G10alkenes

Starter

How many fractions can you name from the fractionating

column ?In order

Without looking

alkenes

a group of hydrocarbons with the general formula, CnH2n where n is the number of carbon atoms in one molecule

contain the C=C functional group (unsaturated hydrocarbons) names of alkenes end with -ene are molecular compounds have low boiling points; gases under room conditions

alkenes

a ball-and-stick model of ethene,

C2H4

alkenes

each member of the series differs from the next by an extra –CH2- group of atoms

alkene homologous series

No. of Carbon Atoms in One

Molecule

Alkene Member

Relative Molecular

Mass

Structure of One Molecule

Boiling Point (°C)

2ethene(C3H4)

28 -104

3propene(C3H6)

42 -48

4butene(C4H8)

56 -6

H

H

C

H

H

C

H

H

C

H

H

H C

H

C

H

H

H C

H

H

C

H

C

H

H

C

alkenes burn in air to form CO2 and H2O

alkenes

C2H4 + 3O2 2CO2 + 2H2O

reactions of alkenes: combustion

incomplete combustion results in the formation of carbon monoxide, CO and soot, C

alkenes tend to produce more soot than alkanes when they burn in air

example: combustion of ethene

alkenes reactions of alkenes: addition reactions

Alkenes undergo addition reaction

This means that a molecule is added to the alkene to make one product

This is possible because of the carbon-carbon double bond (C=C ) in an alkene.

C C

X X

HH

HH

alkenes reactions of alkenes: addition reactions addition of bromine (bromination)

C2H4 + Br2 C2H4Br2

o example: addition reaction of bromine to ethene

o bromine molecule adds to the C=C bond of ethene molecule

H HC

H HC

+Br

Br CH

H

H

H

C Br

Br

reaction with aqueous bromine is used as a test for alkenes and other unsaturated compounds

alkene is shaken with or bubbled through a solution of bromine in water; alkene quickly decolourises the reddish-brown bromine solution

C C

Br Br

HH

HH

alkenes reactions of alkenes: addition reactions addition of hydrogen (hydrogenation)

o alkenes react with hydrogen to produce alkanes in the presence of nickel metal (catalyst)

o example: addition reaction of hydrogen to ethene

o hydrogen molecule adds to the C=C bond of ethene molecule

C2H4 + H2 C2H6

H HC

H HC

+H

H CH

H

H

H

C H

H

used in producing margarine from vegetable oil

C C

H H

HH

HH

alkenes reactions of alkenes: addition reactions addition of water (hydration)

o alkenes react with steam over a catalyst to produce alcohols

o example: addition reaction of water to ethene

o water molecule adds to the C=C bond of ethene molecule

C2H4 + H2O CH3CH2OH

H

H HC

H HC

+OH

H CH

H

H

C OH

H

alcohols are commonly used as solvents and to make other chemicals

C C

H OH

HH

HH

alkenessaturated and unsaturated hydrocarbons

alkenes are unsaturated because extra atoms can be added to their molecules in addition reactions

C C

HH

HHC C

HH

HH

HH

alkanes are saturated because it is not possible to add extra atoms to their molecules

alkenessaturated and unsaturated hydrocarbons

differences between saturated and unsaturated hydrocarbons:

Saturated Hydrocarbons Unsaturated Hydrocarbons

Contains carbon-carbon single bonds (C-C)

Contains carbon-carbon double bonds (C = C)

Reaction with aqueous bromine : do not decolourise aqueous bromine

Reaction with aqueous bromine : decolourises aqueous bromine

have alkane-type formula of CnH2n+2

have alkene-type formula of CnH2n

Extra atoms not added to their molecules in addition reactions

Extra atoms added to their molecules in addition reactions

alkenestesting for unsaturated compounds like alkenes

for liquids: add a solution of bromine in water to an alkene and shake them; reddish-brown colour of bromine disappears in the presence of alkene

bromine becomes colourless

shake

liquid alkene

bromine solution

testing for liquid alkene

poly(ethene)

alkenestesting for unsaturated compounds like alkenes

for gas: bubble the gas through a solution of bromine in water; colour of bromine disappears in the presence of alkene gas

testing for gaseous alkene

bromine becomes colourless

gaseous alkene

bromine solution

Demo - to distinguish between an alkane and an alkene (decolourisation of bromine!).

bromine

hexenehexane

bromine

hexene

hexane

Qn: What will happen if I bubble the gas from a bottle filled with bananas to the bromine water?

