Unit 6: Alkyl and Aryl Halides: Nucleophilic Substitution (SN1, SN2), Elimination (E1, E2) & Organometallics

Haloalkanes (alkyl halides, $\text{R}-\text{X}$) contain a carbon atom covalently bonded to a halogen ($\text{F, Cl, Br, I}$). Due to the electronegativity difference ($\chi_{\text{halogen}} > \chi_{\text{carbon}}$), the carbon-halogen bond is polarized with a partial positive charge on carbon ($^{\delta+}\text{C}-\text{X}^{\delta-}$), rendering the carbon atom an electrophilic target for nucleophiles.

Physical Trends Across the Halogen Series

| Haloalkane | Bond Length (pm) | BDE (kJ/mol) | Dipole Moment (D) | Leaving Group Ability | | :---: | :---: | :---: | :---: | :---: | | $\text{CH}_3-\text{F}$ | $139$ | $452$ | $1.85$ | Extremely Poor (Fluoride is strong base) | | $\text{CH}_3-\text{Cl}$| $178$ | $351$ | $1.87$ | Good | | $\text{CH}_3-\text{Br}$| $193$ | $293$ | $1.81$ | Excellent | | $\text{CH}_3-\text{I}$ | $214$ | $234$ | $1.62$ | Superb (Weakest bond, highly polarizable) |

Leaving Group Thermodynamics & $pK_a$ Correlation

A leaving group ($LG^-$) departs with the electron pair of the $\text{C}-\text{X}$ bond. The fundamental thermodynamic rule of leaving groups states:

The best leaving groups are the conjugate bases of the strongest Brønsted acids.

Weak, stable, highly solvated bases with low charge density and high polarizability leave most readily:

Sulfonate esters (tosylate $\text{OTs}^-$, mesylate $\text{OMs}^-$, triflate $\text{OTf}^-$, $pK_a \approx -14$) are superb leaving groups because negative charge is delocalized over three electronegative oxygen atoms by resonance.

The Bimolecular Nucleophilic Substitution ($S_N2$) is a concerted, single-step reaction discovered by Edward D. Hughes and Christopher Ingold in 1935:

Fundamental Kinetic & Stereochemical Postulates

1. Second-Order Rate Law:

Doubling either the substrate concentration or nucleophile concentration doubles the reaction rate.

2. Backside Attack & Trigonal Bipyramidal Transition State:

The nucleophile donates its lone pair into the empty $\sigma^*(\text{C}-\text{X})$ antibonding orbital, which is oriented at precisely $180^\circ$ relative to the leaving group. In the transition state:

• The central carbon is $sp^2$ hybridized.
• Three spectator groups lie in a planar equatorial geometry.
• The incoming nucleophile and departing leaving group occupy axial positions sharing a single delocalized three-center four-electron (3c-4e) molecular orbital.

3. Stereospecific Walden Inversion (100% Inversion of Configuration):

Backside attack flips the three substituents like an umbrella blown inside out by a gust of wind, resulting in complete inversion of stereochemistry at chiral carbon centers:

Steric Hindrance & Substrate Reactivity

Because the transition state crowds five groups around the central carbon, steric congestion dramatically increases activation energy:

• Tertiary alkyl halides do NOT undergo $S_N2$ substitution under any conditions.
• Neopentyl halides ($(\text{CH}_3)_3\text{C}-\text{CH}_2\text{X}$), although primary, react $10^5$ times slower than ethyl halides due to severe 1,3-steric shielding from the tert-butyl group.

Solvent Acceleration: Polar Aprotic Solvents

Polar protic solvents ($\text{H}_2\text{O}, \text{MeOH}, \text{EtOH}$) form tight hydrogen-bonding cages around small nucleophilic anions, stabilizing their ground state and suppressing their nucleophilicity. Polar aprotic solvents (DMSO, DMF, acetone, acetonitrile, HMPA) solvate cations strongly through dipole-cation interactions while leaving nucleophilic anions 'naked' and unencumbered, accelerating $S_N2$ reaction rates by up to $10^5$ times!

