13.1: Alkenes- Structures and Names Concept Review Exercises Briefly identify the important distinctions between a saturated hydrocarbon and an unsaturated hydrocarbon. Briefly identify the important distinctions between an alkene and an alkane. Classify each compound as saturated or unsaturated. Identify each as an alkane, an alkene, or an alkyne. CH 3 CH 2 C≡CCH 3 Answers Unsaturated hydrocarbons have double or triple bonds and are quite reactive; saturated hydrocarbons have only single bonds and are rather unreactive. An alkene has a double bond; an alkane has single bonds only. saturated; alkane unsaturated; alkyne unsaturated; alkene Exercises Draw the structure for each compound. 2-methyl-2-pentene 2,3-dimethyl-1-butene cyclohexene Draw the structure for each compound. 5-methyl-1-hexene 3-ethyl-2-pentene 4-methyl-2-hexene Name each compound according to the IUPAC system. Name each compound according to the IUPAC system. Answers 2-methyl-1-pentene 2-methyl-2-pentene 2,5-dimethyl-2-hexene 13.2: Cis-Trans Isomers (Geometric Isomers) Concept Review Exercises What are cis-trans (geometric) isomers? What two types of compounds can exhibit cis-trans isomerism? Classify each compound as a cis isomer, a trans isomer, or neither. Answers Cis-trans isomers are compounds that have different configurations (groups permanently in different places in space) because of the presence of a rigid structure in their molecule. Alkenes and cyclic compounds can exhibit cis-trans isomerism. trans (the two hydrogen atoms are on opposite sides) cis (the two hydrogen atoms are on the same side, as are the two ethyl groups) cis (the two ethyl groups are on the same side) neither (flipping the bond does not change the molecule. There are no isomers for this molecule) Exercises Draw the structures of the cis-trans isomers for each compound. Label them cis and trans. If no cis-trans isomers exist, write none. 2-bromo-2-pentene 3-hexene 4-methyl-2-pentene 1,1-dibromo-1-butene 2-butenoic acid (CH 3 CH=CHCOOH) Draw the structures of the cis-trans isomers for each compound. Label them cis and trans. If no cis-trans isomers exist, write none. 2,3-dimethyl-2-pentene 1,1-dimethyl-2-ethylcyclopropane 1,2-dimethylcyclohexane 5-methyl-2-hexene 1,2,3-trimethylcyclopropane Answer a: none. There are two distinct geometric isomers, but since there are there are four different groups off the double bond, these are both cis/trans isomers (they are technically E/Z isomers discussed elsewhere). b: c: d: e: 13.3: Physical Properties of Alkenes Concept Review Exercises Briefly describe the physical properties of alkenes. How do these properties compare to those of the alkanes? Without consulting tables, arrange the following alkenes in order of increasing boiling point: 1-butene, ethene, 1-hexene, and propene. Answers Alkenes have physical properties (low boiling points, insoluble in water) quite similar to those of their corresponding alkanes. ethene < propene < 1-butene < 1-hexene Exercises Without referring to a table or other reference, predict which member of each pair has the higher boiling point. 1-pentene or 1-butene 3-heptene or 3-nonene Which is a good solvent for cyclohexene, pentane or water? Answer 1-pentene 3-nonene 13.4: Chemical Properties of Alkenes Concept Review Exercises What is the principal difference in properties between alkenes and alkanes? How are they alike? If C 12 H 24 reacts with HBr in an addition reaction, what is the molecular formula of the product? Answers Alkenes undergo addition reactions; alkanes do not. Both burn. C 12 H 24 Br 2 Exercises Complete each equation. (CH 3 ) 2 C=CH 2 + Br 2 → \(\mathrm{CH_2\textrm{=C}(CH_3)CH_2CH_3 + H_2 \xrightarrow{Ni}}\) Complete each equation. \(\mathrm{CH_2\textrm{=CHCH=C}H_2 + 2H_2\xrightarrow{Ni}}\) \(\mathrm{(CH_3)_2\textrm{C=C}(CH_3)_2 + H_2O \xrightarrow{H_2SO_4}}\) Answer (CH 3 ) 2 CBrCH 2 Br CH 3 CH(CH 3 )CH 2 CH 3 13.5: Polymers Concept Review Exercises What is a monomer? What is a polymer? How do polymer molecules differ from the molecules we have discussed in earlier sections of this chapter? What is addition polymerization? What structural feature usually characterizes molecules used as monomers in addition polymerization? What is the molecular formula of a polymer molecule formed by the addition polymerization of 175 molecules of vinyl chloride (CH 2 =CHCl)? Answers Monomers are small molecules that can be assembled into giant molecules