Unit 3: Types of Reactions

Unit 3: Types of Reactions

Introduction to Organic Reaction Mechanisms

Organic reaction mechanisms describe the step-by-step sequence of bond-breaking and bond-forming events that occur during a chemical transformation. Understanding mechanisms is essential for predicting reaction outcomes, designing synthetic routes for drug molecules, and understanding drug metabolism and degradation pathways.

Key Concepts in Reaction Mechanisms:

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1. Oxidation Reactions

Definition

Oxidation in organic chemistry involves:

• Increase in oxygen or decrease in hydrogen
• Loss of electrons
• Increase in oxidation state of carbon

Common Oxidation Reactions in Pharmaceuticals

1. Oxidation of Alcohols

Mechanism of Chromium-Based Oxidation:

• Formation of chromate ester
• Elimination via E2-like mechanism
• Carbonyl product formed

Pharmaceutical Example: Oxidation of secondary alcohols in steroid synthesis to produce ketone-containing hormones.

2. Oxidation of Aldehydes to Carboxylic Acids

Aldehydes are easily oxidized to carboxylic acids by:

• Tollens' reagent [Ag(NH₃)₂]⁺ (silver mirror test)
• Fehling's solution (Cu²⁺ in tartrate)
• KMnO₄, K₂Cr₂O₇
• Air oxidation (autoxidation)

Mechanism: Nucleophilic addition of water to aldehyde gives gem-diol, which is oxidized.

Pharmaceutical Relevance: Aldehyde-containing drugs may oxidize during storage, affecting stability. The body oxidizes aldehydes to carboxylic acids in metabolism (e.g., alcohol metabolism).

3. Oxidation of Alkenes

Mechanism of Epoxidation: Electrophilic addition of oxygen to double bond via "butterfly" mechanism.

Pharmaceutical Example: Epoxidation of alkenes in drug metabolism (CYP450 enzymes) can lead to reactive intermediates.

4. Oxidation of Aromatic Compounds

Pharmaceutical Example: Oxidation of toluene side chain in synthesis of benzoic acid preservatives.

5. Baeyer-Villiger Oxidation

Definition: Oxidation of ketones to esters or lactones using peracids or peroxides.

Mechanism: Nucleophilic addition of peracid to carbonyl, followed by migration of one group to oxygen (migratory aptitude: tertiary alkyl > secondary > benzyl > primary > methyl).

Pharmaceutical Application: Synthesis of lactone-containing drugs (e.g., statins, macrolide antibiotics). Also involved in drug metabolism (CYP450-catalyzed oxidations).

6. Oxidation in Drug Metabolism

The body's cytochrome P450 enzymes catalyze various oxidations:

• Hydroxylation of aliphatic and aromatic carbons
• Epoxidation of alkenes
• N-, O-, and S-dealkylation
• Oxidation of alcohols and aldehydes
• Oxidative deamination

Pharmaceutical Significance:

• Converts lipophilic drugs to more water-soluble metabolites for excretion
• Can produce active metabolites (prodrug activation)
• May generate reactive intermediates causing toxicity

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2. Reduction Reactions

Definition

Reduction in organic chemistry involves:

• Decrease in oxygen or increase in hydrogen
• Gain of electrons
• Decrease in oxidation state of carbon

Common Reducing Agents

Common Reduction Reactions

1. Reduction of Carbonyl Compounds

Mechanism of Hydride Reduction:

• Nucleophilic attack by H⁻ on electrophilic carbonyl carbon
• Formation of alkoxide intermediate
• Protonation gives alcohol

Pharmaceutical Application: Synthesis of chiral alcohols for drugs like atorvastatin (Lipitor).

2. Hydrogenation (Reduction of Multiple Bonds)

Mechanism of Catalytic Hydrogenation:

• Adsorption of H₂ on metal surface
• Adsorption of substrate
• Stepwise transfer of H atoms
• Desorption of product

Pharmaceutical Application: Large-scale hydrogenation in drug manufacturing (e.g., reduction of nitro groups in antibiotic synthesis).

