Reaction of Alkenes and Alkynes

8/6/2019 Reaction of Alkenes and Alkynes

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Reactions of Alkenes

and Alkynes



Alkenes Addition to carbon-carbon double bond

MECHANISM Double bond is broken and in its place

single bonds form to two new atoms or

groups of atoms.



Addition Reactions Hydrohalogenation

Hydration

Bromination

Hydrogenation (reduction)



Addition Reaction



Reason for reactivity of C=C One double bond and one single bond (H-

H bond of H2) are replaced by three single

bonds

Net conversion of one bond of the double

bond to two single bonds



Properties of Alkene Reactions

Exothermic

Products are more stable (lower energy)

than the reactants

Rate of reactions depends on the

activation energy (some needs catalyst)



Hydrogenation

The reaction is carried out under pressure

at a temperature of 200 °C in the presence

of a metallic catalyst.

Common industrial catalysts are based

on platinum, nickel or palladium. For

laboratory syntheses, Raneynickel (an alloy of nickel and aluminium) is

often employed.



Hydrogenation

CH2=CH

2+ H

2 CH

3-CH

3

Ethylene Ethane



Halogenation

CH2=CH2 + Br 2 BrCH2

-CH2Br



Hydrohalogenation Reactions of hydrogen halides (HX) with

alkenes produces haloalkanes (alkyl

halide)

Some reactions proceeds to

regioselectivity





Regioselectivity Was first noted by Vladimir Markovnikov

Regioselective reaction takes place when

one direction of bond forming or breaking

occurs in preference to all other directions.

The Markovnikov¶s Rule



Markovnikov¶s Rule In the addition of HX to an alkene,

hydrogen adds to the doubly bonded

carbon that has greater number of hydrogens already bonded to it; halogen

adds to the other carbon.

The rich get

richer





Hydration Addition of Water: Acid-catalyzed

hydration

In the presence of an acid catalyst, most

commonly conc. H2SO4, water adds to the

carbon-carbon double bond to give an

alcohol.



Hydration In simple alkenes, hydration follows the

Markovnikov¶s rule.

That is, H adds to the carbon of the double

bond with the greater number of

hydrogens and OH adds to the carbon

with the smaller number of hydrogens.



Halogenation Addition of bromine and chlorine

Br 2 and Cl2 react with alkenes at room

temperature by addition of halogen atoms

to the carbon atoms of the double bonds



Halogenation Generally carried out either by using pure

reagents or by mixing them in an inert

solvent, such as dichloromethane, CH2Cl2.



Importance Useful qualitative test for the presence of

an alkene.

How?: Br 2 in CCl4 (red to brown) is mixed

with a suspected alkene, the

disappearance of the red color means that

the bromine was added to the doublebond.



Hydrogenation Addition of Hydrogen

All alkenes react quantitatively with H2 in

the presence of a transition metal catalyst

to give alkanes.

Catalysts used are platinum, palladium,

ruthenium and nickel.



Hydrogenation It involves reduction by hydrogen in the

presence of a catalyst, thus; it is also

called as the catalytic reduction or catalytichydrogenation.



Hydrogenation Catalyst ± finely powdered solid

Carried out by dissolving alkene in ethanol

or other non-reactive organic solvent,

adding the catalyst, and exposing the

mixture to hydrogen gas at pressures

ranging from 1 to 150 atm.



P

olymerization Formation of chain-growth polymers

Most important reaction of alkenes

With the presence of initiators, many

alkenes form polymers



P

olymerization Low-density Polyethylene (LDPE)

Transparent polymer from the ethylene

polymerization with peroxide as initiator

Highly branched

Do not pack well together

London dispersion forces are weak



P

olymerization High-density Polyethylene (HDPE)

Does not rely on peroxide initiators

Developed by Karl Ziegler and Guilio Natta

Little chain branching

Chains are pack together more closely Stronger london dispersion forces



Sample 2-methylpropene with HI

2-methylcyclopentene with HCl

Propene with HBr



Sample Hydration of ethene with conc. H

2SO

4

Hydration of propene with conc. H2SO

4

Hydration of 2-methylpropene with conc. H2SO4

Acid-catalyzed hydration of 1-

methylcyclohexene

Acid-catalyzed hydration of 2-methyl-2-butene Acid-catalyzed hydration of 2-methyl-1-butene



Sample Bromination of cyclopentene with CH2Cl2

Chlorination of 2-methylcyclohexene with

CH2Cl2.

Bromination of 3,3-dimethylbutene with

CH2Cl2



Sample Catalytic reduction of trans-2-butene with

Pd

Catalytic reduction of ci s-2-butene with Pd

Catalytic hydrogenation of cyclohexene

with Pd



Benzene and Its

Derivatives



Benzene: Its Properties

Benzene is an organic chemical

compound with the molecular formula

C6H6.

