Alkene Reactions and Synthesis: Addition, Oxidation, Reduction, and Stereochemistry

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Chapter 8: Alkenes – Reactions and Synthesis

Overview of Alkene Addition Reactions

Alkenes are versatile organic compounds that undergo a variety of addition reactions, transforming the double bond into new functional groups. These reactions are fundamental in organic synthesis and are used to prepare alcohols, halohydrins, diols, epoxides, carbonyl compounds, and cyclopropanes.

Addition reactions convert alkenes into more saturated compounds by adding atoms across the double bond.
Elimination reactions can regenerate alkenes from saturated compounds.
Common products include halohydrins, alcohols, alkanes, diols, carbonyl compounds, halides, epoxides, and cyclopropanes.

8.2 Halogenation of Alkenes: Addition of X2

Mechanism and Stereochemistry

Halogenation involves the addition of diatomic halogens (Cl2, Br2) to alkenes, forming vicinal dihalides. Fluorine is too reactive, and iodine does not react with most alkenes.

Chlorine and bromine are synthetically useful for halogenation.
Reaction occurs with anti stereochemistry via a cyclic halonium ion intermediate.
For cycloalkenes, the trans dihalide is formed.

Mechanism:

Alkene reacts with X2 to form a cyclic halonium ion.
Nucleophilic attack by halide ion opens the ring, yielding the anti product.

Relative rates: More substituted alkenes react faster due to stabilization of the halonium ion.

8.3 Halohydrins from Alkenes: Addition of HO–X

Formation and Mechanism

Halohydrins are 1,2-halo alcohols formed by the reaction of alkenes with hypohalous acids (HO–Cl, HO–Br) in the presence of water.

Halohydrin formation: Alkene reacts with Br2 or Cl2 in water, yielding a halohydrin and HX.
Cohalogenation: In nucleophilic solvents, the halonium ion is opened by the solvent (e.g., H2O, MeOH).

Mechanism:

Step 1: Alkene forms a halonium ion with Br2.
Step 2: Water acts as a nucleophile, attacking the more substituted carbon and opening the ring.
Step 3: Deprotonation yields the halohydrin.

8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration

Oxymercuration-Demercuration

Oxymercuration allows hydration of alkenes without carbocation rearrangement, yielding Markovnikov alcohols.

Step 1: Alkene reacts with mercuric acetate (Hg(OAc)2) to form a mercurinium ion.
Step 2: Water attacks, forming an organomercury alcohol.
Step 3: Sodium borohydride (NaBH4) removes mercury, yielding the alcohol.
Advantage: No carbocation rearrangement occurs.

Example: 1-Methylcyclopentene yields 1-methylcyclopentanol (Markovnikov product).

8.5 Hydration of Alkenes: Addition of H2O by Hydroboration

Hydroboration-Oxidation and Anti-Markovnikov Addition

Hydroboration-oxidation hydrates alkenes to give anti-Markovnikov alcohols with syn addition.

Step 1: Alkene reacts with borane (BH3) in THF to form an organoborane intermediate.
Step 2: Oxidation with H2O2, OH– replaces boron with OH.
Syn addition: Both H and OH add to the same face of the double bond.
Anti-Markovnikov: OH attaches to the less substituted carbon.

Example: Cyclohexene yields cyclohexanol (anti-Markovnikov product).

8.6 Reduction of Alkenes: Hydrogenation

Mechanism and Stereochemistry

Hydrogenation reduces alkenes to alkanes by addition of H2 across the double bond, typically using a metal catalyst (Pt, Pd, Ni).

Syn stereochemistry: Both hydrogens add to the same face.
Mechanism: H2 is adsorbed onto the catalyst surface; alkene approaches and is hydrogenated.
Reaction is sensitive to steric environment around the double bond.

Example: 1,2-Dimethylcyclohexene yields cis-1,2-dimethylcyclohexane.

8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation

Epoxidation

Epoxidation converts alkenes to epoxides (cyclic ethers with a three-membered ring) using peroxyacids (e.g., mCPBA).

Mechanism: Concerted addition of oxygen from peroxyacid to the double bond.
Syn addition: Cis groups in the alkene remain cis in the epoxide.
Epoxides can be named as n,n+1-epoxy derivatives or as alkene oxides.

