Organic chemistry for the JEE mains and advanced vol II

Accordingly they are classified as shown below: 1.1.1 aliphatic saturated monohalogen Derivatives These are formed by the displacement of only one hydrogen atom of an alkane by a halogen

I.S.S Raju

Organic Chemistry

for the JEE Main and Advanced

Volume II

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Preface v

Acknowledgment vii

About the Author ix

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Organic Chemistry is one of the most fascinating subjects as it deals with the chemicals related to living organisms The subject has grown to such a level that it is not easy for any person to go through it completely The number of organic compounds known has gone to almost 15 million in the past few years It is further increasing every year Understanding organic chemistry becomes difficult due to this reason In the past few years, its presentation also changed in some cases due to this recent finding, but still there are some ambiguities

Writing a book of Organic Chemistry for students at the 10+2 level is a challenge as there are no boundaries for it

At this level, students have no practical experience Their requirements should be addressed by good books, effectively guided by an able teacher Organic Chemistry has to be presented to such students with crystal clear explanations covering all concepts, while ensuring that the discussion does not digress to topics beyond the scope of 10+2 level

The first volume of Organic Chemistry was published conforming to this requirement.

This second volume of Organic Chemistry adheres to the style followed in the first volume in its delineation of

reaction mechanisms and named reactions An effort has been made to keep the explanations simple to enable students understand and apply the concepts effectively to solve questions in IIT entrance examination Questions framed in the book have been modeled on the syllabus of CBSE and NCERT for the same purpose

Any error noticed in the book may please be brought to my notice so that it can be rectified in the subsequent editions Suggestions for the book’s improvement are welcome

I.S.S Raju

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I thank my family members and friends who encouraged me to write this book at the age of 73 My family members were unfailingly supportive and encouraging during the long months I spent glued into many relevant resources while writing this book

I am especially thankful to my great granddaughter Chy Akshara who plays with me all the time, for not disturbing

me at the time of writing the manuscript

I am obliged to Pearson India Education Services Pvt Ltd for publishing this book In particular, I am grateful to their editors Sri Abhilash Ayyappan and Mrs G Sharmilee whose sincere efforts helped in bringing out this book in record time

I.S.S Raju

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About the Author

I.S.S Raju retired as Principal, with 35 years of experience in teaching graduate and post

graduate students the subject of organic chemistry He currently coaches JEE Main and Advanced aspirants, a vocation that he has been actively pursuing since his retirement 15 years ago

His rich experience in this subject has assisted many students in gaining better command over this subject as well as helping them do well academically His approach in assisting students is methodical and lucid This helps students immensely, especially when it is about understanding the various complex theories and concepts of organic chemistry

The author’s tenure as a principal and his post-retirement contributions, in tirelessly coaching IIT aspirants, has brought him into the crux of celebrating the golden jubilee of his academic career

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Organic compounds formed by the displacement of one or more hydrogen atoms of a hydrocarbon

by halogen atoms are halogen derivatives or halohydrocarbons Hydrocarbons from which they are formed may be aliphatic or aromatic, saturated or unsaturated Accordingly they are classified as shown below:

1.1.1 aliphatic saturated monohalogen Derivatives

These are formed by the displacement of only one hydrogen atom of an alkane by a halogen atom.They are called alkyl halides or haloalkanes Their general formula is CnH2n + 1 X They are generally represented as RX, where R is any alkyl group, and X is any halogen atom

1 CH3Cl methyl chloride or halomethane

2 CH3CH2Br or C2H5Br ethyl bromide or bromoethane

| iso propyl chloride or 2-chloropropane

(3) and (4) are position isomers

| secondary butyl chloride or 2-chlorobutane.

