CHEM 30B  Dr. R. Rinehart    Properties and Reactions of Hydrocarbons

I.  Physical Properties: Record your observations in the tables provided.

            A. Color: this is not a reliable guide to whether or not a compound is organic or inorganic,

 since many compounds of both types are colored, while many are not. The great

 majority of simple organic compounds are colorless. Examine the samples of the

compounds set out.

            B. Odor: It is generally a very poor idea to inhale any unknown substance. Proper technique

 is to first take a breath and hold it in while you lightly fan the vapors above the test

 tube toward you and lightly sniff. Ten drops of substance in a test tube is sufficient for

 this purpose. Very few inorganic compounds have discernable odors, because their

strong ionic attractions mean essentially nothing vaporizes [a prerequisite for us to

be able to smell things]. On the other hand, organic compounds, even those that are

solids, are often suficiently volatile to be smellable. Their odors tend to be rather

strong and are often characteristic. Why this is so will be discussed later in the course. You may want to consult the Smells Database at UC Berkeley at

http://mc2.cchem.berkeley.edu/Smells/  .

            C.  Physical State:  the great majority of inorganic compounds are solids; a few are gases and

 a  very few are liquids at room temperature and normal atmospheric pressure. Because

 ionic bonding is the norm for inorganics, the melting points of these compounds are

 very high. Hydrocarbons with four or fewer carbon atoms per molecule are generally

 gases under normal room conditions. Those with 5-16 carbons are USUALLY liquids.

     The instructor will demonstrate [or attempt to] the melting of the following compounds:

             sodium chloride           potassium iodide           camphene           naphthalene

D.  Solubility:  The “like dissolves like” rule is a useful generalization. Many [but certainly

 not all] inorganic compounds are soluble in water, because its polar nature makes it

capable of  “solvating” ions. On the other hand, the relatively high attractions of water

 molecules for each other [why?] usually lead to the exclusion of nonpolar

 hydrocarbon  molecules. With nonpolar solvents such as cyclohexane, the reverse

 behavior is seen: water and ionic compounds do not dissolve, while other

 hydrocarbons usually dissolve very well.

The instructor will demonstrate (in)solubility in water and in cyclohexane of the following substances:

II.  Reactions of hydrocarbons

            A.  Combustion:  It should hardly be surprising that compounds made up of C and H will burn, but there are some observable differences. Some inorganic compounds are also combustible, but the great majority are not. Watch as the instructor ignites each sample and describe the appearance of both the flame and the smoke produced.

You should be able to write balanced equations for each of these compounds:

            __ C₆H₁₂  + ___ O₂ à   ___ CO₂ +  ___ H₂O

            __ C₆H₁₀  + ___ O₂ à   ___ CO₂ +  ___ H₂O

            __ C₇H₈  + ___ O₂ à   ___ CO₂ +  ___ H₂O

            __ CH₃OH  + ___ O₂ à   ___ CO₂ +  ___ H₂O

            B.  Addition reactions.  These are characteristic of alkenes and alkynes, but not alkanes, cycloalkanes, or aromatic hydrocarbons.

            The red/orange color of elemental bromine disappears when it contacts an alkene [the resultant “vicinal” dibromide is colorless].

RCH=CHR’  +  Br₂  à  RCHBr-CHBrR’

            With potassium permanganate, there is first an addition reaction followed by a redox reaction in which the hydrocarbon winds up split and oxidized, and the purple permanganate ion gets reduced to manganese dioxide, which forms a brown or black precipitate. See details on blackboard.

R₂C=CR₂ + MnO₄^-  à   R₂C=O  +  O=CR₂  +  MnO₂

            These tests work on C=C in multifunctional compounds as well, such as fats.

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©  Ronald W. Rinehart, 2002