Phillips-Kenyon Proof of Walden Inversion & Hughes-Ingold Kinetics

The stereospecific inversion of configuration in bimolecular nucleophilic substitution ($S_N2$) was definitively verified in 1923 by Henry Phillips and Joseph Kenyon through an elegant cycle involving optically active $(+)$-2-octanol:

``` (+)-2-Octanol [alpha] = +10.3 deg | Tosylation | (Retention: C-O bond unbroken) (TsCl / Py) v (+)-2-Octyl Tosylate | Acetate SN2 | (INVERSION: C-O bond broken by AcO-) (AcO- / DMF) v (-)-2-Octyl Acetate [alpha] = -7.0 deg | Saponification| (Retention: Acyl C-O broken, alkyl C-O untouched) (OH- / H2O) v (-)-2-Octanol [alpha] = -10.3 deg (100% Inverted!) ```

Stereochemical Proof Analysis:

1. In Step 1 (tosylation), only the $\text{O}-\text{H}$ bond of octanol is cleaved; the chiral carbon stereocenter is never touched, so configuration is retained ($100\%$).
2. In Step 3 (saponification of the ester), nucleophilic hydroxide attacks the carbonyl carbon, cleaving the acyl-oxygen bond; the chiral alkyl stereocenter remains untouched, ensuring retention ($100\%$).
3. Because the starting material $(+)$-2-octanol is converted into pure $(-)$-2-octanol with an exact reversal of specific rotation, the inversion must have occurred exclusively during the nucleophilic displacement of the tosylate group by acetate!

This experiment established the concerted backside attack geometry of the $S_N2$ mechanism beyond all doubt.

The Unimolecular Nucleophilic Substitution ($S_N1$) is a stepwise, two-step reaction:

Fundamental Kinetic & Stereochemical Postulates

1. First-Order Rate Law:

The rate is strictly independent of nucleophile concentration and identity.

2. Carbocation Stability Governs Reactivity:

The rate-determining step is heterolytic cleavage of the $\text{C}-\text{X}$ bond to generate an intermediate carbocation. Reactivity mirrors carbocation thermodynamic stability:

3. Stereochemical Outcome: Racemization with Partial Inversion:

The carbocation intermediate possesses planar $sp^2$ geometry with an empty $2p_z$ orbital. A nucleophile can attack from either the front or back face with equal probability, leading to racemization. However, in real solutions, Saul Winstein demonstrated that solvolysis proceeds through intimate ion pairs:

The departing leaving group shields the front face of the carbocation, causing backside attack to be slightly favored, yielding net inversion of configuration ($55-70\%$) with partial retention ($30-45\%$).

The Winstein Four-Stage Solvolysis Spectrum

In 1956, Saul Winstein formulated the comprehensive ion-pair solvolysis scheme to explain stereochemical outcomes in $S_N1$ and $E1$ reactions:

1. Intimate (Contact) Ion Pair ($[\text{R}^+ \, \text{X}^-]$):

The covalent bond is severed, but the two ions remain in direct van der Waals contact inside a single solvent cage.

• Attack by nucleophile at this stage occurs exclusively from the back face, resulting in $100\%$ inversion of configuration.
• Internal return ($k_{-1}$) can cause racemization of the starting alkyl halide without substitution!

2. Solvent-Separated Ion Pair ($[\text{R}^+ \| \text{X}^-]$):

One or more solvent molecules have inserted between the carbocation and the leaving group.

• Frontside attack is now partially possible, yielding inversion with significant retention.

3. Free Solvated Ions ($\text{R}^+ + \text{X}^-$):

The ions have diffused apart beyond mutual Coulomb attraction.

• Attack is completely symmetrical from both faces, resulting in complete racemization ($50:50$ enantiomer ratio).

The Special Salt Effect:

