Organic Chemistry Mechanisms: Key Topics for Final Exam (Chapters 13, 14, 18)

Organic Reaction Mechanisms

Overview

This guide summarizes essential organic reaction mechanisms relevant to Chapters 13, 14, and 18, as highlighted for exam preparation. These mechanisms cover the addition, elimination, substitution, and oxidation reactions of alkenes, alkynes, and related compounds.

Addition of HX to Alkenes (Including Rearrangements)

• Definition: Addition of hydrogen halides (HX) to alkenes forms alkyl halides via electrophilic addition.

• Mechanism: Follows Markovnikov's rule; carbocation rearrangements may occur.

• Equation: $RCH=CH_2 + HX \rightarrow RCHX-CH_3$

• Example: Addition of HBr to propene yields 2-bromopropane.

Hydration of Alkenes

• Definition: Addition of water to alkenes to form alcohols, typically acid-catalyzed.

• Mechanism: Electrophilic addition via carbocation intermediate.

• Equation: $RCH=CH_2 + H_2O \xrightarrow{H^+} RCH(OH)-CH_3$

• Example: Hydration of ethene produces ethanol.

1,2-Addition to Alkenes (Vicinal Dihalide Formation)

• Definition: Addition of halogens (e.g., Br2) to alkenes forms vicinal dihalides.

• Mechanism: Anti addition via bromonium ion intermediate.

• Equation: $RCH=CH_2 + Br_2 \rightarrow RCHBr-CH_2Br$

• Example: Bromination of cyclohexene yields trans-1,2-dibromocyclohexane.

Addition of HX to Alkynes

• Definition: Alkynes react with HX to form vinyl halides and then geminal dihalides.

• Mechanism: Follows Markovnikov's rule; two-step addition possible.

• Equation: $RC\equiv CH + 2HX \rightarrow RCHX_2-CH_3$

• Example: Addition of HBr to 1-butyne yields 2,2-dibromobutane.

Addition of CuC and Hg to Alkynes and Alkenes

• Definition: Catalytic addition reactions using copper or mercury salts.

• Application: Hydration of alkynes with HgSO4 yields ketones (Markovnikov addition).

• Equation: $RC\equiv CH + H_2O \xrightarrow{HgSO_4, H_2SO_4} RC(O)-CH_3$

Formation of Halohydrins

• Definition: Reaction of alkenes with halogen and water forms halohydrins.

• Mechanism: Halogen adds first, followed by water attack.

• Equation: $RCH=CH_2 + Br_2 + H_2O \rightarrow RCH(OH)-CH_2Br$

Epoxidation

• Definition: Formation of epoxides from alkenes using peracids (e.g., mCPBA).

• Equation: $RCH=CH_2 + RCO_3H \rightarrow RCH(O)-CH_2$ (epoxide ring)

• Example: Epoxidation of styrene yields styrene oxide.

Enol-Keto Tautomerization

• Definition: Interconversion between enol and keto forms, especially in carbonyl chemistry.

• Equation: $RCH=CH(OH) \rightleftharpoons RC(O)-CH_3$

Electrophilic Aromatic Substitution (EAS)

• Definition: Substitution of an aromatic hydrogen by an electrophile.

• Types: Nitration, sulfonation, halogenation, Friedel-Crafts alkylation/acylation.

• Equation: $C_6H_6 + E^+ \rightarrow C_6H_5E + H^+$

• Example: Nitration of benzene yields nitrobenzene.

Nucleophilic Aromatic Substitution

• Definition: Substitution of an aromatic group by a nucleophile, often requiring electron-withdrawing groups.

• Equation: $C_6H_5X + Nu^- \rightarrow C_6H_5Nu + X^-$

Oxidation of Alkenes (KMnO4, OsO4)

• Definition: Oxidative cleavage or dihydroxylation of alkenes.

• Equation: $RCH=CH_2 + KMnO_4 \rightarrow RCOOH + CO_2$ (cleavage)

• Equation: $RCH=CH_2 + OsO_4 \rightarrow RCH(OH)-CH_2(OH)$ (dihydroxylation)

Wittig Reaction

• Definition: Formation of alkenes from aldehydes/ketones and phosphonium ylides.

• Equation: $R_2C=O + Ph_3P=CR'_2 \rightarrow R_2C=CR'_2 + Ph_3P=O$

Oxidative Cleavage, Alkenes and Alkynes (KMnO4 and O3)

• Definition: Cleavage of double or triple bonds to form carbonyl compounds.

• Equation: $RCH=CH_2 + O_3 \rightarrow RCHO + HCHO$

Formation and Reaction of Acetylide Ions

• Definition: Deprotonation of terminal alkynes forms acetylide ions, which act as nucleophiles.

• Equation: $RC\equiv CH + NaNH_2 \rightarrow RC\equiv C^-Na^+ + NH_3$

• Application: Used in C–C bond formation via alkylation.

Additional info: These mechanisms are foundational for understanding organic synthesis and reactivity, and are commonly tested in college-level Organic Chemistry courses.