Question

Cholesterol can be reduced either to cholestanol or coprostanol. Equilibration of cholesterol with a trace of cholestanone and base gives 90 per cent cholestanol and 10 per cent of a stereoisomer known as epicholestanol. Similar equilibration of coprostanol (in the presence of coprostanone) gives 10 per cent coprostanol and 90 per cent epicoprostanol. Write the configurations of each of these compounds and explain the orders of stablities that are observed.

Short answer

The configurations of cholestanol, epicholestanol, coprostanol, and epicoprostanol are as follows: 1. Cholestanol: \(\alpha\) at C3 and \(\beta\) at C5 2. Epicholestanol: \(\beta\) at C3 and \(\beta\) at C5 3. Coprostanol: \(\alpha\) at C3 and \(\alpha\) at C5 4. Epicoprostanol: \(\beta\) at C3 and \(\alpha\) at C5 The order of stabilities is Cholestanol > Epicoprostanol > Coprostanol > Epicholestanol. This can be attributed to the differences in orientations of the hydroxyl groups, with trans configurations being more stable due to less steric hindrance than cis configurations.

Step-by-step solution

Understanding the compounds

Cholesterol is a sterol, and its structure includes a hydrocarbon chain and a hydroxyl group. To understand the configurations of each of these compounds, we can look at the orientation of the hydroxyl group on C3 and C5 (carbon number 3 and 5) of the steroid ring system. Cholestanol and epicholestanol are stereoisomers of each other, which means they have the same molecular formula but a different spatial arrangement of their atoms. Similarly, coprostanol and epicoprostanol are also stereoisomers. We can denote the orientation of the hydroxyl group using α (alpha) for the upward direction and β (beta) for the downward direction.

Writing the configurations of the compounds

Using the given information and α, β notation, we can write the configurations of the compounds as follows: 1. Cholestanol: \(\alpha\) at C3 and \(\beta\) at C5 2. Epicholestanol: \(\beta\) at C3 and \(\beta\) at C5 3. Coprostanol: \(\alpha\) at C3 and \(\alpha\) at C5 4. Epicoprostanol: \(\beta\) at C3 and \(\alpha\) at C5

Analyzing the equilibriums and determining order of stabilities

Now we can use the given equilibriums to determine the order of stabilities: Equilibrium 1: Cholesterol + Cholestanone + Base → 90% Cholestanol + 10% Epicholestanol This tells us that Cholestanol is more stable than Epicholestanol. Equilibrium 2: Coprostanol + Coprostanone → 10% Coprostanol + 90% Epicoprostanol This tells us that Epicoprostanol is more stable than Coprostanol. Now, we can rank them in order of stability: 1. Cholestanol 2. Epicoprostanol 3. Coprostanol 4. Epicholestanol

Explaining the order of stabilities

The order of stabilities can be explained by the difference in orientations of the hydroxyl groups. In Cholestanol and Epicoprostanol, the hydroxyl groups are in a trans configuration, leading to less steric hindrance and a more stable compound. On the other hand, Coprostanol and Epicholestanol have hydroxyl groups in a cis configuration, which creates more steric hindrance and results in less stable compounds.

Key concepts

Stereoisomers

In the realm of chemistry, stereoisomers are a fascinating concept. These are compounds that share the same molecular formula and sequence of bonded atoms but differ in the three-dimensional orientations of their atoms. Imagine them as siblings who look the same on paper but have different characteristics in reality. In our exercise, both cholestanol and epicholestanol, as well as coprostanol and epicoprostanol, are stereoisomers. This means that even though they have the same chemical formula, their spatial arrangements vary. It is these differences in spatial orientation that impart different physical and chemical properties to the molecules, including their stability and reactivity.

Hydroxyl Group Orientation

The orientation of hydroxyl groups in these sterols is crucial and can be described using the Greek letters alpha (α) and beta (β). The alpha orientation refers to the hydroxyl group extending upward, while beta orientation means it extends downward. This distinction at carbon positions 3 (C3) and 5 (C5) greatly influences the compound's properties.
For instance, cholestanol has an α orientation at C3 and β at C5, which is slightly different from its stereoisomer, epicholestanol, which is β at both positions. This same principle applies to coprostanol and epicoprostanol. Such orientations affect how the molecules interact with one another and their surroundings, playing a critical role in determining their overall stability.

Stability of Isomers

The concept of stability in isomers largely depends on their molecular geometry and the steric hindrance that arises from their atomic configurations. In our given examples, the stability is heavily influenced by the arrangement of hydroxyl groups.
Cholestanol and epicoprostanol both have trans configurations of their hydroxyl groups, which means the groups are positioned opposite to each other, mitigating hindrance and increasing stability. Coprostanol and epicholestanol, however, have the hydroxyl groups in a cis arrangement, meaning they face the same direction. This cis arrangement leads to increased steric hindrance and a decrease in stability.
Thus, in the context of our exercise, cholestanol is the most stable due to its trans configuration, followed by epicoprostanol. Meanwhile, the less stable isomers, coprostanol and epicholestanol, have configurations that create more steric clash, making them less favorable energetically.