or AgF. This reaction is called swarts reaction.

2CH3Cl+Hg2F2 2CH3F+Hg2Cl2

CH3Br+AgF CH3F+AgBr

Methylchloride

MethylBromide

Methylflouride

Methylflouride

When silver salts of fatty acids are treated with Br2 in CCl4 gives corresponding alkyl bromides. This method is called Borodine Hunsdicker method. This reaction follow free radical mechanism. Yield of alkyl chloride is less than alkyl bromide.

CCl4RCOOAg+Br2 RBr+AgBr+CO2

2RCOOAg+I2 RCOOR+CO2+2AgIEster

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Alkyl halides are colorless when pure. However bromides and iodides develop color when exposed to light. Many volatile halogen compounds have sweet smell.

1. Methyl fluoride, chloride and bromide and ethyl chloride are gases at room temperature remaining are color less liquids up to C18 and beyond them are color less solids.

2. Alkyl halides are very slightly soluble in water. Even though these have polar nature they are insoluble in polar solvents as they have no ability of forming H – bonding with water. These are soluble in organic solvents like benzene, ether etc.

Haloalkanes

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Physical properties

Chemical properties

3. Alkyl halides have high boiling points than alkanes equal molecular weights the order is RI >RBr > RCl > RF. With the increase in size of alkyl group boiling points increases

and with the increase in branching boiling points decreases.

1 – Bromo butane → 375K , 2 – Bromo butane → 364K, 2 – bromo – 2 – methyl propane → 346K.

4. Fluoro and chloro compounds are lighter than water where as bromo and iodo compounds are heavier than water. Density decreases with the increase in the size of alkyl group.

5. Dipole moment decreases as electronegativity of halogen decreases. But fluorides have lower value because its small atomic size.

RCl>RF>RBr>RI

Nucleophilicsubstitutionreactions

Eliminationreaction

Reactionwithmetals

Reductionreaction

Friedel-craftalkylation

Nucleophilicsubstitutionreactions:

Alkyl halide on treatment with aqueous NaOH or KOH corresponding alcohol is formed.

RX+KOH ROH+KX(eq)Alkylhalide Alcohol

Haloalkanes

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Alkyl halide on treatment with moist silver oxide alcohol is formed.

RX+AgOH ROH+AgXAlkylhalide

Alkylhalide

Alkylhalide

Alkylhalide

Alkylhalide

Alkylhalide

Alkylhalide

Alcohol

Alkyl halide on treatment with dry silver oxide gives ether.

2RX+Ag2O ROR+2AgXdrysilveroxide Ether

Ether

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Alkyl halides react with sodium alkoxides to give ethers. This reaction is called William son’s synthesis.

RX+R|ONa ROR+NaXSodiumalkoxides

Alkyl halide reacts with KCN and AgCN to give cyanides and isocyanides as the major products respectively. AgCN is covalent in nature and nitrogen free to donate electron pair forming isocyanide. But KCN is ionic in nature and provide cyanide ion in solution because C – C bond is more stable than C – N bond.

R–X+KCN R–CN+KXAlkylcyanide(alc)

R–X+AgCN R–NC+AgXAlkylisocyanide

R–X+KNO2 R–ONO+KX

R–X+AgNO2 R–NO2+AgX

Alkylnitrites

Nitroalkanes

Alkyl halides react with KNO2 and AgNO2 to give alkyl nitrites and nitro alkanes respectively as the major products.

R–X+NH3 R-NH2+HX

RX+R-NH2 R2NH+HX

RX+R2NH R3N+HX

R3N+RX R4N+X-

1oAmine

2oAmine

3oAmine

Quaternaryammoniumsalts

Alkyl halide reacts with ammonia to give a mixture of 1o, 2o, 3o and quaternary ammonium salts.

This reaction is called Hormann ammonolysis of alkyl halides.Alkyl halides react with silver salt of fatty acids to give esters.

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Alkyl halides react with KSH and K2S to give thioalcohol or mercaptan and thioether respectively.

Alkyl halide reacts with sodium salt of hydrazoic acid to give alkylazide.

