An Overview of Organic Reactions

Why this chapter?Why this chapter?

To understand organic and/or biochemistry, it is necessary to know:-What occurs-Why and how chemical reactions take place

We will see how a reaction can be described

2

Kinds of Organic Reactions In general, we look at what occurs and try to learn

how it happensCommon patterns describe the changes

◦ Addition reactions – two molecules combine

◦ Elimination reactions – one molecule splits into two

3

◦ Substitution – parts from two molecules exchange

◦ Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected

What kind of reaction is the transformation shown What kind of reaction is the transformation shown below? below?

1. an elimination reaction

2. a rearrangement reaction

3. a substitution reaction

4. an addition reaction5. none of these

+ HClCl

Learning Check:

What kind of reaction is the transformation shown What kind of reaction is the transformation shown below? below?

1. an elimination reaction

2. a rearrangement reaction

3. a substitution reaction

4. an addition reaction5. none of these

+ HClCl

Solution:

6

How Organic Reactions Occur? Mechanisms

In a clock the hands move but the mechanism behind the face is what causes the movement

In an organic reaction, we see the transformation that has occurred. The mechanism describes the steps behind the changes that we can observe

Reactions occur in defined steps that lead from reactant to product

7

Steps in Mechanisms

We classify the types of steps in a sequence

A step involves either the formation or breaking of a covalent bond

Steps can occur in individually or in combination with other steps

When several steps occur at the same time they are said to be concerted

8

Types of Steps in Reaction MechanismsBond formation or breakage can be symmetrical or

unsymetrical Symmetrical- homolyticUnsymmetrical- heterolytic

Bond Breaking

Bond MakingBond Making

9

Indicating Steps in Mechanisms

Curved arrows indicate breaking and forming of bonds

Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’)

Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)

10

Radical ReactionsNot as common as polar reactionsRadicals react to complete electron octet of valence

shell◦ A radical can break a bond in another molecule and

abstract a partner with an electron, giving substitution in the original molecule

◦ A radical can add to an alkene to give a new radical, causing an addition reaction

11

Three types of steps◦ Initiation – homolytic formation of two reactive species

with unpaired electrons Example – formation of Cl atoms form Cl2 and light

◦ Propagation – reaction with molecule to generate radical Example - reaction of chlorine atom with methane to

give HCl and CH3.

◦ Termination – combination of two radicals to form a stable product: CH3

. + CH3. CH3CH3

Steps in Radical Substitution

12

Steps in Radical Substitution: Monochlorination of Methane

Initiation

Propagation

13

Steps in Radical Substitution

Termination

With excess concentration of Cl2 present continued reaction is probable with formation of dichloro, trichloro, and tetrachloro methanes.

In a radical chain reaction, what would In a radical chain reaction, what would be the best description of the following be the best description of the following reaction?reaction?HH33C• + •Cl → CHC• + •Cl → CH33ClCl

1. propagation2. elimination3. initiation4. termination5. substitution

Learning Check:

In a radical chain reaction, what would In a radical chain reaction, what would be the best description of the following be the best description of the following reaction?reaction?HH33C• + •Cl → CHC• + •Cl → CH33ClCl

1. propagation2. elimination3. initiation4. termination5. substitution

Solution:

16

Radical Substitution: With >1 kind of H

When there is • >1 type of H then there is • >1 option for radical formation and therefore • >1 option for a monohalogenation product.

C

H

H

H

C C

H

H

H

H

H

C

H

H

H

C

H

H

C

Cl

H

H

C

H

H

H

C

Cl

H

C

H

H

H+ Cl2 hv +

1-chloropropane 2-chloropropane

In the reaction of ClIn the reaction of Cl22 with 2-methylbutane, with 2-methylbutane, how many how many monochlorinatedmonochlorinated isomers are isomers are produced?produced?

1. 22. 33. 44. 55. 6

C

H

C C

H

H

H

H

H

C

H

H

H

CHH H

+ Cl2 hv

Learning Check:

In the reaction of ClIn the reaction of Cl22 with 2-methylbutane, with 2-methylbutane, how many how many monochlorinatedmonochlorinated isomers are isomers are produced?produced?

1. 22. 33. 44. 55. 6

C

H

C C

H

H

H

H

H

C

H

H

H

CHH H

+ Cl2 hv

Solution:

19

Polar Reactions

Molecules can contain local unsymmetrical electron distributions due to differences in electro negativities

This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom

The more electronegative atom has the greater electron density

Elements such as O, F, N, Cl more electronegative than carbon

21

22

23

Polarizability

Polarization is a change in electron distribution as a response to change in electronic nature of the surroundings

Polarizability is the tendency to undergo polarization

Polar reactions occur between regions of high electron density and regions of low electron density

p. 144

PolarizabilityBonds inherently polar already can be made more polar by reactions with acids or bases.

p. 144

Polarizability

2.55

2.58

2.55

2.66

Bonds not inherently polar can be polarizable as interactions with solvent or other polar molecules effect the electron distribution.

