Downloaded from Class 12 Chemistry Haloalkanes...though HI breaks, iodine free radicals are unstable...

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HALOALKANES

INTORDUCTION:

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

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

Haloalkanes contain halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group

whereas haloarenes contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl

group.

Many Halogen containing organic compounds are clinically useful & find wide applications in

industry as well as in day-to-day life.

Examples: 1. Chlorine containing antibiotic, chloramphenicol, produced by soil microorganisms is

effective for the treatment of typhoid fever.

2. Our body produces iodine containing hormone, thyroxine, the deficiency of which causes a disease

called goiter.

3. Chloroquine is used for the treatment of malaria;

4. Halothane is used as an anaesthetic during surgery.

5. Certain fully fluorinated compounds are being considered as potential blood substitutes in surgery.

6. Haloalkanes are used as solvents for relatively non-polar compounds and as starting materials for the

synthesis of wide range of organic compounds.

CLASSIFICATION

1.On the Basis of Number of Halogen Atoms-mono, di, or polyhalogen (tri-,tetra-, etc.) compounds

depending on whether they contain 1, 2 or more halogen atoms in their structures.

2. Classification of Monohalo compounds -according to the hybridisation of the C- atom to which the

halogen is bonded:

Compound Containing sp3 C—X Bond (X= F,

Cl, Br, I)

Compounds Containing sp2 C-X Bond

(a) Alkyl halides or haloalkanes (RX): CnH2n+1X,

primary (1º), secondary (2º) or tertiary (3º)

according to the nature of carbon to which halogen

is attached.

(b) Allylic halides: the -X atom is bonded to an

sp3 -hybridised carbon atom next to C=C bond

(a) Vinylic halides are the compounds in

which the -X atom is bonded to an sp2

hybridised carbon atom of a carbon-carbon

double bond (C = C)

(b) Aryl halides

are the compounds in which the -X atom is

bonded to the sp2 hybridised carbon atom

of an aromatic ring.

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(c) Benzylic halides

These are the compounds in which the –X atom is

bonded to an sp3-hybridised carbon atom next to an

aromatic ring.

NOMENCLATURE

1. The common name of alkyl halide =name of the alkyl group followed by the halide (i.e. Alkyl

halide)

2. IUPAC name of Alkyl halide = halo substituted hydrocarbons (i.e. Haloalkane)

3. Haloarenes are the common as well as IUPAC names of aryl halides. For dihalogen derivatives, the

prefixes o-, m-, p- are used in common system but in IUPAC system, the numerals 1,2; 1,3 and 1,4 are

used.

4. The dihalogen derivatives having same type of halogen atoms on the same carbon are known as

geminal dihalides and assigned common name alkylidene halides or alkylidene dihalides.

The dihalogen derivatives having same type of halogen atoms on adjusant carbon atoms are known as

vicinal dihalides and assigned common name alkylene halides or alkylene dihalides.



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ISOMERISM IN HALOALKANES:

1.Chain Isomerism 2. Position Isomerism 3.Optical Isomerism

The haloalknaes with four or more

carbon atoms exhibit this

isomerism.

Example-

The haloalknaes with three or

more carbon atoms exhibit this

isomerism.

Example-

Haloalkanes containing chiral

centres in their molecules can

exhibit enantiomerism or

optical isomerism.

Example-

1-Chloro-2-2methylpropane &

1-Chlorobutane

2-Bromopropane &

1-

Bromopropane

Nature of C-X Bond

Halogen atoms are more electronegative than C , the C-X bond of alkyl halide is polarised; the C

atom bears a +δ charge whereas the halogen atom bears a -δ charge

Since the size of halogen atom increases as we go down the group in the periodic table,

consequently the C-X bond length also increases from C—F to C—I & bond energy

decreases from C—F to C—I .



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Remember:

1. Polarity affects the physical properties like b.pt., physical state etc. More polar compounds have

higher boiling points. That is why B.Pt. of Haloalkanes > B.Pt. of hydrocarbons of comparable

molecular mass.

2. Since the size of halogen atom increases as we go down the group in the periodic table,

consequently for the same alkyl group boiling points of alkyl halides decrease in the order:

RI> RBr> RCl> RF

3. The C atom bears a +δ charge whereas the halogen atom bears a -δ charge. Therefore

haloalkanes undergoes nucleophilic substitution reaction, elimination reaction, reaction with

metals, reduction.

