Q 9P - Organic Chemistry (Mcmurry)

Question

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: $\equiv \mathrm{C}-\mathrm{H}$stretch in the alkyne group, aromatic ring $=\mathrm{C}-\mathrm{H}$  stretch, $-\mathrm{C}\equiv \mathrm{C}-$stretch in the alkyne group, an aromatic ring $-\mathrm{C}=\mathrm{C}-$ 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.

Step 3: Peak is seen in the spectrum

From the given absorption spectrum 4 peaks can be seen:

• Absorption band at 3300 ${\mathrm{cm}}^{-1}$shows the absorption due to $\equiv \mathrm{C}-\mathrm{H}$ stretch in the alkyne group.

• Absorption bands at 3000-3100 ${\mathrm{cm}}^{-1}$ are due to aromatic ring  $=\mathrm{C}-\mathrm{H}$ stretch.

• Absorption at 2100 ${\mathrm{cm}}^{-1}$is due to $-\mathrm{C}\equiv \mathrm{C}-$stretch in the alkyne group.

• Absorption at 1450-1600 ${\mathrm{cm}}^{-1}$ is due to aromatic ring $-\mathrm{C}=\mathrm{C}-$motions.