Chemistry (university) · Part 10
Organic reactions and mechanisms
Curved arrows, substitution, elimination, addition, aromatic and carbonyl reactions.
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- Nucleophiles, electrophiles and curved arrowsA nucleophile is electron-rich and donates a pair of electrons. An electrophile is electron-poor and accepts a pair. A curved arrow shows a pair of electrons moving, from the nucleophile toward the electrophile.
- SN2 reactionSN2 is substitution, nucleophilic, bimolecular. The nucleophile attacks from the back as the leaving group departs, all in one step. Rate = k[substrate][nucleophile]. Stereochemistry is inverted. Methyl and primary carbons react fastest.
- SN1 reactionSN1 is substitution, nucleophilic, unimolecular. The leaving group first departs on its own, forming a carbocation. The nucleophile then attacks. Rate = k[substrate] only. Tertiary substrates react fastest, in polar solvents, and the product is mostly racemic.
- Carbocation stabilityA carbocation is a carbon with three bonds and a positive charge. More alkyl groups on the carbon make it more stable by pushing electron density to it (inductive effect) and by sharing electrons from nearby C–H bonds (hyperconjugation). Order: tertiary > secondary > primary > methyl.
- E2 eliminationIn E2 elimination, a base removes a hydrogen while the leaving group departs from the next carbon, forming a double bond in one step. Rate = k[substrate][base]. The H and the leaving group must be anti to each other. The more substituted alkene usually dominates (Zaitsev's rule).
- Electrophilic addition to alkenesThe pi bond of an alkene is electron-rich and attacks electrophiles such as H–Br. H⁺ adds first, forming a carbocation, and then Br⁻ adds. Markovnikov's rule: H goes to the carbon that already has more hydrogens, so the more stable cation forms.
- Electrophilic aromatic substitutionBenzene's ring is very stable because its electrons are spread around the ring (aromatic). It reacts by substitution, not addition, so the ring survives. An electrophile attacks, a cation forms, then H⁺ is lost to restore the ring.
- Nucleophilic addition to carbonylsThe carbon of a C=O group is partly positive, so nucleophiles attack it, pushing the pi electrons onto oxygen. This gives an alkoxide, which picks up H⁺ to become an alcohol. Aldehydes react faster than ketones.
- Nucleophilic acyl substitutionA carboxylic acid derivative has a leaving group on a carbonyl carbon. A nucleophile adds to C=O, then the C=O reforms and the leaving group leaves. Acid chlorides are most reactive, then anhydrides, esters, amides.
- Oxidation of alcoholsOxidising an alcohol removes hydrogen and adds a C=O. A primary alcohol gives an aldehyde, and with strong oxidant a carboxylic acid. A secondary alcohol gives a ketone. A tertiary alcohol does not oxidise easily because it has no hydrogen on the carbon.