EXPERIMENT 12

[hcrxn.lab]

OBJECTIVES:

1. To learn to safely handle typical organic compounds.

2. To learn how representative organic compounds react with various reagents, and from these reactions, be able to identify an unknown compounds as to type.

DISCUSSION:

In this experiment we are going to compare the properties of a saturated hydrocarbon (hexane) with those of an unsaturated hydrocarbon (cyclohexene) and two aromatic compounds (toluene and chlorobenzene). A saturated hydrocarbon is a compound such as n-hexane:

CH₃-CH₂-CH₂-CH₂-CH₂-CH₃

in which each carbon has four single bonds, joining it to four other atoms (carbon or hydrogen). An unsaturated hydrocarbon is a compound containing carbon and hydrogen and one or more double bonds between adjacent carbon atoms. Examples of unsaturated hydrocarbons include compounds such as ethene and cyclohexene:

H H HC==CH

\ / / \

C = C H₂C CH₂

/ \ \ /

H H H₂C--CH₂

Ethene or Ethylene (C₂H₄) Cyclohexene (C₆H₁₀)

The double bond in an unsaturated compound is generally very reactive. Halogens such as bromine add on at this point to give saturated compounds. The double bond is destroyed.

Example:

H H H H

\ / | |

C = C + Br₂ --> H--C - C--H

/ \ | |

H H Br Br

Ethene Bromine 1,2-Dibromoethane

Double bonds are also attacked by oxidizing agents to give oxidized products.

For example:

H H OH

| | oxidizing agent /

CH₃(CH₂)₂C = C(CH₂)₂CH₃ ---------------> 2 CH₃(CH₂)₂C = O

cis-4-Octene Butanoic acid

When three alternating double bonds are present in a RING system, they are much less reactive than isolated double bonds. Such compounds are called aromatic compounds. Three examples of these are:

HC==CH HC==CH HC==CH

/ \ / \ / \

HC C--CH₃ HC C--Cl HC C--H

\\ // \\ // \\ //

HC--CH HC--CH HC--CH

Toluene Chlorobenzene Benzene

MATERIALS:

Sulfuric acid (H₂SO₄), concentrated Copper wire

Bromine in cyclohexane (Br₂) Petroleum ether

Potassium permanganate solution (KMnO₄) Ultra-violet lamp

EXPERIMENTAL PROCEDURE: Work in pairs.

Use goggles at all times. Each test is run with (A) hexane, (B) cyclohexene, (C) toluene, and (D) chlorobenzene. Write your observations for the following tests on the DATA SHEET. If there is a color/heat change, record "REACTION". A dropper holds about 0.5 mL, so this can be used as a fast measure. This is accurate enough.

1. Ignition. Put 0.5 mL of A (hexane) in an evaporating dish. Try to light it with a match. Does it burn? If so, observe the flame. Is it a "clean" flame or a "dirty" flame? Repeat with B, C and D. Which has the cleanest flame ?

2. Solubility. Add 0.5 mL of A, B, C and D to 4 clean test tubes. (Each in its own tube, not mixed together.)

a. To each tube, add one mL of water and shake. Do you see one layer or two? One layer indicates solubility (record as "soluble"); two layers indicates insolubility (record as "insoluble")

b. Repeat, using petroleum ether as the solvent instead of water.

c. Repeat, using concentrated sulfuric acid as the solvent.

Be sure your test tubes are dry before adding the H₂SO₄. When using H₂SO₄, you may also have a REACTION as well as solubility.

Danger!! Concentrated sulfuric acid will burn you if it gets in contact with skin. After testing, flush H₂SO₄ carefully down the drain with lots of water. Wash the test tubes well!

3. Reactions: with Bromine. (Handling bromine solutions is dangerous.

Be very careful. Keep Br₂ in hood.)

a. Place 1 mL of each compound in dry test tubes. Add 0.5 mL of bromine (in cyclohexane). Shake. Record color of mixture.

3. b. Reaction with bromine, catalyzed by light. (Demonstration) The instructor will irradiate the compounds mixed with bromine with an ultraviolet (UV) lamp. Watch to see if there is a reaction as shown by the disappearance of the Br₂ color. Check after 5 minutes and again after 10 minutes. Any reactions are substitution reactions and HBr is released.

4. Reaction with Potassium permanganate (KMnO₄). Place 1 mL of each compound in test tubes. Add 0.5 mL of KMnO₄ solution. Shake well. What color is the solution? If it is pink or purple, no reaction occurred. A positive test is the appearance of some other color.

5. Beilstein test. Put a little of the compound to be tested on a beaker cover. Put a copper wire into the compound then put the copper wire in a gas burner flame. Hold it there until the wire becomes red hot. Record the color of the flame above the copper. This is a test for the presence of a halogen (except fluorine) in an organic molecule.

Note: we are testing 4 TYPES of compounds.

HEXANE is an alkane or saturated hydrocarbon. Other alkanes (octane; decane; 2-methylpentane; cyclobutane; etc.) would behave the same way.

CYCLOHEXENE is an alkene, or unsaturated hydrocarbon because it has a double bond. Alkynes are unsaturated hydrocarbons with a triple bond. Examples of other unsaturated compounds are: decene; 2,3-dimethyl-1-heptene; cyclopentyne.

TOLUENE represents the aromatic compounds. These do have double bonds, but they do not behave the same way as alkenes do. It is difficult to judge from the name whether or not a compound is aromatic. Simple aromatic compounds are benzene; xylene; naphthalene.

CHLOROBENZENE is an aromatic halogen compound. Any molecule with F, Cl, Br or I in it will be considered first of all a halogen compound, even though it might also be classified as an alkane or aromatic.

EXP 12 - LAB REPORT Name _________________ Sec ____

Exp 12 -- REVIEW QUESTIONS Name _____________________ Sec _____

1. What is the difference between saturated and unsaturated hydrocarbons?

2. You did two different tests for unsaturated hydrocarbons. What are their numbers? What kind of result do you get if the sample is unsaturated?

a. Test # __ :

b. Test # __ :

3. What is an aromatic compound?

4. What is the test that will help you tell the difference between an aromatic compound and a saturated compound?

Tell what happens with a saturated compound as compared with an aromatic compound.

Test # ___ :

5. How would you test to find out if an organic compound has chlorine in it? What would be the result of the test?

Test # ___ :

6. Classify the following as: a) saturated linear or cyclic hydrocarbons, b) unsaturated linear or cyclic hydrocarbons, c) aromatic hydrocarbons, or d) halogen compounds.

a hexane b cyclohexene c toluene

d chlorobenzene heptane bromobenzene

cyclobutane iodotoluene hexyne

fluoropentane nonene benzene

xylene methyldecane dodecene