Chapter 9: The Chemistry of Alkyl Halides – Substitution and Elimination Reactions

Alkyl Halides: Substitution and Elimination Reactions

Overview of Nucleophilic Substitution and β-Elimination Reactions

Alkyl halides are organic compounds containing a halogen atom bonded to an sp3-hybridized carbon. Their reactivity is central to many organic transformations, especially nucleophilic substitution and elimination reactions. These reactions are classified based on the mechanism and the nature of the reactants.

• Nucleophilic Substitution: A nucleophile replaces the leaving group attached to the carbon.

• Elimination (β-Elimination): Two groups (usually a hydrogen and a halide) are removed from adjacent carbons, forming a double bond.

Introducing Nucleophilic Substitution Reactions

Nucleophilic substitution involves the displacement of a leaving group by a nucleophile. The nucleophile donates an electron pair to the electrophilic carbon, resulting in the substitution of the leaving group.

• Electron-pair displacement: Nucleophile donates an electron pair to an electrophile to displace a leaving group.

• Key terms:

  • Electrophile: Alkyl halide (electron acceptor).

  • Nucleophile: Negatively charged or neutral species donating an electron pair.

  • Leaving group: Group displaced by the nucleophile (often a halide).

  • Solvent: Facilitates the reaction by dissolving reactants.

  • Spectator ion: Counter ion for anionic nucleophile, does not participate.

• Example: $ext{CH}_3 ext{CH}_2 ext{I} + ext{NaOH} ightarrow ext{CH}_3 ext{CH}_2 ext{OH} + ext{NaI}$

Intramolecular Nucleophilic Substitution Reactions

Intramolecular reactions occur when the nucleophile and electrophile are within the same molecule, leading to ring closure or rearrangement.

• For both inter- and intramolecular substitution, the carbon bearing the leaving group must be sp3-hybridized.

• Example: Cyclization of a haloamine to form a cyclic amine.

Some Nucleophilic Substitution Reactions

Nucleophilic substitution can occur with various nucleophiles, leading to a wide range of products.

R–X (Alkyl Halide)	Nucleophile	Product
R–Cl	Br-	R–Br
R–Br	OH-	R–OH
R–I	CN-	R–CN
R–Cl	SH-	R–SH
R–Br	NH2-	R–NH2

Equilibria in Substitution Reactions

The favorability of nucleophilic substitution reactions can be predicted by comparing them to Brønsted–Lowry acid-base reactions. The equilibrium favors the release of the weaker base as the leaving group.

• Example: $ext{OH}^- + ext{H}_3 ext{C}- ext{I} ightarrow ext{H}_3 ext{C}- ext{OH} + ext{I}^-$

• If the acid-base reaction is favorable, the substitution is likely favorable.

Equilibria in Substitution Reactions Varies

The equilibrium of nucleophilic substitution depends on the relative strengths of the bases involved.

• Very favorable: Stronger base replaces weaker base.

• Reversible: Similar base strengths.

• Very unfavorable: Weaker base cannot replace stronger base.

Using Le Chatelier's Principle to Drive Unfavorable Equilibria

Unfavorable equilibria can be shifted by removing a product from the reaction mixture, such as by precipitation.

• Example: $ext{R–Cl} + ext{K}^+ ext{I}^- ightarrow ext{R–I} + ext{KCl} ext{(precipitates)}$

• Solubility differences can drive the reaction forward.

Favorable Equilibria and Reaction Rates

Equilibrium constant does not determine reaction rate. Even reactions with favorable equilibria can proceed slowly if the activation energy is high.

• Example: Methyl iodide reacts rapidly, but neopentyl iodide reacts very slowly under similar conditions.

Rate Constant and $riangle G^{ eq}$

The rate of a reaction is determined by the energy barrier ($riangle G^{ eq}$). The rate constant $k$ is related to $riangle G^{ eq}$ by:

• $k ext{ } ext{propto} ext{ } e^{- riangle G^{ eq}/RT}$

• Larger $riangle G^{ eq}$ means slower reaction; smaller $riangle G^{ eq}$ means faster reaction.

The SN2 Reaction

SN2 reactions are bimolecular nucleophilic substitutions, occurring in a single concerted step. Both the nucleophile and the alkyl halide are involved in the rate-determining step.

• Rate law: $ext{rate} = k[ ext{alkyl halide}][ ext{nucleophile}]$

• Reaction proceeds with inversion of configuration at the stereocenter.

• Example: $ext{CH}_3 ext{CH}_2 ext{I} + ext{CH}_3 ext{CH}_2 ext{O}^- ightarrow ext{CH}_3 ext{CH}_2 ext{OCH}_2 ext{CH}_3 + ext{I}^-$

Mechanism Consistent with the Rate Law

The nucleophile attacks the electrophilic carbon from the side opposite the leaving group, leading to a transition state with five groups around the carbon.

• The transition state is crowded; steric hindrance affects the rate.

• Bulky groups slow the reaction.

Observed Results of SN2 Reactions: Inversion of Configuration

SN2 reactions at stereocenters result in inversion of configuration, known as the Walden inversion.

• Example: (R)-2-bromooctane reacts to form (S)-2-ethylthiooctane.

Stereochemistry of the SN2 Reaction

The nucleophile attacks the sp3-hybridized carbon from the side opposite the leaving group, resulting in inversion of stereochemistry.

• Transition state is planar at the reacting carbon.

MO Analysis of the SN2 Reaction

The nucleophile's molecular orbital (MO) containing the donated electron pair interacts with the lowest-energy unoccupied MO (LUMO) of the alkyl halide.

• Opposite-side substitution is favored due to optimal orbital overlap.

