Identifying an unknown compound is a process of elimination using a combination of techniques to determine its physical and chemical properties. This information is then compared to known compounds to narrow down and ultimately confirm its identity.
Here's a breakdown of the common methods used:
1. Preliminary Observations and Physical Properties
- Appearance: Note the color, physical state (solid, liquid, gas), and odor. These simple observations can provide initial clues.
- Solubility: Test the compound's solubility in different solvents (e.g., water, ethanol, ether). Solubility provides information about polarity and potential functional groups.
- Melting Point/Boiling Point: Determine the melting point (for solids) or boiling point (for liquids). These are key physical constants that can be compared to literature values. A sharp melting point suggests a pure compound.
2. Spectroscopic Analysis
Spectroscopic techniques provide detailed information about the structure of the compound.
- Infrared (IR) Spectroscopy: IR spectroscopy identifies functional groups present in the molecule based on the absorption of infrared light. Characteristic peaks correspond to specific bonds and functional groups (e.g., O-H, C=O, N-H).
- Nuclear Magnetic Resonance (NMR) Spectroscopy: NMR provides detailed information about the carbon-hydrogen framework of the molecule. 1H NMR reveals the number and environment of hydrogen atoms, while 13C NMR reveals the number and types of carbon atoms.
- Mass Spectrometry (MS): Mass spectrometry determines the molecular weight of the compound and can provide information about its fragmentation pattern, which can help identify structural features.
- Ultraviolet-Visible (UV-Vis) Spectroscopy: UV-Vis spectroscopy is useful for identifying conjugated systems and aromatic rings.
3. Chemical Tests and Functional Group Analysis
These tests involve specific reactions that reveal the presence or absence of certain functional groups.
- Tests for Alkenes/Alkynes: Bromine test or Baeyer's test (using potassium permanganate) can indicate the presence of unsaturation.
- Tests for Alcohols: Lucas' reagent test differentiates between primary, secondary, and tertiary alcohols.
- Tests for Aldehydes/Ketones: Tollens' test or Fehling's test can differentiate aldehydes from ketones.
- Tests for Carboxylic Acids: Reaction with sodium bicarbonate produces carbon dioxide gas.
- Tests for Amines: Reaction with nitrous acid can differentiate between primary, secondary, and tertiary amines.
4. Derivatization
If the unknown compound is a solid and its melting point doesn't match any known compounds, you can prepare a derivative. A derivative is a new compound synthesized by reacting the unknown with a known reagent. The derivative will have a different, and hopefully more distinctive, melting point. Compare the melting point of the derivative to literature values of derivatives made from similar compounds. This can help confirm the identity of the original unknown.
5. Comparison to Literature Values
Once you have gathered enough data (physical properties, spectroscopic data, functional group information), compare your findings to literature values of known compounds. Databases like SciFinder, Reaxys, and the NIST WebBook are valuable resources.
Summary Table of Methods
| Method | Information Provided |
|---|---|
| Physical Properties | Initial clues about the compound (e.g., polarity, possible functional groups) |
| IR Spectroscopy | Identification of functional groups |
| NMR Spectroscopy | Detailed information about the carbon-hydrogen framework |
| Mass Spectrometry | Molecular weight and fragmentation pattern |
| UV-Vis Spectroscopy | Identification of conjugated systems and aromatic rings |
| Chemical Tests | Presence or absence of specific functional groups |
| Derivatization | Confirmation of identity by comparing the melting point of a derivative to literature values |
| Comparison to Literature | Matching collected data with known compounds |
The process of identifying an unknown compound is iterative. Each piece of information obtained helps to narrow down the possibilities until a conclusive identification can be made.