METHODS OF PREPARATION

1. FROM ALCOHOLS: The -OH group of an ROH can be replaced by -X on reaction with

concentrated HX, PX3, PX5 or SOCl2 (Thionyl chloride).

a. By reaction with halogen acids

(HX):

i. Chloroalkanes are obtained

by treating alcohols with

HCl in the presence of

anhydrous ZnCl2.

The mixture of HCl and anhydrous

ZnCl2 is known as Lucas reagent

Anhydrous ZnCl2 acts as

dehydrating agent and thus

prevents reverse reaction.

1º and 2º alcohols can be converted

into their respective chlorides by

reacting with HCl and anhydrous

ZnCl2 (Groves process)

Tertiary alcohols readily react with

conc HCl even in the absence of

ZnCl2.

ZnCl2

R-OH (alcohol) + HCl R-X + H2O

CH3CH2OH + HCl CH3CH2Cl+ H2O

Ethanol Ethylchloride

ZnCl2

+ HCl +H2O

Isopropanol Isopropyl chloride

tert-Butyl alcohol tert-Butyl chloride

ii. Bromoalkane can be obtained by

heating by heating KBr or NaBr in

the presence of conc.H2SO4 (HBr is

generated in situ ie. In the reaction

mixture)

KBr/NaBr + H2SO4 K/NaHSO4 + HBr

CH3CH2OH + HBr CH3CH2Br + H2O

Ethanol Ethyl bromide

iii. Iodoalkane can be obtained by

heating alcohol with KI and

95%H3PO4(phosphoric acid)

KI + H3PO4 KH2PO4 + HI

Pot.hydrogen phosphate

CH3CH2OH + HI CH3CH2I + H2O

Ethanol Ethyl iodide



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Understand these points:

These are Nucleophilic substitution of –OH by –X.

The order of reactivity of alcohols with a given haloacid is 3°>2°>1°.

o Reason: Greater the number of electron releasing group on α- carbon atom of alcohol,

more is the polarity of C-OH bond. Consequently greater is the ease with which it

cleaves.

The reactivity of halogen acid towards this reaction is :HI> HBr> HCl

o Reason: Bond dissociation energy of H-I is less than that of H-Br which in turn is less

than that of H-Cl.

This method is not suitable for preparation of arylhalide. (Due to partial double bond character

of C-O bond)

b. By reaction with Phosphorous

Halide:

i. Chloro alkanes can be obtained by

reaction of PCl3 orPCl5 with

alcohols

ii. Bromo and Iodo alkane can be

obtained by reaction of alcohol s

with a mixture of red

phosphorous and Br2 or I2.

The phosphorus first reacts with

the Br2 or I2 to give the

phosphorus (III) halide.

These then react with the alcohol

to give the corresponding halo

alkane which can be distilled off.

Propan-1-ol

1-Chloropropane

Butan-1-ol 1-Cholrobutane

c. By reaction with Thionyl Chloride (SOCl2): Alcohols can be converted into corresponding

chloride by refluxing with thionyl chloride in the presence of small amount of pyridine.

The reaction of straight chain primary alcohol with thionyl chloride in the presence or absence

of pyridine is called Darzen’s procedure

Propano-1-ol 1-Cholropropane

Note: The big advantage that this reaction has over the use of either of the phosphorus

chlorides is that the two other products of the reaction (sulphur dioxide and HCl) are both

gases. That means that they separate themselves from the reaction mixture, therefore the

product formed is highly pure

2. FROM HYDROCARBONS: Alkyl halide can be prepared from alkanes through substititution

and from alkenes through addition of halogen acids (HX) or through allylic substitution reactions

a. From Alkanes: Alkanes on reaction with

halogens, chlorine or bromine , in the

presence of light or heat , undergo free

(Note: The extent of halogenations depends upon the



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radical substitution and produce a

mixture of mono, di and poly- haloalkane

amount of halogen used.)

Note the following:

These reactions follow free radical mechanism.

Replacement of H atom of alkane by halogen atom is known as halogenations.

