matriculation chemistry introduction to organic compound part 4.pdf

8/12/2019 Matriculation Chemistry Introduction to Organic Compound part 4.pdf

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REACTIONS OFREACTIONS OFORGANICORGANIC

COMPOUNDSCOMPOUNDS



Lecture 7Lecture 7

12.5 Reactions of Organic12.5 Reactions of Organic

CompoundsCompoundsLearning Outcomes:Learnin

g Outcomes:

At the end of the lesson the studentsAt the end of the lesson the students

should be able to :should be able to : Eplain co!alent bond clea!age:Eplain co!alent bond clea!age:

homol"tichomol"tic

heterol"ticheterol"tic



Types of Covalent BondTypes of Covalent Bond

Cleavage/FissionCleavage/Fission

All chemical reactions in!ol!ed bondAll chemical reactions in!ol!ed bond

brea#ing and bond ma#ing.brea#ing and bond ma#ing.

$%o t"pes of co!alent bond clea!age :& $%o t"pes of co!alent bond clea!age :&

'omol"tic clea!age'omol"tic clea!age 'eterol"tic clea!age'eterol"tic clea!age



a)a) Homolytic CleavageHomolytic Cleavage

Occurs in a non&polar bond in!ol!ing t%oOccurs in a non&polar bond in!ol!ing t%o

atoms of similar electronegati!it".atoms of similar electronegati!it".

A single bond brea#s s"mmetricall" intoA single bond brea#s s"mmetricall" into

t%o e(ual parts) lea!ing each atom %itht%o e(ual parts) lea!ing each atom %ith

one unpaired electron.one unpaired electron.

*ormed free radicals.*ormed free radicals.



+ : ++ : +

Example:

+• , +•

free radicals

XX XX

• •



b) Heterolytic cleavageb) Heterolytic cleavage

Occurs in a polar bond in!ol!ing une(ualOccurs in a polar bond in!ol!ing une(ualsharing of electron pair bet%een t%osharing of electron pair bet%een t%oatoms of different electronegati!ities.atoms of different electronegati!ities.

A single bond brea#s uns"mmetricall".A single bond brea#s uns"mmetricall". -oth the bonding electrons are transferred-oth the bonding electrons are transferred

to the more electronegati!e atom.to the more electronegati!e atom.

*ormed cation and anion.*ormed cation and anion.



A is moreelectronegative.

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 Radical Free Radical

$he" are $he" are unstable and highl" reacti!e.unstable and highl" reacti!e.



a)a) CarbocationCarbocation

Also called carbonium ion.Also called carbonium ion. A !er" reacti!e species %ith aA !er" reacti!e species %ith a

positi!e charge on a carbon atom.positi!e charge on a carbon atom. Carbocation is formed in heterol"ticCarbocation is formed in heterol"tic

clea!age.clea!age.



Eample :Eample :

δ +δ + δ −δ −

C'C'//00//C ClC Cl C'C'//00//CC,, ,, ClCl&&

carbocation anioncarbocation anion

Chlorine is more electronegati!e thanChlorine is more electronegati!e than

carbon and the CCl bond is polar.carbon and the CCl bond is polar.

$he CCl bond brea#s heteroliticall" and $he CCl bond brea#s heteroliticall" and

both the bonding electrons are transferredboth the bonding electrons are transferred

to chlorine atom to form anion andto chlorine atom to form anion and

carbocation.carbocation.



b) Carbanionb) Carbanion

is an anion counterpartis an anion counterpart

a species %ith a negati!e charge on aa species %ith a negati!e charge on acarbon atom.carbon atom.

Carbanion is formed in heterol"ticCarbanion is formed in heterol"tic

clea!age.clea!age.



eample:eample:

δ +δ + δ −δ −

C'C'//00//C LiC Li C'C'//00//CC&&

,, LiLi,,

carbanioncarbanion

#ation#ation



b)b) Free Radical Free Radical

A !er" reacti!e species %ith an unpaired electron.A !er" reacti!e species %ith an unpaired electron.

*ormed in homol"tic clea!age.*ormed in homol"tic clea!age.

Eamples:Eamples:

i0i0

free radicalsfree radicals

Cl Cl ClCl u!u! ClCl33 , Cl , Cl 33



C C C ●+ C●

H3C H H3C● H●+

ii0

iii0



Lecture 7Lecture 7


CompoundsCompoundsLearning Outcomes:Learning Outcomes:

At the end of the lesson the studentsAt the end of the lesson the studentsshould be able to:should be able to:

4tate the relati!e stabilities of primar")4tate the relati!e stabilities of primar")secondar" and tertiar" free radicals)secondar" and tertiar" free radicals)carbocations and carbanions.carbocations and carbanions.