Alkyl halide reacts with (C6H5)3 P to give phosphonium salt.

Alkyl halides are converted to alkenes by treating than with alcohol KOH or NaNH2 or KNH2. The reaction proceeds through E1 or E2 mechanism.

Alkyl halides react with sodium in presence of dry ether to give higher alkanes.

RX+KSH RSH+KX

2RX+K2S RSR+2KX

R-X+NaN3 RN3+NaX

RX+(C6H5)3P [R(C6H5)3P]+X-

CH3-CH2-X CH2=CH2

2RX+2Na R-R+2NaX

Alc.KOH

Dryether

Dryether

Dryether

Ethylene

Phosphoniumsalt

Alkylazide

Thioalcohol

Thioether

Eliminationreaction:

Reactionwirhmetals:

Alkyl halides react with magnesium in presence of dry ether to give Grignard’s reagent. [Reactivity order is RI > RBr > RCl]

Alkyl halides react with lithium in presence of dry ether to give alkyl lithium.

RX+Mg RMgX

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R|X+RCOOAg RCOOR|+AgXAlkylhalide

Alkylhalide

Alkylhalide

Alkylhalide

Alkylhalide

Alkylhalide

Alkylhalides

silveracetate Ester

RX+2Li RLi+LiX

Alkyllithium

Alkyl halide reacts with zinc in presence of dry ether to give higher alkanes. This reaction is called Frankland reaction.

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Ethyl chloride reacts with an alloy of sodium and lead to form TEL (Tetra ethyl lead) which is used as antiknocking agent in petroleum industry.

Alkyl halide on reduction with reducing agents like Zn/acid or LiAlH4 or H2/Ni or Pd gives alkanes.

Alkyl halides undergo Friedel – crafts alkylation in presence of anhydrous AlCl3 with benzene to give alkyl benzene .

RX R-H

2RX+Zn R-R+ZnX2

4C2H5Cl+4Na/Pb (C2H5)4Pb+NaCl+3Pb

Dryether

Zn/HCl

LiAlH4Alkylhalide

Alkylhalides

Alkane

Reactionwirhmetals:

Friedel-Craftalkylation:

+R-X AlCl3

Benzene

R

+HX

Compounds containing CH3CO – group or any compound which on oxidation gives such group reacts with alkali and halogen to give halo form.

CH3-C-R+4NaOH+3X2 R-C-O+CHX3+3NaX+3H2O

= =O O∆ O

Haloform reaction

Reaction of bleaching powder with ethyl alcohol and acetone

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Methods of preparation of tri halogen compound (Chloroform)

Bleaching powder undergoes hydrolysis to give halogen and weak base. Halogen oxidizes ethyl alcohol to trichloro acetaldehyde and acetone to tri chloro acetone which on reaction with base to give chloroform.

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Alkanes react with halogens in presence of sunlight to give different alkyl halides. This reaction is a chain reaction since all the hydrogens are substituted by halogens. This reaction follows free radical substitution

CaOCl2+H2O Ca(OH)2+Cl2

C2H5OH+Cl2 CH3CHO+2HCl

CH3CHO+3Cl2 CCl3CHO+3HCl

2CCl3CHO+Ca(OH)2 2CHCl3+(HCOO)2Ca

CaOCl2+H2O Ca(OH)2+Cl2

CH3COCH3+3Cl2 CCl3COCH3+3HCl

2CCl3COCH3+Ca(OH)2 2CHCl3+(CH3COO)2Ca

(i)

(ii)

CH4+Cl2 CH3Cl+HCl

CH3Cl+Cl2 CH2Cl2+HCl

CH2Cl2+Cl2 CHCl3+HCl

CHCl3+Cl2 CCl4+HCl

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Chloroform

Chloroform is prepared by the reduction of carbon tetra chloride by iron fillings and water.

CCl4+2[H] CHCl3+HClCarbontetrachloride Chloroform

Halogenations of alkanes

Reduction of carbon tetra chloride

From chloral hydrate

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wwPure chloroform is obtained by the distillation of chloral hydrate with NaOH(aq).

