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General formula: CnH 2

n unsaturated ALKENE have double , hydrocarbon bonds, C=C where n = 2, 3, 4...

Naming for alkenes: -ane change to -ene

– Melting/boiling point: low – Density: less dense than water –

Alkenes are more reactive than alkanes. Alkenes are unsaturated hydrocarbons Have double bond, C=C Alkenes can undergo: – combustion reaction – addition

Complete combustion Product- CO 2

and H 2

O Example: Butene (C 4

H 8

)

In an addition reaction: i) a molecule is “added” to the two carbons of a double bond. ii) One of the bonds that are part of the double bond between the carbons, C is broken. iii) The product is (1) ADDITION OF saturated HYDROGEN GAS, H compound (alkane). 2

(Hydrogenation) – catalysts used: platinum, Pt and nickel, Ni – heated at 180ºC – Hydrogen molecule, H 2

added to the alkene molecule. An alkane is produced.

temperature 150ºC to 180ºC

catalyst C

C

H

H

(Ethene)

(Ethane)

2

2

4

6

REMEMBER: Hydrogenation = adding of hydrogen, H 2

which can also be written as, H–H

Alkene reacts with halogens (GROUP 17) such as chlorine, Cl 2

and bromine, Br 2

. The halogen molecule added to the alkene molecule. The product is a haloalkane . Example: X X || ~C=C~ + X–X ~C –C~ Chlorine H H Propene H (C HHH 1,2–dichloropropane (Cl | | | | | |3 2 H ) (a) H–C=C–C–H + Cl–Cl H–C–C–C–H 6 |||| ) H Cl Cl H

+

A mixture of alkene and hydrogen halides (such as hydrogen chloride, HCl or hydrogen bromide, HBr) reacts at room condition. The product is a haloalkane. Example: HHHHHH |||||| a) H–C=C–C–H + H–Cl H–C–C–C–H |||| H H Cl H Propene + Hydrogen Chloride ––> 2–chlorobutane

Catalyst used: phosphoric acid, H 3

PO 4

. Heated at: 300ºC , under 60 atm pressure. Water molecule is added to the alkene molecule. Product is alcohol . Example: Pentene + Water Pentanol HHHHHHHHHH |||||||||| H–C=C–C–C–C–H + H–OH H–C–C–C–C–C–H

An alkene reacts with KMnO 4

solution acidified with sulphuric acid, H 2

SO 4

at room condition. The purple color of KMnO 4

is decolourized . The product is an alcohol (contain ~two OH–diol) Example: HHHHHH |||||| H–C=C–C–H

To form polymer = a long chain of monomer. Monomer is an alkene molecule that is broken to double bonds, joint or links together with another to form a polymer. The condition for polymerization to take place are: – temperature: 100ºC to 300ºC – pressure: n is any number 100 atm Polymerization process in general: 50between to 20 000 HH || n

~C=C~ ~{~C–C~}~ n

|| HH alkene polymer

Steps to make a polymer. Step 1 : focus to double bonds, and bend the other carbon atom Step 2 : Break the double bonds Step 3

C–C=C–C Step 1: Focus to double bonds and bend the other carbon atom C=C || CC Step 2: Break the double bonds ~C–C~ || CC Step 3: Make a bracket between the monomer and put n. ~{~C–C~}~ {||} { H H }n Step 4: Complete the hydrogen atoms {HH} {||} ~{~C–C~}~ {||} {CH3 CH3}n