The broad stretching of an OH (hydroxyl) group on an infrared (IR) spectrum is primarily due to hydrogen bonding. In a molecule containing an OH group, the presence of hydrogen bonding can significantly affect the vibrational frequencies of the bond.
When the OH group forms hydrogen bonds with neighboring molecules or even within the same molecule, the stretch of the OH bond becomes less sharp and more diffuse. This is because the hydrogen bond creates a range of environments for the OH group, which leads to a distribution of energies for the stretching vibration. Consequently, instead of a distinct peak, you see a broad absorption band in the IR spectrum.
Another factor contributing to the broadening is the presence of different types of hydroxyl functionalities in a mixture. For example, alcohols, phenols, and carboxylic acids all have OH groups but may engage in varying degrees of hydrogen bonding, further contributing to the width of the absorption peak.
In summary, the broad stretching observed in the IR spectrum for an OH group is mainly due to hydrogen bonding interactions that create a range of vibrational energies, leading to a broad absorption feature instead of a sharp peak.