OC Ch 02 — 15-Minute Revision

Last hour. Seven cards = seven slide topics. Read straight through with the past-paper traps in mind.

1 · Hydrocarbon classification + alkane structure

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Three skeletons

Saturated open chain (alkane), saturated ring (cycloalkane), unsaturated (alkene/alkyne/aromatic).

Alkane structure

Every C is sp³. C–C 154 pm, C–H 110 pm, angles 109.5°. Free rotation about every σ. Formula C_nH_2n+2.

Cycloalkane formula

C_nH_2n — same formula as an alkene (constitutional isomers).

2 · Conformations: ethane & butane

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Newman projection

Look along a C–C bond. Front C = dot with 3 bonds. Back C = circle with 3 bonds.

Bond	Conformation	Energy
Ethane	Eclipsed	+12 kJ/mol (torsion + VdW)
Ethane	Staggered (60° offset)	min (~99% pop)
Butane (C2–C3)	Eclipsed Me/Me (0°)	+19 kJ/mol
	Gauche (60°)	+3.8 kJ/mol
	Eclipsed Me/H (120°)	+16 kJ/mol
	Anti (180°)	min

✨ Rule of thumb

Big groups want to be as far from each other as possible. Anti for butane; staggered for ethane.

3 · IUPAC naming algorithm

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C1–C10

meth, eth, prop, but, pent, hex, hept, oct, non, dec. All end "-ane".

6 rules

Longest chain = parent.
Name parent.
Number for lowest locants.
Write substituent + locant.
Alphabetise (di/tri skipped).
Ring → "cyclo" prefix.

⚠ 2019 PP III.3

"Parent chain = longest C chain." TRUE for alkanes; for compounds with FG, parent must contain the FG.

Worked names

2,3,5-trimethylhexane (not 2,4,5) · 2,2,5-trimethylhexane (not 2,5,5) · 5-ethyl-2,2-dimethyloctane (e before m for alphabetising).

4 · Carbon types & alkyl groups

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Type	C-neighbours	H's on that C	Example
1° (primary)	1	3	terminal CH₃
2° (secondary)	2	2	middle CH₂
3° (tertiary)	3	1	(CH₃)₃CH
4° (quaternary)	4	0	(CH₃)₄C

Common alkyl groups

Me, Et, n-Pr, iPr, n-Bu, sec-Bu, iBu, tBu — you must recognise all 8 on sight.

5 · Physical properties

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Forces

Only London dispersion — alkanes are non-polar.

Trends

b.p. rises with chain length (bigger e– cloud). Even-C members pack better → slightly higher m.p. Density < 1. Insoluble in water; soluble in organic solvents.

⚔ Clinical

Cholesterol has long hydrocarbon scaffold → insoluble in plasma → carried by HDL/LDL.

6 · Reactions: combustion + radical halogenation

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Combustion

C_nH_2n+2 + O₂ → CO₂ + H₂O + heat. Radical chain.

Radical halogenation

R–H + X₂ —UV / 300°C→ R–X + HX. Three-step chain: initiation (UV homolyses X₂) · propagation (Cl· + RH → HCl + R·, then R· + Cl₂ → RCl + Cl·) · termination (2 radicals combine).

Radical stability

3° > 2° > 1° > methyl by hyperconjugation. Radical is sp² planar, unpaired e– in p orbital.

⚠ 2019 PP III.8

Cyclopropane vs propene by Br₂/CCl₄: propene decolourises (addition); cyclopropane does not. TRUE.

7 · Cycloalkanes & cyclohexane chair

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Baeyer strain

Assumed rings are planar → predicted cyclopropane very strained (TRUE: 60° angle), cyclopentane low strain (TRUE). Wrong for rings ≥ 6 (they pucker).

cis vs trans (rings)

Two substituents on different ring C's can be on same face (cis) or opposite faces (trans). Stereoisomers, not interconvertible.

Cyclohexane chair

Strain-free. 6 axial + 6 equatorial positions, alternating. Boat ~30 kJ/mol higher.

⚠ 2019 PP III.5

"Large substituent in axial more stable than equatorial." FALSE. EQUATORIAL avoids 1,3-diaxial repulsion. The bigger the group, the stronger the preference. t-Bu → essentially 100% equatorial.

📝 Full Notes ✅ Q-Bank ⚡ 5-min 🌙 Day-Before 🏠 All OC Units

McMurry & Ballantine 8e Ch 13 · TMU Slide pp.1–65 · Past Papers 2019–2022