Matriculation Chemistry ( Introduction to Organic Compound ) part 4

REACTIONS OF REACTIONS OF ORGANIC ORGANIC

COMPOUNDSCOMPOUNDS

Lecture 7Lecture 712.5 Reactions of Organic 12.5 Reactions of Organic

CompoundsCompoundsLearning Outcomes:Learning Outcomes:

At the end of the lesson the students At the end of the lesson the students should be able to :should be able to :

Explain covalent bond cleavage:Explain covalent bond cleavage:homolytic homolytic heterolytic heterolytic

Types of Covalent Bond Types of Covalent Bond Cleavage/FissionCleavage/Fission

All chemical reactions involved bond All chemical reactions involved bond breaking and bond making. breaking and bond making.

Two types of covalent bond Two types of covalent bond cleavage :-cleavage :-

Homolytic cleavageHomolytic cleavage

Heterolytic cleavageHeterolytic cleavage

a)a) Homolytic CleavageHomolytic Cleavage

Occurs in a non-polar bond involving Occurs in a non-polar bond involving two atoms of similar electronegativity.two atoms of similar electronegativity.

A single bond breaks symmetrically A single bond breaks symmetrically into two equal parts, leaving each into two equal parts, leaving each atom with one unpaired electron.atom with one unpaired electron.

Formed free radicals.Formed free radicals.

X : X X : X

Example:

X + X

free radicals

XX XX

b) Heterolytic cleavageb) Heterolytic cleavage

Occurs in a polar bond involving unequal Occurs in a polar bond involving unequal sharing of electron pair between two sharing of electron pair between two atoms of different electronegativities.atoms of different electronegativities.

A single bond breaks unsymmetrically.A single bond breaks unsymmetrically. Both the bonding electrons are transferred Both the bonding electrons are transferred

to the more electronegative atom.to the more electronegative atom.

Formed cation and anion.Formed cation and anion.

A is more electronegative.

B is more electronegative.

A:- + B+

anion cation

A : B

A+ + B:-

cation anion

Reaction IntermediatesReaction Intermediates

a)a) CarbocationCarbocation b) Carbanionb) Carbanion c)c) Free RadicalFree Radical

They are They are unstable and highly reactive.unstable and highly reactive.

a)a) CarbocationCarbocation

Also called carbonium ion.Also called carbonium ion. A very reactive species with a A very reactive species with a

positive charge on a carbon atom.positive charge on a carbon atom. Carbocation is formed in heterolytic Carbocation is formed in heterolytic

cleavage.cleavage.

Example :Example :

(CH(CH33))33C — Cl C — Cl (CH(CH33))33CC++ + + ClCl--

carbocation anion carbocation anion

Chlorine is more electronegative than Chlorine is more electronegative than carbon and the C—Cl bond is polar. carbon and the C—Cl bond is polar.

The C—Cl bond breaks heterolitically and The C—Cl bond breaks heterolitically and both the bonding electrons are transferred both the bonding electrons are transferred to chlorine atom to form anion and to chlorine atom to form anion and carbocation.carbocation.

b) Carbanionb) Carbanion

is an anion counterpartis an anion counterpart a species with a negative charge on a species with a negative charge on

a carbon atom. a carbon atom. Carbanion is formed in heterolytic Carbanion is formed in heterolytic

cleavage. cleavage.

example:example:

(CH(CH33))33C — Li C — Li (CH(CH33))33CC-- + +

LiLi++ carbanion carbanion kationkation

b)b) Free RadicalFree Radical

A very reactive species with an unpaired electron.A very reactive species with an unpaired electron.Formed in homolytic cleavage.Formed in homolytic cleavage.Examples:Examples:

i) i)

free radicalsfree radicals

Cl – ClCl – Cl uv uv ClCl●● + Cl + Cl ●●

C C C ●+ C●

H3C H H3C● H●+

ii)

iii)

Lecture 7Lecture 712.5 Reactions of Organic 12.5 Reactions of Organic

CompoundsCompoundsLearning Outcomes:Learning Outcomes:

At the end of the lesson the students At the end of the lesson the students should be able to:should be able to:

State the relative stabilities of primary, State the relative stabilities of primary, secondary and tertiary free radicals, secondary and tertiary free radicals, carbocations and carbanions. carbocations and carbanions.