It is optically active, C2 is asymmetric

− − tertiary butyl chloride or 2-chloro-2-methyl propane

(5) and (6) are position isomers (5) and (7) are chain isomers, (6) and (8) are chain isomers, (7) and (8) are position isomers

These alkyl halides are classified as: (a) primary, (b) secondary, and (c) tertiary alkyl halides based on the nature of carbon atom to which halogen atom is connected

(a) Primary carbon: It is that, which is connected to only one more carbon If halogen is

connected to any primary carbon, that is primary alkyl halide Alternatively if the carbon

holding the halogen atom is connected to two H-atoms, that is primary alkyl halide General formula is R CH

X

2

−

| denoted as °1.

In the alkyl halides shown above—(1), (2), (3), (5), and (7) are primary alkyl halides

(b) Secondary carbon: It is that, which is connected to two other carbon atoms If halogen

is connected to a secondary carbon holding only one H-atom that is a secondary halogen derivative or alkyl halide General formula R CH R

X

1

| denoted as °2.

(4) and (6) are secondary alkyl halides

(c) Tertiary carbon: It is connected to three other carbon atoms If halogen atom is connected

to tertiary carbon, that is tertiary alkyl halide It does not have a H-atom on the carbon

holding halogen It is denoted as°3 General formula

′

− − ′′

R

R C RX

|

| (8) is tertiary alkyl halide

1.1.2 Dihalogen Derivatives

If two hydrogen atoms of a hydrocarbon are displaced by two halogen atoms, the compound formed is dihalogen derivative General formula CnH2nX2

These are classified into three types:

1 Gem dihalogen derivatives: If both the halogen atoms are connected to the same carbon, they are geminal dihalogen derivatives or dihaloalkanes-idene is suffixed to the name of alkyl group in writing their names

(a) CH2Cl2 methylidene chloride or 1, 1-dichloromethane

(b) CH3–CHCl2 ethylidene chloride or 1, 1-dichloroethane

(c) CH3CH2CHCl2 propylidene chloride or 1, 1-dichloropropane

3− − 3isopropylidene chloride or 2, 2-dichloropropane

(c) and (d) are position isomers

2 Vicinal dihalogen derivatives: If two halogen atoms are present on adjacent carbon atoms—they are vicinal dihalogen derivatives They are named as addition product of corresponding alkene

| | 1, 2-dichloropropane or propylene chloride.

3 Isolated dihalogen derivatives: If halogen atoms scatter in the carbon skeleton, they are isolated

halogen derivatives If they are on terminal carbon atoms, they are also called polymethylene halides.

If three H-atoms are displaced by three halogen atoms, they are trihalogen derivatives General formula

is CnH2n–1X3 Among them those which are having the three halogen atoms on the same carbon are important

CHCl3 chloroform, trichloromethane

CHBr3 bromoform, tribromo methane

CHI3 iodoform, triiodomethane

2

| 1, 1, 2-tribromopropane.

1.1.4 tetrahalogen Derivatives

If 4 H-atoms are displaced by 4 halogen atoms—they are tetrahalogen derivatives CnH2n–2X4

CCl4—carbon tetrachloride or tetrachloromethane

1.1.6 unsaturated Halogen Derivatives

They are named as derivatives of alkenes or alkynes They are haloalkenes or halo alkynes (alkenyl halides

or alkynyl halides)

These are classified into two categories:

1 Vinyl type: These have halogen connected to a doubly bonded carbon

|Cl

2= vinyl chloride chloroethene

CH CH CH

|Cl

3− = 1-chloropropene

CH C CHCl

3− = 2

CH3–C ≡ C–Cl 1-chloropropyne

The doubly bonded carbon to which X is connected is called vinylic carbon.

