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 11

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 12

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 13

Addition Reactions 1,2 dibromo ethane 14

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 15

Addition Reactions This reaction is used to turn a pourable oil into a spreadable fat (margarine) 16

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) 17

Other Addition Reactions Alkenes can add to each other to form polymers (very large molecules) polyethylene ethene 18