Explain the inductive effect of alkyl group Explain the inductive effect of alkyl group towards the stability of carbocations and towards the stability of carbocations and carbanions.carbanions.

Define electrophile and nucleophile.Define electrophile and nucleophile.

Relative Stabilities of Carbocations,Relative Stabilities of Carbocations,

Carbanions and Free RadicalsCarbanions and Free Radicals Carbocation, carbanion and free Carbocation, carbanion and free

radical can be classified into:radical can be classified into: PrimaryPrimary SecondarySecondary TertiaryTertiary

The alkyl groups (electron-releasing The alkyl groups (electron-releasing group) stabilise the positive charge group) stabilise the positive charge on the carbocation.on the carbocation.

The stability of carbocation increases The stability of carbocation increases with the number of alkyl groups with the number of alkyl groups present. present.

Carbocation Stability

Carbocation Stability:Carbocation Stability:

H

C HH+

R

C RH+

R

C RR+

H

C RH+

< < <

Methylcation

Primary10

Secondary20

Tertiary30

Increasing stability

Carbanion Stability

o Alkyl group and other electron-Alkyl group and other electron-donating groups destabilise donating groups destabilise carbanions.carbanions.

o Electron withdrawing group (e.g: Electron withdrawing group (e.g: halogen) stabilise carbanions through halogen) stabilise carbanions through the inductive withdrawal of electron the inductive withdrawal of electron density density

Carbanion Stability:Carbanion Stability:

H

C HH-

R

C RH-

R

C RR -

H

C RH -< < <

Methylanion

Primary10

Secondary20

Tertiary30

Increasing stability

Free-radical stabilityFree-radical stability

The stability of free radical increases The stability of free radical increases as more alkyl groups are attached to as more alkyl groups are attached to the carbon atom with unpaired the carbon atom with unpaired electron.electron.

H R

C RH . C RH .

H

C HH . < < <

methylradical

Primary10

Secondary20

R

C RR .

Tertiary30

Increasing stability

Free Radical Stability :Free Radical Stability :

Reagents and sites of organic Reagents and sites of organic reactionsreactions

A) ElectrophileA) Electrophile Means ‘electron loving’.Means ‘electron loving’. An electron-deficient species and An electron-deficient species and

accepting electron from an attacking accepting electron from an attacking nucleophile.nucleophile.

Can be either neutral or positively Can be either neutral or positively chargedcharged

Examples of electrophiles :Examples of electrophiles : cations such as Hcations such as H++, H, H33OO++, NO, NO22

++ etc. etc. carbocations.carbocations. Lewis acids such as AlClLewis acids such as AlCl33, FeCl, FeCl33, BF, BF33

etc.etc. oxidizing agents such as Cloxidizing agents such as Cl22, Br, Br22 and and

etc.etc.

electrophilic sites electrophilic sites areare molecules with molecules with low electron density around a polar low electron density around a polar bond bond

Examples:Examples:

+ + - - + + - -

-C = O (carbonyl) ; -C – X (haloalkanes)-C = O (carbonyl) ; -C – X (haloalkanes)

+ + --

-C – OH (hydroxy compounds)-C – OH (hydroxy compounds)

i) ii)

iii)

b)b) NucleophileNucleophile means ‘nucleus loving’means ‘nucleus loving’ An electron-rich species and An electron-rich species and

electron-pair donor.electron-pair donor. A nucleophile can be either neutral A nucleophile can be either neutral

or negatively charged.or negatively charged.

Examples of nucleophiles :Examples of nucleophiles :

anions such as OHanions such as OH--, RO, RO--, Cl, Cl--, CN, CN-- etc. etc.

carbanions ( species with a negative carbanions ( species with a negative charge on carbon atoms ).charge on carbon atoms ).

Lecture 7Lecture 712.5 Reactions of Organic 12.5 Reactions of Organic

CompoundsCompoundsLearning Outcomes:Learning Outcomes:

At the end of the lesson the students shouldAt the end of the lesson the students should

be able to ebe able to explain the main types of organic xplain the main types of organic reactions:reactions:

addition: electrophilic and nucleophilicaddition: electrophilic and nucleophilic substitution: electrophilic, nucleophilic and substitution: electrophilic, nucleophilic and

free radical free radical eliminationelimination rearrangement rearrangement

4 Types of Organic 4 Types of Organic ReactionsReactions

Addition Addition

Substitution Substitution

Elimination Elimination

Rearrangement Rearrangement

I) Addition ReactionI) Addition Reaction

A reaction in which atoms or groups A reaction in which atoms or groups addadd to adjacent atoms of a multiple to adjacent atoms of a multiple bond.bond.

