Chapter 4Chapter 4

Unsaturated HydrocarbonsUnsaturated Hydrocarbons

ObjectivesObjectives

Bonding in AlkenesBonding in Alkenes Constitutional isomers in alkenesConstitutional isomers in alkenes Cis-trans stereoisomers in alkenesCis-trans stereoisomers in alkenes Addition, Polymerization and Oxidation Addition, Polymerization and Oxidation

reactions of alkenes.reactions of alkenes. Structure of Alkynes and AromaticsStructure of Alkynes and Aromatics Constitutional isomers of aromaticsConstitutional isomers of aromatics Reactions of aromaticsReactions of aromatics

Bonding in unsaturated Bonding in unsaturated hydrocarbonshydrocarbons

Unsaturated hydrocarbon

Alkene Alkyne Aromatic

• The unsaturated hydrocarbon families, the alkene, alkyne and the aromatic contain C-C multiple bonds.• Alkene has double bond and alkyne has triple bond.• Aromatic compound has alternative double and single bond.

CharacteristicsCharacteristics

Alkenes have fewer hydrogen compared Alkenes have fewer hydrogen compared to alkanes, therefore referred as to alkanes, therefore referred as unsaturated.unsaturated.

Presence of multiple bonds enhances the Presence of multiple bonds enhances the reactivity.reactivity.

Aromatic compounds are generally stable Aromatic compounds are generally stable compared to alkenes and alkynescompared to alkenes and alkynes

General formula for alkene is CGeneral formula for alkene is CnnHH2n2n

Compounds having C-C double Compounds having C-C double bondbond

Animal fats and vegetable oils, some proteins, Animal fats and vegetable oils, some proteins, nucleic acids and vitamins.nucleic acids and vitamins.

Cis-11 retinal, visual perceptionCis-11 retinal, visual perception Smoke – fused aromatic compoundsSmoke – fused aromatic compounds Sex hormone – aromatic ring structureSex hormone – aromatic ring structure Spices contain aromatic compoundsSpices contain aromatic compounds Pesticides – DDTPesticides – DDT Synthetic compounds (drugs, plastics, synthetic Synthetic compounds (drugs, plastics, synthetic

rubbers, dyes, soaps and cosmetics).rubbers, dyes, soaps and cosmetics).

Structure of alkane, alkene and Structure of alkane, alkene and cycloalkanecycloalkane

Table 12.1Table 12.1

Bonding in AlkenesBonding in Alkenes

The two bonds in the double bonds in The two bonds in the double bonds in alkene are not equivalent and have alkene are not equivalent and have different bond strength.different bond strength.

One bond (sigma) is strong and other One bond (sigma) is strong and other bond (pi) is weak.bond (pi) is weak.

The pi bond is responsible for the The pi bond is responsible for the characteristic behavior of alkene.characteristic behavior of alkene.

The orbitals used to form the C-C double The orbitals used to form the C-C double bonds are of spbonds are of sp22 type. type.

Formation of Formation of spsp22 orbitals for carbon–carbon double bonds orbitals for carbon–carbon double bonds..

Double-bond formation in etheneDouble-bond formation in ethene

Pi-bond formation in ethenePi-bond formation in ethene..

Trigonal bond angles of Trigonal bond angles of spsp22 carbons in the alkene carbons in the alkene double bond.double bond.

Ball-and-stick and space-filling models of ethene.Ball-and-stick and space-filling models of ethene.

Constitutional isomers of alkenesConstitutional isomers of alkenes

More number of constitutional isomers are More number of constitutional isomers are formed for alkenes and other families compared formed for alkenes and other families compared to alkanes.to alkanes.

With alkenes different C skeleton are possibleWith alkenes different C skeleton are possible With alkenes different placement of double bond With alkenes different placement of double bond

is possibleis possible Two isomers for CTwo isomers for C44HH1010 but there are three C but there are three C44HH88

isomers.isomers. There are five C-5 alkenes but there are only There are five C-5 alkenes but there are only

three C-5 alkanes. three C-5 alkanes.

Structural isomers of CStructural isomers of C44HH1010 and C and C44HH88

Drawing constitutional isomersDrawing constitutional isomers

Naming alkenesNaming alkenes

Identify the longest chain containing both C of Identify the longest chain containing both C of the double bond.the double bond.