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Dihalocarbene is a good electrophile and most of the reactions are attribute to it.

Reactivity order is CHI3 > CHBr3 >CHCl3 >>CHF3

Chloroform auto oxidises in presence of sunlight with air only to form phosgene gas (carbonyl chloride). Phosgene gas is poisonous in nature so now a day chloroform is not used as an anesthetic. To prevent this reaction 1% C2H5OH is used. The purity of chloroform can be tested by the addition of AgNO3. If precipitate is seen then chloroform is impure.

CHCl3 and CHBr3 are sickly smelling liquid, while CHI3 is in the form of yellow hexagonal plates.

Boiling points increases with increase in molecular weight.[CHCl3 → 334K, CHBr3 → 422.5K, CHI3 → 392K]

CHCl3, CHBr3 are sparingly soluble in water, but CHI3 is in soluble in waters.

CCl3CH(OH)2+NaOH(aq) CHCl3+HCOONa+H2O

:CCl3 :CCl2+Cl

CHCl3+OH :CCl3+H2OO

O O

O

CHCl3+12 O2 COCl2+2HCl

Chloroform

Chloroform

Carbonylchloride

COCl2+2C2H5OH (C2H5O)2CO+2HCl

Ethylcarbonate

Chloroform on reduction with Zn/HCl in ethyl alcohol gives dichloro methane.

Chloroform on reduction with Zn/H2O gives methane

Oxidation

Reduction

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Physical properties

Chemical properties of Haloform

CHCl3 CH2Cl2Dichloromethane

Zn/HCl-C2H5OH

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Chloroform undergoes hydrolysis in presence of a base like NaOH or KOH to give sodium formate or potassium formate.

Chloroform reacts with nitric acid to form chloropicrin which is used as insecticide and a war gas. Chloroform is used as a tear gas.

Chloroform reacts with acetone in alkaline medium to form chloretone. Which is used as a hypnotic.

CHCl3 CH4Chloroform Methane

Zn/H2O

ChloroformCHCl3+4NaOH HCOONa+3NaCl+2H2O

CHCl3+3NaOH CH(OH)3+3NaCl

CH(OH)3 HCOOH HCOONaNaOH-H2O

CHCl3+HO-NO2 CCl3NO2+H2OChloropicrinChloroform Nitricacid

CHCl3+CH3-C-CH3 C

OHCH3

H3C CCl3Chloroform

Chloroform

AcetoneChloretone

Chloroform reacts with chlorine in presence of sunlight to give carbon tetrachloride (Pyrene).

Chloroform reacts with silver to give acetylene.

Hydrolysis

Nitration

With acetone

Halogenation

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Chloroform

O

CHCl3 CCl4Cl2/hu

Carbontetrachloride

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2CHCl3+6Ag CHCH+6AgCl

Chloroform reacts with phenol in presence of a base to give salicylaldehyde or ortho-hydroxy benzaldehyde. The electrophile in this reaction is dichloro carbene.

Step-1: Formation of dichloro carbene.

Acetylene

+CHCl3+3NaOH +3NaCl+2H2O

OH OH

CHO

SalicylaldehydePhenol

Chloroform

O:CCl3 :CCl2+Cl

CHCl3+OH- :CCl3+H2OO

OH O

O

OOOH

O O

H H

+H+

O

O

O

OO

+ :CCl2

CCl2 CHCl2

CHCl2CHOCHO

H2O

H+

Hydrolysiswithalkali

Reimer - Tiemann reaction

Mechanism

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Step-2:Attack of the electrophile on to benzene ring followed by hydrolysis with alkali.

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1) Chloroform react with primary amine (both aliphatic and aromatic ) to give isocyanides

This reaction has been found to proceed by two different mechanisms.

They are SN1 and SN2

In this type rate depends on the concentration of both the reactants

The attacking nucleophile interacts with alkyl halide leading to the breakage of carbon halogen bond and form a new carbon oxygen (C-OH) bond.

These two process takes place simultaneously without formation of an intermediate.

