In a 1H NMR spectrum the area under each peak is proportional to the number of hydrogen atoms in that chemical environment. Comparing the areas tells you how many protons are in each environment.The spectrometer often shows the areas as an integration trace: a line that rises in a step at each peak. The height of each step is proportional to the area of that peak. Exam spectra may instead print the relative areas as numbers next to the peaks.Relative peak areas give a ratio, not the actual number of protons. To find actual numbers, simplify the ratio to whole numbers and scale it so that the total matches the number of hydrogen atoms in the molecular formula.For example, ethanol, CH3CH2OH, has three proton environments with 3, 2 and 1 hydrogen atoms, so its three peaks have relative areas of 3 : 2 : 1.Worked example: a compound C2H4O has two peaks with integration step heights of 3.6 cm and 1.2 cm.
• Ratio: 3.6 : 1.2 = 3 : 1
• Total hydrogen atoms = 4, and 3 + 1 = 4, so the peaks represent 3 H and 1 H.
• The 3 H peak is a CH3 group and the 1 H peak (at about 9.8 ppm) is a CHO proton, so the compound is ethanal, CH3CHO.If the ratio total does not match the formula (for example 3 : 1 for a molecule with 8 H), multiply up: 6 : 2.
• Ratio: 3.6 : 1.2 = 3 : 1
• Total hydrogen atoms = 4, and 3 + 1 = 4, so the peaks represent 3 H and 1 H.
• The 3 H peak is a CH3 group and the 1 H peak (at about 9.8 ppm) is a CHO proton, so the compound is ethanal, CH3CHO.If the ratio total does not match the formula (for example 3 : 1 for a molecule with 8 H), multiply up: 6 : 2.