Organic Chemistry – Some Basic Principles and Techniques

(a) Four C–C σ bonds and six C–H σ bonds: 10σ total; one π bond from C=C and two from C≡C: 3π total. (b) Three C–C σ bonds and six C–H σ bonds: 9σ total; two π bonds: 2π total.

(a) sp³. (b) The two methyl carbons are sp³; the carbonyl carbon is sp². (c) CH₃ carbon is sp³; nitrile carbon is sp. (d) Carbonyl carbon is sp². (e) In order along the chain: sp³, sp², sp², sp.

(a) Carbon is sp²; the atoms around it are trigonal planar. (b) Carbon is sp³; the shape around carbon is tetrahedral. (c) Carbon is sp; the molecule is linear.

(a) CH₃–CH₂–C(=O)–CH₂–CH₃. (b) CH₃–CH=CH–CH₂–CH₂–CH₂–CH₃.

(a) HOCH₂CH₂CH₂CH(CH₃)CH(CH₃)CH₃. (b) HOCH(CN)₂, also written NC–CH(OH)–CN.

(a) C₆H₁₁CH(CH₃)₂ (propan-2-ylcyclohexane). (b) CH₃(CH₂)₆CH₃ (octane). (c) HC≡CCH₂CH(OH)CH₃. (d) CH₃CH(CH₃)CH(CH₃)CH₃.

(a) HO–CH₂–CH₂–CH₂–CH(CH₃)–CH(CH₃)₂. (b) HO–CH(C≡N)₂: the central carbon is attached to –OH, H and two cyano groups.

(i) 6-Methyloctan-3-ol. (ii) Hexane-2,4-dione. (iii) 5-Oxohexanoic acid. (iv) Hexa-1,3-dien-5-yne.

(i) CH₃–CH(Cl)–CH₂–CH₂–CH₂–CH₃. (ii) CH₃–CH(OH)–CH₂–CH=CH₂. (iii) Cyclohex-1-ene with –NO₂ on carbon 3. (iv) Cyclohex-2-en-1-ol: a six-membered ring with OH at C1 and C2=C3. (v) OHC–CH₂–CH₂–CH₂–CH₂–CH(OH)–CH₃.

Place ring groups at the named positions; for (c), use a five-carbon chain with phenyl at C1 and Br at C2 and C3.

(a) The S–CH₃ bond breaks with the pair going to sulfur: CH₃S⁻ + CH₃⁺. (b) The C–CN bond breaks with the pair going to CN: CH₃⁺ + CN⁻. (c) The C–Cu bond breaks with the pair going to carbon: CH₃⁻ + Cu⁺.

Nucleophiles: HS⁻, C₂H₅O⁻, (CH₃)₃N and H₂N⁻; they can donate an electron pair. Electrophiles: BF₃, CH₃⁺, CH₃CO⁺ and NO₂⁺; they can accept an electron pair.

In each molecule, the carbon attached to the more electronegative atom is electrophilic: the carbonyl carbon in CH₃CHO, the nitrile carbon in CH₃CN and the carbon bonded to iodine in CH₃I.

(a) C–Br. (b) C–O. (c) C–O. In each pair, the bond to the more electronegative atom is more polar.

It is least in the C₂–C₃ bond, farthest from bromine, because the inductive effect weakens as it travels through more bonds.

Two equivalent forms share the negative charge over both oxygens.

The neutral structure is the major contributor; charge-separated forms are minor.

Neither I nor II is a major contributor: both have charge separation, and structure I also contains a carbon with an incomplete octet. The neutral structure of methyl acetate is more stable.

Methyl groups stabilise a nearby positive carbon by donating electron density through hyperconjugation. (CH₃)₃C⁺ has nine adjacent C–H bonds available for this effect, CH₃CH₂⁺ has three, and CH₃⁺ has none. So (CH₃)₃C⁺ is most stable and CH₃⁺ least stable.

Carbon mass=(12/44)(0.198)=0.0540 g, so %C=(0.0540/0.246)×100≈21.95%. Hydrogen mass=(2/18)(0.1014)=0.01127 g, so %H=(0.01127/0.246)×100≈4.58%.

Dry N₂ pressure=715−15=700 mm. Volume at STP=50×(700/760)×(273/300)=41.9 mL. Nitrogen mass=(28/22400)×41.9=0.0524 g. Percentage N=(0.0524/0.30)×100≈17.5%.

10 mL of 1 M H₂SO₄ neutralises 20 mL of 1 M NH₃; this is 0.28 g N.

Mass of Br=(80/188)×0.12=0.0511 g. Percentage Br=(0.0511/0.15)×100≈34.0%.

Mass of S=(32/233)×0.4813=0.0661 g. Percentage S=(0.0661/0.157)×100≈42.1%.

Single-bond carbon: sp³; alkene/carbonyl/aromatic carbon: sp²; nitrile/linear carbon: sp.

Count each single bond as σ; each double bond contributes one additional π.

Place the functional group at the numbered carbon, then add the correct carbon chain and branches.

Add one –CH₂– unit at a time and keep the functional group unchanged.

Build the named parent structure and mark each suffix group.

The –NO₂ group has a strong electron-withdrawing (−I) effect.

Adjacent C–H σ electrons delocalise into the π system (hyperconjugation).

Nucleophile donates an electron pair; electrophile accepts one.

OH⁻ and CN⁻ donate electron pairs; CH₃CO⁺ accepts one.

Check whether a group is replaced, added across a bond, or removed to form a multiple bond.

Electron-withdrawing groups stabilise the carboxylate conjugate base; the inductive effect weakens with distance.

Crystallisation uses solubility; distillation uses boiling point; chromatography uses different phase affinities.

Use an immiscible solvent and a separating funnel.

Normal pressure; lowered pressure for heat-sensitive/high-boiling liquids; steam for volatile water-immiscible compounds.

Sodium fusion changes covalently bonded elements into water-soluble ions for testing.

Dumas measures N₂ gas; Kjeldahl measures ammonia released from ammonium salt.

Convert each element into a measurable stable precipitate.

Components partition differently between stationary and moving phases.

Remove CN⁻ and S²⁻, which would also react with Ag⁺.

Fusion changes covalent elements into detectable water-soluble ions.

Camphor sublimes; calcium sulfate remains solid.

Water and organic vapour pressures add to reach atmospheric pressure sooner.

CCl₄ contains covalent C–Cl bonds, not free Cl⁻ ions.

KOH removes CO₂ by converting it into carbonate.

Avoid adding sulfate, which precipitates lead as PbSO₄.

CO₂: 0.20×0.69×44/12=0.506 g; H₂O: 0.20×0.048×9=0.0864 g.

n(N)=0.050−0.030=0.020 mol; m(N)=0.28 g; %N=56.0%.

%Cl=0.5740×35.45/143.32/0.3780×100≈37.6%.

%S=0.668×32.06/233.39/0.468×100≈19.6%.

C₂ is sp²; C₃ is sp³.

The blue complex is ferric ferrocyanide.

Chromatography separates compounds by their different phase affinities.

OH⁻ replaces the leaving group I⁻.

(a) 2,5-Dimethylheptane. (b) 1-Bromo-3-methylpentane. (c) 3-Chloro-4-methylhexane. (d) 2-Methylbutan-1-ol. (e) 3,3-Dimethylbutanal. (f) 2,2-Dichloroethan-1-ol.

(i) 2,2-Dimethylpentane. (ii) 2,4,7-Trimethyloctane. (iii) 2-Chloro-4-methylpentane. (iv) But-3-yn-1-ol.

(a) Vanillin: aldehyde, ether (methoxy) and phenolic –OH. (b) The amino-aromatic ester: primary aromatic amine, ester and tertiary amine. (c) The unsaturated nitro compound: alkene and nitro group.

(a) Phenol: an oxygen lone pair is donated into the ring, placing negative charge at ortho and para positions while oxygen becomes positive. (b) Nitrobenzene: the nitro group withdraws ring electron density, placing positive charge at ortho/para positions. (c) CH3CH=CHCHO: conjugation shares electron density across C=C–C=O; a contributor has O− and positive charge at the β-carbon. (d) Benzaldehyde: ring donation toward –CHO gives O− and positive charge at ortho/para ring positions. (e) Benzyl cation: positive charge is shared between benzylic carbon and ortho/para ring positions. (f) Allyl cation: positive charge is shared between the two terminal carbons.

(a) Structural isomers. (b) Geometrical (cis/trans) isomers. (c) Resonance contributors.

(a) C–C bond splits evenly: homolysis, giving two methyl radicals. (b) C–Cl bond electrons go to chlorine: heterolysis, giving CH3+ and Cl−. (c) tert-Butyl bromide splits with both electrons to Br: heterolysis, giving a tertiary carbocation and Br−. (d) The C–Mg bond pair stays with carbon: heterolysis, giving a carbanion-like CH3− and MgBr+.

The tert-butyl carbocation, (CH3)3C+, is most stable because it is tertiary and is stabilised by electron donation from three alkyl groups.