2 Allyl type: In these compounds—the halogen is connected to a saturated carbon of alkene (which

is connected to doubly bonded carbon) The saturated carbon connected to doubly bonded carbon

1.1.7 nomenclature of aliphatic Halogen Derivatives

1 The longest continuous carbon chain containing halogen atom is selected as parent chain If a double or triple bond is present—the parent chain should have both double or triple bond as well

as halogen atom as far as possible But double or triple bond takes priority over halogen atom as it

is considered as a substituent as far as naming is concerned

2 The carbon atoms of the parent chain are allotted serial numbers from left to right or right to left

in such a manner that the carbon holding the halogen gets lowest serial number if the molecule is saturated

If there is a double or triple bond, priority given to the double or triple bond while allotting serial numbers

3 An appropriate word root is written depending on the number of carbon atoms present in the parent chain

4 A primary suffix like ane, ene or yne is included depending on the nature of C–C bonds in the parent chain

5 The names of halogen atoms present in the molecule are prefixed before word root in alphabetical order.

6 Numerical prefixes like di, tri are used if two or more number of halogen atoms are present Their positions in the parent chain are shown in front of their names

7 The serial number of the carbon atom holding two or more same halogen atom is repeated twice, thrice like that

8 Names and numbers are separated by hyphens, numbers are separated by commas

9 If alkyl groups are also present as substituents, their names are also prefixed to word root along with the serial number of the carbon atom to which they are connected in the parent chain in alphabetical order

1.1.8 aromatic Halogen Derivatives

These are classified into two types: (1) Nuclear substituted halogen derivatives, and (2) Side chain

substituted halogen derivatives

1.1.8.1 Nuclear Substituted Halogen Derivatives

These are compounds in which halogen atom is connected to a carbon atom of the benzene ring These are haloarenes or aryl halides

1.1.8.2 Side Chain Substituted Halogen Derivatives

They are aralkyl halides Halogen atom is present in the side chain

1.2 isomerism

1.2.1 chain isomers

Two or more alkyl halides having the same formula, however differing in the arrangement of carbon atoms in the parent chain, without a difference in the position of X are chain isomers

These two differ only in the carbon skeleton They are chain isomers

No difference in the position of halogen atom in both

2.

all the four are position isomers 1, 3-dichloropropane including stereo-isomers, these are 5

these 4 are position isomers

1.2.3 ring-chain isomerism

An unsaturate halogen derivative and a cycloalkyl halide are ring chain isomers

Chlorocyclopropane and allyl chloride—both have the same formula C3H5Cl, but one has open chain structure and the other has ring structure

The one in which similar atoms or groups are on the same side is cis isomer The other in which they are on opposite side is trans isomer These two don’t interchange as the double bond does not allow

the bond to rotate at room temperature

They have different physical and chemical properties:

■trans has higher melting point and more stable.

But trans is more polar than cis isomer, in this case as positive and negative pole are separated

or centre of symmetry It should be chiral 2-chlorobutane is optically active as C2 is asymmetric

These two molecules have object-mirror image relationship, and don’t superimpose on each other

One rotates plane polarised light towards right side That is d-isomer The other rotates it towards left side That is l-isomer.

Unsaturated halogen derivatives also can exhibit optical isomerism, provided they have a chiral centre

It exits in four stereo isomeric forms cis and its mirror image, trans and its mirror image.

Halogen derivatives of cycloalkanes also exhibit optical isomerism

Both cis and trans isomers of 1, 4-dichlorocyclohexane are inactive as former has plane of

symmetry, latter has centre of symmetry

In fact both don’t have asymmetric carbon atoms

1.2.6 conformational isomerism

It arises due to C–C bond rotation When C–C bond rotates, the atoms or groups on one carbon, arrange themselves in different positions in space around that carbon, with respect to those on adjacent carbon Such different forms of a molecule, with different spatial arrangement of atoms of groups around a carbon atom, arising due to C–C bond rotation are conformers

The angle through which the C–C bond rotates is dihedral angle If it is 60°—1, 2 dichloroethane exists in 6 different conformers

Among these—antiform is more stable as it does not have both torsional strain and steric strain.

anti > gauche > half eclipsed > cis eclipsed cis eclipsed has both torsional and steric strain and become least stable.

