Addition Reactions

The carbon-carbon double bond is the defining functional group of alkenes and the site of their characteristic reactivity. The $\pi$ bond, formed by lateral overlap of two$p$ orbitals, is weaker than the underlying $\sigma$ bond ($\sim 264 \;\text{kJ/mol}$ for $\pi$ vs.$\sim 347 \;\text{kJ/mol}$ for $\sigma$) and its electron density lies above and below the molecular plane, making it accessible to electrophiles.

In an addition reaction, the $\pi$ bond breaks and two new $\sigma$ bonds form to the two carbons of the former double bond. The general equation is:

Because one $\pi$ bond ($\sim 264 \;\text{kJ/mol}$) is replaced by two$\sigma$ bonds ($\sim 2 \times 347 \;\text{kJ/mol}$), addition reactions are typically exothermic by about $\sim 80$–$180 \;\text{kJ/mol}$, depending on the reagent.

2.1 Mechanism

The addition of hydrogen halides ($\text{HCl}$, $\text{HBr}$,$\text{HI}$) to alkenes proceeds through a two-step electrophilic addition mechanism:

1. Step 1 (rate-determining): The $\pi$ electrons of the alkene attack the electrophilic proton of HX. A new C–H $\sigma$ bond forms, generating a carbocation intermediate:
   $$\text{C=C} + \text{H-X} \xrightarrow{\text{slow}} \text{C}^+\text{-CH} + \text{X}^-$$
2. Step 2 (fast): The halide ion (nucleophile) attacks the electron-deficient carbocation:
   $$\text{C}^+ + \text{X}^- \xrightarrow{\text{fast}} \text{C-X}$$

2.2 Markovnikov's Rule

Markovnikov's rule states that when HX adds to an unsymmetrical alkene, the hydrogen adds to the carbon with more hydrogen atoms (the less substituted carbon), and the halide adds to the carbon with fewer hydrogen atoms (the more substituted carbon):

The modern restatement: the proton adds to form the more stable carbocation. Carbocation stability follows the order:

This stability order arises from hyperconjugation — electron donation from adjacent C–H $\sigma$ bonds into the empty$p$ orbital of the carbocation — and from inductive effects of alkyl groups.

2.3 Carbocation Rearrangements

Because the reaction passes through a carbocation intermediate, rearrangements can occur. A secondary carbocation can rearrange to a more stable tertiary carbocation via:

• 1,2-Hydride shift: a hydrogen migrates with its bonding electrons to the adjacent carbocationic center
• 1,2-Methyl shift: a methyl group migrates similarly

These rearrangements are fast (barrierless) when they convert a less stable carbocation to a more stable one, and they can divert the product to an unexpected regiochemistry.

The addition of water to an alkene in the presence of an acid catalyst (typically$\text{H}_2\text{SO}_4$ or $\text{H}_3\text{PO}_4$) produces an alcohol. The mechanism closely parallels hydrohalogenation:

1. Protonation of the alkene by $\text{H}_3\text{O}^+$to form a carbocation (Markovnikov regiochemistry).
2. Nucleophilic attack by water on the carbocation to form an oxonium ion ($\text{R-OH}_2^+$).
3. Deprotonation by water to regenerate the acid catalyst and yield the alcohol product.

The product is a Markovnikov alcohol — the OH ends up on the more substituted carbon. This reaction is the reverse of acid-catalyzed dehydration, and the equilibrium position is governed by Le Chatelier's principle: excess water drives the equilibrium toward the alcohol, while removal of water (or use of concentrated acid) favors the alkene.

