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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 Windsor University School of Medicine

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

3 Prefix Names/Substituents Chain lengthPrefix name 1Meth- 2Eth- 3Prop- 4But- 5Pent- 6Hex- Prefix names & chain lengthSome nonmetal substituents Element or group Name of group (halide) -F Fluoro- -ClChloro- -BrBromo- -IIodo- -NH2Amino-

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

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

6 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.

7 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).

8 Substitution reactions Cont’d.

9 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.

10 One possible reaction involving the hydrocarbon C 15 H 32 might be:

11 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).

12 Cracking: catalytic vs. thermal  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 Catalytic cracking Thermal cracking

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

14 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)

15 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.

16 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.

17 Hydrogenation of Alkenes  Alkenes can be reduced to alkanes with H 2 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.

18 Halogenation of Alkenes  overall transformation : C=C to X-C-C-X  Reagent : normally the halogen (e.g. Br 2 ) in an inert solvent like methylene chloride, CH 2 Cl 2.

19 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).

20 Hydration of Alkenes  When treated with aq. acid, most commonly H 2 SO 4, alkenes form alcohols.

21 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

22 See attached folders (5 worksheets) Naming alkanes/alkenes

23 Quality is never an accident. It is always the result of high intention, sincere effort and skillful execution. It represents the wise choice of many alternatives. unknown


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