3. Reductive Amination

Definition: One-pot conversion of aldehydes or ketones to amines using ammonia or amines and a reducing agent.

Mechanism:

1. Carbonyl + amine → imine (or iminium ion)
2. Imine reduced to amine by NaBH₃CN, NaBH₄, or H₂/catalyst

Pharmaceutical Significance:

• Most common method for synthesizing amine drugs
• Used in synthesis of antidepressants, antihistamines, etc.
• Mimics biological transamination reactions

4. Clemmensen Reduction

Definition: Reduction of aldehydes or ketones to methylene (CH₂) using zinc amalgam and HCl.

Mechanism: Involves carbenoid intermediates on metal surface.

Application: Conversion of ketones to alkyl chains in steroid synthesis.

5. Wolff-Kishner Reduction

Definition: Reduction of aldehydes or ketones to methylene using hydrazine and strong base (KOH) at high temperature.

Mechanism: Formation of hydrazone, then deprotonation, loss of N₂, and protonation.

Application: Reduction of ketones to alkanes where acidic conditions (Clemmensen) would damage sensitive groups.

6. Birch Reduction

Definition: Reduction of aromatic rings to 1,4-cyclohexadienes using Na or Li in liquid NH₃ with an alcohol.

Mechanism: Dissolving metal reduction via radical anion intermediates.

Application: Synthesis of partially reduced aromatic compounds for drug intermediates.

7. Reduction in Drug Metabolism

• Azo reduction: By gut microflora, activates certain prodrugs
• Nitro reduction: To amines (can be toxic or therapeutic)
• Carbonyl reduction: To alcohols (phase I metabolism)
• Disulfide reduction: Cleaves disulfide bonds in proteins

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3. Acylation Reactions

Definition

Acylation is the introduction of an acyl group (R–CO–) into a molecule. It is a fundamental reaction in pharmaceutical synthesis for introducing amide, ester, and ketone functionalities.

Common Acylating Agents

Types of Acylation Reactions

1. Acylation of Alcohols (Esterification)

R–OH + R'–CO–X → R'–COO–R + HX

Mechanism (with acid chloride) :

• Nucleophilic attack of alcohol on carbonyl
• Tetrahedral intermediate
• Elimination of leaving group

Pharmaceutical Examples:

• Aspirin synthesis: Acetylation of salicylic acid with acetic anhydride
• Prodrug formation: Esterification of carboxylic acid drugs to improve bioavailability

2. Acylation of Amines (Amide Formation)

R–NH₂ + R'–CO–X → R'–CONH–R + HX

Mechanism: Similar to esterification; amines are more nucleophilic than alcohols.

Pharmaceutical Examples:

• Paracetamol synthesis: Acetylation of p-aminophenol
• Peptide bond formation in protein synthesis
• Penicillin structure contains amide bonds

3. Friedel-Crafts Acylation

Definition: Introduction of acyl group onto aromatic rings using acid chloride and Lewis acid catalyst (AlCl₃).

Ar–H + R–COCl + AlCl₃ → Ar–CO–R + HCl + AlCl₃

Mechanism:

1. Formation of acylium ion (R–C⁺=O)
2. Electrophilic attack on aromatic ring
3. Loss of proton to restore aromaticity

Pharmaceutical Examples:

• Synthesis of aromatic ketones (e.g., propiophenone derivatives)
• Intermediate for many drug syntheses

4. Acylation of Carbon Nucleophiles

Enolates and other carbanions undergo acylation to give β-diketones, β-keto esters, etc. (Claisen condensation).

Pharmaceutical Application: Synthesis of β-diketone antibiotics (tetracyclines).

Pharmaceutical Significance of Acylation

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4. Esterification

Definition

Esterification is the formation of esters from carboxylic acids and alcohols. It is a specific type of acylation.

Fischer Esterification

Reaction: R–COOH + R'–OH ⇌ R–COO–R' + H₂O

Conditions: Acid catalyst (conc. H₂SO₄, HCl, or p-toluenesulfonic acid), heat, often with removal of water to drive equilibrium.

Mechanism:

Factors Affecting Fischer Esterification:

• Alcohol structure: Reactivity: 1° > 2° > 3° (steric hindrance)
• Acid structure: Less hindered acids react faster
• Equilibrium: Can be shifted by using excess reagent or removing water

Other Esterification Methods

Pharmaceutical Applications of Esterification

1. Prodrug Design

Esterification of carboxylic acid or alcohol drugs to improve:

• Oral absorption (increased lipophilicity)
• Masking of unpleasant taste
• Prolonged duration of action
• Site-specific delivery

Examples:

• Enalapril: Ethyl ester prodrug of enalaprilat (ACE inhibitor)
• Bambuterol: Prodrug of terbutaline (bronchodilator)
• Oseltamivir (Tamiflu): Ethyl ester prodrug
• Aspirin: Acetyl ester of salicylic acid (improves GI tolerance)

2. Drug Stability

Esterification can protect labile groups from oxidation or other degradation.

3. Formulation

Esters are used as:

• Solvents (ethyl acetate, isopropyl myristate)
• Plasticizers in coatings
• Penetration enhancers in topical formulations

4. Metabolism

Ester hydrolysis (by esterases) is a common metabolic pathway, often activating prodrugs or inactivating drugs.

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5. Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve replacement of a leaving group (L) by a nucleophile (Nu). They are among the most important reactions in organic synthesis and drug metabolism.

Types: SN1 and SN2

SN1 Mechanism

Steps:

1. Slow step (rate-determining) : Leaving group departs, forming carbocation.
2. Fast step: Nucleophile attacks carbocation.
3. Deprotonation (if nucleophile was neutral).

Characteristics:

• Carbocation stability determines rate: 3° > 2° > 1° > methyl
• Racemization at chiral centers (attack from both sides)
• Rearrangements possible via hydride or alkyl shifts
• Favored by polar protic solvents (stabilize carbocation and leaving group)

Pharmaceutical Relevance:

• Some drug degradation pathways (e.g., hydrolysis of tertiary alkyl halides)
• Synthesis of certain drug intermediates

SN2 Mechanism

Characteristics:

• Concerted: bond formation and bond breaking occur simultaneously
• Backside attack by nucleophile
• Inversion of configuration (Walden inversion)
• Steric hindrance slows reaction: CH₃ > 1° > 2° >> 3°
• Favored by polar aprotic solvents (DMF, DMSO, acetone)
• Strong nucleophiles favored

Transition State: Pentacoordinate carbon with partial bonds to nucleophile and leaving group.

Pharmaceutical Relevance:

• Alkylation reactions in drug synthesis
• Methylation of drugs in metabolism
• Synthesis of quaternary ammonium compounds

Nucleophiles and Leaving Groups

Common Nucleophiles (in order of strength) :

Leaving Group Ability: Good leaving groups are weak bases (stable anions): I⁻ > Br⁻ > Cl⁻ >> F⁻ Also good: tosylate (OTs), mesylate (OMs), triflate (OTf)

Pharmaceutical Relevance: In alkylating agents (anticancer drugs), chloride is the leaving group that allows DNA alkylation.

Factors Affecting Nucleophilic Substitution

Pharmaceutical Examples

1. Synthesis of Ethers (Williamson Synthesis) : SN2 reaction of alkoxide with primary alkyl halide.

2. Alkylation of Amines : SN2 reaction of amine with alkyl halide (but overalkylation is a problem).

3. Anticancer Alkylating Agents : Nitrogen mustards (e.g., cyclophosphamide) undergo SN2 reactions with DNA bases, crosslinking DNA.

