Functional Groups of Organic Compounds and Key Organic Reaction Mechanisms

Types of Organic Reactions

Overview of Organic Reactions

Organic reactions are chemical transformations involving organic compounds, which are molecules containing carbon. Understanding the main types of organic reactions is fundamental to organic chemistry, as these reactions form the basis for the synthesis and transformation of organic molecules.

• Substitution Reactions (SN1 and SN2): Replacement of one atom or group by another.

• Elimination Reactions: Removal of two substituents from a molecule, forming a multiple bond.

• Addition Reactions: Addition of atoms or groups to a molecule, typically across a multiple bond.

• Oxidation Reactions: Increase in the oxidation state of carbon, often by adding oxygen or removing hydrogen.

Elimination Reactions

General Mechanism

Elimination reactions involve the removal of two substituents from a molecule, resulting in the formation of a double or triple bond. The two main mechanisms are E1 (unimolecular) and E2 (bimolecular).

• E1 Reaction: Two-step mechanism; formation of a carbocation intermediate.

• E2 Reaction: One-step mechanism; simultaneous removal of a proton and a leaving group.

Elimination (E1) Reaction

• Mechanism: Occurs in two steps:

  1. Leaving group departs, forming a carbocation intermediate.

  2. Base removes a proton from the adjacent carbon, forming a double bond.

• Rate Law: $\text{Rate} = k[\text{RX}]$ (first-order, unimolecular)

• Carbocation Stability: Tertiary > Secondary > Primary > Methyl

Energy Diagram: The rate-determining step is the loss of the leaving group, which has the highest activation energy.

Elimination (E2) Reaction

• Mechanism: Single-step, concerted process where the base removes a proton as the leaving group departs.

• Rate Law: $\text{Rate} = k[\text{RX}][\text{Base}]$ (second-order, bimolecular)

• Substrate Reactivity: Tertiary > Secondary > Primary

• Base Strength: Strong bases are required; weak bases are not favorable.

• Stereochemistry: Base and leaving group must be antiperiplanar (on the same plane but opposite sides).

• Zaitsev's Rule: The most substituted alkene is the major product unless a bulky base is used (Hofmann product).

Comparison of E1 and E2 Reactions

• Base Strength: E2 favored by strong bases, E1 by weak bases.

• Rate Dependence: E2 depends on both substrate and base; E1 depends only on substrate.

• Competition: Strong, bulky bases (e.g., tert-butoxide) favor E2 over SN2.

Base	Relative Reaction Rate	$pK_a$ of Base-H
Phenol	1	4.75
Acetate	5	5.2
Water	40	15.7
Ethanol	60	16.0
t-Butoxide	353	18.0

Additional info: Table shows that stronger bases (higher $pK_a$) lead to faster E2 reactions.

Addition Reactions

Addition Reaction of Alkenes

Addition reactions involve the addition of atoms or groups to the carbon atoms of a double or triple bond. Alkenes react with hydrogen halides (HBr, HCl, HI), halogens, water, and other reagents.

• Markovnikov's Rule: In the addition of HX to an alkene, the hydrogen attaches to the carbon with more hydrogens, and the halide attaches to the more substituted carbon.

• Mechanism: Typically a two-step process involving a carbocation intermediate.

Energy Diagram: Shows two transition states and a carbocation intermediate. The first step (carbocation formation) is rate-determining.

• Anti-Markovnikov Addition: In the presence of peroxides, HBr adds to alkenes via a free radical mechanism, resulting in the halide attaching to the less substituted carbon.

Addition Reaction of Alkynes

• Halogenation: Alkynes react with halogens to form dihaloalkenes and tetrahaloalkanes.

• Hydrogenation: Alkynes react with H2 (Pt/Pd/Ni catalyst) to form alkenes, then alkanes.

• Hydration: Alkynes react with water (H2SO4/HgSO4 catalyst) to form carbonyl compounds via keto-enol tautomerism.

• Hydrohalogenation: Alkynes react with HX to form geminal dihalides (both halogens on the same carbon).

Addition Reaction of Carbonyl Group

Nucleophilic Addition to Carbonyls (Aldehydes and Ketones)

• Mechanism: Nucleophile attacks the electrophilic carbonyl carbon, changing its hybridization from sp2 to sp3.

• Irreversible Addition: Strong nucleophiles (e.g., hydride, alkyl groups) lead to irreversible addition (e.g., NaBH4 reduction).

• Reversible Addition: Weaker nucleophiles (e.g., water, alcohols, cyanide) lead to reversible addition.

• Electronic Effects: Electron-withdrawing groups increase reactivity; electron-donating groups decrease reactivity.

• Steric Effects: Bulky groups near the carbonyl carbon hinder nucleophilic attack.

Properties of the Carbonyl (C=O) Group

• Electrophilicity: The carbonyl carbon is highly electrophilic due to the polarization of the C=O bond (oxygen is more electronegative).

• Resonance: The resonance hybrid shows partial positive charge on carbon and partial negative on oxygen.

Electrophilic Aromatic Substitution (EAS)

General Mechanism

In EAS, an aromatic ring reacts with an electrophile, replacing a hydrogen atom with the electrophile. The reaction proceeds via a resonance-stabilized carbocation intermediate (arenium ion).

• Key Steps: Formation of the arenium ion (slow, rate-determining), followed by deprotonation to restore aromaticity.

• Common EAS Reactions: Chlorination, bromination, nitration, sulfonation, Friedel-Crafts alkylation, Friedel-Crafts acylation.

• Acid Catalyst: Required to generate a strong enough electrophile for reaction with the aromatic ring.

Step	Bond Broken	Bond Formed
1. Electrophile attack	C-H	C-E
2. Deprotonation	H	Restores aromaticity

Summary Table: Key Organic Reaction Types

Reaction Type	Main Mechanism	Key Features
Elimination (E1)	Two-step, carbocation intermediate	Unimolecular, first-order, favors tertiary substrates
Elimination (E2)	One-step, concerted	Bimolecular, second-order, strong base required
Addition (Alkene)	Carbocation intermediate	Markovnikov/Anti-Markovnikov, two-step
Addition (Alkyne)	Stepwise, via alkene intermediate	Halogenation, hydrogenation, hydration, hydrohalogenation
Nucleophilic Addition (Carbonyl)	Nucleophile attacks C=O	Irreversible/reversible, affected by electronic/steric effects
Electrophilic Aromatic Substitution	Arenium ion intermediate	Requires acid catalyst, restores aromaticity