REACTIONS OF HYDROCARBONS REACTIONS OF ALKANES the single covalent bonds between carbon atoms in saturated and unsaturated hydrocarbons are difficult to break and therefore alkanes are relatively unreactive. REACTION WITH O 2 (COMBUSTION) All the alkanes burn in air. In burning, the alkanes react with the oxygen in air to produce carbon dioxide and water. For example, methane burns in an excess of oxygen as follows: Hydrocarbon + Oxygen --  Water + Carbon dioxides CH 4 (g) + 2O 2 (g)  2H 2 O (l) + CO 2 (g) C 3 H 8 (g) + 5O 2 (g)  4H 2 O (l) + 3CO 2(g)

SUBTITUTION REACTION Methane and other alkanes react with chlorine or another halogen compounds in sunlight or at high temperature in a substitution reaction. C 2 H 6 (g) + Br 2 (l)  CH 3 ---CH 2 Br (l) + HBr (l) CH 4 (g) + Cl 2 (g)  CH 3 Cl (g) + HCl (g) A substitution reaction is a reaction in which one atom (or group of atoms) substitutes for another atom (or group of atoms) in a molecule. The reaction of an alkane with other halogens (such as bromine) results in a range of substitution products called haloalkanes. Chloromethane is an example of a haloalkane.

REACTIONS OF ALKENES AND ALKYNES alkenes and alkynes exhibit much greater chemical reactivity due to the presence of the double or triple bonds. REACTION WITH O 2 (COMBUSTION) Alkenes and Alkynes also undergo combustion reactions. For example, propene burns in a supply of air to give carbon dioxide and water: 2C 3 H 6 (g) + 9O 2 (g)  6CO 2 (g) + 6H 2 O (g)

If there is not enough oxygen available, propene will burn with a yellow flame and produce soot (carbon) and carbon monoxide (CO). Ethyne burns in air with a luminous flame and produces a lot of soot. If a good supply of oxygen is available, ethyne burns with a colorless flame. This reaction is: 2C 2 H 2 (g) + 5O 2 (g)  4CO 2 (g) + 2H 2 O (g)

ADDITION REACTION The alkenes and alkynes are generally more reactive than the alkanes. The reactions they undergo are mostly addition reactions. An addition reaction is a reaction in which parts of a reactant molecule are added to each carbon atom of a carbon-carbon double or triple bond. This characteristic reaction of alkenes and alkynes is called an addition reaction as atoms are added to the molecule with no loss of hydrogen atoms.

Addition of H 2 to alkenes and alkynes: Alkenes and alkynes react with hydrogen in the presence of a metal catalyst such as palladium to yield the corresponding saturated alkane products – called hydrogenation. Addition of Cl 2 and Br 2 to alkenes (Halogenation): Alkenes react with the halogens Br 2 and Cl 2 to give 1,2- dihaloalkane.

Addition of HCl and HBr to alkenes - Hydrohalogenation ( with hydrogen halides ): Alkenes react with the hydrogen bromide and hydrogen chloride to give alkyl bromide or alkyl chloride product. Markovnikov rule: In the addition of HX to an alkene, the H becomes attached to the carbon that already has the most H’s, and X becomes attached to the carbon that has fewer H’s.

Addition of water to alkenes: Hydration Alcohol is produced on treatment of the alkene with water in the presence of a strong acid catalyst, such as H 2 SO 4. Markovnikov’s rule can be used to predict the product when water adds to an unsymmetrically substituted alkene.