Alkanes and Alkenes

Both have low melting and boiling points. Their melting and boiling points increase gradually with increasing number of carbon atoms per molecule.

Changes from gas to liquid to solid as the number of carbon atoms per molecule increase.

Both have low densities. The gases and liquids are less dense than water. Density increase gradually with increasing number of carbon atoms per molecule.

Both are insoluble in water but soluble in organic solvents.

Both do not conduct electricity in any state.

Alkanes are saturated hydrocarbons, unreactive and undergo substitution reaction.

Alkenes are unsaturated hydrocarbons, reactive and undergo addition reaction.

Alkanes burn in air, producing yellow sooty flame.

Alkenes burn in air, producing yellow and very sooty flame.

Alkanes has no reaction with bromine in the dark.

Alkenes react with bromine in the dark and decolourises reddish-brown bromine.

Chemical Reactions of Alkenes

Combustion reaction- Similar to combustion reaction of alkanes.

Alkenes burn completely in excess oxygen to produce carbon dioxide and water.

(Refer to alkanes' combustion for a clearer picture of how combustion works)

However, alkenes burn with sootier flames as compared to alkanes. This is because alkenes have more % of carbon in their molecules than alkanes. Hence, alkanes are cleaner than alkenes.

Addition Reaction- Is a reaction in which atoms or groups of atoms are added to each carbon atom of a carbon-carbon multiple bond to form a single product. The reason why alkenes are able to do this is due to their reactivity due to their double bond.

Addition of hydrogen- Ethene reacts with hydrogen at 180 degrees in the presence of nickel or platinum catalyst to produce ethane. This process is called catalytic hydrogenation.

CH2=CH2 + H2 -> CH3-CH3

This hydrogenation reaction is used to convert an unsaturated compound to a saturated compound. It is used in the industry to change vegetables oils which are unsaturated into saturated fats for making margarine.

Addition of water(Hydration)- Alkenes do not react with cold water under normal conditions. When a mixture of an alkene and steam is passed over a catalyst, a molecule of water is added to the carbon-carbon double bond to produce an alcohol. For example, ethane reacts with steam at 300 degrees and with phosphoric acid as a catalyst to form ethanol.

Addition of halogens- Addition of halogens to alkenes. Chlorine, bromine, iodine react readily with alkenes. No catalyst or uv light is needed. This is different from alkanes.

Testing for unsaturation- Alkenes react with aqueous bromine (bromine water) turning it from red to colourless. A colourless liquid, 1,2-dibromoethane is formed. Hence, bromine water is a reagent used as a test for the presence of a carbon-carbon double bond in organic molecules.

Addition of hydrogen halide- Hydrogen halides react readily with alkenes. No catalyst is required. The hydrogen halides that react with alkenes include hydrogen chlorides (HCI), hydrogen bromide (HBr) and hydrogen iodide. (HI)

Isomerism in Alkenes

Isomerism in alkenes in due to the branching of the carbon chain and the different positioning of the double bond.

Ethane and propene do not exhibit isomerism because they do not have enough carbon atoms.

Unsaturated hydrocarbons

General formula=C_nH_2n.

They are known as alkenes and every alkene has a double C-C bond.

The double bond determines the chemical reactions of alkenes.

Chemical properties of Alkanes

Alkanes are generally unreactive due to strong C-C bonds and C-H bonds. Under ordinary conditions, alkanes do not react with most chemicals such as acids, alkalis, oxidising agents or reducing agents.

However, alkanes react in two ways- Combustion and Halogenation.

Combustion- Complete combustion (in the presence of sufficient oxygen) to produce carbon dioxide and water.

-Incomplete combustion (insufficient oxygen) to produce carbon dioxide, water, carbon monoxide or even carbon.

Alkanes are used as fuel because large amount of heat are liberated when they are burnt.

Halogenation- Substitution reaction of alkanes with halogens. For example, each hydrogen atom in the alkane molecule is substituted by a halogen atom. This reaction is initiated by ultraviolet light. For example, when a mixture of methane and chlorine is exposed to ultraviolet light, substitution reaction occurs. The hydrogen atoms in methane are replaced one by one by chlorine atoms to produce 4 different products.

*Apart from combustion and the reaction with chlorine, alkanes are generally unreactive.

Physical properties of Alkanes

Boiling and melting points- Alkanes are covalent compounds which consist of simple molecules. These molecules are held by weak van der waals forces. Hence. alkanes generally have low melting and boiling points.

However, as the molecules become larger, there is a gradual increase in the melting and boiling points. The surface area of the molecule as well as the number of electrons surrounding the molecule increases as the size of the molecule increases. This leads to a stronger van der waals force. Larger energy is required to overcome the stronger van der waals force between the molecules.

Density- Alkanes are less dense than water/ They form a colourless oily layer floating on top of water. The densities of the alkanes increase gradually with increasing molecular mass.

Viscosity- The alkanes become more viscous as their molecular size increase. This is because the van der waals force are stronger as the alkane molecules become bigger. This makes it difficult for the liquid to flow.

Solubility- Alkanes are insoluble in water, but are soluble in organic solvents such as ether.

Electrical conductivity- Alkanes are simple covalent molecules. The absence of mobile ions makes them unable to conduct electricity.

Flammability- As the molecular size of the alkane molecules increases, the percentage of carbon in the alkane molecules also increases. As a result, alkanes become less flammable. They also produce a smokier flame due to the incomplete combustion of alkane molecules.

Isomerism in Alkanes

Isomerism is a phenomenon whereby two or more molecules are found to have the same molecular formula, but with different structural formula.

They have different structural formulas, hence different physical properties for example different melting and boiling points.

Isomerism in alkanes must start with molecules that have more than 3 carbon atoms, so methane, ethane and propane do not have isomers.

General formula=C_nH_2n+1

Isomerism in Butane: