Classification Tests for Hydrocarbons

Jasareno, K.L.S., Juguilon, I.M.M., Limsi, K.E.H., Llanita, A.J.I., Lopez, S.S.C., and Macaranas, J.D.P.

2B-PH, Group No. 5, Department of Pharmacy, Faculty of Pharmacy, University of Santo Tomas, España Boulevard, 1015 Manila, Philippines

Abstract

In this experiment the group used 5 sample organic compounds, Hexane, Heptane, Cyclohexane, Cyclohexene, Benzene and Toluene. These compounds were observed in terms of intrinsic physical properties and chemical properties in terms of structure and behavior. The compounds were subjected to the several tests available to classify hydrocarbons. The main objective of this experiment was for the group to be familiar with the different properties and reaction mechanisms that characterize the different classifications of hydrocarbons.

Introduction

Hydrocarbons are one of the basic organic compounds there is.[6] A hydrocarbon, as its name implies, consists of only covalent hydrogen-carbon bonds. These organic compounds are mainly classified as aliphatic or aromatic. Aliphatic hydrocarbons are classified according to its carbon chain skeleton, acyclic (open-chained) or cyclic (closed-chained). Also, according to the bonds present, saturated hydrocarbons which only have single covalent bonds or unsaturated hydrocarbon which have at least a double or triple covalent bond in its structure. Aromatic hydrocarbons are mainly composed of benzenes (Fig. I.5.), those that contain alkyl side chains are called arenes.[] In the experiment, six different organic compounds were used namely, hexane, heptane, cyclohexane, cyclohexene, benzene and toluene. For every classification of the given samples of hydrocarbons, there is a corresponding test that would help identify it. The importance of classifying hydrocarbons is that it enables researchers to identify the certain characteristic that would

either be beneficial or harmful towards its intended use.[6]

The intended learning outcomes of this experiment are to analyze and classify hydrocarbons, differentiate hydrocarbons in terms of their intrinsic physical properties and chemical properties based on their structure and behavior.[1]

Methodolgy

For this experiment, the group used the following organic compounds: hexane, heptane, cyclohexane, cyclohexene, benzene and toluene to differentiate each with the aid of several tests.

A. Physical State, Color and Solubility

First, the physical state, color and odor of the samples were taken note of.

B. Solubility in conc. H2SO4

1 mL of concentrated H2SO4 was placed in 6 different test tubes, then one drop of each sample was added to the test tubes. Any color, odor and formation of precipitates or layers were then taken note of after mixing.[1]

C. Ignition Test

3-5 drops of each liquid sample was placed in a small evaporating dish, for the solid samples a pinch amount was used, the samples were then lit with a match. Flammability, formation of soot and flame color was noted for each sample afterwards.[1]

D. Test for Active Unsaturation

a. Baeyer’s Test

5 drops of the sample was placed in a dry test tube. 2 drops of KMnO4 was added to the solution. The test tube was shaken vigorously and the rate and extent of decorization was observed. The formation of a brown precipitate was taken note of and the sample was compared with water as the negative control. Decolorization under a minute was recorded as immediate.[1]

b. Bromine Test

10 drops of 0.5% Br2 in CCl4 reagent was added to 5 drops of the sample in a dry test tube. The test tube was shaken vigorously and the rate of color change was observed. Water was used as a negative control to compare. Samples that did not decolorize within a minute were exposed to sunlight. Results were taken note of.[1]

E. Test for Aromaticity: Nitration

For the formation of the nitrating mixture, 2 mL of concentrated HNO3 was placed in an Erlenmeyer flask and was immersed in an evaporating dish that contained water. 2 mL of concentrated

H2SO4 was gradually added. The resulting mixture was cooled at room temperature.

8 drops of the nitrating mixture was added to 5 drops of the sample and was shaken to ensure complete mixing. The formation of a yellow oily layer or globule was noted and the solution was diluted with 20 drops of water. Solutions that did not react within a minute were placed in a water bath for 10 minutes and diluted with 20 drops of water. The results were taken note of after.[1]

F. Basic Oxidation

4 drops of the sample in a dry test tube was added with a drop of 2% KMnO4 solution, 7 drops of distilled water and 3 drops of 10% NaOH solution. Each test tube was warmed in a water bath for 2 minutes. Any color change was observed.[1]

Results and Discussion

A. Physical State, Color and Solubility

Based on the observations, all six samples were clear, colorless liquids at room temperature that had strong, gasoline-like odors. Hexane, heptane and cyclohexane were saturated hydrocarbons while cyclohexene was an aliphatic unsaturated hydrocarbon. Benzene and toluene were aromatic hydrocarbons.

Hexane Heptane Cyclohexane

condensed structural formula

CH3(CH2)4CH3 CH3(CH2)5CH3

A.Physical state at RT

liquid liquid liquid

Appearance clear clear clear Color colorless colorless colorless

Odor strong, gasoline-like

strong, gasoline-like

strong, gasoline-like

Hexane Heptane Cyclohexane

B. Solubility in conc. H2SO4

immiscible immiscible immiscible

Inference formed a layer formed a layer formed a layer

Cyclohexene Benzene Toluene

B. Solubility in conc. H2SO4

miscible immiscible immiscible

Inference orange solution formed a layer formed a layer

B. Solubility in conc. H2SO4

Sulfuric acid reacts with both alkenes and aromatic hydrocarbons, although in different ways. Sulfuric acid adds to cyclohexene via an acid catalyzed addition. Sulfuric acid adds to toluene via electrophilic aromatic substitution.[3] On the other hand, alkanes are immiscible and do not produce a reaction since all the bonds are filled and there would be no room for the sulfuric acid to attach or react to it.