Adding non-nucleophilic lithium perchlorate ($\text{LiClO}_4$) traps the solvent-separated ion pair:

Perchlorate is a non-nucleophilic counter-ion, shutting down internal return ($k_{-2}$) and accelerating the rate of solvolysis. This 'special salt effect' confirmed Winstein's multi-stage ionization spectrum!

The Winstein Ion-Pair Continuum & Racemization vs Inversion

In unimolecular nucleophilic substitution ($S_N1$), the rate-determining step is heterolytic dissociation of the carbon-halogen bond. In 1956, Saul Winstein proved that ionization does not instantaneously generate free, independent ions, but proceeds through an equilibrium cascade of ion pairs:

Mechanistic & Stereochemical Consequences:

1. Intimate Ion Pair ($[\text{R}^+ \, \text{X}^-]$):

• The carbocation and leaving group anion are in direct van der Waals contact, enclosed within a common solvent cage.
• The departing anion blocks the front face of the planar carbocation.
• If the nucleophile captures the carbocation at this stage, attack occurs exclusively from the rear face, resulting in net stereochemical inversion ($60-90\%$ inversion)!

2. Solvent-Separated Ion Pair ($[\text{R}^+ \parallel \text{X}^-]$):

• One or more solvent molecules have inserted between the cation and anion.
• Frontside attack is now partially accessible, yielding diminished stereospecificity.

3. Free Dissociated Ions ($\text{R}^+ + \text{X}^-$):

• Both ions diffuse independently through the bulk solution.
• Attack occurs with equal probability from either face of the planar carbocation, producing complete racemization ($50\% R, 50\% S$).
• Solvent Effect: In highly ionizing, polar protic solvents (e.g., water, trifluoroacetic acid), dissociation to free ions is rapid, leading to predominantly racemic products. In less polar solvents (e.g., acetone, ether), intimate ion pair collapse dominates, resulting in substantial net inversion ($10-30\%$ optical activity retained)!

1. The $E2$ Elimination

Concerted bimolecular elimination requiring anti-periplanar geometry ($\phi = 180^\circ$):

• Favored by strong bases ($\text{OH}^-, \text{OR}^-, \text{NH}_2^-$).
• Unhindered base $\to$ Zaitsev (more substituted alkene).
• Bulky base ($t\text{-BuO}^-$) $\to$ Hofmann (least substituted alkene).

2. The $E1$ Elimination

Stepwise unimolecular elimination sharing the identical first step as $S_N1$:

• Occurs concurrently with $S_N1$ whenever tertiary halides are heated in weak nucleophile/base solvents (solvolysis).
• Always yields the thermodynamically most stable Zaitsev alkene (trans > cis) because deprotonation occurs from an unconstrained planar carbocation.

3. The $E1cB$ (Conjugate Base) Elimination

Operates when the substrate possesses a relatively acidic $\beta$-hydrogen and a poor leaving group:

1. Deprotonation by base yields a stabilized carbanion conjugate base:

1. The carbanion expels the poor leaving group in step 2:

Prevalent in biochemical pathways and aldol dehydration (expelling $\text{OH}^-$).

The $E1\text{cB}$ Mechanism: Carbanion Intermediates & Kinetic Criteria

The Elimination Unimolecular conjugate Base ($E1\text{cB}$) pathway operates when the substrate possesses an unusually acidic $\beta$-hydrogen and a relatively poor leaving group:

Kinetic Regimes:

1. $(E1\text{cB})_{\text{reversible}}$ ($k_{-1}[\text{Base-H}^+] \gg k_2$):

• Carbanion formation is fast and reversible. The rate-determining step is the subsequent expulsion of the leaving group ($k_2$).
• Deuterium exchange experiments in $\text{MeOD} / \text{MeO}^-$ show that unreacted starting material incorporates deuterium rapidly prior to elimination:

