Reaction of Alkenes and Alkynes for Students

7/31/2019 Reaction of Alkenes and Alkynes for Students

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Reactions of Alkenes

and Alkynes


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Alkenes

Addition to carbon-carbon double bond

MECHANISM

Double bond is broken and in its placesingle bonds form to two new atoms orgroups of atoms.


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Addition Reactions

Hydrohalogenation

Hydration

Bromination

Hydrogenation (reduction)


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Addition Reaction


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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


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Properties of Alkene Reactions

Exothermic

Products are more stable (lower energy)

than the reactants

Rate of reactions depends on theactivation energy (some needs catalyst)


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Hydrogenation

The reaction is carried out under pressureat 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) isoften employed.


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Hydrogenation

CH2=CH2 + H2 CH3-CH3

Ethylene Ethane


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Halogenation

CH2=CH2 + Br2 BrCH2-CH2Br


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Hydrohalogenation

Reactions of hydrogen halides (HX) withalkenes produces haloalkanes (alkyl

halide) Some reactions proceeds to

regioselectivity


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Regioselectivity

Was first noted by Vladimir Markovnikov

Regioselective reaction takes place when

one direction of bond forming or breakingoccurs in preference to all other directions.

The Markovnikovs Rule


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Markovnikovs Rule

In the addition of HX to an alkene,hydrogen adds to the doubly bonded

carbon that has greater number ofhydrogens already bonded to it; halogenadds to the other carbon.

The rich getricher


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Hydration

Addition of Water: Acid-catalyzedhydration

In the presence of an acid catalyst, mostcommonly conc. H2SO4, water adds to thecarbon-carbon double bond to give an

alcohol.


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Hydration

In simple alkenes, hydration follows theMarkovnikovs rule.

That is, H adds to the carbon of the doublebond with the greater number ofhydrogens and OH adds to the carbon

with the smaller number of hydrogens.


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Halogenation

Generally carried out either by using purereagents or by mixing them in an inert

solvent, such as dichloromethane, CH2Cl2.


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Importance

Useful qualitative test for the presence ofan alkene.

How?: Br2 in CCl4 (red to brown) is mixedwith a suspected alkene, thedisappearance of the red color means that

the bromine was added to the doublebond.


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Hydrogenation

Addition of Hydrogen

All alkenes react quantitatively with H2 in

the presence of a transition metal catalystto give alkanes.

Catalysts used are platinum, palladium,

ruthenium and nickel.


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Hydrogenation

It involves reduction by hydrogen in thepresence of a catalyst, thus; it is also

called as the catalytic reduction or catalytichydrogenation.


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Hydrogenation

Catalyst finely powdered solid

Carried out by dissolving alkene in ethanol

or other non-reactive organic solvent,adding the catalyst, and exposing themixture to hydrogen gas at pressures

ranging from 1 to 150 atm.


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Polymerization

Formation of chain-growth polymers

Most important reaction of alkenes

With the presence of initiators, manyalkenes form polymers


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Polymerization

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


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Polymerization

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


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Benzene and Its

Derivatives


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Benzene: Its Properties

Benzene is an organic chemicalcompound with the molecular formula

C6H6. It is sometimes abbreviated PhH.

Benzene is a colorless and highly

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


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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 crude

oil, and may be synthesized from othercompounds present in petroleum.


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Benzene is an aromatic hydrocarbon

It is also related to the functional group

arene which is a generalized structure ofbenzene.

Arene is use to describe aromatic

hydrocarbon.


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A group derived from the removal of an Hfrom a arene is called an aryl group (Ar-).

Discovered by Michael Faraday

Conflict arose between chemists becauseof the molecular formula


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Has few hydrogens for its six carbons(compare to hexane); chemists argued it

to be unsaturated. However, benzene does not behave like

alkenes


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Benzene: Its Structure

First structure was proposed by FriedrichAugust Kekul

Consist of six-membered ring withalternating single and double bonds, withone hydrogen bonded to each carbon


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Benzene: Its Structure

Kekul proposal was consistent with manychemical properties of benzene

But was objected because it does notundergo reactions similar to alkenes.


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Resonance Structure of Benzene

In answer to the objections to the structureproposed by Kekul, resonance hybrid of

the molecular benzene was used.


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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 rathermidway between the two.


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The closed loop of six electrons ( two fromthe second bond of each double bond)

characteristic of a benzene ring issometimes called an aromatic sextet.


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The benzene ring is greatly stabilized byresonance, which explains why it does not

undergo the addition reactions of typicalalkenes.


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Nomenclature of Aromatic

Hydrocarbons Aromatic hydrocarbon is evident with the

presence of a benzene ring.

Benzene has six carbon atoms arearranged in a ring with alternating doublebonds.


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Hydrocarbons The substituent group derived by loss of

an H from benzene is called a phenyl

group (Ph-). In molecules containing other functional

groups, phenyl groups are commonly

assigned as substituents.


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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 1,4relationships of substituents on the

benzene ring.


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Hydrocarbons For three or more substituents, specify

their locations by numbers (smallest set).

If one of the substituents imparts a specialname, then name the molecule as aderivative of that parent molecule.


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Polynuclear Aromatic

Hydrocarbons (PAHs) Molecules containing two or more

benzene rings, with each pair of rings

sharing two adjacent carbon atoms. Naphthalene, anthracene, phenanthrene


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Characteristic Reactions of

Benzene The most characteristic reaction of

aromatic compounds is substitution at a

ring carbon (aromatic substitution). Halogenation

Nitration

Sulfonation


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Halogenation

Cl2 and Br2 noramlly do not react withbenzene but with an Fe catalyst, halogens

reacts rapidly to give halobenzene andHCl.


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Sample

Chlorination of benzene in FeCl3

Bromination of benzene in FeCl3


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Nitration

Heating benzene or its derivative withconc. nitric acid, one of the hydrogen

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


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Sample

Nitration of benzene

Nitration of toluene


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Sulfonation

Heating an aromatic compound with conc.sulfuric acid results in the formation of an

arenesulfonic acid and water. Major use is in the preparation of synthetic

detergents


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PHENOLS

The functional group of phenols is ahydroxy group bonded to a benzene ring

Subtituted phenols are named either asderivatives of phenols or by commonnames.


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Sample

Phenol

3-methylphenol (m-cresol)

1,2-benzenediol


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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


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Acidity and Properties

Weak acids

Insoluble in water

Reacts with strong bases like NaOh andKOH forming water-soluble salts

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Reaction of Alkenes and Alkynes for Students