Based on the results from the experiment, only cyclohexene was miscible with sulfuric acid. Both benzene and toluene failed to yield results and just formed layers which the group interpreted as miscible. The lack of reaction may have been caused by the contamination of the reagents, lack of standing time or human error during the procedure.

C. Ignition Test

Generally, most hydrocarbons burn over a flame- the hydrocarbons react with oxygen to produce carbon dioxide and water.[3] The number of carbon atoms in a hydrocarbon only affected the sootiness (production of CO2) of the flame. The degree of luminosity remained the same but compounds were produced at different energies.

The results gathered showed that all six samples were flammable. The first three, hexane, heptane and cyclohexane produced yellow flames without any soot. While the following three, cyclohexene, benzene and toluene produced yellow-orange flames with soot.

Hexane Heptane Cyclohexane

C. Ignition test flammable without soot

flammable without soot

flammable without soot

Inference yellow flame yellow flame yellow flame

Cyclohexene Benzene Toluene

C. Ignition test flammable with soot

flammable with soot

flammable with soot

Inference yellow flame orange flame orange flame

Cyclohexene Benzene Toluene

condensed structural formula

A.Physical state at RT

liquid liquid liquid

Appearance clear clear clear

Color colorless colorless colorless

Odor strong, gasoline-like

strong, gasoline-like

strong, gasoline-like

Hexane Heptane Cyclohexane

D. Baeyer's test purple solution

purple solution

purple solution

Cyclohexene Benzene Toluene

D. Baeyer's test decolorized purple solution

purple solution

Hexane Heptane Cyclohexane

Bromine test orange solution

light orange solution

orange solution

Cyclohexene Benzene Toluene

Bromine test

clear, colorless

solution with brown ppt

orange solution

orange solution

Inference oily layer oily layer oily layer

Cyclohexene Benzene Toluene

Inference oily layerslightly yellow

oily layerslightly yellow

oily layer

E. Test for aromaticity: Nitration

immiscible immiscible immiscible

D. Test for Active Unsaturation

a. Baeyer’s Test

Baeyer’s test was conducted to test for the presence of active unsaturation. The reagent used was KMnO4. Potassium permanganate reacts with anything unsaturated. KMnO4 reacts with alkenes to form vicinal diols. KMnO4 will also react with alkyl benzenes, such as toluene, to form benzoic acids. It does not react with alkanes.[3] The compound that had a positive reaction showed decolorization, cyclohexene.

b. Bromine Test

The bromine test was another test conducted to identify the actively unsaturated hydrocarbons. Bromine, the reagent used in the test, was an orange-brown aqueous solution that can only be decolorized by alkenes. Bromine, when added to alkanes and benzenes, would not react since alkanes only react to bromine

under free radical conditions and benzenes need a strong Lewis acid catalyst.[3]

In the results gathered, it was observed that only cyclohexene was able to decolorize the reagent and produce a brown precipitate. This indicated that among the six samples only cyclohexane is an actively unsaturated hydrocarbon.

E. Test for Aromaticity: Nitration

Nitration is a chemical reaction in which a nitro group is added to a hydrocarbon compound replacing a hydrogen. In nitration, sulfuric acid and

nitric acid, together served as the nitrating mixture, was reacted with the samples. The aromatic hydrocarbons react with warm sulfuric acid to form a sulfonic acid which will then dissolve and then precipitate when the nitro group is precipitated.[3]

In the results, the yellow globules indicated that the hydrocarbon was aromatic which was observed for benzene and toluene while the others that did not produce any results were identified as aliphatic.[4]

Hexane Heptane Cyclohexane

Inference oily layer oily layer oily layer

E. Test for Aromaticity:

Nitrationimmiscible immiscible immiscible

F. Basic Oxidation

In basic oxidation, a strong oxidizing agent, KMnO4, was used. Its reaction to the sample determined the presence of oxidation.[6] A color change was the indicator that the reaction between the sample and oxidizing occurred. For this experiment, there was an observed color change in cyclohexene. The other five samples showed no signs of decolorization and it was assumed that they did not undergo oxidation.[5]

In conclusion, it was observed that the saturated hydrocarbons were hexane, heptane and cyclohexane. The only actively unsaturated hydrocarbon was cyclohexene and that benzene and toluene are both aromatic but only toluene may be classified as an arene.

Sources

[1] (Bathan, et al., 2014, Laboratory Manual in Organic Chemistry, pp. 81-84)

[2] Experiment 7: Classification Tests for Hydrocarbons. (n.d.) retrieved November 20, 2015 from: http://www.scribd.com/doc/37898377/Formal-Report-Experiment-7-Classification-test-for-hydrocarbons

[3] Properties of Hydrocarbons (n.d.) retrieved November 20, 2015 from: http://www.mendelset.com/articles/689/properties_hydrocarbons

[4] Aromaticity test: Nitration (n.d.) retrieved November 20, 2015 from: https://ph.answers.yahoo.com/question/index?qid=20111119173932AAoWlxS

[5] (Poon, et. al., 2014, Introduction to Organic

Chemistry, pp. 305-308)

[6] Hydrocarbons and Its Identification Tests for Classification (n.d.) retrieved November 20, 2015 from: http://documents.mx/documents/hydrocarbons-and-its-identification-tests-for-classification.html

Hexane Heptane CyclohexaneF. Basic oxidation

no decolorization

no decolorization

no decolorization

Inference no oxidation no oxidation no oxidation

Cyclohexene Benzene Toluene

F. Basic oxidation

decolorized no decolorization

no decolorization

Inference

color change with presence

of brown precipitate

no oxidation no oxidation