Organic Chemistry MCAT Practice Exam

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In IR spectroscopy, what range indicates the presence of an O-H bond?

  1. 3200-2600 cm-1

  2. 1700 cm-1

  3. 1600 cm-1

  4. 2100-2260 cm-1

The correct answer is: 3200-2600 cm-1

The presence of an O-H bond in infrared (IR) spectroscopy is indicated by a strong, broad absorption typically found in the range of 3200-3600 cm-1. This range arises due to the stretching vibrations of the O-H bond, which can appear broader than many other functional groups due to hydrogen bonding effects. In contrast, the other ranges listed do not correspond to the O-H bond. The peak at 1700 cm-1 is typically associated with C=O (carbonyl) stretching, common in ketones and aldehydes. The 1600 cm-1 range is often indicative of C=C (alkene) stretching or bending vibrations, while the range of 2100-2260 cm-1 is associated with C≡C (alkyne) and C≡N (nitrile) functional groups. Therefore, the range of 3200-2600 cm-1 is specific to O-H bonds and reflects their characteristic absorption patterns in IR spectroscopy.

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