4.1 Mechanism via Halonium Ion

Addition of $\text{Br}_2$ or $\text{Cl}_2$ to an alkene proceeds through a cyclic halonium ion intermediate:

1. Step 1: The $\pi$ electrons attack one halogen atom of $\text{X}_2$, displacing $\text{X}^-$ and forming a three-membered cyclic halonium ion:
   $$\text{C=C} + \text{X}_2 \longrightarrow \begin{bmatrix} \text{C--X--C} \end{bmatrix}^+ + \text{X}^-$$
2. Step 2: The halide ion ($\text{X}^-$) attacks the halonium ion in an S$_\text{N}$2-like fashion from the back side, opening the three-membered ring:
   $$\text{X}^- + \begin{bmatrix} \text{C--X--C} \end{bmatrix}^+ \longrightarrow \text{X-C-C-X}$$

4.2 Anti Addition Stereochemistry

Because the nucleophilic halide attacks from the face opposite to the halonium ion bridge, the two halogen atoms end up on opposite faces of the former double bond. This is anti addition (anti-periplanar stereochemistry).

For a cyclic alkene like cyclohexene, bromination gives exclusively the trans-1,2-dibromide — no cis product is formed. This stereospecificity is strong evidence for the halonium ion mechanism.

4.3 Halohydrin Formation

When the halogenation is performed in water, the water molecule can serve as the nucleophile instead of the halide ion:

The product is a halohydrin (a $\beta$-halo alcohol). The stereochemistry is again anti (water attacks the opposite face from the bridging halogen). The regiochemistry follows Markovnikov's rule: water attacks the more substituted carbon of the halonium ion (where there is more positive charge).

Hydroboration-oxidation, discovered by Herbert C. Brown in 1956, is one of the most important reactions in organic synthesis. It achieves the anti-Markovnikov addition of water across a double bond with syn stereochemistry — the opposite regiochemical and stereochemical outcome compared to acid-catalyzed hydration.

5.1 Hydroboration Step

Borane ($\text{BH}_3$), typically used as the THF complex ($\text{BH}_3 \cdot \text{THF}$) or as the dimer diborane ($\text{B}_2\text{H}_6$), adds to the alkene in a single concerted step through a four-membered cyclic transition state:

Key features of hydroboration:

• Syn addition: B and H add to the same face of the double bond (cis stereochemistry)
• Anti-Markovnikov regiochemistry: boron attaches to the less substituted carbon (steric control)
• No carbocation intermediate: no rearrangements occur
• Each $\text{BH}_3$ reacts with three equivalents of alkene to form a trialkylborane ($\text{R}_3\text{B}$)

5.2 Oxidation Step

The trialkylborane is then oxidized with alkaline hydrogen peroxide:

The C–B bond is replaced by a C–OH bond with retention of configuration(the OH replaces the B on the same face). Combined with the syn hydroboration, the overall result is syn, anti-Markovnikov addition of water.

Catalytic hydrogenation adds $\text{H}_2$across the double bond in the presence of a heterogeneous metal catalyst (Pt, Pd, Ni, or Rh):

6.1 Mechanism on the Metal Surface

The reaction occurs on the catalyst surface in four steps:

1. Adsorption of $\text{H}_2$ on the metal surface (dissociative chemisorption into two M–H bonds)
2. Adsorption of the alkene via its $\pi$ bond to the surface
3. Sequential transfer of one hydrogen atom from the surface to each carbon
4. Desorption of the saturated product from the surface

Since both hydrogens are delivered from the same face of the metal surface, the stereochemistry is syn addition. For a cyclic alkene, this produces the cis product.

6.2 Heats of Hydrogenation and Stability

The heat of hydrogenation ($\Delta H_{\text{hyd}}$) measures the enthalpy released when one mole of $\text{H}_2$ adds to one mole of double bond. Less negative $\Delta H_{\text{hyd}}$ indicates a more stable (lower energy) alkene:

The data confirm that more substituted alkenes are more stable(hyperconjugation) and that trans alkenes are more stable than cis(reduced steric strain).