Conformers cannot be separated They interchange at a rapid rate, as the energy required for interchange is less, and more energy is available during collisions The actual number of conformers is infinite

1.3 preparation of alipHatic Halogen Derivatives

HI is a strong reducing agent and reduces the alkyl iodide formed back into alkane To prevent

backward reaction, oxidising agents like conc HNO3, H3PO4, HIO3 or HgO are added

R H+I IR I+I H

5HI+HIO3 →3I2 +3H O22HI+HNO3 →NO2 +H O2 +I22HI+HgO →I2 +Hg+H O2Reactivity of halogen F2 > Cl2 > Br2 > I2

Mechanism: It is a free radical substitution and chain reaction:

It is a chain initiation reaction

Second stage is more endothermic during bromination Energy of activation of chlorination is

3 kcal/mole whereas for bromination it is 18 kcal/mole Hence, chlorination takes place more easily than bromination Eact is, the energy required for the formation of transition state, i.e [R H X] As this value increases rate of reaction decreases Thus, reactivity is fluorination > chlorination > bromination

> iodination Transition state collapses into R⋅ subsequently

Selectivity: Bromine being less reactive prefers to displace those H-atoms which are easily placed Rates of displacement of 1°, 2° and 3° H-atoms are:

Halogenation (mono) of an alkane produces a number of isomers including stereoisomers

Totally they are 6, including stereoisomers

If the product is optically active—racemic mixture is formed

Sulphuryl chloride and t-butoxy chloride are other reagents used for chlorination.

1 R′–O–O–R′ → 2R′O

2

3.

4 SO Cl2 →SO2+Cl⋅

5 Cl RH HCl R⋅ + → + ⋅ stages 3, 4, 5 are propagation stages

(C2H5)4Pb initiates chlorination even in dark at 150°C

O2 slows down chlorination

1.3.2 formation of unsaturated Halogen Derivatives

Alkenes undergo allylic substitution (in a-position) which is also a free radical substitution

2− = 2 + 2→ 2− = 2 +

(2) and (3) are chain propagation stages

This is allylic rearrangement

The rearranged product is more stable as it has more number of a-H-atoms and stabilized by more number of hyper conjugate resonating structures Free radicals don’t undergo hydride or alkyl shifts

4 Allylic bromination is carried out with N-bromosuccinimide (NBS)

(a)

(b)

(c)

The bromine so formed should be in small quantity then substitution takes place in allyl position Addition takes place only when it is in larger quantity which is not possible under these circumstances of this experiment

1.3.3 from alkenes and alkynes

Alkenes undergo addition with H–X, and form alkyl halides It is Grooves process

It is electrophilic addition H⊕ acting as an electrophile initiates the reaction An intermediate carbocation is formed It combines with X

Rearrangements, ring expansion are common whereever possible

If the product is optically active—a racemic mixture is formed

Presence of electron releasing groups increase the rate of reaction Withdrawing groups decrease the rate of reaction

If the alkene is unsymmetrical addition takes place via Markovnikov’s rule

Alkenes give vicinal dihalogen derivatives with halogens

cis alkene gives racemic mixture if the product is optically active and transisomer gives meso

dihalogen derivative

Alkynes give gem dihalogen derivatives with H–X

1.3.3.1 Anti Markovnikov Addition

Only addition of HBr takes place against Markovnikov rule in presence of peroxide

Addition of HCl or HI does not take place against Markovnikov rule

No rearrangements like hydride shift take place during free radical addition Only p bond can shift

if rearranged species is more stable

1.3.4 from alcohols

1 Alcohols: It give alkyl halides when treated with H–X The reaction is catalysed by Lewis acid like ZnCl2

If the alcohol is primary or secondary—this reaction takes place via SN2 mechanism

Secondary and tertiary alcohols undergo this reaction via SN1 mechanism

As this reaction is accompanied by formation of carbocation—a racemic mixture is formed if the product is optically active.

