Characterize the chemical reactivities of the functional groups in organic compounds.

Transcription

1 Chapter 03 Experiment: Organic Compounds OBJECTIVES: Characterize the chemical reactivities of the functional groups in organic compounds. INTRODUCTION: Carbon has the unusual property of bonding to itself. Although atoms of other elements bond to themselves in compounds such as S 8, P 4 S 4 and B 10 H 14, the extent to which carbon bonds to itself is nearly endless. More than three million different carbon-containing compounds, called organic compounds, have been reported in the literature. As a result, a complete knowledge of the unique chemical and physical properties of these compounds is very difficult to acquire. The complexity of organic chemistry is lessened somewhat by the fact that organic compounds form natural classes according to their chemical properties. An atom or group of atoms in an organic molecule, commonly referred to as a functional group, often governs the chemical properties of the compound. Each functional group imparts certain characteristic chemical properties to the entire organic molecule. In this experiment, the chemistry of hydrocarbons (alkanes, alkenes, alkynes, aromatics), alcohols, aldehydes, acids, bases, and esters is investigated. Hydrocarbons, the simplest of the classes, are compounds containing only carbon and hydrogen atoms. According to their chemicals properties, the hydrocarbons are subdivided into three groups: saturated hydrocarbons, unsaturated hydrocarbons, and aromatic hydrocarbons. Saturated hydrocarbons are also called alkanes. In the alkanes all carbon-carbon bonds are single bonds with a bond energy of about 350 kj/mol; the carbon-hydrogen bonds have a bond energy of about 420 kj/mol. Thus the alkanes are relatively inert chemically. By far their most important reaction is combustion, forming carbon dioxide and water as products. An example is the combustion of propane, a substance used extensively as a fuel. CH 3 -CH 2 -CH 3 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(g) kj Unsaturated hydrocarbons have two subgroups, the alkenes and the alkynes. The alkenes have one or more carboncarbon double bonds in the molecule and the alkynes have one or more carbon-carbon triple bonds. These double and triple bonds, called unsaturated bonds, are chemically quite reactive. For example, bromine readily reacts by addition to the carbon-carbon double bond of propene. CH 3 -CH=CH 2 + Br 2 CH 3 -CHBr-CH 2 Br The red-brown color of bromine disappears as this addition reaction occurs, confirming the presence of the C=C bond.

2 Figure 1: Functional Groups in Organic Compounds

3 Alkenes and alkynes also react with oxidizing agents, such as potassium permanganate, to produce alcohols (Baeyer s test). The deep purple permanganate ion is reduced to a brown manganese (IV) oxide precipitate, which may also appear as a brown suspension. 3 CH 2 =CH MnO H 2 O 3 CH 2 OH-CH 2 OH + 2 MnO OH Aromatic hydrocarbons are generally characterized by the presence of the six-member carbon ring called benzene. Although somewhat resistant to chemical attack, aromatic compounds are more reactive than the alkanes, but less reactive than the alkenes or alkynes. For example, bromine displaces hydrogen from the benzene ring, but only in the presence of a catalyst. An alcohol is a hydrocarbon in which an OH group substitutes for a hydrogen atom. For example, the substitution of an OH group for an H atom in ethane forms ethanol, the alcohol formed in the fermentation of grains (CH 3 -CH 3 to CH 3 - CH 2 OH). An alcohol is also like water in than an alkyl group replaces a hydrogen atom in a water molecule. The substitution of a methyl group, CH 3 -, for an H atom in water forms methanol, CH 3 OH, commonly called wood alcohol. Therefore, alcohols have properties that are intermediate between those of hydrocarbons and water. In this experiment, potassium dichromate, K 2 Cr 2 O 7, is used to oxidize the (primary) alcohols to the corresponding aldehydes. A color change from a brilliant orange (Cr 2 O 7 2 ) to green (Cr 3+ ) or a change in color in odor determines whether or not a reaction has occurred. For ethanol, the corresponding aldehyde is acetaldehyde (ethanal). 3 CH 3 -CH 2 OH + Cr 2 O H + 3 CH 3 -CHO + 2 Cr H 2 O Stronger oxidizing agents, such as potassium permanganate, KMnO 4, oxidize an alcohol to its corresponding acid. For ethanol, the corresponding acid is acetic (ethanoic) acid, the acid of household vinegar: 5 CH 3 -CH 2 OH + 4 MnO H + 5 CH 3 -COOH + 4 Mn H 2 O An aldehyde has the CHO grouping of atoms in the organic molecule. A characteristic test of an aldehyde is its solubility in an aqueous sodium bisulfite, NaHSO 3, solution. The bisulfite ion adds across the carbon-oxygen double bond, forming a water soluble salt. Acetaldehyde reacts with the HSO 3 ion to produce the corresponding salt. An organic acid has the COOH grouping of atoms in the organic molecule. An acid can be prepared by oxidation of its alcohol with a strong oxidizing agent, such as potassium permanganate. Organic acids react readily with sodium bicarbonate, NaHCO 3, releasing carbon dioxide gas: CH 3 COOH + NaHCO 3 CH 3 COO Na + + H 2 O + CO 2 Organic bases can be considered organic derivatives of ammonia, NH 3, in which one or more alkyl groups replace one or more hydrogens; the resulting compound is an amine. Examples include methylamine, CH 3 NH 2, where a methyl group, CH 3, replaces one hydrogen atom in NH 3, ethylamine, C 2 H 5 NH 2, and dimethylamine, (CH 3 ) 2 NH. The electron pair on the nitrogen atom serves as a Lewis base. Most organic compounds have a low solubility in water. Those that are soluble generally form neutral solutions; however, the acids and bases have the expected low and high ph, respectively. This experiment uses litmus paper for testing. An ester forms from the reaction of an organic acid with an alcohol. Esters have the -COO- grouping of atoms, consisting of one carbon-oxygen double and one carbon-oxygen single bond. For acetic (ethanoic) acid and methanol, the corresponding ester is methyl acetate.