Epoxidation converts an alkene to an epoxide (oxirane) — a three-membered ring containing an oxygen atom. The most common reagent is a peroxyacid (peracid), such asm-chloroperoxybenzoic acid (mCPBA):

7.1 Concerted Mechanism

The epoxidation proceeds through a concerted, "butterfly" transition state in which the oxygen atom of the peracid is delivered to both carbons of the double bond simultaneously. The key features:

• Syn addition: the oxygen is delivered to one face of the alkene
• Stereospecific: cis-alkenes give cis-epoxides; trans-alkenes give trans-epoxides
• Electrophilic oxygen: more electron-rich alkenes react faster
• No carbocation intermediate — no rearrangements

7.2 Sharpless Asymmetric Epoxidation

K. Barry Sharpless (Nobel Prize, 2001) developed a method for the enantioselective epoxidation of allylic alcohols using titanium(IV) isopropoxide, tert-butyl hydroperoxide, and a chiral tartrate ester (diethyl tartrate or diisopropyl tartrate). The catalyst delivers the oxygen preferentially to one face of the alkene, achieving enantiomeric excesses above 90% in many cases. This was one of the first practical catalytic asymmetric reactions and remains widely used in total synthesis.

Dihydroxylation adds two hydroxyl groups across the double bond to form a 1,2-diol (vicinal diol). Two complementary methods provide opposite stereochemical outcomes:

8.1 Syn Dihydroxylation (OsO$_4$)

Osmium tetroxide ($\text{OsO}_4$) reacts with alkenes through a concerted [3+2] cycloaddition to form a cyclic osmate ester, which is then hydrolyzed to the cis-diol:

Since both oxygen atoms come from the same osmium species and are delivered to the same face, the stereochemistry is syn. In practice,$\text{OsO}_4$ is used in catalytic amounts with a co-oxidant (NMO = N-methylmorpholine N-oxide) to regenerate the osmium reagent (Upjohn dihydroxylation).

8.2 Anti Dihydroxylation

Anti dihydroxylation can be achieved by a two-step sequence: epoxidation followed by acid- or base-catalyzed ring opening of the epoxide. Since epoxidation is syn and ring opening is anti (S$_\text{N}$2), the overall result is anti addition of two OH groups — yielding the trans-diol:

8.3 Sharpless Asymmetric Dihydroxylation

Sharpless also developed an enantioselective version of osmium-catalyzed dihydroxylation using chiral ligands derived from cinchona alkaloids (the AD-mix reagents). This allows the synthesis of chiral diols with high enantiomeric excess from prochiral alkenes.

The thermodynamics of addition reactions can be analyzed using bond dissociation energies (BDEs). For the general addition $\text{C=C} + \text{X-Y} \to \text{X-C-C-Y}$:

Note that we only break the $\pi$ component of the double bond (the $\sigma$bond remains intact). Using typical BDE values:

• $D(\text{C=C})_\pi \approx 264 \;\text{kJ/mol}$
• $D(\text{H-H}) = 436 \;\text{kJ/mol}$
• $D(\text{H-Br}) = 366 \;\text{kJ/mol}$
• $D(\text{Br-Br}) = 193 \;\text{kJ/mol}$
• $D(\text{C-H}) \approx 411 \;\text{kJ/mol}$
• $D(\text{C-Br}) \approx 285 \;\text{kJ/mol}$

Hydrogenation

Bromination

Both reactions are significantly exothermic, consistent with the conversion of a weak$\pi$ bond into strong $\sigma$ bonds.

Mastering addition reactions requires tracking both regiochemistry(where the atoms add) and stereochemistry (which face they add to). The following table summarizes the outcomes for all major addition reactions:

Key Patterns

• Reactions with carbocation intermediates (HX, acid-catalyzed hydration) give Markovnikov products and are non-stereospecific because the planar carbocation can be attacked from either face.
• Reactions with cyclic intermediates (halonium ions, osmate esters, epoxides) are stereospecific because the intermediate constrains the geometry of nucleophilic attack.
• Concerted reactions (hydroboration, epoxidation, hydrogenation) deliver both atoms from the same face, giving syn addition.
• Anti-Markovnikov products require either a radical mechanism (HBr + ROOR) or hydroboration-oxidation. Ionic HX addition always gives Markovnikov products.

The following simulation calculates reaction thermodynamics, compares heats of hydrogenation, models the energy profile along the reaction coordinate, and analyzes Markovnikov selectivity based on carbocation stability.