Presence of releasing groups increase the rate of reaction withdrawing groups decrease the rate

of reaction,

This reaction is useful in detecting—primary, secondary tertiary alcohols—Lucas test

A mixture of conc HCl anhydrous ZnCl2 is called Lucas reagent When this reagent is added

to tertiary alcohol—a turbidity is formed immediately Secondary alcohols form turbidity after

5 minutes Primary alcohols don’t give turbidity at room temperature The reason is—the reaction proceeds via SN1 mechanism The intermediate carbocation formed from tertiary alcohols is more stable and more easily formed Primary alcohols don’t generate the primary carbocation unless heated as it is less stable So, primary alcohols don’t form turbidity at room temperature

2 From alcohols by the action of PX 3 : Alcohols give an alkyl halide with PX3.

3 Alkyl chlorides from PCl 5 : In solid state PCl5 exists as PCl6 and PCl⊕4 Alcohols form alkyl chlorides with PCl5.

The probable mechanism is

4 (i) From thionyl chloride: It is a Darzen’s method Thionyl chloride also converts an alcohol

into the corresponding alkyl chloride In the absence of pyridine, the reaction proceeds via

SNi mechanism without inversion

The configuration of the parent is retained as the insertion of nucleophile takes place on the same side from which leaving group leaves The nucleophile is not coming from outside

It is an atom present in the substrate It is intramolecular substitution, nucleophilic (SNi)

(ii) If thionyl chloride attacks the alcohol in presence of pyridine—it becomes SN2 reaction, accompanied by inversion of configuration

HCl released interacts with pyridine and forms pyridine hydrochloride, which is an ionic pound Cl present in isolated state attacks the carbon holding the leaving group So, configuration

com-is inverted as shown above

Thus, alcohols react with SOCl2 in two different ways:

1 In the absence of pyridine SNi—Parent and product have the same configuration No inversion takes place

2 In the presence of pyridine—Reaction is SN2 Inversion takes place

Preparation of alkyl chlorides by this method is convenient as the other products are gases and escape, leaving the product only

1.3.5 from carboxylic acids

1.3.5.1 Borodine-Hunsdiecker reaction

When the silver salt of a carboxylic acid is treated with Cl2/CCl4 or Br2/CCl4, alkyl chloride or bromide with one carbon less than those in the acid is formed Silver salt of an aromatic acid gives aryl halide Iodides and fluorides cannot be prepared by this method

Bromides are more easily obtained than chloride

Primary alkyl bromide or chloride is more easily formed than secondary or tertiary

If the alkyl group in the acid is tertiary—instead of a halide, an alkene is formed

Mechanism:

When silver salt of an acid is treated with iodine—instead of RI—an ester is formed It is Birnbaum Simonini reaction

NaCl, NaBr are insoluble in acetone

SWARTS reaction: An alkyl chloride or bromide, when heated with AgF or Hg2F2, or SbF5 or COF3 give fluorides

These reactions are electrophilic substitution reactions Halogen interacts with Lewis acid and generates halonium ion X⊕ acts as electrophile and gets connected to a carbon atom of the ring forming a carbocation Even though it is stabilized by resonance, it is not aromatic as it is not planar

and does not have (4n + 2) p electron To regain the aromaticity lost, a H⊕ is released The s bond pair connecting C and H enters the ring forms a 3rd p bond and aromaticity is restored

2 Preparation of side chain substituted halogen derivatives: When toluene is treated with Cl2

in presence of light or sulphuryl chloride/peroxide—halogen is introduced in the benzene ring Benzyl chloride is formed It is free radical substitution

(a) Cl Cl− hv →2Cl⋅

(b)

(c)

Free radical also is stabilisd by resonance

1.4.2 from Diazonium salts

1 Sandmeyer reaction: When benzene diazonium chloride is treated with HCl/CuCl—chlorobenzene

is formed If treated with CuBr/HBr—bromobenzene is formed This reaction is Sandmeyer reaction

