Properties of Hydrocarbons

Physical Properties of Alkanes

Non polar compounds

Insoluble in water

Less dense than water and so will float on top of the water

Dissolve in organic solvents (eg dry cleaning fluid) and in each other (eg Petrol is a mixture of alkanes)

Low melting and boiling points

Chemical Properties of Alkanes

Very unreactive

Don’t react with concentrated sulfuric acid or concentrated bases

Don’t react with strong oxidants (dichromate or permanganate ions)

Will react with bromine but only if heated or in strong sunlight

Substitution Reactions

Where an atom or group of atoms is displaced by an atom or group of atoms

CH4 + Br2 → CH3Br + HBr

Non specific reaction. Can not control which hydrogen is substituted or how many are substituted

CH3Br + Br2 → CH2Br2 + HBr

Substitution reactions

Very slow with heat or sunlight

Product is a mixture of bromoalkanes

Products would need to be separated by fractional distillation giving only a very small yield of each

Combustion of Alkanes

Very useful group as fuels

Fuels burn completely when there is sufficient oxygen, forming CO2 and H2O

If oxygen is limited then incomplete combustion occurs and carbon (soot) and carbon monoxide (CO) are formed as well as CO2 & H2O

Combustion of Alkanes

Incomplete combustion is less efficient and so produces less energy per mole that complete combustion

Balancing Combustion Equations

Write correct formula Alkane + O2 → CO2 + H2O

Balance carbon

Balance hydrogen

Balance oxygen (Fractional coefficients are acceptable in combustion equations)

Physical Properties of Alkenes

Non polar molecules so they are insoluble in water

Less dense than H2O, so floats on water

Low melting and boiling points

Chemical Properties of Alkenes

Combustion is incomplete in air but will burn completely in pure O2

Burn with a smoky flame in air due to unburnt carbon. Higher carbon to hydrogen ratio means some carbon remains unburnt

More reactive than alkanes because the can undergo addition and oxidation reactions

Addition Reactions

The colour of bromine rapidly disappears when shaken with an alkene

C2H4 + Br2 → C2H4Br2

The reaction site is the alkene double bond. One bromine adds to the carbon on one end of the double bond and the other bromine to the other end. The double bond becomes a single bond

Addition Reactions

C

H

H

C

H

H

C

H

H

C

H

H

+

Br Br

Br Br

C

H

H

Br C

H

H

Br

C

H

H

Br C

H

H

Br

1,2 dibromo ethane

Addition Reactions

An alkene can be converted to the corresponding alkane by the addition of hydrogen gas

The reaction conditions for this to occur are heat, high pressure and a nickel catalyst

This reaction is called catalytic hydrogenation

Addition Reactions

C

H

H

H C

H

C H

H

C

H

H

H C

H

C H

H

+

H H

H H

Ni Cat HEAT PRESSURENi Cat

Ni Cat

HEAT PRESSURE

HEAT PRESSURE

C

H

H

H C

H

H

C

H

H

H

C

H

H

H C

H

H

C

H

H

H

This reaction is used to turn a pourable oil into a spreadable fat (margarine)

Other Addition Reactions

Alcohols can also be made from alkenes by the addition of water in the presence of an acid catalyst (conc. Sulfuric acid)

C

H

H

C

H

H

C

H

H

C

H

H

+

H O H

H O H

H+ CatH+ Cat

H+ Cat

C

H

H

H C

H

H

O H

C

H

H

H C

H

H

O H

Other Addition Reactions

Alkenes can add to each other to form polymers (very large molecules)

C

H

H

C

H

H

C

H

H

C

H

H

CatCat

Cat

C

H

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

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

.

ethene polyethylene