LAB #1     Reactions of Hydrocarbons     Return

Hydrocarbons, compounds which contain only carbon and hydrogen, can be classified into several types, depending on their structure.  Aliphatic hydrocarbons are divided into three classes: alkanes have only single bonds, and are said to be saturated; alkenes and alkynes have carbon-carbon double or triple bonds, and are said to be unsaturated.Aromatic hydrocarbons are cyclic compounds whose structure is related to that of benzene, with six -electrons in a six-membered ring.

The following experiments illustrate some of the fundamental reactions of saturated, unsaturated, and aromatic hydrocarbons. The three classes sometimes react differently toward the same reagent, in which case it may be used to distinguish between them.

Aliphatic Hydrocarbons
All tests should be carried out in dry test tubes, and observations should be recorded on the report sheet as each experiment is performed.

1. Bromine or chlorine water
Alkanes react slowly or not at all with bromine at room temperature in the dark, but in the presence of sunlight, substitution is fairly rapid:

             R-H + Br2 --light--> R-Br + HBr (a substitution rxn)

The reaction is easily detected by loss of the bromine colour and by evolution of hydrogen bromide.

On the other hand, bromine adds rapidly at room temperature to alkenes in a reaction which does not require light. Since the product is colourless, the bromine is rapidly decolorized when added to an alkene.

Put 1 mL portions of cyclohexane into two separate test tubes.   Add 10-15 drops of bromine or chlorine water.  After shaking the tubes, place one in the dark, (under the counter) and expose the other to sunlight or a bright light for a few minutes.  Then compare the colour of the two tubes. Test for the presence of hydrogen bromide or hydrogen chloride by holding a piece of moist blue litmus at the mouth of each tube.

Add 1 mL of cyclohexene to a clean test tube. Add 10-15 drops of bromine or chlorie water, shake the tube and observe the result. Test for the presence of hydrogen bromide or hydrogen chloried evolution.

2. Aqueous Potassium Permanganate (Baeyer's Test)
Alkanes are relatively inert to chemical oxidizing agents such as neutral or alkaline permanganate, where alkenes are readily oxidized at room temperature. The change in colour can be used as a test for a double bond, provided the molecule contains no easily oxidizable group.

Place 5mL of potassium permanganate solution in two separate test tubes.  Add about five drops of alkane to one tube and the same of alkene to the other test tube.   Shake the tubes well for 1 -2 minutes, and note the results.

3. Sulphuric Acid
Although alkanes are inert to cold, concentrated sulphuric acid, alkenes react by addition. The product, alkyl hydrogen sulphate, is soluble in concentrated sulphuric acid.

To each of two 2 mL portions of concentrated sulphuric acid in separate test tubes add about 10 drops of alkane and alkene, respectively. Shake the tubes well and note the results. Discard the contents by pouring them into a beaker containing at least 50 mL of water.

Rxns of Aromatic Hydrocarbons
Toluene will be used in each of the following experiments. Although formally unsaturated, C6H6CH3 in the sense that it has multiple carbon-carbon double bonds, toluene does not give the usual reactions expected of an alkene. It is not easily oxidized, and preferably undergoes substitution rather than addition reactions.

1. Bromine
Add 1 mL of toluene to a small test tube. To another, add a few iron filings, followed by 1 mL of toluene, using the latter to rinse down any filings stuck on the test tube walls. To each test tube add three drops of bromine. Place the tubes in a beaker of warm water for 15 minutes. Observe the colour of each tube, and whether or not hydrogen bromide was evolved and record the results.

2. Aqueous Potassium permanganate
To a test tube containing 1 mL of toluene add 2 mL of dilute (2.0% w/w) potassium permanganate solution, shake and record the results.

Rxns of Alcohols and Phenols
The following tests and experiments are designed to illustrate some properties and reactions of alcohols and phenols.
     R-OH        Ar-OH
    alcohol        phenol

1. Solubility
The presence of a hydroxyl group in alcohols and phenols permits hydrogen bonding between then and the similar substance water, H-OH. This leads to appreciable water solubility for these classes of compounds, particularly the lower members of the series.

