FreeMCATPrep Logo

More Than 475 Free MCAT Questions with Detailed Answers!

Click HERE for your Random Question from our MCAT Question A Day Archive

Organic Chemistry - Spectroscopy

You should have a little familiarity with infrared and nuclear magnetic resonance spectroscopy and their use in structural identification of organic compounds.

Infrared spectroscopy

The infrared spectrometer is used to detect the presence or absence of functional groups. So, the infrared spectrum helps to reveal the structure of a compound by telling you what groups are present or absent from the molecule. This is how it works. Think of a molecule as containing atoms connected by springs. Imagine that the molecule is constantly vibrating and that its bonds are always stretching and bending. Now, remember that when the molecule absorbs infrared light, each bond absorbs energy and increases the amplitude of its vibration. When the molecule is somehow connected to an IR spectrometer the increase amplitude shows up as a set of absorption bonds and the nature of each absorption bond indicate the presence of a particular functional group. An important point is that molecules that have dipole moments showed deep absorption bonds. A carbonyl group, for example, which has dipole moment will show a deep absorption bond just remember that a given functional group produces a distinct absorption bond. So, there tends to be a commonality among the infrared spectra of all alcohols, carboxylic acids, aldehydes and esters.

Here are lists of bonds and their IR frequency ranges, listen, but do not try to memorize them:

Aldehyde 1740 to 1720

Ketone 1725 to 1680

Carboxylic acid 1725 to 1720

Esters 1700 to 1630

Alkanes 3095 to 2850

Alkynes near 3300

Aromatic near 3030

Amine 3500 to 3300

Amide 3500 to 3140

Alcohol 3650 to 3200

Nuclear magnetic resonance (NMR) spectroscopy

NMR spectroscopy helps identify molecules on the basis of CH bonds. In regard to NMR, just remember these three little facts:

  1. NMR spectroscopy works through subjecting the molecule to a) magnetic fields and b) electro magnetic radiation.
  2. When a molecule is subjected to NMR testing, it produces signals which show up as peaks on a graph. Identical groups of hydrogen atom show identical peaks and are said to be equivalent.
  3. NMR spectroscopy works by measuring the spin-spin interactions between hydrogen atoms that are attached to adjacent carbon atoms. If two peaks in an NMR graph are split, it is called spin-spin splitting or spin-spin coupling. So, instead of getting two true peaks, we get two peaks each of which is split. The splitting is due to the fact that the hydrogens on one carbon are affected by the hydrogens on neighboring carbons.