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AS Organic Chemistry Revision

Alkenes and Alkanes

Jacob Rowe

on 6 February 2013

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Transcript of AS Organic Chemistry Revision

Organic Chemistry Alkanes Combustion Complete n 2n+2 Alkane + Oxygen --> Carbon Dioxide + Water Alkane + Oxygen --> Carbon + Water (not excess) Incomplete Alkane + Oxygen --> Carbon Dioxide + Carbon Monoxide + Carbon + Water Cl 2Cl 2 UV . (excess) Naming Methane CH
Ethane C H
Propane C H
Butane C H
Pentane C H
Hexane C H 4 4 2 6 3 8 10 5 12 6 14 General Formula C H n 2n+2 Functional Group -CH - 2 -CH - methyl
-C H - ethyl
-C H -propyl No. identical groups: 2 di-
3 tri-
4 tetra-
5 penta-
6 hexa- 2 2 5 3 7 Alkyl Groups sooty flame Clean flame Overall: Reactivity Alkanes are generally unreactive because: C-C and C-H bonds are very strong, so Alkanes are quite thermodynamically stable

C-C and C-H bonds are non-polar, so do not attract any polar molecules or ions Reaction with Halogens Free Radical Substitution Mechanism CH + Cl ----> CH Cl + HCl 4 2 3 Methane Cl Cl 2Cl . (free radical) CH + Cl CH + HCl 4 . 3 3 . C H H H H Cl . C H H H . H CH + Cl CH Cl + Cl 2 3 . Initiation Propagation . C H H H . Cl Cl C Cl H H H H Cl Cl . Termination Cl + Cl Cl + Cl CH + CH . . . . . . 3 3 3 CH Cl 3 Cl C H 2 2 6 Eg. Alkane + Oxygen --> Carbon Monoxide + Water (not excess) Alkenes General Formula C H 2n n Functional Group Naming
Ethene C H
Prop-1-ene C H
But-1-ene C H
Pent-1-ene C H
Hex-1-ene C H 4 2 4 3 6 8 5 10 6 12 The lowest-numbered carbon with the double bond is given the number in the alkene
eg. alk-X-ene Reactivity Alkenes are insoluble in water as thy are non-polar When there is more than 1 C-C double bond, the name changes slightly:
eg. hex-1,3-diene C C H H H H -Saturated Hydrocarbons -Contain single bonds only Structural Isomerism - Used widely as fuels
- obtained from crude oil -Contain a double bond formed from a
and a bond -Unsaturated Hydrocarbons E-Z Isomerism The double bond prevents free rotation around the bonds, so geometric isomers are formed C C How a double bond is formed: sigma bond C C Interaction Interaction p-orbitals p-orbitals overlap side-on to form a pi bond C C sigma bond pi bond The pi bond prevents the groups on either side from rotating relative to each other The large no. of electrons forming the bond causes alkenes to have a high electron density at the double bond eg. but-2-ene C CH C 3 H CH 3 H C CH C 3 H CH H 3 Z-but-2-ene E-but-2-ene Z (Cis) - same side E (trans) - opposite side In order for Geometric isomerism to occur, there must be two different groups on both sides of the double bond Eg. C C CH H H H would not show geometric isomerism Reactions Alkenes are more reactive than alkanes because the pi component of the double bond is more easily broken than a single bond The double bond is made up of four electrons, so attracts electrophiles An electrophile is a positively charged ion (or +ve end of a dipole) which is attracted to regions of high electron density. They accept a pair of electrons to form a dative covalent bond Alkenes undergo addition reactions In general: C C R R R R + X Y Alkyl group C C R R R R X Y C C H H H H Ethene + Br Br C C H H H H Br Br Bromine 1,2-dibromoethane Addition reaction of Ethene and Halogens Alkenes react with Halogens at room temperature
The reaction with iodine is very slow Electrophillic addition mechanism C C H H H H Ethene + H Br electrophile C C H H H H H Br area of high electron density C C H H H H H Br a full curly-arrow represents the movement of 2 electrons C C H H H H H Br Carbocation C C H H H H H Br Overall Reaction With Ethene and Hydrogen Bromide Initial stage Intermediate Stage Final stage 3 Major and Minor Products C C H H H C H Br Propene + Hydrogen bromide H H H There are two possible products for this reaction C C H H H C Br H H H H 1-bromopropane 2-bromopropane Alkyl groups have an inductive effect which makes different isomers more or less stable: The more alkyl groups around the carbon with the functional group, the more stable it is. ie. the most stable carbocations are tertiary, then secondary; the least stable are primary. C C H H H C H Br H H H In this example, 2-bromopropane is the major product as it is formed from a secondary carbocation C C H H H C Br H H H H Likewise, 1-bromopropane is the minor product as it is formed from a primary carbocation Electrophillic addition mechanism C C H H H H Ethene + H H C C H H H H H H Overall Reaction With Ethene and Hydrogen Hydrogen Ethane Nickel

catalyst Alkene + Hydrogen Alkane Nickel

catalyst In General: In general: Alkene + Halogen di-substituted halogenoalkane In general: Alkene + Hydrogen halide mono-substituted halogenoalkane Testing for Alkenes:
Adding bromine water to an alkene will turn the solution from yellow to clear This reaction takes place by bubbling the Hydrogen halide gas through the alkene at 100 C o the reaction with HI is very rapid, but for HCl is slow, so a catalyst (Aluminium Chloride) is often used Oxidation of Alkenes C C H H H H Ethene + Potassium Manganate (VII) (oxidising agent) C C H H H H OH OH Ethan-1,2-diol In General: Alkene diol Oxidising

Agent Polymerisation Alkene Poly(alkene) In General: n ( ) C C R R R R Alkene [ ] C C R R R R Poly(alkene) n n ( ) C C H H H H Ethene [ ] C C H H H H Poly(ethene) n Polymerisation of Ethene n ( ) C C H H Cl H Chloroethene [ ] C C Cl H H H Poly(chloroethene) n Polymerisation of chloroethene to form PVC or Poly(vinylchloride) PVC Ecomic importance of Polymers All plastics are polymers - we use them every day for many things-
plastic bags, carpet fibres, bottles, electric insulation, etc.
Polymers are produced in huge quantities from crude oil, a limited natural recource
Due to their chemistry, they are non-biodegradable, so are a danger to the environment C C Alkanes and Alkenes By Jacob Rowe & Lewis Keeble
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