19.4 Alkyl Halides (SN1/SN2), Grignard Reagents & Functional-Group Tests

Alkyl Halides (SN1/SN2), Grignard Reagents & Functional-Group Tests

In the Intermediate Chemistry Part II syllabus and the UET ECAT entry test, oxygen- and halogen-containing organic functional groups make up a large part of the organic syllabus. Section 19.5 and Section 19.6 extend this section with the preparation and reactions of alcohols, phenols, ethers, carbonyl compounds and carboxylic acids. ECAT candidates must demonstrate absolute precision in differentiating $\text{S}_\text{N}1$ from $\text{S}_\text{N}2$ mechanisms, predicting Grignard addition products, interpreting Lucas and iodoform qualitative tests, and navigating condensation mechanisms such as the Aldol and Cannizzaro reactions.

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1. Alkyl Halides: Nucleophilic Substitution Mechanisms ($\text{S}_\text{N}1$ vs. $\text{S}_\text{N}2$)

Alkyl halides (haloalkanes, $\text{R}-\text{X}$) contain a polar carbon-halogen bond ($\text{C}^{\delta+}-\text{X}^{\delta-}$). Because halogen atoms are more electronegative than carbon, electron density is pulled away from carbon, making the $\alpha$-carbon electrophilic and susceptible to attack by electron-rich nucleophiles ($\text{Nu}^-$). The reaction proceeds by displacing the halogen as a leaving group (halide ion, $\text{X}^-$):

$\text{Nu}^- + \text{R}-\text{X} \rightarrow \text{R}-\text{Nu} + \text{X}^-$

Leaving group ability parallels the stability of the departing anion (weaker conjugate base = better leaving group): $\text{I}^- > \text{Br}^- > \text{Cl}^- \gg \text{F}^- \quad (\text{R}-\text{I} \text{ is most reactive})$

Nucleophilic substitution bifurcates into two distinct mechanisms:

Bimolecular Nucleophilic Substitution ($\text{S}_\text{N}2$)

1. Kinetics: The rate depends simultaneously on the concentration of both the substrate and the attacking nucleophile (second-order kinetics): $\text{Rate} = k[\text{RX}][\text{Nu}^-]$
2. Mechanism: A single concerted step with no intermediate. The nucleophile attacks the $\alpha$-carbon from the backside ($180^\circ$ directly opposite the departing halogen) while the carbon-halogen bond is simultaneously breaking.
3. Transition State: Proceeds through an unstable, high-energy pentacoordinate transition state where the central carbon is $sp^2$ hybridized, with three coplanar equatorial bonds and two partial apical bonds to the incoming nucleophile and the departing halide.
4. Stereochemistry: 100% Walden inversion of configuration (the three remaining groups flip over like an umbrella blown inside out by a gust of wind).
5. Substrate Reactivity: Governed strictly by steric hindrance: $\text{Methyl} > 1^\circ\text{ (primary)} > 2^\circ\text{ (secondary)} \gg 3^\circ\text{ (tertiary, virtually unreactive})$
6. Solvent Preference: Favored by polar aprotic solvents (acetone, DMSO, DMF, acetonitrile) which solvate metal cations but leave nucleophilic anions unencumbered and highly reactive.

Unimolecular Nucleophilic Substitution ($\text{S}_\text{N}1$)

