ORGANIC CHEMISTRY Introduction It was originally thought that the formation of organic compounds could only be achieved by the influence of a ‘vital force’ which was present in nature, and the word organic applied only to those substances that were produced by living organisms. However, in 1828, Woehler found that when the inorganic compound, ammonium cyanate was heated, the organic compound, urea was obtained. NH4CNO CO(NH2)2 Woehler Later, in 1845, Kolbe synthesised ethanoic acid from carbon, hydrogen and oxygen. Today, the distinction between organic and inorganic chemistry is ill-defined, but usually organic chemistry refers to the chemistry of carbon compounds other than oxides, metal carbonates and related compounds. The bewildering variety of forms in which the 5 million known carbon compounds exist arises from a relatively small number of structural features, which include: carbon atoms linked by strong single, double and triple bonds strong bonds to other elements, particularly H, O and N straight carbon chains of varying length branched carbon chains of varying length aliphatic homocyclic rings of varying size aliphatic heterocyclic rings of varying size aromatic homocyclic rings of varying size aromatic heterocyclic rings of varying size functional groups fused ring systems TOPIC 12.4: ORGANIC CHEMISTRY 1 Some of these structural features are illustrated in the following compounds: H C CH3(CH2)7CH=CH(CH2)7COOH H octadec-9-enoic acid (oleic acid) H H methane cyclohexane O O N H coumarin HOCH2 H pyrrole O OH H OH H H HO H OH cholesterol D-glucose HO CH2.CO.NH Penicillin G carotene TOPIC 12.4: ORGANIC CHEMISTRY 2 O H H C C C N S CH3 C CH3 CH COOH N H N N H OH N porphyrin OH O HO OH OH HO O N HO CH3 morphine TOPIC 12.4: ORGANIC CHEMISTRY 3 O quercetin Empirical & Molecular Formulae Microanalysis of an organic compound gives the percentage by mass of each element present in the compound. From these data, the empirical formula of the compound can be calculated. (See Topic 12.1: Amount of Substance) The EMPIRICAL FORMULA is the simplest WHOLE number ratio of the atoms of each element present in a compound. The MOLECULAR FORMULA is the actual number of atoms of each element present in one molecule of the compound. The two are related by the expression: Molecular formula = Empirical formula x n where n is a whole number. For example: The molecular formulae of methane and ethane are CH4 and C2H6 respectively. The empirical formula of methane is CH4; in the above expression n = 1. The empirical formula of ethane is CH3, which is the simplest whole number ratio of carbon to hydrogen atoms. In the above expression n = 2. Structural Formulae The STRUCTURAL FORMULA of a molecule shows the number and type of each atom present and how they are joined together. The DISPLAYED FORMULA of a molecule shows all how of the atoms are arranged and all all of the bonds between them. The full structural formulae of the first few alkanes are shown below. Although shown in two dimensions, these structures are in reality three-dimensional. The formulae shown in brackets are partial structural formulae. Partial structural formulae are acceptable as long as they are unambiguous and can only represent one isomer. TOPIC 12.4: ORGANIC CHEMISTRY 4 H methane (CH4) H C H ethane (CH3.CH3) H H propane (CH3.CH2.CH3) H butane (CH3.CH2.CH2.CH3) H H H C C C H H H H pentane (CH3.CH2.CH2.CH2.CH3) H H H C C H H H Note that each carbon atom has formed four single bonds and is joined to four other atoms. Each hydrogen atom has formed one bond. H H H H H C C C C H H H H H H H H H H C C C C C H H H H H H Functional Groups A FUNCTIONAL GROUP is an atom or group of atoms which, when present in different molecules, gives them similar chemical properties. The functional group determines the chemical properties of a compound. Some compounds contain more than one functional group. C-H -OH C=O C=C -COOH -OR -COOR -NH2 -Cl -CONH2 -CN alkane hydroxyl group (alcohols) carbonyl group (aldehydes & ketones) alkene carboxyl group (carboxylic acids) alkoxy (ethers) ester amino (amines) chloro (haloalkanes) amide nitrile TOPIC 12.4: ORGANIC CHEMISTRY 5 Homologous Series To simplify the study of the huge number of organic compounds, they are divided into homologous series. A HOMOLOGOUS SERIES is a group of compounds with the same general formula and similar chemical properties. The similarity of chemical properties arises because members of a series contain the same functional group. Since the number of carbon atoms increases steadily down a homologous series, there is a gradual change in physical properties such as boiling point and density. Structural Isomerism For a given molecular formula, there may be more than one possible structure, giving rise to isomerism. STRUCTURAL ISOMERS are compounds with the same molecular formula but with different structural formulae. Structural isomers must have at least one bond in their molecules which is different. Structural isomers are sometimes sub-divided into chain isomers, position isomers and functional group isomers. The number of structural isomers rises rapidly as the number of carbon atoms increases. Chain Isomers These isomers differ only in the arrangement of the carbon skeleton of the molecule. Examples of chain isomers (of C4H10) are butane and methyl propane. H H H H H C C C C H H H H butane TOPIC 12.4: ORGANIC CHEMISTRY 6 H H H H H C C C H CH3 H methylpropane H For the molecular formula C5H12, there are three chain isomers: H H H H H H C C C C C H H H H H pentane (b.p. 36oC) H 2,2-dimethylpropane (b.p. 10oC) H H H H H H C H C C C H C H H H H H C C C C H H C H H H H H 2-methylbutane (b.p. 28oC) H H H H Position Isomers These isomers have the same carbon skeleton and the same functional group(s) but differ in the position in which the functional group is attached to the carbon skeleton. Examples of position isomers are propan-1-ol and propan-2-ol. H H H H C C C H OH H H H H H H C C C H propan-1-ol H OH H propan-2-ol Functional Group Isomers These isomers have different functional groups and therefore have different chemical properties. Examples of functional group isomers are ethanol and methoxymethane. H H H C C H OH H H ethanol TOPIC 12.4: ORGANIC CHEMISTRY 7 H C H H O C H methoxymethane H Stereoisomerism STEREOISOMERS have the same structural formula. They have the same number and types of bonds and differ only in their orientation in space. There are two types of stereoisomerism: geometrical isomerism and optical isomerism. Only geometrical isomerism will be considered in this module. Geometrical Isomerism Geometrical isomerism arises when there is restricted rotation about a bond, for example around the C=C double bond in alkenes and around C-C single bonds in cycloalkanes. Alkenes There is always restricted rotation around a C=C double bond. The double bond is a orbital formed by the overlap of two 2p-orbitals on the two carbon atoms. If rotation about a double bond were to take place, it would require the -bond to be broken This requires an amount of energy not possessed by molecules at room temperature. -orbital C 2p rotation C C 2p C -bond broken Restricted rotation gives rise to geometrical isomers only if there are two different atoms or groups attached to both of the double bond carbon atoms. For example, but-2-ene exhibits geometrical isomerism, but but-1-ene does not. H two different groups CH3 C C CH3 H two different groups E-but-2-ene CH3 C CH3 C H H Z-but-2-ene These two molecules are identical apart from their orientation in space. Both of the double bond carbon atoms are attached to two different groups. The prefixes E and Z indicate that the two methyl groups are on opposite sides or on the same side of the double bond respectively. H two identical C atoms H CH2CH3 C but-1-ene H TOPIC 12.4: ORGANIC CHEMISTRY 8 Geometrical isomerism is not possible because one of the double bond carbon atoms is attached to two identical atoms. Cycloalkanes There is always restricted rotation around a C-C single bond in a ring, but this gives rise to geometrical isomers only if there are two different atoms or groups attached to at least two different carbon atoms. For example, 1,2-dimethylcyclopentane exhibits geometrical isomerism but 1,1dimethylcyclopentane does not. two different groups CH3 CH3 H CH3 two different groups H H CH3 Z-1,2-dimethylcyclopentane E-1,2-dimethylcyclopentane H CH3 H H two identical groups CH3 1,1-dimethylcyclopentane TOPIC 12.4: ORGANIC CHEMISTRY 9 Nomenclature Organic compounds are named according to IUPAC rules. Some of the simpler rules are given below: 1. The naming of a compound is based on the longest straight carbon chain present in the molecule, and the first step in naming is to select this longest chain. Where a functional group is present, the longest straight carbon chain containing or attached to the functional group is selected. 2. The chain is named according to the number of carbon atoms and functional groups it contains. No. of C atoms 1 2 3 4 5 6 7 8 Prefix methethpropbutpenthexheptoct- Functional group C-H C=C -OH C=O H-C=O -COOH -CONH2 -COCl Suffix -ane -ene -ol -one -al -oic acid -amide -oyl chloride 3. The carbon atoms of this longest chain are numbered from one end to the other, starting from the end which gives the functional group the lower possible number. If there is no functional group, the chain is numbered so as to give the alkyl substituents the lower possible number(s). 4. Carboxyl groups (-COOH), aldehydes –(CHO) and nitriles (-CN) can only ever appear at the end of a chain, i.e. at carbon number 1. This number is usually omitted. 5. The position of the functional group and any chain branches are indicated by the number of the carbon atom to which the functional group or the substituent is attached followed by the name of the functional group or substituent. Formula -CH3 -CH2CH3 -CH2CH2CH3 Name methyl ethyl propyl 6. If two or more of the same substituent are present in a molecule, the number of them is indicated by multipliers: Number of identical substituents Multiplier 2 di3 tri4 tetra5 penta6 hexaTOPIC 12.4: ORGANIC CHEMISTRY 10 7. In unsaturated compounds which contain one double bond (alkenes), the double bond is formed between two numbered carbon atoms. The position of the double bond is indicated by the lower of these two numbers. 8. Most names are written as a single word, with commas separating numbers and hyphens separating numbers and letters. 9. When there is more than one functional group present in a molecule, the groups have an order of priority; the more important appear as suffixes, the less important as prefixes. If functional groups appear as prefixes, they have the following names: Functional group -Cl -Br -I -OH -OR C=O -NH2 Prefix chloro bromo iodo hydroxy alkoxy oxo amino 10. When there is more than one prefix, the prefixes (ignoring the multiplier) are listed in alphabetical not numerical order. For example, tribromo- appears before dichloro-. Example 1: Br Br Br H 5 4 3 C C C H CH3 H H 2 C H 1 C OH H The longest carbon chain has five carbons. H The chain is numbered to give the most important functional group (OH) the lower possible number. This gives ......pentan-2-ol There are four substituent groups attached to this chain: a methyl group is attached to carbon no. 4 4-methyl three bromines are attached, at carbons no. 3, 4 and 5 3,4,5-tribromo The name of this compound is therefore 3,4,5-tribromo-4-methylpentan-2-ol Note the following: commas between numbers hyphens between numbers and letters tribromo is alphabetically before methyl TOPIC 12.4: ORGANIC CHEMISTRY 11 Example 2: H H H H H C C 2 H H 1 HO 4 C H C C H 5 C H 3 C C Cl O H H H H The longest carbon chain in the molecule has six carbons, but the longest carbon chain of which the most important functional group (COOH) forms part has five carbons. The chain is numbered to give this functional group the lower possible number. This gives ......pentanoic acid There are three substituent groups attached to this chain: a methyl group is attached to carbon no. 3 3-methyl a chlorine atom is attached to carbon no. 3 3-chloro an ethyl group is attached to carbon no. 2 2-ethyl The name of this compound is therefore 3-chloro-2-ethyl-3-methylpentanoic acid Example 3: A carboxyl group takes precedence over an aldehyde or ketone, which take precedence over an alcohol. CH3CH(OH)COOH is 2-hydroxypropanoic acid; the less important group (OH) appears as a prefix. CH3COCH2COOH is 3-oxobutanoic acid; the less important group (C=O) appears as a prefix. CH3CH(OH)CHO is 2-hydroxypropanal; the less important group (OH) appears as a prefix. CH3CHClCH2OH is 2-chloropropan-1-ol. The OH group is now the more important group and appears as a suffix. HOCH2CH=CHCH2COOH is 5-hydroxypent-3-enoic acid. OH again appears as a prefix, but the alkene, as is usual, appears as a suffix in front of other suffixes. TOPIC 12.4: ORGANIC CHEMISTRY 12 PETROLEUM & ALKANES PETROLEUM Petroleum (crude oil) was formed over millions of years from the accumulated remains of sea creatures which became buried on the ocean bed. The conditions required for the formation of petroleum (and natural gas) are: high temperature high pressure (compression by overlying sediments) absence of oxygen Petroleum is a complex mixture of hydrocarbons, mostly alkanes. ALKANES The alkanes are a homologous series of saturated hydrocarbons which all have the general formula CnH2n+2. A HOMOLOGOUS SERIES is a group of compounds which have: the same general formula similar chemical properties HYDROCARBONS are compounds which are made from ONLY carbon and hydrogen atoms. In a SATURATED compound, there are only single bonds between carbon atoms. Carbon atoms form the spine of hydrocarbon molecules. Each carbon atom forms four covalent bonds; each hydrogen forms one covalent bond. When the carbon atoms are joined only by single covalent bonds, the molecule contains the maximum possible number of hydrogen atoms for its particular number of carbon atoms. This is why the molecule is said to be saturated. Physical Properties The alkanes have simple molecular structures. The carbon and hydrogen atoms within each molecule are joined by strong covalent bonds; there are weak van der Waals’ forces between molecules. The strength of the van der Waals’ forces increases as the surface area of the molecule increases. Down a homologous series, the molecular mass and therefore the boiling point increases. Thus the lower alkanes are gases at room temperature; the higher members are liquids and solids. TOPIC 12.4: ORGANIC CHEMISTRY 13 Straight chain alkanes Name methane ethane propane butane pentane hexane heptane octane nonane decane undecane dodecane tridecane tetradecane pentadecane hexadecane heptadecane octadecane nonadecane eicosane Formula CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 C11H24 C12H26 C13H28 C14H30 C15H32 C16H34 C17H36 C18H38 C19H40 C20H42 m.p. /oC b.p. /oC -182 -162 -183 -89 -188 -42 -138 -0.5 -130 36 -95 69 -91 98 -57 126 -54 151 -30 174 -26 196 -10 216 -5.5 235 6 254 10 271 18 287 22 302 28 316 32 330 37 343 Density /g.cm-3 0.626 0.659 0.684 0.703 0.718 0.730 0.740 0.749 0.756 0.763 0.769 0.773 0.778 0.782 0.786 0.789 Isomeric alkanes Isomers have different boiling points because these depend on the strength of the intermolecular forces. The strength of the intermolecular forces, and therefore the boiling point, decreases as the amount of chain branching increases. Straight chain alkanes are approximately sausage shaped, but as the amount of branching increases, the shape becomes more spherical. This can be seen in the diagrams below: pentane b.p. 36oC 2-methylbutane b.p. 28oC 2,2-dimethylpropane b.p. 10oC The more spherical the structure, the smaller the surface area is and so the weaker the van der Waals’ forces are. Therefore the boiling point decreases. Chemical Properties Alkanes contain only C-C and C-H bonds, which are strong and non-polar. Alkanes are, therefore, unreactive towards acids, alkalis, electrophiles and nucleophiles. They do, however, readily undergo combustion and are important as fuels. TOPIC 12.4: ORGANIC CHEMISTRY 14 FRACTIONAL DISTILLATION The properties of each substance in a mixture are unchanged. This makes it possible to separate substances in a mixture by physical methods including distillation. The complex mixture of hydrocarbons in crude oil can be separated into simpler mixtures or fractions by fractional distillation. Fractions contain molecules with a similar number of carbon atoms and have a narrow boiling point range. The crude oil is heated to about 400oC and the liquid/vapour mixture is then pumped into a tall tower called a fractionating column. Most of the hydrocarbons have been converted to vapour by the heating and start to rise up the column. The lower the boiling point of a hydrocarbon, the further it will rise up the column before it cools enough to condense. In this way, the different fractions are collected at different points up the column. The number of different fractions which are collected and the amount of each which is produced depends on the source of the crude oil. Most of the fractions from crude oil are burned as fuels. The residue from this primary distillation contains useful materials such as lubricating oil and waxes. If these were distilled at atmospheric pressure, the temperature needed to vaporise them would be so high that thermal decomposition would occur. Therefore, the residue is distilled in a separate column under reduced pressure; reducing the pressure lowers the boiling point and prevents decomposition. The quantities of the different fractions produced by fractional distillation do not usually match up with the market requirements for each fraction. There is a shortage of light fractions, especially gasoline and an excess of the heavier fractions. To resolve this problem, some of the heavier fractions (larger molecules) are converted into lighter, higher value fractions (smaller molecules) by cracking. CRACKING In the process of cracking, large hydrocarbon molecules are broken down ("cracked") to produce smaller, more useful molecules. Molecules may break down in more than one way and will give a mixture of products which can be separated by a further distillation process. During cracking carbon-carbon bonds are broken; in addition to smaller alkane molecules, alkenes and hydrogen are produced. For example: C14H30 alkane C14H30 alkane There are two main types of cracking: TOPIC 12.4: ORGANIC CHEMISTRY 15 C7H16 + C3H6 + 2C2H4 alkane alkenes C12H24 + C2H4 + H2 alkenes thermal cracking catalytic cracking FRACTIONAL DISTILLATION PETROLEUM (bottled gases) GASES 100oC b.p.decreases Mr decreases size of molecule decreases viscosity decreases volatility increases easier to ignite GASOLINE (fuel for cars) NAPHTHA (feedstock for petrochemicals )KEROSINE 200oC (fuel for jet aircraft) GAS OIL (diesel: fuel for cars & large vehicles) vapour 300oC LUBRICATING OIL & WAXES CRUDE OIL VAPOUR liquids FUEL OIL (fuel for ships & industrial heating) 360oC BITUMEN (road surfacing) TOPIC 12.4: ORGANIC CHEMISTRY 16 Thermal Cracking In this process, the bonds are broken by heating the hydrocarbon vapour to a high temperature under a high pressure for a few seconds. Temperature: Pressure: 400 – 900oC 7MPa The higher the temperature at which the cracking is carried out, the closer to the end of the chain the C-C bond breaks. Homolytic fission of the carbon-carbon bond takes place, forming two alkyl radicals. HOMOLYTIC FISSION When a bond breaks homolytically, each of the bonded atoms takes one electron from the shared pair, forming two particles with unpaired electrons called (free) radicals. e.g. CH3Cl CH3. + Cl. Thermal cracking produces a high percentage of alkenes. Catalytic Cracking In this process, the bonds are broken by heating the hydrocarbon vapour to a high temperature under a high pressure for a few seconds. Temperature: Pressure: Catalyst: 450oC slight zeolite (crystalline aluminosilicates) Catalytic cracking proceeds by a carbocation (C+) mechanism; heterolytic fission of the carbon-carbon bond takes place, forming two ions. HETEROLYTIC FISSION When a bond breaks heterolytically, one of the bonded atoms takes both electrons from the shared pair, forming a positive ion and a negative ion. e.g. (CH3)3CCl (CH3)3C+ + ClCatalytic cracking is used mainly to produce motor fuels (branched-chain alkanes) and aromatic hydrocarbons. Economics of Cracking The lower Mr branched-chain alkanes produced by the cracking of heavy fractions are more useful as fuels and are therefore of higher value. The alkenes produced by cracking can be used to make plastics (polymers) such as poly(ethene) and poly(propene). TOPIC 12.4: ORGANIC CHEMISTRY 17 COMBUSTION OF ALKANES Most of the hydrocarbon fractions obtained from petroleum are used as fuels, because their combustion reactions are very exothermic. The products of combustion depend on whether the combustion is complete or incomplete. Complete Combustion When alkanes burn in a plentiful supply of air or oxygen, complete combustion takes place, forming carbon dioxide and water. CH4 + 2O2 CO2 + 2H2O C8H18 + 121/2O2 8CO2 + 9H2O H = -890 kJ.mol-1 H = -5512 kJ.mol-1 A graph of enthalpy of combustion against no. of carbon atoms for straight chain alkanes is a straight line. Incomplete Combustion When the supply of air or oxygen is restricted, incomplete combustion of alkanes takes place, forming water together with carbon monoxide or carbon. The design of the burner affects the product of incomplete combustion. Bunsen burners, which are intended for use in open laboratories, produce carbon when combustion is incomplete (the luminous flame obtained when the air hole is closed is sooty). CH4 + O2 C + 2H2O The design of gas fires is such that if the flue becomes blocked, restricting the air supply, incomplete combustion takes place to form carbon monoxide. Carbon monoxide is toxic. Every year there are a number of accidental deaths caused by carbon monoxide from poorly maintained gas fires and central heating boilers. CH4 + 11/2O2 CO + 2H2O Pollutants from Combustion The principal products of the internal combustion engine are carbon dioxide and water. Carbon dioxide is a greenhouse gas and contributes to global warming. Sulphur-containing compounds are often present as impurities in alkanes obtained by the fractional distillation of petroleum. When these hydrocarbons are burned in air or oxygen, the sulphur is oxidised to sulphur(IV) oxide, SO2, and possibly to sulphur(VI) oxide, SO3. Both these oxides are toxic and also dissolve in atmospheric moisture, causing acid rain. This happens on a massive scale when power stations burn fossil fuels to produce electricity. Flue Gas Desulphurisation is a process used to prevent SO2 escaping into the atmosphere. Waste gases containing SO2 are passed through a flue (chimney) TOPIC 12.4: ORGANIC CHEMISTRY 18 containing calcium oxide (CaO) which absorbs the SO2 producing calcium sulphite (CaSO3). CaO + SO2 CaSO3 This can easily be oxidised to to make hydrated calcium sulphate (CaSO 4), also known as gypsum, which is used to make plasterboard for the building industry. Carbon monoxide (petrol engine) and carbon (diesel engine) are formed as a result of incomplete combustion. Carbon monoxide is toxic; carbon particles are irritant. Unburned hydrocarbons pass through the engine and enter the exhaust gases. At the high temperatures produced in the engine (up to 2500 oC), the nitrogen and oxygen molecules in air have enough energy to combine to form nitrogen oxide. N2 + O2 2NO On cooling and in the presence of more oxygen, nitrogen oxide reacts to form other oxides of nitrogen (NOx), especially nitrogen dioxide, NO2. With water and more oxygen, nitrogen dioxide reacts to form nitric acid, which contributes to acid rain. 2NO + O2 4NO2 + 2H2O + O2 2NO2 4HNO3 Oxides of nitrogen are irritant, toxic gases. They combine with unburned hydrocarbons in the presence of sunlight to form photochemical smog. This is a particular problem in Los Angeles. Catalytic Converters Catalytic converters are fitted to the exhaust systems of cars to remove pollutant gases. They consist of a honeycomb of ceramic material which is coated with a thin layer of a catalyst containing platinum (Pt) and rhodium (Rh). Up to 90% of pollutant gases are removed. The catalyst system catalyses two important reactions: the reaction between carbon monoxide and nitrogen oxide, forming carbon dioxide and nitrogen 2NO + 2CO TOPIC 12.4: ORGANIC CHEMISTRY 19 N2 + 2CO2 the reaction between nitrogen oxide and unburned hydrocarbon fuel, forming carbon dioxide and nitrogen C8H18 + 25NO 8CO2 + 121/2N2 + 9H2O The principal exhaust gases are therefore carbon dioxide, nitrogen and water vapour. These gases are harmless, but carbon dioxide causes environmental problems. TOPIC 12.4: ORGANIC CHEMISTRY 20