Fission of a Covalent Bond

All organic molecules are built from covalent bonds, formed by two atoms mutually sharing a pair of electrons. Since a reaction is fundamentally about breaking old bonds and making new ones, understanding organic reaction mechanisms starts with understanding exactly how a covalent bond can break.

A covalent bond can break in only two fundamentally different ways:

• Homolytic cleavage (symmetrical splitting) -- each atom keeps one of the two bonding electrons.
• Heterolytic cleavage (unsymmetrical splitting) -- one atom keeps both bonding electrons, and the other keeps none. …

Homolytic cleavage is the process in which a covalent bond breaks symmetrically, so that each of the two previously bonded atoms keeps exactly one of the shared electrons. Because this splits one bonding pair into two separate, unpaired electrons, it is denoted with a special notation: a half-headed arrow (a 'fish-hook' arrow, with only a single barb), used precisely because only a single electron -- not a pair -- is moving with each fish-hook.

When it happens. Homolytic cleavage needs an energetic push: it occurs under high temperature or in the presence of UV light, and it happens most readily in bonds that are non-polar -- i.e. formed between atoms of similar electronegativity, where there is no built-in pull of the electron pair toward one side. Since there is no electronegativity difference to guide the electrons to one atom, the only symmetric, 'fair' outcome is for each atom to keep one electron.

What it produces. Homolysis of this kind generates free radicals -- species with an unpaired electron. Free radicals are short-lived and highly reactive (an unpaired electron is inherently unstable and 'wants' to pair up), and this reactivity is exactly what makes them useful as reaction intermediates.

Free-radical initiators. Reagents specifically chosen to trigger homolytic cleavage in a substrate are called free-radical initiators. Two classic examples are:

• AIBN (azobisisobutyronitrile)
• Peroxides, such as benzoyl peroxide

Both decompose homolytically (often photochemically, i.e. under UV light) to generate radicals that then go on to start chain reactions -- most importantly, polymerisation reactions, where the initial radical adds to a monomer's double bond and the resulting new radical adds to the next monomer, and so on.

Worked example -- photolysis of benzoyl peroxide. Benzoyl peroxide, $C_6H_5$-CO-O-O-CO-$C_6H_5$, contains a relatively weak O-O (peroxide) linkage. Under UV light ($h\nu$), this O-O bond cleaves homolytically -- each oxygen keeps one electron -- giving two benzoyloxy radicals. These are themselves unstable and immediately lose $CO_2$ (decarboxylate), so the net effect is two phenyl free radicals plus two molecules of $CO_2$. These phenyl radicals are what actually initiate the polymerisation.

Stability order of alkyl free radicals. Organic reactions frequently involve homolytic fission of C-C bonds to give alkyl free radicals, and their relative stability follows a now-familiar pattern -- more alkyl substitution means more stability:

(tertiary > secondary > primary > methyl). The reasoning is the same electron-donating (hyperconjugative and inductive) stabilisation that alkyl groups provide to carbocations, discussed later in this unit -- each additional alkyl group around the radical centre helps disperse the destabilising effect of the unpaired electron. …

Heterolytic cleavage is the process in which a covalent bond breaks unsymmetrically -- one of the two bonded atoms keeps both electrons of the bonding pair, while the other keeps none. Of the two atoms, the more electronegative one is the one that ends up keeping the electron pair, becoming an anion; the other atom, left electron-deficient, becomes a cation. This is denoted with a curved arrow pointing toward the more electronegative atom -- the destination of the departing electron pair.

Worked example -- hydrolysis of tert-butyl bromide (C-Br heterolysis). In tert-butyl bromide, $(CH_3)_3C$-Br, the C-Br bond is polar because bromine is considerably more electronegative than carbon. The bonding electrons sit closer to bromine (marked $\delta^-$ on Br, $\delta^+$ on the attached carbon). On reaction with water, the C-Br bond breaks heterolytically: bromine departs with both bonding electrons as $Br^-$, and the carbon -- now with only three bonds and no share of that electron pair -- becomes the electron-deficient, planar tert-butyl cation, $(CH_3)_3C^+$.

Worked example -- carbanion formation from a C-H bond (aldol condensation). Now consider heterolysis in the other direction, at a carbon-hydrogen bond. Carbon is more electronegative than hydrogen, so heterolytic cleavage of a C-H bond leaves the bonding electrons on carbon, generating a carbanion (a carbon atom bearing a formal negative charge). In aldol condensation, hydroxide ion ($OH^-$) abstracts an $\alpha$-hydrogen from an aldehyde: the C-H bond breaks, the hydrogen leaves as part of a newly formed water molecule, and the bonding electron pair stays behind on the $\alpha$-carbon, generating the resonance-stabilised enolate-type carbanion that goes on to attack a second aldehyde molecule in the aldol mechanism.

Hybridisation and shape of a carbocation. In a carbocation, the positively-charged carbon is $sp^2$ hybridised, and so the ion is planar (trigonal) in shape: three $sp^2$ orbitals form ordinary sigma bonds to the three attached groups, and the remaining, unhybridised p-orbital is empty (this empty p-orbital is precisely why the carbon is electron-deficient and positively charged). Because this empty p-orbital has two lobes, one above and one below the plane of the three sigma bonds, a negatively-charged nucleophile attacking the carbocation can approach from either face with equal ease.

Shape of a carbanion. A carbanion is, by contrast, generally pyramidal: the carbon is $sp^3$ hybridised, and the extra lone pair of electrons (the source of the negative charge) occupies one of the four $sp^3$ orbitals, exactly as a lone pair occupies an $sp^3$ orbital on nitrogen in ammonia.

Shape of a free radical. An alkyl free radical is structurally intermediate between the two: it may be either pyramidal or planar. When it adopts the planar arrangement, it closely resembles the carbocation's $sp^2$ geometry, except that the p-orbital holds a single unpaired electron rather than being completely empty.

Relative stability orders. Putting the three species side by side for a simple set of alkyl substituents (methyl through tertiary-butyl) reveals a consistent pattern: alkyl groups stabilise an electron-deficient centre (by donating electron density inductively and hyperconjugatively) but destabilise an electron-rich centre (by concentrating even more negative charge onto an already electron-rich carbon). So the orders run in opposite directions for cations/radicals versus anions: …