Lesson 6 of 13 · 7 min
Bond cleavage and reactive intermediates
NCERT § "Fundamental Concepts in Organic Reaction Mechanism"
When acetic acid dissolves in water, its O–H bond breaks and the bonding pair stays with oxygen. When sunlight hits a chlorine molecule, the Cl–Cl bond breaks and the pair is split, one electron each. Two ways to break a bond, two very different products.
The lesson in notes
In short
In an organic reaction an attacking reagent reacts with the substrate; the bond to the carbon under attack breaks and a new one forms.
Heterolytic cleavage leaves both bonding electrons on one fragment, giving ions; homolytic cleavage gives one electron to each fragment, forming free radicals.
A carbocation has a carbon with only six valence electrons and a positive charge; it is sp² hybridised and trigonal planar, with an empty p orbital perpendicular to the plane.
Carbocation stability: (CH₃)₃C⁺ (3°) > (CH₃)₂CH⁺ (2°) > CH₃CH₂⁺ (1°) > CH₃⁺, because alkyl groups release electrons by inductive and hyperconjugation effects.
A carbanion carries a negative charge on carbon with eight valence electrons; it is sp³ hybridised and pyramidal.
Free radicals have an unpaired electron; alkyl radicals follow the stability order tertiary > secondary > primary > methyl.
A nucleophile (electron-pair donor, 'nucleus-seeking') attacks electron-poor centres: OH⁻, CN⁻, carbanions, and neutral molecules with lone pairs such as H₂O and R₃N.
An electrophile (electron-seeking) attacks electron-rich centres: carbocations, and neutral species with an electron-deficient atom such as the carbonyl carbon or the carbon bonded to halogen in an alkyl halide.