1 of 30 © Boardworks Ltd 2009 2 of 30 © Boardworks Ltd 2009 Molecular and empirical formulae There are many ways of representing organic compounds by using different formulae. The molecular formula of a compound shows the number of each type of atom present in one molecule of the compound. The empirical formula of a compound shows the simplest ratio of the atoms present. Molecular formula C2H6 Empirical formula CH3 C6H12O6 CH2O C2H4O2 CH2O Neither the molecular nor empirical formula gives information about the structure of a molecule. 3 of 30 © Boardworks Ltd 2009 Displayed formula of organic compounds The displayed formula of a compound shows the arrangement of atoms in a molecule, as well as all the bonds. Single bonds are represented by a single line, double bonds with two lines and triple bonds by three lines. The displayed formula can show the different structures of compounds with the same molecular formulae. ethanol (C2H6O) 4 of 30 methoxymethane (C2H6O) © Boardworks Ltd 2009 Structural formula of organic compounds The structural formula of a compound shows how the atoms are arranged in a molecule and, in particular, shows which functional groups are present. Unlike displayed formulae, structural formulae do not show single bonds, although double/triple bonds may be shown. CH3CHClCH3 H2C=CH2 CH3C≡N 2-chloropropane ethene ethanenitrile 5 of 30 © Boardworks Ltd 2009 Displayed and structural formula activity 6 of 30 © Boardworks Ltd 2009 Skeletal formula of organic compounds The skeletal formula of a compound shows the bonds between carbon atoms, but not the atoms themselves. Hydrogen atoms are also omitted, but other atoms are shown. 7 of 30 © Boardworks Ltd 2009 Types of formulae 8 of 30 © Boardworks Ltd 2009 Functional groups and homologous series A functional group is an atom or group of atoms responsible for the typical chemical reactions of a molecule. A homologous series is a group of molecules with the same functional group but a different number of –CH2 groups. methanoic acid (HCOOH) ethanoic acid (CH3COOH) propanoic acid (CH3CH2COOH) Functional groups determine the pattern of reactivity of a homologous series, whereas the carbon chain length determines physical properties such as melting/boiling points. 9 of 30 © Boardworks Ltd 2009 Functional groups 10 of 30 © Boardworks Ltd 2009 Homologous series and general formulae The general formula of a homologous series can be used to calculate the molecular formula of any member of the series by substituting n for the number of carbon atoms. For example, the general formula of a halogenoalkane is CnH2n+1X, where X is a halogen. Example: what is the molecular formula of chloroethane? 1. Write down the general formula: CnH2n+1X 2. Write down the value of n: n=2 3. Substitute n into the general formula: C2H5Cl 11 of 30 © Boardworks Ltd 2009 Homologous series 12 of 30 © Boardworks Ltd 2009 13 of 30 © Boardworks Ltd 2009 What is isomerism? Isomers are molecules with the same molecular formula (i.e. the same number and type of atoms) but in which the atoms are arranged in a different way. There are two main categories of isomerism: structural isomerism and stereoisomerism. Structural isomers have different structural formulae. Three types of structural isomerism are chain isomerism, positional isomerism and functional group isomerism. Stereoisomers have the same structural formula, but the 3D arrangement of atoms is different. Two types are cis–trans isomerism and optical isomerism. 14 of 30 © Boardworks Ltd 2009 Chain isomerism in alkanes In chain isomers, the carbon chain is arranged differently. For example, hexane has several chain isomers, all with the molecular formula C6H14: hexane 2,3-dimethylbutane 3-methylpentane 15 of 30 © Boardworks Ltd 2009 Positional isomerism In positional isomers, the functional group is attached to a different carbon atom. For example, chloropentane has several positional isomers, all with the molecular formula C5H11Cl: 1-chloropentane 2-chloropentane 3-chloropentane 16 of 30 © Boardworks Ltd 2009 Positional isomerism in alkenes Positional isomerism also exists in alkenes with four or more carbon atoms. hex-1-ene For example, hexene has several positional isomers, all with the molecular formula C6H12: hex-2-ene hex-3-ene 17 of 30 © Boardworks Ltd 2009 Functional group isomerism Functional group isomers contain different functional groups and so are members of different homologous series. For example, both alcohols and ethers have the general formula CnH2n+2O so they may be functional group isomers: propanol (C3H8O) an alcohol 18 of 30 methoxyethane (C3H8O) an ether © Boardworks Ltd 2009 Structural isomers activity 19 of 30 © Boardworks Ltd 2009 20 of 30 © Boardworks Ltd 2009 Rotation around the C=C bond in alkenes Molecules can rotate freely around single C-C covalent bonds, but not around C=C double bonds. This leads to type of stereoisomerism called cis–trans isomerism, in which isomers differ in the arrangement of the groups attached to the carbons in the double bonds. is not the same as These isomers cannot be superimposed on each other because the arrangement of the methyl groups is different. 21 of 30 © Boardworks Ltd 2009 Cis–trans isomerism If an alkyl group or atom other than hydrogen is attached to each carbon then the isomers can be named either cis (‘on the same side’) or trans (‘on the opposite side’). cis-but-2-ene cis-1,2-dichloroethene 22 of 30 trans-but-2-ene trans-1,2-dichloroethene © Boardworks Ltd 2009 Limitations of cis–trans isomerism In more complex organic compounds, in which multiple hydrogens have been substituted by different groups, isomers cannot be defined using the cis–trans notation. For example, is it possible to identify which of these halogenoalkanes is the cis isomer and which is the trans isomer? Instead, a different system is used for these type of molecules: E–Z notation. 23 of 30 © Boardworks Ltd 2009 E–Z isomerism The E–Z notation is used to identify stereoisomers that cannot be called cis or trans. Isomers are identified as either E or Z depending on what ‘priority’ is given to the groups attached to the carbon atoms in the double bond. The priority of these groups is determined by a complex series of rules. E represents the German word ‘entgegen’, and corresponds to trans isomers. The highest priority groups are on the opposite side of the double bond. Z represents the German word ‘zusammen’, and corresponds to cis isomers. The highest priority groups are on the same side of the double bond. 24 of 30 © Boardworks Ltd 2009 Optical isomerism Another form of stereoisomerism is optical isomerism, in which a molecule can exist as two isomers that are nonsuperimposable, mirror images of each other, just like a left hand and right hand. optical isomers of the amino acid alanine Optical isomers have the same physical properties, but they rotate polarized light in opposite directions. 25 of 30 © Boardworks Ltd 2009 Stereoisomerism: true or false? 26 of 30 © Boardworks Ltd 2009 27 of 30 © Boardworks Ltd 2009 Glossary 28 of 30 © Boardworks Ltd 2009 What’s the keyword? 29 of 30 © Boardworks Ltd 2009 Multiple-choice quiz 30 of 30 © Boardworks Ltd 2009 31 of 37 © Boardworks Ltd 2009 32 of 37 © Boardworks Ltd 2009 What are alkanes? The alkanes are a homologous series of hydrocarbons with the general formula CnH2n+2 and names ending –ane. Alkanes contain only single carbon–carbon bonds and so are saturated. No. of carbon atoms 33 of 37 Molecular formula Name 1 CH4 methane 2 C2H6 ethane 3 C3H8 propane 4 C4H10 butane 5 C5H12 pentane 6 C6H14 hexane © Boardworks Ltd 2009 Alkanes and isomersim Alkanes with four or more carbon atoms display structural isomerism because the carbon chain may be either straight or branched. pentane: straight chain 2-methylbutane: branched chain The naming of alkanes depends on whether they are straight or branched. 34 of 37 © Boardworks Ltd 2009 Naming branched chain alkanes 35 of 37 © Boardworks Ltd 2009 Naming the alkanes activity 36 of 37 © Boardworks Ltd 2009 37 of 37 © Boardworks Ltd 2009 Trends in boiling points 38 of 37 © Boardworks Ltd 2009 Trends in boiling points The boiling point of straight-chain alkanes increases with chain length due to increasing van der Waals forces between molecules. As the length of the chain increases, so does its surface area, and so the van der Waals forces are stronger. Branched-chain alkanes have lower boiling points because the chains cannot pack as closely together. There are fewer points of contact between molecules so the van der Waals forces are weaker. 39 of 37 © Boardworks Ltd 2009 Crude oil and alkanes Crude oil is a mixture composed mainly of straight and branched chain alkanes. It also includes lesser amounts of cycloalkanes and arenes, both of which are hydrocarbons containing a ring of carbon atoms, as well as impurities such as sulfur compounds. The exact composition of crude oil depends on the conditions under which it formed, so crude oil extracted at different locations has different compositions. 40 of 37 © Boardworks Ltd 2009 Fractional distillation 41 of 37 © Boardworks Ltd 2009 Uses of fractions 42 of 37 © Boardworks Ltd 2009 Fractions and boiling point 43 of 37 © Boardworks Ltd 2009 44 of 37 © Boardworks Ltd 2009 Supply and demand The demand for lower boiling point (shorter chain) fractions is greater than the proportion found in crude oil. Crude oil contains more higher boiling point (longer chain) fractions, which are in lower demand and are less economically valuable. There is therefore a shortage of shorter chain fractions and a surplus of longer chain ones. 45 of 37 © Boardworks Ltd 2009 What is cracking? Cracking is a process that splits long chain alkanes into shorter chain alkanes, alkenes and hydrogen. C10H22 → C7H16 + C3H6 Cracking has the following uses: it increases the amount of gasoline and other economically important fractions it increases branching in chains, an important factor for petrol it produces alkenes, an important feedstock for chemicals. There are two main types of cracking: thermal and catalytic. 46 of 37 © Boardworks Ltd 2009 Thermal cracking 47 of 37 © Boardworks Ltd 2009 Catalytic cracking 48 of 37 © Boardworks Ltd 2009 Thermal vs. catalytic cracking Catalytic cracking has several advantages over thermal cracking: it produces a higher proportion of branched alkanes, which burn more easily than straight-chain alkanes and are therefore an important component of petrol the use of a lower temperature and pressure mean it is cheaper it produces a higher proportion of arenes, which are valuable feedstock chemicals. However, unlike thermal cracking, catalytic cracking cannot be used on all fractions, such as bitumen, the supply of which outstrips its demand. 49 of 37 © Boardworks Ltd 2009 Other products from cracking Alkenes such as ethene are always produced in cracking. They are an important feedstock for use in the chemical industry, particularly in the production of plastics. Arenes such as benzene are also produced during catalytic cracking. Benzene is added in small quantities to petrol as a replacement for the lead compounds. It too is now the subject of health concerns, and its use is being reduced. 50 of 37 © Boardworks Ltd 2009 Cracking: true or false? 