In six separate test tubes place 0.5 mL or 0.2 -0.5 g of each of the following:  ethanol, n-butyl alcohol, t-butyl alcohol, cyclohexanol, iso-amyl alcohol, and phenol. (Caution: Avoid any skin contact with phenol. In case of a burn, wash thoroughly with water for 15 minutes sand notify the teacher.) Add 2 mL of water to each tube, mix, and observe. Record the results on the report sheet.

2. Reaction with Alkali
Phenols are more acidic than alcohols and may be converted to their sodium salts by reaction with aqueous sodium hydroxide. The sodium salts are usually soluble in water.

In four separate test tubes place 0.5 mL or 0.2-0.5 g of each of the following:   n-butyl alcohol, cyclohexanol, phenol, and 1-naphthol. Add 5 mL of 10% sodium hydroxide to each tube, observe, and record the result.

3. Reaction with Metallic Sodium
Just as with water, the hydrogen atom of the hydroxyl group in alcohols and phenols can be displaced by sodium:

       2 R-OH + 2 Na ------->  2 R-O-Na+ + H2
                                                                 an alkoxide
The resulting alkoxides are strong bases, useful when a basic catalyst is needed for an organic reaction.

Place 2 mL of each of the following in separate dry test tubes: ethanol, 1-propanol, 2 -propanol, and orcinol. Add a small piece of sodium metal to each test tube and note the result. Add a few drops of universal indicator to the solution and record the result. (Caution:DO NOT discard the contents of any tube with unreacted sodium down the drain; sodium reacts violently with water. Add sufficient ethanol to destroy any unreacted sodium, then rinse the solution down the drain.)

Tests for the Three Classes of Alcohols
Alcohols are classified as primary, secondary, or tertiary, depending on whether the hydroxyl group is attached to a carbon that is bound to one, two or three other carbon atoms. Alcohols treated with a particular reagent may differ in the rates that they react, or indeed even in the type of product obtained, depending upon the class to which they belong. Tests which distinguish among the three classes can be useful in determining the structure of an unknown alcohol.

The Lucas Test
The reagent is a solution of zinc chloride in concentrated hydrochloric acid. The test is based on the different rates at which primary, secondary and tertiary alcohols are converted to their chlorides.

               R-OH + HCl ---------> R-Cl + H2O

Tertiary alcohols react instantly, giving an insoluble alkyl chloride which appears as a cloudy dispersion or as a separate layer. Secondary alcohols dissolve to give a clear solution (provided R does not have too many carbon atoms in the chain.), then form chlorides (cloudy solution) within five minutes. Primary alcohols are not converted to chloride for several hours at room temperature with this reagent.

Place 2 mL of Lucas reagent in each of four test tubes. Add about five drops of the alcohol to be tested, shake , and note the length of time it takes for the mixture to become cloudy or separate into two layers. Test 1-butanol, 2-butanol, cyclohexanol, n-amyl alcohol, benzyl alcohol and t-butyl alcohol and record the results.

Chromic Acid Oxidation (Bordwell-Wellman Test)
Primary and secondary alcohols are oxidized rapidly by chromic acid, where tertiary alcohol are not. In this test, an acetone solution of the alcohol to be tested is treated with a solution of chromic anhydride (Cr5+) in sulphuric acid. Alcohols which are oxidized reduce the chromium to Cr3+, causing the solution to become opaque and take on a greenish cast.

Place 1 mL of acetone in each of five separate test tubes.
To each tube, add one drop of a liquid alcohol or a few crystals (10 mg.) of a solid alcohol to be tested and shake until the solution is clear. The add, with shaking, one drop of the reagent. Test the following alcohols: 1-butanol, 2-butanol, t-butyl alcohol, n-amyl alcohol, benzyl alcohol, and cholesterol.

Reactions of Phenols
A. With Bromine Water
The hydroxyl group of phenols activates the ring to electrophilic substitution, so that reaction occurs under very mild conditions. With bromine water and phenol, the product is 2,4,6-tribromophenol, which has such a low solubility in water that it is often used not only as a qualitative test for phenol but also as a quantitative measure of the amount of phenol present.

To about 0.1 g of phenol dissolved in 3 mL of water add bromine water with shaking, until the yellow colour persists. Observe the results.

