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Hydrocarbon pre-lab lecture

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1 Hydrocarbon pre-lab lecture
Chemistry 2412L Hydrocarbon pre-lab lecture

2 Properties of Hydrocarbons
The purpose of this lab is to study the chemical and physical properties of hydrocarbon compounds Remember the typical reactions of hydrocarbons: Class of Hydrocarbon Type of Reaction(s) Saturated Combustion; halogenation (with heat or light) Unsaturated Addition; oxidation Aromatic Substitution

3 Procedure Notes You will observe results of 6 tests on known compounds, and use your observations to identify an unknown compound. Unknowns are either saturated, unsaturated, or aromatic. Test compounds for all 6 tests: Known saturated hydrocarbon (cyclohexane) Known unsaturated hydrocarbon (cyclohexene) Known aromatic hydrocarbon (toluene) 2 unknowns (1a and 1b, 2a and 2b, etc.) Make sure you are using the same number for your two unknowns!!

4 Procedure Notes The 6 tests are: Solubility Density Flammability Br2

5 Procedure Notes Solubility Density Flammability
If two compounds are insoluble they will form two layers (or be cloudy) when mixed Density If the hydrocarbon is not soluble in water, you can determine relative density by determining which compound is the top layer Flammability Hydrocarbons will undergo combustion in the presence of air when ignited. Compounds with a high carbon-to-hydrogen ratio (i.e. those with a high degree of unsaturation) burn at a low temperature and are characterized by a yellow sooty flame

6 Procedure Notes Br2 Bromine readily adds across the double bond of an alkene through an electrophilic addition reaction. A colorless vicinal halide is produced: This addition reaction will not occur with aromatic compounds Bromine will react with saturated hydrocarbons in the presence of heat or light (halogenation). Observe the color of the solution when Br2 is added to the saturated hydrocarbon quickly, before the substitution reaction has time to occur.

7 Procedure Notes KMnO4 This test is positive for double and triple bonds but not for saturated hydrocarbons or aromatic rings. It depends on the conversion of the purple MnO4- ion to a brown precipitate of MnO2 following the oxidation of an unsaturated compound.

8 Procedure Notes H2SO4 Concentrated sulfuric acid will react with unsaturated hydrocarbons to form alkyl hydrogen sulfates. In the process the alkene will appear to dissolve in the concentrated sulfuric acid. Reaction is indicated by a change in color, production of heat, or formation of insoluble material. Alcohols can be produced by heating these products with water. Aromatic compounds will react with sulfuric acid when heated to make soluble sulfonic acids.

9 Safety Notes Concentrated acids and bromine can cause burns. Wash immediately with water if any of these are spilled on the skin. Bromine and methylene chloride should not be inhaled. Use these chemicals in the hood. KMnO4 will stain! As always, be careful with flames!

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