Organic Chemistry
Organic chemistry is the study of carbon-containing compounds, covering how they are named using IUPAC rules, the different types of isomerism they show, and the mechanisms (free-radical substitution, electrophilic addition, nucleophilic substitution and elimination) by which alkanes, alkenes and halogenoalkanes react. It is one of the largest topics in A Level Chemistry, spanning both years of the course.
Method
- To name an organic compound, identify the longest continuous carbon chain containing the principal functional group, number the chain to give the functional group and substituents the lowest possible locants, then name substituents as prefixes in alphabetical order.
- To identify the type of isomerism between two compounds, first check they share the same molecular formula; if the carbon skeleton differs it is chain isomerism, if the functional group position differs it is positional isomerism, and if the functional group itself differs it is functional group isomerism.
- To draw a curly-arrow mechanism, identify the electrophile (an electron pair acceptor) and the nucleophile (an electron pair donor), then draw arrows starting from a lone pair or bond and ending where the new bond forms.
- For a free-radical substitution mechanism, write the initiation step (homolytic fission of the halogen molecule under UV light) as a stand-alone step, then write the two propagation steps in sequence, checking a radical is regenerated in the second propagation step.
- For electrophilic addition to an unsymmetrical alkene, apply Markovnikov's rule: predict the major product by identifying which carbocation intermediate (secondary/tertiary rather than primary) is more stable, because more alkyl groups donate electron density and disperse the positive charge.
- When comparing reaction conditions, match the reagent and solvent to the mechanism required: aqueous conditions with heat favour nucleophilic substitution of halogenoalkanes, while ethanolic (alcoholic) conditions with heat favour elimination.
Worked example
An alkene, Z, is a gas at room temperature and contains only carbon and hydrogen. Complete combustion of 0.420 g of Z produces 1.32 g of carbon dioxide and 0.540 g of water. Given that Z has a relative molecular mass (Mr) of 42, calculate the molecular formula of Z.
- Calculate moles of CO2 (= moles of C): 1.32/44 = 0.0300 mol.
- Calculate moles of H2O and hence moles of H: 0.540/18 = 0.0300 mol H2O, so moles H = 0.0600 mol.
- Check for oxygen: mass of C + mass of H = (0.0300 x 12) + (0.0600 x 1) = 0.360 + 0.0600 = 0.420 g, equal to the sample mass, confirming Z contains no oxygen.
- Find the simplest mole ratio C : H = 0.0300 : 0.0600 = 1 : 2, giving empirical formula CH2 (empirical mass = 14).
- Divide the given Mr by the empirical formula mass: 42/14 = 3, so the molecular formula is 3 x CH2 = C3H6.
- Final answer: molecular formula of Z = C3H6
Practice questions
Try each question, then tap to reveal the answer.
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
Give the IUPAC name of the following compound: CH3-CH(CH3)-CH2-CH2-OH
Propan-1-ol is heated with excess concentrated sulfuric acid at a high temperature, undergoing acid-catalysed dehydration. Name the type of reaction mechanism, give a suitable catalyst, write the structural formula of the alkene product, and explain why only one alkene product (not a mixture) is formed.
A student prepared a sample of bromoethane from ethanol using the following method: ethanol is heated under reflux with a mixture of sodium bromide and concentrated sulfuric acid; the crude product is separated by distillation and purified. The student reacted 9.20 g of ethanol (Mr = 46.0) with excess sodium bromide and concentrated sulfuric acid, and obtained 14.5 g of pure bromoethane (Mr = 109.0). Write an equation for the formation of bromoethane from ethanol and hydrogen bromide, and calculate the percentage yield of bromoethane.
Free printable worksheet
Want more practice on paper? Download the organic chemistry worksheet pack - 14 pages of exam-style questions with a full mark scheme. No sign-up, no email wall - just the PDF, free for personal and classroom use.
Build a full practice pack.
This topic is one of hundreds in the library - pick the ones a student needs and generate a printable PDF in minutes.