4.1.3 Alkenes

Alkenes are unsaturated hydrocarbons General formula is CnH2nAlkenes contain a carbon-carbon double bondsomewhere in their structure

C C

H

H

H

H

EtheneC C

H

H H

C

H

H

HPropene

Numbers need to beadded to the name whenpositional isomers canoccur

C

H

H

C C C

H H H

H

HHBut-1-ene But-2-ene

C=C double covalent bondconsists of one sigma (σ)bond and one pi (π) bond.

π bonds are exposed and have highelectron density.

They are therefore vulnerable to attackby species which ‘like’ electrons: thesespecies are called electrophiles.

C C C CH

H

H H

H

H

H

H

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The π bond is formed bysideways overlap of two porbitals on each carbon atomforming a π-bond above andbelow the plane of molecule.

The π bond is weaker thanthe σ bond.

p orbitals

C-C sigma bondC-C pi bond

C-C sigma bond C-C pi bond

The arrangement of bonds around the>C=C< is planar and has the bond angle 120o

Formation of π bond

C C

H

H

H

H

C C

H

CH3H

H

Stereoisomerism

Stereoisomers have the same structural formulaebut have a different spatial arrangement of atoms.

Alkenes can exhibit a type of isomerismcalled E-Z stereoisomerism

E-Z isomers exist due to restrictedrotation about the C=C bond

Single carbon-carbon covalentbonds can easily rotate

E-Z stereoisomers arise when:(a) There is restricted rotation around the C=C double bond.(b) There are two different groups/atoms attached both ends of

the double bond.

C C

H

H

C

H

C

H

H

H

H

HC C

H

C

H

C

H

H H H H

HBut-1-ene

two identical groups attached toone end of the restricted doublebond – no E-Z isomers

two different groupsattached either end ofthe restricted doublebond- leads to EZisomers

CH

H

C C

H

H

CH

H

H

H

Z- but-2-ene

E -but-2-ene

These are twoisomers as the lackof rotation aroundthe double bondsmeans one cannotbe switched to theother.

Naming E-Z stereoisomers

Priority Group: The atom with the biggeratomic number is classed as the priority atom

Prioritygroupside 1

Prioritygroupside 2

If the priority atom is on the same side of thedouble bond it is labelled Z from the germanzusammen (The Zame Zide!)

If the priority atom is on the opposite side ofthe double bond it is labelled E from thegerman entgegen (The Epposite side!)

Z-1,2-dichloroetheneE-1,2-dichloroethene

But-1-ene is a structural isomer of But-2-ene but does not show E-Z isomerism.

2

First determine the priority groups on both sides ofthe double bond

Cl

C C

Cl

H H

Cl

C C

H

H Cl

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1. Compare the atomic number of theatoms directly attached to each side of thedouble bond; the group having the atom ofhigher atomic number receives higherpriority.

Cl

C C

Br

H Cl

priority

priority

2. If there is a tie, consider the atoms atdistance 2 from the double bond. Make a listfor each group of the atoms bonded to theone directly attached to the double bond.Arranged list in order of decreasing atomicnumber. Compare the lists atom by atom; atthe earliest difference, the group containingthe atom of higher atomic number receiveshigher priority

CH3

CH2

C CCH3CH3

H

priority

priority

Cahn–Ingold–Prelog (CIP) priority rules.

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cis-trans isomerism is a special case of EIZ isomerism inwhich two of the substituent groups are the same.

C C

H

C

H

C

H

H H H H

H

E- but-2-ene

Can also be called

trans- but-2-ene

CH

H

C C

H

H

CH

H

H

H

Z- but-2-ene

Can also be called

Cis- but-2-ene

The alkenes are relatively reactive because of the relatively low bond enthalpy of the π-bond.

Addition reactions of alkenes

Addition reaction: a reaction where two molecules reacttogether to produce one

Change in functional group: alkene alkaneReagent: hydrogenConditions: Nickel CatalystType of reaction: Addition/Reduction

1. Reaction of Alkenes with Hydrogen

C C

H

H

H

H

+ H2

ethane

C C

H

H

H H

H

H

ethene

Electrophilic Addition Reactions of Alkenes

Definition Electrophile: an electron pair acceptorThe double bonds in alkenes are areas with highelectron density. This attracts electrophiles and thealkenes undergo addition reactions

2. Reaction of alkenes with bromine/chlorine

Change in functional group:alkene dihalogenoalkane

Reagent: BromineConditions: Room temperature (not in UV light)Mechanism: Electrophilic AdditionType of reagent: Electrophile, Br+

Type of Bond Fission: Heterolytic

C C

H

H

Br Br

H

H

C C

H

H

H

H

+ Br2

1,2-dibromoethane

As the Br2 moleculeapproaches the alkene,the pi bond electronsrepel the electron pair inthe Br-Br bond. ThisINDUCES a DIPOLE. Br2becomes polar andELECTROPHILIC (Brδ+).