Effect of Alkyl Halide Substitution on SN2 Reaction Rates

Increasing substitution at the reacting carbon slows SN2 reactions due to steric hindrance.

Alkyl Group	Relative Rate
Methyl	1.45
Ethyl	1.0
Isopropyl	0.0075
tert-Butyl	0.0005

Leaving-Group Effects in the SN2 Reaction

The best leaving groups are those that form the weakest bases. Good leaving groups facilitate faster SN2 reactions.

• Order of leaving group ability: I- > Br- > Cl- > F-

• Relative reactivity: R–F << R–Cl < R–Br < R–I

Dependence of SN2 Rates on Basicity of the Nucleophile

Nucleophilicity generally increases with basicity for period 2 atoms. Stronger bases are stronger nucleophiles and increase the rate of SN2 reactions.

Nucleophile	pKa of Conjugate Acid	k (M-1s-1)
CH3O-	15.5	2.5 × 106
HO-	15.7	7.5 × 105
CN-	9.4	4.6 × 105

Dependence of SN2 Rates on Basicity of the Nucleophile (Group 6A and 7A)

Polarizability also affects nucleophilicity. Highly polarizable atoms can be good nucleophiles even if they are weak bases.

Nucleophile	pKa of Conjugate Acid	k (M-1s-1)
PH2-	6.2	1.1
F-	-10	14.0 × 10-2

Solvents Can Affect Nucleophilicity

Solvent choice is crucial for SN2 reactions. Protic solvents form hydrogen bonds with nucleophiles, slowing the reaction. Polar aprotic solvents enhance nucleophilicity and increase reaction rates.

• Common polar aprotic solvents: DMF, DMSO, acetonitrile, acetone.

Polar, Aprotic Solvents Enhance Nucleophilicity

Nucleophile	pKa	In Methanol: Reaction Time	In DMF: Reaction Time
I-	-10	17 min	2.7 min
Br-	-8	12 h	2.7 h
Cl-	-6	13 days	1.4 days
CN-	9.3	1.5 h	0.011 h

The E2 Reaction (β-Elimination Reaction)

E2 reactions are bimolecular eliminations where a base removes a β-hydrogen and the leaving group departs simultaneously, forming a double bond.

• Rate law: $ext{rate} = k[ ext{alkyl halide}][ ext{base}]$

• Dominant for 3° alkyl halides with strong base and protic solvent.

• Order of leaving group reactivity: I- > Br- > Cl- > F-

Stereochemistry of the E2 Reaction

E2 reactions require the hydrogen and leaving group to be anti-periplanar (dihedral angle = 180°) for optimal orbital overlap.

• Anti elimination: H and X are anti-periplanar.

• Syn elimination: H and X are syn-periplanar (less common).

Multiple Products Possible for E2 Reactions

If more than one β-hydrogen is available, multiple alkene products can form, including cis- and trans-isomers.

• Example: 2-bromobutane can form cis-2-butene, trans-2-butene, and 1-butene.

Regioselectivity of E2: Most Stable Alkene Isomer Dominates

With simple alkoxide bases, the most substituted (and thus most stable) alkene is the major product (Zaitsev's rule).

• Example: $ext{CH}_3 ext{CH}_2 ext{CHBrCH}_3 + ext{KOEt} ightarrow ext{CH}_3 ext{CH}= ext{C}( ext{CH}_3)_2 + ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{CH}_3$

Regioselectivity of E2: Effect of Base Structure

Highly branched bases (e.g., tert-butoxide) favor formation of less substituted alkenes due to steric hindrance.

E2 Reaction of Cyclohexyl Halides

For cyclohexyl halides, the hydrogen and leaving group must be trans and both axial for E2 elimination to occur.

• Only the chair conformation with trans, diaxial arrangement allows elimination.

Competition between SN2 and E2

The pathway (substitution vs. elimination) depends on the structure of the alkyl halide and the base/nucleophile.

• Strong, unhindered nucleophiles favor SN2 for primary halides.

• Strong bases favor E2 for secondary and tertiary halides.

• Bulky bases favor E2 elimination.

SN1 and E1 Reactions

SN1 and E1 reactions proceed via a two-step mechanism involving carbocation formation. The rate-determining step is the dissociation of the alkyl halide to form the carbocation.

• Rate law: $ext{rate} = k[ ext{alkyl halide}]$

• Both reactions share a common intermediate: the carbocation.

• SN1: Substitution, unimolecular.

• E1: Elimination, unimolecular.

Factors Affecting SN1 and E1 Reactivity

• Most rapid with tertiary alkyl halides, slower with secondary, never with primary.

• Leaving group ability: I- > Br- > Cl- > F-

• Nucleophile/base: Weak base and poor nucleophile favor SN1/E1.

• High temperature increases E1 product.

• Polar, protic solvents favor SN1/E1 by stabilizing ions.

Rearrangements in SN1 Solvolysis

Carbocation rearrangements (e.g., 1,2-methyl shift, 1,2-hydride shift) can occur to form more stable carbocations.

• Example: Secondary carbocation rearranges to tertiary via methyl or hydride shift.

Stereochemistry of the SN1 Reaction

SN1 reactions lead to racemization at the stereocenter due to planar carbocation intermediate, allowing attack from either side.

• Equal amounts of enantiomers are formed.

Stereochemistry of E1 vs E2

E1 eliminations can form both cis and trans isomers, while E2 eliminations are more stereospecific due to the anti-periplanar requirement.

• E2: Stereochemistry determined by anti-periplanar geometry.

• E1: Less stereospecific, can form mixtures.

Additional info: These notes summarize the key mechanistic, stereochemical, and kinetic aspects of alkyl halide substitution and elimination reactions, suitable for college-level Organic Chemistry study.