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Chapter 8: Problem 71

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

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

Step by step solution

01

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: \[ \text{CH}_3 \;/ = \backslash; \text{H} \backslash;/ = \;/ \text{H} \;/ = \backslash; \text{CH}_3 \]
02

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.
03

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: \[ \begin{array}{c} \text{H}_3\text{C} - \text{CH} \begin{array}{c} | \hspace{-0.6em}/ \hspace{-0.6em} \backslash| \hspace{0.3em} \text{OH}\ \backslash|\hspace{-0.6em} | \hspace{-0.6em}/ \text{OH} \hspace{0.3em} \text{CH}_3 \end{array} \end{array} \]
04

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: \[ \begin{array}{c} \text{CH}_3 - \text{CH} \begin{array}{c} | \hspace{-0.6em}/ \hspace{-0.6em} \backslash| \hspace{0.3em} \text{OH}\ \backslash|\hspace{-0.6em} | \hspace{-0.6em}/ \text{OH} \hspace{0.3em} \text{CH}_3 \end{array} \end{array} \]
05

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.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

cis-2-butene
Cis-2-butene is an isomer of 2-butene. In this form, both methyl groups (CH鈧�) 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鈥攈ow atoms are arranged in three-dimensional space鈥攊s 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.

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Most popular questions from this chapter

The cis and trans isomers of 2 -butene give different cyclopropane products in the Simmons-Smith reaction. Show the structures of both, and explain the difference.

Draw the structure of an alkene that yields only acetone, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O},\) on ozonolysis followed by treatment with Zn.

Compound A has the formula \(\mathrm{C}_{8} \mathrm{H}_{8}\). It reacts rapidly with \(\mathrm{KMnO}_{4}\) to give \(\mathrm{CO}_{2}\) and a carboxylic acid, \(\mathrm{B}\left(\mathrm{C}_{7} \mathrm{H}_{6} \mathrm{O}_{2}\right),\) but reacts with only 1 molar equivalent of \(\mathrm{H}_{2}\) on catalytic hydrogenation over a palladium catalyst. On hydrogenation under conditions that reduce aromatic rings, 4 equivalents of \(\mathrm{H}_{2}\) are taken up and hydrocarbon \(\mathrm{C}\left(\mathrm{C}_{8} \mathrm{H}_{16}\right)\) is produced. What are the structures of \(\mathbf{A}, \mathbf{B},\) and \(\mathbf{C} ?\) Write the reactions.

Simmons-Smith reaction of cyclohexene with diiodomethane gives a single cyclopropane product, but the analogous reaction of cyclohexene with 1,1 -diiodoethane gives (in low yield) a mixture of two isomeric methylcyclopropane products. What are the two products, and how do they differ?

How would you distinguish between the following pairs of compounds using simple chemical tests? Tell what you would do and what you would see. (a) Cyclopentene and cyclopentane (b) 2 -Hexene and benzene
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