2. $(E1\text{cB})_{\text{irreversible}}$ ($k_2 \gg k_{-1}[\text{Base-H}^+]$):

• Proton abstraction by base is rate-determining, and the carbanion collapses immediately to alkene.
• Exhibits clean second-order kinetics ($v = k_1 [\text{Substrate}][\text{Base}]$) and a large primary kinetic isotope effect ($k_H / k_D \approx 3 - 6$), but zero deuterium exchange in recovered starting material.
• Classic Substrates: Aldol condensation dehydration ($\beta$-hydroxy carbonyls losing $\text{OH}^-$), eliminations of $\beta$-fluoro sulfones, and eliminations with $\beta$-nitro groups.

Predicting which mechanism dominates is one of the foundational skills of organic synthesis. The competition is decided by four interdependent parameters:

The 4-Way Decision Grid

| Substrate | Strong Base / Nucleophile ($\text{OH}^-, \text{OMe}^-$) | Bulky Strong Base ($t\text{-BuO}^-, \text{LDA}$) | Weak Base / Good Nucleophile ($\text{I}^-, \text{CN}^-, \text{RS}^-$) | Weak Base / Poor Nucleophile ($\text{H}_2\text{O}, \text{ROH}$) | | :---: | :---: | :---: | :---: | :---: | | Methyl | $S_N2$ exclusively | $S_N2$ | $S_N2$ exclusively | No reaction (slow solvolysis) | | Primary ($1^\circ$) | $S_N2$ major ($E2$ trace) | $E2$ major (Hofmann) | $S_N2$ exclusively | No reaction | | Secondary ($2^\circ$)| $E2$ major (Zaitsev) | $E2$ exclusively | $S_N2$ (in aprotic) / $S_N1$ (in protic) | $S_N1 / E1$ mixture (slow) | | Tertiary ($3^\circ$) | $E2$ exclusively | $E2$ exclusively | $S_N1$ | $S_N1 / E1$ mixture |

Thermodynamic Temperature Control

• Elimination reactions ($E1, E2$) produce three product molecules from two reactants ($\Delta S_{\text{rxn}} > 0$).
• Substitution reactions ($S_N1, S_N2$) produce two molecules from two reactants ($\Delta S_{\text{rxn}} \approx 0$).

By the Gibbs equation $\Delta G = \Delta H - T\Delta S$, elevated temperatures ($T \uparrow$) dramatically favor elimination over substitution!

Aryl halides ($\text{Ar}-\text{X}$) are completely inert toward classical $S_N2$ displacement:

1. Backside attack is geometrically impossible because the nucleophile would have to pass directly through the aromatic ring.
2. The $sp^2$ carbon forms a shorter, stronger $\text{C}-\text{X}$ bond with partial double-bond character from halogen resonance back-donation.

However, aryl halides undergo substitution via two non-classical mechanisms:

1. Nucleophilic Aromatic Substitution ($S_NAr$ Addition-Elimination)

Operates when the aryl halide bears strong electron-withdrawing groups (such as $-\text{NO}_2$) at positions ortho or para to the halogen:

• Step 1 (Addition, RDS): The nucleophile attacks the ipso-carbon, generating a resonance-stabilized cyclohexadienyl carbanion, the Meisenheimer Complex:

The negative charge is delocalized onto the electronegative oxygens of the ortho and para nitro groups.

• Step 2 (Elimination, Fast): Expulsion of the halide leaving group re-establishes the aromatic system.
• Meta-nitrochlorobenzene is unreactive because negative charge cannot delocalize directly onto the nitro group.

2. Elimination-Addition via Benzyne Intermediates

When an unactivated aryl halide is treated with an exceptionally strong base (such as sodium amide in liquid ammonia, $\text{NaNH}_2 / \text{NH}_3$ at $-33^\circ\text{C}$):

• Step 1 (Elimination): Amide ion abstracts an ortho-proton, followed by expulsion of chloride to form a neutral, highly strained intermediate: Benzyne ($ ext{C}_6\text{H}_4$).
• Benzyne contains a formal triple bond in a six-membered ring!
