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Chapter 19: Problem 451

1,2 Cyclopentanedione exists primarily as a monoenol whereas biacetyl exists primarily as the keto form. Remembering conformational variances between the two substances; (a) explain this occurrence, (b) To what degree would Bicyclo [2.2.1]heptan-2, 3-dione enolize? Explain.

Short Answer

Expert verified
The primary form of 1,2-cyclopentanedione is monoenol due to the stabilizing effect of intramolecular hydrogen bonding, while biacetyl exists mainly in its keto form due to the absence of such bonding. Bicyclo[2.2.1]heptan-2,3-dione exhibits minimal enolization as ring strain and conformational restrictions prevent intramolecular hydrogen bonding.

Step by step solution

01

Understanding the Keto-Enol Tautomerism

Keto-enol tautomerism is an equilibrium between a keto form (a compound containing a carbonyl group) and an enol form (a compound containing an alcohol and a double bond). Structural differences and stability of these forms play an essential role in determining the predominant form.
02

Analyzing 1,2-Cyclopentanedione

In 1,2-cyclopentanedione, there are two adjacent carbonyl groups (C=O) within a five-membered ring. These functional groups can be part of an enolization reaction, leading to a more stable enol structure. The enol form of 1,2-cyclopentanedione can have intramolecular hydrogen bonding, which stabilizes the enolic form. This hydrogen bonding explains why 1,2-cyclopentanedione exists primarily as the monoenol.
03

Analyzing Biacetyl

Biacetyl consists of two carbonyl groups separated by a methylene (-CH2-) group. In this case, due to the separation of carbonyl groups, the enol form can't undergo intramolecular hydrogen bonding. Consequently, the keto form is more stable than the enol form. This lack of intramolecular hydrogen bonding explains why biacetyl exists mainly in its keto form.
04

Investigating Bicyclo[2.2.1]heptan-2,3-dione Enolization

In the case of bicyclo[2.2.1]heptan-2,3-dione, there is a bicyclic structure that constrains the molecule. Due to the ring strain and the inability of the molecule to form intramolecular hydrogen bonds, the enolization of this compound is minimal and not favorable.
05

Conclusion

1,2-cyclopentanedione exists primarily as a monoenol because of the intramolecular hydrogen bonding, which stabilizes the enolic form. In contrast, biacetyl exists primarily as the keto form due to the lack of intramolecular hydrogen bonding. In the case of bicyclo[2.2.1]heptan-2,3-dione, enolization is minimal because of the ring strain and the conformational restrictions preventing intramolecular hydrogen bonding.

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