51 of 37 © Boardworks Ltd 2009 52 of 37 © Boardworks Ltd 2009 Complete combustion In excess oxygen, short chain alkanes can undergo complete combustion: alkane + oxygen → carbon dioxide + water For example: propane + oxygen → carbon dioxide + water C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) The combustion of alkanes is a highly exothermic process. This makes them good fuels because they release a relatively large amount of energy per gram of fuel. 53 of 37 © Boardworks Ltd 2009 Incomplete combustion If oxygen is limited then incomplete combustion will occur: alkane + oxygen → carbon monoxide + water alkane + oxygen → carbon + water For example: propane + oxygen → carbon monoxide + water C3H8(g) + 3½O2(g) → 3CO(g) + 4H2O(g) propane + oxygen → carbon + water C3H8(g) + 2O2(g) → 3C(s) + 4H2O(g) 54 of 37 © Boardworks Ltd 2009 The internal combustion engine: carbon Alkanes with chain lengths of 5–10 carbon atoms are used as fuels in internal combustion engines. This releases carbon dioxide into the atmosphere: nonane + oxygen → carbon dioxide + water C9H20(g) + 14O2(g) → 9CO2(g) + 10H2O(g) Although modern internal combustion engines are more efficient than in the past, incomplete combustion still occurs: nonane + oxygen → carbon monoxide + water 2C9H20(g) + 19O2(g) → 18CO(g) + 20H2O(g) 55 of 37 © Boardworks Ltd 2009 The internal combustion engine: nitrogen The temperature in an internal combustion engine can reach over 2000 °C. Here, nitrogen and oxygen, which at normal temperatures don’t react, combine to form nitrogen monoxide: N2(g) + O2(g) → 2NO(g) Nitrogen monoxide reacts further forming nitrogen dioxide: 2NO(g) + O2(g) → 2NO2(g) Nitrogen dioxide gas reacts with rain water and more oxygen to form nitric acid, which contributes to acid rain: 4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq) 56 of 37 © Boardworks Ltd 2009 The catalytic converter 57 of 37 © Boardworks Ltd 2009 Sulfur contamination of fossil fuels Sulfur is found as an impurity in crude oil and other fossil fuels. It burns in oxygen to form sulfur dioxide: S(s) + O2(g) → SO2(g) Sulfur dioxide may be oxidized to sulfur trioxide: 2SO2(g) + O2(g) → 2SO3(g) Both of these oxides dissolve in water forming acidic solutions: SO2(g) + H2O(l) → H2SO3(aq) SO3(g) + H2O(l) → H2SO4(aq) 58 of 37 © Boardworks Ltd 2009 What is acid rain? Acid rain is caused by acidic non-metal oxides such as sulfur oxides and nitrogen oxides dissolving in rain water. Rain water is naturally acidic because carbon dioxide dissolves in it, forming weak carbonic acid. However, sulfur and nitrogen oxides form more acidic solutions, which can damage trees and affect aquatic life in lakes and rivers. 59 of 37 © Boardworks Ltd 2009 Removing sulfur dioxide pollution Sulfur dioxide emissions from vehicle fuels such as petrol and diesel are reduced by removing nearly all of the sulfur impurities from the fuel before it is burnt. Removing the sulfur from coal before it is burnt is not practical. Instead, the acidic sulfur oxides are removed from the waste gases using a base such as calcium oxide. 60 of 37 © Boardworks Ltd 2009 Carbon dioxide in the atmosphere Burning fossil fuels releases carbon dioxide into the atmosphere. Fossil fuels are being burned faster than they are being formed, which means that their combustion leads to a net increase in the amount of atmospheric carbon dioxide. It has been suggested that increases in the amount of carbon dioxide and other greenhouse gases may be responsible for apparent changes to the climate. 61 of 37 © Boardworks Ltd 2009 Greenhouse gases Carbon dioxide, water vapour and methane have been described as the main greenhouse gases. This is because these have been suggested as the gases responsible for the majority of the greenhouse effect. The greenhouse effect is a theory that has been suggested to explain apparent rises in the average temperature of the Earth. Increasing the amount of any of the greenhouse gases traps more heat energy from the Sun in the Earth’s atmosphere, raising the average temperature. 62 of 37 © Boardworks Ltd 2009 Pollutant gases 63 of 37 © Boardworks Ltd 2009 64 of 37 © Boardworks Ltd 2009 Glossary 65 of 37 © Boardworks Ltd 2009 What’s the keyword? 66 of 37 © Boardworks Ltd 2009 Multiple-choice quiz 67 of 37 © Boardworks Ltd 2009 68 of 34 © Boardworks Ltd 2009 69 of 34 © Boardworks Ltd 2009 What are alkenes? The alkenes are a homologous series of hydrocarbons with the general formula CnH2n and names ending –ene. Alkenes contain a carbon – carbon double bond and so are unsaturated. 