The attacking nucleophile will come from one end and the leaving nucleophile leave from the other end.

This process is called inversion of configuration.

In transition state the structure cannot be isolated because it is unstable.

If these are bulky groups on carbon atom then they inhibit this type of mechanism.

This is an identification test for primary amines, secondary amines and tertiary amines do not give this test.

Isocyanides have an offensive smell. The electrophile in this reaction is dichloro carbene Chloroform reacts with aniline in presence of a base to give phenyl isocyanide

NH2 NC

CHCl3+3KOH+ +3KCl+3H2OChloroform

PhenylisocyanideAniline

Substitutionnucleophilicbi-molecular(SN2)

Nucleophilic substitutions reaction

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Carbylamine reaction

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The order of reactivity of alkyl halides towards SN2 is methyl>primary>secondary>tertiary.

In this type rate depend only on the concentration of alkyl halide.

These reactions are generally carried out in polar protic solvent (water, alcohol, acetic acid etc).

This mechanism occurs in two steps. In step-1: The bond between carbon and halogen cleaves which results in the formation of a carbocation.

In step-2: The carbocation formed is attached by the nucleophile.

Since step-1 is slow and reversible rate of reaction depends only on the concentration of alkyl halide.

Substitutionnucleophilicuni-molecular(SN1)

(CH3)3CBr

(CH3)3COH

Step - 1

Step - 2

+

+

+Br-

+OH-

CH3

CH3CH3

CH3

CH3

CH3

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More the stability of carbocation greater will be the ease of formation of carbocation and and faster will be the rate of reaction.

The order of reactivity of alkyl halide towards SN1 is tertiary > secondary > primary >methyl

Allylic and benzylic halides show high reactivity towards the SN1 reaction since carbocation formed gets resonance stabilized.

For a given alkyl group the reactivity of halide follows the order same in both the mechanisms they are R-I > R-Br >R-Cl > R-F

A SN2 reaction proceeds with complete stereo chemical inversion while a SN1 reaction proceeds with racemisation.

In case of optically active alkyl halide the product formed by SN2 mechanism has inverted configuration when compared to the reactant. This is because the attacking nucleophile attacks from one end and the outgoing nucleophile will leave from the other end.

H2C=CH CH2 H2C+ CH=CH2

+ +

++

CH2 CH2 CH2CH2

Stereochemicalaspectsofnucleophilicsubstitutionreaction

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Eg: When 2-bromo octane is made to react with sodium hydroxide (+) - octan-2-ol is formed.

SN2 reactions are accompanied by inversion in configuration.

In case of optically active alkyl halide the product formed by SN1 mechanism is accompanied by racemisation.This is because the attacking nucleophile can attack carbocation from either side resulting in a mixture of products one having the same configuration and other inversion.

Example:

When 2-bromo butane is treated with sodium hydroxide butan-2-ol formation takes place.

When haloalkanes with β-hydrogen is treated with alcoholic KOH there is elimination of β-hydrogen and halogen from α-carbon resulting in the formation of an alkene.

Since β-hydrogen is eliminated it is often called β-elimination.

If there is availability of more than one β-hydrogen (i.e., β-carbon) then the major product will be according to Zaitsev rule(Alexander Zaitsev a Russian chemist formulated this rule, also pronounced as saytzeff)

According to this rule in dehydrohalogenation reactions, the preferred product is that alkenes which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.

HO HOOH- OH-

C2H5 C2H5C2H5

CH3 CH3

CH3

H H

CH3 CH2 CH2 CH CH3

CH3 CH2 CH2 CH CH2CH3 CH2 CH CH CH2Pent 2 ene Pent 1 ene

(81%) (19%)

OH-

Elimination reaction

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Br

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Benzene undergo electrophilic substitution of halogen in presence of lewis acids like FeCl3 or AlCl3 etc to form halobenzene.

When a primary aromatic amine dissolved in cold aqueous mineral acid is treated with sodium nitrite, a diozonium salt is formed.

When diozonium salt is treated with cuprous chloride or cuprous bromide gives chloro-benzene or bromobenzene.

