Experiment 2

Experiment 2 : Hydrocarbons Objectives : After completing this experiment, students should have learned : 1. The physical properties of alkanes, alkenes and alkynes. 2. The chemical reactions of alkanes, alkenes and alkynes. 3. To differentiate alkanes, alkenes and alkynes based on their physical and chemical properties. Theory : a) Alkane Alkanes are saturated hydrocarbon because they contain only single bones between carbon atoms. Like most organic compounds, the alkanes are combustible. The products of their complete combustion are carbon dioxide and water. The reactions of the alkanes are of the substitution type ; that is some atom or group of atoms is substituted for one or more of the hydrogen atoms in the hydrocarbon molecule. CH4

+

Br2

Methane

CH3Br + HBr Ultraviolet

Methyl bromide

b) Alkenes Alkenes are unsaturated hydrocarbon because they contain a carbon-carbon double bond in their structures. The functional group for alkenes is the carbon-carbon double bond (C=C) ; it is a point of high reactivity. Alkenes undergo addition-type reactions which are the other groups are added to the double bond, causing the molecule to be saturated.

CH2 = CH2

+

Br2

CH2 CH2 Br

Br

The disappearance of the red-brown color of free bromine can be the evidence to the reactions. Other reactions of olefin also show that increased reactivity of the alkenes over the alkanes. Unsaturated hydrocarbons can be oxidized by potassium permanganate. The reaction is called Baeyar test for saturation. The evidence that reactions has occurred is the rapid disappearance of the purple colour of the permanganate ion. Potassium permanganate is a very strong oxidizing agent and gives similar results when reacted with other oxidizable substances, such as alcohols. c) Alkynes These unsaturated hydrocarbons are also called acetylene. Alkynes contain a carboncarbon triple bond in their structures. Acetylene can be prepared from calcium carbide and water. Mixtures of acetylene and air are explosive. The alkynes undergo additiontype reactions similar to those of the alkenes.

Results : A) COMBUSTION 0RGANIC COMPOUND Heptane Cyclohexene

COLOUR OF FLAME No sooty flame Sooty flame

RESIDUE None (all used) Oily left

B) SOLUBILITY OF HYDROCARBON ORGANIC COMPOUND Heptane

Cyclohexene

WATER 1. Insoluble

1- BUTANOL 1. Soluble

2. The mixture forming

2. Forming unicous

2 layers 1. In soluble

mixture 1. Soluble

2. The forming of 2

2. None layer formed.

layers.

C) ACTION OF BROMINE IN 1,4 – DIOXANE ON HYDROCARBONS ORGANIC COMPOUND Heptane

Cyclohexene

DARK 1. No reaction occur

LIGHT 1. Reaction occur

2. Bromine color does

2. Bromine color

not change. 1. No bromine color

decolorized. 1. No reaction occur.

decolorized

2. Bromine color does not change.

D) REACTION OF HYDROCARBON WITH POTASSIUM PERMANGANATE ORGANIC COMPOUND

COLOR OF POTTASIUM PERMANGANATE

Heptane Cyclohexene

1. Change from purple to green precipitate A) Change from purple to dark brown B) Slightly decolorized

E) CHEMICAL PROPERTIES OF ACETLYENE ORGANIC COMPOUND

BROMIN IN 1,4-

POTASSIUM

Acetlyene

DIOXANE 1. Transparent /

PERMANGANATE 1. Dark brown

colorless solution formed Discussions : a) CH3CH2C ≡ CCH3

+

O2

b) CH3CH2CH = CHCH3 + c)

O2

CH3 heat, KMNO4 CH3

d)

CH(CH3)2 heat KMNO4

e) heat KMNO4

solution formed

f)

CH2 Br2 1,4 - dioxane

g) Br2 C ≡ CH 1,4 - dioxane

h) CH3CH2CH = CHCH2CH = CH2

Br2 1,4 - dioxane