Question

Propose structures for compounds that have the following formulas and IR absorptions: (a) \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{2}, 1735 \mathrm{~cm}^{-1}\) (b) \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{NO}, 1650 \mathrm{~cm}^{-1}\) (c) \(\mathrm{C}_{4} \mathrm{H}_{5} \mathrm{ClO}, 1780 \mathrm{~cm}^{-1}\)

Short answer

(a) Methyl pentanoate; (b) N,N-dimethylacetamide; (c) Acetyl chloride with vinyl group.

Step-by-step solution

Analyze the given formula (a)

For compound (a), the molecular formula is \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{2}\) and there is an IR absorption at \(1735 \, \mathrm{cm}^{-1}\). This absorption typically indicates the presence of a carbonyl group such as an ester or a saturated aldehyde.

Determine possible structures for (a)

Given the formula \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{2}\) and IR absorption indicating a carbonyl group, one possible structure is an ester like hexanoate, specifically methyl pentanoate: \(\text{CH}_3\text{-(CH}_2)_4\text{-C(=O)-O-CH}_3\).

Analyze the given formula (b)

For compound (b), the molecular formula is \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{NO}\), with an IR absorption at \(1650 \, \mathrm{cm}^{-1}\). This absorption is characteristic of an amide group (\(\text{C=O-N}\)).

Determine possible structures for (b)

Given the molecular formula and the IR absorption, a possible structure for compound (b) is N,N-dimethylacetamide: \(\text{CH}_3\text{-C(=O)-N(CH}_3)_2\).

Analyze the given formula (c)

For compound (c), with the formula \(\mathrm{C}_{4} \mathrm{H}_{5} \mathrm{ClO}\) and an IR absorption at \(1780 \, \mathrm{cm}^{-1}\), the absorption is typical of an acid chloride.

Determine possible structures for (c)

Considering the molecular formula and IR absorption for an acid chloride, a possible structure is acetyl chloride with a vinyl group: \(\text{CH}_2=\text{CH-COCl}\).

Key concepts

IR Spectroscopy

IR Spectroscopy, or infrared spectroscopy, is a powerful analytical tool used to determine the molecular structure of organic compounds. It involves measuring the absorption of infrared radiation by a molecule as a function of frequency or wavelength. Each absorption band in an IR spectrum corresponds to a specific vibrational mode of the molecule. Different chemical bonds and functional groups absorb infrared light at different characteristic frequencies, providing a unique "fingerprint" for identifying compounds.

In the exercise, the IR absorption frequencies provided are crucial clues for determining the type of carbonyl compounds present.

• The absorption at 1735 cm^-1 in compound (a) suggests the presence of an ester or a similar carbonyl-containing group.
• For compound (b), an absorption at 1650 cm^-1 typically indicates an amide group.
• Compound (c)'s absorption at 1780 cm^-1 is characteristic of an acid chloride.

Thus, IR spectroscopy helps us piece together the puzzle of a molecule's identity by focusing on these specific absorptions.

Carbonyl Compounds

Carbonyl compounds include a variety of organic molecules that contain a carbonyl group, which is a carbon atom double-bonded to an oxygen atom (C=O). This group is polar and reactive, making it essential in various chemical reactions and synthesis processes.

In organic chemistry, the carbonyl group appears in multiple functional groups:

• Esters: As seen in compound (a), esters typically show an IR absorption around 1735 cm^-1. An example is methyl pentanoate, which was identified in our solution.
• Amides: These are characteristic of absorbing around 1650 cm^-1, as demonstrated in compound (b), which corresponds to N,N-dimethylacetamide.
• Acid Chlorides: With an absorption near 1780 cm^-1, they include compounds like acetyl chloride, identified in compound (c).

Recognizing these absorptions is key to determining the molecular structure and understanding chemical behavior and reactions of carbonyl compounds.

Molecular Structure Determination

Molecular structure determination is an essential aspect of organic chemistry. It involves identifying the precise arrangement of atoms within a compound. This determination is often accomplished using a combination of analytical techniques, of which IR spectroscopy is foundational.

Determining the molecular structure typically follows these steps:

• Empirical and Molecular Formula: Provide insights into the number and types of atoms present in the compound.
• IR Spectroscopy: Offer detailed information on specific functional groups through characteristic absorptions.
• Complementary Techniques: Methods such as NMR spectroscopy, mass spectrometry, and X-ray crystallography can augment the understanding of molecular structure further.

Through the step-by-step solution in the exercise, we utilized IR spectroscopy to hypothesize the structural configuration of compounds based on absorption frequencies. This logical method serves as a roadmap for determining unknown structures, crucial for both academic and industrial applications in organic chemistry.