CN Alkanes-alkenes 11

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Introduction to organic chemistry.:

Introduction to organic chemistry . The following terms must be known and understood. Empirical formula Molecular formula Structural formula Displayed formula Structural isomers Functional group Homologous series

Alkanes :

Alkanes Homologous series C n H 2n+2 Names of first six MUST be known

The separation of petroleum into fractions:

The separation of petroleum into fractions Fractional distillation:- Crude oil/cracked products vaporised by heat Vapour passed into fractionating tower Tower has a temperature gradient, hot at bottom, cooler at top Fraction separate due to different boiling points Boiling points linked to M r and shape vdw attraction between molecules

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Method of cracking Reaction conditions Products Thermal High temp. (450 - 900 o C) High pressure (70 atm, 7000 kPa) Mostly alkenes Catalytic Slight pressure High temperature (450 o C) Catalyst (zeolite) Mostly branched chain alkanes, cycloalkanes and aromatics. Used mainly to make motor fuels

Economic reasons for cracking :

Economic reasons for cracking Thermal: makes alkenes which are used to make polymers and alcohols. Catalytic: makes shorter alkanes, more valuable as fuels

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The reactivity of alkanes C-C and C-H bonds are very strong Alkanes are unreactive, only react if (i) a lot of energy used (ii) a very reactive reagent used.

The combustion of alkanes:

The combustion of alkanes Complete combustion. e.g. C 3 H 8 + 5O 2  3CO 2 + 4H 2 O Incomplete combustion e.g. C 3 H 8 + 3 1 / 2 O 2  3CO + 4H 2 O C 3 H 8 + 2O 2  3C + 4H 2 O Pollution due to nitrogen NO x e.g. N 2 + O 2  2NO

Combustion products causing problems:

Combustion products causing problems (i) CO and NO x Catalytic converters metal catalysts; Pt. Pd and Rh ceramic honeycomb support removes 90% harmful gases e.g. 2CO + 2NO  2CO 2 + N 2 C 8 H 18 + 25NO  8CO 2 + 12 1 / 2 N 2 + 9H 2 O

Combustion products causing problems:

Combustion products causing problems SO 2 As coal and petroleum are formed from plant an animal remains. Some compounds contain sulfur atoms. Burning produces sulfur dioxide. Impurities removed form motor fuels.

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SO 2 produced by burning these fuels forms SO 3 in the upper atmosphere. This dissolves in rain water. SO 2 produced in power stations by burning coal is ‘scrubbed’ out using calcium oxide. CaO + SO 2  CaSO 3 Note The CaO is produced by heating CaCO 3

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Global Warming

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North Sea Gas; a HSW discussion. Consider a mixture containing methane and ethane. Can this mixture be fed directly into the gas main? How can the mixture be separated? Why is this separation process very expensive? What compounds are then made from the ethane? How are these compounds made from ethane? What are these compounds used for? Where are they used and how are they transported?

The chlorination of methane :

The chlorination of methane

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Chlorination Mechanism Initiation Cl 2  2Cl . 2 Propagation Cl . + CH 4  CH 3 . + HCl CH 3 . + Cl 2  CH 3 Cl + Cl . e.t.c. 3 Termination Cl . + CH 3 .  CH 3 Cl CH 3 . + CH 3 .  CH 3 CH 3 e.t.c.

These reactions can be represented as follows :

These reactions can be represented as follows

Alkenes :

Alkenes a double covalent bond double bond arrangement is planar catalytic hydrogenation used in the manufacture of margarine. electrophilic addition alcohols produced industrially by hydration form addition polymers

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(i) Hydrogen bromide

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Unsymmetrical alkenes form more than one carbonium ion intermediate; carbonium ion stability determines the amounts formed the inductive effect of alkyl groups mean stability increases in the order + + + R -C- H < R -C- R < R -C- R | | | H H R i.e. Primary < Secondary < Tertiary

Other reactions of alkenes :

Other reactions of alkenes 1. Hydration 2. Polymerisation Examples Poly(ethene) Polychloroethene, PVC Polyphenylethene, poly(styrene) Polytetrafloroethene, PTFE Poly(acrylonitriles)

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