IGCSE Organic Chemistry IGCSE Organic Chemistry I Homologous Series and Functional Groups There are many different organic compounds compounds which contain carbon Chemists have placed organic compounds which share the same chemical properties into groups called homologous series Each homologous series has its own set of chemical properties because each homologous series has its own group of atoms which are arranged or bonded in a certain way which is different from the other homologous series The group of atoms is called the functional group It is the functional group which determines the chemical properties of an organic compound A homologous series is a group of organic compounds which share the following Each compound has the same functional group and therefore the same general formula characteristics All compounds do the same chemical reactions Members differ from the previous member by one CH group There is a gradual change in physical properties e g boiling points colour and viscosity Examples of homologous series are alkanes alkenes and alcohols The functional group can be defined a group of atoms which are arranged or bonded in a particular way and determine how an organic compound will react The functional group in alkenes is C C and in alcohols it is always the R OH single bond When naming an organic compound we want to give a lot of information in its name which is why the name of an organic compound consists of two parts shown in the following examples Prefix This part tells us how many carbon atoms there are in the molecule Meth 1 carbon atom Eth 2 carbon atoms Prop 3 carbon atoms But 4 carbon atoms Pent 5 carbon atoms Suffix This part tells us the functional group it has or which homologous series is belongs to ane alkanes ene alkenes anol alcohol Alkanes general formula for alkanes is CnHn All alkanes have the same functional group and the same chemical reactions However they do have different physical properties They are said to be saturated as they only have single bonds in the molecule These are hydrocarbons meaning that they are made up of hydrogen and carbon atoms The Molecular Formula Structural Formula Melting Point Boiling Point State at rtp C C 162 183 183 188 89 42 Gas Gas Gas Alkane Methane Ethane Propane In organic compounds it is possible to have different structure for the same molecular formula as there are different ways in which atoms which are stated in the molecular formula can be arranged We call this phenomena isomerism Isomers are compounds with the same molecular formula buy different structures As a result their properties can differ 1 IGCSE Organic Chemistry The chemical properties would not differ as they have the same functional group The isomer of pentane that would have the lowest boiling point is 2 2 dimethyl propane as it cannot get close to its neighbouring molecules and therefore cannot establish strong intermolecular forces The molecular structure of 2 2 dimethyl propane The unreactive alkanes also get involved in another reaction that is not combustion A substitution reaction is an endothermic reaction during which hydrogen is replaced by another atom This reaction only happens in sunlight which has plenty of UV light because a lot of energy is needed to break the covalent bonds in the molecules being substituted It is the breaking of the bond that gets the reaction started If a large excess of bromine is used in the reaction with methane more than one and eventually all of the hydrogen atoms will be substituted into bromines Combustion Alkanes are quite unreactive as they have many single strong covalent bonds but they do carry out certain reactions The shorter alkanes are used as fuels because they react easily with oxygen Whenever an alkane burns in sufficient oxygen complete combustion occurs and carbon dioxide and water are given as by products Incomplete combustion occurs when their insufficient oxygen present and the by products are carbon monoxide carbon and water Carbon monoxide is poisonous as it binds with haemoglobin preventing red blood cells from carrying oxygen There is then a lack of oxygen reaching the cells as carbon monoxide has bonded with haemoglobin to form carboxyl haemoglobin Carbon monoxide is especially dangerous in small and enclosed spaces as there will be a high amount of carbon monoxide and the body will take it in The two types of combustion can be illustrated using a standard Bunsen burner When the air hole is open to provide plenty of air complete combustion occurs and carbon dioxide is given off However when the hole is half open or completely closed there is a lack of oxygen and incomplete combustion takes place so there is a thick smoky flame We should not heat things with a yellow flame as poisonous carbon monoxide will be produced Crude Oil Crude oil is a finite substance it will run out that contains petrol kerosene and fuel oil It is a natural organic compound that can be separated The components in a mixture are not chemically combined so the different parts can be separated quite easily using physical methods Crude oil contains a large number of different liquids which have different boiling points These types of mixtures can be separated by fractional distillation by exploiting their different boiling points If crude oil is heated the liquids with the lowest boiling points evaporate first and pass up the flask and fractioning column This vapour then passes down and through the condenser to cool down the vapour and so that it condenses into a liquid which forms in the conical flask If the mixture is heated very gently between a numbers of temperature ranges different liquids can be collected at different temperatures These are called fractions As the boiling point of the fraction increases The appearance becomes darker in colour i 2 IGCSE Organic Chemistry ii iii iv The substance will become harder to light The flame when the substance is lit will become smokier and thicker The viscosity will increase Crude oil is refined separated into its constituent fractions by fractional distillation on a much larger scale in an industrial fractioning column The crude oil is heated and then it enters the fractioning column The fraction that is liquid is collected at the bottom The gaseous fractions rise up the column As they rise up the column the temperature becomes cooler When the temperature becomes just below the boiling point of a substance that substance will condense and a liquid will