1. Kinetics: The rate depends exclusively on the concentration of the alkyl halide substrate (first-order kinetics): $\text{Rate} = k[\text{RX}]$
2. Mechanism: Proceeds across two distinct steps:
   • Step 1 (Slow, Rate-Determining Step): Heterolytic fission of the polar $\text{C}-\text{X}$ bond generates a planar carbocation intermediate ($\text{R}^+$) and a halide anion: $\text{R}-\text{X} \xrightarrow{\text{slow, RDS}} \text{R}^+ + \text{X}^-$
   • Step 2 (Fast): The nucleophile attacks the planar carbocation from either the front or rear face with equal probability: $\text{R}^+ + \text{Nu}^- \xrightarrow{\text{fast}} \text{R}-\text{Nu}$
3. Stereochemistry: Results in racemization (formation of an equimolar mixture of retention and inversion products, producing an optically inactive racemic mixture if the original substrate was chiral).
4. Substrate Reactivity: Governed strictly by carbocation stability ($3^\circ > 2^\circ \gg 1^\circ > \text{methyl}$): $3^\circ\text{ (tertiary)} > 2^\circ\text{ (secondary)} \gg 1^\circ\text{ (primary)} > \text{methyl}$ Tertiary carbocations are stabilized by inductive electron release ($+I$) from three alkyl groups and hyperconjugation with nine $\alpha$-hydrogens. Allylic and benzylic halides also undergo rapid $\text{S}_\text{N}1$ substitution due to resonance stabilization of their carbocations.
5. Solvent Preference: Favored by polar protic solvents ($\text{H}_2\text{O}, \text{ROH}, \text{RCOOH}$) whose high dielectric constants and hydrogen-bonding capabilities stabilize both the developing carbocation and the halide anion.

Elimination Reactions: Saytzeff's Rule

When an alkyl halide possessing $\beta$-hydrogens is treated with a strong base at elevated temperatures, elimination of $\text{HX}$ competes with substitution, yielding an alkene. In unsymmetrical substrates, dehydrohalogenation follows Saytzeff's (Zaitsev's) rule:

In $\beta$-elimination reactions, hydrogen is eliminated preferentially from the $\beta$-carbon atom bearing the fewer number of hydrogen atoms, yielding the more highly substituted, more alkylated, and thermodynamically more stable alkene as the major product.

For example, heating 2-bromobutane with alcoholic potassium hydroxide yields $80\%$ but-2-ene (disubstituted alkene) and only $20\%$ but-1-ene (monosubstituted alkene):

$\text{CH}_3-\text{CH}_2-\text{CH(Br)}-\text{CH}_3 \xrightarrow{\text{alc. KOH}, \Delta} \underset{\text{But-2-ene (major, 80\%)}}{\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3} + \underset{\text{But-1-ene (minor, 20\%)}}{\text{CH}_3-\text{CH}_2-\text{CH}=\text{CH}_2}$

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2. Grignard Reagents: Synthesis & Organometallic Transformations

Discovered by French chemist Victor Grignard in 1900, organomagnesium halides ($\text{RMgX}$) are prepared by reacting an alkyl or aryl halide with pure magnesium turnings in anhydrous, dry diethyl ether:

$\text{R}-\text{X} + \text{Mg} \xrightarrow{\text{dry ether}} \text{R}-\text{Mg}-\text{X}$

Reactivity follows the halide bond weakness: $\text{R}-\text{I} > \text{R}-\text{Br} > \text{R}-\text{Cl}$. Dry ether is indispensable because ether oxygen atoms coordinate their lone pairs to the electron-deficient magnesium atom, stabilizing the organometallic complex.

Polar Nature & Carbanionic Character:

Carbon is more electronegative ($2.55$) than magnesium ($1.31$). Consequently, the carbon-magnesium bond possesses substantial polar covalent character ($\approx 35\%$ ionic character), placing a strong partial negative charge on carbon:

$\overset{\delta-}{\text{R}}-\overset{\delta+}{\text{Mg}}-\overset{\delta-}{\text{X}}$

The alkyl carbon acts as a potent carbanionic nucleophile or powerful Bronsted base.

