Hydrocarbons Lab

5/13/2018 Hydrocarbons Lab

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The Physical and Chemical Properties of Hydrocarbons

Taken from: Experiments for. .. Atoms, Molecules, and Life- An Introduction to General, Or-ganic and Biological Chemistry, Michael S. Matta and Antony C. Wilbraham,Benjamin/Cummings Publishing Co., 1981.

andTaken from: Laboratory Manual For the Allied Health Sciences by Roger D. Bauer, James A.

Campbell, Robert Loeschen, and Jack Wolf, pp 96-202, Prentice-Hall, EnglewoodCliffs, 1980.

Although all hydrocarbons contain only carbon and hydrogen, they have a variety of differentstructural features. On the basis of their structural features, hydrocarbons can be divided intothree groups: saturated hydrocarbons (alkanes), unsaturated hydrocarbons (alkenes and al-kynes), and aromatic hydrocarbons (arenes). You will study the differences in reactivity amongthese classes of compounds in this experiment.In this experiment heptane will represent the saturated class of hydrocarbons, cyclohexene willrepresent the unsaturated class of hydrocarbons, and toluene will represent the aromatic class ofhydrocarbons. You will determine how each of these react with a variety of reagents. You willrun an unknown through a similar set of tests and from its behavior classify it as saturated, un-saturated, or aromatic.HYPOTHESISThe physical properties and chemical reactivity of a hydrocarbon reflect the presence or absenceof certain chemical bonds.OBJECTIVES

1. To examine the physical properties of hydrocarbons.2. To observe reactions characteristic of each of the three main classes of

hydrocarbons.MATERIALS AND EQUIPMENTHeptane, cyclohexene, toluene, hydrocarbon unknown, I % bromine in dichloromethane [meth-ylene chloride], ethanol, dichloromethane [methylene chloride], 0.05 % KMn04 in H20, 5% so-dium carbonate solution, eye dropper, small test tubes, large test tubes, evaporating dish, wiregauze, tongs, wood splints, matches.


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R R Br Br\ I CH2Cl2 \ IC = C + Br2 --------> R - C - C-RI \ I \R R R R

THEORY

Hydrocarbons are organic compounds that contain only two kinds of elements, carbon and hy-drogen. In spite of the fact that only two kinds of elements are found in hydrocarbons, there aremany different kinds of hydrocarbons. One method of classification divides the hydrocarbonsinto three classes: saturated hydrocarbons, unsaturated hydrocarbons, and aromatic hydrocar-bons. Saturated hydrocarbons do not contain any carbon-carbon double or carbon-carbon triplebonds; consequently, they are not able to undergo any addition reactions. Unsaturated hydrocar-bons do contain carbon-carbon double bonds or carbon-carbon triple bonds and are able to un-dergo addition reactions. For our purposes aromatic hydrocarbons will either be represented as asix membered ring with alternating double bonds around the ring or as a six membered ring witha circle inside the ring. Aromatic compounds appear to be unsaturated when one looks at theirstructure, but they behave as though the were saturated hydrocarbons.In chemistry there is a saying that, "like dissolves like". You will use this fact to determine thepolarity of hydrocarbons. You will attempt to dissolve hydrocarbons in a nonpolar solvent,methylene chloride, a slightly polar solvent, ethanol, and a polar solvent, water. Based upon thesolubility of a hydrocarbon in the three solvents you will be able to classify is as nonpolar,slightly polar, or polar.Combustion of hydrocarbons - When plenty of oxygen is available, the different classes ofhy-drocarbons burn differently. Saturated hydrocarbons undergo complete combustion giving twocolorless, odorless gases, CO2 and H20. Unsaturated hydrocarbons undergo incomplete combus-tion and produce in addition to CO2 and H20 some CO and C. Aromatic hydrocarbons burn withvery incomplete combustion and produce a lot ofC (lamp black).Reaction with bromine - Unsaturated hydrocarbons react rapidly with Br2 in methylene chlorideto give addition products that are colorless. This is shown in the following equation.

Saturated hydrocarbons and aromatic hydrocarbons will not react with bromine in methylenechloride without special conditions. The bromine reacting with an unsaturated hydrocarbon willlose its color; whereas, the bromine in the presence of a saturated hydrocarbon or an aromatic hy-drocarbon will retain its color.Potassium permanganate, KMn04 , reacts with unsaturated hydrocarbons but not with saturatedhydrocarbons or aromatic hydrocarbons. Evidence of this reaction with unsaturated hydrocar-bons is the disappearance of the purple KMn04 and the formation of brown manganese dioxide,Mn02


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5. Of the following compounds, which react(s) with bromine: ethane, ethene, andpropane?

PRELABORATORY QUIZ - The answers are in your class notes and in the text.

I. What is the difference between a saturated hydrocarbon and an unsaturatedhydrocarbon?

2. Give the name and structure of a saturated hydrocarbon and of an unsaturated hy-drocarbon, each having five carbons.

3. What characteristic structural features allow you to differentiate between an al-kane, an alkene, an alkyne, and an arene?