METHODS OF PREPARATIONS OF ARYL HALIDES

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From benzene

From diazonium salt

Iodobenzene can’t prepared by this method since the reaction is reversible, the formed HI must be oxidized by using oxidising agents like HNO3, HIO4 etc...Fluorobenzene can be prepared by this method because Fluorine being more reactive.

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When diozonium salt reacts with copper powder in presence of HCl or HBr form chrolobenzene or bromobenzene.

When diozonium salt is treated with potassium iodide iodobenzene is formed.

When diozonium salt is treated with fluoroboric acid fluorobenzene is formed.

The commercial method of preparation chlorobenzene is by this process.

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Gattermann reaction

Preparation of iodobenzene

Preparation of fluoro benzene

Raschig process

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In this process silver salt of benzoic acid is distilled with Br2 in presence of CCl4 at 350K to give bromobenzene.

Aryl halides are heavier than water, through polar but immiscible with water. In Aryl halides the halogen is attached to sp2 hybridized carbon, the C-X in aryl halides have double bond character due to conjugation resulting in 3 resonating structures.

Due to this nature the reactions are not normal i.e., nucleophilic substitution reactions are difficult. Even the length of carbon-halogen bond is less (169pm) when compared to haloalkanes (177pm).

These reactions do not take under ordinary conditions. Possibility of SN1 mechanism is ruled out because the phenyl cation formed will not be resonance stabilized.

Chloro benzene can be converted into phenol by heating in aqueous sodium hydroxide at a temperature of 623K and a pressure of 300 atmospheres

NaOH,623K300atm

Cl OH

Nucleophilic substitution reaction

Re-placement by hydroxyl group

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Borodine - Hunsdicker reaction

Physical properties and Chemical properties

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If there is any electron withdrawing group on benzene ring at ortho and para position the reactivity increases. But such type of effect is not observed when electron withdrawing group is present at meta position.

Presence of – NO2 group at ortho and para positions withdraws electron density from benzene ring facilitating the attack of nucleophile, the carbanion formed is stabilized by resonance.

The negative change at ortho and para position with respect to halogen is stabilized by - NO2 group. But this type of stabilization is not there if - NO2 group is present at meta position.

When brome benzene is treated with copper cyanide in presence of dimethyl form amide at a temperature of 470K gives cyano benzene.

NO2

NO2

NO2 NO2

NO2O2N O2N NO2

NO2

NO2

NO2 NO2

NaOH,443K

NaOH,386K

NaOHH2O

H+

H+

Cl

Cl

OH

OH

Cl OH

Re-placement by cyano group

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Br CN

+CuCN +CuBrDMF470K

2

Cl NH2

+2NH3+Cu2O 2 +2CuCl+H2O475K

60atm

When chloro benzene is treated with ammonia in cuprous oxide at a temperature of 475k and at a pressure of 60 atom give aniline.

Halogen atom on benzene ring is deactivating but ortho para directing. Halogen atom increases electron density at ortho and para positions due to +R(resonance effect) and +M(mesomeric effect) effect. But these have –I effect so the electrophilic substitution reactions are slow in chloro benzene when compared to benzene.

Chloro benzene reacts with chlorine in presence of anhydrous Ferric chloride to give 1,4 – Di chloro benzene (major) and 1,2 –di chloro benzene (minor)

Replacement by amino group

Electrophilic substitution on benzene ring

Halogenation

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Chloro benzene reacts with nitric acid (Nitration mixture -1:1 HNO3 and H2SO4) to give 1-chloro- 4-nitro benzene (major) and 1-chloro -2- nitro benzene (minor)

Chloro benzene when treated with concentrated H2SO4 gives 4-chloro benzene sulphonic acid (major) and 2-chloro benzene sulphonic acid (minor).

Chloro benzene reacts with methyl chloride in presence of anhydrous AlCl3 to give 1-chloro – 4- methyl bengene (major )and 1-chloro -2-methyl benzene (minor).

Nitration

Sulphonation

Friedel - craft reactions

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Chloro benzene reacts with acetyl chloride to give 4 chloro acetophenone (major) and 2-chloro acetophenone(minor).

