Chemistry - Alkanes and Alkenes

Characteristics of Alkanes

- Found naturally in crude oil (petrol, gasoline) - Name of an alkane ends with 'ane' - Each member of the alkane homologous series differs from the next by a CH2 unit - General formula of CnH2n+2 - Structural formula: Straight chain with CH3 as beginning and end groups (except methane) - Called saturated hydrocarbons because they contain only carbon-carbon single covalent bonds --> All the outer electrons are used in forming single bonds with four other atoms

Physical properties of alkenes

- Gases at room temperature - Boiling point of an alkene increases as the number of carbon atoms in the molecule increases

Physical properties of alkanes

- Low melting and boiling points (most alkanes are gases at r.t.p.) - Insoluble in water - Soluble in organic solvents

Characteristics of alkenes

- Name of an alkene ends with 'ene' - Each member of the alkene homologous series differs from the next by a CH2 unit - General formula of CnH2n NOTE! No alkene has only one carbon atom in its molecule as no double bonds would be possible for the molecule - Called unsaturated hydrocarbons because they contain carbon-carbon double bonds - Functional Group - Carbon-Carbon double bonds

What are alkanes?

Alkanes are a homologous series of hydrocarbons that contain only carbon-carbon single covalent bonds.

Chemical properties of alkanes

Alkanes are generally unreactive because their single C-C and C-H bonds are strong and difficult to break. However, they undergo: - Combustion: When ignited by a spark or flame, alkanes burn readily in excess air (oxygen) to produce carbon dioxide and water vapour CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g) This reaction is highly exothermic, and thus, alkanes make good fuels 2CnH2n+2 + (3n+1)O2 --> 2nCO2 + (2n+2)H2O - Substitution reactions: One or more atoms of an organic compound is replaced with one or more other atoms - Alkanes react with gaseous halogens (chlorine, bromine) in the presence of sunlight CH4 (g) + Cl2 (g) → CH3Cl (g) + HCl (g) Hydrogen atoms in the alkanes are replaced by halogen atoms to form substituted alkanes and hydrogen halide

What are alkenes?

Alkenes are a homologous series of hydrocarbons that contain one or more carbon-carbon double bonds.

Addition Reactions - Hydrogenation

At 200℃ and in the presence of a catalyst (e.g. nickel) Alkenes react with hydrogen to form alkanes Alkene + Hydrogen -- 200℃, Nickel → Alkane Ethene + Hydrogen -- 200℃, Ni → Ethane Manufacture of margarine (Vegetable oil has carbon double bonds, but when it is hydrogenated, it is converted into solid margarine)

Ethane

C2H6 (Structural formula CH3CH3) 2 carbon atoms

Propane

C3H8 (Structural formula CH3CH2CH3) 3 carbon atoms

Butane

C4H10 (Structural formula CH3CH2CH2CH3) 4 carbon atoms

Methane

CH4 (Structural formula CH4) 1 carbon atom

Chemical properties of alkenes

Combustion: Alkenes burn in excess air (oxygen) to form carbon dioxide and water vapour Alkene + Oxygen → Carbon Dioxide + Water Vapour C2H4 (g) + 3O2 (g) → 2CO2 (g) + 2H2O (g) Addition Reaction: Reaction in which an unsaturated organic compound combines with another substance to form a single new compound (no other byproduct) The carbon-carbon double bond breaks to form new single bonds, and as a result, an unsaturated hydrocarbon becomes a saturated organic compound Types of Addition Reactions: - Addition of Hydrogen (Hydrogenation) - Addition of Bromine (Bromination) - Addition of Steam (Hydration) - Addition polymerisation

Cracking of Alkanes

Cracking of alkanes is the breaking down of long-chain hydrocarbons (alkanes) into smaller molecules. - Can form both alkanes and alkenes Cracking is used for the manufacture of petrol, hydrogen and petroleum gases from petroleum

Uses of Ethene

Ethanol Ethanoic Acid Poly(ethene) Other chemicals for making plastics

Addition Reactions - Bromination

If an alkene is added to a solution of bromine, the reddish-brown colour disappears (decolourises) immediately and a colourless liquid is formed Alkenes react with bromine to form alkanes Ethene + Bromine → 1, 2-dibromoethane / C2H4 (g) + Br2 (aq) → C2H4Br2 (ℓ) Chemical test for the presence of unsaturated hydrocarbons - Used to distinguish between an alkane and an alkene: Alkanes do not decolourise bromine solutions, but alkenes decolourise bromine solutions rapidly

What are isomers?

Isomers are compounds that have the same molecular formula, but have different structural formulae. - They have different arrangements of atoms, and hence, have different structural formulae - They have different physical properties such as melting and boiling points

Molecular Formula VS Structural Formula

Molecular formula shows the actual number of atoms in a molecule Structural formula shows the arrangements of atoms in the formula

Addition Reactions - Hydration

Temperature of 300℃, pressure of 60 atm, and presence of a catalyst (Phosphoric Acid H3PO4) Alkenes can react with steam to produce alcohols Alkene + Steam -- 300℃, 60 atm, H3PO4 → Alcohol Ethene + Steam -- 300℃, 60 atm, H3PO4 → Ethanol C2H4 (g) + H2O (g) --- 300℃, 60 atm, H3PO4 → C2H5OH (ℓ)

Making of margarine from vegetable oil

Vegetable oils (liquid at room temperature) contain many carbon- carbon double bonds - Said to be unsaturated Margarine (solid at room temperature) is made by reacting vegetable oil with hydrogen in the presence of a nickel catalyst - Process known as hydrogenation (addition reaction) Margarine has more hydrogen atoms hence it has stronger intermolecular forces of attraction than vegetable oil. More energy is absorbed to overcome these forces and margarine has a higher melting point than vegetable oil. Thus, margarine is a solid while vegetable oil is liquid at room temperature