Master Synthetic Applications of Grignard Reagents:

1. Reaction with Active Hydrogen Compounds: Grignard reagents react violently with water, alcohols, phenols, carboxylic acids, amines, or terminal alkynes, abstracting a proton to yield the corresponding alkane quantitatively: $\text{R}-\text{MgX} + \text{H}_2\text{O} \rightarrow \text{R}-\text{H} + \text{Mg(OH)X}$ $\text{CH}_3\text{CH}_2\text{MgBr} + \text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{CH}_3 + \text{Mg(OC}_2\text{H}_5)\text{Br}$
2. Addition to Formaldehyde (Methanal, $\text{HCHO}$): Yields a primary ($1^\circ$) alcohol with one additional carbon: $\text{R}-\text{MgX} + \text{H}-\overset{\overset{\text{O}}{\parallel}}{\text{C}}-\text{H} \rightarrow \left[\text{R}-\text{CH}_2-\text{OMgX}\right] \xrightarrow{\text{H}_3\text{O}^+} \text{R}-\text{CH}_2-\text{OH} + \text{Mg(OH)X}$
3. Addition to Other Aldehydes ($\text{R}'\text{CHO}$): Yields a secondary ($2^\circ$) alcohol: $\text{R}-\text{MgX} + \text{R}'-\overset{\overset{\text{O}}{\parallel}}{\text{C}}-\text{H} \rightarrow \left[\text{R}'-\text{CH(R)}-\text{OMgX}\right] \xrightarrow{\text{H}_3\text{O}^+} \text{R}'-\text{CH(OH)}-\text{R}$
4. Addition to Ketones ($\text{R}'\text{COR}''$): Yields a tertiary ($3^\circ$) alcohol: $\text{R}-\text{MgX} + \text{R}'-\overset{\overset{\text{O}}{\parallel}}{\text{C}}-\text{R}'' \rightarrow \left[\text{R}'-\text{C(R)(R}'')-\text{OMgX}\right] \xrightarrow{\text{H}_3\text{O}^+} \text{R}'-\text{C(OH)(R)}-\text{R}''$
5. Addition to Carbon Dioxide (Dry Ice, $\text{CO}_2$): Yields a carboxylic acid with one additional carbon atom: $\text{R}-\text{MgX} + \text{O}=\text{C}=\text{O} \rightarrow \text{R}-\overset{\overset{\text{O}}{\parallel}}{\text{C}}-\text{OMgX} \xrightarrow{\text{H}_3\text{O}^+} \text{R}-\text{COOH} + \text{Mg(OH)X}$

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3. Alcohols & Phenols: Discrimination Tests & Acidity Profiles

The Lucas Test (Distinguishing $1^\circ, 2^\circ, 3^\circ$ Alcohols)

The Lucas reagent is an equimolar solution of concentrated hydrochloric acid ($\text{conc. HCl}$) and anhydrous zinc chloride ($\text{anh. ZnCl}_2$) catalyst. Alcohols react by converting into insoluble alkyl chlorides, causing visible cloudiness or turbidity in the aqueous reaction medium:

$\text{R}-\text{OH} + \text{HCl} \xrightarrow{\text{anh. } \text{ZnCl}_2} \text{R}-\text{Cl} \downarrow + \text{H}_2\text{O}$

Because the reaction proceeds via an $\text{S}_\text{N}1$ mechanism involving a carbocation intermediate, the rate of turbidity formation reflects carbocation stability:

• Tertiary ($3^\circ$) Alcohols: Form an insoluble oily layer/turbidity immediately at room temperature.
• Secondary ($2^\circ$) Alcohols: Produce turbidity within 5 to 10 minutes at room temperature.
• Primary ($1^\circ$) Alcohols: Show no turbidity at room temperature; cloudiness appears only upon prolonged heating in a boiling water bath.

