Windsor University School of Medicine Properties of alkanes & alkenes It is better to keep your mouth closed and be thought a fool than to open your mouth and remove all doubt.  Mark Twain Ch.14 J.C. Rowe

Chain Length & the State of Matter of Organic Compounds (# of carbons in the chain) State of matter 1-4 gas 5-15 liquid Greater than 15 solid

Prefix Names/Substituents Prefix names & chain length Some nonmetal substituents Chain length Prefix name 1 Meth- 2 Eth- 3 Prop- 4 But- 5 Pent- 6 Hex- Element or group Name of group (halide) -F Fluoro- -Cl Chloro- -Br Bromo- -I Iodo- -NH2 Amino-

Structures : alkanes vs. alkenes ALKANE (has single bonds) ALKENE (has double bonds)

REACTIONS of ALKANES Combustion of alkanes Substitution of alkanes Cracking of alkanes

Combustion of alkanes You can burn them - destroying the whole molecule. Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water. Incomplete combustion (where there isn't enough oxygen present) can lead to the formation of carbon or carbon monoxide.

Substitution of alkanes You can react them with some of the halogens, breaking carbon-hydrogen bonds. Substitution reactions happen in which hydrogen atoms in the methane are replaced one at a time by chlorine atoms. You end up with a mixture of chloromethane, dichloromethane, trichloromethane (chloroform) and tetrachloromethane (carbon tetrachloride).

Substitution reactions Cont’d.

Cracking of alkanes You can crack them, breaking carbon-carbon bonds Cracking is the name given to breaking up large hydrocarbon molecules into smaller and more useful bits. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst.

One possible reaction involving the hydrocarbon C15H32 might be:

Or, showing more clearly what happens to the various atoms and bonds: The ethene and propene are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petrol (gasoline).

Cracking: catalytic vs. thermal Catalytic cracking Thermal cracking The alkane is brought into contact with the catalyst at a temperature of about 500°C and moderately low pressures. high temperatures(typically in the range of 450°C to 750°C) and pressures (up to about 70 atmospheres) are used to break the large hydro-carbons into smaller ones. Thermal cracking gives mixtures of products containing high proportions of hydrocarbons with double bonds - alkenes

REACTIONS of ALKENES Combustion of alkenes Substitution of alkenes Addition reactions : Hydrogenation Hydrohalogenation Hydration Polymerisation.

Combustion of alkenes The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water. For example, ethene burns as follows : C2H4 (g)+ 3 O2 (g) ==> 2 CO2 (g) + 2 H2O (g)

Substitution of alkenes A substitution reaction involves replacings the hydrogen atoms of the alkene molecule with halogen atoms. Note that the double bond does not break during this reaction. Substitution reactions are rare for alkenes, therefore extreme conditions, such as high temperature & pressure &/or a catalyst, are needed to force the reaction.

Addition reactions of alkenes Alkenes contain the unsaturated C=C functional group which characteristically undergo addition reactions.  This is driven by the conversion of the weaker π bond into 2 new, stronger σ bonds.

Hydrogenation of Alkenes Alkenes can be reduced to alkanes with H2 in the presence of metal catalysts such as Pt, Pd, Ni or Rh. The two new C-H (sigma) σ bonds are formed simultaneously from H atoms absorbed into the metal surface.

Halogenation of Alkenes overall transformation :  C=C to X-C-C-X Reagent : normally the halogen (e.g. Br2) in an inert solvent like methylene chloride, CH2Cl2.

Reaction of Alkenes with Hydrogen Halides When treated with HX alkenes form alkyl halides. Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity order).

Hydration of Alkenes When treated with aq. acid, most commonly H2SO4,  alkenes form alcohols.

Polymerisation of alkenes During the polymerisation of ethene, thousands of ethene molecules join together to make poly(ethene) - commonly called polythene. The number of molecules joining up is very variable, but is in the region of 2000 to 20000.

Naming alkanes/alkenes See attached folders (5 worksheets)

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