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Ch. 21 - Conjugated Systems I: Stability and Addition Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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All textbooksMullins 1st EditionCh. 21 - Conjugated Systems I: Stability and Addition ReactionsProblem 60b
Chapter 20, Problem 60b

The reactivity of cyclopropanes often mimics that of alkenes.
(b) Besides opening the three-membered ring, what is the driving force for this reaction?
Chemical reaction diagram showing the transformation of a cyclopropane derivative with ethyl groups and sodium methylthio in water.

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Cyclopropanes are highly strained due to their three-membered ring structure, which forces bond angles to deviate significantly from the ideal tetrahedral angle of 109.5°. This strain makes cyclopropanes reactive and prone to ring-opening reactions.
In the given reaction, the nucleophile (NaSCH₃) attacks the cyclopropane ring, leading to the opening of the strained three-membered ring. This is facilitated by the electrophilic nature of the carbon atoms in the cyclopropane ring.
The driving force for this reaction is the relief of ring strain. When the cyclopropane ring opens, the bond angles increase closer to the ideal tetrahedral angle, significantly reducing the strain energy.
Additionally, the formation of new bonds with the nucleophile (SCH₃) and stabilization of the product contribute to the thermodynamic favorability of the reaction.
The product is more stable due to the absence of ring strain and the presence of functional groups (CO₂Et) that can further stabilize the molecule through resonance or inductive effects.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Cyclopropane Structure and Strain

Cyclopropane is a three-membered cyclic alkane characterized by significant angle strain due to its bond angles of approximately 60 degrees, which deviate from the ideal tetrahedral angle of 109.5 degrees. This strain makes cyclopropanes highly reactive, as they seek to relieve this tension through various reactions, often resembling the reactivity patterns of alkenes.
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Ring Opening Reactions

Ring opening reactions involve the breaking of the cyclic structure of compounds like cyclopropanes, leading to the formation of more stable, acyclic products. This process is often driven by the release of strain energy, making the reaction thermodynamically favorable. The resulting open-chain products can further react, enhancing the overall reactivity of the initial compound.
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Electrophilic Addition Mechanism

The reactivity of cyclopropanes can be likened to that of alkenes, particularly in electrophilic addition reactions. In these reactions, an electrophile attacks the π bond of the alkene or the strained C-C bonds of the cyclopropane, leading to the formation of a more stable carbocation intermediate. The driving force for these reactions often includes the formation of new, stronger bonds and the stabilization of the reaction products.
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Related Practice
Textbook Question

Predict the product of the following reactions.

(j)

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Textbook Question

The following covalent inhibitor blocks function in a protease found in the porcine epidemic diarrhea virus by reacting with a cysteine amino acid residue (shown below) in the active site. Draw the expected complex that forms between the inhibitor and the enzyme active site (J. Med. Chem. 2017, 60, 3212–3216.) [Assume the presence of active site bases if you need them.]

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Textbook Question

The following transformation was found to occur in areas with large NO₂ emissions. Suggest a mechanism for the reaction (J. Phys. Chem. 2013, 117, 14132–14140). [Hint: Use the fishhook arrows associated with radical reactions.]

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Textbook Question

The reactivity of cyclopropanes often mimics that of alkenes.

(a) On the basis of this, suggest a mechanism for the following reaction.

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Textbook Question

The following reaction was used in the synthesis of aculeatin A, a natural product that is active against KB cell lines. Although it only worked under acidic conditions, a mechanism can be drawn where the reaction might proceed under basic conditions. Suggest this mechanism (J. Org. Chem. 2014, 79, 1498–1504). [The most acidic proton is indicated . . . and number your carbons!]

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Textbook Question

In Chapter 9, electron-rich alkenes were oxidized under acidic conditions with mCPBA. Conjugated alkenes can be oxidized using the same reagent, but under basic conditions. Suggest a mechanism for this reaction. [Think about what is electron-rich and what is electron-poor in the reaction. Also, identify the bonds formed and broken.]


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