The reactivity order for halogens shows the order : F2 > CI2 > Br2 > I2

F2 reacts violently even in dark and reaction may be controlled by diluting fluorine with N2,

whereas iodination is very slow and reversible. Therefore iodination is made in presence of HgO

or HIO3 (oxidants which decompose HI)

[CH4 + I2 ↔ CH3I + HI] × 5

5HI + HIO3 → 3I2 + 3H2O

4CH4 + 2I2 + HIO3 → 5CH3I + 3H2O

The ease of substitution of different types of hydrogen atom is: Benzylic, allylic > tertiary >

secondary > primary > vinylic, aryl

Propene Allyl chloride

b. From alkenes: Haloalkenes can be

prepared from alkenes by addition of

halogen acids (HCl, HBr, HI)

The order of reactivity of various

halogen acids is HI >HBr > HCl

CH2=CH2 + HI CH3CH2I

Ethene Iodo ethane

Markovnikov’s rule: "when an unsymmetrical

alkene reacts with a hydrogen halide to give an

alkyl halide, the hydrogen adds to the carbon of

the alkene that has the greater number of

hydrogen substituents, and the halogen to the

carbon of the alkene with the fewer number of

hydrogen substituents"

Reason: the reaction occurs via protonation to give the more stable carbocation.

Propene after protonation gives two different carbocations, one is 2o and the other is 1o. Formation of

the more stable 2o carbocation is preferred.

The carbocation then reacts with the nucleophile to give the alkyl bromide and hence 2-bromopropane is

the major product.



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Peroxide Effect or Kharasch effect is a

special case of a specific halogen acid, namely

HBr, adding to unsymmetrical alkene in such a

way that the negative part of the addendum

attaches itself to that carbon of the multiple bond

which has the least number of hydrogen atoms.

IMPORTANT:

HCl and HI cannot exhibit this effect because

HCl's bond dissociation enthalpy is too high for

its bond to break in normal conditions and

though HI breaks, iodine free radicals are

unstable and combine to form iodine molecule

instead.

Mechanism of Peroxide Effect or Kharasch

effect

3. BY HALIDE EXCHANGE:

i. Finkelstein reaction: Alkyl iodides can be

prepared by the reaction of alkyl

chlorides/ bromides with NaI in dry

acetone. This reaction is known as

Finkelstein reaction.

ii. Swarts reaction Alkyl fluorides can be

synthesized by heating an alkyl

chloride/bromide in the presence of a

metallic fluoride such as AgF, Hg2F2,

CoF2 or SbF3. The reaction is termed as

Swarts reaction.

4. FROM SILVER SALT OF FATTY ACIDS:

a. Borodine Hunsdiecker reaction:

Bromoalkane can be prepared by refluxing

silver salt of fatty with bromine in CCl4

(Note: iodo alkane cannot be prepared by

this reaction)

PREPARATION OF HALOARENES:



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1. From Arenes (by

Electrophilic Substitution ):

ArCl & ArBr can be prepared

by electrophilic substitution of

arenes with Cl2 and Br2

respectively in the presence of

Lewis acid catalysts like Fe or

FeCl3.

Note:

Reactions with I2 are reversible in nature and require the presence of an oxidising agent (HNO3,

HIO4) to oxidise the HI formed during iodination.

Fluoro compounds are not prepared by this method due to high reactivity of F2.

2. From diazonium compounds:

Primary aromatic amines on reaction

with nitrous acid (NaNO2 + HCl) at 0º-

4ºC produce benzene diazonium chloride,

the reaction is known as Diazotization. i. Sandmeyer’s reaction: Mixing the

solution of diazonium salt with

cuprous chloride (Cu2Cl2) or cuprous

bromide (Cu2Br2) results in the

replacement of the diazonium group

by –Cl or –Br.

ii. Gatterman’s reaction: This is

modification of Sandmeyer reaction.

Mixing the solution of diazonium salt

with HCl/HBr in the presence of

copper powder results in the

replacement of the diazonium group

by –Cl or –Br.

iii. Balz –Schiemann reaction:

Fluorobenzene can be prepared by

treating benzene diazonium chloride

with fluoroboric acid. The reaction

produces diazonium fluoroborate

which, on heating, produces

fluorobenzene.

iv. Preparation of Iodo benzene by

warming benzene diazonium salt with

KI.

3. From Benzene (Commercial method)

a. Raschig Process: chlorobenzene is



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prepared by passing a mixture of

benzene, hydrogen chloride and

oxygen over CuCl2 as catalyst at

about 525ºC

b. Friedel Craft’s Halogenations

reaction: Benzene on reaction with

halogen in the presence of Lewis

catalyst like FeCl3/ AlCl3 gives halo

benzene

Note: aryl fluorides cannot be prepared

by this method because of high affinity of

fluorine for hydrogen.

+ HCl + ½ O2 + H2O

PHYSICAL PROPERTIES

1.COLOUR: RCl is Colourless when pure.

RBr & RI-develop colour when exposed to light (Colour develops due to atmospheric oxidation)

2. PHYSICAL STATE: CH3Cl, CH3Br, C2H5Cl and some chlorofluoromethanes are gases at room

temperature.