Mechanism: The exact mechanism is not certain It is believed that diazonium ion is reduced by cuprous ion

There is an alternative mechanism

2 Iodobenzene: It is prepared by treating benzene diazonium chloride with KI.

2 They are insoluble in water but soluble in organic solvents, like benzene, ether, chloroform.

3 They are Polar However, they are insoluble in water as they are unable to form H-bands with water.Polarity order CH3Cl > CH3F > CH3Br > CH3I

Methyl chloride is more polar than fluoride as Cl is large and C–Cl bond length is larger While going from CH3F to CH3Cl– charge component decreases But distance between positive, negative poles increases The decrease in charge is less than the increase in the distance Hence, dipole moment increases from CH3F to CH3Cl

CH Cl CH Cl CHCl CCl Dipole moment decreases

CCl CHCl CH Cl CH Cl Size decreases as molecular weight

4 > 3 > 2 2 > 3

ddecreases inter molecular attractions decrease Boiling pointts decrease

Structure: The sp3 hybrid orbital of carbon overlaps with the p orbital of halogen when C–X bond

is formed, so alkyl halides are tetrahedral

1.5.1 chemical reactions

Alkyl halides undergo the following major reactions:

1 Nucleophilic substitution: Several nucleophiles displace the halogen atom forming different products

2 Elimination: A b–H atom and halogen are removed from alkyl halide, and p bond is formed between a, b-carbons An alkene is formed

3 Reduction: Alkyl halides are reduced to alkanes when treated with a reducing agent

4 Formation of Grignard reagent: When refluxed with magnesium metal in pressure of dry ether,

they form alkyl magnesium halides called Grignard reagent

5 Wurtz reaction: They form higher alkanes when heated with sodium metal in dry ether

6 Friedel-crafts reaction: They are useful in the introduction of an alkyl group in the benzene ring

in the place of hydrogenation, when heated with benzene in presence of a Lewis acid like AlCl3 Arenes are formed

1.5.2 nucleophilic substitution reactions

A nucleophilie is a lone pair donor A base is also a lone pair donor But base donates lone pair to a proton (H+) and accepts it

A nucleophile donates lone pair to an atom other than H⊕ generally to a carbon atom and displaces

an existing atom or group, present on that carbon The displaced atom or group is called leaving group

The nucleophile occupying its place is entering group

The leaving group leaves with the bond pair The entering group provides a lone pair for the formation of the new bond with the carbon holding the leaving group The leaving group is also a base But it is a weaker base than the entering group

Thus, in any nucleophilic substitution reaction, a strong base displaces a weak base but not the reverse

Two types of nucleophilic substitution: SN2 and SN1 whenever a primary alkyl halide is involved

in a nucleophilic substitution, the rate of displacement was found to be depending on the concentration

of both substrate and nucleophile

If the concentration of any one is increased, rate increases If the concentration of any one of these two is decreased, rate also decreases As the rate depends on the concentration of two species, it is a bimolecular reaction Two species are involved in its rate determining stage The mechanism proposed for it is—SN2

SN2—stands for nucleophilic substitution—bimolecularwhen tertiary alkyl halide is involved in a nucleophilic substitution—it was noticed that rate depends on the concentration of only substrate It was noticed that rate of this reaction is independent of concentration

of nucleophile So, only one species involves in its rate determining stage and it becomes unimolecular reaction A different mechanism is proposed for it It is SN1

SN1—stands for—nucleophilic substitution unimolecular

It was also noticed that—a secondary alkyl halide involves not only in SN2 reaction, but also

SN1 reaction depending on the condition of reaction

1.5.3 s n 2-substitution, nucleophilic—bimolecular

The nucleophile attacks the carbon, holding the leaving group, on the rear side Its orbital starts

overlapping with the smaller lobe of sp3 hybrid orbital, of C-atom whose larger lobe is involved in the formation of a bond with the leaving group on the opposite side

