3.3 Organic chemistry

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

Content

Opportunities for skills development

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.
 

3.3.1.2 Reaction mechanisms

Content

Opportunities for skills development

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.
 

3.3.1.3 Isomerism

Content

Opportunities for skills development

Structural isomerism.

Stereoisomerism.

EZ 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 EZ 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

Content

Opportunities for skills development

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

Content

Opportunities for skills development

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.
 

3.3.2.3 Combustion of alkanes

Content

Opportunities for skills development

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.
 

3.3.2.4 Chlorination of alkanes

Content

Opportunities for skills development

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.
 

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

Content

Opportunities for skills development

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

Content

Opportunities for skills development

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.
 

3.3.3.3 Ozone depletion

Content

Opportunities for skills development

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

Content

Opportunities for skills development

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

Content

Opportunities for skills development

Electrophilic addition reactions of alkenes with HBr, H2SO4 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

Content

Opportunities for skills development

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

Content

Opportunities for skills development

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

Content

Opportunities for skills development

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

Content

Opportunities for skills development

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

Content

Opportunities for skills development

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

Content

Opportunities for skills development

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.
 

3.3.6.3 Infrared spectroscopy

Content

Opportunities for skills development

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.

3.3.7 Optical isomerism (A-level only)

Compounds that contain an asymmetric carbon atom form stereoisomers that differ in their effect on plane polarised light. This type of isomerism is called optical isomerism.

Content

Opportunities for skills development

Optical isomerism is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to molecules with a single chiral centre.

An asymmetric carbon atom is chiral and gives rise to optical isomers (enantiomers), which exist as non super-imposable mirror images and differ in their effect on plane polarised light.

A mixture of equal amounts of enantiomers is called a racemic mixture (racemate).

Students should be able to:

  • draw the structural formulas and displayed formulas of enantiomers
  • understand how racemic mixtures (racemates) are formed and why they are optically inactive.

MS 4.1, 4.2 and 4.3

Students could be asked to recognise the presence of a chiral centre in a given structure in 2D or 3D forms. They could also be asked to draw the 3D representation of chiral centres in various species.

Students understand the origin of optical isomerism.

AT a and k

PS 1.2

Passing polarised light through a solution of sucrose.

3.3.8 Aldehydes and ketones (A-level only)

Aldehydes, ketones, carboxylic acids and their derivatives all contain the carbonyl group which is attacked by nucleophiles. This section includes the addition reactions of aldehydes and ketones.

Content

Opportunities for skills development

Aldehydes are readily oxidised to carboxylic acids.

Chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.

Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using NaBH4 in aqueous solution. These reduction reactions are examples of nucleophilic addition.

The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce hydroxynitriles.

Aldehydes and unsymmetrical ketones form mixtures of enantiomers when they react with KCN followed by dilute acid.

The hazards of using KCN.

Students should be able to:

  • write overall equations for reduction reactions using [H] as the reductant
  • outline the nucleophilic addition mechanism for reduction reactions with NaBH4 (the nucleophile should be shown as H)
  • write overall equations for the formation of hydroxynitriles using HCN
  • outline the nucleophilic addition mechanism for the reaction with KCN followed by dilute acid
  • explain why nucleophilic addition reactions of KCN, followed by dilute acid, can produce a mixture of enantiomers.

AT b, d and k

PS 2.2

Students could carry out test-tube reactions of Tollens’ reagent and Fehling’s solution to distinguish aldehydes and ketones.

3.3.9 Carboxylic acids and derivatives (A-level only)

Carboxylic acids are weak acids but strong enough to liberate carbon dioxide from carbonates. Esters occur naturally in vegetable oils and animal fats. Important products obtained from esters include biodiesel, soap and glycerol.

3.3.9.1 Carboxylic acids and esters (A-level only)

Content

Opportunities for skills development

The structures of:

  • carboxylic acids
  • esters.

Carboxylic acids are weak acids but will liberate CO2 from carbonates.

Carboxylic acids and alcohols react, in the presence of an acid catalyst, to give esters.

Common uses of esters (eg in solvents, plasticisers, perfumes and food flavourings).

Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol).

Esters can be hydrolysed in acid or alkaline conditions to form alcohols and carboxylic acids or salts of carboxylic acids.

Vegetable oils and animal fats can be hydrolysed in alkaline conditions to give soap (salts of long-chain carboxylic acids) and glycerol.

Biodiesel is a mixture of methyl esters of long-chain carboxylic acids.

Biodiesel is produced by reacting vegetable oils with methanol in the presence of a catalyst.

AT b, d, g and k

PS 4.1

Students could make esters by reacting alcohols with carboxylic acids, purifying the product using a separating funnel and by distillation.

AT b, d, g, h and k

Students could identify an ester by measuring its boiling point, followed by hydrolysis to form the carboxylic acid, which is purified by recrystallisation, and determine its melting point.

AT b, c, d and k

Students could make soap.

AT b and k

Students could make biodiesel.

3.3.9.2 Acylation (A-level only)

Content

Opportunities for skills development

The structures of:

  • acid anhydrides
  • acyl chlorides
  • amides.

The nucleophilic addition–elimination reactions of water, alcohols, ammonia and primary amines with acyl chlorides and acid anhydrides.

The industrial advantages of ethanoic anhydride over ethanoyl chloride in the manufacture of the drug aspirin.

Students should be able to outline the mechanism of nucleophilic addition–elimination reactions of acyl chlorides with water, alcohols, ammonia and primary amines.

AT d and k

PS 2.2

Students could record observations from reaction of ethanoyl chloride and ethanoic anhydride with water, ethanol, ammonia and phenylamine.

AT b, d, g and h

PS 2.1, 2.3 and 4.1

Students could carry out the preparation of aspirin, purification by recrystallisation and determination of its melting point.

Students could carry out the purification of impure benzoic acid and determination of its melting point.

Required practical 10

Preparation of:
  • a pure organic solid and test of its purity
  • a pure organic liquid.
 

3.3.10 Aromatic chemistry (A-level only)

Aromatic chemistry takes benzene as an example of this type of molecule and looks at the structure of the benzene ring and its substitution reactions.

3.3.10.1 Bonding (A-level only)

Content

Opportunities for skills development

The nature of the bonding in a benzene ring, limited to planar structure and bond length intermediate between single and double.

Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene.

Students should be able to:

  • use thermochemical evidence from enthalpies of hydrogenation to account for this extra stability
  • explain why substitution reactions occur in preference to addition reactions.
 

3.3.10.2 Electrophilic substitution (A-level only)

Content

Opportunities for skills development

Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions.

Nitration is an important step in synthesis, including the manufacture of explosives and formation of amines.

Friedel–Crafts acylation reactions are also important steps in synthesis.

Students should be able to outline the electrophilic substitution mechanisms of:

  • nitration, including the generation of the nitronium ion
  • acylation using AlCl3 as a catalyst.

AT b, d, g and h

PS 2.1, 2.3 and 4.1

Students could carry out the preparation of methyl 3-nitrobenzoate by nitration of methyl benzoate, purification by recrystallisation and determination of melting point.

3.3.11 Amines (A-level only)

Amines are compounds based on ammonia where hydrogen atoms have been replaced by alkyl or aryl groups. This section includes their reactions as nucleophiles.

3.3.11.1 Preparation (A-level only)

Content

Opportunities for skills development

Primary aliphatic amines can be prepared by the reaction of ammonia with halogenoalkanes and by the reduction of nitriles.

Aromatic amines, prepared by the reduction of nitro compounds, are used in the manufacture of dyes.

 

3.3.11.2 Base properties (A-level only)

Content

Opportunities for skills development

Amines are weak bases.

The difference in base strength between ammonia, primary aliphatic and primary aromatic amines.

Students should be able to:

  • explain the difference in base strength in terms of the availability of the lone pair of electrons on the N atom.
 

3.3.11.3 Nucleophilic properties (A-level only)

Content

Opportunities for skills development

Amines are nucleophiles.

The nucleophilic substitution reactions of ammonia and amines with halogenoalkanes to form primary, secondary, tertiary amines and quaternary ammonium salts.