Higher members are liquids or solids.

3. BOILING POINTS:

i. B.Pt. of Haloalkanes > B.Pt. of hydrocarbons of comparable molecular mass

REASON: Due to greater polarity & higher molecular mass as compared to the parent hydrocarbon, the

intermolecular dipole-dipole and van der Waals forces of attraction are stronger in the halogen

derivatives.

ii. For the same alkyl group boiling points of alkyl halides decrease in the order: RI> RBr> RCl> RF

Example: B.Pt CH4<CH3F< CH3Cl< CH3Br< CH3I

REASON: with the increase in size of halogen atom, the magnitude of van der Waal forces increases

iii. For the same halogen atom boiling points increases with increase in the size of alkyl group.

Example: B.Pt CH4<CH3CH2Cl< CH3(CH2)2Cl< CH3(CH2)3Cl

REASON: with the increase in size of alkyl group, the magnitude of van der Waal forces increases.

iv. The boiling points of chloro, bromo, & iodo compounds increases with the increase in the number of

halogen atoms.

Example: B.Pt CH3Cl<CH2Cl2< CHCl3< CCl4

Reason: with the increase in the number of halogen atoms, the magnitude of van der Waal forces

increases

v. The boiling points of isomeric Haloalkanes decrease with increase in branching



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Example:

Reason: With increase in branching surface area decreases causing decrease in van der Waals forces

of attraction

vi. Boiling points of isomeric dihalobenzenes are very nearly the same.

However, the p-isomers are high melting as compared to their o- and m-isomers.

Example:

Reason: It is due to symmetry of para isomers that fits in crystal lattice better as compared to

ortho- and meta-isomers.

NUCLEOPHILIC SUBSTITUTION:

NUCLEOPHILE: Negatively charged or neutral but electron rich species which attacks on electron

deficient or positively charged site of the substrate.

Neutral Electron rich nucleophile:H2O,ROH,ROR,NH3,RNH2,RSH

Negatively charged nucleophile: X-(Halide),CN-,OH-,RCOO-,NO2-

NUCLEOPHILIC SUBSTITUTION REACTION: a substitution reaction which starts with the initial

attack of a nucleophile is called nucleophilic substitution reaction.

The stronger nucleophile displaces a weaker nucleophile, the atom or group which departs with its bond

pair of electrons is called the leaving group.

Nucleophilic Substitution of Alkyl Halides (R–X):

A nucleophile reacts with haloalkane (the substrate) having a partial positive charge on the carbon

atom bonded to halogen. Halogen atom, called leaving group departs as halide ion.

R—X + Nu- R—Nu + X-

Name/Type of

Reaction

Reagent Nucleophile

(Nu–)

Reaction Class of main

product

Replacement by

hydroxyl group

NaOH(aq)

or KOH(aq)

HO– R-X + KOH(aq) ROH +

KX

Alcohol (ROH )

H2O H2O R-X + H2O ROH + HX

Replacement by NaOR’ R’O- R-X + NaOR’ ROR’ + Ether (ROR’)



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alkoxy group-

Williamson

synthesis

NaX

Replacement by

iodide group

NaI I– R-X + NaI R-I + NaX Alkyl iodide (R—I)

Replacement by

amino group-

Hoffmann

ammonolysis

NH3 NH3 R-X + NH3 RNH2 + HX Primary

amine(RNH2)

R’R’’NH R’R’’NH RNH2 + HX R2NH + HX Tert. Amine

(RNR’R’’)

Replacement by

cyano group

KCN -CN R-X + KCNRCN + KX Nitrile (cyanide)

RCN

Replacement by

isocyano group

AgCN Ag-CN: R-X + AgCNRNC + AgX Isonitrile ,RNC

(isocyanide)

Replacement by

nitrite group

KNO2 O=N—O R-X + KNO2R-O-N=O +

KX

Alkyl nitrite (R-O-

N=O)

Replacement by

nitro group

AgNO2 Ag—Ö-

N=O

R-X + AgNO2RNO2 +

AgX

Nitroalkane (R-

NO2)

Replacement by

carboxylate

group

R’COOAg R’COO– R-X + R’COOAg

R’COOR + AgX

Ester (R’COOR)

Replacement by

hydride ion

LiAlH4 H- 4R-X + LiAlH4 4RH +

LiX + AlX3

Hydrocarbon (RH)

Replacement by

alkyl group

R’– M+ R’– R-X + R’-M R-R’ + MX Alkane (RR’)

Replacement by

mercaptide (-SR)

group

NaSR RS- R-X + NaSR’ R-S-R’ +

NaX

Thioether

Ambident nucleophile: Nucleophiles which have more than one site through which reaction can occur

is called ambident nucleophiles.

