Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Problem 108

Chapter 9, Problem 108

In 1935, J. Bredt, a German chemist, proposed that a bicycloalkene could not have a double bond at a bridgehead carbon unless one of the rings contains at least eight carbons. This is known as Bredt's rule. Explain why there cannot be a double bond at this position.

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Bredt's rule states that a double bond cannot exist at a bridgehead carbon in a bicyclic system unless one of the rings contains at least eight carbons. This is due to the geometric constraints imposed by the bicyclic structure.

In a bicyclic system, the bridgehead carbon is part of two rings and is constrained in its spatial orientation. For a double bond to form, the involved atoms must adopt a planar geometry to allow proper overlap of the π orbitals.

The bridgehead carbon in small bicyclic systems (rings with fewer than eight carbons) cannot achieve the required planar geometry due to the strain caused by the rigid structure of the rings. This strain prevents the π orbitals from overlapping effectively, making the double bond at the bridgehead carbon highly unstable.

In larger bicyclic systems (rings with eight or more carbons), the increased flexibility and reduced strain allow the bridgehead carbon to adopt a planar geometry, enabling the formation of a stable double bond.

The image provided shows a bicyclic structure with a bridgehead carbon. If the rings are small (e.g., six carbons or fewer), the geometric constraints would prevent the formation of a double bond at the bridgehead carbon, consistent with Bredt's rule.

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Key Concepts

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

Bredt's Rule

Bredt's Rule states that a bicycloalkene cannot have a double bond at a bridgehead carbon unless one of the rings contains at least eight carbon atoms. This is due to the geometric constraints imposed by the bicyclic structure, which can lead to angle strain and destabilization if a double bond is formed at the bridgehead.

Bicyclic Compounds

Bicyclic compounds are organic molecules that contain two interconnected rings. The structure of these compounds can create unique steric and electronic environments, influencing their reactivity and stability. Understanding the arrangement of atoms in bicyclic systems is crucial for predicting the behavior of these compounds under various conditions.

Angle Strain

Angle strain occurs when the bond angles in a molecule deviate from the ideal values, leading to increased energy and instability. In bicyclic compounds, particularly those with smaller rings, the presence of a double bond at a bridgehead carbon can exacerbate angle strain, making the molecule less stable and less likely to form.

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Related Practice

Textbook Question

Draw the resonance contributors of the cyclooctatrienyl dianion.

a. Which of the resonance contributors is the least stable?

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Textbook Question

The experiment shown next and discussed in Section 8.13 shows that the proximity of the chloride ion to C-2 in the transition state causes the 1,2-addition product to form more rapidly than the 1,4-addition product.

a. Why was it important for the investigators to know that the preceding reaction was being carried out under kinetic control?

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Textbook Question

Identify the product of the following reaction.

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Textbook Question

As many as 18 different Diels–Alder products can be obtained by heating a mixture of 1,3-butadiene and 2-methyl-1,3-butadiene. Identify the products.

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Textbook Question

While attempting to recrystallize maleic anhydride, a student dissolved it in freshly distilled cyclopentadiene rather than in freshly distilled cyclopentane. Was her recrystallization successful?

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Textbook Question

On a single graph, draw the reaction coordinate diagram for the addition of one equivalent of HBr to 2-methyl-1,3-pentadiene and for the addition of one equivalent of HBr to 2-methyl-1,4-pentadiene. Which reaction is faster?

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