Why a Carbocation Is Flat but a Carbanion Is a Pyramid

Every organic reaction starts with a bond breaking, and the way it breaks decides which short-lived species form next — a radical, a positive ion or a negative ion. Before any of that chemistry can be studied, the compound itself must be purified and its elements identified.

This lesson covers bond fission and reactive intermediates, purification and elemental analysis, and the calculations that give the percentage of each element.

In homolytic fission a covalent bond breaks evenly, each atom keeping one electron and forming free radicals, while in heterolytic fission one atom takes both electrons, forming a carbocation or a carbanion.

Homolytic fission:

- Favoured by heat, light or peroxides, and by non-polar bonds: $\mathrm{C}{\mathrm{l}}_2\to 2C{\mathrm{l}}^{\bullet }$
- Free radicals such as $\mathrm{C}{\mathrm{H}}_3^{\bullet }$ have an unpaired electron, are neutral and very reactive
- Stability: tertiary > secondary > primary > methyl

Heterolytic fission:

- Favoured by polar bonds and polar solvents: $(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{CBr}\to (\mathrm{C}{\mathrm{H}}_3{)}_3{\mathrm{C}}^{+}+\mathrm{B}{\mathrm{r}}^{-}$
- Carbocation — carbon with only six electrons and a positive charge, $\mathrm{s}{\mathrm{p}}^2$ hybridised and planar; an electrophile
- Carbanion — carbon with eight electrons, including a lone pair, and a negative charge, $\mathrm{s}{\mathrm{p}}^3$ hybridised and pyramidal; a nucleophile

Stability orders:

- Carbocations: tertiary > secondary > primary > methyl, because alkyl groups donate electrons by the +I effect and hyperconjugation
- Carbanions: methyl > primary > secondary > tertiary, because alkyl groups push more electrons onto an already negative carbon

Electrophiles and nucleophiles. Electrophiles such as ${\mathrm{H}}^{+}$, $\mathrm{N}{\mathrm{O}}_2^{+}$ and carbocations seek electrons; nucleophiles such as $\mathrm{O}{\mathrm{H}}^{-}$, $\mathrm{C}{\mathrm{N}}^{-}$, $\mathrm{N}{\mathrm{H}}_3$ and carbanions donate an electron pair.

An everyday example. Cooking oil left in a hot kitchen turns rancid partly through free radical reactions, as oxygen attacks the oil molecules in a chain of radical steps.

The substance. Carbanions follow the reverse stability order of carbocations — the same electron-donating alkyl groups that calm a positive charge make a negative charge worse.

Organic compounds are purified by crystallisation, sublimation, distillation or chromatography, chosen according to their physical properties, and their elements are detected by converting them into simple ions, as in Lassaigne's test.

Purification methods:

- Crystallisation — dissolving in a hot solvent and cooling, as for impure benzoic acid in hot water
- Simple distillation — for liquids with widely different boiling points, such as chloroform and aniline
- Fractional distillation — for liquids with close boiling points, using a fractionating column
- Steam distillation — for steam-volatile liquids that do not mix with water, such as aniline
- Chromatography — separating components by their different adsorption or partition between a stationary and a mobile phase; in thin-layer chromatography each spot has a retention factor, $R_f=\frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$

Detecting elements:

- Carbon and hydrogen — heating with copper(II) oxide gives carbon dioxide, which turns lime water milky, and water, which turns anhydrous copper sulphate blue
- Lassaigne's test — fusing with sodium converts nitrogen, sulphur and halogens into NaCN, $\mathrm{N}{\mathrm{a}}_2\mathrm{S}$ and NaX
- Nitrogen — the extract boiled with iron(II) sulphate and acidified gives Prussian blue
- Sulphur — sodium nitroprusside gives a violet colour, or lead acetate a black precipitate
- Halogens — after boiling with nitric acid, silver nitrate gives white AgCl, pale yellow AgBr or yellow AgI

An everyday example. The refinery complex at Jamnagar separates crude oil into petrol, kerosene and diesel by fractional distillation, the same principle used in a school laboratory.

The substance. Steam distillation lets a liquid distil below its own boiling point — the liquid and water together reach atmospheric pressure at a lower temperature than either would alone.