Problem 12 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

An optically active organic compound has the molecular formula \(\mathrm{C}_{7} \mathrm{H}_{10}\) (A). On reaction with \(\mathrm{H}_{2}+\mathrm{Pt}\) it forms an optically inactive compound (B). Then, compound (A) will be

Step-by-Step Solution

Answer

Compound (A) is likely 3-methylcyclohexene.

Step 1: Analyzing the Compound

The compound (A) has the molecular formula \( \mathrm{C}_7\mathrm{H}_{10} \). It is optically active, which suggests that it contains chiral centers. Additionally, the formula indicates a level of unsaturation (such as a double bond, triple bond, or ring structure).

Step 2: Determining Degrees of Unsaturation

Calculate the degrees of unsaturation for compound \( \mathrm{C}_7\mathrm{H}_{10} \) using the formula \( (2C + 2 + N - H - X)/2 \). For \( \mathrm{C}_7\mathrm{H}_{10} \), this results in \((2 \times 7 + 2 - 10)/2 = 3\). This suggests there are 3 points of unsaturation, which could be multiple bonds or rings.

Step 3: Reaction Characteristics

The reaction with \( \mathrm{H}_2 + \mathrm{Pt} \) is a hydrogenation process, which typically reduces double or triple bonds. Since the resulting compound (B) is optically inactive, it indicates that the reaction removes elements of asymmetry (such as double bonds adjacent to a chiral center).

Step 4: Propose Structure for Compound A

Given the unsaturation and optical activity, consider possible structures for \( \mathrm{C}_7\mathrm{H}_{10} \). A typical structure would be a cyclic compound like a cycloalkene. Consider a cyclohexene ring (6-membered ring) with a single methyl group (to satisfy the 7 carbons), providing a chiral center adjacent to the double bond making it optically active.

Step 5: Propose a Cycloalkene Structure

One feasible structure for compound (A) is a chiral cyclohexene, such as 3-methylcyclohexene. This structure has a chiral center at the carbon with the methyl group, which is removed upon hydrogenation, making the compound optically inactive.

Step 6: Reaction Outcome Confirmation

On hydrogenation, the double bond in 3-methylcyclohexene is reduced, resulting in methylcyclohexane, a saturated compound that is optically inactive due to the lack of chiral centers or asymmetric elements.

Key Concepts

Chiral Centers

A chiral center is a carbon atom that is attached to four different groups. This unique arrangement makes the carbon atom sp3 hybridized, and it leads to optical activity. Optical activity is a property where a compound can rotate plane-polarized light, a phenomenon observed in chiral substances.

• Chirality results in two possible configurations, called enantiomers, which are non-superimposable mirror images of each other.
• Enantiomers have the same physical properties except for their interaction with plane-polarized light and reactions in a chiral environment.

For compound (A), the presence of a chiral center suggests it can exist in such enantiomeric forms, which makes it optically active. After hydrogenation, the compound becomes optically inactive because the asymmetry is lost, indicating the chiral center is no longer present.

Degrees of Unsaturation

Degrees of unsaturation indicate the total number of rings, double bonds, or triple bonds present in a compound. It is calculated using the formula:\[\text{Degrees of Unsaturation} = \frac{2C + 2 + N - H - X}{2}\]where:

• \(C\) is the number of carbon atoms
• \(N\) is the number of nitrogen atoms
• \(H\) is the number of hydrogen atoms
• \(X\) is the number of halogens

For compound \( \mathrm{C}_7\mathrm{H}_{10} \), the calculation \[ \frac{2(7) + 2 - 10}{2} = 3 \]reveals three degrees of unsaturation. This can mean the compound has a combination of rings and multiple bonds, such as double or triple bonds. Understanding the degrees of unsaturation helps determine the structure and possible types of bonding in a compound.

Hydrogenation Process

Hydrogenation is a chemical reaction in which hydrogen is added to unsaturated bonds in a molecule, typically in the presence of a catalyst like platinum (Pt). This process reduces double or triple bonds to single bonds. Key features of hydrogenation include:

• It converts unsaturated organic compounds into saturated forms.
• The reaction is commonly used to remove asymmetry when it involves chiral centers adjacent to double bonds.
• A unsaturated cycloalkene, when hydrogenated, may lose its optical activity due to the loss of a chiral center.

For compound (A), the hydrogenation leads to a loss in optical activity, indicating that the structure becomes more symmetrical, likely saturating a double bond near the chiral center, thus rendering compound (B) optically inactive.

Cycloalkene Structure

A cycloalkene is a type of hydrocarbon that consists of a cyclic (ring-shaped) structure with one or more carbon-carbon double bonds. Cycloalkenes are a sub-category of unsaturated compounds and they often have degrees of unsaturation due to the presence of these double bonds. Notable aspects of cycloalkenes:

• They have a basic alkene name such as ethene, propene, modified by adding "cyclo" for the ring structure, e.g., cyclohexene.
• The presence of a double bond in the ring imparts unsaturation, influencing the optical properties of the compound.
• Chiral centers can arise in cycloalkenes if a substituent group, like a methyl group, attaches to one of the carbons in the double bond.

In the case of compound (A), the suggested structure is a cyclohexene with a chiral center due to an additional group making the compound optically active. Upon hydrogenation, this structure becomes fully saturated, resulting in a cycloalkane with no chiral center, hence its optical inactivity.