Demo – while ripening, bananas give off ethene gas!

bananasbananasbromine bromine

manufacture of alkenes

a process to break up big alkane molecules into smaller molecules; done by passing big alkane molecules over a solid catalyst at a high temperature

example: cracking of big petroleum (hydrocarbon) molecules

big hydrocarbon molecules

hydrocarbon molecules break up on the

surface of the catalyst

small hydrocarbon molecules

CATALYST

cracking Alkene molecules can be made by cracking alkanes

an ________mo

lecule

C18H38 X + C10H20

an ________molecule

big alkane

cracking

cracking of alkanes

C20H42 C12H26 + C8H16

an alkenemolecule

an alkanemolecule

big alkane

Uses:

C16H34 H2 + C6H12 + C10H20

cracking

cracking of alkanes

Hydrogen can be one of the products in the cracking of alkanes

Hydrogen is used as fuel for big rockets and to make ammonia for fertilisers

Uses:

o cracking of big petroleum molecules into smaller hydrocarbon molecules to produce fuel for motor vehicles;

cracking

cracking of alkanes

• This is important as the amount of petrol produced from fractional distillation of petroleum is insufficient. So the extra petrol needed is produced by cracking

polymers

Macromolecules are very large numbers of small molecules joined together.

Polymerisation is the chemical reaction when large numbers of similar small molecules (monomers) are joined together to form a big molecule (polymer).

polymerisation

H

H

CC

H

H

H

H

CC

H

H

H

H

CC

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

polymers

polyethene (polythene) one of the simplest addition polymer used to make ‘cling film’ for wrapping fresh vegetables

and meat, plastic bottles, plastic bags, buckets and pipes

can be made up of 500 to 20,000 ethene molecules

model of polyethene molecule

carbon atom

hydrogen atom

polymers

polyethene (polythene) deriving the polymerisation equation:

H

C C

H H

H H

C C

H H

HH

C C

H H

H H

C C

H H

HH

C C

H H

H

o write out the structural formulae of several ethene molecules in a row

C

H

C

H H

H H

C C

H H

HH

C C

H H

HH

C C

H H

HH

C C

H H

H

o open the C=C double bonds

polymers

polyethene (polythene) deriving the polymerisation equation:

o join the molecules together

H

H

C C

H H

H H

C C

H H

HH

C C

H H

H H

C C

H

H H

C C

H H

HH

C C

H H

H

formula of polyethene can be simply written as:

C

H

C

H

H H n

polymers

examples of addition polymer

Chemical Name of Polymer

Monomer Polymer Uses

Poly(ethene) or polythene

Plastic film for wrapping fresh vegetables and meat; plastic bottles; plastic bags, buckets and pipes

Poly(chloroethene)or PVC

For water pipes, waterproof plastic sheet, insulation for electric cables and wires

Poly(phenylethene)or polystyrene

For packaging and containers for electrical goods, disposable cups and food containers used in ‘fast food’ restaurants and hawker centres

H

H

C

Cl

H

C

H

H

C

C6H5

H

C

H

H

C

Cl

H

C

n

H

H

C

C6H5

H

C

n

H

H

C

H

H

C

H

H

C

H

H

C

n

unsaturated foods

polyunsaturated means that organic molecules contain C=C bonds

example: vegetable oils are polyunsaturated oils

manufacture of margarine by addition reaction; vegetable oil reacts with hydrogen in presence of nickel catalyst; H2 molecules add to the C=C bonds of vegetable oil

after addition reaction, vegetable oil has more hydrogen atoms and hence, greater relative molecular mass and higher melting point; becomes solid (solidify) at room conditions

margarine (a healthy product) made from vegetable oil by addition reaction

Demo - Testing for unsaturation

bromine

After five droplets of each to bromine solution, shaken and stirred…

butter margarine ghee olive oil

Butter - did not decolorized Br soln (very minimal) - mostly saturated

Margarine - decolorized Br soln (largely) - mostly polyunsaturated

Ghee - did not decolorized Br soln(least) - highly saturated

Olive oil -decolorized Br soln(completely) - highly monounsaturated