B. With Ferric Chloride
Phenols and compounds with a hydroxyl group attached to an unsaturated carbon atom(enols) give a coloration (pink, green, or violet depending on the structure of the phenol or enol) with ferric chloride. This is due to the formation of certain coordination complexes with the iron. Ordinary alcohols do not react. This test may be used to distinguish most phenols from alcohols.

In three separate test tubes dissolve one or two crystals, or one or two drops of the compounds to be tested in 5 mL of water. To each tube, add one or two drops of 1% ferric chloride solution, shake, and observe the results. Test phenol, resorcinol and 2-propanol, and record the results.

Reactions of Aldehydes and Ketones
Aldehydes and ketones both have, as their functional group, a carbon-oxygen double bond (carbonyl group). Accordingly, they undergo similar reactions. With the same reagent, aldehydes usually react faster than ketones, mainly because there is less crowding at the carbonyl carbon.   Aldehydes are also more easily oxidized than ketones.

Aldehydes are easily oxidized to acids which have the same number of carbon atoms.

An analogous reaction is not possible for ketones, whose oxidation to an acid requires the rupture of a carbon-carbon bond and results in two acids, each with fewer carbon atoms than the original ketone.

Several simple tests which are based on this difference in reactivity toward oxidizing agents are used to distinguish between the two classes of carbonyl compounds.

Aldehyde and Ketone Tests
1. Tollen's Silver Mirror Test
Tollens' reagent is an ammoniacial solution of silver ion prepared by dissolving silver oxide in ammonia.

          ---->R-C-OH + HO-C-R'

Several simple tests which are based on this difference in reactivity toward oxidizing agents are used to distinguish between the two classes of carbonyl compounds.
The reagent is reduced to metallic silver by aldehydes which, in turn, are oxidized to the corresponding acids. Ketones are not oxidized by the reagent.

            O                                                   O
            ||                                                     ||
       R-C-H + 2 Ag(NH3)2+OH- -----> R-C-O-NH4+ + 2 Ag + 3 NH3 + H2O

If the test is carried out with dilute solutions of reagents, in scrupulously clean glassware, the silver deposits finely in the form of a mirror on the walls of the beaker or test tube. Otherwise, the silver is deposited as a black precipitate.

Tollin's reagent is explosive on standing for long periods of time, therfore the reagent is prepared fresh just before it is needed as follows: Clean a test tube thoroughly with soap and water, and rinse with distilled water. To 2 mL of 5% silver nitrate solution add two drops of 5% sodium hydroxide solution and mix thoroughly. Then add drop by drop, and with stirring, only enough 2% ammonium hydroxide (concentrated ammonium hydroxide is 28%) to dissolve the precipitate. The test will fail if excess ammonia is added.

Prepare four test tubes with Tollen's reagent. Test each of the following by adding two drops of the substance: benzaldehyde, acetone, ethyl methyl ketone, and acetaldehyde. Shake the mixture, then allow it to stand for 10 minutes. If no reaction has occurred place the tube in a beaker of warm water (35-50oC) for five minutes. Record your observations.

2. Fehling's or Benedict's Test
The reagent in each of these tests is cupric ion, Cu2+ in an alkaline solution. To keep the copper from precipitating as the hydroxide, it is complexed with either tartrate ion (Fehling's reagent) or citrate ion (Benedict's reagent). For simplicity's sake we will write the equation with just the cupric ion. Aldehydes reduce the copper; the solutions usually turn from blue to muddy green, and gradually a reddish precipitate of cuprous oxide, Cu2O is formed. Most simple ketones do not react, although certain hydroxyketones and carbohydrates do.

             O                                              O
             ||                                                ||
        R-C-H + 2 Cu2+ + 5 OH- ----> R-C-O- + Cu2O + 3 H2O

To each of four test tubes add either 5 mL of Benedicts reagent or 5 mL of freshly prepared Fehling's reagent (made by mixing 10 mL of Fehling's A and 10 mL of Fehling's B solutions). To each tube add a few drops of the substance to be tested. Place the tubes in a beaker of boiling water and observe the changes which occur during 10-15 minutes. Test acetaldehyde, acetone and methyl isobutyl ketone and glucose.