Key Concepts in Nucleophilic Substitution and Elimination Reactions (CHEM 3305 Exam 3)

Nucleophilic Substitution and Elimination: Overview

Main Reaction Types

Organic Chemistry I covers several fundamental reaction mechanisms involving alkyl halides, alcohols, and epoxides. These include nucleophilic substitution (SN2, SN1) and elimination (E2, E1) reactions, each with distinct mechanistic and selectivity features.

• Nucleophilic substitution: SN2 (bimolecular), SN1 (unimolecular)

• Elimination: E2 (bimolecular), E1 (unimolecular)

• Selectivity: Stereospecificity, regioselectivity (Zaitsev rule), stereoselectivity

• Carbocation rearrangements

• Reactions of alcohols and epoxides

Leaving Groups in Nucleophilic Substitution

Defining Characteristic of X-atom

The ability of a halogen atom (X) to act as a leaving group is crucial in substitution and elimination reactions. The polar C–X bond makes the carbon atom electron-deficient, allowing nucleophilic attack.

• Electrophiles: Alkyl halides (R–X) are electrophilic and react with nucleophiles.

• Leaving group: The group that departs with a pair of electrons during substitution or elimination.

Competing Elimination Reactions

Alkyl halides can also undergo elimination with Brønsted-Lowry bases, forming alkenes and conjugate acids.

Leaving Group Ability

Leaving group ability depends on:

• Bond strength (BDE): Strength of the C–X bond

• Stability of the X– anion: pKa of the conjugate acid (HX)

Starting Material	Leaving Group	Conjugate Acid	pKa
R–Cl	Cl–	HCl	-6
R–Br	Br–	HBr	-9
R–I	I–	HI	-10
R–OH2+	H2O	H3O+	-2

The 'good' leaving groups have low pKa values (strong acids).

Starting Material	Leaving Group	Conjugate Acid	pKa
R–F	F–	HF	3.2
R–OH	OH–	H2O	14
R–NH2	NH2–	NH3	35
R–H	H–	H2	50

The 'bad' leaving groups have high pKa values (weak acids).

Nucleophile Strength and Common Nucleophiles

Factors Affecting Nucleophilicity

• Charge: Negatively charged nucleophiles are stronger than their neutral counterparts.

• Basicity: For nucleophiles with the same atom, stronger bases are stronger nucleophiles.

• Periodicity: Nucleophilicity decreases across a row and increases down a group in the periodic table.

• Steric hindrance: Bulky nucleophiles are less nucleophilic.

	Negatively Charged Nucleophiles	Neutral Nucleophiles
Oxygen	OH–, OCH3–	H2O, ROH
Nitrogen	NH2–, CN–	NH3, RNH2
Carbon	CN–, HC≡C–
Halogen	Br–, I–
Sulfur	HS–, RS–	H2S, RSH

Mechanisms of Nucleophilic Substitution: SN2 vs SN1

SN2 Mechanism (Bimolecular)

Bond breaking and bond making occur simultaneously in a single concerted step.

• Transition state: Trigonal bipyramidal

• Rate law: $ext{rate} = k[ ext{RCH}_2 ext{X}][ ext{Nu}]$

SN1 Mechanism (Unimolecular)

Bond breaking occurs before bond making, forming a carbocation intermediate.

• Intermediate: Trigonal planar carbocation

• Rate law: $ext{rate} = k[ ext{R}_3 ext{CX}]$

Kinetics and Energy Profiles

SN2 Features

• Second-order kinetics: Bimolecular process

• Rate depends on: Concentration of both electrophile and nucleophile

• Energy profile: Single transition state

SN1 Features

• First-order kinetics: Unimolecular process

• Rate depends on: Only the concentration of the electrophile

• Energy profile: Two transition states, carbocation intermediate

SN2 vs SN1: Nucleophile and Stereochemistry

• Strong nucleophiles: Favor SN2 (negatively charged, e.g., RO–)

• Weak nucleophiles: Favor SN1 (neutral, e.g., ROH)

• Stereochemistry: SN2 leads to inversion of configuration; SN1 leads to racemization

General Features of Elimination

E2 Mechanism (Bimolecular)

• Concerted process: HX elimination and alkene formation in a single step

• Rate law: $ext{rate} = k[ ext{R–X}][ ext{B}]$

E1 Mechanism (Unimolecular)

• Stepwise process: C–X bond breaks to form carbocation, then deprotonation

• Rate law: $ext{rate} = k[ ext{R–X}]$

Energy Profiles

• E2: Single transition state, partial C=C double bond character

• E1: Two transition states, carbocation intermediate, partial C=C double bond character in TS2

E2/E1: Strength of Base

• Strong bases: Favor E2 reactions

• Weak bases: Favor E1 reactions

• Both E1 and SN1 can occur under similar conditions, often yielding mixtures of products

E2 Stereospecificity and Selectivity

Stereospecific Anti-Periplanar Arrangement

• E2 elimination requires anti-periplanar arrangement of H and X

• Stereospecificity: Stereochemistry of product determined by reaction pathway

• Regioselectivity: Zaitsev rule favors more substituted (internal) alkene

• Stereoselectivity: Trans alkene favored over cis

Summary Table: SN2, SN1, E2, E1 Mechanisms

Alkyl Halide Type	Reaction With	Mechanism
1° RCH2X	Strong nucleophile	SN2
1° RCH2X	Strong bulky base	E2
2° R2CHX	Strong base and nucleophile	SN2 and E2
2° R2CHX	Strong bulky base	E2
2° R2CHX	Weak base and nucleophile	SN1 and E1
3° R3CX	Weak base and nucleophile	SN1 and E1
3° R3CX	Strong base	E2

General Reactivity of Alcohols, Ethers, and Epoxides

Alcohols and Ethers

• OH– and OR– are poor leaving groups; reactions require strong acid conditions

• Elimination (E1) and substitution (SN1) possible under acidic conditions

• Similar trends apply to ethers

Epoxides

• Epoxides are highly strained and readily undergo ring-opening by nucleophiles or acids

• Ring-opening is stereospecific (backside attack)

Carbocation Rearrangements

Wagner–Meerwein Rearrangements

• Carbocations can rearrange to more stable forms via 1,2-hydride or 1,2-alkyl shifts

• Secondary carbocation can become tertiary via hydride or methyl shift

• Leads to 'unexpected' products in reactions involving carbocation intermediates

Reactions of Alcohols

• H2SO4: E1 elimination, Zaitsev regioselectivity

• POCl3, pyridine: E2 elimination, stereospecific

• HX (Cl, Br, I): SN1 substitution, possible carbocation rearrangement

• SOCl2 or PBr3: SN2 substitution

• TsCl, pyridine: SN2 substitution

Reactions of Epoxides

• Epoxide ring-opening occurs via nucleophilic substitution under acidic or basic conditions

• Backside attack (SN2) leads to regioselective and stereospecific products

• Under acidic conditions, attack occurs at the more substituted carbon

• Under basic conditions, attack occurs at the less substituted carbon

Example: 2,2-Dimethyloxirane

• Backside attack by nucleophile opens the ring

• Mechanism can be SN2 or SN1 depending on conditions

Additional info: These notes summarize the key mechanistic and selectivity features of nucleophilic substitution and elimination reactions, including the role of leaving groups, nucleophile strength, base strength, and carbocation rearrangements, as well as the reactivity of alcohols and epoxides. The tables and energy diagrams provide a concise reference for predicting reaction outcomes and understanding mechanistic pathways.