Example: cyanides [-C N :C=N-], and nitrites [–O—N =O] possess two nucleophilic centres .

Linkage of [CN]- through C atom gives alkyl cyanides and through N atom gives isocyanides.

Linkage of nitrite ion through O atom results in alkyl nitrites while through N atom, it gives

nitroalkanes.

MECHANISM OF SUBSTITUTION REACTION OF ALKYL HALIDE (R-X):

Unimolecular Nucleophilic Substitution

Reaction(SN1)

Bimolecular Nucleophilic Substitution

Reaction (SN2)

Mechanism:

STEP1.Dissociation of C-X bond formation of a

carbo cation intermediate.-slow step

STEP2.Attack of nucleophile on the carbocation-

Mechanism:atach of nucleophile & cleavage

of C-X bond takeplace simultaneously



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formation of product, fast step.

follows the first order kinetics

r=k[R—X ]

Generally carried out in polar protic

solvents (like H2O, ROH, CH3COOH, etc.).

Formation of racemic mixture.

The order of reactivity: Greater the stability of

carbocation, greater will be its ease of formation

from alkyl halide and faster will be the rate of

reaction. In case of alkyl halides, 3º alkyl halides

undergo SN1 reaction very fast because of the high

stability of 3ºcarbocations.

follows a second order kinetics,

r=k[R—X ][Nu-]

The rate depends upon the

concentration of both the reactants

Inversion of configuration.

Tertiary halides are the least reactive

because bulky groups hinder the

approaching nucleophiles.

(Steric effects in SN2 reaction)

The order of reactivity followed is:

1ºhalide > 2º halide > 3º halide

FACTORS AFFECTING SN1 AND SN2 MECHANISM:

1. Nature of Alkyl halide: 1ºalkyl halide reacts through SN2 & 3ºalkyl halide reacts through

SN1mechanism. 2ºalkyl, 1ºallylic & 1º benzylic may react either by SN2 or SN1 or by both

mechanism depending upon reaction conditions.

Allylic and benzylic halides show high reactivity towards the SN

1 reaction. The carbocation thus formed

gets stabilised through resonance as shown below:

2.Nature of Nucleophile: strong nucleophile favors SN2 & weak nucleophile favors SN1

3.Concentration of nucleophil: high concentration of nucleophile favors SN2 & low concentration of

nucleophile favors SN1



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4.Nature of solvent: polar solvents favor SN2 &solvents of low polarity favor SN1 mechanism

5. For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both the

mechanisms: R–I> R–Br>R–Cl>>R–F.

SOME USEFUL TERMS RELATED WITH OPTICAL ACTIVITY:

1. Monochromatic light: A beam of light of single wavelength is called monochromatic light.

Ordinary light can be turned into a monochromatic light by passing it through a filter

2. Plane Polarised light: Light whose propagation takes place along only one plane only is called

plane polarized light. Ordinary monochromatic light can be turned into a plane polarized light by

passing through Nicol Prism. Nicol Prism is made of calcite ( special crystalline form of CaCO3)

3. Optically active substance: The substance which rotate the plane of polarization of plane

polarized light is called optically active substance.

Example:

4. Optical activity: The ability of optically active substance to cause rotation in the plane of

poarisation of the plane polarized light is called optical activity.

5. Optical rotation (αobs): The angle through which plane of plane polarized light is rotated either

towards left or towards right is called optical rotation.

It depends upon-

i. λ of light

ii. Length of the sample tube

iii. Concentration of solution

iv. Solvent used

v. Temperature of sample

6. Polarimeter: The instrument used to measure the extent of optical rotation by a substance is

called Polarimeter.

7. Dextrorotatory substance: The substances which rotate the plane of the plane polarized light in

the clockwise direction , i.e., towards right. This is indicated by putting a letter d or (+) before

the name of the substance. e.g.; d-lactic acid or (+)-lactic acid.

8. Laevororatorysubstance: The substances which rotate the plane of the plane polarized light in

the anticlockwise direction , i.e., towards left. This is indicated by putting a letter l or (-) before

the name of the substance. e.g.; l-lactic acid or (-)-lactic acid.

