Problem 16 - Advanced Organic Chemistry. Part A. Structure and Mechanisms

Question

The reaction of both \(E\) - and \(Z\)-2-butene with acetic acid to give 2-butyl acetate is catalyzed by various strong acids. With \(\mathrm{DBr}, \mathrm{DCl}\), and \(\mathrm{CH}_{3} \mathrm{SO}_{3} \mathrm{H}\) in \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{D}\), the reaction proceeds with largely \((84 \pm 2 \%)\) anti addition. If the reaction is stopped short of completion, there is no incorporation of deuterium into unreacted alkene, nor any interconversion of the \(E=\) and \(Z\)-isomers. When the catalyst is changed to \(\mathrm{CF}_{3} \mathrm{SO}_{3} \mathrm{H}\), the recovered butene shows small amounts of 1-butene and interconversion of the 2 -butene stereoisomers. The stereoselectivity of the reaction drops to 60-70\% anti addition. How can you account for the changes that occur when \(\mathrm{CF}_{3} \mathrm{SO}_{3} \mathrm{H}\) is used as the catalyst, as compared with the other acids?

Step-by-Step Solution

Answer

CF3SO3H creates a reactive environment allowing isomerization, reducing anti addition selectivity.

Step 1: Identify the Basics of Anti-Addition

In the given reaction of 2-butene with acetic acid, anti addition implies that the two different substituents (the hydrogen and the acetic acid group) are added to opposite sides of the double bond. This means the stereochemistry of the reaction product is opposite of that in the starting material when viewed from the plane of the double bond.

Step 2: Analyze the Role of Catalysts

The original reaction using `DBr`, `DCl`, and `CH3SO3H` as catalysts results in high anti addition (84% ± 2%). This indicates that these catalysts help maintain a stable transition state that favors the anti addition.

Step 3: Examine Effect of CF3SO3H Catalyst

When `CF3SO3H` is used as the catalyst, the reaction produces some 1-butene and leads to interconversion of the E and Z isomers, reducing stereoselectivity to 60-70%. This means that `CF3SO3H` allows isomerization of 2-butene, disrupting the stability needed for consistent anti addition.

Step 4: Understand Isomerization and Reaction Environment

The presence of 1-butene and isomerization suggests that `CF3SO3H` may create a carbocation intermediate under the reaction conditions, which promotes rearrangement and thus interconversion of isomers. This results in lower stereoselectivity because the reactive intermediate can rotate freely, mixing up the configurations.

Step 5: Summarize Changes and Conclusion

Using `CF3SO3H` leads to a more vigorous reaction environment that promotes isomerization and the formation of different by-products like 1-butene. The catalyst's strength and the potential for forming a separate carbocationic pathway could account for the lowered anti addition selectivity and changes observed.

Key Concepts

Stereochemistry

Stereochemistry is a sub-discipline of chemistry, focusing on the spatial arrangement of atoms in molecules. In organic reactions, stereochemistry is crucial because the way atoms are positioned affects the properties and reactions of a compound.
In the reaction of 2-butene with acetic acid, the concept of anti addition plays a key role. Anti addition means that the two substituents add on opposite sides of the double bond.
This results in specific stereochemistry. For example:

• A hydrogen atom and an acetic acid group attach to opposite sides of the butene's double bond.
• This spatial arrangement leads to a particular stereochemical configuration in the product, different from the starting material's configuration.

Stereochemistry can determine the physical and chemical properties of a compound. For students, understanding how to predict the stereochemical outcome of a reaction is crucial.

Catalysis in Organic Chemistry

Catalysts are substances that speed up chemical reactions without being consumed. In organic chemistry, catalysts can profoundly influence reaction outcomes.
For the 2-butene reaction with acetic acid, different catalysts lead to varying results. Strong acids like `DBr`, `DCl`, and `CH3SO3H` favor high anti addition by stabilizing transition states during the reaction process.

• These acids likely maintain high stereoselectivity, meaning the reaction predominantly produces one stereochemical form of the product.
• When `CF3SO3H` is used, the selectivity drops due to its ability to allow isomerization, or the change from one isomer to another.

Catalysts not only determine the rate but also the pathway a reaction might take. Therefore, choosing the correct catalyst is essential for achieving the desired product with the correct stereochemistry.

Carbocation Intermediates

Carbocation intermediates are positively charged carbon species that can form during reactions. They are crucial because they are highly reactive and can lead to different products.
In our butene reaction, when `CF3SO3H` is the catalyst, it likely creates a situation where carbocation intermediates are more common.

• This carbocation can rotate freely, making it prone to isomerization and producing different structural isomers of butene.
• The presence of a carbocation means there's potential for rearrangement in the molecule, complicating product predictability.

Understanding carbocation intermediates helps explain why certain catalysts cause changes in reaction pathways. Predicting when such intermediates might form allows chemists to control reactions more precisely.