Other Reactions of Alkenes

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. Their reactivity is mainly due to the presence of this double bond, which can participate in a variety of reactions. In addition to the well-known addition reactions, alkenes can undergo a range of other reactions that can be used to synthesize different organic compounds. Below, we explore some of these reactions in detail.

Hydroboration-Oxidation

Hydroboration-oxidation is a two-step reaction process that converts alkenes into alcohols. The first step involves the addition of borane (BH3) to the double bond, followed by oxidation with hydrogen peroxide (H2O2) in the presence of a base.

Reaction Steps:

1. Hydroboration: The alkene reacts with BH3 to form a trialkylborane.
2. Oxidation: The trialkylborane is oxidized by H2O2 in the presence of NaOH to form an alcohol.

Mechanism:

The hydroboration step occurs with syn-addition, meaning that both the hydrogen and the boron atom add to the same side of the double bond. The oxidation step replaces the boron atom with a hydroxyl group.

Example:

$$ \text{CH}_2{=}CH_2 + \text{BH}_3 \rightarrow \text{CH}_3\text{CH}_2\text{BH}_2 $$ $$ \text{CH}_3\text{CH}_2\text{BH}_2 + \text{H}_2\text{O}_2/\text{NaOH} \rightarrow \text{CH}_3\text{CH}_2\text{OH} $$

Ozonolysis

Ozonolysis is a reaction where an alkene is cleaved by ozone (O3) to form carbonyl compounds such as aldehydes, ketones, or carboxylic acids, depending on the subsequent workup.

Reaction Steps:

1. Ozone addition: Ozone adds to the double bond to form a molozonide, which is unstable.
2. Molozonide rearrangement: The molozonide rearranges to a more stable ozonide.
3. Reductive workup: The ozonide is reduced, typically with zinc and acetic acid or dimethyl sulfide, to form carbonyl compounds.
4. Oxidative workup: Alternatively, oxidative workup with hydrogen peroxide can lead to carboxylic acids.

Example:

$$ \text{CH}_3\text{CH}={CH}_2 + \text{O}_3 \rightarrow \text{CH}_3\text{CO} + \text{HCHO} $$

Epoxidation

Epoxidation is the reaction where an alkene is converted to an epoxide (a three-membered cyclic ether) by the addition of an oxygen atom across the double bond.

Reaction Steps:

1. Oxygen transfer: A peracid, such as mCPBA (meta-chloroperoxybenzoic acid), transfers an oxygen atom to the alkene to form the epoxide.

Example:

$$ \text{CH}_2{=}CH_2 + \text{mCPBA} \rightarrow \text{CH}_2\text{OCH}_2 $$

Dihydroxylation

Dihydroxylation is the addition of two hydroxyl groups across the double bond of an alkene, resulting in a glycol.

Reaction Steps:

1. Osmium tetroxide addition: Osmium tetroxide (OsO4) adds to the double bond to form an osmate ester.
2. Reductive cleavage: The osmate ester is reduced, typically with sodium sulfite (Na2SO3), to form the glycol.

Example:

$$ \text{CH}_2{=}CH_2 + \text{OsO}_4 + \text{Na}_2\text{SO}_3 \rightarrow \text{CH}_2(\text{OH})\text{CH}_2(\text{OH}) $$

Comparison Table

Reaction	Reagents/Conditions	Products	Stereochemistry
Hydroboration-Oxidation	BH3, H2O2/NaOH	Alcohols	Syn-addition
Ozonolysis	O3, Zn/AcOH or (CH3)2S	Aldehydes, Ketones, or Carboxylic acids	-
Epoxidation	mCPBA or other peracids	Epoxides	Syn-addition
Dihydroxylation	OsO4, Na2SO3	Glycols	Syn-addition

These reactions are important tools in organic synthesis, allowing chemists to transform alkenes into a variety of functionalized products. Understanding the mechanisms and conditions for each reaction is crucial for predicting the outcome of alkene transformations and for designing synthetic pathways for complex molecules.