Question

IR spectroscopy is normally used to distinguish between: A. neighboring protons on different compounds, B. neighboring protons on the same compound. C. different functional groups on the same compound. D. acids and bases.

Short answer

C. Different functional groups on the same compound.

Step-by-step solution

- Understand IR Spectroscopy

Infrared (IR) spectroscopy is a technique used to identify chemical compounds based on the way they absorb infrared light. Different functional groups absorb specific wavelengths of infrared light, creating a unique spectrum for each compound.

- Analyze the Options

Review the answer choices provided: A. Neighboring protons on different compounds.B. Neighboring protons on the same compound.C. Different functional groups on the same compound.D. Acids and bases. Determine which of these characteristics IR spectroscopy is best suited to distinguish.

- Eliminate Irrelevant Options

IR spectroscopy works by identifying vibrations between atoms within a molecule. It does not provide information about protons or distinguishing between acids and bases directly. Therefore, we can eliminate options A, B, and D.

- Select the Correct Option

The technique is particularly useful for identifying different functional groups within the same compound. These functional groups absorb infrared radiation at different characteristic frequencies, allowing for their identification. Therefore, the correct answer is C.

Key concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical properties of those molecules. They are vital in organic chemistry because they participate in chemical reactions in predictable ways.

The most common functional groups include:

• Hydroxyl (-OH)
• Carbonyl (C=O)
• Carboxyl (-COOH)
• Amino (-NH2)

By identifying functional groups, you can predict the behavior of a molecule in a reaction.

In IR spectroscopy, these functional groups have unique IR absorption patterns, making it easier to identify them within a compound.

Infrared Absorption

Infrared absorption is the process where a molecule absorbs infrared light at specific wavelengths. This absorption causes the molecules to vibrate in particular modes such as stretching or bending.

Different bonds and functional groups will absorb infrared light at specific, predictable frequencies.
This characteristic absorption is reported in units called wavenumbers (cm^-1).

For example:

• The hydroxyl group (-OH) generally absorbs around 3200-3600 cm^-1.
• The carbonyl group (C=O) typically absorbs near 1700 cm^-1

Being able to interpret these absorption peaks allows chemists to identify the presence of different functional groups in a sample.

Chemical Identification

Chemical identification using IR spectroscopy involves understanding the spectrum that results when a sample is exposed to infrared light.

Each functional group within a molecule absorbs infrared light at specific frequencies, thus creating a 'fingerprint' unique to that molecule.
This 'fingerprint' is displayed as a spectrum with peaks corresponding to the frequencies where absorption occurs.

By comparing this spectrum to known values, chemists can identify the functional groups present and, consequently, identify the molecule.

• Step 1: Collect the IR spectrum of the sample.
• Step 2: Observe the peaks in the spectrum.
• Step 3: Match the observed peaks to known absorption frequencies of functional groups.
• Step 4: Identify the functional groups and infer the structure of the compound.

This technique is powerful because it provides a non-destructive means of chemical identification, making it invaluable in both research and quality control settings.