Monday, March 15, 2010

solubility test



Solubility and Functional Group Tests:

Each functional group has a particular set of chemical properties that allow it to be identified.
Some of these properties can be demonstrated by observing solubility behavior, while others
can be seen in chemical reactions that are accompanied by color changes, precipitate formation,
or other visible effects.
1. Solubility Tests
The solubility of an organic compound in water, dilute acid, or dilute base can provide
useful information about the presence or absence of certain functional groups.

Water Solubility:
Place 0.05 mL or 25 mg of compound in a small test tube, and add 0.75 mL of water in small portions. Shake test tube vigorously after the addition of each portion of solvent. If water soluble, go on to step 2; otherwise proceed to step 3.
Ether Solubility:
Place 0.05 mL or 25 mg of compound in a small test tube, and add 0.75 mL of diethyl ether in small portions. Shake test tube vigorously after the addition of each portion of solvent. If the compound is both water and ether soluble, the acid-base properties of the compound should be determined with litmus.
litmus turns red - water soluble acidic compound (class Sa)
litmus turns blue - water soluble basic compound (class Sb)
litmus neutral - water soluble general compound (class Sg)
If the compound is not ether soluble it is a salt, amino acid, or contains many hydrophilic functionalities (class S)
5% NaOH Solubility:
Place 0.05 mL or 25 mg of compound in a small test tube, and add 0.75 mL of NaOH solution in small portions. Shake test tube vigorously after the addition of each portion of solvent. If NaOH soluble, go on to step 4; otherwise proceed to step 5.
5% NaHCO3 Solubility:
Place 0.05 mL or 25 mg of compound in a small test tube, and add 0.75 mL of NaHCO3 solution in small portions. Shake test tube vigorously after the addition of each portion of solvent. If NaHCO3 soluble, then it is a strong organic acid (class As). If not NaHCO3 soluble, then it is a weak organic acid (class Aw).
5% HCl Solubility:
Place 0.05 mL or 25 mg of compound in a small test tube, and add 0.75 mL of HCl solution in small portions. Shake test tube vigorously after the addition of each portion of solvent. If HCl soluble, then it is an organic base (class B). If not HCl soluble and (from elemental analysis) is found to contain nitrogen or sulfur, then it is a miscellaneous neutral compound (class Nm). If not HCl soluble, then go on to step 6.
96% H2SO4 Solubility:
Place 0.6 mL of H2SO4 in a small test tube, and add 0.05 mL or 25 mg of compound. Shake test tube vigorously. If H2SO4 soluble, then it is a neutral compound (class N). If not H2SO4 soluble, then it is an inert compound (class I).


Procedure:
Into 4 labeled test tubes, add 1 mL of : water; 5% aq HCl; 5% aq NaHCO3; and 5% aq. NaOH. To each tube, add approx. 30 mg of the unknown. Shake or stir for a few seconds. Record your observations. It may take several minutes for the unknown to appear to dissolve or react. After suitable time, heat the four test tubes in a water bath and observe. Look for color changes, evolution of gas, any evidence of reaction such as precipitates, and enhanced solubility. Record your observations.
Repeat the solubility test for the other unknown.

Most organic compounds are not water soluble. Only low molecular weight and/or highly polar compounds will be water soluble to 3%. Salts of acids or bases will often be soluble.
If the compound is not soluble in water, solubility in acid suggests that it is basic (eg, amines), solubility in strong base suggests that it is at least weakly acidic (eg, phenols) and solubility in weak base (bicarbonate) suggests that it is a stronger acid (eg, carboxylic acids).
Whatever determination you make from this solubility test, you will corroborate that suggestion with other data including class tests and analysis of the compound's infrared spectrum.

Compounds which are insoluble in all of the above liquids can be tested for solubility and color change in concentrated sulfuric acid. Compounds giving a color change or showing solubility include neutral compounds such as alkenes, alkynes, alcohols, ketones, aldehydes, esters, ethers, amides and nitro compounds. Compounds that appear inert include alkanes, alkyl halides, and simple aromatic hydrocarbons. Dispose of the sulfuric acid solution carefully by pouring it into a beaker of cold water, carefully rinsing the test tube with water, and washing both down the sink.

No comments:

Post a Comment

Post a Comment