In Example 5.4.2, the "thought" molecules were enantiomers of each other. These protons are considered enantiotopic. The relationship between the two molecules is that they are enantiomers. ![]() Start with the same thought experiment that we did with methane and exchange one of the hydrogens with X to make one "thought molecule" and then repeat with the other hydrogen. Are the protons of the CH 2 chemically equivalent?Īre the protons in bromochloromethane, C 2H 4BrCl, homotopic, enantiotopic, or diastereotopic? ![]() The next example we will consider is bromochloromethane. In NMR terms, this means that all 12 protons in cyclohexane are equivalent. In this sense, NMR is like a camera that takes photographs of a rapidly moving object with a slow shutter speed - the result is a blurred image. It turns out that, except at extremely low temperatures, this rotational motion occurs on a time scale that is much faster than the time scale of an NMR experiment. Remember, though, that the molecule rotates rapidly between its two chair conformations, meaning that any given hydrogen is rapidly moving back and forth between equatorial and axial positions. In fact, an axial hydrogen is in a different electronic environment than an equatorial hydrogen. You might expect that the equatorial and axial hydrogens in cyclohexane would be non-equivalent, and would have different resonance frequencies. Both "thought molecules" are identical, so the protons are homotopic. The next step is to determine the relationship between these two molecules. Doing the same with H b, the resulting molecule is. If we exchange H a for X, then the molecule would be. Now exhange replace each with X to form two separate "thought molecules". Two of the hydrogens in methane have been labeled a and b. This means that they will show up at the same location in the NMR spectrum.Īre the protons in methane, CH 4, homotopic, enantiotopic, or diastereotopic? Homotopic protons are identical protons and will be chemically equivalent. If the protons in the two "thought molecules" are identical, then the protons are said to be homotopic. In the thought exercise, you replace hydrogens by X to determine what the "thought molecules'" relationship would be to each other. The same can be said for the three H b protons.Ī good test to determine if hydrogens are chemically equivalent is by doing a thought exercise. ![]() They have identical resonance frequencies. On the other hand, the three H a protons are all in the same electronic environment, and are chemically equivalent to one another. The three protons labeled H a have a different - and easily distinguishable – resonance frequency than the three H b protons, because the two sets of protons are in non-identical environments: they are, in other words, chemically nonequivalent. In methyl acetate, below for example, there would be two peaks in the 1H NMR spectrum, which means there are two types of protons. In an external magnetic field of a given strength, protons in different locations in a molecule have different resonance frequencies, because they are in non-identical electronic environments. ![]() Fortunately for organic chemists, resonance frequencies are not uniform for all protons in a molecule. If all protons in all organic molecules had the same resonance frequency in an external magnetic field of a given strength, 1H NMR would not be terribly useful to organic chemists. Make certain that you can define, and use in context, the key terms below.
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