Two types of addition :-Two types of addition :-

a)a) Electrophilic AdditionElectrophilic Addition

b)b) Nucleophilic AdditionNucleophilic Addition

a)Electrophilic Additiona)Electrophilic Addition Initiated by an electrophile accepting Initiated by an electrophile accepting

electron from an attacking nucleophile.electron from an attacking nucleophile. Typical reaction of unsaturated Typical reaction of unsaturated

compounds such as alkenes and alkynes.compounds such as alkenes and alkynes.

Example :Example :

CHCH33CH=CHCH=CH22 + Br + Br22 CH CH33CHBrCHCHBrCH22BrBr

electrophileelectrophile

Room temperature

CCl4

b)b) Nucleophilic AdditionNucleophilic Addition

Initiated by a nucleophile, which Initiated by a nucleophile, which attacks an electrophilic site of a attacks an electrophilic site of a molecule. molecule. – Typical reaction of carbonyl compounds.Typical reaction of carbonyl compounds.

CCH3 CH3

O

+ HCN CH3 C CH3

CN

OH

II)II) Substitution ReactionSubstitution Reaction A reaction in which an atom or group in A reaction in which an atom or group in

a molecule is replaced by another atom a molecule is replaced by another atom or group.or group.

Three types of substitution :-Three types of substitution :-

a)a) free radical substitution.free radical substitution.

b)b) electrophilic substitution.electrophilic substitution.

c)c) nuclephilic substitution.nuclephilic substitution.

a)a) Free-radical SubstitutionFree-radical Substitution Substitution which involves free Substitution which involves free

radicals as intermediate species. radicals as intermediate species. Example :Example :

CHCH33CHCH33 + Cl + Cl22 uv lightuv light CH CH33CHCH22Cl + Cl + HClHCl

b)b) Electrophilic SubstitutionElectrophilic Substitution

Typical reaction of aromatic Typical reaction of aromatic compounds.compounds.– The aromatic nucleus has high electron The aromatic nucleus has high electron

density, thus it is nucleophilic and is prone density, thus it is nucleophilic and is prone to electrophilic attack.to electrophilic attack.

+ Br+ Br22 Br + Br + HBrHBr

electrophileelectrophile

Fecatalyst

Example:

c) c) Nucleophilic SubstitutionNucleophilic Substitution

Typical reaction of saturated organic Typical reaction of saturated organic compounds bearing polar bond as functional compounds bearing polar bond as functional group, such as haloalkane with alcohol. group, such as haloalkane with alcohol.

Example :Example :

CHCH33CHCH22Br + OH-(aq) CHBr + OH-(aq) CH33CHCH22OH + Br-OH + Br-

(aq)(aq) nucleophilenucleophile

Δ

III) Elimination ReactionIII) Elimination Reaction– An atoms or groups are removed from An atoms or groups are removed from

adjacent carbon atoms of a molecule to adjacent carbon atoms of a molecule to form a multiple bond (double or triple form a multiple bond (double or triple bond).bond).

– Results in the formation of unsaturated Results in the formation of unsaturated molecules.molecules.

– Example :Example :

CHCH33CHCH22OH OH conc. H conc. H22SOSO44 CH CH22= CH= CH22 + H + H22OO

IV) Rearrangement IV) Rearrangement ReactionReaction

A reaction in which atoms or groups in a A reaction in which atoms or groups in a molecule molecule change positionchange position..

Occurs when a single reactant reorganizes Occurs when a single reactant reorganizes the bonds and atoms.the bonds and atoms.

Example :Example :

C CC C

OHH

H R R

H

H

H

O

tautomerisme

ExercisesExercises

1.1. Explain how the free radicals are Explain how the free radicals are formed in homolytic cleavage. formed in homolytic cleavage.

2.2. Write an equation for the bromine-Write an equation for the bromine-bromine bond cleavage in the bromine bond cleavage in the bromination of methane. State the bromination of methane. State the type of bond cleavage.type of bond cleavage.

3.3. Which would you expect to be Which would you expect to be the the most stable free radical ?most stable free radical ?

CHCH22CHCH33 , (CH , (CH33))22 CH CH , , CH3 , CH3 , CH3