Phenol Acidity vs. Alcohols & Carboxylic Acids

Phenol ($\text{C}_6\text{H}_5\text{OH}$, carbolic acid) is a weak Bronsted acid ($pK_a \approx 10.0$):

$\text{C}_6\text{H}_5\text{OH} + \text{H}_2\text{O} \rightleftharpoons \text{C}_6\text{H}_5\text{O}^- + \text{H}_3\text{O}^+$

1. Comparison with Aliphatic Alcohols ($pK_a \approx 16\text{-}18$): Phenol is roughly one million times more acidic than ethanol. In aliphatic alkoxide ions ($\text{RO}^-$), the negative charge is localized strictly on the oxygen atom and is further destabilized by the $+I$ inductive electron push of the alkyl group. In the phenoxide ion ($\text{C}_6\text{H}_5\text{O}^-$), the unshared lone pair on oxygen is delocalized over the aromatic ring via resonance, spreading the negative charge across the ortho and para ring carbons across four contributing structures. Phenoxide resonance delocalization stabilizes the conjugate base, driving dissociation. Phenol dissolves readily in aqueous sodium hydroxide ($\text{NaOH}$) to form sodium phenoxide, whereas alcohols do not react with aqueous alkali.
2. Comparison with Carboxylic Acids ($pK_a \approx 4.8$): Carboxylic acids are substantially more acidic than phenol ($pK_a \sim 4.8$ vs. $10.0$). In the carboxylate anion ($\text{RCOO}^-$), the negative charge is delocalized symmetrically between two highly electronegative oxygen atoms. In phenoxide, the negative charge is delocalized predominantly onto less electronegative carbon atoms. Consequently, carboxylic acids react with weak bases like sodium bicarbonate ($\text{NaHCO}_3$) to effervesce carbon dioxide gas vigorously, whereas phenol gives NO reaction with $\text{NaHCO}_3$.

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4. Carbonyl Compounds: Nucleophilic Addition & Diagnostic Testing

Aldehydes and ketones contain a planar carbonyl group ($\text{C}=\text{O}$). Because oxygen is significantly more electronegative than carbon, the carbonyl $\pi$-bond is strongly polarized: $\overset{\delta+}{\text{C}}=\overset{\delta-}{\text{O}}$. The primary reaction pathway is nucleophilic addition ($\text{A}_\text{N}$), where a nucleophile attacks the electrophilic carbonyl carbon to form a tetrahedral alkoxide intermediate.

Reactivity: Aldehydes are more reactive toward nucleophiles than ketones because:

1. Steric Factor: Aldehydes possess only one alkyl substituent adjacent to the carbonyl carbon, leaving it open to backside attack; ketones have two bulky alkyl groups that sterically hinder nucleophilic approach.
2. Electronic Factor: In ketones, two electron-donating alkyl groups ($+I$) disperse and diminish the partial positive charge on the carbonyl carbon; in aldehydes, only one alkyl group is present.

