11.3 Alkenes and Alkynes: Addition Reactions, Markovnikov Rule, and Stereospecificity

Alkenes and Alkynes: Addition Reactions, Markovnikov Rule, and Stereospecificity

Alkenes and alkynes are fundamental unsaturated hydrocarbons characterized by carbon-carbon $\pi$ bonds. Their high electron density makes them prime nucleophiles in electrophilic addition reactions. For the PRC Chemist Licensure Examination, candidates must master the stereoelectronic principles dictating regiochemical outcomes (Markovnikov vs. anti-Markovnikov additions), stereospecific additions (syn vs. anti pathways), carbocation rearrangement pathways, and alkyne synthetic methodologies.

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1. Electronic Structure, Bonding, and Thermodynamic Stability

Orbital Architecture

• Alkene Carbon ($sp^2$): Possesses planar trigonal geometry ($120^\circ$ bond angles). One $\sigma$ bond forms via end-on overlap of $sp^2-sp^2$ hybrid orbitals ($C-C$ length $\approx 1.34\text{ \AA}$), while one $\pi$ bond forms through lateral side-by-side overlap of unhybridized $2p_z$ orbitals. The $\pi$ bond possesses an electron density nodal plane in the molecular plane, resulting in a rotational barrier of $\approx 264\text{ kJ/mol}$ ($63\text{ kcal/mol}$), preventing room-temperature rotation and establishing geometric $(E)/(Z)$ isomerism.
• Alkyne Carbon ($sp$): Possesses linear geometry ($180^\circ$ bond angles). A carbon-carbon triple bond ($C\equiv C$ length $\approx 1.20\text{ \AA}$) comprises one $\sigma$ bond ($sp-sp$ overlap) and two mutually perpendicular $\pi$ bonds ($2p_y-2p_y$ and $2p_z-2p_z$ overlap), creating a cylindrical sheath of $\pi$ electron density.

Alkene Stability and Heats of Hydrogenation ($-\Delta H^\circ_{\text{hydrog}}$)

The thermodynamic stability of alkenes increases systematically with the degree of alkyl substitution: $\text{Tetrasubstituted} > \text{Trisubstituted} > \text{Disubstituted} > \text{Monosubstituted} > \text{Ethylene}$ Among disubstituted isomers: $\text{trans} > \text{geminal disubstituted (1,1-)} > \text{cis}$.

Physical Rationale: Alkyl groups stabilize the electron-deficient $\pi^*$ antibonding orbital via hyperconjugation (donation of electron density from adjacent $\sigma_{C-H}$ or $\sigma_{C-C}$ bonds into the empty $\pi^*$ orbital). Additionally, $C_{sp^2}-C_{sp^3}$ bonds have greater $s$-character than $C_{sp^3}-C_{sp^3}$ bonds, resulting in shorter, thermodynamically stronger bonds. In cis-alkenes, steric clash between alkyl groups raises ground-state potential energy.

Heats of catalytic hydrogenation directly measure relative ground-state thermodynamic stabilities (lower $-\Delta H^\circ_{\text{hydrog}}$ indicates greater thermodynamic stability):

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2. Electrophilic Addition Reactions of Alkenes

Electrophilic addition is initiated by the attack of the nucleophilic $\pi$ bond on an electrophilic species, generating either an open carbocation or a bridged three-membered onium intermediate.

Halogenation ($X_2 = Br_2, Cl_2$): Stereospecific Anti-Addition

Halogenation in non-nucleophilic solvents ($CH_2Cl_2, CCl_4$) proceeds through a three-membered cyclic halonium ion (e.g., bromonium ion):

1. Polarization of $X-X$ by the alkene $\pi$ cloud leads to electrophilic attack, expelling $X^-$ and forming a bridged halonium ring.
2. The halide ion ($X^-$) attacks the halonium ion from the backside ($S_N2$-like anti-periplanar trajectory), inverting configuration at that carbon.
3. The net stereochemical outcome is stereospecific anti-addition:
   • cis-2-Butene $+ Br_2 \implies$ $(\pm)$-racemic 2,3-dibromobutane ($(2R, 3R)$ and $(2S, 3S)$).
   • trans-2-Butene $+ Br_2 \implies$ meso-2,3-dibromobutane ($(2R, 3S)$).