Replace –ane to –ene.Replace –ane to –ene. The longest chain is numbered from the end The longest chain is numbered from the end

nearest the double bond.nearest the double bond. The compound having two double bonds ends in The compound having two double bonds ends in

–adiene.–adiene. A cyclic compound is named as cycloalkene.A cyclic compound is named as cycloalkene. Numbering in cycloalkene starts from the carbon Numbering in cycloalkene starts from the carbon

containing double bond.containing double bond.

ExampleExample

Exercise – IUPAC namesExercise – IUPAC names

Common namesCommon names

Ethene = EthyleneEthene = Ethylene Propene = PropylenePropene = Propylene They are important industrial chemicals.They are important industrial chemicals. Plastic polyethylene and polypropylene Plastic polyethylene and polypropylene

are synthesized from these organic are synthesized from these organic chemicals.chemicals.

Importance of alkenesImportance of alkenes

Ethene produced as hormone in plants.Ethene produced as hormone in plants. Ethene used in fruit industry to ripen the Ethene used in fruit industry to ripen the

fruits.fruits. Other alkenes distinguishes the odor in Other alkenes distinguishes the odor in

different plantsdifferent plants

Geraniol is extracted from roses and used in perfume Geraniol is extracted from roses and used in perfume formulationsformulations..

Box 12.1 (b)Box 12.1 (b)

Cis-trans isomerismCis-trans isomerism

ConceptConcept Geometric isomers are possible only when each Geometric isomers are possible only when each

carbon of the double bond is a stereocenterscarbon of the double bond is a stereocenters The C - C double bond has restricted rotationThe C - C double bond has restricted rotation There are two 2-butene compounds, cis-2-butene and There are two 2-butene compounds, cis-2-butene and

trans-2-butene.trans-2-butene. Substituents are placed on the same or opposite side Substituents are placed on the same or opposite side

of the double bond.of the double bond. The pi bond presents a significant barrier to rotation.The pi bond presents a significant barrier to rotation. Two isomers have different physiological behavior.Two isomers have different physiological behavior.

Ball-and-stick and space-filling models of 2-Ball-and-stick and space-filling models of 2-butene.butene.

Cis-trans isomerism of 11-retinal is responsible for vision.Cis-trans isomerism of 11-retinal is responsible for vision.

Vitamin AVitamin A

Box 12.2 (b)Box 12.2 (b)

Bombykol’s pheromone action as a sex attractant for the Bombykol’s pheromone action as a sex attractant for the silkworm moth depends on geometric isomerism.silkworm moth depends on geometric isomerism.

Addition ReactionsAddition Reactions

Addition reaction is the Addition reaction is the most characteristic most characteristic reaction in alkene reaction in alkene family.family.

Pi bond is responsible Pi bond is responsible for high reactivity in for high reactivity in alkenes.alkenes.

Pi bond undergo Pi bond undergo reactions with several reactions with several chemical reagents.chemical reagents.

Hydrogen (HHydrogen (H22))

Halogen (FHalogen (F22, Cl, Cl22, Br, Br22))

Hydrogen halide (HCl, Hydrogen halide (HCl, HBr, HI)HBr, HI)

Water (HWater (H22O)O)

Addition of Symmetric ReagentAddition of Symmetric Reagent

Hydrogen and Halogens are symmetric Hydrogen and Halogens are symmetric reagents. H H reagents. H H

CHCH22= CH= CH22 + H H + H H H H22C CHC CH22

PtPt or Ni or Ni This reaction is catalytic hydrogenation.This reaction is catalytic hydrogenation. Hydrogenation is a reduction reaction. Hydrogenation is a reduction reaction.

There is an increase in the number of There is an increase in the number of hydrogen atoms.hydrogen atoms.

HalogenationHalogenation

Addition of halogen is Halogenation.Addition of halogen is Halogenation. Does not require any catalyst, takes place Does not require any catalyst, takes place

in ambient temperature.in ambient temperature. Halogenation in alkanes is of substitution Halogenation in alkanes is of substitution

type, whereas, in alkenes it is addition type, whereas, in alkenes it is addition type of reaction.type of reaction.

CHCH22= CH= CH22 + Br- Br Br Br + Br- Br Br Br

HH22C CHC CH22

The deep red color of BrThe deep red color of Br22 is decolorized as it reacts with the is decolorized as it reacts with the

double bond of 1-hexene.double bond of 1-hexene.

Addition of Asymmetric ReagentsAddition of Asymmetric Reagents

Hydrogen halides and HHydrogen halides and H22O are asymmetric O are asymmetric reagents.reagents.

Addition of hydrogen halide is called Addition of hydrogen halide is called hydrohalogenation.hydrohalogenation.