Large atoms with loosely held electrons are more polarizable than small atoms with few tightly held electrons.So: SS is more polarizable than OO

II is more polarizable than ClCl

26

Generalized Polar ReactionsAn electrophileelectrophile, an electron-poor species,

combines with a nucleophilenucleophile, an electron-rich species

An electrophile is a Lewis acidA nucleophile is a Lewis baseThe combination is indicate with a curved arrow from

nucleophile to electrophile

p. 146

Learning Check:Which of the following is likely to be a nucleophile and which an electrophile?

p. 146

Solution:Which of the following is likely to be a nucleophile and which an electrophile?

E EN N

p. 146

Is BF3 is likely to be a nucleophile or an electrophile?

Learning Check:

p. 146

Is BF3 is likely to be a nucleophile or an electrophile?

Solution:

E

Which of the following is expected Which of the following is expected to be the worst nucleophile?to be the worst nucleophile?

1. NH3

2. H2O3. BH3

4. ethylene5. (CH3) 3P

Learning Check:

Which of the following is expected Which of the following is expected to be the worst nucleophile?to be the worst nucleophile?

1. NH3

2. H2O3. BH3

4. ethylene5. (CH3) 3P

Solution:

34

An Example of a Polar Reaction: Addition of HBr to Ethylene

HBr adds to the part of C-C double bondThe bond is e- rich, allowing it to function as a nucleophileH-Br is electron deficient at the H since Br is much more

electronegative, making HBr an electrophile

35

Mechanism of Addition of

HBr to EthyleneHBr electrophile is attacked by electrons of

ethylene (nucleophile) to form a carbocation intermediate and bromide ion

Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br bond

The result is that ethylene and HBr combine to form bromoethane

All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

36

p. 142

Learning Check:What product would you expect?

p. 142

Solution:What product would you expect?

Br

40

Using Curved Arrows in Polar Reaction Mechanisms

Curved arrows are a way to keep track of changes in bonding in polar reaction

The arrows track “electron movement” Electrons always move in pairsCharges change during the reactionOne curved arrow corresponds to one

step in a reaction mechanism

41

Rules for Using Curved ArrowsThe arrow (electrons) goes from the

nucleophilic reaction site (Nu: or Nu:Nu: or Nu:-- ) to the electrophilic reaction site (sink, E or Esink, E or E++)

The nucleophilic site can be neutral or negative

42

The nucleophilic site can be negative or neutralRules for Using Curved ArrowsRules for Using Curved Arrows

43

The electrophilic site can be positive or neutral

44

The octet rule must be followed

The hydrogen already has two e-s so when another pair moves in the 2 already owned have to leave.

What is the role of the alkene in the What is the role of the alkene in the reaction above? reaction above?

1. electrophile2. nucleophile3. free radical4. catalyst5. Lewis acid

+ H3O + H2O

Learning Check:

What is the role of the alkene in the What is the role of the alkene in the reaction above? reaction above?

1. electrophile2. nucleophile3. free radical4. catalyst5. Lewis acid

+ H3O + H2O

Solution:

Learning Check:Add curved arrows to indicate the flow of electrons:

Solution:Add curved arrows to indicate the flow of electrons:

Add curved arrows to indicate the flow of electrons:Learning Check:

50

Add curved arrows to indicate the flow of electrons:Solution:

p. 142

Learning Check:What carbocation intermediate is consistent with the product formed? Propose a mechanism. (Add curved arrows to indicate the flow of electrons.)

Reaction Mechanism Reaction Mechanism • Detailed description of sequence

of steps involved in group from reactantsto products.

• Reactant intermediate

product

Bond Cleavage

B:AB:A A : B A– : B+

+vely charged ion – carbocation-vely charged ion – carbaanion

Heterolytic Cleavage

Homolytic Cleavage

BAB:A

Free radicals.

Carbonium ion• Planar – sp2 hybridised bond

angle 120o

• Has six electrons• Stabilized by resonance or inductive

effect or hyperconjugation

C

Empty unhybridisedp-orbital

sp2 Hybridisation ofcarbon

Planar Strucutre of carbnion

+

Examples of carbonium ion

Benzyl cation

CH2 CH2 CH2

etc

+

+ +

CH 2 CH CH 2 CH 2 CH CH 2 + +

Stabilised through resonance

Allyl cation

H2C CH+

Vinyl cation

no resonance hence unstable.

