Chapter 5,6

CHAPTER 5 REACTION OF ALKENES,ALKYNES AND AROMATIC COMPOUNDS

ADDITION REACTIONS OF ALKENES 1) 2) 3) 4) 5)

Addition of hydrogen halides Halogenation Hydration Hydrogenation Oxydation

Addition of Hydrogen Halides 

The reaction of HX (X = Cl, Br, I) with double bond of alkene.



Example:

Markovnikov’s Rule: In addition of HX to an

C=C of an unsymmetrical alkene, the H attaches to the carbon with fewer R substituents and the X attaches to the carbon with more R substituents.

Mechanism of the reaction:



Since carbocations are involved as intermediates in the reaction, Markovnikov’s rules can be restated. - In the addition of HX to an alkene, the more

highly substituted carbocation is formed as an intermediate rather than less highly substituted one.







Anti-Markovnikov’s rule: In addition of HX

to an C=C of an unsymmetrical alkene, the H attaches to the carbon with more R substituents and the X attaches to the carbon with few R substituents. Example: H2O2 CH3CH=CH2 + HBr CH3CH2CH2Br Free radical mechanism .

Exercise

exercise

Halogenation 

Addition of X2 (X = Cl, Br, I) in the presence of CH2Cl2 to an C=C of an alkene. X2 X

X = Cl,Br,I

X

•Mechanism of the reaction H3C

Cl

H

CH3

H3C

H

Cl

CH3

Cl

H

ClH

Cl

CH3

ClH

H3C

H

H

H

Cl

CH3

Cl

CH3



Halohidrin – the reaction of an alkene with X2 in the presence of H2O to give a molecule with both OH and X.

Hydration 

The reaction of an alkene with water in presence of light sulphuric acid as a catalyst to yield an alcohol.



Markovnikov’s rule is also applied in this reaction.

Example: CH3CH=CH2 + H2O





H2SO4

CH3CH(OH)CH3

Mechanism of the reaction:

Exercise

Hydrogenation Reduction of alkenes  React with H2 in the presence of catalyst such as Pt, Pd, Ni to form corresponding saturated alkanes. 



Example:

Oxidation a)

Combustion -Reaction of an alkene with O2 CH2=CH2 + O2

d)

2CO2 + H2O

Hydroxylation -React with warm and concentrated KMnO4 to form two carbonyl compounds - C=C will break into two fragments

Hydroxylation 1)

2)

-React with cold KMnO4 to form 1,2-diol or glycol KMnO CH3CH=CH2

4

cold

CH3CH(OH)CH2(OH) 1,2-propanediol OH

KMnO4 cold OH

CH3C(CH3)=CH(CH3)

KMnO4 cold

H3C

CH3

H

C

C

OH

OH

CH3

C) Ozonolysis The reaction of an alkene with ozone, O3 to give aldehyde and ketone.

O3

H2O Zn

O

In ozonolysis, aldehyde does not undergoes further reaction of oxidation to give carboxylic acid.

O



Mechanism of the reaction

ADDITION REACTION OF ALKYNES 1) Hydrogenation H3C

C

CH

2H2

H3C

H

H

C

C

H

H

H

2) Halogenation H3C

C

CH

Br2

H3C

C

C

H

Br Br

3) Hydrohalogenation H3C

C

CH

2HCl

H3C

Cl

H

C

C

Cl

H

H



Electrophililic addition mechanism reaction: H3C

H3C

C

C

H3C

2HCl

CH

CH

H

H3C

Cl

H

C

C

Cl

H

C

H

CH2

Cl

H3C

C

CH2

Cl H3C

C Cl

CH2

H+

H3C

C Cl

CH3

Cl -

Cl

H3C

C Cl

CH3

4) Addition of water to alkynes (hydration)

AROMATIC COMPOUNDS 

Benzene, C6H6 – aromatic hydrocarbon Resonance form of benzene- Kekule structure

Electrons delocalization – resonance effect.  Resonance – differ in the position of electrons, not in atom 

Nomenclature of the aromatic compounds 

Nomenclature:

Reaction: Electrophile Substitution

Mechanism of Electrophile Substitution

a)Halogenation

b) Friedel Craft Alkylation

Friedel Craft Acylation

Exercise

c) Nitration

d)sulfonation

Orientation of Substitution Directive effects Ortho-para * R = CH3, C2H5 1)

•

OH, OCH3

•

X = F, Cl, Br

•

Ph

Meta NO2, C=OR, C=OOR, SO3H CN, NR3

2) Activating and Deactivating Groups Activating group -group that increases the reactivity of aromatic compound to electrophile substitution. -Ortho & para director Deactivating group -group that decreases the aromatic reactivity compound to electrophile substitution -meta director

Example:

Example: