Chapter 8: Organic Chemistry: Some Basic Principles and Techniques (Organic Chemistry)

Rule: four single bonds → sp³; one double bond → sp²; two double bonds or a triple bond → sp.

CH₂=C=O: CH₂ sp², central C sp. CH₃CH=CH₂: sp³, sp², sp². (CH₃)₂CO: CH₃ sp³, C=O carbon sp². CH₂=CHCN: sp², sp², sp (C≡N). C₆H₆: all six carbons sp².

As listed

C₆H₆: 12σ (6 C–C + 6 C–H), 3π. C₆H₁₂ (cyclohexane): 18σ (6 C–C + 12 C–H), no π. CH₂Cl₂: 4σ. CH₂=C=CH₂: 6σ (2 C–C + 4 C–H), 2π. CH₃NO₂: 6σ (3 C–H, 1 C–N, 2 N–O), 1π (N=O). HCONHCH₃: 8σ, 1π (C=O).

C₆H₆ 12σ 3π; C₆H₁₂ 18σ; CH₂Cl₂ 4σ; allene 6σ 2π; CH₃NO₂ 6σ 1π; HCONHCH₃ 8σ 1π

Isopropyl alcohol (propan-2-ol, CH₃CH(OH)CH₃): a "V" of two lines with –OH on the middle vertex.

2,3-Dimethylbutanal (OHC–CH(CH₃)–CH(CH₃)–CH₃): a four-carbon zig-zag with the –CHO at C1 and methyl branches on C2 and C3.

Heptan-4-one (CH₃CH₂CH₂COCH₂CH₂CH₃): a seven-carbon zig-zag with =O on the fourth (middle) carbon.

As described

(b) The nitrile carbon is C1. (d) Prefixes in alphabetical order: bromo before chloro.

(a) Propylbenzene (b) 3-methylpentanenitrile (c) 2,5-dimethylheptane (d) 3-bromo-3-chloroheptane (e) 3-chloropropanal (f) 2,2-dichloroethanol

(a) Each methyl needs its own locant. (b) Lowest locant set: 2,4,7 is lower than 2,5,7 at the first point of difference. (c) Locant sets {2, 4} tie, so the group first in alphabetical order (chloro) gets the lower number. (d) –OH is the principal group (suffix) and gets the lowest number.

(a) 2,2-Dimethylpentane (b) 2,4,7-Trimethyloctane (c) 2-Chloro-4-methylpentane (d) But-3-yn-1-ol

Each next member adds –CH₂–.

(a) HCOOH, CH₃COOH, CH₃CH₂COOH, CH₃(CH₂)₂COOH, CH₃(CH₂)₃COOH (b) CH₃COCH₃, CH₃COCH₂CH₃, CH₃CO(CH₂)₂CH₃, CH₃CO(CH₂)₃CH₃, CH₃CO(CH₂)₄CH₃ (c) CH₂=CH₂, CH₃CH=CH₂, CH₃CH₂CH=CH₂, CH₃(CH₂)₂CH=CH₂, CH₃(CH₂)₃CH=CH₂

(a) (CH₃)₃CCH₂CH(CH₃)₂: a five-carbon zig-zag with two methyls on C2 and one on C4; no functional group (alkane).

(b) HOOCCH₂C(OH)(COOH)CH₂COOH (citric acid): three carboxyl groups and one hydroxyl group.

(c) OHC(CH₂)₄CHO: a six-carbon chain with –CHO at both ends: two aldehyde groups.

(a) none (b) 3 –COOH + 1 –OH (c) 2 –CHO

(a) Aldehyde, ether (methoxy), phenolic –OH (b) primary aromatic amine, ester, tertiary amine (c) nitro group and C=C (alkene), with a phenyl ring

O₂NCH₂CH₂O⁻. The –NO₂ group has a strong −I effect: it withdraws electron density and disperses the negative charge on oxygen. In CH₃CH₂O⁻ the ethyl group (+I) pushes electrons towards O⁻, intensifying the charge.

O₂NCH₂CH₂O⁻ (−I effect of NO₂ disperses the charge)

By hyperconjugation: the σ electrons of the C–H bonds on the carbon next to the π system delocalise into the π orbital. For propene, H–CH₂–CH=CH₂ ↔ H⁺ CH₂=CH–CH₂⁻ (a "no-bond" resonance form). This raises the electron density of the π system, so the alkyl group acts as an electron donor.

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

(a) Phenol: a lone pair on O moves into the ring (+R): three structures with O⁺ and a negative charge at the two ortho carbons and the para carbon, plus the two Kekulé forms.

(b) Nitrobenzene: ring π electrons move towards NO₂ (−R): structures with a positive charge at the two ortho and the para carbons (O⁻ on the nitro group).

(c) CH₃CH=CH–CH=O ↔ CH₃CH⁺–CH=CH–O⁻ (the C=C π pair moves to C–C, the C=O π pair onto O).

(d) Benzaldehyde: the C=O π pair moves onto O, then the ring donates: positive charge at the ortho and para carbons.

(e) Benzyl cation: ring π electrons move towards CH₂⁺: positive charge at the two ortho and the para carbons (four structures).

(f) CH₃CH=CH–CH₂⁺ ↔ CH₃CH⁺–CH=CH₂ (allylic resonance).

As described (curved arrows move π pairs or lone pairs one step at a time)

Electrophile ("electron-loving"): an electron-deficient reagent that accepts an electron pair; positive ions (H⁺, NO₂⁺, CH₃⁺) or neutral molecules with an electron-poor centre (BF₃, AlCl₃, SO₃, the carbon of C=O). It attacks electron-rich sites.

Nucleophile ("nucleus-loving"): a reagent that donates an electron pair; negative ions (OH⁻, CN⁻, Cl⁻, RO⁻) or neutral molecules with lone pairs (H₂O, NH₃, R₃N). It attacks electron-poor centres, e.g. OH⁻ + CH₃Cl → CH₃OH + Cl⁻.

Electrophiles accept, nucleophiles donate an electron pair.

(a) HO⁻ donates a pair to the acidic H. (b) ⁻CN attacks the carbonyl carbon. (c) The acylium ion CH₃CO⁺ attacks the π electrons of benzene.

(a) Nucleophile (b) nucleophile (c) electrophile

(a) HS⁻ replaces Br⁻. (b) H⁺ adds first (Markovnikov). (c) H and Br are removed from adjacent carbons. (d) The 1° carbocation rearranges (methyl shift) to a 3° carbocation, then Br⁻ attacks.

(a) Nucleophilic substitution (b) electrophilic addition (c) β-elimination (d) nucleophilic substitution with rearrangement

(a) Same formula C₅H₁₀O, C=O in different positions. (b) Same connectivity, different arrangement about C=C. (c) Same positions of nuclei; only the π electrons differ.

(a) Structural (position) isomers (b) geometrical isomers (cis–trans) (c) resonance contributors

(a) Two half-arrows (fish-hooks) from the O–O bond, one to each O. (b) OH⁻ takes the α-H; the C–H pair stays on carbon. (c) The C–Br pair goes with Br. (d) A ring π pair forms a bond to E⁺.

(a) Homolysis: free radicals (b) heterolysis: carbanion (c) heterolysis: carbocation (d) heterolysis: carbocation (arenium ion)

Inductive effect: permanent displacement of σ electrons along a chain due to a polar bond (−I: halogens, NO₂; +I: alkyl groups).

Electromeric effect: temporary, complete transfer of a π pair to one atom of a multiple bond when an attacking reagent is present.

Both orders are explained by the inductive effect: (a) each Cl (−I) withdraws electrons and stabilises the carboxylate ion, so more Cl makes a stronger acid. (b) Alkyl groups (+I) push electrons and destabilise the carboxylate, so more alkyl groups make a weaker acid.

Inductive effect in both

(a) Crystallisation: the compound and the impurities have different solubilities in a solvent; the compound is sparingly soluble cold but quite soluble hot. A hot saturated solution is filtered and cooled, and pure crystals form (benzoic acid from hot water).

(b) Distillation: liquids with different boiling points vaporise at different temperatures; the vapours are condensed separately (chloroform, b.p. 334 K, from aniline, b.p. 457 K).

(c) Chromatography: the components distribute differently between a stationary phase and a mobile phase, so they move at different rates (separating the dyes of ink on paper).

Difference in solubility; in boiling point; in distribution between two phases.

Fractional crystallisation: dissolve the mixture in hot S to make a saturated solution and cool it. The less soluble compound crystallises first and is filtered off; the mother liquor is concentrated and cooled again to obtain the more soluble one. Repeating gives each pure.

Fractional crystallisation

Simple distillation: for liquids that boil without decomposition and differ widely in boiling point, at atmospheric pressure.

Under reduced pressure: the pressure is lowered so the liquid boils below its normal boiling point; used for liquids that decompose at their boiling points (e.g. glycerol).

Steam distillation: for steam-volatile, water-immiscible substances (aniline, essential oils). Steam is passed through the mixture, which boils when the sum of the vapour pressures equals atmospheric pressure, below 373 K.

Normal pressure; lowered pressure (decomposing liquids); with steam (steam-volatile, immiscible liquids).

The compound is fused with sodium, converting covalent N, S and X into ionic NaCN, Na₂S and NaX. The fused mass is boiled with water and filtered (sodium fusion extract).

N: boil with FeSO₄, acidify with H₂SO₄: $6\mathrm{CN}{}^{-}+\mathrm{Fe}{}^{2+}\to [\mathrm{Fe}(\mathrm{CN}){}_6]{}^{4-}$; $3[\mathrm{Fe}(\mathrm{CN}){}_6]{}^{4-}+4\mathrm{Fe}{}^{3+}\to \mathrm{Fe}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]{}_3$ (Prussian blue).

S: acetic acid + lead acetate → black PbS; or sodium nitroprusside → violet colour.

N and S together: NaSCN forms; with Fe³⁺ a blood-red colour ([Fe(SCN)]²⁺).

Halogens: boil with HNO₃, add AgNO₃: white AgCl (soluble in NH₃), pale yellow AgBr (sparingly soluble), yellow AgI (insoluble).

Sodium fusion converts N, S, X into ions that give characteristic colour or precipitate tests.

(i) Dumas: the compound is heated with CuO in an atmosphere of CO₂; its nitrogen is set free as N₂ gas, collected over KOH and its volume measured. Works for all nitrogen compounds.

(ii) Kjeldahl: the compound is digested with conc. H₂SO₄, converting N into ammonium sulphate; NH₃ is liberated with NaOH, absorbed in a known excess of standard acid and the excess is back-titrated. Not suitable for nitro, azo or ring nitrogen.

Dumas: N₂ volume; Kjeldahl: NH₃ absorbed in standard acid.

Halogens (Carius method): heat a known mass with fuming HNO₃ and AgNO₃ in a sealed tube; the halogen forms AgX, which is filtered, dried and weighed.

Sulphur: heat with fuming HNO₃: S → H₂SO₄; precipitate as BaSO₄ with BaCl₂ and weigh.

Phosphorus: heat with fuming HNO₃: P → H₃PO₄; precipitate as ammonium phosphomolybdate, or as MgNH₄PO₄ ignited to Mg₂P₂O₇, and weigh.

X as AgX, S as BaSO₄, P as phosphomolybdate or Mg₂P₂O₇ (gravimetric).

It is partition chromatography. Water held in the paper is the stationary phase; a solvent rising by capillary action is the mobile phase. The components partition differently between the two phases, so they travel different distances. Spots are located directly, by a spraying reagent or under UV light, and identified by $R_f=\frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$.

Partition between stationary water and a moving solvent.

If N or S is present, the extract contains NaCN and Na₂S, which would also give precipitates with AgNO₃ (AgCN white, Ag₂S black). Boiling with HNO₃ decomposes them: $\mathrm{NaCN}+\mathrm{HNO}{}_3\to \mathrm{NaNO}{}_3+\mathrm{HCN}$↑; $\mathrm{Na}{}_2\mathrm{S}+2\mathrm{HNO}{}_3\to 2\mathrm{NaNO}{}_3+\mathrm{H}{}_2\mathrm{S}$↑.

To remove CN⁻ and S²⁻, which would interfere.

In organic compounds these elements are covalently bonded and give no ions in water, so ionic tests fail. Fusion with Na converts them into ionic, water-soluble NaCN, Na₂S and NaX.

To convert covalent N, S, X into ionic sodium salts.

Camphor sublimes on heating and collects on a cool surface; CaSO₄ does not sublime.

Sublimation

The liquid and water are immiscible, so each exerts its own vapour pressure. The mixture boils when $p_{\text{organic}}+p_{\text{water}}$ = atmospheric pressure, so the organic liquid's vapour pressure need only be well below 1 atm: the mixture boils below its boiling point.

The two vapour pressures add to reach atmospheric pressure.

No. Chlorine in CCl₄ is covalently bonded; CCl₄ does not give Cl⁻ ions, so no AgCl forms.

No (no free Cl⁻ ions)

CO₂ is acidic and is absorbed completely by the strong base: $2\mathrm{KOH}+\mathrm{CO}{}_2\to \mathrm{K}{}_2\mathrm{CO}{}_3+\mathrm{H}{}_2\mathrm{O}$. The increase in mass of the KOH tube gives the mass of CO₂.

KOH absorbs CO₂ completely; its gain in mass is weighed.

With H₂SO₄, lead acetate would give a white precipitate of PbSO₄, masking the black PbS. Acetic acid acidifies without adding sulphate ions.

H₂SO₄ would precipitate white PbSO₄.

C $=0.69\times 0.20=0.138$ g; CO₂ $=0.138\times \frac{44}{12}=0.506$ g.

H $=0.048\times 0.20=0.0096$ g; H₂O $=0.0096\times \frac{18}{2}=0.0864$ g.

CO₂ 0.506 g; H₂O 0.0864 g

H⁺ taken $=50\times 0.5\times 2=50$ mmol; NaOH used $=60\times 0.5=30$ mmol, so 30 mmol H⁺ were left. H⁺ neutralised by NH₃ $=20$ mmol $=20$ mmol NH₃.

N $=20\times 10^{-3}\times 14=0.28$ g; $\%\text{N}=\frac{0.28}{0.50}\times 100$.

56%

$\%\text{Cl}=\frac{35.5\times 0.5740\times 100}{143.5\times 0.3780}$

37.57%

$\%\text{S}=\frac{32\times 0.668\times 100}{233\times 0.468}$

19.6% (NCERT key 19.66%)

C2 is a double-bond carbon (sp²); C3 is saturated (sp³).

(c)

Prussian blue is iron(III) hexacyanoferrate(II).

(b)

The tertiary cation is stabilised by three +I methyl groups and nine hyperconjugating C–H bonds.

(b)

Chromatography works even for very small amounts and for compounds with very similar properties.

(d)

OH⁻ (a nucleophile) replaces I⁻.

(b)