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Hydrocarbons

Alkanes, addition to alkenes and alkynes, ozonolysis, aromaticity and substitution on benzene. Most questions ask which carbon the new group goes to.

Questions in the bank
165
q/paper in 2025–26
1.56
Calculation
23%
Notes pages
8

Strand: Reactions

When you’ll see it

An alkane, alkene, alkyne or benzene ring meets a reagent, and you must say where the new group goes, how many products form, or whether a ring is aromatic.

How this chapter is tested

Most questions reduce to one decision: which carbon gets the new group. For an alkene it is the carbon that leaves the more stable carbocation, after any hydride or methyl shift, or the more stable radical when HBr has a peroxide. For a substituted benzene it is the position the group already on the ring directs to.

The numeric answers are counts: monochloro products of an alkane, carbonyl fragments from ozonolysis, π electrons in a ring, moles of H₂ taken up. Each count has a routine. Apply it in full, because one missed isomer loses the four marks and a wrong entry costs one more.

Marks slip on conditions, not on the reactions. Cold dilute KMnO₄ gives a diol, hot KMnO₄ cuts the C=C; Cl₂ in the dark adds, Cl₂ in light substitutes; Lindlar gives the cis alkene, Na in liquid NH₃ the trans. Read the reagent line twice before choosing.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Alkanes: preparation and conformations

    Wurtz and Kolbe double the chain, soda lime removes one carbon, RMgX + H₂O gives RH; staggered ethane is most stable, fully eclipsed butane least.

  • Free-radical halogenation

    Monohalo products = sets of non-equivalent H; add enantiomers when the stem counts stereoisomers; Br₂ strongly prefers a 3° H, Cl₂ does not.

  • Alkene stability and HX or water addition

    More alkyl groups on C=C, more stable; HX adds Markovnikov after any shift; HBr + peroxide adds anti-Markovnikov; hydroboration gives the anti-Markovnikov alcohol with no shift.

  • Halogens, KMnO₄ and ozonolysis

    Br₂ adds anti (trans-but-2-ene → meso); cold KMnO₄ → syn diol, hot KMnO₄ → acids, ketones and CO₂; O₃ then Zn/H₂O → aldehydes and ketones.

  • Alkynes

    Alc. KOH then NaNH₂ makes the C≡C; a terminal C–H gives acetylides and a white AgNO₃ precipitate; Lindlar → cis, Na/liq. NH₃ → trans; HgSO₄ hydration gives a methyl ketone (ethanal from ethyne).

  • Benzene and aromaticity

    Cyclic, planar, fully conjugated, 4n + 2 π electrons; count only π electrons inside the ring; aromatic > non-aromatic > antiaromatic in stability.

  • Electrophilic substitution on benzene

    NO₂⁺, Cl⁺, SO₃ and R⁺ from acids or AlCl₃; lone-pair groups and alkyls direct ortho-para, NO₂, CN, CHO, COOH meta; halogens deactivate yet direct ortho-para.

  • Friedel–Crafts, side-chain oxidation and step order

    Alkylation rearranges, acylation does not; hot KMnO₄ cuts any chain with a benzylic H to COOH; do Friedel–Crafts before adding NO₂, COR or SO₃H.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • The unshifted product

    When the first cation is 2° next to a 3° or quaternary carbon, a hydride or methyl shift comes first. The product without the shift is always among the options; oxymercuration and hydroboration are the routes that never shift.

  • Peroxide on the wrong acid

    Only HBr adds anti-Markovnikov with a peroxide. HCl and HI still give the Markovnikov product.

  • A ring alkene split into two molecules

    Ozonolysis or hot KMnO₄ of a ring C=C opens the ring into one chain with a carbonyl at each end. Cyclohexene gives one product, hexanedioic acid with hot KMnO₄.

  • Aromatic by count alone

    cis-[10]annulene and cyclooctatetraene fail on planarity, cyclopentadiene on its sp³ CH₂. Judge the ion where the question is about acidity: the cyclopentadienyl anion is aromatic.

  • Halobenzenes ranked above benzene

    Halogens direct ortho-para but slow the ring. Chlorobenzene sits just below benzene in rate of electrophilic substitution.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Hydrocarbons notes

Drill every Hydrocarbons question

165 questions from the bank, across 8 subtopics.

Drill the 165 questions

Drill one subtopic at a time

The 8 subtopics, in teaching order.

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