Question

Hydroxylation of \(c i s-2\) -butene with \(\mathrm{O} \mathrm{s} \mathrm{O}_{4}\) yields a different product than hydroxylation of trans-2-butene. Draw the structure, show the stereochemistry of each product, and explain the difference between them.

Short answer

Cis-2-butene forms a meso compound; trans-2-butene forms a racemic mixture.

Step-by-step solution

Draw the Reactants

Firstly, we need to identify the structure of cis-2-butene and trans-2-butene. - **Cis-2-butene**: Both methyl groups are on the same side of the double bond. The structure can be represented as: \[ \text{H}_3\text{C} - \text{CH} = \text{CH} - \text{CH}_3 \] - **Trans-2-butene**: The methyl groups are on opposite sides of the double bond: \[ \text{CH}_3 \;/ = \backslash; \text{H} \backslash;/ = \;/ \text{H} \;/ = \backslash; \text{CH}_3 \]

Understand the Reaction

Both cis-2-butene and trans-2-butene undergo hydroxylation reactions with osmium tetroxide (\(\text{OsO}_4\)). This reaction adds two hydroxyl groups (\(OH\)) across the double bond in a syn-addition configuration, meaning both \(OH\) groups are added to the same side of the double bond.

Draw the Products for Cis-2-butene

For cis-2-butene:- Both \(OH\) groups are added on the same side, forming a meso-compound. - The structure of the product is as follows, where both \(OH\) groups are on the same face of the molecule: \[ \begin{array}{c} \text{H}_3\text{C} - \text{CH} \begin{array}{c} | \hspace{-0.6em}/ \hspace{-0.6em} \backslash| \hspace{0.3em} \text{OH}\ \backslash|\hspace{-0.6em} | \hspace{-0.6em}/ \text{OH} \hspace{0.3em} \text{CH}_3 \end{array} \end{array} \]

Draw the Products for Trans-2-butene

For trans-2-butene:- The hydroxyl groups are added to the same side of the alkene carbon atoms, forming (2R,3R)-2,3-butanediol. - The structure of the product is as follows, the \(OH\) groups being syn to each other: \[ \begin{array}{c} \text{CH}_3 - \text{CH} \begin{array}{c} | \hspace{-0.6em}/ \hspace{-0.6em} \backslash| \hspace{0.3em} \text{OH}\ \backslash|\hspace{-0.6em} | \hspace{-0.6em}/ \text{OH} \hspace{0.3em} \text{CH}_3 \end{array} \end{array} \]

Explain the Stereochemical Difference

- **Cis-2-butene** results in a meso compound because the addition of the \(OH\) groups maintains symmetry, thus creating no new stereocenters.- **Trans-2-butene**, on the other hand, results in a pair of enantiomers or a racemic mixture. This is because the addition of the \(OH\) groups creates chiral centers.

Key concepts

cis-2-butene

Cis-2-butene is an isomer of 2-butene. In this form, both methyl groups (CH₃) are positioned on the same side of the double bond. The presence of these groups on the same side lends the molecule unique properties.

• In chemistry, the term "cis" indicates that similar groups in a molecule are on the same side.
• This isomeric form means that when reactions occur, like hydroxylation, stereochemistry—how atoms are arranged in three-dimensional space—is an important consideration.

Cis-2-butene is a key player in stereochemistry experiments because its symmetry can affect the products formed in chemical reactions like syn-additions.

trans-2-butene

Trans-2-butene is the geometric isomer of 2-butene where the methyl groups are on opposite sides of the double bond. This opposite arrangement leads to different chemical behavior compared to cis-2-butene.

• The term "trans" denotes that similar groups are across from each other, or on opposite sides.
• The trans configuration often leads to a more stretched-out and less compact molecular shape, which influences physical properties like boiling and melting points.

When it comes to reactivity, trans-2-butene demonstrates distinct patterns, especially in reactions like syn-addition where stereochemical outcomes, such as chiral centers, become significant.

syn-addition reaction

A syn-addition reaction involves the addition of two substituents to the same side of the double bond in a molecule. This type of reaction is crucial in organic chemistry as it impacts the three-dimensional conformation of molecules.

• In the case of cis and trans-2-butene with osmium tetroxide, a syn-addition reaction leads to the addition of hydroxyl groups (OH) to the same side of the molecule.
• This can control the formation of chiral centers, thus influencing whether the final product will be a meso compound or a racemic mixture.

Understanding syn-addition is vital when predicting the outcomes of such reactions, especially with compounds like butenes.

osmium tetroxide

Osmium tetroxide (\(\text{OsO}_4\)) is a powerful oxidizing agent commonly used in organic chemistry for dihydroxylation reactions. It adds two hydroxyl groups to alkene bonds, facilitating syn-addition.

• When it reacts with alkenes like 2-butene, \(\text{OsO}_4\) does so in a manner that dictates the spatial arrangement of the added groups.
• The reagent is highly effective for creating vicinal diols, where two hydroxyl groups are attached to adjacent carbon atoms.

Osmium tetroxide's selectivity and efficiency make it an invaluable tool for studying stereochemical outcomes in various compounds.

meso-compound

A meso-compound is a molecule with multiple stereocenters that is superimposable on its mirror image. Essentially, it's an achiral compound despite having stereocenters.

• In the hydroxylation of cis-2-butene, the result is a meso compound because the hydroxyl groups add symmetrically across the molecule.
• This symmetry removes the potential for chirality, as the molecules can be folded upon itself without discrepancies.

Meso-compounds are fascinating as they defy typical stereochemistry expectations, maintaining internal symmetry that negates chirality.

racemic mixture

A racemic mixture is a 50:50 blend of two enantiomers, which are mirror images of each other but are not superimposable. This is a common outcome in reactions that produce chiral centers.

• For trans-2-butene, when undergoing hydroxylation, the resulting compound is a racemic mixture.
• This mixture arises because the reaction creates new stereocenters, leading to an equal mix of both enantiomers (2R,3R and 2S,3S configuration).

Racemic mixtures are notable in chemistry for their implications in chirality and biological activity, as both enantiomers can have different effects.