ORGANIC CHEMISTRY-OWL V2 ACCESS
ORGANIC CHEMISTRY-OWL V2 ACCESS
8th Edition
ISBN: 9781305582422
Author: Brown
Publisher: CENGAGE L
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Chapter 20, Problem 20.44P

(a)

Interpretation Introduction

Interpretation: The structural formula for the Diels-Alder adduct formed by cyclopentadiene has to be drawn.

Concept Introduction:

Diels-Alder reaction:

It is the reaction of conjugated dienes with double or triple bonded compounds which are known as “dienophiles”. The reaction is a 4+2 cycloaddition reaction that results in the formation of a six membered cyclic product which is known as “adduct”.

Example:

ORGANIC CHEMISTRY-OWL V2 ACCESS, Chapter 20, Problem 20.44P , additional homework tip  1ORGANIC CHEMISTRY-OWL V2 ACCESS, Chapter 20, Problem 20.44P , additional homework tip  2

This mechanism shown that three π-bonds are broken to form two new σ -bonds and a new π-bond.

Diels-Alder reaction to form bicyclic system:

The Diels-Alder adduct formed in the Diels-Alder reaction can also be a bicyclic system which will be obtained when cylopentadiene is used as the diene as shown here:

ORGANIC CHEMISTRY-OWL V2 ACCESS, Chapter 20, Problem 20.44P , additional homework tip  3

In this reaction, the cylopentadiene acts as both diene and dienophile and formed a bicyclic system. When it is heated to 170oC, a reverse Diels-Alder reaction takes place in which the bicyclic system will be decomposed and gives back the cylopentadiene.

(a)

Expert Solution
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Explanation of Solution

The given reaction is:

ORGANIC CHEMISTRY-OWL V2 ACCESS, Chapter 20, Problem 20.44P , additional homework tip  4

The structural formula for the Diels-Alder adduct can be found using the following mechanism of the given Diels-Alder reaction:

ORGANIC CHEMISTRY-OWL V2 ACCESS, Chapter 20, Problem 20.44P , additional homework tip  5

In this Diels-Alder reaction, one molecule of cyclopentadienone acts as a Diene whereas the other molecule of cyclopentadienone acts as a Dienophile. The resulted Diels-Alder adduct is a tricyclic product

(b)

Interpretation Introduction

Interpretation: The difference in the stability of the given ketones has to be accounted.

Concept Introduction:

Stability based on aromaticity:

The term aromaticity means “extreme stability”. So, aromatic compounds are highly stable compounds whereas anti-aromatic compounds are highly unstable compounds.

The aromatic compounds and anti-aromatic compounds can be distinguished based on Huckel’s rule of aromaticity.

Huckel’s rule of aromaticity is 4n+2=πe

If n=0,1,2,...., then the given compound is an aromatic compound.

If πe=4n, then the given compound is an anti-aromatic compound.

(b)

Expert Solution
Check Mark

Answer to Problem 20.44P

The difference in the stability of the given ketones has been accounted as:

Cycloheptatrienone is more stable than cyclopentadienone.

Explanation of Solution

The given ketones are:

ORGANIC CHEMISTRY-OWL V2 ACCESS, Chapter 20, Problem 20.44P , additional homework tip  6

The stability of these two ketones can be distinguished based on aromaticity as discussed below:

ORGANIC CHEMISTRY-OWL V2 ACCESS, Chapter 20, Problem 20.44P , additional homework tip  7

There are two π- bonds inside the cyclopentane ring. Each π- bond has two electrons. Hence, there are a total of 2×2=4-π-electrons(πe) which is equal to 4n.

Huckel’s rule of aromaticity:

4n+2=πeπe=44nAntiAromatic.

Therefore, the given molecule has 4n number of π-electrons. So, it is an anti-aromatic compound which means it is a highly unstable compound.

ORGANIC CHEMISTRY-OWL V2 ACCESS, Chapter 20, Problem 20.44P , additional homework tip  8

There are three π- bonds inside the cycloheptane ring. Each π- bond has two electrons. Hence, there are a total of 3×2=6-π-electrons(πe).

Huckel’s rule of aromaticity:

  4n+2=πeπe=6

  n=πe-24n=6 - 24=44=1

  n=1Aromatic

Therefore, the given molecule satisfies the Huckel’s rule of aromaticity. So, it is an anti-aromatic compound which means it is a highly stable compound.

Hence, the overall discussion makes it clear that cycloheptatrienone is more stable than cyclopentadienone.

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Chapter 20 Solutions

ORGANIC CHEMISTRY-OWL V2 ACCESS

Ch. 20.6 - Prob. 20.11PCh. 20.6 - Prob. 20.12PCh. 20 - If an electron is added to 1,3-butadiene, into...Ch. 20 - Prob. 20.15PCh. 20 - Predict the structure of the major product formed...Ch. 20 - Predict the major product formed by 1,4-addition...Ch. 20 - Predict the structure of the major 1,2-addition...Ch. 20 - Prob. 20.19PCh. 20 - Prob. 20.20PCh. 20 - Prob. 20.21PCh. 20 - Prob. 20.22PCh. 20 - Prob. 20.23PCh. 20 - Pyridine exhibits a UV transition of the type n at...Ch. 20 - Prob. 20.25PCh. 20 - Prob. 20.26PCh. 20 - Prob. 20.27PCh. 20 - Write the frontier molecular orbital analysis for...Ch. 20 - Prob. 20.29PCh. 20 - Draw structural formulas for the products of...Ch. 20 - Propose structural formulas for compounds A and B...Ch. 20 - Under certain conditions, 1,3-butadiene can...Ch. 20 - Prob. 20.33PCh. 20 - Prob. 20.34PCh. 20 - The following triene undergoes an intramolecular...Ch. 20 - Prob. 20.36PCh. 20 - Prob. 20.37PCh. 20 - Prob. 20.38PCh. 20 - Prob. 20.39PCh. 20 - The Diels-Alder reaction is not limited to making...Ch. 20 - The first step in a synthesis of dodecahedrane...Ch. 20 - Bicyclo-2,5-heptadiene can be prepared in two...Ch. 20 - Prob. 20.43PCh. 20 - Prob. 20.44PCh. 20 - Following is a retrosynthetic scheme for the...Ch. 20 - Prob. 20.46PCh. 20 - Prob. 20.47PCh. 20 - Prob. 20.48PCh. 20 - Prob. 20.49PCh. 20 - Prob. 20.50PCh. 20 - What reaction presented in this chapter is...Ch. 20 - Claisen rearrangement of an allyl phenyl ether...Ch. 20 - Prob. 20.53PCh. 20 - Prob. 20.54PCh. 20 - We now continue the use of organic chemistry...Ch. 20 - Write the products of the following sequences of...
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