9. Specific rotation



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Observed rotation or angle of rotation

[α]λt= ________________________________________________________

Length of the sample tube(dm)x concentration of solution(g/mL)

10. Enantiomers: The stereoisomers related to each other as non super imposable mirror images are

called enantiomer. The enantiomer rotates the plane of the plane polarized light through same

angle but in opposite direction. If one enantiomer is dextrorotatory, the other would be

laevorotatory.

11. Chirality centre: A carbon atom bonded to four different atoms/groups in the molecule is called

chirality centre. Chirality centre in a molecule is represented by asterisk(*)

The term” Chirality centre” was approved by IUPAC in 1996. Earlier such a carbon was called

asymmetric carbon/stereo centre/ chiral carbon.

A molecule (or an object) is said to be chiral or dissymmetric, if it is does not possess any element of

symmetry and not superimposable on its mirror image and this property of the molecule to show non-

superimposability is called chirality.

On the other hand, a molecule (or an object) which is superimposable on its mirror image is called

achiral (non-dissymmetric or symmetric).

To understand the term chiral and achiral let us consider the alphabet letters ‘P’ and ‘A’ whereas ‘P’ is

chiral, ‘A’ is achiral as shown in fig

12. Dissymmetric object/molecule: A molecule (or an object ) which does not have a plane of

symmetry is called a chiral molecule (or object ).The chiral molecules are also called dissemtric

molecule.

13. Plane of Summetry: An imaginary plane which divides the molecule ( or an object) into two

identical halves each being the mirror image of the other is called plane of symmetry ( also

known as mirror plane)

14. Cetre of Symmetry: A point (atom) within the molecule, such that if an imaginary straight lineis

drawn from any point of the molecule through that point , meets a similar feature at equal

distance on either side of the point, is called centre of symmetry.



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15. Alternating axis of symmetry: A molecule is said to possess an alternating axis of symmetry if

an orientation indistinguishable from the original is obtained when molecule is rotated degree

around an axis passing through the molecule and the rotated molecule is reflected in a mirror that

is perpendicular to the axis of rotation in step (I).

16. Symmetric, Asymmetric and Dissymmetric molecules

(a) Symmetric molecules: If any symmetry is present in the molecule then molecule will be

symmetric molecule.

(b) Dissymmetric molecules: Molecule will be a dissymmetric molecule if it has no plane of

symmetry, no centre of symmetry and no alternating axis of symmetry.

(c) Asymmetric molecules: Dissymmetric molecule having at least one asymmetric carbon is

known as asymmetric molecule. All asymmetric molecules are also dissymmetric molecules but

the reverse is not necessarily true.

17. Racemic Mixture: Equimolar mixture of the two enantiomers is called racemic mixture. A

racemic mixture has zero optical rotation, as the rotation due to one isomer is cancelled by the

rotation due to the other isomer.

Racemic mixture is indicated by prefixing dl or (±) before the name of the substance.

e.g.; dl-butan-2-ol or (±)-butan-2-ol.

18. Racemization: The process of conversion of an enantiomer into a racemic mixture is called

racemization. Racemic mixture is optically inactive due to external compensation.

19. Configuration: The spatial arrangement of atoms or groups around an asymmetric carbon that

characterizes a particular enantiomer is called configuration of the enantiomer

20. Inversion, retention and racemization:

There are three outcomes for a substitution reaction at an asymmetric carbon atom. Consider the

replacement of a group X by Y in the following reaction;

If (A) is the only compound obtained; the process is called retention of configuration.



Page 16 of 23

If (B) is the only compound obtained, the process is called inversion of configuration.

If a 50:50 mixture of the above two is obtained then the process is called racemization and the product

is optically inactive, as one isomer will rotate light in the direction opposite to another.

If during a reaction, no bond to the stereo centre is broken, the product will have the same general

configuration of groups around the stereo center as that of the reactant- this is retention of configuration.

If product has configuration opposite to that of the reactant-inversion of configuration.

If there is formation of two enantiomers in equal amount-the process is called racemization.

Example: of Racemization

21.Diastereomers: Stereoiosomers which are not mirror image of each other are called diastereomers

and the phenomenon is called diastereoisomerism.

22. Meso Compound: compounds which do not show optical activity in spite of the presence of chiral

carbon atoms are called meso compounds. Meso compounds consists of parts, the lower half part is

mirror image of the upper part half part. The rotation caused by the upper half part of the molecule is

cancelled by the equal and opposite rotation caused by the lower half of the molecule. Meso compounds

are optically inactive due to internal compensation.