Hydrogen is added to one carbon of the Hydrogen is added to one carbon of the double bond and halogen is added to the double bond and halogen is added to the other carbon.other carbon.

CHCH22= CH= CH22 + H-Cl H Cl + H-Cl H Cl HH22C CHC CH22

HydrationHydration

Addition of water is Hydration.Addition of water is Hydration. Requires strong acid catalyst for the Requires strong acid catalyst for the

reaction to occur. (Hreaction to occur. (H22SOSO44))

Product of hydration is alcohol.Product of hydration is alcohol. Industrially an important reactionIndustrially an important reaction In the living system, this reaction is In the living system, this reaction is

important in metabolism. H OHimportant in metabolism. H OH HH22C=CHC=CH22 + H-OH + H-OH H+H+ H H22C CHC CH22

Symmetric Vs Asymmetric alkeneSymmetric Vs Asymmetric alkene

Symmetric alkenes and Asymmetric Symmetric alkenes and Asymmetric alkenes.alkenes.

Only one product is possible by the Only one product is possible by the addition of symmetric or asymmetric addition of symmetric or asymmetric reagent.reagent.

Two products are formed by the addition Two products are formed by the addition of asymmetric reagent to the asymmetric of asymmetric reagent to the asymmetric alkene.alkene.

Major and Minor ProductsMajor and Minor Products

Markovnikov ruleMarkovnikov rule

Non-selective Vs selective type of reactionNon-selective Vs selective type of reaction Major and minor productsMajor and minor products Asymmetric reagent and asymmetric Asymmetric reagent and asymmetric

alkene follows selective type of reactionalkene follows selective type of reaction Markovnikov ruleMarkovnikov rule

The major product is the one formed when the The major product is the one formed when the hydrogen from the asymmetric reagent adds hydrogen from the asymmetric reagent adds to the carbon of the double bond that already to the carbon of the double bond that already had more hydrogens prior to the reaction.had more hydrogens prior to the reaction.

Mechanism of alkene addition reactionMechanism of alkene addition reaction

HCl interacts with the electron pair of the weak pi bond of ethene. The H-Cl bond breaks to form H+ and Cl-, and the H+ adds to ethene.

Mechanism of addition reactionMechanism of addition reaction

Bond formation between the positive carbocation and the negative chloride ion yields the final product.

Tertiary, secondary and primary carbocationsTertiary, secondary and primary carbocations

Carbocation stability and Markovnikov ruleCarbocation stability and Markovnikov rule

Addition PolymerizationAddition Polymerization

Addition polymerization is a self – addition Addition polymerization is a self – addition reaction, in which thousands of alkene reaction, in which thousands of alkene molecules add to one another to form polymer molecules add to one another to form polymer molecule.molecule.

Natural polymers – Carbohydrates, proteins, Natural polymers – Carbohydrates, proteins, nucleic acidsnucleic acids

Synthetic polymers – Plastics, rubber and fiber.Synthetic polymers – Plastics, rubber and fiber. CHCH22= CH= CH22 + CH + CH22= CH= CH22 + +

-----CH-----CH22-CH-CH22--CHCH22-CH-CH22--CHCH22-CH-CH22--------

Important industrial dienesImportant industrial dienes

Commercial products created through addition Commercial products created through addition polymerizationpolymerization..

Commercial products created through addition Commercial products created through addition polymerizationpolymerization..

Commercial products created through addition Commercial products created through addition polymerization.polymerization.

Commercial products created through addition Commercial products created through addition polymerizationpolymerization..

Commercial products created through addition Commercial products created through addition polymerization.polymerization.

Commercial products created through addition Commercial products created through addition polymerizationpolymerization..

Table 12.3Table 12.3

Oxidation of alkenesOxidation of alkenes

Alkenes oxidize with sufficient oxygen to Alkenes oxidize with sufficient oxygen to produce COproduce CO22 and H and H22O. (Combustion)O. (Combustion)

Selective Oxidation – Only the carbon of Selective Oxidation – Only the carbon of the double bond receives oxygen.the double bond receives oxygen.

Oxidizing reagents like (MnOOxidizing reagents like (MnO44--) and ) and

dichromate (Crdichromate (Cr22OO772-2-) ions, Oxygen and ) ions, Oxygen and

Ozone from the atmosphere.Ozone from the atmosphere. The products are aldehydes, ketones, The products are aldehydes, ketones,

alcohols and carboxylic acids.alcohols and carboxylic acids.