Question

Propose structures for compounds that meet the following descriptions: (a) An optically active compound \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}\) with an IR absorption at \(1730 \mathrm{~cm}^{-1}\) (b) An optically inactive compound \(\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{~N}\) with an IR absorption at \(2215 \mathrm{~cm}^{-1}\)

Short answer

(a) 2-Pentanone; (b) 3-Pentenenitrile.

Step-by-step solution

Understand Optically Active Compounds

An optically active compound must contain a chiral center. This means there will be a carbon atom attached to four different groups. We must also look for a significant IR absorption at \(1730 \mathrm{~cm}^{-1}\), which indicates the presence of a carbonyl group (C=O).

Propose Structure for Compound (a)

To satisfy the molecular formula \(\mathrm{C}_5 \mathrm{H}_{10} \mathrm{O}\) and ensure optical activity, we propose a ketone that is chiral. One possible structure is 2-pentanone, where a chiral center is present at the second carbon atom, and the C=O group (giving the IR absorption) is on the second carbon. Structure: \(\text{CH}_3\text{CH}_2\overset{*}{\text{C}}\text{(O)}\text{CH}_2\text{CH}_3\).

Understand Optically Inactive Compounds

An optically inactive compound lacks a chiral center, meaning it does not have a carbon atom connected to four different groups. The IR absorption at \(2215 \mathrm{~cm}^{-1}\) indicates the presence of a cyanide group \(-\mathrm{C} \equiv \mathrm{N}\).

Propose Structure for Compound (b)

For the formula \(\mathrm{C}_5 \mathrm{H}_9 \mathrm{~N}\), and to be optically inactive, the structure must be symmetric, with a nitrile group. One possible structure is 3-pentenenitrile, which is linear and contains a \(-\mathrm{C} \equiv \mathrm{N}\) group at one end, ensuring no chirality. Structure: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}=\text{CHC} \equiv\text{N}\).

Key concepts

Chiral Compounds

Chiral compounds are fascinating topics in organic chemistry. When a molecule is chiral, it means it has a non-superimposable mirror image—much like how your left and right hands are mirror images but can't be perfectly overlaid. This unique property of chirality usually occurs because of a specific structural feature known as a chiral center. A chiral center is typically a carbon atom that is bonded to four different groups. When it exists, it imparts optical activity to the compound, meaning the compound can rotate plane-polarized light. This rotation can be either to the right (dextrorotatory) or the left (levorotatory), depending on the configuration of the molecule. In the context of the given exercise, to propose a structure for an optically active compound with a molecular formula of

• a C=O group, indicated by the IR absorption at 1730 cm⁻¹
• a ketone structure that possesses chirality.

One such example that fulfills these conditions is 2-pentanone with a chiral center at the second carbon atom. Its structure can be described as CH₃CH₂C*(O)CH₂CH₃, where the asterisk denotes the chiral center.

Infrared Spectroscopy

Infrared (IR) spectroscopy is a powerful analytical tool used to identify specific functional groups within a molecule based on the absorption of infrared light at various wavelengths. Different functional groups absorb IR light at characteristic frequencies, providing a fingerprint that can reveal the presence of certain bonds. For instance, in our task, we observe two significant IR absorptions:

• 1730 cm⁻¹, which is indicative of a carbonyl group (C=O) typical in ketones or aldehydes.
• 2215 cm⁻¹, which correlates to a nitrile group (C≡N), showcasing its presence in the molecule.

By identifying these absorptions, chemists can infer the functional groups and propose potential structures for a compound, as for compound (a) with a carbonyl group and compound (b) with a nitrile group. Thus, IR spectroscopy serves as an essential method for understanding and unveiling molecular structure details.

Molecular Structure

The molecular structure of a compound provides a comprehensive picture of how atoms are arranged and bonded in a molecule. It's not just about which atoms are present but their spatial arrangement matters greatly. A basic understanding includes concepts like:

• The presence of chiral centers or lack thereof, which determines optical activity.
• Functional groups like carbonyls (C=O), alkenes (C=C), alkynes (C≡C), or nitriles (C≡N) that can affect chemical behavior and physical properties.
• Linear versus branched chains, which influence stability and reactivity.

For the given problem, compound (a) is 2-pentanone, a ketone with a chiral center, giving it optical activity. Its carbonyl group is crucial both for reactivity and identifying the IR absorption. On the other hand, compound (b), 3-pentenenitrile, lacks chirality due to its linearity and symmetrical distribution of atoms, confirmed by the absence of a chiral center. Understanding these structural components helps in predicting and explaining the behavior of molecules in various chemical contexts.