4. Prodrug Activation : Some prodrugs are activated by SN2 reactions (e.g., with glutathione).

5. Drug Degradation : Hydrolysis of alkyl halides and esters can occur via SN1 or SN2 depending on structure.

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6. Elimination Reactions

Elimination reactions involve loss of two atoms or groups from a molecule to form a multiple bond (usually an alkene).

Types: E1 and E2

E1 Mechanism

Steps:

1. Slow step: Leaving group departs, forming carbocation.
2. Fast step: Base abstracts a β-hydrogen, forming alkene.

Characteristics:

• Same carbocation intermediate as SN1
• Follows Zaitsev's rule (more substituted alkene is major product)
• Rearrangements possible
• Favored by polar protic solvents
• Weak bases favor E1 over E2

E2 Mechanism

Characteristics:

• Concerted: Base abstracts β-H while leaving group departs
• Anti-periplanar geometry required (H and leaving group must be 180° apart)
• Strong base favors E2
• Stereospecific: gives specific alkene stereochemistry
• Follows Zaitsev's rule with small bases; with bulky bases (e.g., tert-butoxide), Hofmann product (less substituted alkene) may dominate

Transition State: Partial bonds to H (being removed) and leaving group, double bond forming.

Zaitsev's Rule vs. Hofmann Rule

Competition: Substitution vs. Elimination

Pharmaceutical Relevance

1. Drug Synthesis: Elimination reactions are used to introduce double bonds into drug molecules (e.g., steroid synthesis).

2. Drug Degradation: Some drugs undergo elimination under basic conditions or at high temperatures, leading to loss of activity.

3. Metabolism: Dehydrogenation (oxidative elimination) is a metabolic pathway catalyzed by enzymes.

4. Prodrug Design: Some prodrugs are designed to undergo elimination to release the active drug (e.g., β-elimination of phosphate prodrugs).

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7. Addition Reactions

Addition reactions occur when two molecules combine to form a single product, typically across a multiple bond (C=C, C≡C, C=O). They are the reverse of elimination reactions.

Types of Addition Reactions

1. Electrophilic Addition to Alkenes

This is the most common addition reaction, where an electrophile adds to the π bond, forming a carbocation that is then attacked by a nucleophile.

General Mechanism:

1. Slow step: Electrophile adds to π bond, forming carbocation.
2. Fast step: Nucleophile attacks carbocation.

Examples:

Markovnikov's Rule: In addition of HX to unsymmetrical alkene, H adds to the carbon with more H atoms (less substituted), and X adds to more substituted carbon. Reason: formation of more stable carbocation.

Anti-Markovnikov addition: Occurs with HBr in presence of peroxides (free radical mechanism).

Pharmaceutical Relevance:

• Synthesis of alcohol, halide, and epoxide intermediates
• Epoxidation in drug metabolism (can form reactive intermediates)
• Hydrogenation used in large-scale drug manufacturing

2. Electrophilic Addition to Alkynes

Similar to alkenes but can add one or two equivalents.

• With one equivalent HX → vinyl halide (Markovnikov)
• With two equivalents HX → geminal dihalide
• With one equivalent X₂ → dihaloalkene (trans usually)
• With two equivalents X₂ → tetrahaloalkane

3. Nucleophilic Addition to Carbonyl Compounds

Aldehydes and ketones undergo nucleophilic addition due to electrophilic carbonyl carbon.

General Mechanism:

1. Nucleophile attacks carbonyl carbon.
2. Tetrahedral intermediate forms.
3. Protonation gives alcohol (or elimination gives derivative).

Examples:

Pharmaceutical Relevance:

• Imine formation is involved in many enzyme mechanisms (e.g., pyridoxal phosphate)
• Schiff bases are intermediates in drug synthesis
• Grignard reactions are fundamental in building complex drug molecules

4. Michael Addition (Conjugate Addition)

Definition: Nucleophilic addition to α,β-unsaturated carbonyl compounds (enones), where addition occurs at the β-carbon.

Mechanism: Nucleophile attacks β-carbon, forming enolate, which is then protonated.

