Infrared Spectroscopy

If you look at an IR spectrum of 1-butanol, you will see:

• there are sp³ C-H stretching and CH₂ bending modes at 2900 and 1500 cm^-1.
• there is a strong C-O stretching mode near 1000 cm^-1.
• there is a very large peak around 3400 cm^-1. O-H peaks are usually very broad like this one.

Peak shapes are sometimes very useful in recognizing what kind of bond is present. The rounded shape of most O-H stretching modes occurs because of hydrogen bonding between different hydroxy groups. Because protons are shared to varying extent with neighboring oxygens, the covalent O-H bonds in a sample of alcohol all vibrate at slightly different frequencies and show up at slightly different positions in the IR spectrum. Instead of seeing one sharp peak, you see a whole lot of them all smeared out into one broad blob. Since C-H bonds don't hydrogen bond very well, you don't see that phenomenon in an ether, and an O-H peak is very easy to distinguish in the IR spectrum.