Qualitative Diagnostic Tests Distinguishing Aldehydes from Ketones

1. Tollens' Test (Silver Mirror Test): Tollens' reagent is ammoniacal silver nitrate solution ($[\text{Ag}(\text{NH}_3)_2]^+\text{OH}^-$). Aldehydes reduce $\text{Ag}^+$ to elemental silver, forming a brilliant reflective silver mirror on the clean inner glass wall of the test tube. Ketones do not react: $\text{R}-\text{CHO} + 2[\text{Ag}(\text{NH}_3)_2]^+ + 3\text{OH}^- \rightarrow \text{R}-\text{COO}^- + 2\text{Ag} \downarrow + 4\text{NH}_3 + 2\text{H}_2\text{O}$
2. Fehling's & Benedict's Tests: Fehling's solution contains cupric ions ($\text{Cu}^{2+}$) complexed with alkaline tartrate; Benedict's solution complexed with alkaline citrate. Aliphatic aldehydes reduce the deep blue $\text{Cu}^{2+}$ complex to a brick-red precipitate of cuprous oxide ($\text{Cu}_2\text{O}$): $\text{R}-\text{CHO} + 2\text{Cu}^{2+} + 5\text{OH}^- \rightarrow \text{R}-\text{COO}^- + \underset{\text{Brick-red ppt}}{\text{Cu}_2\text{O} \downarrow} + 3\text{H}_2\text{O}$ (Ketones and aromatic aldehydes like benzaldehyde do NOT reduce Fehling's solution!)
3. The Iodoform Test (Haloform Reaction):
   • Structural Criteria: Identifies compounds containing either a methyl carbonyl group ($\text{CH}_3-\text{CO}-$) or a methyl carbinol group ($\text{CH}_3-\text{CH(OH)}-$, which oxidizes in situ to a methyl carbonyl).
   • Reagent: Iodine dissolved in aqueous sodium hydroxide ($\text{I}_2 + \text{NaOH}$ or $\text{NaOI}$).
   • Positive Observation: Formation of a canary-yellow crystalline precipitate of iodoform ($\text{CHI}_3$), characterized by a medicinal antiseptic odor and melting point of $119^\circ\text{C}$: $\text{CH}_3-\overset{\overset{\text{O}}{\parallel}}{\text{C}}-\text{R} + 3\text{I}_2 + 4\text{NaOH} \rightarrow \underset{\text{Yellow ppt}}{\text{CHI}_3 \downarrow} + \text{R}-\text{COONa} + 3\text{NaI} + 3\text{H}_2\text{O}$
   • Positive Compounds: Acetaldehyde (the only aldehyde giving a positive test!), all methyl ketones (acetone, butan-2-one), ethanol (the only primary alcohol giving a positive test!), and secondary methyl carbinols (propan-2-ol, butan-2-ol).

Condensation Mechanisms: Aldol vs. Cannizzaro

1. Aldol Condensation

• Requirement: Aldehydes or ketones containing at least one $\alpha$-hydrogen atom.
• Reagent: Dilute aqueous alkali ($10\%\, \text{NaOH}$ or $\text{Ba(OH)}_2$).
• Mechanism: Hydroxide abstracts an acidic $\alpha$-proton to form a resonance-stabilized enolate carbanion, which attacks the carbonyl carbon of a second aldehyde molecule. Proton transfer yields a $\beta$-hydroxyaldehyde (aldol) or $\beta$-hydroxyketone (ketol). Heating dehydrates the product into an $\alpha,\beta$-unsaturated carbonyl compound: $2\text{CH}_3\text{CHO} \xrightarrow{\text{dil. NaOH}} \underset{\text{3-Hydroxybutanal (Aldol)}}{\text{CH}_3-\text{CH(OH)}-\text{CH}_2-\text{CHO}} \xrightarrow{\Delta, -\text{H}_2\text{O}} \underset{\text{Crotonaldehyde}}{\text{CH}_3-\text{CH}=\text{CH}-\text{CHO}}$

2. Cannizzaro Reaction

• Requirement: Aldehydes that possess NO $\alpha$-hydrogen atoms (e.g., formaldehyde $\text{HCHO}$, benzaldehyde $\text{C}_6\text{H}_5\text{CHO}$, trimethylacetaldehyde $(\text{CH}_3)_3\text{C}-\text{CHO}$).
• Reagent: Concentrated alkali ($50\%\, \text{NaOH}$ or $\text{KOH}$).
• Mechanism: A self-oxidation-reduction (disproportionation) reaction where one molecule is reduced to an alcohol, while the second molecule is oxidized to a carboxylic acid salt: $2\text{HCHO} + \text{NaOH} \rightarrow \underset{\text{Methanol}}{\text{CH}_3\text{OH}} + \underset{\text{Sodium formate}}{\text{HCOONa}}$ $2\text{C}_6\text{H}_5\text{CHO} + \text{KOH} \rightarrow \underset{\text{Benzyl alcohol}}{\text{C}_6\text{H}_5\text{CH}_2\text{OH}} + \underset{\text{Potassium benzoate}}{\text{C}_6\text{H}_5\text{COOK}}$

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5. Master Diagnostic Tests Summary Table