• The third bond is formed by sideways overlap of two $sp^2$ hybrid orbitals in the molecular plane, resulting in severe bond angle strain.
• Step 2 (Addition): Amide ion attacks either carbon of the triple bond with equal probability.
• Isotopic Proof (John D. Roberts, 1953): Treating $^{14}\text{C}$-labeled chlorobenzene with $\text{NaNH}_2$ yields an exact $50:50$ mixture of 1-$^{14}\text{C}$-aniline and 2-$^{14}\text{C}$-aniline, proving the existence of the symmetrical benzyne intermediate!

Experimental Verification of the Benzyne Intermediate: Isotope Labeling & Trapping

When unactivated chlorobenzene is treated with potassium amide in liquid ammonia at $-33^\circ\text{C}$, aniline is formed rapidly. In 1953, John D. Roberts conclusively verified the existence of the transient symmetrical benzyne intermediate using carbon-14 isotopic labeling:

Roberts' $^{14}\text{C}$ Isotopic Experiment:

1. Roberts synthesized chlorobenzene labeled exclusively at the C1 position with carbon-14 ($[1\text{-}^{14}\text{C}]\text{-chlorobenzene}$):

1. If substitution occurred via direct displacement, the amino group would be attached exclusively to the labeled carbon ($100\% [1\text{-}^{14}\text{C}]\text{-aniline}$).
2. Experimental Result: The isolated aniline showed that:

• $48.5\%$ of the $^{14}\text{C}$ label was at C1 (bearing the amino group).
• $51.5\%$ of the $^{14}\text{C}$ label was at C2 (adjacent to the amino group)!

4. Mechanistic Explanation:

Deprotonation by amide at C2 followed by loss of chloride generates a symmetrical 1,2-dehydrobenzene (benzyne) intermediate:

Amide ion attacks either the labeled C1 or the unlabeled C2 with exactly equal probability ($50:50$), proving the intermediacy of benzyne!

Diels-Alder Trapping of Benzyne:

Because the in-plane $\pi$ bond of benzyne is formed by overlap of $sp^2$ orbitals tilted away from parallel by $60^\circ$, it is intensely strained ($\sim 210\text{ kJ/mol}$ of strain energy) and acts as an ultra-reactive dienophile. Generating benzyne in the presence of furan traps it quantitatively in a $[4+2]$ cycloaddition to form 1,4-epoxy-1,4-dihydronaphthalene (endoxide) in $>85\%$ yield!

Discovered by François Auguste Victor Grignard in 1900 (Nobel Prize 1912), alkyl- and arylmagnesium halides (Grignard Reagents, $\text{RMgX}$) are synthesized by inserting metallic magnesium into carbon-halogen bonds in anhydrous ether:

The Schlenk Equilibrium

In ethereal solution, Grignard reagents exist as a dynamic multi-species equilibrium:

Solvation by diethyl ether or THF (coordinating lone pairs into empty magnesium orbitals) is indispensable: Grignard reagents cannot be prepared in hydrocarbon solvents without coordinating Lewis bases.

Umpolung (Reversal of Polarity) & Synthetic Reactivity

In alkyl halides, carbon is electrophilic ($^{\delta+}\text{C}-\text{X}^{\delta-}$). In Grignard reagents, magnesium is electropositive ($\chi_{\text{Mg}} = 1.31$ vs $\chi_{\text{C}} = 2.55$), inverting carbon into a powerful carbanionic nucleophile and superbase:

1. Reactions with Protic Acids: Grignard reagents react explosively with water, alcohols, amines, and terminal alkynes, abstracting protons to yield alkanes:

2. Nucleophilic Carbon-Carbon Bond Formations:

• Formaldehyde $\longrightarrow$ Primary Alcohol ($1^\circ$)
• Aldehydes $\longrightarrow$ Secondary Alcohol ($2^\circ$)
• Ketones $\longrightarrow$ Tertiary Alcohol ($3^\circ$)
• Carbon Dioxide ($\text{CO}_2$) $\longrightarrow$ Carboxylic Acid ($ ext{R}-\text{COOH}$)
• Epoxides $\longrightarrow$ Alcohol extended by two carbons.

Stereochemical Models of Carbonyl Addition: Felkin-Anh Transition State