4. Explain how it is possible to distinguish between an alkane and an arene by thecombustion test.


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9. lfan unknown compound dissolves in a nonpolar solvent, but does not dissolve ina polar solvent, what does that tell you about the unknown compound?

6. How can you tell whether a compound reacts with bromine?

7. Explain how it is possible to distinguish between an alkane and an alkene by theKMn04 test.

8. How can you tell whether a compound reacts with KMn04?


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Solubility

PROCEDURE (50-60 MINUTES)

Perform the tests described in Parts A, B, C, and D on each of these compounds: heptane,cyclohexene, and toluene. Carry out the same tests with your unknown hydrocarbon.

A. Solubility. Place 10 drops of each hydrocarbon [heptane, cyclohexene, toluene,and unknown] in small separate clean, dry test tubes. Cautiously compare theirodors. Add about 15 drops of methylene chloride to each tube. Shake the tubeswith the vortex mixer, let stand for 2 minutes, and note in the data table whetherthe compound is soluble, insoluble, or slightly soluble. Ifthe compound issoluble, the two layers will blend together to form a single homogeneous layer. Ifthe compound is insoluble, two distinct layers will form. Ifthe compound isslightly soluble, the mixture may appear cloudy. Repeat the solubility test usingethanol. Repeat the solubility test a third time using water as solvent.CAUTION: methylene chloride is toxic. Avoid inhalation of the vapors orcontact with the liquid. Remember to use clean, dry test tubes for each test.

Hydrocarbon Odor Solubility in Solubility in Solubility inMethylene Ethanol WaterChloride

Heptane

Cyclohexene

Toluene

Unknown


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COMBUSTION TEST RESULTS

B. Combustion. CAUTION: Remove all your hydrocarbon samples to a safe placebefore starting this test. If possible, work in the hood. Place about 8 drops of heptane ina clean, dry WHITE evaporating dish on a wire gauze, and ignite it with a burning splint.(Keep another wire gauze at hand to put over the evaporating dish to extinguish the flameif necessary). Observe the color of the flame, the smoke given off, and whether a residueis left. Record your observations, and repeat the test for each of the other hydrocarbons[cyclohexene, toluene, and unknown]. Burn the hydrocarbons in the following order:heptane, first; cyclohexene, second; your unknown, third; toluene, last. To clean theevaporating dish wipe the dish with a dry paper towel to remove all residue beforewashing it with soap and water.The combustion test should distinguish between a saturated hydrocarbon and an aromatichydrocarbon very clearly.

HYDROCARBON FLAME SMOKE RESIDUE

Heptane

Cyclohexene

Toluene

Unknown


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Bromine Test Results

C. Reaction with Bromine. Place 10 drops of each hydrocarbon [heptane,cyclohexene, toluene, and unknown] in separate clean, small, dry test tubes. To eachsample add 2 drops of a 1% solution of bromine solution. Shake each test tube, andrecord the results. CAUTION: Avoid breathing fumes from the 1 % bromine solution.Observe whether the color of the bromine is lost or retained when the bromine solution ismixed with the hydrocarbon.This test will tell whether a compound is unsaturated or not.

HYDROCARBON OBSERVATION

Heptane

Cyclohexene

Toluene

Unknown


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Unknown No. _ Unknown Identity

D. Reaction with Potassium Permanganate. Place 20 drops of each hydrocarbon[heptane, cyclohexene, toluene, and unknown] in separate large test tubes. Add 10 dropsof a 0.05% potassium permanganate solution and two drops of sodium carbonate solutionto each test tube and shake with the vortex mixer. Observe the color of the colored layerthat separates out. Make your observations within 30 seconds of mixing. Record yourobservations on the data sheet.

This test will tell whether a compound is unsaturated or not.Potassium Permanganate Test Results

HYDROCARBON OBSERVATION

Heptane

Cyclohexene

Toluene

Unknown

Use your test results to classify your unknown hydrocarbon as a saturated hydrocarbon,unsaturated hydrocarbon, or aromatic hydrocarbon.


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Water ~

QUESTIONS

1. Write the full structural formulas for the three known hydrocarbons used in thisexperiment.Heptane ~

Cyclohexene -

Toluene ~

2. Write the full structural formulas of the three solvents used and label each as po-lar, nonpolar, or slightly polar.Methylene Chloride ~

Ethanol ~


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2 . Balance the equation for the combustion ofnonane.

3. Write a balanced chemical equation for each of the known hydrocarbons that re-acted with the bromine solution.

4. When the KMn04 reacted to give a brown solid, what was the identity of thebrown solid? Give both the formula and the name of the compound.

5. Based upon your solubility tests do you consider the hydrocarbons to be mostlypolar or nonpolar? Explain your reasoning.

6. When hydrocarbons bum in such a manner that a lot of soot is produced, what isthe chemical name and formula of the soot?

7. An unknown hydrocarbon dissolved in methylene chloride, but was not soluble inwater. It reacted with Br2 to decolorize the bromine solution. It turned theKMn04 solution from a purple solution to brown solid. It burned to produce asmall amount of smoke and some black residue. What can you conclude aboutthe nature of the hydrocarbon from these results?

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Hydrocarbons Lab