Question

Complete the following equations showing the stereo chemistry of the product wherever applicable. (i) Cis-2-pentene \(\stackrel{\mathrm{H}_{2} / \mathrm{Pt}}{\longrightarrow}\) (ii) Cis-2-butene \(\stackrel{\mathrm{Br}_{2} / \mathrm{CCl}_{4}}{\longrightarrow}\) (iii) Cis-2-butene \(\stackrel{\mathrm{m}-\mathrm{CPBA}}{\longrightarrow}\) (iv)

Short answer

Answer: Pentane

Step-by-step solution

(i) Hydrogenation of Cis-2-pentene

First, identify the reactants and the reagents. Cis-2-pentene is an alkene, and the reagent is \(\mathrm{H}_{2}\) (hydrogen gas) in the presence of a platinum catalyst (Pt). The reaction involves hydrogenation, where the double bond in cis-2-pentene breaks and forms single bonds with two hydrogen atoms. Since this reaction takes place on the surface of the catalyst, the two hydrogen atoms add from the same side, giving a single stereoisomer product. In this case, the product is pentane. Cis-2-pentene \(\stackrel{\mathrm{H}_{2} / \mathrm{Pt}}{\longrightarrow}\) Pentane

(ii) Bromination of Cis-2-butene

In this reaction, cis-2-butene reacts with \(\mathrm{Br}_{2}\) (bromine) in \(\mathrm{CCl}_{4}\) (carbon tetrachloride) solvent. This reaction is called bromination, and the two bromine atoms add across the double bond of the alkene. This results in an anti-addition process due to the formation of bromonium ion. As the bromine adds to the opposite faces of the pi bond, the stereochemistry at both carbons is inverted. Cis-2-butene \(\stackrel{\mathrm{Br}_{2} / \mathrm{CCl}_{4}}{\longrightarrow}\) (2R,3S)-2,3-dibromobutane

(iii) Epoxidation of Cis-2-butene

The reagent in this reaction is m-CPBA (meta-Chloroperoxybenzoic acid), which is an epoxidation agent. It turns the double bond of cis-2-butene into an epoxide through a syn-addition mechanism. As a result, the oxygen atom will be added to the same face of the double bond. The positions of the stereocenters in cis-2-butene are preserved. Cis-2-butene \(\stackrel{\mathrm{m}-\mathrm{CPBA}}{\longrightarrow}\) (2R,3S)-2,3-epoxybutane

(iv) Hydrogenation of Trans-2-pentene

Similar to the first reaction, trans-2-pentene undergoes hydrogenation with \(\mathrm{H}_{2}\) in the presence of a nickel catalyst (Ni). The double bond in trans-2-pentene breaks, and two hydrogen atoms add to the same side due to the catalyzed reaction mechanism. The product, in this case, is also pentane. Trans-2-pentene \(\stackrel{\mathrm{H}_{2} / \mathrm{Ni}}{\longrightarrow}\) Pentane

Key concepts

Stereochemistry

Stereochemistry is a subfield of organic chemistry that focuses on the three-dimensional arrangement of atoms in molecules and the impact of this arrangement on the chemical reactivity and physical properties of these molecules.
When a reaction occurs on a molecule that contains chiral centers or double bonds, the stereochemistry can determine the specific isomers that are formed as products. For example, in the hydrogenation of alkenes, the addition of hydrogen atoms can occur on the same side of the molecule (syn-addition) or on opposite sides (anti-addition), leading to different stereoisomers.

Importance of Stereochemistry in Reactions

Stereochemistry plays a crucial role in determining the biological activity and properties of molecules, especially in pharmaceuticals, where the precise arrangement of atoms can significantly impact the drug's effectiveness.

Hydrogenation of Alkenes

Hydrogenation of alkenes is a chemical reaction that involves the addition of hydrogen (H₂) across the carbon-carbon double bond, converting an alkene into an alkane. This reaction typically requires a catalyst such as palladium, platinum, or nickel to occur under mild conditions.
When dealing with cis- or trans-alkenes, the stereochemistry of the starting material can influence the outcome of the reaction. For example, hydrogenation of cis-2-pentene will lead to the formation of pentane, where the two new hydrogen atoms will add to the same side of the molecule.

Application in Industry

Hydrogenation reactions are widely used in the food industry to convert liquid vegetable oils into solid or semi-solid fats, like margarine and shortening, by reducing their degree of unsaturation.

Bromination of Alkenes

Bromination of alkenes involves the addition of bromine (Br₂) across a carbon-carbon double bond to form vicinal dibromides. This reaction often proceeds through the formation of a cyclic bromonium ion intermediate, leading to anti-addition where the bromine atoms attach to opposite sides of the original double bond.
For instance, when cis-2-butene undergoes bromination in the presence of carbon tetrachloride (CCl₄), the product is (2R,3S)-2,3-dibromobutane, showing that the stereochemistry of the reactants influences the configuration of the product.

Selectivity in Bromination

Unlike some other halogenation reactions, bromination is highly selective, often resulting in a single stereoisomer, which makes it a valuable tool in synthetic chemistry for constructing complex molecules.

Epoxidation

Epoxidation is the process of transforming a carbon-carbon double bond into an epoxide ring, a three-membered ring containing oxygen. Reagents like meta-chloroperoxybenzoic acid (m-CPBA) facilitate this transformation through a mechanism that is typically syn-addition, meaning both atoms add to the same side of the alkene.
The stereochemistry of the reactant alkene is maintained in the product, resulting in the formation of chiral epoxides when the starting alkene is substituted asymmetrically. For example, epoxidation of cis-2-butene with m-CPBA yields (2R,3S)-2,3-epoxybutane.

Applications of Epoxides

Epoxides serve as important intermediates in the synthesis of a variety of chemical products, including pharmaceuticals, due to their high reactivity, which allows further derivatization.