The INTERMEDIATEformed, which has apositive charge on acarbon atom is called aCARBOCATION

C C

H

H

H

H

C C

H

H

Br Br

H

H

C+

C

Br

H

HH

H

Br

Br

δ+

δ-

:Br -

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3. Reaction of Hydrogen Bromide with Alkenes

Change in functional group: alkenehalogenoalkaneReagent: HCl or HBrConditions: Room temperatureMechanism: Electrophilic AdditionType of reagent: Electrophile, H+

+ HBrC

H

H

C C C

H H H

H

HH C C

H

C

H Br

C

H

H

H

H

H

H

H

But-2-ene 2-bromobutane

HBr is a polarmolecule becauseBr is moreelectronegativethan H. The H δ + isattracted to theelectron-rich pibond.

This reaction canlead to twoproducts when thealkene isunsymmetrical

Majorproduct90%

Minorproduct10%

If the alkene isunsymmetrical,addition of hydrogenbromide can lead totwo isomericproducts.

‘Markownikoff’s Rule’In most cases, bromine will be added to thecarbon with the fewest hydrogens attached to it

C C

H

H

H

C

H

H

H

H

+

This carbocation intermediateis more stable because themethyl groups on either sideof the positive carbon areelectron releasing and reducethe charge on the ion whichstabilises it.

WHY?

The order of stability for carbocations is

tertiary > secondary >primary

In electrophilic addition to alkenes, the major product isformed via the more stable carbocation intermediate.

In exam answers•Draw out both carbocations and identify as primary, secondaryand tertiary•State which is the more stable carbocation e.g. secondary morestable than primary•State that the more stable carbocation is stabilised because themethyl groups on either (or one) side of the positive carbon areelectron releasing and reduce the charge on the ion.•(If both carbocations are secondary then both will be equallystable and a 50/50 split will be achieved)

C+

C

H

CH3

HH

CH3 C C

H

CH3

HH

CH3

Br

C C CH3

HH

CH3

δ+

δ- :Br -

H

Br

CH2 CH CH2 CH3

H Br

CH3

C+

CH2

CH3

H

C+

C CH2 CH3

H

HH

H

:Br -

:Br -δ+ δ-

CH2 CH2 CH2 CH3

Br

C C

H

C

H Br

C

H

H

H

H

H

H

H

4. Reaction of alkenes with steam to form alcohols

Industrially alkenes are converted to alcohols in one step. They arereacted with steam in the presence of an acid catalyst.

CH2=CH2 (g) + H2O (g) CH3CH2OH (l)

This reaction can be calledhydration: a reaction wherewater is added to a molecule

Reagent : steam

Essential Conditions

High temperature 300 to 600°C

High pressure 70 atm

Catalyst of concentrated H3PO4

The high pressures needed mean this cannot bedone in the laboratory. It is preferred industrially,however, as there are no waste products and so hasa high atom economy. It would also mean separationof products is easier (and cheaper) to carry out.

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Ethene

Monomer Polymerpolyethene

n

Addition PolymersAddition polymers are formed from alkenes Poly(alkenes) like alkanes are unreactive due to

the strong C-C and C-H bonds

C C

H

CH3H

H

propenen

poly(propene)

C C C C C C

CH3

H H

H CH3

H

H

H

CH3

H

H

H

be able to recognise the repeating unit in a poly(alkene)

Poly(propene) is recycled

This is called addition polymerisation

Add the n’s if writing an equation showing thereaction where ‘n’ monomers become ‘n’ repeatingunits

n C C

H CH3

HH

CH3 CH CH CH3CH3

C C

CH3

H

HYou should be ableto draw the polymerrepeating unit for anyalkene

It is best to first draw outthe monomer with groupsof atoms arranged aroundthe double bond

e.g. For but-2-ene

C C

CH3 H

CH3H

Industrial importance of alkenesThe formation of polymers from ethene based monomersis a major use of alkenes.The manufacture of margarine by catalytichydrogenation of unsaturated vegetable oils usinghydrogen and a nickel catalyst is another importantindustrial process.

Liquid vegetable oils are generallypolyunsaturated alkenes. Hydrogenation bythe reaction of hydrogen using a nickelcatalyst converts the double bonds tosaturated single bonds. This increases themelting point of the oil making it harder andmore solid.

Waste polymers can be processed in several ways.

Separation and recyclingThe waste is sorted into each different type of

polymer (ie PTFE, PVC, PET) and then eachtype can be recycled by melting and remoulding.

Combustion for energy productionWaste polymers can be incinerated and the heat releasedcan be used to generate electricity.Combustion of halogenated plastics (ie PVC) can lead tothe formation of toxic, acidic waste products such as HCl.Chemists can minimise the environmental damage of thisby removing the HCl fumes formed from the combustionprocess.

Feedstock for CrackingWaste polymers can be used as a feedstock forthe cracking process allowing for the newproduction of plastics and other chemicals.

Chemists have also been developing a rangeof biodegradable polymers, compostablepolymers, soluble polymers andphotodegradable polymers.

Dealing with waste polymers

Polymers formed from isoprene (2-methyl-1,3-butadiene), maize and starch arebiodegradable