70 of 34 No. of carbon atoms Molecular formula 2 C2H4 ethene 3 C3H6 propene 4 C4H8 butene 5 C5H10 pentene 6 C6H12 hexene Name © Boardworks Ltd 2009 Naming alkenes Alkenes with four or more carbon atoms display positional isomerism because the carbon–carbon double bond may appear between different carbon atoms. If there are two or more possible positions for the double bond, a number is used before the –ene to indicate the first carbon involved. but-1-ene 71 of 34 but-2-ene © Boardworks Ltd 2009 Structure of alkenes 72 of 34 © Boardworks Ltd 2009 Bonding in alkenes 73 of 34 © Boardworks Ltd 2009 Isomerism in alkenes Rotation around the double bond in alkenes is restricted by the presence of the bond. This leads to E–Z stereoisomerism in some alkenes. E-pent-2-ene Z-pent-2-ene The two E-Z stereoisomers have the same structure; the only difference between them is the arrangement of the atoms in space. 74 of 34 © Boardworks Ltd 2009 Structure of alkenes summary 75 of 34 © Boardworks Ltd 2009 76 of 34 © Boardworks Ltd 2009 Double bonds and electrophiles The double bond of an alkene is an area of high electron density, and therefore an area of high negative charge. The negative charge of the double bond may be attacked by electron-deficient species, which will accept a lone pair of electrons. These species have either a full positive charge or slight positive charge on one or more of their atoms. They are called electrophiles, meaning ‘electron loving’. Alkenes undergo addition reactions when attacked by electrophiles. This is called electrophilic addition. 77 of 34 © Boardworks Ltd 2009 Electrophilic addition mechanism: 1 In the first stage of electrophilic addition, the positive charge on the electrophile is attracted to the electron density in the double bond. δ+ As the electrophile approaches the double bond, electrons in the A–B bond are repelled towards B. δ- The pi bond breaks, and A bonds to the carbon, forming a carbocation – an ion with a positively-charge carbon atom. The two electrons in the A–B bond move to B forming a B- ion. 78 of 34 © Boardworks Ltd 2009 Electrophilic addition mechanism: 2 In the second stage of electrophilic addition, the B- ion acts as a nucleophile and attacks the carbocation. The lone pair of electrons on the B- ion are attracted towards the positivelycharged carbon in the carbocation, causing B to bond to it. Because both electrons in the bond that joins B- to the carbocation ion come from B-, the bond is a co-ordinate bond. 79 of 34 © Boardworks Ltd 2009 What is hydrogenation? Hydrogen can be added to the carbon–carbon double bond using a nickel catalyst in a process called hydrogenation. C2H4 + H2 C2H6 Vegetable oils are unsaturated and may be hydrogenated to make margarine, which has a higher melting point. As well as a nickel catalyst, this requires a temperature of 200 °C and a pressure of 1000 kPa. 80 of 34 © Boardworks Ltd 2009 Testing for alkenes The presence of unsaturation (a carbon– carbon double bond) can be detected using bromine water, a red/orange coloured solution of bromine. A few drops of bromine water are added to the test liquid and shaken. If a carbon–carbon double bond is present, the bromine adds across it and the solution turns colourless. 81 of 34 © Boardworks Ltd 2009 More on the bromine water test A simple equation for the bromine water test with ethene is: CH2=CH2 + Br2 + H2O CH2BrCH2Br + H2O However, because water is present in such a large amount, a water molecule (which has a lone pair) adds to one of the carbon atoms, followed by the loss of a H+ ion. CH2=CH2 + Br2 + H2O CH2BrCH2OH + HBr The major product of the test is not 1,2-dibromoethane (CH2BrCH2Br) but 2-bromoethan-1-ol (CH2BrCH2OH). 