19 Hydrocarbons

What is an isomer

A compound with the same general formula but different structural formula

Alkanes

No functional group, suffix ane, general formula CnH2n+2

Alkenes

Functional group C=C, suffix ene, general formula CnH2n

Alcohols

Functional group OH hydroxyl, suffix ol, general formula CnH2n+1OH

Carboxylic acids

Functional group COOH, suffix oic acid, general formula CnH2n+1COOH

How does melting and boiling point change as carbon atoms increases

Larger molecules have stronger intermolecular forces of attraction, so more energy is required to overcome these stronger intermolecular forces of attraction. Hence there is an increase in melting and boiling point.

How does flammability change as the number of carbon atoms increases.

Larger molecules have stronger intermolecular forces of attraction and are less volatile. Larger molecules also have a higher percentage composition of carbon, hence requires more oxygen to burn and is less flammable.

How does viscosity change as the number of carbon atoms increases.

Larger molecules have stronger intermolecular forces of attraction hence it is harder for liquids containing larger molecules to flow.

How does solubility change?

All hydrocarbons are insoluble in water. Alkanes and alkenes are hydrocarbons which are non-polar hence no interactions with water molecules to dissolve.

What chemical reactions do hydrocarbons go through

Substitution, addition, condensation, hydrolysis, elimination

Describe the combustions of alkanes and alkenes

The alkane/alkene react with oxygen. A complete combustion produces carbon dioxide and water. An incomplete combustion produces carbon monoxide and water, when o2 is the limiting reagent.

Describe substitution in alkanes

UV light is used to break the covalent bond in the chlorine molecule. Produces Chloro-(hydrocarbon) and HCl

Describe the cracking of alkanes

Occurs at a high temperature with aluminium oxide and silicon dioxide as a catalyst.

Why is cracking needed

To produce short chain alkanes and alkenes from heavier fractions in crude oil, which are higher in demand than long chains.

Describe the addition of hydrogen to alkenes

Occurs at a high temperature (150-200C) with a nickel catalyst. The C=C bond is broken and hydrogen is bonded. Produces an alkane.

Describe addition of halide to an alkene

Occurs at rtp. C=C bond is brond and halogen atom is bonded.

How can addition of halide be used to differentiate alkane from alkene

Add few drops of aqueous bromine into the unknown substance. Shake test tube thoroughly. Observe any changes. If the reddish-brown bromine solution remains reddish-brown, it is an alkane. If it turns colourless, it is an alkene.

Describe the addition of steam for alkenes

High temperature (300C) and pressure (60atm), phosphoric acid catalyst. alkene + water → alcohol