ELIMINATION REACTION:

Haloalkanes having β hydrogen atom on heating with alcoholic KOH undergo dehydrohalogenation to

form alkene. The se reactions are called β –eliminations reaction because the hydrogen atom present at β

position of the halogen is removed.



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Saytzeff Rule: In case the haloalkane can eliminate hydrogen halide in two different ways, the preferred

alkene is the one which carries more number of alkyl groups attached to the doubly bonded carbon

atoms.

Reason:

The more substituted alkene is more stable and hence is formed at faster rate;

Cis trans isomerism of Saytzeff’s product: if the alkene formed is during Saytezeffs elimination is

capable of showing cis-trans isomerism , then the trans alkene being more stable is formed as the major

product.

Order of reactivity in Elimination reaction:



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1. Reactivity of haloalkanes towards elimination : tertiary> secondary> primary

Reason: the alkyl halide which form more stable alkene will be more reactive.

2. Among various halides with same alkyl group: RI>RBr>RCl

Reason: Bigger halogen is better leaving group

Elimination versus substitution:

An alkyl halide with a-hydrogen atoms when reacted with a base or a nucleophile has two competing

routes: substitution (SN1 and SN2) and elimination. Which route will be taken up depends upon the

nature of alkyl halide, strength and size of base/nucleophile and reaction conditions. Thus, a bulkier

nucleophile will prefer to act as a base and abstracts a proton rather than approach a tetravalent carbon

atom (steric reasons) and vice versa. Similarly, a primary alkyl halide will prefer a SN2, a secondary

halide- SN2 or elimination depending upon the strength of base/nucleophile and a tertiary halide- SN1 or

elimination depending upon the stability of carbocation or the more substituted alkene

MISCILLANAEOUS REACTIONS:

Organometallic compounds: compounds having metal carbon bonds are called organometallic

compounds.

Eg; Alkylmagnesium halide(RMgX), vinylhalide (CH2=CHX) and aryl magnesium halide(ArMgX)

known as Grignard reagent.

Preparation of Alkyl magnesium halide (RMgX): by the action of alkyl halide with magnesium in

dry ether medium.

Eg; CH3CH2Br + Mg –(dry ether) CH3CH2MgBr

Characterstic of Grignard reagent: due to high electronegativity of C than Mg , C-Mg bond is polar

as shown below.

Due to this bond polarity Grignard reagent reacts with any compound having acidic hydrogen (like H2O,

ROH, NH3, HCΞ CH, acids etc.) to give hydrocarbons. It also reacts with O2 and CO2.

This is why Grignard reagent should be prepared & used in the absence of moisture and air.

Grignard reagent is used for preparation of alcohols, aldehydes, ketones, carboxylic acids etc.



Page 19 of 23

Wurtz reaction: Alkyl halide on reaction with sodium in dry ether gives hydrocarbon containing double

the number of carbon atoms present in the alkyl halide

R-X + 2Na + X-R R-R + 2NaX

CH3 –Br + 2Na + Br-CH3 CH3- CH3 + 2NaBr

Note:

1. Tertiary halide donot undergo this reaction

2. When a mixture of two different alkyl halide is used , all together three possible alkanes are

formed.

Example:

CH3-Br + Na + Br-C2H5 CH3- C2H5 + CH3- CH3 + C2H5-C2H5

Propane Ethane Butane

Reduction:

Alkyl halides can be reduced by following reagents into corresponding alkane.

i. LiAlH4/Ether solution ii. H2/Ni, Pt iii.Na/Moist ether

iv.H2/Zn-Cu

v. Na/C2H5OH

Isocyanide test (Carbyl amine reaction): Chloroform on reaction with primary amine in the presence

of ethanolic solution of KOH/NaOH gives a foul smelling compound named isocyanide (carbylamine)

R-NH2 + CHCl3 + 3KOH R-N≡C + 3KCl + 3H2O

Significance of Isocyanide test carbyl amine reaction: the reaction is used to distinguisg-

i. Trihalomethane from other halogenated methane

ii. 1º amine from 2º & 3º amines

iii. Preparation of isocyanide

O OH

‖ │

Haloform reaction: Componds having –C-CH3 or –CH-CH3 structure on reaction with Br2 / Cl2 in the

presence of alkali (like NaOH, KOH, NaHCO3 etc.) give bromoform (CHBr3)/ chlororform (CHCl3)

Example:

O OH

‖ │

Iodoform Test: Componds having –C-CH3 or –CH-CH3 structure on reaction with I2 in the presence of

NaOH give yellow coloured precipitate of iodoform (CHI3)



Page 20 of 23

Eg;

Significance of Iodoform reaction: The reaction is used –

i. For structural illucidation

ii. To distinguish pairs of alcohols, aldehydes and ketones

iii. For preparation of Iodoform

REACTIONS OF HALOARENES:

Electrophilic substitution reaction: those reactions in which strong electrophile replace a weaker

electrophile are called electrophilic substitution reaction.