Stability of Cabocation

(i) By inductive effect

The resonance effect is always more predominant than the inductive effect in stabilizing an ion.

CH3 C

CH3

CH3

CH3 C

CH3

H

> > H C

CH3

H

> >

3° 2°

+ +

1°

+

Stability of Cabocation(ii) By hyperconjugation

H3C — C

CH3

CH2 — H

H3C — C

CH3

CH2H

H3C — C

CH2H

C

CH3

CH3CH3

CH2H

+

+

+

etc. +

Thus, tertiary carbocation is more stable than secondary and so on.

Carbanion• Pyramidal - sp3 hybridised

bond angle 109.28

• Has eight electrons• Stabilized by resonance or by

inductive effect. . . sp3 hybrid orbital

containing lone pair

Tetrahedral structure of carboanion

Stability of Carbanion(i) By resonance

H

-

H

-

H

Cyclopentadienyl carbanion

Stability of Carbanion

(ii) By inductive

CH3 C H

CH3

CH3

CH3 C

CH3

H

C

CH3

H

3° 2° 1°

Stability of Carbanion(iii) Electron-donating groups destabilize a carbanion while electron-withdrawing groups stabilize it.

N O 2 3OC H

>

2CH

2CH

Free Radical• Planar or Pyramidal

• Has seven electrons• Stabilized by resonance or by inductive

effect.• Order of stability of free radical 3o >2o> 1o

C(

Unhybridised orbitalcontaining odd electron

120oC

sp2 hybridised carbon

Planar Sturcutre

+

Classification of ReagentsNucleophilic Reagents (Nucleophiles)• Attacks the positive end of a polar bond or nucleus-

loving is known as nucleophile. • Generally, negatively charged or electron rich

species are nucleophilic. 3 3 2 3e.g. OH , OCH , CN , I , CH COO , NH , CH

2 3 3 2H O, NH , NH — NH

N..

..NH 3,CH 3 — O — CH 3,

..

. ...

C 2H 5 — O H,. ...H 2 O,

• All nucleophiles are in general Lewis bases.

Classification of ReagentsElectrophilic Reagents (Electrophiles)• Attacks a region of high electron density

or electron-loving is known as electrophile.

• All positively charged or electron deficient species are electrophilic.

3 2H , CH , NO , Cl , Br , Ag

Classification of Reagents• Neutral reagents which contain an

electron-deficient atom are also electrophiles.

AlCl3, SO3, BF3, SOCl2, POCl3, FeCl3, ZnCl2

• All electrophiles are in general Lewis acids.

Carbenes• Divalent carbon compound. • Carbon atom is linked to two adjacent

groups by covalent bonding.• A carbene is neutral and possesses two

free electrons, i.e. a total of six electrons. • Electron deficient.

CarbenesCarbene is of two types

(i) Singlet carbene:

(ii)Triplet carbene: Triplet carbene is more stable

than single carbene.

CH2 hybridisation sp2

it is v-shaped

CH2 hybridisation spit is linear shaped

Types of Organic Reactions

Substitution Addition

Elimination Rearrangement

Condensation Isomerisation

Types of Organic ReactionsSubstitution ReactionReplacement of an atom or group by other atom of group

Nucleophilic substitution:

R X OH R OH X

SN1 Reaction: Unimolecular nucleophilic substitution reaction.

Types of Organic Reaction - SN1 Reaction

CH3 — C — CH2Cl

CH3

CH3

OH–

slow

CH3 — C — CH2

CH3

CH3SN1 +

CH3 — C — CH2

CH3

CH3

CH3 — C — CH2 – CH3

CH3

+

+

1, 2-Methyl anionshift

Fast OH–CH3 — C — CH2CH3

OH

CH3

(1)

(2)

Types of Organic Reaction - SN2 ReactionSN2 Reaction: This is called bimolecular nucleophilic substitution and it is one-step process.

H — C — Br + OH

CH2CH3

CH3

–OH C Br

H CH3

.

CH2CH3

FastHO — C — H

CH3

CH2CH3

– –

Transition state unstable

slow

Addition ReactionsThe reagent often adds to

bond and the bond is converted into bond. Can be electrophilic addition or nucleophilic addition.

C C , C O or C N C C ,

Cl22 2 2 2CCl4CH CH Cl CH CH Cl

OH2H

OH

+

(Hydration)

Elimination ReactionsTwo groups on adjacent atoms are lost as a double bond is formed.

