Q9-P - Organic Chemistry (Mcmurry)

Question

The IR spectrum of phenylacetylene is shown in Figure 12-28. What absorption bands can you identify?

Step-by-Step Solution

Answer

From the given absorption spectrum 4 peaks are seen: \( \equiv C - H\) stretch in the alkyne group, aromatic ring  \( = C - H\) stretch, \( - C \equiv C - \)stretch in the alkyne group, an aromatic ring \( - C = C - \) motions.

Step 1: Absorption spectrum

The absorption spectrum depicts the presence of dark lines in the spectrum when transmitted through a substance which indicates absorption at specific wavelengths of electromagnetic radiation.

Step 2: Chart for absorption by specific functional group

The given chart tells the value of absorption of particular wavelength radiation by a specific functional group.

                                      Functional group absorption band

Step 3: Peak is seen in the spectrum

From the given absorption spectrum 4 peaks can be seen:

• Absorption band at 3300 \(c{m^{ - 1}}\)shows the absorption due to \( \equiv C - H\) stretch in the alkyne group.
• Absorption bands at 3000-3100 \(c{m^{ - 1}}\) are due to aromatic ring  \( = C - H\) stretch.
• Absorption at 2100 \(c{m^{ - 1}}\) is due to \( - C \equiv C - \) stretch in the alkyne group.
• Absorption at 1450-1600 is due to aromatic ring \( - C = C - \) motions.