3.3.1 Introduction to organic chemistry
Organic chemistry is the study of the millions of covalent compounds of the element carbon.
These structurally diverse compounds vary from naturally occurring petroleum fuels to DNA and the molecules in living systems. Organic compounds also demonstrate human ingenuity in the vast range of synthetic materials created by chemists. Many of these compounds are used as drugs, medicines and plastics.
Organic compounds are named using the International Union of Pure and Applied Chemistry (IUPAC) system and the structure or formula of molecules can be represented in various different ways. Organic mechanisms are studied, which enable reactions to be explained.
In the search for sustainable chemistry, for safer agrochemicals and for new materials to match the desire for new technology, Chemistry plays the dominant role.
3.3.1.1 Nomenclature
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Organic compounds can be represented by: - empirical formula
- molecular formula
- general formula
- structural formula
- displayed formula
- skeletal formula.
The characteristics of a homologous series, a series of compounds containing the same functional group. IUPAC rules for nomenclature. Students should be able to: - draw structural, displayed and skeletal formulas for given organic compounds
- apply IUPAC rules for nomenclature to name organic compounds limited to chains and rings with up to six carbon atoms each
- apply IUPAC rules for nomenclature to draw the structure of an organic compound from the IUPAC name limited to chains and rings with up to six carbon atoms each.
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3.3.1.2 Reaction mechanisms
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Reactions of organic compounds can be explained using mechanisms. Free-radical mechanisms: - the unpaired electron in a radical is represented by a dot
- the use of curly arrows is not required for radical mechanisms.
Students should be able to: - write balanced equations for the steps in a free-radical mechanism.
Other mechanisms: - the formation of a covalent bond is shown by a curly arrow that starts from a lone electron pair or from another covalent bond
- the breaking of a covalent bond is shown by a curly arrow starting from the bond.
Students should be able to: - outline mechanisms by drawing the structures of the species involved and curly arrows to represent the movement of electron pairs.
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3.3.1.3 Isomerism
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Structural isomerism. Stereoisomerism. E – Z isomerism is a form of stereoisomerism and occurs as a result of restricted rotation about the planar carbon–carbon double bond. Cahn–Ingold–Prelog (CIP) priority rules. Students should be able to: - define the term structural isomer
- draw the structures of chain, position and functional group isomers
- define the term stereoisomer
- draw the structural formulas of E and Z isomers
- apply the CIP priority rules to E and Z isomers.
| MS 4.2 Students could be given the structure of one isomer and asked to draw further isomers. Various representations could be used to give the opportunity to identify those that are isomeric. MS 4.1, 4.2 and 4.3 Students understand the origin of E – Z isomerism. Students draw different forms of isomers. |
3.3.2 Alkanes
Alkanes are the main constituent of crude oil, which is an important raw material for the chemical industry. Alkanes are also used as fuels and the environmental consequences of this use are considered in this section.
3.3.2.1 Fractional distillation of crude oil
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Alkanes are saturated hydrocarbons. Petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation. | AT a, d and k PS 1.2 Fractional distillation of a crude oil substitute. |
3.3.2.2 Modification of alkanes by cracking
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Cracking involves breaking C–C bonds in alkanes. Thermal cracking takes place at high pressure and high temperature and produces a high percentage of alkenes (mechanism not required). Catalytic cracking takes place at a slight pressure, high temperature and in the presence of a zeolite catalyst and is used mainly to produce motor fuels and aromatic hydrocarbons (mechanism not required). Students should be able to: - explain the economic reasons for cracking alkanes.
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3.3.2.3 Combustion of alkanes
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Alkanes are used as fuels. Combustion of alkanes and other organic compounds can be complete or incomplete. The internal combustion engine produces a number of pollutants including NOx , CO, carbon and unburned hydrocarbons. These gaseous pollutants from internal combustion engines can be removed using catalytic converters. Combustion of hydrocarbons containing sulfur leads to sulfur dioxide that causes air pollution. Students should be able to: - explain why sulfur dioxide can be removed from flue gases using calcium oxide or calcium carbonate.
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3.3.2.4 Chlorination of alkanes
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The reaction of methane with chlorine. Students should be able to: - explain this reaction as a free-radical substitution mechanism involving initiation, propagation and termination steps.
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3.3.3 Halogenoalkanes
Halogenoalkanes are much more reactive than alkanes. They have many uses, including as refrigerants, as solvents and in pharmaceuticals. The use of some halogenoalkanes has been restricted due to the effect of chlorofluorocarbons (CFCs) on the atmosphere.
3.3.3.1 Nucleophilic substitution
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Halogenoalkanes contain polar bonds. Halogenoalkanes undergo substitution reactions with the nucleophiles OH– , CN– and NH3 Students should be able to: - outline the nucleophilic substitution mechanisms of these reactions
- explain why the carbon–halogen bond enthalpy influences the rate of reaction.
| AT a, b and k PS 4.1 Students could follow instructions when carrying out test-tube hydrolysis of halogenoalkanes to show their relative rates of reaction. AT d, g and k Students could prepare a chloroalkane, purifying the product using a separating funnel and distillation. |
3.3.3.2 Elimination
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The concurrent substitution and elimination reactions of a halogenoalkane (eg 2-bromopropane with potassium hydroxide). Students should be able to: - explain the role of the reagent as both nucleophile and base
- outline the mechanisms of these reactions.
