Problem 6 - Basic Principles of Organic Chemistry

Question

Indicate the approximate positions of \(\mathrm{C}=\mathrm{O}\) and \(\mathrm{N}-\mathrm{H}\) absorptions you would expect in the infrared spectra of (a) \(\mathrm{ClH}_{3}^{\oplus} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{H}\) and (b) \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{Na}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{C}=\mathrm{O}\) at \(1700 \text{ cm}^{-1}\), \(\mathrm{N}-\mathrm{H}\) at \(3300-3500 \text{ cm}^{-1}\); (b) \(\mathrm{COO}^{-}\) at \(1600 \text{ cm}^{-1}\) and \(1400 \text{ cm}^{-1}\), \(\mathrm{N}-\mathrm{H}\) at \(3300-3500 \text{ cm}^{-1}\).

Step 1: Identify Functional Groups

The first step in determining the approximate positions of absorptions in an IR spectrum is to identify the functional groups within the molecule. (a) For \(\mathrm{ClH}_{3}^{\oplus} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{H}\), the relevant functional groups are the amine group \(\mathrm{NH}\) and carbonyl \(\mathrm{C}=\mathrm{O}\).(b) For \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{Na}\), we similarly identify the amine \(\mathrm{NH}\) group and a carbonyl salt \(\mathrm{COO}^{-}\).

Step 2: Determine IR Absorption Regions

Now, we determine the approximate IR absorption regions for the identified functional groups:- The \(\mathrm{C}=\mathrm{O}\) (carbonyl) stretch typically appears around \(1700 \text{ cm}^{-1}\).- The \(\mathrm{NH}\) (amine) stretch appears around \(3300-3500 \text{ cm}^{-1}\). For primary amines, two stretches can be observed; however, the exact position might vary slightly depending on the molecular environment.

Step 3: Predict IR Absorption for Compound (a)

For compound (a) \(\mathrm{ClH}_{3}^{\oplus} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{H}\):- The \(\mathrm{C}=\mathrm{O}\) stretch is expected around \(1700 \text{ cm}^{-1}\).- The \(\mathrm{N}-\mathrm{H}\) stretch is expected in the range of \(3300-3500 \text{ cm}^{-1}\).

Step 4: Predict IR Absorption for Compound (b)

For compound (b) \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{Na}\):- The \(\mathrm{COO}^{-}\) stretch for carboxylate ions may appear in the regions around \(1600 \text{ cm}^{-1}\) (asymmetric) and \(1400 \text{ cm}^{-1}\) (symmetric).- The \(\mathrm{N}-\mathrm{H}\) stretch will be in the usual \(3300-3500 \text{ cm}^{-1}\) region.

Key Concepts

Functional Groups

Functional groups are specific groupings of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Each functional group has its own distinctive properties and reactive behavior. In infrared (IR) spectroscopy, these groups are recognized by their unique absorption patterns in the IR spectrum. By identifying them, chemists can predict how a molecule will behave in chemical reactions. For instance, common functional groups include:

• Amines, characterized by the presence of nitrogen (N) bonded to hydrogen (H) - \(\mathrm{NH}\)\.
• Carbonyls, which involve a carbon (C) atom double-bonded to an oxygen (O) - \(\mathrm{C}=\mathrm{O}\)\.

This identification is the first step in analyzing the IR spectrum of any molecule.

Amine Group

The amine group contains nitrogen bonded to one or more alkyl or aryl groups. In IR spectroscopy, the presence of an amine group is detected through the N-H stretching vibration. Primary amines, which contain the formula \(\mathrm{RNH}_2\), usually show two distinct absorption bands in the region of \(3300-3500 \, \mathrm{cm}^{-1}\). This occurs due to symmetric and asymmetric N-H stretching. Such bands are often medium to strong in intensity, making them easily noticeable.
These absorption peaks are crucial for identifying the presence of amine groups in various organic compounds, aiding in structural analysis and understanding the molecule's overall chemical properties.

Carbonyl Stretch

The carbonyl stretch in IR spectroscopy is one of the most identifiable bands, appearing typically around \(1700 \, \mathrm{cm}^{-1}\). The carbonyl group consists of a carbon atom double-bonded to an oxygen atom (\(\mathrm{C}=\mathrm{O}\)).
This band is strong and sharp due to the change in dipole moment during the stretching motion of the carbonyl bond. Its exact position can shift depending on neighboring groups or the presence of certain substituents, which can slightly increase or decrease the frequency. For instance, in carboxylic acids or esters, the stretching frequency might be adjusted due to electronic effects. Understanding the carbonyl stretch is vital for distinguishing between various carbonyl-containing functional groups in organic compounds.

IR Absorption Regions

Infrared absorption regions in an IR spectrum represent the specific ranges where different functional group vibrations appear. These regions are expressed in parts per million (ppm) or more commonly in terms of wavenumbers \(\mathrm{cm}^{-1}\). Different bonds and functional groups vibrate at characteristic frequencies, allowing chemists to deduce the structure of unknown compounds.

• The carbonyl stretch, identified in the \(1680-1750 \, \mathrm{cm}^{-1}\) range, helps confirm the presence of carbonyl groups.
• The N-H stretch of amines typically falls in the \(3300-3500 \, \mathrm{cm}^{-1}\) range.
• Carboxylate ions exhibit absorption in the \(1400-1600 \, \mathrm{cm}^{-1}\) range for their asymmetric and symmetric stretches.

By locating these regions within the spectrum, scientists can infer specific functional group data, allowing a deeper understanding of the molecular structure and composition.