Hydrocarbons

• Alkane: strong, non-polar bonds, so only a radical can start anything, and every radical step moves one electron. Alkene: a π bond full of electrons, so an electrophile attacks it, and every arrow moves a pair, starting at the π bond.
• Strong, non-polar C–H: only radicals, fish-hooks, UV. Exposed π bond: electrophiles, arrows from the π bond, planar carbocation. H goes where the H's are, because the more stable carbocation wins.

• 14.1.1 I can recall — recall the reactions (reagents and conditions) by which alkanes can be produced: (a) addition of hydrogen to an alkene in a hydrogenation reaction, H2(g) and Pt/Ni catalyst and heat (b) cracking of a longer chain alkane, heat with Al2O3
• 14.1.2 I can describe — describe: (a) the complete and incomplete combustion of alkanes (b) the free-radical substitution of alkanes by Cl2 or Br2 in the presence of ultraviolet light, as exemplified by the reactions of ethane
• 14.1.3 I can describe — describe the mechanism of free-radical substitution with reference to the initiation, propagation and termination steps
• 14.1.4 I can suggest — suggest how cracking can be used to obtain more useful alkanes and alkenes of lower Mr from heavier crude oil fractions
• 14.1.5 I can understand — understand the general unreactivity of alkanes, including towards polar reagents in terms of the strength of the C–H bonds and their relative lack of polarity
• 14.1.6 I can — recognise the environmental consequences of carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the combustion of alkanes in the internal combustion engine and of their catalytic removal
• 14.2.1 I can recall — recall the reactions (including reagents and conditions) by which alkenes can be produced: (a) elimination of HX from a halogenoalkane by ethanolic NaOH and heat (b) dehydration of an alcohol, by using a heated catalyst (e.g. Al2O3) or a concentrated acid (e.g. concentrated H2SO4) (c) cracking of a longer chain alkane
• 14.2.2 I can describe — describe the following reactions of alkenes: (a) the electrophilic addition of (i) hydrogen in a hydrogenation reaction, H2(g) and Pt/Ni catalyst and heat (ii) steam, H2O(g) and H3PO4 catalyst (iii) a hydrogen halide, HX(g), at room temperature (iv) a halogen, X2 (b) the oxidation by cold dilute acidified KMnO4 to form the diol (c) the oxidation by hot concentrated acidified KMnO4 leading to the cleavage of the carbon-carbon double bond and the identities of the subsequent products to determine the position of alkene linkages in larger molecules (d) addition polymerisation exemplified by the reactions of ethene and propene
• 14.2.3 I can describe — describe the use of aqueous bromine to show the presence of a C=C bond
• 14.2.4 I can describe — describe the mechanism of electrophilic addition in alkenes, using bromine/ethene and hydrogen bromide/propene as examples
• 14.2.5 I can describe — describe and explain the inductive effects of alkyl groups on the stability of primary, secondary and tertiary cations formed during electrophilic addition (this should be used to explain Markovnikov addition)

Where the two routes meet. Hydrogenation uses an alkene to make an alkane; cracking makes alkenes from alkanes. The same heated Al2O3 that cracks an alkane also dehydrates an alcohol (Section G), so “heat with Al2O3” alone does not tell you which reaction is meant: the starting material does.

• “In electrophilic addition the first arrow goes from the bromine to the carbon; in a radical step the arrow has a full head.” Correct The first arrow of electrophilic addition starts at the π bond and ends at the δ+ atom of the electrophile; a curly arrow with a full head moves a pair of electrons. A radical step moves one electron and is drawn with a fish-hook (single-barbed) arrow. (Sections C and M.)
• “Alkanes are unreactive because they are saturated.” Correct Saturation describes alkanes; it does not explain them. The reason is the strong C–H bond (410 kJ mol−1) and its very low polarity (2.5 − 2.1 = 0.4), which leave a polar reagent nothing to attack. (Section E.)
• “Initiation: C2H6 → C2H5• + H•.” Correct Ultraviolet light breaks the weaker bond: Cl–Cl at 242 kJ mol−1, not C–H at 410. Initiation is Cl2 → 2Cl•. (Section C.)
• “Propene and HBr give 1-bromopropane, because the bromine goes on the end.” Correct The hydrogen joins the CH2 carbon, leaving the more stable secondary carbocation, so 2-bromopropane is the major product. (Sections M and N.)
• “Hot concentrated KMnO4 turns but-1-ene into butanoic acid.” Correct Hot concentrated acidified KMnO4 cuts the C=C: the =CH2 end becomes CO2 and water and the =CHCH2CH3 end becomes propanoic acid. Only cold, dilute KMnO4 keeps the carbon chain whole, as a diol. (Sections I and J.)
• “The catalytic converter removes carbon dioxide.” Correct It converts CO, NO and unburnt hydrocarbons into CO2, N2 and H2O. Carbon dioxide is one of its products. (Section F.)
• “Bromine water turns orange with an alkene.” Correct Bromine water is already orange; an alkene decolourises it, orange to colourless. (Section L.)
• “Ethanol is dehydrated by NaOH in ethanol.” Correct Ethanolic NaOH eliminates HX from a halogenoalkane. An alcohol is dehydrated by heated Al2O3 or by concentrated H2SO4. (Section G.)
• “Alkanes are unreactive because they are saturated.” Repair Saturation describes them; the reason is the strong (410 kJ mol−1) and almost non-polar (0.4 difference) C–H bond, which gives polar reagents nothing to attack.
• “Initiation: C2H6 → C2H5• + H•.” Repair The weaker Cl–Cl bond (242 kJ mol−1) breaks, not C–H (410): Cl2 → 2Cl•.
• “Propagation: Cl• + C2H6 → C2H5Cl + H•.” Repair The chlorine radical takes a hydrogen atom, making HCl and the ethyl radical. Each propagation step consumes one radical and makes one, and the two steps sum to the overall equation.
• “A radical mechanism drawn with full-headed curly arrows.” Repair Radical steps move one electron at a time: fish-hook (single-barbed) arrows. A full head means an electron pair.
• “Cracking produces two alkanes.” Repair Hydrogen is conserved; two alkanes would need two more hydrogen atoms than the starting alkane has, so at least one product is an alkene.
• “The catalytic converter removes carbon dioxide.” Repair It converts CO, NO and hydrocarbons into CO2, N2 and H2O; CO2 is a product.
• “The nitrogen in NO comes from the petrol.” Repair Alkanes contain no nitrogen. N2 and O2 from the air combine at the high temperature in the cylinder.
• “Bromine water is used to test for alkenes; the bromine turns orange.” Repair The bromine water is decolourised: orange to colourless.
• “Ethene + Br2: the arrow starts on Br and points at the carbon.” Repair The first arrow starts at the π bond and points to the δ+ bromine; the second starts at the Br–Br bond. A curly arrow always starts at a bond or a lone pair.
• “The carbocation is drawn with the + on the bromine” Repair The syllabus mechanism goes through a carbocation: the carbon that did not gain the Br has three bonds at 120° and carries the + sign.
• “Propene + HBr gives 1-bromopropane because the bromine goes on the end carbon.” Repair The H adds to the carbon with more hydrogens, giving the more stable secondary carbocation and 2-bromopropane as the major product.
• “A tertiary carbocation is more stable because it is bigger.” Repair Three alkyl groups each release electron density towards the positive carbon (positive inductive effect), reducing and spreading its charge.
• “But-1-ene with hot concentrated KMnO4 gives butanoic acid.” Repair The C=C is cleaved: propanoic acid, plus CO2 and water from the =CH2 end.
• “Ethanol is dehydrated by NaOH in ethanol.” Repair Ethanolic NaOH eliminates HX from a halogenoalkane; an alcohol is dehydrated by heated Al2O3 or concentrated H2SO4.
• “Poly(propene): –[CH2–CH2–CH2]n–.” Repair Only the two carbons of the C=C form the backbone; the CH3 is a side group: –[CH2–CH(CH3)]n–.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. Topic 15 (halogenoalkanes): re-answer why ethanolic NaOH gives an alkene and why tertiary carbocations matter for SN1. Topic 16 (alcohols): re-answer how propene becomes propan-2-ol. Topic 20 (polymers): redraw the poly(propene) repeat unit. Topic 30: contrast the reactivity of the alkene π bond with the aromatic ring you meet there. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 14 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A multiple-choice item can turn on one exact reagent or condition, on classifying a radical step, or on picking the major product of an addition. A structured question asks you to describe a mechanism with the right arrows, explain the unreactivity of alkanes or Markovnikov's rule, and deduce an alkene from its cleavage products.
• Balancing combustion, cracking, converter and [O] equations; masses from cracking equations; bond-energy arithmetic for propagation steps. The bond energies (C–H 410, Cl–Cl 242 kJ mol−1) and electronegativities (C 2.5, H 2.1) are given in the Data section, so you use them rather than recall them.
• Paper 3 can ask for the bromine water or acidified KMnO4 test with the syllabus's colour words, and for gas collection over water, read to 0.5 cm3 on a measuring cylinder. A Paper 5 item might ask you to plan a bromine-water titration and name its largest error.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.