The use of quaternary ammonium salts as cationic surfactants.

The nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides and acid anhydrides.

Students should be able to outline the mechanisms of:

  • these nucleophilic substitution reactions
  • the nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides.
 

3.3.12 Polymers (A-level only)

The study of polymers is extended to include condensation polymers. The ways in which condensation polymers are formed are studied, together with their properties and typical uses. Problems associated with the reuse or disposal of both addition and condensation polymers are considered.

3.3.12.1 Condensation polymers (A-level only)

Content

Opportunities for skills development

Condensation polymers are formed by reactions between:

  • dicarboxylic acids and diols
  • dicarboxylic acids and diamines
  • amino acids.

The repeating units in polyesters (eg Terylene) and polyamides (eg nylon 6,6 and Kevlar) and the linkages between these repeating units.

Typical uses of these polymers.

Students should be able to:

  • draw the repeating unit from monomer structure(s)
  • draw the repeating unit from a section of the polymer chain
  • draw the structure(s) of the monomer(s) from a section of the polymer
  • explain the nature of the intermolecular forces between molecules of condensation polymers.

AT k

PS 1.2

Making nylon 6,6

3.3.12.2 Biodegradability and disposal of polymers (A-level only)

Content

Opportunities for skills development

Polyalkenes are chemically inert and non-biodegradable.

Polyesters and polyamides can be broken down by hydrolysis and are biodegradable.

The advantages and disadvantages of different methods of disposal of polymers, including recycling.

Students should be able to:

  • explain why polyesters and polyamides can be hydrolysed but polyalkenes cannot.
Research opportunity

Students could research problems associated with the disposal of different polymers.

3.3.13 Amino acids, proteins and DNA (A-level only)

Amino acids, proteins and DNA are the molecules of life. In this section, the structure and bonding in these molecules and the way they interact is studied. Drug action is also considered.

3.3.13.1 Amino acids (A-level only)

Content

Opportunities for skills development

Amino acids have both acidic and basic properties, including the formation of zwitterions.

Students should be able to draw the structures of amino acids as zwitterions and the ions formed from amino acids:

  • in acid solution
  • in alkaline solution.
 

3.3.13.2 Proteins (A-level only)

Content

Opportunities for skills development

Proteins are sequences of amino acids joined by peptide links.

The importance of hydrogen bonding and sulfur–sulfur bonds in proteins.

The primary, secondary (α-helix and β–pleated sheets) and tertiary structure of proteins.

Hydrolysis of the peptide link produces the constituent amino acids.

Amino acids can be separated and identified by thin-layer chromatography.

Amino acids can be located on a chromatogram using developing agents such as ninhydrin or ultraviolet light and identified by their Rf values.

Students should be able to:

  • draw the structure of a peptide formed from up to three amino acids
  • draw the structure of the amino acids formed by hydrolysis of a peptide
  • identify primary, secondary and tertiary structures in diagrams
  • explain how these structures are maintained by hydrogen bonding and S–S bonds
  • calculate Rf values from a chromatogram.
 

3.3.13.3 Enzymes (A-level only)

Content

Opportunities for skills development

Enzymes are proteins.

The action of enzymes as catalysts, including the concept of a stereospecific active site that binds to a substrate molecule.

The principle of a drug acting as an enzyme inhibitor by blocking the active site.

Computers can be used to help design such drugs.

Students should be able to:

  • explain why a stereospecific active site can only bond to one enantiomeric form of a substrate or drug.
 

3.3.13.4 DNA (A-level only)

Content

Opportunities for skills development

The structures of the phosphate ion, 2-deoxyribose (a pentose sugar) and the four bases adenine, cytosine, guanine and thymine are given in the Chemistry Data Booklet.

A nucleotide is made up from a phosphate ion bonded to 2-deoxyribose which is in turn bonded to one of the four bases adenine, cytosine, guanine and thymine.

