Organic Reactions — AI Study Guide

Mastering Organic Reactions

Organic reactions are classified by mechanism type, which predicts patterns of reactivity across different substrates and reagents. The four major reaction types are: substitution (one group replaced by another), elimination (atoms removed to form a double bond), addition (atoms added across a double bond), and oxidation-reduction (change in oxidation state). Learning these categories and their characteristics is more valuable than memorizing individual reactions in isolation.

Nucleophilic substitution reactions (SN1 and SN2) involve replacement of a leaving group by a nucleophile. SN2 reactions are bimolecular, proceed via backside attack (inverting stereochemistry), favor primary substrates and strong nucleophiles, and show second-order kinetics. SN1 reactions are unimolecular, proceed via carbocation intermediate (racemization possible), favor tertiary substrates and polar protic solvents, and show first-order kinetics. Understanding which mechanism operates under given conditions is essential.

Elimination reactions (E1 and E2) compete with substitution. E2 reactions are bimolecular, require anti-periplanar geometry between the leaving group and the beta-hydrogen, favor bulky strong bases, and give Zaitsev (more substituted) or Hofmann (less substituted) products depending on conditions. E1 reactions proceed via carbocation intermediate and are favored by weak bases and tertiary substrates. Temperature, base strength, and substrate structure determine substitution/elimination competition.

Addition reactions across double and triple bonds include electrophilic addition (to alkenes via carbocation intermediates), nucleophilic addition (to carbonyls), and radical addition. Markovnikov's rule for electrophilic addition: in HX addition to an asymmetric alkene, the H adds to the carbon with more hydrogens (giving the more stable carbocation intermediate). Anti-Markovnikov addition occurs with radical addition (peroxide conditions) or hydroboration-oxidation (syn addition, anti-Markovnikov).

Frequently Asked Questions: Organic Reactions

How do I predict SN1 vs SN2?

Key factors: Substrate: primary → SN2; tertiary → SN1; secondary → either, depending on conditions. Nucleophile: strong, non-bulky nucleophile → SN2; weak or hindered → SN1. Solvent: polar aprotic (acetone, DMF, DMSO) → SN2; polar protic (water, alcohols) → SN1. Leaving group quality affects rate of both. In general: primary substrates undergo SN2; tertiary substrates undergo SN1 (unless poor leaving group or too hindered even for SN2); secondary substrates require considering all factors.

What is Markovnikov's rule?

Markovnikov's rule states that in the addition of HX (or H2O with H+) to an asymmetric alkene, the hydrogen adds to the carbon bearing more hydrogen atoms, and the X (or OH) adds to the carbon bearing fewer hydrogens. Mechanistically, this reflects the formation of the more stable (more substituted) carbocation intermediate. Anti-Markovnikov addition (H adds to the less-hydrogen-bearing carbon) occurs with radical mechanisms or hydroboration-oxidation.

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