Eplain the inducti!e effect of al#"l groupEplain the inducti!e effect of al#"l group

to%ards the stabilit" of carbocations andto%ards the stabilit" of carbocations andcarbanions.carbanions.

efine electrophile and nucleophile.efine electrophile and nucleophile.



Relative Stabilities of Carbocations,Relative Stabilities of Carbocations,

Carbanions and Free RadicalsCarbanions and Free Radicals

Carbocation) carbanion and freeCarbocation) carbanion and free

radical can be classified into:radical can be classified into:

PrimaryPrimary

SecondarySecondary

TertiaryTertiary



$he al#"l groups electron&releasing $he al#"l groups electron&releasinggroup0 stabilise the positi!e chargegroup0 stabilise the positi!e charge

on the carbocation.on the carbocation.

$he stabilit" of carbocation increases $he stabilit" of carbocation increases

%ith the number of al#"l groups%ith the number of al#"l groups

present.present.

Carbocation Stability



Carbocation Stability:Carbocation Stability:

H

C HH

+

R

C RH

+

R

C RR

+

H

C RH

+

< < <

Methl

cation

!rimar

"#

$econ%ar

&#

'ertiar

3#

(ncreasing sta)ilit



Carbanion Stability:Carbanion Stability:

H

C HH-

R

C RH-

R

C RR-

H

C RH-< < <

Methl

anion

!rimar

"#

$econ%ar

&#

'ertiar

3#

(ncreasing sta)ilit



Freeradical stability Freeradical stability

$he stabilit" of free radical increases $he stabilit" of free radical increases

as more al#"l groups are attached toas more al#"l groups are attached to

the carbon atom %ith unpairedthe carbon atom %ith unpaired

electron.electron.



H R

C RH.

C RH.

H

C HH.

< < <

methl

ra%ical

!rimar

"#

$econ%ar

&#

R

C RR.

'ertiar

3#

(ncreasing sta)ilit

Free Radical Stability :Free Radical Stability :



!"amples of electrop#iles :!"amples of electrop#iles :

cations such as 'cations such as ',,) ') '//OO,,) 9O) 9O22

,, etc.etc.

carbocations.carbocations.

Le%is acids such as AlClLe%is acids such as AlCl//) *eCl) *eCl//) -*) -*// etc.etc.

oidiing agents such as Cloidiing agents such as Cl22) -r) -r22 and etc.and etc.



electrop#ilic siteselectrop#ilic sites areare molecules withmolecules with

low electron density around a polar bondlow electron density around a polar bondExamples:Examples:

δδ ,, δδ && δδ ,, δδ &&

&C ; O carbon"l0 < &C + haloal#anes0&C ; O carbon"l0 < &C + haloal#anes0

δδ ,, δδ &&

&C O' h"dro" compounds0&C O' h"dro" compounds0

i0 ii0

iii0



b)b) $%cleop#ile$%cleop#ile

means nucleus lo!ing8means nucleus lo!ing8

An electron&rich species andAn electron&rich species andelectron&pair donor.electron&pair donor.

A nucleophile can be either neutralA nucleophile can be either neutral

or negati!el" charged.or negati!el" charged.



Examples of nucleophiles :Examples of nucleophiles :

anions such as O'anions such as O'&&) RO) RO&&) Cl) Cl&&) C9) C9&& etc.etc.

carbanions species %ith a negati!ecarbanions species %ith a negati!e

charge on carbon atoms 0.charge on carbon atoms 0.

L t 7L t 7



Lecture 7Lecture 7


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 eplain the main t"pes of organicplain the main t"pes of organic

reactions:reactions: addition: electrophilic and nucleophilicaddition: electrophilic and nucleophilic

substitution: electrophilic) nucleophilic and freesubstitution: electrophilic) nucleophilic and free

radicalradical

eliminationelimination rearrangementrearrangement



& Types of 'rganic & Types of 'rganic

ReactionsReactions

AdditionAddition

4ubstitution4ubstitution

EliminationElimination RearrangementRearrangement



I) (ddition ReactionI) (ddition Reaction

A reaction in %hich atoms or groupsA reaction in %hich atoms or groups add add to ad=acent atoms of a multiple bond.to ad=acent atoms of a multiple bond.

$%o t"pes of addition :& $%o t"pes of addition :&

a)a) !lectrop#ilic (ddition!lectrop#ilic (ddition

b)b) $%cleop#ilic (ddition$%cleop#ilic (ddition



a)!lectrop#ilic (dditiona)!lectrop#ilic (ddition

>nitiated b" an electrophile accepting>nitiated b" an electrophile acceptingelectron from an attac#ing nucleophile.electron from an attac#ing nucleophile.