Halo arenes react with metals like magnesium and lithium to form organo metallic compounds.

Aryl halides react with alkyl halides in presence of sodium in dry ether to give alkyl benzene. This reaction is called wurtz - fittig reaction.

Br

Cl

MgBr

Li

+Mg

+2Li

Ether

Ether

∆

∆

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Reaction with metals

Wurtz - fitting reaction

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Aryl halides react among themselves in presence of sodium in dry ether to give biphenyl. This reaction is called Fitting reaction.

Two moles of iodo benzene reacts with copper at a temperature of 100°C to 350°C in presence of nitro benzene to give biphenyl.

Chloro benzene reacts with chloral to give DDT ( dichloro diphenyl trichloro ethane).

Widely used as a solvent, as a paint remover, as a solvent in the manufacture of drugs, as a propellent in aerosols.

USES OF SOME POLYHALOZEN COMPOUND

Fitting reaction

Ullmann reaction

Reaction with chloral

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Dichloro methane (Methylene chloride)

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Used as a solvent in metal furnishing and cleaning.

This effects the human central nervous system and when exposed it causes slight impaired hearing and vision.

Higher level of methylene chloride causes dizziness, nausea, tingling and numbness in fingers and toes. Direct contact with eyes can burn the cornea.

Drugs Aerosols

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Direct contact with skin causes intense burning and mild redness of the skin.

Previously this is used as an antiseptic. But this nature is not due to iodoform but due to liberation of iodine. Due to its smell this has been replaced by other compounds containing iodine.

Used as a solvent for fats, alkaloids, iodine etc...

Used In the production of freon refrigerant R-22.

Inhaling the vapours depress central nervous system. Breathing 900 parts of chloroform per million parts of air for a short time causes dizziness, fatigue and headache.

Previously this is used as an anesthetic but not now because of the formation of a poisonous gas phosgene (carbonyl chloride).

Used as a solvent.

Used in the manufacture of refrigerants and propellents for aerosol cans.

Used as a feed stock in the preparation of chlorofluoro carbons.

The effect of CCI4 are dizziness, light headache, nausea, vomiting, permanent damage to

nerve cells and if it is severe it causes stupor, coma, unconsciousness or death.

Exposure may irritate eyes, heart beat becomes irregular or may even stop.

If released into air depletes ozone layers.

Thesse are unstable, unreactive, non-toxic, non-corrosive and easily liquefiable gases.freon 12 ( CCl2F2) is most common.

When realised into stratosphere may deplete ozone layer.

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Trichloromethane (Chloroform)

Tri iodo methane (Iodoform)

Tetra chloro methane (Carbon tetrachloride)

Freons(Chloro floro compounds of mathane and ethane are called freous)

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Used as an insecticide.

It is toxic to fish when mixed with water.

DDT is not metabolised very rapidly by animals so it is deposited and stored in fatty tissues.

Alkyl and aryl halides may be classified as mono, di tri poly halo compounds depending upon the no.of halogen atoms.

The carbon halogen bond is polarized because halogen is more electronegative than carbon.

Alkyl halides are prepared by the replacement of –OH group of alcohols using PCl3 or PCl5 or H or SOCl2, addition of halogen acids to alkanes, halogenation of alkanes.

Alkyl halides are prepared by electrophilic substitution or from benzene diazonium salts using proper reagents.

The boiling points of aryl and alkyl halogen compounds are higher than the corresponding hydrocarbons.These are slightly soluble in water but more soluble in organic, solvents.

Alkyl and aryl halides undergo nucleophilic substitution, elimation reactions, reaction with metals, and some miscellaneous reactions.

Nucleophilic substitution is of two types SN1 and SN2.

In SN1 racemisation is seen and in SN2 inversion in configuration is seen if the alkyl halides contain chiral carbon.

Different types of polyhalogen compounds are discussed with their uses and their environmental hazards.

SUMMARY

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FDDT (Dichloro diphenyl trichloro ethane)

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haloalkanes and haloarenes - science...

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