A single strand of DNA (deoxyribonucleic acid) is a polymer of nucleotides linked by covalent bonds between the phosphate group of one nucleotide and the 2-deoxyribose of another nucleotide. This results in a sugar-phosphate-sugar-phosphate polymer chain with bases attached to the sugars in the chain.

DNA exists as two complementary strands arranged in the form of a double helix.

Students should be able to:

  • explain how hydrogen bonding between base pairs leads to the two complementary strands of DNA.
 

3.3.13.5 Action of anticancer drugs (A-level only)

Content

Opportunities for skills development

The Pt(II) complex cisplatin is used as an anticancer drug.

Cisplatin prevents DNA replication in cancer cells by a ligand replacement reaction with DNA in which a bond is formed between platinum and a nitrogen atom on guanine.

Appreciate that society needs to assess the balance between the benefits and the adverse effects of drugs, such as the anticancer drug cisplatin.

Students should be able to:

  • explain why cisplatin prevents DNA replication
  • explain why such drugs can have adverse effects.
 

3.3.14 Organic synthesis (A-level only)

The formation of new organic compounds by multi-step syntheses using reactions included in the specification is covered in this section.

Content

Opportunities for skills development

The synthesis of an organic compound can involve several steps.

Students should be able to:

  • explain why chemists aim to design processes that do not require a solvent and that use non-hazardous starting materials
  • explain why chemists aim to design production methods with fewer steps that have a high percentage atom economy
  • use reactions in this specification to devise a synthesis, with up to four steps, for an organic compound.
 

3.3.15 Nuclear magnetic resonance spectroscopy (A-level only)

Chemists use a variety of techniques to deduce the structure of compounds. In this section, nuclear magnetic resonance spectroscopy is added to mass spectrometry and infrared spectroscopy as an analytical technique. The emphasis is on the use of analytical data to solve problems rather than on spectroscopic theory.

Content

Opportunities for skills development

Appreciation that scientists have developed a range of analytical techniques which together enable the structures of new compounds to be confirmed.

Nuclear magnetic resonance (NMR) gives information about the position of 13C or 1H atoms in a molecule.

13C NMR gives simpler spectra than 1H NMR.

The use of the δ scale for recording chemical shift.

Chemical shift depends on the molecular environment.

Integrated spectra indicate the relative numbers of 1H atoms in different environments.

1H NMR spectra are obtained using samples dissolved in deuterated solvents or CCl4

The use of tetramethylsilane (TMS) as a standard.

Students should be able to:

  • explain why TMS is a suitable substance to use as a standard
  • use 1H NMR and 13C NMR spectra and chemical shift data from the Chemistry Data Booklet to suggest possible structures or part structures for molecules
  • use integration data from 1H NMR spectra to determine the relative numbers of equivalent protons in the molecule
  • use the n+1 rule to deduce the spin–spin splitting patterns of adjacent, non-equivalent protons, limited to doublet, triplet and quartet formation in aliphatic compounds.

Students should be able to use data in the Chemistry Data Booklet to suggest possible structures for molecules.

3.3.16 Chromatography (A-level only)

Chromatography provides an important method of separating and identifying components in a mixture. Different types of chromatography are used depending on the composition of mixture to be separated.

Content

Opportunities for skills development

Chromatography can be used to separate and identify the components in a mixture.

Types of chromatography include:

  • thin-layer chromatography (TLC) – a plate is coated with a solid and a solvent moves up the plate
  • column chromatography (CC) – a column is packed with a solid and a solvent moves down the column
  • gas chromatography (GC) – a column is packed with a solid or with a solid coated by a liquid, and a gas is passed through the column under pressure at high temperature.

Separation depends on the balance between solubility in the moving phase and retention by the stationary phase.

Retention times and Rf values are used to identify different substances.

The use of mass spectrometry to analyse the components separated by GC.

Students should be able to:

  • calculate Rf values from a chromatogram
  • compare retention times and Rf values with standards to identify different substances.

AT a, i and k

PS 1.2, 3.2 and 4.1

Students could use thin-layer chromatography to identify analgesics.

Students could use thin-layer chromatography to identify transition metal ions in a solution.

Required practical 12

Separation of species by thin-layer chromatography.