$"pical reaction of unsaturated compounds $"pical reaction of unsaturated compounds

such as al#enes and al#"nes.such as al#enes and al#"nes.

Eample :Eample :

C'C'//C';C'C';C'22 , -r, -r22 C' C'//C'-rC'C'-rC'22-r-r

electrophileelectrophile

Roomtemperature

CCl



b)b) $%cleop#ilic (ddition$%cleop#ilic (ddition

>nitiated b" a nucleophile) %hich>nitiated b" a nucleophile) %hich

attac#s an electrophilic site of aattac#s an electrophilic site of a

molecule.molecule.

$"pical reaction of carbon"l compounds. $"pical reaction of carbon"l compounds.

δ −δ −

CCH3 CH3

*

δ ++ HC CH3 C CH3

C

*H



II)II) S%bstit%tion ReactionS%bstit%tion Reaction A reaction in %hich an atom or group inA reaction in %hich an atom or group in

a molecule is replaced b" another atoma molecule is replaced b" another atom

or group.or group.

$hree t"pes of substitution :& $hree t"pes of substitution :&

a0a0 free radical substitution.free radical substitution.

b0b0 electrophilic substitution.electrophilic substitution.

c0c0 nuclephilic substitution.nuclephilic substitution.



a)a) Freeradical S%bstit%tionFreeradical S%bstit%tion 4ubstitution %hich in!ol!es free4ubstitution %hich in!ol!es free

radicals as intermediate species.radicals as intermediate species.

Eample :Eample :

C'C'//C'C'

// , Cl , Cl

22 uv light uv light C' C'

//C'C'

22Cl , 'ClCl , 'Cl



b)b) !lectrop#ilic S%bstit%tion!lectrop#ilic S%bstit%tion

$"pical reaction of aromatic compounds. $"pical reaction of aromatic compounds.

$he aromatic nucleus has high electron $he aromatic nucleus has high electron

densit") thus it is nucleophilic and is pronedensit") thus it is nucleophilic and is prone

to electrophilic attac#.to electrophilic attac#.

, -r, -r22 -r , '-r -r , '-r electrophileelectrophile

,ecatalst

Eample:



c)c) $%cleop#ilic S%bstit%tion$%cleop#ilic S%bstit%tion

$"pical reaction of saturated organic $"pical reaction of saturated organic

compounds bearing polar bond as functionalcompounds bearing polar bond as functionalgroup) such as haloal#ane %ith alcohol.group) such as haloal#ane %ith alcohol.

Eample :Eample :

δ +δ + δ −δ −

C'C'//C'C'

22-r , O'&a(0 C'-r , O'&a(0 C'//C'C'

22O' , -r&a(0O' , -r&a(0

nucleophilenucleophile



III) !limination ReactionIII) !limination Reaction An atoms or groups are remo!ed fromAn atoms or groups are remo!ed from

ad=acent carbon atoms of a molecule to formad=acent carbon atoms of a molecule to forma multiple bond double or triple bond0.a multiple bond double or triple bond0.

Results in the formation of unsaturatedResults in the formation of unsaturatedmolecules.molecules.

Eample :Eample :

C'C'//C'C'22O'O' conc. 'conc. '224O4O?? C' C'22; C'; C'22 , ', '22OO

∆∆



I) RearrangementI) Rearrangement

ReactionReaction

A reaction in %hich atoms or groups in aA reaction in %hich atoms or groups in a

moleculemolecule change positionchange position..

Occurs %hen a single reactant reorganiesOccurs %hen a single reactant reorganiesthe bonds and atoms.the bonds and atoms.

Eample :Eample :

C CC C

*HH

H R R

H

H

H

*

tatomerisme



EercisesEercises

1.1. Eplain ho% the free radicals areEplain ho% the free radicals are

formed in homol"tic clea!age.formed in homol"tic clea!age.

2.2. @rite an e(uation for the bromine&@rite an e(uation for the bromine&

bromine bond clea!age in thebromine bond clea!age in the

bromination of methane. 4tate thebromination of methane. 4tate the

t"pe of bond clea!age.t"pe of bond clea!age.



/./. @hich %ould "ou epect to be the@hich %ould "ou epect to be the

most stable free radical most stable free radical

C!C!""C!C!

## $ %C!$ %C!

##&&"" C!C! $$ C!# $C!# $

C!#

matriculation chemistry introduction to organic compound part 4.pdf

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