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Lab IV: Part 2 of 3-Week Dual Unknown Lab

NOTE: As stated in Lab 1, each unknown will contain one or more of the following major functional groups: ALCOHOL, ALDEHYDE, AMINE, CARBOXYLIC ACID, KETONE, NITRILE, PHENOL.

The unknown might also contain one of these other functional groups: alkene, alkyne, amide, aromatic, ester, ether, halide, and nitro.

Pre-lab Work

Reading Assignment:

  1. Introduction to Functional Groups:
    Hornback; pp. 50-53
  2. Infrared Spectroscopy:
    Appendix & Mohrig, Technique 18.1-18.4, pp. 228-236 & 18.6-18.9, pp. 243-267.
  3. Polarimetry:
    Appendix & Mohrig, Technique 14.1-14.4, pp. 165-172.
  4. Refractometry
    Appendix & Mohrig, Technique 13 pp. 161-164.
  5. Solubility Tests:

Solubility Tests
These tests help you to determine the nature of your unknown compounds. They show if you have a strong acid, a weak acid, a strong base or a neutral substance. With this information, you can narrow down the possible major functional groups that could exist as components of your unknown compounds.

Click here for specific procedures.

Instrumental Techniques
It is often necessary in organic chemistry to use instrumental techniques to identify compounds or characteristics of compounds. Many of these techniques are used to determine physical properties of compounds in order to make the identification of unknown compounds easier. For Part 2 of your Dual Unknown lab, you will be using infrared spectroscopy, refractometry, and polarimetry to collect more physical property information about your unknowns.

Experimental Work
1. To determine the solubility/reactivity of your compounds.
2. To learn how to use infrared spectroscopy, polarimetry, and refractometry to obtain information about the physical properties of a compound.

Experimental Procedures
The determination of the solubility in water is the starting point, and is perhaps the most important solubility test. The pH of the water-solubility test should always be determined. (Why?)

Record your solubility data neatly in a table in your notebook.

Sample Handling for Instrumental Analysis


  • For infrared and refractive index, use pure (neat or undiluted) samples.
  • For polarimetry: Prepare approximately a 1% (weight/volume, g/mL) solution of sample in ethanol. Use a balance (recording the amount you weigh out) and a 25.00 mL volumetric flask. Calculate the exact concentration of the solution you prepare.

For both unknowns:
Prepare a 1% ethanolic solution of your unknown for the polarimeter and use your pure unknown for the infrared spectrophotometer. Please see your Instructor for directions on how to use the instruments.

Calculate the specific rotation for each unknown using the exact concentration you calculated previously for your 1% solution.

Prepare an IR data table for each unknown. List ALL possible functional groups and, using the other data you have collected on your unknown, highlight the functional groups which are most likely to be present.

For liquid unknowns only:
Obtain a refractive index for you liquid unknown making sure to note the temperature as well.

Calculate the refractive index for your liquid unknown at 20oC.

**Be sure to share a copy of your IR spectrum as well as your calculated values for specific rotation and refractive index with your partner.

Post-lab Work

1. Write equations to show what happens when...
(a.)...an amine dissolves in dilute, aqueous HCl.
(b.)...a carboxylic acid dissolves in dilute, aqueous NaOH.
(c.)...a carboxylic acid dissolves in dilute, aqueous NaHCO3.
(d.)...a phenol dissolves in dilute, aqueous NaOH.

Type the answers to the following questions(#2-6) and tape the answers into your notebook.

2. From the solubility data, identify the functional groups that are possible for each unknown.

3. From the FT-IR data, identify the functional groups that are possible for each unknown.

4. What does each instrument tell us about a compound?

5. Discuss any errors in your experiment and what you would do differently if you were to do this lab again.

6. Summarize the information about each unknown compound.