Nucleophiles: Enolates (giving 1,5-dicarbonyls), amines, thiols, etc.

Pharmaceutical Relevance:

• Key reaction in steroid synthesis
• Involved in biosynthesis of natural products
• Some drugs act as Michael acceptors (e.g., certain anticancer agents alkylate proteins via Michael addition)

5. Diels-Alder Reaction

Definition: [4+2] cycloaddition between a conjugated diene and a dienophile (alkene or alkyne) to form a six-membered ring.

Mechanism: Concerted, pericyclic; stereospecific (cis principle retained).

Pharmaceutical Relevance:

• Synthesis of complex natural products (steroids, alkaloids)
• Synthesis of aromatic drug intermediates
• Used in combinatorial chemistry for drug discovery

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8. Rearrangement Reactions

Rearrangement reactions involve migration of an atom or group from one atom to another within the same molecule, resulting in a structural isomer.

Common Rearrangements in Pharmaceutical Chemistry

1. Carbocation Rearrangements

In SN1 and E1 reactions, carbocations may rearrange via hydride shift or alkyl shift to form more stable carbocations.

Example: 3,3-dimethyl-1-butene with HCl gives rearranged product due to hydride shift.

Pharmaceutical Relevance: Explains unexpected products in drug synthesis and metabolism.

2. Pinacol Rearrangement

Definition: Acid-catalyzed rearrangement of vicinal diols (1,2-diols) to carbonyl compounds.

Mechanism: Protonation, loss of water, carbocation formation, migration of alkyl group, deprotonation.

Pharmaceutical Example: Synthesis of steroids and terpenes.

3. Beckmann Rearrangement

Definition: Acid-catalyzed rearrangement of ketoximes to N-substituted amides.

Mechanism: Protonation of oxime OH, migration of anti group to nitrogen, loss of water, formation of nitrilium ion, hydration to amide.

Pharmaceutical Application: Synthesis of caprolactam (for nylon) and amide drugs. Also involved in drug metabolism.

4. Hofmann Rearrangement

Definition: Reaction of primary amides with Br₂/NaOH to give primary amines with one less carbon.

Mechanism: N-bromination, deprotonation, migration of R group to nitrogen (with loss of CO₂), protonation.

Pharmaceutical Application: Synthesis of amine drugs from carboxylic acids.

5. Curtius Rearrangement

Definition: Thermal decomposition of acyl azides to give isocyanates, which can be trapped with nucleophiles to yield amines, urethanes, or ureas.

Mechanism: Loss of N₂, migration of R group to nitrogen, formation of isocyanate.

Pharmaceutical Application: Synthesis of amines and derivatives.

6. Claisen Rearrangement

Definition: [3,3]-sigmatropic rearrangement of allyl vinyl ethers to γ,δ-unsaturated carbonyls.

Pharmaceutical Application: Synthesis of natural products and complex molecules.

7. Cope Rearrangement

Definition: [3,3]-sigmatropic rearrangement of 1,5-dienes.

8. Fries Rearrangement

Definition: Rearrangement of phenolic esters to ortho- or para-hydroxy ketones under Lewis acid catalysis.

Pharmaceutical Application: Synthesis of hydroxy ketone intermediates.

9. Benzilic Acid Rearrangement

Definition: Base-catalyzed rearrangement of 1,2-diketones to α-hydroxy acids.

10. Wagner-Meerwein Rearrangement

Definition: Carbocation rearrangement involving migration of alkyl groups in terpene and steroid chemistry.

Pharmaceutical Significance of Rearrangements

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9. Polymerization Reactions

Polymerization is the process of combining many small molecules (monomers) to form large chain-like molecules (polymers). In pharmacy, polymers are essential for drug delivery systems, excipients, and biomedical applications.

Types of Polymerization

1. Addition Polymerization (Chain Growth)

Monomers with double bonds (alkenes) polymerize via free radical, cationic, or anionic mechanisms.