Halohydrin Formation ($X_2 + H_2O$)

When halogenation occurs in water or alcohol, solvent molecules act as the competing nucleophile:

• The cyclic halonium ion opens by nucleophilic attack of $H_2O$ exclusively at the more substituted carbon because that carbon carries greater partial positive charge (better carbocation-like character).
• Subsequent deprotonation delivers a halohydrin with $-OH$ on the more substituted carbon and $-X$ on the less substituted carbon, exhibiting both Markovnikov regioselectivity and anti-stereospecificity.

Hydrohalogenation ($HX = HCl, HBr, HI$) and Carbocation Rearrangements

1. Markovnikov's Rule: The electrophile ($H^+$) adds to the carbon of the double bond bearing more hydrogen atoms, generating the more substituted, thermodynamically stable carbocation intermediate ($3^\circ > 2^\circ > 1^\circ$).
2. Rearrangement Hazard: Because open planar carbocations are formed, skeletal rearrangements (1,2-hydride or 1,2-alkyl shifts) occur whenever a more stable carbocation can be generated: $(CH_3)_3C-CH=CH_2 + HCl \to \left[(CH_3)_3C-\overset{+}{C}H-CH_3 \xrightarrow{\text{1,2-methyl shift}} (CH_3)_2\overset{+}{C}-CH(CH_3)_2\right] \to (CH_3)_2C(Cl)-CH(CH_3)_2$ Major Product: 2-chloro-2,3-dimethylbutane (rearranged $3^\circ$ chloride), rather than 3-chloro-2,2-dimethylbutane ($2^\circ$).
3. Peroxide Effect (Free-Radical Anti-Markovnikov Addition): In the presence of peroxides ($ROOR$) or light, only $HBr$ adds in an anti-Markovnikov orientation. Peroxide initiates formation of a bromine radical ($Br^\bullet$), which adds to the less substituted carbon to generate the more stable carbon radical ($3^\circ > 2^\circ > 1^\circ$). Subsequent hydrogen abstraction from $HBr$ completes the chain. Exam Note: $HCl$ and $HI$ do NOT exhibit the peroxide effect because one of the propagation steps is strongly endothermic (for $HCl$, $H-\text{abstraction}$ from $HCl$ is too endothermic; for $HI$, $I^\bullet$ addition to the double bond is endothermic).

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3. Hydration Methodologies: Comparative Regio- and Stereochemistry

Preparing alcohols from alkenes is heavily tested through three distinct synthetic routes:

                                  ALKENE
                                    |
        +---------------------------+---------------------------+
        |                           |                           |
 Acid-Catalyzed               Oxymercuration-             Hydroboration-
   Hydration                   Demercuration                Oxidation
(H2SO4 / H2O)              (1. Hg(OAc)2; 2. NaBH4)     (1. BH3·THF; 2. H2O2, NaOH)
        |                           |                           |
• Markovnikov               • Markovnikov               • Anti-Markovnikov
• Carbocation formed        • Mercurinium ion           • Concerted 4-center TS
• REARRANGEMENTS OCCUR      • NO REARRANGEMENTS         • Syn-addition
• Non-stereospecific        • Anti-addition (step 1)    • Retention in oxidation

1. Acid-Catalyzed Hydration ($H_2SO_4, H_2O$)

• Reagents: Catalytic dilute $H_2SO_4$ in water.
• Regiochemistry: Markovnikov.
• Mechanism: Protonation generates an open carbocation, which is trapped by $H_2O$.
• Limitations: Subject to carbocation rearrangements and polymerization under strongly acidic conditions.

2. Oxymercuration-Demercuration

• Reagents: Step 1: $Hg(OAc)_2$ in $THF/H_2O$; Step 2: $NaBH_4, NaOH$.
• Regiochemistry: Markovnikov ($-OH$ adds to more substituted carbon; $-H$ to less substituted).
• Mechanism: Electrophilic attack forms a cyclic mercurinium ion, which undergoes backside attack by $H_2O$ at the more substituted carbon (anti-addition). Subsequent reduction with $NaBH_4$ replaces $-HgOAc$ with $-H$.
• Advantage: Zero carbocation rearrangements; high yields of Markovnikov alcohols from alkenes with quaternary adjacent carbons (e.g., 3,3-dimethyl-1-butene yields 3,3-dimethyl-2-butanol).

