GCSE Chemistry Organic Chemistry Introduction Alkanes o Crude oil Combustion Halogenation Fractional distillation Cracking Reforming Alkenes o Structural formulae o Homologous series o Names o Reactions o Names o Addition reactions With bromine With hydrogen With potassium manganate VII Addition polymerization Polymer properties Alcohols o Names o Reactions Fermentation Dehydration Oxidation Esterification o Uses of ethanol Carboxylic acids o Names o Esterification o Condensation polymerisation o Natural condensation polymers o Saponification Flowchart Summary of Reactions Organic Chemistry Introduction 1 Structural formulae As well as using a normal type of molecular formula to describe an organic molecule they can be represented by drawing out their structure i e by showing how the atoms are connected or bonded to each other In order to do this a few rules have to be followed i carbon atoms must be bonded four times ii oxygen atoms must be bonded twice iii hydrogen atoms must bond only once 2 Homologous Series A homologous series is a group of organic compounds with similar chemical properties and structural formula and a gradual change in physical properties e g melting point and boiling point Below is a graph of boiling point against number of carbon atoms for the various homologous series covered in GCSE O level Organic Chemistry 1 2 3 4 5 6 7 8 9 10 alkanes alkenes alcohols carboxylic acids 200 150 100 50 0 50 100 150 200 250 300 From the graph above it can be seen that as the number of carbon atoms in the organic compound increases the boiling points increase Also the boiling points tend to follow a straight line with the higher members of each group i e the difference between boiling points tends towards a single value The four homologous series studied at IGCSE are alkanes alkenes alcohols and carboxylic acids The names and formulae of these compounds will be dealt with in separate sections The members of each series differ from each other by the number of carbon atoms contained in the molecule Organic Chemistry Alkanes 1 Names Alkanes are the simplest homologous series of compounds and their names follow this pattern CH4 methane C2H6 ethane C3H8 propane C4H10 butane C5H12 pentane i e they have a prefix meth eth prop but etc which depends on the number of carbon atoms in the molecule and a common suffix ane The general chemical formula for an alkane is CnH2n 2 Branched alkanes When the alkane is not just a simple straight chain of carbon atoms joined together the names become a little more complex The longest connected chain of carbon atoms must be found as before and the alkane name generated as usual Then the name for the pendent group is found again by counting the number of carbon atoms present and used as a prefix Enter the name here Enter the name here The numbers used to indicate the positions of the pendent groups must be the lowest numbers possible so always check them from both ends of the molecule Example Some examples to try out 2 Reactions of alkanes i Combustion Alkanes along with all other types of hydrocarbon will burn in an excess of oxygen to give carbon dioxide and water only as the products e g CH4 g 2O2 g CO2 g 2H2O g in general CnH2n 2 g 1 5n 0 5 O2 g nCO2 g n 1 H2O g If there is not enough oxygen present then instead of carbon dioxide carbon monoxide CO is produced Carbon monoxide is particularly toxic and absorbed into blood through respiration very easily For domestic heating systems it is particularly important that enough air can get to the flame to avoid carbon monoxide being generated in the home Car engines also require a lot of air and there is a lot of research going on to make the internal combustion engine more efficient and so put out less carbon monoxide Note also that both alkanes and carbon dioxide are green house gases i e they trap infra red i r radiation inside the Earth s atmosphere gradually increasing global temperatures Global Warming ii Halogenation The only other reaction that an alkane will undergo is a reaction with a halogen chlorine or bromine typically with UV light present as an initiator of the reaction e g CH4 g Br2 g CH3Br g HBr g The UV light causes the formation of free radical halogen atoms by providing enough energy for the bond between the two halogen atoms to break A halogen atom attacks the alkane substituting itself for a hydrogen atom This substitution may occur many times in an alkane before the reaction is finished A similar process occurs high up in the earth s atmosphere when CFC s and other organic solvents react with intense sunlight to produce free radicals chlorine atoms in this case These attack molecules of ozone O3 depleting ozone s concentration and leading to the holes 3 Crude oil Introductory videos from the Periodic Table of Videos What happens when Crude Oil spills into the sea More Crude Oil i Fractional distillation Crude oil is a mixture of many different hydrocarbon compounds some of them liquid and some of them gases These compounds can be separated because the different length of alkanes will have different boiling points The crude oil is heated up to about 350 C and is fed into a fractionating column as in the diagram below The vapours with the lowest boiling points pass all the way up the column and come off as gases e g methane ethane and propane The temperature of the column gradually decreases the higher up the vapours go and so various fractions will condense to liquids at different heights The fractions with the highest boiling points do not vaporize and are collected at the bottom of the fractionating column e g bitumen Here is a table with the range of carbon atoms and suggested boiling point range for the most common fractions Fraction name Refinery gases Gasoline Naphtha Kerosene Number of C atoms 1 to 4 5 to 9 5 to 10 10 to 16 Diesel oil Fuel oil Bitumen 14 to 20 20 to 70 more than 70 ii Cracking In industry the fractions obtained from the fractional distillation of crude oil are heated at high pressure in the presence of a catalyst to produce shorter chain alkanes and alkenes e g C10H22 C5H12 C5H10 iii Reforming This is a process where straight chain alkanes are turned into branched alkanes and cyclic alkanes are turned into aromatic compounds Both these reactions result in the formation of chemicals that improve the performance of fuels as well as enable more exotic compounds to be made Organic Chemistry Alkenes 1 Names Alkenes all have a C C double bond in their
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