Halogen atom besides being slightly deactivating is o, p directing; therefore, further substitution occurs

at ortho- and para positions with respect to the halogen atom.

Reason:

Due to resonance as shown above, the electron density increases more at ortho- and para-positions than

at meta-positions. Further, the halogen atom because of its –I effect has some tendency to withdraw

electrons from the benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene

and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic

conditions as compared to those in benzene.

Name of reaction Reaction Elctrophile(E+)

i.Halogenation:

Reagent: Halogen

Catalyst: Lewis acid

like FeCl3/

Anhyd.AlCl3

Haloniumion(X+)

eg;Cl+, Br+



Page 21 of 23

ii.Nitration:

Reagent: Conc.HNO3

Catalyst:Conc.H2SO4

Nitronium

ion(NO2)+

iii.Sulphonation:

Reagent:Conc.

H2SO4

Catalyst:Conc.H2SO4

Sulphonium ion

(HSO3)+

iv.Friedel-Crafts

reaction:

Reagent:

Alkylhalide/acylhalide

Catalyst: Lewis acid

like

FeCl3/Anhyd.AlCl3

Carbonium ion

(R+) like +CH3 in

alkylation,

acylinium ion

(R-C+=O)in

acylation.

Mechanism of Electrophilic substitution reaction:

Slow fast

The reaction of the electrophile E+ with the arene is the slow step since it results in the loss of

aromaticity even though the resulting cation is still resonance stablised.

Nucleophilic substitution:

Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following

reasons:

i. Resonance effect: due to resonance C-X bond acquires partial double bond character, hence cleavage

ii. Difference in hybridisation of carbon atom in C—X bond:



Page 22 of 23

The sp2 hybridised carbon with a greater s-character is more electronegative and can hold the electron

pair of C—X bond more tightly than sp3-hybridised carbon in haloalkane with less s-chararcter.

Replacement by hydroxyl group:

Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a

temperature of 623K and a pressure of 300 atmospheres.

The presence of an electron withdrawing group (-NO2) at ortho- and para-positions increases the

reactivity of haloarenes

Vinyl halides are less reactive towards nucleophilic substitution reaction: this is due to resonance C-

X bond acquires partial double bond character.

Reaction with metals:

1. Reaction with magnesium:

2. Wurtz-Fittig reaction: A mixture of an alkyl halide and aryl halide gives an alkylarene when treated

with sodium in dry ether and is called Wurtz-Fittig reaction.

3. Fittig reaction: Aryl halides also give analogous compounds when treated with sodium in dry

ether, in which two aryl groups are joined together. It is called Fittig reaction.

4. Wurtz fittig reaction: Mixture of alkyl halide and aryl halide on reaction with sodium in dry

ether medium gives alkyl aryl hydrocarbon



Page 23 of 23

5. Ullman reaction: Iodobenze when heated with copper in a sealed tube gives diphenyl.

POLYHALOGEN COMPOUNDS:

COMPOUND PREPARATION PROPERTIES USES

Methylenechloride

(Dichlorormethane)

CH2Cl2

By direct chlorination of

methane in the presence of

light

CH4 + 2Cl2 –hν CH2Cl2

+ 2HCl

Colourless, sweet

smelling, volatile,

liquid, B.Pt. 40ºC,low

inflammability

Physiological effect:

Harms human central

nervous system, contact

with eye burns cornea

Solvent

extraction,

solvent in paint

remover,

propellant in

aerosol, metal

cleansing and

finishing solvent

Chloroform/

trichlorormethane

(CHCl3)

i. By the action of

bleaching

powder on

ethanol or

acetone

ii. Reduction of CCl4:

CCl4 + H2 –Fe/H2O

CHCl3 + HCl

iii.CH4 + Cl2 –hv

CHCl3 + HCl

1.Colourless liquid,

sweet smell,B.Pt61C,

insoluble in water

,miscible in organic

solvent

2. in the presence of air

& sunlight it oxidises

into phosgene (COCl2)

which is poisonous.

2CHCl3 + O2 –

air/hv2COCl2 +2HCl



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