Bonding and reactivity in carbon compounds

MDCAT Chemistry · Chemistry · Fundamental principles of organic chemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Carbon always forms four bonds, but it can mix its orbitals in three ways. You find the hybridisation of a carbon atom by looking at its bonds: only single bonds means sp³, one double bond means sp², and one triple bond (or two double bonds) means sp.

Organic reactions start when a covalent bond breaks. In homolytic fission each atom keeps one electron, so two free radicals form. In heterolytic fission one atom takes both electrons, so a positive ion and a negative ion form.

The species that attack are named by what they look for. An electrophile accepts an electron pair. A nucleophile donates an electron pair.

Rules to remember

sp³ carbon: 4 σ bonds, tetrahedral, 109.5° (methane, ethane)
sp² carbon: 3 σ bonds + 1 π bond, trigonal planar, 120° (ethene, benzene, C=O)
sp carbon: 2 σ bonds + 2 π bonds, linear, 180° (ethyne, the carbon of C≡N)
Homolytic fission: A–B → A• + B•. It is favoured by non-polar bonds, ultraviolet light or heat
Heterolytic fission: A–B → A⁺ + B⁻. It is favoured by polar bonds and polar solvents; the more electronegative atom takes both electrons
Electrophiles are positive or electron-deficient: H⁺, NO₂⁺, Br⁺, AlCl₃, BF₃, SO₃
Nucleophiles have a lone pair to give: OH⁻, CN⁻, Cl⁻, NH₃, H₂O

Common mistake

Students think a nucleophile must carry a negative charge. Neutral molecules with a lone pair, such as NH₃ and H₂O, are nucleophiles too, and neutral molecules that are short of electrons, such as AlCl₃ and BF₃, are electrophiles.

Worked example

State the hybridisation of each carbon atom, from left to right, in CH₃–CH=CH–C≡CH.

Carbon 1 (CH₃) has only single bonds, so it is sp³.
Carbons 2 and 3 each have one double bond, so each is sp².
Carbons 4 and 5 are joined by a triple bond, so each is sp.

Answer: sp³, sp², sp², sp, sp

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