Trích:
Question 1: Account for the conclusions in syntheses by intramolecular cyclization:
1.Large ring with more than six Carbon are stable but difficult to prepare.
2.Cyclopropanes have the greatest ring strain but they are readily prepared.
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To syntheses cycloalkanes by intramolecular cyclizations depend on two factors:
-Thermal stablity which is determined by ring strain
-The entropy factor decrease with chain length, making it’s difficult to syntheses rings of increasing size.
So we can using numbers to represent ring sizes, for determining the ease of ring closure for cycloalkanes:
Thermal stability: 6 > 7,5 > 8,9 >> 4 > 3
Entropy (probability) of ring closure: 3 > 4 > 5 > 6 > 7 > 8 > 9
Therefore, following descending order: 5 > 3,6 > 4,7,8,9
Note that: This order only apply to intramoleclar cyclization.
In successful cyclopropane syntheses the favorable entropy factor is more important than thermal stability. With rings larger than six C’s, ring stability is outweighed by the more important, higly unfavorable entropy factor.
Trích:
Question 2: Draw the most stable conformation of cis-1,4-di-t-butylcyclohexane (A)
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Conformation of cyclohexane have four shape usual, they are chair, half-chair, skew or twist boat, and boat, with the energy relative, in kcal/mol, are 0, 11, 5.5, 7.1 . So the most stable conformation is chair, in descending order twist boat, boat, half chair.
To determine the most stable conformation of A, you must knowing why conformations have that order??? Why the twist boat conformation is more stable than the boat conformation, and why the half chair is least stable conformation?
Twisting the boat to the twist-boat conformation move the “flagpole” H’s away from each other and reduces eclipsing of the gunwale H’s. Therefore, the twist boat structure is more stable than the boat conformation.
The half chair, formed by raising the footrest of the chair, has five of the six C’s in a plane with one C out of the plane. So, it has both angle and eclipsing strain, it is the least stable.
Return the question, as we know, in general, chair conformation with axial t-Bu groups do not exist.
Cis-t-butylcyclohexan have at least one t-butyl group in axial, which is very unstable. To overcome this strain, the molecule assumes a more stable twist-boat with a quasi (ee)conformation.
Noone answer the first question, I hope you should more active in the second question.
Have fun!!!