Hydroxylation

Epoxides undergo acid-catalyzed ring opening with water to yield 1,2-diols.
Direct hydroxylation with osmium tetroxide (OsO4) gives syn diols without carbocation intermediates.

8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds

Ozonolysis and Other Oxidative Cleavage

Ozonolysis splits alkenes into carbonyl compounds (aldehydes, ketones) by reaction with ozone (O3).

Step 1: Ozone adds to the double bond, forming a molozonide.
Step 2: Molozonide rearranges to ozonide.
Step 3: Reductive workup (e.g., Zn, (CH3)2S) cleaves the ozonide, yielding carbonyl compounds.
Other oxidants (e.g., KMnO4) can also cleave double bonds, producing carboxylic acids and CO2 if hydrogens are present.

Example: Isopropylidenecyclohexane yields cyclohexanone and acetone.

8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis

Carbene Addition and Stereochemistry

Carbenes (neutral molecules with a divalent carbon) add to alkenes to form cyclopropanes.

Carbene generation: Deprotonation of chloroform (CHCl3) yields dichlorocarbene.
Stereospecificity: Addition to cis-alkenes yields a single stereoisomer.
Simmons-Smith reaction: Uses diiodomethane and Zn(Cu) to generate carbenoid species for cyclopropanation.

Example: Cyclohexene reacts with CH2I2/Zn(Cu) to yield bicyclo[4.1.0]heptane.

Anti-Markovnikov Addition with HBr

Radical Mechanism

In the presence of peroxides (ROOR), HBr adds to alkenes via a radical mechanism, yielding anti-Markovnikov products.

Initiation: Peroxide bond cleaves to form alkoxy radicals.
Propagation: Alkoxy radical abstracts H from HBr, generating Br radical.
Br radical adds to alkene, forming the more substituted radical intermediate.
Final abstraction of H from HBr yields the product and regenerates Br radical.

Example: 1-butene with HBr and peroxides yields 1-bromobutane (anti-Markovnikov).

8.12 Reaction Stereochemistry: Addition of H2O to an Achiral Alkene

Formation of Racemic Mixtures

Addition of water to an achiral alkene via acid-catalyzed hydration produces a racemic mixture of enantiomers.

Carbocation intermediate can be attacked from either side, yielding both R and S products in equal amounts.
Transition state involves partial bond formation to both faces.

Example: 1-butene yields (R)-2-butanol and (S)-2-butanol (50:50 mixture).

Retrosynthesis and Synthetic Pathways

Planning Alkene Transformations

Retrosynthetic analysis is used to plan the synthesis of target molecules from available starting materials.

Alcohols can be dehydrated to alkenes, which can then be epoxidized.
Forward synthesis involves sequential application of reactions to reach the target.

Summary Table: Alkene Addition Reactions

Reaction Type	Reagents	Product	Stereochemistry	Regioselectivity
Halogenation	Cl2, Br2	Vicinal dihalide	Anti	Not applicable
Halohydrin formation	Br2/Cl2 + H2O	Halohydrin	Anti	OH to more substituted C
Hydration (acid-catalyzed)	H2O, H+	Alcohol	Mixed	Markovnikov
Oxymercuration	Hg(OAc)2, H2O; NaBH4	Alcohol	Anti	Markovnikov
Hydroboration-oxidation	BH3, H2O2, OH–	Alcohol	Syn	Anti-Markovnikov
Hydrogenation	H2, Pt/Pd/Ni	Alkane	Syn	Not applicable
Epoxidation	Peroxyacid (mCPBA)	Epoxide	Syn	Not applicable
Hydroxylation	OsO4, H2O2	1,2-diol	Syn	Not applicable
Ozonolysis	O3, Zn/(CH3)2S	Aldehyde/ketone	Cleavage	Not applicable
Cyclopropanation	CHCl3/Zn(Cu), CH2I2/Zn(Cu)	Cyclopropane	Stereospecific	Not applicable

Key Equations

Halogenation:
Halohydrin formation:
Oxymercuration: , then
Hydroboration-oxidation: , then
Hydrogenation:
Epoxidation:
Ozonolysis:
Cyclopropanation:

Additional info:

All mechanisms involve specific stereochemical outcomes, which are crucial for predicting product structures.
Retrosynthetic analysis is a powerful tool for planning multi-step organic syntheses.