When a nucleophile or Grignard reagent attacks a chiral carbonyl compound possessing an adjacent stereocenter with substituents categorized by size (Large $L$, Medium $M$, Small $S$):

The stereochemical outcome is governed by the Felkin-Anh Model (1968, 1976):

1. Conformational Alignment: The largest group ($L$) aligns perpendicular ($90^\circ$) to the carbonyl $\text{C}=\text{O}$ double bond to maximize $\sigma^*_{\text{C}-L} \to \pi^*_{\text{C}=\text{O}}$ hyperconjugative stabilization.

2. Bürgi-Dunitz Angle of Attack: The incoming nucleophile approaches the carbonyl carbon at an obtuse angle of $\alpha_{\text{BD}} \approx 107^\circ$ to maximize overlap with the $\pi^*_{\text{C}=\text{O}}$ LUMO while minimizing repulsion with the oxygen lone pairs.

3. Steric Trajectory: The nucleophile attacks from the face bearing the Small ($S$) substituent rather than the Medium ($M$) substituent.

This transition-state model accurately predicts the diastereomeric ratio and absolute configuration of major alcohol products across hundreds of complex natural product syntheses!

Asymmetric Phase-Transfer Catalysis (PTC)

Phase-transfer catalysis enables reactions between nucleophiles dissolved in an aqueous or solid inorganic phase and organic electrophiles dissolved in an immiscible organic solvent (e.g., toluene, dichloromethane):

1. Mechanism of Phase Transfer:

1. In the interfacial boundary, an inorganic base deprotonates the organic pronucleophile (such as a protected glycine ester).
2. The lipophilic chiral quaternary ammonium cation ($\text{Q}^{*+}$, derived from cinchona alkaloids or Maruoka $C_2$-symmetric binaphthyl catalysts) pairs with the enolate anion to form a lipophilic ion pair:

1. The neutral, lipophilic ion pair extracts across the phase boundary into the non-polar organic phase.
2. Stereochemical Induction: The bulky, rigid chiral pocket of $\text{Q}^{*+}$ shields one face of the planar enolate through attractive $\pi-\pi$ stacking and directed $\text{C}-\text{H}\cdots\text{O}$ hydrogen bonding.
3. The incoming alkyl halide electrophile can only attack from the exposed face, producing non-proteinogenic $\alpha$-amino acids in up to $99\%$ enantiomeric excess ($ee$)!

Organocopper Reagents: The Gilman Reagent ($\text{R}_2\text{CuLi}$)

While organolithium and Grignard reagents are hard nucleophiles that attack carbonyl carbons directly ($1,2$-addition), lithium dialkylcuprates (Gilman reagents) are soft nucleophiles that exhibit unique selectivity:

1. Conjugate 1,4-Addition to $\alpha,\beta$-Unsaturated Carbonyls:

• HSAB Rationale: The copper(I) center ($d^{10}$) and its alkyl ligands are soft, polarizable species. According to Klopman-Salem frontier orbital analysis, soft nucleophiles react preferentially with the soft $\beta$-carbon of $\alpha,\beta$-unsaturated enones (large LUMO coefficient at C4) rather than the hard carbonyl carbon (C2).
• The reaction proceeds via initial single-electron transfer to form a copper(III) metallacyclic intermediate, which undergoes fast reductive elimination to deliver the 1,4-addition product upon aqueous protonation.

2. Corey-Posner-Whitesides-House Cross-Coupling:

Gilman reagents cleanly displace primary and secondary alkyl halides, vinyl halides, and aryl halides without $\beta$-elimination:

• Unlike Grignard reagents, Gilman reagents tolerate diverse functional groups (esters, ketones, amides, nitriles) in the coupling partner!

Organoboron Precursors: Miyaura Borylation & Pinacol Boronates

Aryl and alkyl boronic esters serve as the indispensable organometallic partners for Suzuki-Miyaura cross-couplings:

