EXERCISE 6 HYDROCARBONS I.
INTRODUCTION Hydrocarbons are compounds containing only hydrogen and carbon atoms. They are classified based on the presence of certain functional groups in their structures: alkanes (no functional group), alkenes (C = C), alkynes (C C), and aromatic hydrocarbons (pi system of electrons). ≡
Alkanes and their cyclic analogues have only two types of bonds in their molecules: C – C s sigma igma bonds and C – H sigma bonds. Both types of bonds may also be present in other hydrocarbons. All carbon atoms in alkanes are sp 3 hybridized. Also since both C – C and C – H bonds are non – polar, ionic reactions are unusual for alkanes under ordinary conditions. Instead, alkanes typically undergo reactions involving free radicals. An example of a reaction of alkanes is halogenation, which is a free radical substitution reaction involving C – H bonds. The reaction of a halogen with an alkane is initiated by heat or UV light and is given by: UV light or
R–H + X–X
High temp.
R–X + H–X
When the halogen used is bromine (Br 2), the reaction is accompanied by loss of the red – orange color of molecular bromine and the evolution of a gas, hydrogen bromide. These are signs signs indicating that a free radical substitution reaction has occurred. Alkenes and their cyclic analogues contain at least one C = C in their molecule. The double bond contains a pi bond which, arises from the p orbital of each carbon atom (C p – C p) and a sigma bond which, is due to Csp2 – Csp2 orbital overlap. Since the double bond bond is a site of high electron density (i.e. it is electron rich), alkenes react with electrophillic and oxidizing reagents. Alkenes react with halogens in a reaction known as electrophillic addition. This reaction takes place at the double bond and does not require the presence of heat or light. Free radical addition also occurs but at a slower rate. The general reaction for electro[phillic electro[philli c addition of halogen is:
+
X–X X X
Alkene
alkyl halide (vicinal dihalide)
The presence of C = C mab be detected using bromine. As in the halogenation of alkanes, the loss of the red – orange color of bromine (even at room temperature or in the dark may indicate the presence of C = C); hydrogen bromide is not evolved. Another test for the C = C bond is the reaction with Baeyer’s reagent (cold, dilute, neutral aqueous potassium permanganate). The general reaction is:
+ KMnO4 + H2O
+ MnO2 + KOH OH OH
Alkene
glycol
brown Precipitate
In a positive test, the purple color if the reagent disappears completely as the permanganate ion is reduced and a brown precipitate (MnO2) if formed. Alkynes possess at least one C C in their molecules. The triple bond consists of two pi bonds and one sigma bond. Like the C=C , the C C is electron rich. Consequently, alkynes undergo the same type of reactions as alkenes: −
≡
−
−
−
≡
−
Halogenation:
+
Alkyne
X
X
X
X
2X–X
tetrahalide
−
Oxidation O
KmnO4 H2O
+
R
MnO2 +
KOH
OH
brown precipitate Whether the C C is found at the end of the carbon chain (terminal alkyne) or not can be determined by the test with ammoniacal or alcoholic silver nitrate. The test involve the release of the terminal proton of the alkyne so that the resulting acetylide ion can form an insoluble precipitate with Ag+: ≡
AgNO3
R –C
≡
C– R
R –C C Ag+ + HNO3 ≡
ethanol
−
terminal alkyne R –C C– R ≡
Ag(NH3)
+
R –C C Ag+ + HNO3 ≡
−
terminal alkyne
Aromatic hydrocarbons are characterized by the presence of the pi system of electrons. Although the pi system is also a region of high electron density, aromatic hydrocarbons do not undergo the same types of reactions as alkenes or alkynes.
II.
OBJECTIVES 1. To develop powers of observations for detection of the various signs of chemical change. 2. To observe how hydrocarbons types may be detected and differentiated from each other by means of simple chemical tests (test tube reactions) 3. To compare the reactivity of alkanes, alkenes, alkynes and aromatic hydrocarbons towards selected chemical reagents.
III.
EXPERIMENTAL The following representative compounds will be used in this exercise: CH3
H –C C– H ≡
cyclohexane cyclohexene
benzene
toluene
acetylene
A. Investigation of Hydrocarbons Note down the color, clarity and physical state of the above representative compounds. Use cyclohexane, cyclehexene, benzene and toluene for the tests below. 1. Solubility behavior a. Place two mL carbon tetrachloride in a test tube b. Add 15 drops of the sample c. Shake to mix d. Examine the mixture e. Repeat steps a – d using H 2O, 10% NaOH and then concentrated H2SO4 as solvents. 2. Halogenation: Reaction with bromine (BROMINE CAUSES PAINFUL BURNS. IF SPILLED ON THE SKIN, FLUSH OFF WITH PLENTY OF WATER WITHOUT RUBBING THE SKIN AND TREAT IMMEDIATELY WITH GLYCERINE) a. Place 15 drops of the sample in each of two test tubes, one of which is wrapped in carbon paper. b. Add 5 drops of the reagent (0.05 M Br 2 /CCl4). Shake to mix. c. Expose the unwrapped test tube to light for 10 minutes. Observe the appearance of the solution. d. Unwrap the other test tube and compare its contents with that of the tube exposed to light. 3. Oxidation: Reaction with Baeyer’s Reagent a. Place two mL of Baeyer’s reagent in a test tube. b. Add 5 drops of the sample. Shake to mix. c. Observe for color changes. (Partial reduction of KmnO 4 can be due to impurities. Therefore if the purple color of the reagent is not completely destroyed, the test is to be considered negative.
4. Reaction with ammoniacal silver nitrate Perform this test on cyclohexane, cyclohexene and acetylene. a. Place 10 drops of the reagent in a test tube. b. Add 2 drops of the sample. Shake to mix. c. Observe ANY SOLID PRECIPITATE FORMED IN THIS TEST MAY BE EXPLOSIVE WHEN DRY. TRANSFER ANY PRECIPITATE INTO A VESSEL CONTAINING HNO3. B. Preparation and Testing of Acetylene Gas (CAUTION: NO OPEN FLAME SHOULD BE PRESENT!) a. Introduce about 5 grams of calcium carbide into a dry 250 mL distilling flask b. Assemble the set – up as shown in the figure c. Add 100 mL of distilled water to the separatory funnel. d. Allow the water to fall on the solids dropwise to generate acetylene gas. Observe e. Perform tests 1 – 4 (Part A) on the acetylene gas.
water
acetylene gas
Calcium carbide
Set – up for the generation of acetylene gas
IV.
QUESTIONS 1. What is the function of CCl4 in the bromination reactions? How can it fulfill this role? 2. Explain why terminal alkynes are acidic. 3. Explain the bromination reaction of benzene, toluene, and cyclohexane. 4. Give the reagent and the reaction conditions that would distinguish between the following compounds. Write the equations for the reactions involved. a. b. c. d.
benzene and ethylbenzene 1-butyne and 2-butyne 2-methylpentane and 2-methyl-2-pentene toluene and 1-methylcyclohexene
