Question

Draw the structural formula for the product formed by treatment of cholesterol with \(\mathrm{H}_{2} / \mathrm{Pd}\); then do the same for \(\mathrm{Br}_{2}\).

Short answer

Answer: When cholesterol is treated with (1) \(\mathrm{H}_{2} / \mathrm{Pd}\), the product formed is a hydrogenated cholesterol with a single bond between carbons 5 and 6. When treated with (2) \(\mathrm{Br}_{2}\), the major product formed is a di-bromo derivative with bromine atoms at carbon 5 and carbon 6, anti to each other.

Step-by-step solution

Recognize the structure of cholesterol and the type of reactions with the given reagents

Cholesterol is a sterol with a complex structure. It has three cyclohexane rings (A, B, and C) and a cyclopentane ring (D) joined together. There is also an unsaturated bond between carbons 5 and 6, and an OH group (hydroxyl group) in position 3. The given reagents typically participate in the following reactions: 1. \(\mathrm{H}_{2} / \mathrm{Pd}\): This reagent is used for the hydrogenation of unsaturated carbon-carbon bonds (alkenes). 2. \(\mathrm{Br}_{2}\): This reagent performs the halogenation reaction, where it adds across the unsaturated carbon-carbon bonds (alkenes).

Draw the product formed by treating cholesterol with \(\mathrm{H}_{2} / \mathrm{Pd}\)

The \(\mathrm{H}_{2} / \mathrm{Pd}\) reagent hydrogenates the unsaturated bond between carbons 5 and 6 in cholesterol's structure. The addition of hydrogen in this reaction is syn-addition, which means that both hydrogen atoms will add to the same face of cholesterol. After the hydrogenation reaction, the unsaturated bond between carbons 5 and 6 will be saturated, giving a single bond. Here is the resulting [structure after hydrogenation](https://www.drugfuture.com/chemdata/Hydrogenated-Cholesterol.html) of cholesterol with \(\mathrm{H}_{2} / \mathrm{Pd}\).

Draw the product formed by treating cholesterol with \(\mathrm{Br}_{2}\)

The \(\mathrm{Br}_{2}\) reagent can add across the unsaturated bond between carbons 5 and 6, resulting in the formation of a di-bromo derivative of cholesterol. In this reaction, the bromine atoms add to the unsaturated bond with anti-addition, which means that the two bromine atoms add to opposite faces of the molecule. The addition of the bromine atoms to the unsaturated bond can occur at different positions, forming different regioisomeric products. However, due to sterical hindrance and the rigidity of the cholesterol structure, there is a higher preference to add the bromine atoms at the outer, less crowded positions. The major product formed in this reaction will have both bromine atoms at carbon 5 and carbon 6, anti to each other. Here is the [resulting structure after bromination](https://pubchem.ncbi.nlm.nih.gov/compound/5_6-Dibromocholesterol) of cholesterol with \(\mathrm{Br}_{2}\).

Key concepts

Cholesterol Chemical Reactions

Cholesterol, the waxy substance found in our body, undergoes various chemical reactions. In organic chemistry, the focus is often on reactions involving the functional groups within the cholesterol molecule. Featuring prominently in these discussions is the double bond between carbons 5 and 6, as well as the hydroxyl group at position 3. These sites are reactive and can participate in several common organic reactions.

For example, when cholesterol is treated with hydrogen in the presence of a palladium catalyst (denoted by \(\mathrm{H}_{2} / \mathrm{Pd}\)), a hydrogenation reaction occurs at the double bond. This essentially converts the double bond into a single bond by adding hydrogen atoms to each of the carbon atoms involved in the double bond. Hydrogenation reactions are vital in biochemistry as they can influence the fluidity of cell membranes and the structure of steroids. The resulting hydrogenated cholesterol has enhanced stability and altered physical properties compared to its unsaturated precursor.

Hydrogenation of Alkenes

Alkenes are hydrocarbons with one or more carbon-carbon double bonds and are known for their reactivity. A common reaction involving alkenes is hydrogenation – the addition of hydrogen to the carbon atoms of the double bond. This reaction is not only crucial in the context of cholesterol transformation but also in the production of margarine from vegetable oils.

The hydrogenation process is typically catalyzed by metals such as palladium, platinum, or nickel, and takes place under specific conditions of temperature and pressure. The outcome is an alkane, where what was once a double bond is now a single bond with added hydrogen atoms (\(\mathrm{C=C} \rightarrow \mathrm{C-C}\)). Importantly, this reaction follows a syn-addition pattern, meaning that both hydrogen atoms add to the same side of the double bond, leading to the saturation of the alkene and a decrease in unsaturation.

Halogenation Reaction

Another pivotal reaction in organic chemistry is halogenation, which involves the addition of halogens, such as bromine or chlorine, to a substrate. When applied to alkenes like the double bond in cholesterol, halogenation typically results in vicinal dihalides – compounds where two halogen atoms are attached to adjacent carbon atoms.

The reaction mechanism involves the cyclic halonium ion intermediate. This occurs when the diatomic halogen molecule approaches the electron-rich double bond and is polarized, leading to the formation of a three-membered ring with a positive charge on the halogen. Subsequent attack by a halide ion from the opposite face (anti-addition) gives the vicinal dihalide. Notably, the regioselectivity and the stereochemistry of the added halogens depend on the structure of the alkene and the reaction conditions. For cholesterol, this reaction can be used to introduce bromine atoms onto the steroid nucleus, altering its reactivity and biological function.