Day-Before Sheet

A hydrocarbon contains only C and H. Saturation determines the family: alkane (single bonds, C_nH_2n+2), cycloalkane (single, in a ring, C_nH_2n), alkene (one C=C, C_nH_2n), alkyne (one C≡C, C_nH_2n−2), aromatic (benzene ring).

Every carbon in alkanes/cycloalkanes is sp³, tetrahedral, angle 109.5°. Bond lengths: C–C 154 pm, C–H 110 pm. Single bonds rotate freely.

"Saturated" = every C carries the max possible H count. No double/triple bonds. No Br₂ addition. Alkanes are the most chemically inert family.

Looking down a C–C bond, the front C is a dot (3 lines to substituents) and the back C is a circle (3 lines from edge). The dihedral angle = torsion between front-substituent and back-substituent.

Ethane: staggered (60° offset, ~99% pop) is the energy minimum. Eclipsed (0°) is +12 kJ/mol higher (torsional + Van der Waals strain — eclipsed H–H 227 pm < 2 × VdW radius 240 pm).

Butane (down C2–C3): anti (180°, methyls maximally far) < gauche (60°, +3.8 kJ/mol) < Me/H eclipsed (120°, +16) < Me/Me eclipsed (0°, +19). The lesson: large groups prefer to be as far apart as possible. Long-chain alkanes therefore adopt extended zig-zag shapes.

C1–C10 prefixes: meth, eth, prop, but, pent, hex, hept, oct, non, dec. All alkanes end "-ane".

1. Find the longest continuous chain — that's the parent. Tie-break: pick the chain with more substituents.
2. Name parent: prefix + "-ane".
3. Number to give the lowest set of locants at the first point of difference.
4. Each substituent gets "locant-name" written in front of the parent: "2-methyl", "3-bromo".
5. List substituents alphabetically. Multiplier prefixes di, tri, tetra are ignored for alphabetisation when directly attached (so "ethyl" comes before "dimethyl"). They are counted when embedded inside a complex substituent name (1,1-dimethylethyl).
6. For rings: prefix "cyclo" (cyclohexane). If the side chain is bigger than the ring, the ring becomes "cyclohexyl-" substituent.

Halogens etc: F = fluoro, Cl = chloro, Br = bromo, I = iodo, NO₂ = nitro, NH₂ = amino.

Carbon types (1°/2°/3°/4°): count the other carbons attached to that C. Predicts radical/cation/alcohol stability, SN1/SN2 selectivity, oxidation outcomes.

C–C and C–H are essentially nonpolar → only London dispersion (induced dipole — induced dipole) holds alkanes together. Trends:

• b.p. rises with chain length (bigger electron cloud → bigger transient dipole → stronger London).
• m.p. rises with chain length; even-C members slightly higher than the odd-C neighbours because even chains pack tighter in the crystal.
• Density < 1 g/mL. Insoluble in water (no H-bonding), soluble in nonpolar organic solvents.

Alkanes are inert at room temperature — no reaction with acids, bases, oxidants. They react only under forcing conditions:

Combustion (high O₂): C_nH_2n+2 + O₂ → CO₂ + H₂O + heat. Three benchmarks: CH₄ 882 kJ/mol, C₂H₆ 1538, C₃H₈ 2199. Radical chain mechanism (not acid-base).

Radical halogenation: R–H + X₂ —UV / 300°C→ R–X + HX. Three-step mechanism — memorise:

1. Initiation: X₂ —UV→ 2 X· (homolytic cleavage; electrons split evenly).
2. Propagation: (a) X· + R–H → HX + R· (abstracts H). (b) R· + X₂ → R–X + X· (regenerates X radical; chain continues).
3. Termination: any radical–radical recombination (Cl·+Cl·, Cl·+R·, R·+R·); chain stops.

General formula C_nH_2n. Named by "cyclo" + alkane name with the same C count.

cis/trans isomerism on rings: two substituents on different ring C's can sit on the same face (cis) or opposite faces (trans). They are stereoisomers — can't interconvert without breaking the ring. If both substituents are on the same carbon, no cis/trans.

Baeyer (1885): assumed every ring is planar; calculated strain from the angle deviation from 109.5°. Correctly predicted high strain in cyclopropane (60°) and cyclobutane. Wrong for ≥ 6-membered rings, which actually pucker out of the plane.

Cyclopropane is unstable not just from bond-angle strain but also torsional strain — all its H–H pairs are eclipsed (it has to be planar).

Chair conformation is strain-free: all 6 C–C–C ≈ 111°, all adjacent C–H staggered. The boat is ~30 kJ/mol higher (flagpole H's eclipsed). The two equivalent chairs interconvert by ring flip; axial substituents swap to equatorial and vice versa.

Each ring carbon carries one axial H (perpendicular to ring plane) and one equatorial H (around the equator). They alternate around the ring — 3 axial-up + 3 axial-down + 3 equatorial-up + 3 equatorial-down.

Worked: tert-butylcyclohexane → t-Bu equatorial (essentially only conformer). trans-1-isopropyl-3-methylcyclohexane → both equatorial in one of the chairs. cis-1-isopropyl-4-methylcyclohexane → cis-1,4 always one-axial/one-equatorial; place the bigger (iPr) equatorial.

1. Name an alkane? Longest chain → number for lowest locants → list substituents alphabetically (ignore di/tri) → write final name.
2. Identify a chiral / non-chiral carbon? Count distinct groups bonded.
3. Classify a C? Count other carbons attached. 1°/2°/3°/4°.
4. Most stable conformation of a chain? Anti for butane; staggered for ethane.
5. Most stable chair? The one with the BIGGEST substituent EQUATORIAL.
6. Predict the radical-halogenation product? The hydrogen most easily abstracted is the one whose loss gives the most stable radical: 3° > 2° > 1°.
7. Tell cyclopropane from propene? Br₂/CCl₄ (decolourises propene only).
8. T/F on cyclohexane chair? Default: examiners invert the equatorial rule. Read the verb.