Slowly the size of that smaller lobe goes on increasing and that of larger one goes on decreasing

A new bond begins to form between that carbon and nucleophile The existing bond between that carbon and leaving group begins to break At one stage the leaving group as well as entering group both are

held by the same carbon when the size of both the lobes become equal as if it becomes p-orbital It is a

transition state The leaving group, the nucleophile and the carbon holding them lie in the same plane in this state The other existing groups extend above and below this plane

Soon the bond between carbon and leaving group breaks A new bond is formed between that carbon and nucleophile on the backside of leaving group The other groups existing on the carbon are pushed to opposite side The configuration of product molecule is opposite of parent molecule It is

called Walden inversion It is compared to an umbrella in a gale.

Thus, a SN2 reaction is a single stage reaction Departure of the leaving group and insertion of nucleophile in its place are simultaneous and not one after the other No intermediate is formed So, a

SN2 reaction is not accompanied by rearrangements and ring expansion

The nucleophile and substrate—both are involved in the only stage, which is rate determining stage So, it is a bimolecular reaction If the concentration of any one is increased—collision frequency increases, transition state is formed soon, rate increases If concentration of any one decreases, rate decreases

Due to rear side attack of nucleophile on the carbon holding the leaving group, it gets connected to that carbon on the opposite side of leaving group Other groups on the carbon are pushed to the side of leaving groups during its departure It leads to inversion of configuration

Thus, the mechanism proposed explained all observed facts

1.5.3.1 Factors Affecting Rate of S N 2 Reaction

1 Nature of substrate: As crowding around the carbon holding the leaving group increases, rear side attack of nucleophile becomes difficult and rate of SN2 goes on decreasing So, primary alkyl

halides with two H-atoms on the carbon holding leaving group is more reactive as H-atoms are the smallest atoms and which don’t occupy much space.

Tertiary alkyl halide is least reactive with 3 alkyl groups on the carbon holding the leaving group There is much crowding around it Nucleophile cannot attack it so easily due to lack of sufficient space

Presence of a p bond between b–γ carbons or presence of an atom with lone pair in b-position increases rate of SN2 reaction, due to delocalization of electron cloud in transition state It increases its stability and transition state is formed more easily Rate increases

A halogen at bridge head carbon cannot be displaced

Steric effect is responsible for this Nucleophile cannot approach the carbon holding the leaving group

Vinyl halide and Haloarenes are almost inert to nucleophilic substitution, due to the stability acquired by them via resonance

|

− −C X bond acquires partial double bond character and its displacement requires breaking of strong bond and more energy is necessary

If the double bond of groups like− −C

O||

are present in b-position—the above effect does not appear

Neopentyl chloride is a primary alkyl chloride, but the tertiary butyl group connected to the carbon holding halogen atom is so bulky that nucleophile is unable to attack the carbon to displace –Cl

So, neopentyl chloride is less reactive than tertiary butyl chloride

Stability of transition state when double bond of oxygen or group are present in

b-position

2 Effect of nucleophile: Nucleophile is a lone pair donor A nucleophile which can donate the lone pair easily is a soft nucleophile, that which cannot donate it so easily is a hard nucleophile The ability to donate a lone pair is nucleophilicity It also affects the rate of SN2 reaction

In the above examples, the first reaction takes place more easily The reason is −OH is a better nucleophile than C6H5O– which is stabilized by resonance and becomes a hard nucleophile Nucleophilicity depends on several factors:

(a) If the same atom, in two species, donates lone pair, the negatively charged one is better nucleophile than neutral one

CH O3−:

 is better nucleophile than CH OH3

(b) In a period from left to right—nucleophilicity decreases

is a better nucleophile than

As size of the donor decreases—lone pair is close to its nucleus and more tightly held

by it and less readily donated Above all—electronegativity increases from left to right So, nucleophilicity decreases

Hence,

(c) In a group—from top to bottom—nucleophilicity increases as size increases and ativity decreases