Mechanism:

• Initiation: Formation of active center (radical, cation, anion)
• Propagation: Sequential addition of monomers
• Termination: Combination, disproportionation, or chain transfer

Examples:

2. Condensation Polymerization (Step Growth)

Monomers with two functional groups react with loss of small molecule (H₂O, HCl, etc.).

Examples:

3. Ring-Opening Polymerization

Cyclic monomers (lactones, lactams, epoxides) polymerize by ring opening.

Examples:

• Polylactic acid (PLA) from lactide
• Polycaprolactone (PCL) from ε-caprolactone
• Poly(ethylene oxide) from ethylene oxide
• Poly(lactic-co-glycolic acid) (PLGA) from lactide and glycolide

Pharmaceutical Applications of Polymers

Biodegradable Polymers in Drug Delivery

Polymerization in Pharmaceutical Manufacturing

1. Synthesis of Polymeric Excipients: Controlled polymerization to achieve specific molecular weights and properties.

2. Cross-linking: To form hydrogels and insoluble matrices for controlled release.

3. Copolymerization: To tune properties (e.g., PLGA combines fast-degrading PGA and slow-degrading PLA).

4. Graft Polymerization: To modify surfaces or create functional polymers.

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Summary Tables

Table 1: Comparison of Substitution and Elimination Reactions

Table 2: Common Addition Reactions to Alkenes

Table 3: Oxidation Reactions in Pharmacy

Table 4: Reduction Reactions in Pharmacy

Table 5: Polymer Types in Pharmaceutical Applications

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References

1. Brown, W. H., & Bursten, M. L. (2014). Introduction to organic chemistry (8th ed.). Wiley. (Chapters on reaction mechanisms)

2. Vollhardt, K. P. C., & Schore, N. E. (2014). Organic chemistry: Structure and function (7th ed.). W. H. Freeman and Company. (Chapters 6-14 covering reaction types)

3. Carey, F. A. (2007). Advanced organic chemistry: Part A: Structure and mechanisms (5th ed.). Wiley. (In-depth coverage of reaction mechanisms)

4. Silverman, R. B. (2014). The organic chemistry of drug design and drug action (2nd ed.). Academic Press. (Chapter 4: Drug metabolism and reactions)

5. Lemke, T. L., & Williams, D. A. (2013). Foye's principles of medicinal chemistry (7th ed.). Lippincott Williams & Wilkins. (Chapters on drug metabolism and synthesis)

6. Clayden, J., Greeves, N., & Warren, S. (2012). Organic chemistry (2nd ed.). Oxford University Press. (Comprehensive coverage of reactions)

7. Patrick, G. L. (2013). An introduction to medicinal chemistry (5th ed.). Oxford University Press. (Chapter 4: Drug metabolism)

8. Smith, M. B., & March, J. (2007). March's advanced organic chemistry: Reactions, mechanisms, and structure (6th ed.). Wiley. (Definitive reference)

9. Solomons, T. W. G., & Fryhle, C. B. (2011). Organic chemistry (10th ed.). Wiley. (Student-friendly explanations)

10. McMurry, J. (2015). Organic chemistry (9th ed.). Cengage Learning. (Clear mechanisms and examples)

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Recommended Textbooks for Further Reading:

• Brown, W. H., & Bursten, M. L. (2014). Introduction to organic chemistry (8th ed.). Wiley. (Excellent for foundational reaction mechanisms)
• Vollhardt, K. P. C., & Schore, N. E. (2014). Organic chemistry: Structure and function (7th ed.). W. H. Freeman and Company. (Strong on reaction mechanisms and their applications)
• Silverman, R. B. (2014). The organic chemistry of drug design and drug action (2nd ed.). Academic Press. (Connects reactions to drug action and metabolism)
• Lemke, T. L., & Williams, D. A. (2013). Foye's principles of medicinal chemistry (7th ed.). Lippincott Williams & Wilkins. (Practical applications in medicinal chemistry)