82 of 34 © Boardworks Ltd 2009 Electrophilic addition reactions 83 of 34 © Boardworks Ltd 2009 Addition to unsymmetrical alkenes When an electrophile (e.g. HBr) attacks an alkene with three or more carbon atoms (e.g. propene), a mix of products is formed. This is because these alkenes are unsymmetrical. minor product: 1-bromopropane HBr major product: 2-bromopropane Unequal amounts of each product are formed due to the relative stabilities of the carbocation intermediates. 84 of 34 © Boardworks Ltd 2009 Structure of carbocations A chain of carbon atoms can be represented by R when drawing organic structures. This is an alkyl group (general formula CnH2n+1). Primary (1°) carbocations have one alkyl group attached to the positively-charged carbon. Secondary (2°) carbocations have two alkyl groups attached to the positively-charged carbon. Tertiary (3°) carbocations have three alkyl groups attached to the positively-charged carbon. 85 of 34 © Boardworks Ltd 2009 Stability of carbocations The stability of carbocations increases as the number of alkyl groups on the positively-charged carbon atom increases. primary secondary tertiary increasing stability The stability increases because alkyl groups contain a greater electron density than hydrogen atoms. This density is attracted towards, and reduces, the positive charge on the carbon atom, which has a stabilizing effect. 86 of 34 © Boardworks Ltd 2009 Structure of carbocations 87 of 34 © Boardworks Ltd 2009 Electrophiles: true or false? 88 of 34 © Boardworks Ltd 2009 89 of 34 © Boardworks Ltd 2009 Polyalkenes Alkenes can undergo addition reactions with themselves to form a long chain polymer molecule. This reaction is addition polymerization. The polymer can be represented by showing the repeating unit with square brackets around it. The n stands for a unspecified number of monomer units. 90 of 34 © Boardworks Ltd 2009 Polymerization of ethene 91 of 34 © Boardworks Ltd 2009 LDPE and HDPE The reaction conditions under which ethene polymerizes affect the structure and properties of the poly(e)thene. Low-density polythene (LDPE) is formed under a high pressure (1400 atm) and a temperature of about 170 °C. These conditions cause a high level of branching, meaning that the polymer chains cannot pack tightly together. High-density polythene (HDPE) is formed with a catalyst, a pressure of 2 atm and a temperature of about 70 °C. Little branching occurs under these conditions, resulting in chains that can pack tightly together to create a denser material. 92 of 34 © Boardworks Ltd 2009 Other polyalkenes Propene undergoes addition polymerization to form polyproprene: Chloroethene undergoes addition polymerization to form polychloroethene: 93 of 34 © Boardworks Ltd 2009 Which alkene? 94 of 34 © Boardworks Ltd 2009 More about LDPE LDPE is a soft, flexible and stretchy plastic, with a melting point of about 120 °C. It is used to make: plastic bags squeezable bottles, and general purpose containers and trays other items that need to be soft and flexible, such as tubing. LDPE has the recycling symbol ‘4’. 95 of 34 © Boardworks Ltd 2009 More about HDPE HDPE is a tough and flexible plastic, with a melting point of about 130 °C. It is used to make: containers such as milk and detergent bottles rigid items such as folding tables, chairs and pipes. HDPE has the recycling symbol ‘2’. 96 of 34 © Boardworks Ltd 2009 More about polypropene Polypropene is a tough and flexible plastic, with a melting point of about 160 °C. It is used to make: ropes, carpets, rugs and other textiles medical, laboratory and kitchen items that need to withstand temperatures in autoclaves and dishwashers certain bottles, buckets, containers and other items such as bottle tops and moulded fittings. Polypropene has the recycling symbol ‘5’. 97 of 34 © Boardworks Ltd 2009 98 of 34 © Boardworks Ltd 2009 Glossary 99 of 34 © Boardworks Ltd 2009 What’s the keyword? 100 of © Boardworks Ltd 2009 Multiple-choice quiz 101 of © Boardworks Ltd 2009