4 Esters are usually volatile and have a pleasant odor. The natural scents of many flowers and the flavors of fruits are due to one or more esters. Some important natural esters are fats (butter, lard, tallow) and oils (linseed, cottonseed, olive) used in the synthesis of oleomargarine, peanut butter, and vegetable shortening. PROCEDURE: Qualitative testing of an array of organic compounds is used to observe the characteristic chemical reactions of several functional groups. An unknown organic compound is tested to identify its functional group. If you are to identify the functional group in an unknown sample, place the sample in a test tube alongside that containing the known compound. As you progress through the procedure, perform the same test on both solutions and make comparative observations. Check ( ) the findings on the Report Sheet. Disposal: Dispose of all test solutions in the Waste Organics container, or ask your instructor about proper disposal. Caution: A number of concentrated acids are used for testing these organic compounds. Handle each of these solutions with care. Caution: Many organic compounds are flammable, rapidly reacting with oxygen in the presence of a flame or spark. Therefore, extinguish all flames in the laboratory before beginning the experiment. Test for Hydrocarbons: Complete the following test on these samples: 4-5 drops of pentane (an alkane) 4-5 drops of 1-pentene or 2-pentene (an alkene) (vegetable oil will also work) 4-5 drops toluene (an aromatic) Reactivity with Potassium Permanganate. In a 150 mm test tube dissolve a sample in about 1 ml of acetone. Add drops of 1 % (0.06M) KMnO 4. (Caution: Avoid skin contact) Agitate the mixture. Observe the reaction mixture for several minutes. Note: Remember, bromine and potassium permanganate may react with subsequent groups of compounds. Because a reaction may occur for your unknown here, the evidence may not be conclusive. Keep checking! Test for Primary Alcohols: Reactivity with Mild Oxidizing Agent. Place 1 ml of 0.1 M Na 2 Cr 2 O 7 (caution: avoid skin contact) in a 150-mm test tube and slowly add drops of concentrated H 2 SO 4. (Caution: this reaction is very exothermic, swirl after each drop; concentrated H 2 SO 4 is a severe skin irritant and also causes clothes to disappear!) Swirl the mixture. Slowly add 1 ml of the ethanol to the mixture. Note any color change and odor. Compare its odor with that of the ethanol. First test for Organic Acids and Organic Bases: Effect of Litmus. Place 4-5 drops of acetic acid in a 75-mm test tube and add 4-5 drops of water. Test the solution with litmus paper. If your unknown sample is insoluble in water, add drops of ethanol until it dissolves. Repeat the litmus test on a similarly prepared sample of diphenhydramine.

5 Second test for Organic Acids: Reactivity with Sodium Barcarbonate. Again place 4-5 drops of acetic acid in a 75-mm test tube and add 4-5 drops of water. Add drops of 10% NaHCO 3. A distinct fizzing sound is detectable, even if the visual evolution of CO 2 is questionable. Third test for Organic Acids: Reactivity with Potassium Permanganate. Place 1 ml of 1% (0.06 M) KMnO 4 in a 150-mm test tube. Slowly add 1 ml of ethanol. Watch for a color change. Compare its odor with those of ethanol and acetic acid. What s your conclusion?

6 Prelaboratory Assignment NAME: Answer the following questions: 1. Write the balanced equation for the combustion of octane, C 8 H Discuss why an alcohol may have properties that are intermediate between those of a hydrocarbon and water. 3. a. What structurally distinguishes a saturated hydrocarbon from an unsaturated hydrocarbon? b. Indicate the (minimum) number and type of unsaturated bonds present in an alkane, an alkene, and an alkyne. 4. a. How can organic acids be prepared in the laboratory? b. Which functional groups must be a part of the reactant molecules for the preparation of an organic ester in the laboratory? 5. Explain the statement: alcohols are to water as amines are to ammonia. 6. Write the structural formula of the a. Acid formed from the oxidation of methanol, CH 3 OH b. Aldehyde formed from the oxidation of methanol, CH 3 OH c. Ester formed from the reaction of butanoic acid, C 3 H 7 COOH, and ethanol, CH 3 CH 2 OH.