CH3 – CH – CH – CH3

OH H

Conc. H2SO4

– H2OCH3 — CH CH – CH3

We divide elimination reactions into three classes.

(1) E1 (2) E1 CB (3) E2

Rearrangement• Migration of a group takes place within the same

molecule.

C6H5

C6H5

C = N

OHC6H5 — C — N — C6H5

O

H

Hether

OHH+

(Beckmann rearrangement)

(Dehydration and rearrangement)

Condensation

H3C — C — CH3 + H3C — C — CH3

O

H3C — C — CH — C — CH3

CH3O O

dil.

NaOH,

Two molecules of same or different reactants combine to give a new product with the elimination of simple byproducts like H2O, NH3, etc,

Isomerisation

C

H

H3C

CH

CH3

C

H

H3C

CCH3

H

h

Class Test

Class Exercise – 1Class Exercise – 1Select the most stable carbocation among the following.

CH3

CH3HC

6 5 3(C H ) C

3 2 2CH CH CH

3 3(CH ) C

(a) (b)

(c) (d)

Solution:

C — C6H5

C6H5

C6H5

C+ +

This carbocation is highly stabilized through resonancewith three benzene rings.

Hence answer is (b).

Class Exercise - 2Which of the following is an addition reaction?

3 3CH CH CH|Br

OH2 3Alcohol

CH CH CH

h3 3 2 3 2CH CH Cl CH CH Cl

(a)

(b)

3 2 3 2CH CH Br CN CH CH CN Br(c)

3 2 3 3|Br

CH CH CH H CH C H CHBr(d)

Solution:H3C — CH — CH3

Br

Alcohol H2C CH — CH3

H3C — CH3 + Cl2 CH3CH2Cl + HCl

H3C — CH2Br + CN H3C — CH2CN + Br

H3C CH CH2 + HBr H3C CH CH3

Br

OH–

(Elimination)

h

–

(Substitution)

Substitution

–

Addition

Hence answer is (d).

Class Exercise - 3

Which of the following is the most effective group in stabilizing a free radical inductively?(a) F (b) I(c) Br (d) Cl

Solution:Since free radical is electron deficient, any substituent with more electron releasing and less electron withdrawing ability will stabilize the radical inductively.The decreasing order of electronegativity of halogens is: F > Cl > Br > I

Hence answer is (b).

Class Exercise - 4

Which of the following is not a nucleophile?(a) CN– (b) BF3

(c) RNH2 (d) OH–

Solution:

Among the following, BF3 is only electron deficient. Hence, it will not act as a nucleophile.

Hence answer is (b).

Class Exercise - 5

Which of the following is the correct order regarding –I effect of the substituents? (a) –NR2 > –OR > –F(b) –NR2 > –OR < –F(c) –NR2 < –OR < –F(d) –OR > –NR2 > –F

Solution:–I effect increases with electronegativity of atom. The decreasing order of electronegativity isF > O > NThe correct order for –I effect is–NR2 < –OR < –F

Hence answer is (c).

Class Exercise - 6The least stable carbonium ion is

(a) (b)

(c) (d)

3 2H C CH

6 5 2 2C H — CH — CH

6 5 2C H — CH

6 5 6 5C H — CH — C H

Solution:

Among the following, (a) is stabilized through +I effect and (b) is destabilized through –I effect of phenyl ring. Other two are stabilized through resonance.

Hence answer is (c).

Class Exercise - 7Arrange the following ions in the decreasing order of stability.

2HC

CH3 CH3 CH3

++

+

(a) (b) (c) (d)

Solution:CH2

+. It is a primary cation. Hence, minimum stability.

CH3CH3

+and +

(c) (b)

are secondary cations.

Hence, stabilized through +I effect of –CH3 group which decreases with distance. (c) is more stable as compared to (b).

(d) is most stable as it is tertiary cation and stabilized through +I effect of –CH3 group and hyper conjugation.

The order is (d) > (c) > (b) > (a)

Class Exercise - 8

Arrange the following radicals in order of their decreasing stability

3 2 3 3 6 5 2 2 2CH C H , (CH ) C, C H C H , CH CH C H

Solution:Radicals are stabilised through electron releasing resonance and inductive effect.

CH2CH2etc.

More resonating structure

H2C CH — CH2 H2C — CH CH2

Solution:One resonating structure, although both are primary radicals.

Among and , later is a tertiary radical. Hence, more stable.

The decreasing order of stability is

H3C — CH2 (CH3)3C

C6H5CH2 > H2C CH — CH2 > (CH3)3C > H3CCH2