Alkenes

Alkenes & ethene

Occurs naturally in small quantity From crude oil – product of cracking & catalytic reforming Raw materials – polymers, detergents, solvents, etc. Unsaturated HC (has at least 1 C=C) For 1 C=C, less 2 H atoms General formula – CnH2n

Mp & bp increase with increase # of C

C=C C-C (σ bond) in ethane

e cloud symmetrical about central axis of molecule Rotate about the axis – 2 ends of ethane free to rotate

C=C (σ and π bonds) in ethene π bond above & below the plane of molecule Does not allow rotation around axis e density in π bond covers large area around the nuclei of the C

atoms C=C less than twice as strong as C-C (1 σ bond) in alkane Bond enthalpy for σ bond is 347, for π bond is 265

C=C Properties of molecules with C=C

e rich – positively polarised groups (electrophiles) more attracted to it

π bond provides e pair to form new bond with electrophile

Geometric isomerism – rotation around bond axis not possible

Naming & geometric isomerism Geometric isomers = occur when components of molecules

arranged on diff sides of molecule. Traditional way – cis-trans isomerism Example:

Diff physical props (mp, bp, density) Often same chemical reactivity

H

CH3 CH3

H H

CH3 H

CH3

cis-but-2-ene trans-but-2-ene

E-Z Isomerism Current IUPAC system Works for all geometric isomers Groups attached around C=C are ranked

Highest number, highest priority E = engegen = opposite Z = zussamen = together

Reaction – addition Energy to break π bond is lower Heterolytic fission Can be attacked by electrophiles & oxidising

agents

H

CH3 CH3

H

H

CH3 H

CH3

Z-but-2-ene

E-but-2-ene

E-Z Isomerism

Br

I Cl

F Br

I F

Cl

F < Cl < Br < I

2. Find the mainisomers of pentene & give them correct E-Z nomenclature

1. Give correct E-Z nomenclature

Electrophiles & addition rxn Electrophiles

Electron deficient species Most common is H+

More attracted to C atom (has e pair)

Rxn of alkenes w H2

condition – moderately high T (abt 200 ºC), Ni catalyst product - alkane

H

H H

H

+ H2 HH

H H

H HN i

Electrophiles & addition rxn Rxn w halogens

decolourise Br2 (orange) product – disubstituted halogenoalkane

rate of reaction decrease down halogen groupF > Cl > Br > I

H

H H

H

+ HH

H H

Br BrN i

Br Br

Addition rxn mechanism

HH

H H

Br BrC

+

H H

H

BrHBr

-

H

H

Br

Br

H

H

2 e in π bond form bond with H atom

H-Br bond break, 2 e goes to Br

Carbocation (carbonium ion)

Nucleophile

Testing for alkenes Bromine water (HBr) – test for alkene Orange bromine water turn to colourless Major product:

Minor product:

+ Br Br + OH2

H

H H

H

H

OH Br

H

HH + H Br

+ Br Br + OH2

H

H H

H

H

Br Br

H

HH

Rxns of alkenes with HX Product – monosubstituted haloalkane Rxn of HX with

Symmetrical alkenes (ethene) – 1 product Asymmetrical alkenes (propene) – 2 products

Asymmetrical alkene – Markovnikov’s rule: Major product form has H added to C with greater number of H

Rxns of symmetrical alkenes w HXHH

H H

H BrC

+

H H

H

HHBr

-

H

H

H

Br

H

H

Rxn of asymmetrical alkene w HXCH3

H H

H

H Br

C+

CH3

H H

H

H

Br-

CH3

H

Br

H

H

H

CH3

H H

H

H Br

C+

H

H H

HCH3

Br- H

H H

HCH3

Br

Major product

Minor product

Rxn with KMnO4/H+

Addition & oxidation Product – alkanediols Purple KMnO4 turn to colourless Can be used to differentiate btwn alkenes & alkanes

H

H H

H

H H

OH OH

HHKM nO 4/d il. H 2SO 4

Polymerisation Very large molecule made fr monomers (smaller units) Synthesis

Addition rxn btwn monomers with C=C or C≡C Ethene → polyethene Propene → polypropene Chloroethene → polycholoroethene (polyvinylchloride,

PVC)

H

H H

H H

H H

H H

H H

H

.

H

H

H

H

H

H

H

H

.

H

H

H

H

Polymerisation

LDPE• Highly branched• Soft & malleable• Low mp

HDPE• Few branched chains• Rigid & dense• High mp

Properties of polymer Depend on their monomers, various properties

•Tensile strength & mp ↑ when length ↑ until about 500 units

Avg chain length•Branched chains cannot pack closely – low

tensile strength, mp & density

Branching

•Strong forces – high mp•Due to side groups

Intermolecular forces

•Hold chains together – very rigid, hard, brittle, high mp

Cross links

Polymer problems & solutions

Energy

Fr fossil fuels

Energy to make polymers fr the

same source

Resources

Fossil fuel – limited supply,

costly

Need for new resource

Disposal

Waste problem, toxic gases

Endanger animals

C footprint of plastics C footprint = a measure of impact fr human activities in terms of

CO2

Purpose – to try to reduce C footprint as much as possible Source of CO2 – burning of fuels, waste plastics Solutions:

Use renewable energy Reduce use of polymer products, reuse Recycle – reduce diposal & demand to produce new ones Recycle thermoplastics – PET – carpets, textiles, waterproofing

materials

How are plastics recycled?

Mechanical• Melted, shredded, turned into granules• Sorting – done by hand & expensive

Chemical• aka feedstock recycling• Brkdown of waste to monomers – burning, thermal

cracking, & gasification• Expensive – use energy

Biodegradable polymers Development of renewable resources – biopolymers, bioplastics,

‘green plastics’ Modified starch or cellulose (fr plants)

Use CO2 during growth – reduce C footprint Bacterial fermentation – break down plant materials to monomers Broken down by microbes – biodegradable

Land use to grow plants for polymers – decrease land to grow food crops

Energy recovery Recovering some energy that was used into making polymers Example:

burn waste polymer to generate electricity – use incinerators at high T to prevent release of dioxin

Pyrolysis – break down polymer by heat in absence of O2

Syngas – gasification involves brkdown of solid HC in limited O2

Product – mainly mixture of H2 & CO Most process are not efficient & expensive

Life cycle analysis Use by industry – quantify effect to environment Product examined fr extraction of raw materials to cost of recycling

careful look at energy & material use & emission to environment to improve process & reduce impact to environment compare 2 diff polymers & decide which to be used based on their life cycle

End of alkenes