Hydrocarbons. Table Alkanes

Transcription

1 Hydrocarbons Hydrocarbons may be divided into two groups, aliphatic and aromatic. The aliphatics can be subdivided into i) alkanes, ii) alkenes, and iii) alkynes. The alkanes contain single covalent bonds between each carbon atom and generally are relatively unreactive. The alkenes contain at least one double bond between two carbon atoms somewhere in the molecule and are more reactive. Aromatic hydrocarbons (arenes) generally react under conditions quite different from the considered offer a means by which the above classes of hydrocarbons can be qualitatively determined. The following chemical tests will be performed on each type of hydrocarbon: A) 1% Br 2 (in CCl 4 ) - - any products that do form are colorless; B) 0.05% KMnO 4 (in H 2 O) - - a brownish MnO 2 forms if a reaction occurs; C) combustion (in an evaporating dish) - - in the hood. Alkanes Alkanes are called by three other common names; paraffins, aliphatics and saturated hydrocarbons. The general formula for any noncyclic alkane is given by C n H( 2n+2 ). For example, n-octane has eight carbons, that is, n = 8, so that 2n + 2 = 18 for the hydrogen count. Therefore, the formula for n-octane is C 8 H1 8. The simplest member of this alkane series is methane, CH 4. This series as displayed in Table-1 is known as a homologous series; that is, each member differs from its immediate neighbor by one CH 2 unit. Table Alkanes Reaction of Alkanes: A) A solution of bromine reacts slowly or not at all with alkanes in the dark, but when catalyzed by light, the following reaction occurs to produce a colorless bromoalkane: (λ=light added) Br 2 + R-H λ HBr + R-Br Add 10 drops of n-hexane to two small dry test tubes, then add 5 drops of 1% Br 2 to each. Put

2 one test tube in the dark. Record the time it takes for the color to disappear in room light. (In sunlight, the reaction is even faster.) Check the other test tube for any changes. Dispose of the liquids in appropriate waste bottles. B) Add 10 drops of alkane to 10 drops of 0.05% KMnO 4 and look for any color changes. Warm the mixture in a hot water bath to see if the alkane can be oxidized. C) In the hood, place a few drops of an alkane in your evaporating dish propped onto a ring stand. Carefully light the liquid with a match, and observe how it burns. Is the flame smokey? Is there any residue? Alkenes Alkenes are also known as unsaturated hydrocarbons or as olefins. A general formula for noncyclic alkenes is C n H 2n. Hence for n = 7, one possibility for C 7 H 14 is 1-heptene. Table 2 displays some physical constants of a homologous series of the alkenes. Table Alkenes Reactions of Alkenes A) Alkenes react readily with bromine by an addition reaction as illustrated by the following general reaction: Br R 2 C=CR 2 + Br 2 R 2 -C-C-R 2 Br

3 R 2 may be either a hydrogen or some hydrocarbon residue. As the bromine reacts with the alkene, a colorless 1, 2-dibromo compound is produced. Therefore, a good qualitative test for alkene is the disappearance of the reddish bromine color. Place 10 drops of 1-hexene or 10 drops of cyclohexene in a test tube. Add 5 drops of 1% Br 2 and note how long it takes for the color to disappear. B) Alkenes are easily oxidized by neutral permanganate to the 1,2-glycols. For example: 3 CH 2 =CH 2 + KMnO 2 +4H 2 O 2 KOH + 2 MnO CH 2 -CH 2 OH OH ethylene glycol Place 10 drops each of 1-hexene or cyclohexene in a test tube. Add 10 drops of 0.05% aqueous KMnO 4 solution. Mix it well and note any color changes. Is heating necessary to cause a color change? C) In the hood, on a ring stand, burn a few drops of an alkene in your evaporating dish. Note its burning characteristics. Arenes Arenes are generally known as the aromatic hydrocarbons. The simplest, common member of this series is benzene, C 6 H 6. One Lewis structure for benzene sets the carbons in a hexagon having three double bonds alternating with three single bonds. In reality, each carbon-carbon bond is identical in length, and reactivity of the bond is intermediate between that of a single and a double bond. This cyclic structure is so stable, in fact, that benzene and its

4 derivatives, in general, undergo substitution reactions on the ring instead of addition reactions. Toluene (also called methylbenzene) will be used to represent this family. A) To a test tube place 10 drops of toluene, then add 5 drops of 1% Br 2 to it. How long does the reaction take? B) In another test tube, to 10 drops of toluene, add 10 drops of 0.05% KMnO 4, warm in the water bath, and observe when the KMnO 4 color changes. A change to brown MnO 2 denotes an oxidation of toluene. C) In the hood, on a ring stand, as before, burn a few drops of toluene, and make note of its burning characteristics. Liquid Unknown The information you have obtained from your observations can be used to determine the identity of an unknown liquid. This liquid will be either an alkane, an alkene, or an arene. Procedure 1. Perform as many chemical tests as needed to determine to which family your unknown belongs. Keep track of your observations. 2. Your final report should contain the conclusions of the kind of hydrocarbon you have found. Miscellaneous Thoughts (as directed by your instructor) 1. Write equations for each reaction you observed. (The oxidation of toluene is a special type.) 3. Draw all the possible isomers of the noncyclic C 6 H 14 alkanes and C 6 H 12 alkenes. Name each isomer. 4. Describe any differences in odors among the alkanes, alkenes, and arenes you have investigated. 5. How would you evaluate the relative reactivity of an alkane vs an alkene vs an arene? For example, compare the time taken for the oxidation reactions using KmnO 4.

5 Lab section Reaction of Alkanes Alkane with bromine In light In dark Hydrocarbons Name Alkane with 0.05% KMnO 4 After heating Did it oxidize? How do you know? Combustion of alkane Is the flame smokey? Is there any residue? Reactions of Alkenes Alkene with bromine What alkene did you use? How long did it take for the reaction to occur? Alkene with 0.05% KMnO 4 Did any color changes occur? Is heating necessary to cause a color change? Combustion of alkene

6 Is the flame smokey? Is there any residue? Arenes Arene with bromine How long did it take for the reaction to occur? Arene with 0.05% KMnO 4 Did any color changes occur? If so what product would form the brown color? Combustion of arene Is the flame smokey? Is there any residue? Liquid Unknown Br 2 results Unknown number KMnO 4 results Combustion results Unknown is: Alkane Alkene Arene