It is sometimes abbreviated Ph±H.

Benzene is a colorless and highly

flammable liquid with a sweet smell and arelatively high melting point.




Because it is a known carcinogen, its useas an additive in gasoline is now limited,

but it is an important industrial solvent andprecursor in the production of drugs,plastics, synthetic rubber, and dyes.

Benzene is a natural constituent of crudeoil, and may be synthesized from other compounds present in petroleum.




Benzene is an aromatic hydrocarbon

It is also related to the functional group

arene which is a generalized structure of

benzene.

Arene is use to describe aromatic

hydrocarbon.




A group derived from the removal of an H

from a arene is called an aryl group (Ar-).

Discovered by Michael Faraday

Conflict arose between chemists because

of the molecular formula




Has few hydrogens for its six carbons

(compare to hexane); chemists argued it

to be unsaturated.

However, benzene does not behave like

alkenes



Benzene: Its Structure

First structure was proposed by Friedrich

August Kekulé

Consist of six-membered ring with

alternating single and double bonds, with

one hydrogen bonded to each carbon



Benzene: Its Structure

Kekulé proposal was consistent with many

chemical properties of benzene

But was objected because it does not

undergo reactions similar to alkenes.





Resonance Structure of Benzene

Experimentally, the length of the carbon-

carbon bond in benzene is not as long as

a C-C nor as short as a C=C, but rather midway between the two.




The closed loop of six electrons ( two from

the second bond of each double bond)

characteristic of a benzene ring issometimes called an aromatic sextet.




The benzene ring is greatly stabilized by

resonance, which explains why it does not

undergo the addition reactions of typicalalkenes.



Nomenclature of Aromatic

Hydrocarbons Aromatic hydrocarbon is evident with the

presence of a benzene ring.

Benzene has six carbon atoms are

arranged in a ring with alternating double

bonds.



Sample

Ethylbenzene

Methylbenzene (toluene)

Phenylethene (styrene) Hydroxybenzene (phenol)

Anisole

Aniline

Benzaldehyde

Benzoic acid





Sample

1-phenylcyclohexene

4-phenylbutene




Hydrocarbons Monosubstituted benzenes are named by

combining the substituents name with the

word benzene.

For disubstituted benzenes the 3 possible

isomers are named using the prefixes

ortho, metha and para (o, m, p) todesignate the 1,2- , 1,3- and 1,4

relationships of substituents on the

benzene ring.



Sample

4-bromobenzoic acid (p-bromobenzoic

acid)

3-chloroaniline (m-aniline)

1,3-dimetyhlbenzene (m-xylene or m-

dimethylbenzene)

1-chloro-4-ethylbenzene (p-chloroethylbenzene)




Hydrocarbons For three or more substituents, specify

their locations by numbers (smallest set).

If one of the substituents imparts a special

name, then name the molecule as a

derivative of that parent molecule.



Sample

4-chloro-1-methyl-2-nitro benzene (4-

chloro-2-nitrotoluene)

2,4,6-tribromo-1-hydroxybenzene (2,4,6-

tribromophenol)

2-bromo-1-ethyl-4-nitrobenzene

1-methyl-2,4,6-trinitrobenzene (2,4,6-trinitrotoluene or TNT)



Sample

m-iodotoluene

3,5-dibromobenzoic acid

P-chloroaniline



Polynuclear Aromatic

Hydrocarbons (PAH¶s) Molecules containing two or more

benzene rings, with each pair of rings

sharing two adjacent carbon atoms.

Naphthalene, anthracene, phenanthrene





Characteristic Reactions of

Benzene The most characteristic reaction of

aromatic compounds is substitution at a

ring carbon (aromatic substitution).

Halogenation

Nitration

Sulfonation



Halogenation

Cl2 and Br 2 noramlly do not react with

benzene but with an Fe catalyst, halogens

reacts rapidly to give halobenzene andHCl.



Sample

Chlorination of benzene in FeCl3

Bromination of benzene in FeCl3



Nitration

Heating benzene or its derivative with

conc. nitric acid, one of the hydrogen

atoms bonded to the ring is replaced by anitro (-NO2) group.



Sample

Nitration of benzene

Nitration of toluene





PHENOLS

The functional group of phenols is a

hydroxy group bonded to a benzene ring

Subtituted phenols are named either asderivatives of phenols or by common

names.



Sample

Phenol

3-methylphenol (m-cresol)

1,2-benzenediol



Sources

Widely distributed in nature

Phenol and the isomeric cresol (o-, m-,

and p-) are found in coal tar.

Thymol and vanillin are important

constituents of thyme and vanilla beans.

Urushiol is the main component of poisonivy ± an irritating oil



Acidity and Properties

Weak acids

Insoluble in water

Reacts with strong bases like NaOh and

KOH forming water-soluble salts

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Reaction of Alkenes and Alkynes

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