``` MIYAURA BORYLATION CATALYTIC CYCLE: Pd(0)L2 | OA | Ar-X (Aryl Halide) v Ar-Pd(II)L2-X | Ligand| KOAc (Displaces Halide by Acetate) Exch. v Ar-Pd(II)L2-OAc | TM | B2pin2 (Bis(pinacolato)diboron) v Ar-Pd(II)L2-Bpin + AcO-Bpin | RE v (Arylboronate Ar-Bpin Released) Pd(0)L2 (Regenerated) ```

Why Potassium Acetate ($\text{KOAc}$) is Essential:

• If a strong base like $\text{K}_2\text{CO}_3$ or $\text{KOH}$ is used, the generated $\text{Ar}-\text{Bpin}$ product undergoes premature Suzuki cross-coupling with remaining unreacted $\text{Ar}-\text{X}$, forming unwanted biaryl dimers ($\text{Ar}-\text{Ar}$).
• Potassium acetate is weakly basic enough to facilitate transmetalation of $\text{B}_2\text{pin}_2$ while completely suppressing subsequent Suzuki coupling, ensuring $>90\%$ isolated yields of pure boronic esters!

The Simmons-Smith Reaction & Zinc Carbenoids

Discovered by Howard Simmons and Ronald Smith in 1958, the reaction of alkenes with diiodomethane and zinc-copper couple ($\text{Zn(Cu)}$) cleanly synthesizes cyclopropanes with strict stereospecific syn-addition:

The Active Reagent: Iodomethylzinc Iodide ($\text{ICH}_2\text{ZnI}$):

• Does NOT generate free carbene ($:\text{CH}_2$).
• Exists as an organozinc carbenoid where zinc coordinates both iodine and carbon.
• Diastereoselective Directed Cyclopropanation:

When an allylic alcohol is cyclopropanated, the zinc atom coordinates to the hydroxyl oxygen prior to delivering the methylene group:

This internal chelation delivers methylene exclusively to the same face as the hydroxyl group ($>99\%$ diastereoselectivity)!

The Master Four-Way Mechanistic Decision Algorithm

To determine whether an alkyl halide reacts via $S_N2, S_N1, E2$, or $E1$, evaluate the four fundamental parameters in strict sequential order:

``` STEP 1: SUBSTRATE STRUCTURE (Primary, Secondary, Tertiary, Benzylic/Allylic) STEP 2: NUCLEOPHILE / BASE CLASS (Strong/Bulky Base, Strong Nucleophile/Weak Base, Weak Nu/Base) STEP 3: SOLVENT PROTICITY (Polar Protic vs Polar Aprotic) STEP 4: TEMPERATURE (Elevated Temperature Favors Eliminations: Delta S > 0) ```

| Substrate Class | Strong Nucleophile / Strong Base (e.g., $\text{OH}^-, \text{EtO}^-$) | Bulky Strong Base (e.g., $t\text{-BuO}^-, \text{LDA}$) | Strong Nucleophile / Weak Base (e.g., $\text{I}^-, \text{RS}^-, \text{CN}^-$) | Weak Nucleophile / Weak Base (e.g., $\text{H}_2\text{O}, \text{MeOH}$) | | :---: | :---: | :---: | :---: | :---: | | Methyl ($\text{CH}_3\text{X}$) | $S_N2$ exclusively | $S_N2$ | $S_N2$ exclusively | No reaction | | Primary ($1^\circ$) | $S_N2$ major ($E2$ minor) | $E2$ exclusively (Hofmann) | $S_N2$ exclusively | No reaction | | Secondary ($2^\circ$) | $E2$ major ($S_N2$ minor) | $E2$ exclusively (Hofmann) | $S_N2$ exclusively (Inversion) | Slow $S_N1 / E1$ | | Tertiary ($3^\circ$) | $E2$ exclusively (Zaitsev) | $E2$ exclusively (Hofmann) | $S_N1 / E1$ | $S_N1 / E1$ | | Allylic / Benzylic ($1^\circ/2^\circ$) | $S_N2$ fast ($E2$ with base) | $E2$ | $S_N2$ extremely fast | $S_N1$ fast |