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3.3.3.3 Ozone depletion
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Ozone, formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation. Chlorine atoms are formed in the upper atmosphere when ultraviolet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break. Chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer. Appreciate that results of research by different groups in the scientific community provided evidence for legislation to ban the use of CFCs as solvents and refrigerants. Chemists have now developed alternative chlorine-free compounds. Students should be able to: - use equations, such as the following, to explain how chlorine atoms catalyse decomposition of ozone:
Cl• + O3 → ClO• + O2 and ClO• + O3 → 2O2 + Cl• | Research opportunity Students could investigate the role of chemists in the introduction of legislation to ban the use of CFCs and in finding replacements. |
3.3.4 Alkenes
In alkenes, the high electron density of the carbon–carbon double bond leads to attack on these molecules by electrophiles. This section also covers the mechanism of addition to the double bond and introduces addition polymers, which are commercially important and have many uses in modern society.
3.3.4.1 Structure, bonding and reactivity
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Alkenes are unsaturated hydrocarbons. Bonding in alkenes involves a double covalent bond, a centre of high electron density. | |
3.3.4.2 Addition reactions of alkenes
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Electrophilic addition reactions of alkenes with HBr, H2 SO4 and Br2 The use of bromine to test for unsaturation. The formation of major and minor products in addition reactions of unsymmetrical alkenes. Students should be able to: - outline the mechanisms for these reactions
- explain the formation of major and minor products by reference to the relative stabilities of primary, secondary and tertiary carbocation intermediates.
| AT d and k PS 4.1 Students could test organic compounds for unsaturation using bromine water and record their observations. |
3.3.4.3 Addition polymers
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Addition polymers are formed from alkenes and substituted alkenes. The repeating unit of addition polymers. IUPAC rules for naming addition polymers. Addition polymers are unreactive. Appreciate that knowledge and understanding of the production and properties of polymers has developed over time. Typical uses of poly(chloroethene), commonly known as PVC, and how its properties can be modified using a plasticiser. Students should be able to: - draw the repeating unit from a monomer structure
- draw the repeating unit from a section of the polymer chain
- draw the structure of the monomer from a section of the polymer
- explain why addition polymers are unreactive
- explain the nature of intermolecular forces between molecules of polyalkenes.
| AT k PS 1.2 Making poly(phenylethene) from phenylethene. |
3.3.5 Alcohols
Alcohols have many scientific, medicinal and industrial uses. Ethanol is one such alcohol and it is produced using different methods, which are considered in this section. Ethanol can be used as a biofuel.
3.3.5.1 Alcohol production
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Alcohols are produced industrially by hydration of alkenes in the presence of an acid catalyst. Ethanol is produced industrially by fermentation of glucose. The conditions for this process. Ethanol produced industrially by fermentation is separated by fractional distillation and can then be used as a biofuel. Students should be able to: - explain the meaning of the term biofuel
- justify the conditions used in the production of ethanol by fermentation of glucose
- write equations to support the statement that ethanol produced by fermentation is a carbon neutral fuel and give reasons why this statement is not valid
- outline the mechanism for the formation of an alcohol by the reaction of an alkene with steam in the presence of an acid catalyst
- discuss the environmental (including ethical) issues linked to decision making about biofuel use.
| AT a, d and k PS 1.2 Students could produce ethanol by fermentation, followed by purification by fractional distillation. |
3.3.5.2 Oxidation of alcohols
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Alcohols are classified as primary, secondary and tertiary. Primary alcohols can be oxidised to aldehydes which can be further oxidised to carboxylic acids. Secondary alcohols can be oxidised to ketones. Tertiary alcohols are not easily oxidised. Acidified potassium dichromate(VI) is a suitable oxidising agent. Students should be able to: - write equations for these oxidation reactions (equations showing [O] as oxidant are acceptable)
- explain how the method used to oxidise a primary alcohol determines whether an aldehyde or carboxylic acid is obtained
- use chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.
| AT b, d and k Students could carry out the preparation of an aldehyde by the oxidation of a primary alcohol. Students could carry out the preparation of a carboxylic acid by the oxidation of a primary alcohol. |
3.3.5.3 Elimination
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Alkenes can be formed from alcohols by acid-catalysed elimination reactions. Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oil. Students should be able to: - outline the mechanism for the elimination of water from alcohols.
| AT b, d, g and k PS 4.1 Students could carry out the preparation of cyclohexene from cyclohexanol, including purification using a separating funnel and by distillation. |
Required practical 5 Distillation of a product from a reaction. | |
3.3.6 Organic analysis
Our understanding of organic molecules, their structure and the way they react, has been enhanced by organic analysis. This section considers some of the analytical techniques used by chemists, including test-tube reactions and spectroscopic techniques.
3.3.6.1 Identification of functional groups by test-tube reactions
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The reactions of functional groups listed in the specification. Students should be able to: - identify the functional groups using reactions in the specification.
| AT b, d and k PS 2.2, 2.3 and 4.1 Students could carry out test-tube reactions in the specification to distinguish alcohols, aldehydes, alkenes and carboxylic acids. |
Required practical 6 Tests for alcohol, aldehyde, alkene and carboxylic acid. | |
3.3.6.2 Mass spectrometry
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Mass spectrometry can be used to determine the molecular formula of a compound. Students should be able to: - use precise atomic masses and the precise molecular mass to determine the molecular formula of a compound.
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3.3.6.3 Infrared spectroscopy
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Bonds in a molecule absorb infrared radiation at characteristic wavenumbers. ‘Fingerprinting’ allows identification of a molecule by comparison of spectra. Students should be able to: - use infrared spectra and the Chemistry Data Sheet or Booklet to identify particular bonds, and therefore functional groups, and also to identify impurities.
The link between absorption of infrared radiation by bonds in CO2 , methane and water vapour and global warming. | Students should be able to use data in the Chemistry Data Sheet or Booklet to suggest possible structures for molecules. |
