Alkenes

Alkenes

• Alkenes are family of hydrocarbons containing at least one carbon-carbon double bond

• Alkenes are unsaturated hydrocarbons with the presents of double bond ( ═ )

Formulae for Alkene

• General formula : CnH2n

Molecular formulae Ethene Structural formula

C2H4

H — C ═ C — H

H H| |

Naming AlkenesUsing IUPAC nomenclature

RootDenotes the longest

carbon chain containing double bond

EndingDenotes the family

name and the position of the double bond

Tells how many carbon atomsIn the continuous carbon chain

Tells the family of the compounds

Structural Formulae

1 2 3 1 2 3 4 5

Propene Pent-1-ene

Non-1-ene

1 2 3 4 5 6 7 8 9

n Molecularformula

Name Structural formula

2

3456789

10

C2H4 Ethene

PropeneBut-1-ene

Pent-1-eneHex-1-eneHept-1-eneOct-1-eneNon-1-eneDec-1-ene

H — C ═ C — H

H H| |

Physical Properties

Electrical conductivityCannot conduct electricity

SolubilityObeys “like dissolve like” rule• Dissolve in organic solvents• Insoluble in water

DensityLess dense than water

Melting and boiling points• Alkanes are covalent compounds• They are held together by weak intermolecular forces• Low melting and boiling points

NAME Formula M.P./oC B.P./oC Density/g cm -3(20oC)

Ethene C2H4 -169 -103 0.0011

Propene C3H6 -185 -48 0.0018

But-1-ene C4H18 -185 -6 0.0023

Pent-1-ene C5H10 -165 30 0.6430

Hex-1-ene C6H12 -140 64 0.6750

Hept-1-ene C7H14 -119 93 0.6980

Oct-1-ene C8H16 -104 122 0.7160

Non-1-ene C9H18 -94 146 0.7310

Dec-1-ene C10H20 -87 171 0.7430

When the number of carbon atoms increases :

• The physical properties increase• The molecules get bigger and the

molecular mass increase. So then, the densities of the alkanes increase

• The molecules get bigger and the intermolecular force of attraction becomes stronger. More energy is needed to overcome the strong forces of attraction. The melting and boiling point increase.

Chemical Properties

Addition of Hydrogen

Addition of Halogens

Addition of Hydrogen Halides

Addition of Water

Addition of Hydroxyl Groups

Combustion Reaction Addition Reaction

Combustion reaction

Alkene + excess O2 → CO2 + H2O

e.g:

C2H4 + 3O2 → 2CO2 + 2H2O

• Alkenes burn with sootier flames as compared to alkanes

• Because alkenes have a higher percentage of carbon than alkanes

Addition Reaction

The carbon-carbon double bond is converted

into two single bonds

C ═ C + A — B | |— C — C — | |

→

A B

Addition of Hydrogen

• Catalytic hydrogenation

• Temp : 180°C

• Catalyst : nickel

+ H — H | |H — C — C — H | |

→

H H

H — C ═ C — H

H H| |

H H

Addition of Halogen• Known as Halogenation

• E.g : Ethene, C2H4 is bubbled through a solution of bromine in tetrachloromethane, CCl4 in the dark. Reddish-brown bromine is decolourized and a colourless liquid 1,2-dibromoethane is formed

• Bromination is used to test the presence of a carbon-carbon double bond

+ Br — Br | |H — C — C — H | |

→

Br Br

H — C ═ C — H

H H| |

H H

Addition of Hydrogen Halides

• Rapid reaction at room temperature

• No catalyst

• Hydrogen halides : HCl, HBr, HI

+ H — Cl | |H — C — C — H | |

→

H Cl

H — C ═ C — H

H H| |

H H

Addition of HX to propene

+ H — Br

| | |H — C — C — C— H | | |

→Br H H

H — C ═ C — C — H

H H H| | |

H H H

H|

| | |H — C — C — C— H | | |

H Br H

H H H→

1-bromopropane

2-bromopropane

Propene Hydrogenbromide

1 2 3

1 2 3

Addition of Water

• Called hydration• Alkene + steam → alcohol

+ H — OH CH2 — CH2

| |H OH

CH2 ═ CH2

H3PO4

300°C, 60atm

Addition of Hydroxyl Groups

• Ethene,C2H4 reacts with dilute acidified solution of potassium manganate(VII), KMnO4 to produce ethane-1,2-diol

• Ethene decolourizes the purple solution of KMnO4

• Test for a double bond

+ H — OH + [ O ] CH2 — CH2

| |OH OH

CH2 ═ CH2

acidic KMnO4

Ethane-1,2-diol

Polymerization Reaction

Addition polymerization :

• Reaction to form a polymer from alkene monomers.

• Hundreds or thousands of alkenes molecules link together to form long chains called polymers

Polymerization of ethene, C2H4

• Temp : about 200°C• Pressure : 1200 atm• Presence of traces of oxygen• Product : poly(ethene) or polythene• Uses :

• making film and plastic sheets for bag and wrappers

• Making moulded articles (toys, bottles, squeeze bottles

C ═ C

H H| |

H H

| |

C ═ C

H H| |

H H

| |

C ═ C

H H| |

H H

| |

Polymerization of Ethene

+ +

| | | | | | — C — C — — C — C — — C — C— | | | | | |

| | | | | | — C — C — C — C — C — C — | | | | | |

H H H H H H

H H H H H H

H H H H H H

H H H H H H

| | — C — C — | | n

or

Poly(ethene)

Part of double bond in C2H4

‘break open’ to produceTwo free bonds

The free bonds link with one another to form a chain

Comparing Properties of Alkanes and Alkenes

Chemical Properties

Differences

Alkanes Alkenes

Reactivity • Unreactive • Undergo substitution reaction

• Reactive• Undergo addition reaction

Reaction with oxygen Burn in air, producing yellow sooty flame

Burn in air, producing yellow and very sooty flame

Reaction with bromine No reaction Decolourise reddish-brown bromine

Recation with acidified potassium manganate(VII) solution

No reaction Decolourise acidified potassium manganate(VII) solution

Homologous SeriesIs a group or family of organic compounds that hasthe following characteristics:

• Members can be represented by a general formula

• Successive members differ from each other by – CH2

• Members can be prepared by similar methods• Physical properties change regularly with

increasing number of carbon atoms• Member have similar chemical properties

because they have the same functional group

Functional group

• Is a special group of atoms attached to an organic molecule

• This group determines the chemical properties of the molecules because it represents the most common site of chemical reactivity

• Chemical reactions occur at the functional group• C ═ C is the functional group in alkenes• It causes the chemical properties of alkenes to

differ from alkanes

Name of the homologous series

Functional group

Alkane C — H

Alkene C ═ C

Alcohol — C — O — H

Carboxylic acid — C ═ O | H