3. Hydroboration-Oxidation

• Reagents: Step 1: $BH_3 \cdot THF$ (or $B_2H_6$); Step 2: $H_2O_2, NaOH, H_2O$.
• Regiochemistry: Anti-Markovnikov ($-OH$ attaches to less substituted carbon).
• Mechanism: Boron is electrophilic (empty $2p$ orbital). Addition across the double bond occurs in a single concerted 4-membered cyclic transition state: $\left[\begin{matrix} R-CH & = & CH_2 \\ \vdots & & \vdots \\ H & \cdots & BH_2 \end{matrix}\right]^\ddagger$ Boron attaches to the less hindered carbon due to steric factors and partial positive charge stabilization. Both $-H$ and $-BH_2$ add from the same face (stereospecific syn-addition).
• Oxidation: Alkaline hydroperoxide ($HOO^-$) coordinates boron, followed by 1,2-alkyl migration with complete retention of stereochemical configuration at the migrating carbon.

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4. Epoxidation, Dihydroxylation, and Oxidative Cleavage

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5. Alkyne Chemistry and Synthetic Transformations

Terminal Alkyne Acidity

The terminal proton of an alkyne exhibits remarkable acidity compared to other hydrocarbons: $R-C\equiv C-H \quad (pK_a \approx 25) \quad \text{vs.} \quad R_2C=CH_2 \quad (pK_a \approx 44) \quad \text{vs.} \quad R-CH_3 \quad (pK_a \approx 50)$ Physical Rationale: The $C_{sp}-H$ bonding orbital has $50\%$ $s$-character (compared to $33\%$ in $sp^2$ and $25\%$ in $sp^3$). Electrons in $sp$ orbitals are held closer to the positively charged carbon nucleus, strongly stabilizing the resulting carbanion conjugate base—the acetylide ion ($R-C\equiv C^-$).

Terminal alkynes are quantitatively deprotonated by strong bases such as sodium amide ($NaNH_2$) in liquid ammonia ($pK_a$ of conjugate acid $NH_3 \approx 38$). Alkoxide bases ($-OR$, conjugate acid alcohol $pK_a \approx 16$) and hydroxide ($OH^-$, water $pK_a = 15.7$) are far too weak to deprotonate terminal alkynes.

Carbon-Carbon Bond Formation via Acetylide Alkylation

Acetylide anions act as powerful carbon nucleophiles in $S_N2$ displacements with unhindered primary alkyl halides ($1^\circ$ or methyl): $R-C\equiv C:^- Na^+ + R'-CH_2-Br \xrightarrow{S_N2} R-C\equiv C-CH_2-R' + NaBr$ Limitation: With secondary ($2^\circ$) or tertiary ($3^\circ$) alkyl halides, acetylide ions act primarily as strong Brønsted bases, causing predominant $E_2$ elimination to yield alkenes.

Stereoselective Reduction of Alkynes

Alkynes can be selectively converted into either geometric isomer of an alkene:

1. Reduction to cis-Alkene (Z-Alkene): Catalytic hydrogenation using Lindlar's catalyst ($Pd / CaCO_3$ poisoned with lead acetate and quinoline). Poisoning moderates catalytic activity, stopping reduction cleanly at the alkene stage via stereospecific syn-addition of hydrogen.
2. Reduction to trans-Alkene (E-Alkene): Dissolving metal reduction using Sodium (or Lithium) in liquid ammonia ($Na / NH_3(l)$) at $-33^\circ\text{C}$. The reaction proceeds via sequential single-electron transfer (SET) to generate a radical anion. The radical anion adopts a trans configuration to minimize mutual Coulombic and steric repulsion between the electron pairs, delivering exclusively the trans-alkene.
3. Exhaustive Hydrogenation: Hydrogenation over active catalysts ($Pt, Pd/C, Ni$) under excess $H_2$ reduces alkynes completely to alkanes.

Hydration of Alkynes: Methyl Ketones vs. Aldehydes

• Mercuric-Catalyzed Hydration ($HgSO_4, H_2SO_4, H_2O$): Follows Markovnikov's rule. Addition of water across the triple bond generates an unstable enol intermediate with $-OH$ at the internal carbon. Rapid keto-enol tautomerization produces a methyl ketone.
• Hydroboration-Oxidation of Terminal Alkynes: Using sterically hindered dialkylboranes such as disiamylborane ($Sia_2BH$) or $9\text{-BBN}$ (to prevent double hydroboration), boron attaches to the terminal carbon (anti-Markovnikov). Alkaline peroxide oxidation generates an enol with $-OH$ at the terminal carbon, which tautomerizes to an aldehyde.