Question

Write a mechanism for the reaction of trans-2-butene with trifluoroperoxoethanoic acid to give trans-2,3dimethyloxacyclopropane that is consistent with the fact that the reaction is first order in each participant and gives suprafacial addition.

Short answer

The reaction involves a concerted mechanism forming an epoxide intermediate via a suprafacial addition on trans-2-butene.

Step-by-step solution

Understanding the Reaction

We are asked to write the mechanism for a reaction involving trans-2-butene and trifluoroperoxoethanoic acid. The product is trans-2,3-dimethyloxacyclopropane and the reaction follows a first-order rate law in both reactants with suprafacial addition, implying that both oxygens add to the same face of the butene.

Initiation of the Mechanism

The mechanism begins with the formation of an electrophilic species from the trifluoroperoxoethanoic acid. In this case, the peracid transfers an oxygen atom to the pi bond of trans-2-butene, forming an epoxide intermediate, a common feature of this type of reaction.

Formation of the Epoxide

Trans-2-butene reacts with the oxygen of the peracid to form a three-membered epoxide ring. Since the reaction is suprafacial, both bonds form on the same face of the double bond. Therefore, the epoxide retains the trans configuration of the original alkene, forming trans-2,3-dimethyloxacyclopropane.

Explanation of Reaction Order

The reaction order is first order in each reactant, meaning that the rate of reaction depends linearly on the concentration of trans-2-butene and trifluoroperoxoethanoic acid. This supports the mechanism involving a direct and concerted transition state where both reactants interact directly to form the product.

Key concepts

Epoxide Formation

Epoxide formation is an intriguing chemical reaction where a three-membered cyclic ether is produced. This specific type of chemical is also known as an oxirane. The compound forms when an alkene reacts with a peracid.
In the exercise, trans-2-butene reacts with trifluoroperoxoethanoic acid to create an epoxide. The reaction begins with the transfer of an oxygen atom from the peracid to the alkene's double bond. This step results in the breaking of the double bond in the alkene and the formation of an epoxide ring.
The transformation is notable because it conserves the stereochemistry of the alkene; in this case, the trans configuration is maintained in the resulting epoxide. This reaction is suprafacial, signifying that both bonds to oxygen form on the same side of the alkene.

Reaction Kinetics

Understanding reaction kinetics helps us comprehend how fast or slow a chemical reaction occurs. The rate of reaction provides insight into the changes in concentration of reactants and products over time.
For this particular epoxide formation, the reaction is first order in each reactant. This implies that the reaction rate is directly proportional to the concentration of each reactant. As such, the change in concentration of trans-2-butene or trifluoroperoxoethanoic acid will directly affect the rate of product formation.
Reaction kinetics not only reveal how the concentrations influence the reaction speed, but also hint at the reaction mechanism. The first-order nature suggests a concerted process, where each reactant directly influences the rate-determining step.

Suprafacial Addition

Suprafacial addition is a fascinating facet of organic reactions involving double bonds. It indicates that both new bonds form on the same face of a molecule.
The concept of suprafaciality ensures the stereospecificity of the reaction. In the case of trans-2-butene, this means that the epoxide formed retains the same stereochemistry as the starting material.
Such specificity is crucial for producing the desired compound without rearrangements. Thus, the mechanism aligns beautifully with the requirements for both the stereochemistry and the kinetic observations noted in the exercise.

Electrophilic Attack

An electrophilic attack is a chemical process where an electron-poor species, known as an electrophile, seeks out an electronic-rich region to react with. In this exercise, the epoxidation involves an electrophilic attack by the oxygen atom from the peracid on the alkene's double bond.
This attack is crucial in forming the epoxide intermediate, as it generates a highly strained three-membered ring useful in further synthesis steps.

• The electrophile: the oxygen in trifluoroperoxoethanoic acid.
• The nucleophile: the double bond in trans-2-butene.

The electrophile initially forms a bond with one of the carbon atoms in the double bond, leading to the breaking of the π bond and subsequent formation of the epoxide. This attack is part of the concerted mechanism, thus related directly to the observed reaction kinetics.