referred to as polymers, which are much larger than the molecules we discussed earlier in this chapter. In addition polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers. C 350 H 525 Cl 175 Exercises Write the condensed structural formula of the monomer from which Saran is formed. A segment of the Saran molecule has the following structure: CH 2 CCl 2 CH 2 CCl 2 CH 2 CCl 2 CH 2 CCl 2 . Write the condensed structural formula for the section of a molecule formed from four units of the monomer CH 2 =CHF. Answer H 2 C=CCl 2 13.6: Alkynes Concept Review Exercises Briefly identify the important differences between an alkene and an alkyne. How are they similar? The alkene (CH 3 ) 2 CHCH 2 CH=CH 2 is named 4-methyl-1-pentene. What is the name of (CH 3 ) 2 CHCH 2 C≡CH? Do alkynes show cis-trans isomerism? Explain. Answers Alkenes have double bonds; alkynes have triple bonds. Both undergo addition reactions. 4-methyl-1-pentyne No; a triply bonded carbon atom can form only one other bond. It would have to have two groups attached to show cis-trans isomerism. Exercises Draw the structure for each compound. acetylene 3-methyl-1-hexyne Draw the structure for each compound. 4-methyl-2-hexyne 3-octyne Name each alkyne. CH 3 CH 2 CH 2 C≡CH CH 3 CH 2 CH 2 C≡CCH 3 Answers H–C≡C–H 1-pentyne 2-hexyne 13.7: Aromatic Compounds- Benzene Concept Review Exercises How do the typical reactions of benzene differ from those of the alkenes? Briefly describe the bonding in benzene. What does the circle mean in the chemist’s representation of benzene? Answers Benzene is rather unreactive toward addition reactions compared to an alkene. Valence electrons are shared equally by all six carbon atoms (that is, the electrons are delocalized). The six electrons are shared equally by all six carbon atoms. Exercises Draw the structure of benzene as if it had alternate single and double bonds. Draw the structure of benzene as chemists usually represent it today. Answer 13.8: Structure and Nomenclature of Aromatic Compounds Concept Review Exercises Briefly identify the important characteristics of an aromatic compound. What is meant by the prefixes meta , ortho , or para ? Give the name and draw the structure for a compound that illustrates each. What is a phenyl group? Give the structure for 3-phenyloctane. Answers An aromatic compound is any compound that contains a benzene ring or has certain benzene-like properties. meta = 1,3 disubstitution; (answers will vary) ortho = 1,2 disubstitution para = 1,4 disubstitution or 1-bromo-4-chlorobenzene phenyl group: C 6 H 5 or 3-phenyloctane: Exercises Is each compound aromatic? Is each compound aromatic? Draw the structure for each compound. toluene m -diethylbenzene 3,5-dinitrotoluene Draw the structure for each compound. p -dichlorobenzene naphthalene 1,2,4-trimethylbenzene Name each compound with its IUPAC name. Name each compound with its IUPAC name. Answers yes no ethylbenzene isopropylbenzene o -bromotoluene 3,5-dichlorotoluene Additional Exercises Classify each compound as saturated or unsaturated. CH 3 C≡CCH 3 Classify each compound as saturated or unsaturated. Give the molecular formula for each compound. When three isomeric pentenes—X, Y, and Z—are hydrogenated, all three form 2-methylbutane. The addition of Cl 2 to Y gives 1,2-dichloro-3-methylbutane, and the addition of Cl 2 to Z gives 1,2-dichloro-2-methylbutane. Draw the original structures for X, Y, and Z. Pentane and 1-pentene are both colorless, low-boiling liquids. Describe a simple test that distinguishes the two compounds. Indicate what you would observe. Draw and name all the alkene cis-trans isomers corresponding to the molecular formula C 5 H 10 . (Hint: there are only two.) The complete combustion of benzene forms carbon dioxide and water: C 6 H 6 + O 2 → CO 2 + H 2 O Balance the equation. What mass, in grams, of carbon dioxide is formed by the complete combustion of 39.0 g of benzene? Describe a physiological effect of some PAHs. What are some of the hazards associated with the use of benzene? What is wrong with each name? Draw the structure and give the correct name for each compound. 2-methyl-4-heptene 2-ethyl-2-hexene 2,2-dimethyl-3-pentene What is wrong with each name? 2-bromobenzene 3,3-dichlorotoluene 1,4-dimethylnitrobenzene Following are line-angle formulas for three compounds. Draw the structure and give the name for each. Following are ball-and-stick molecular models for three compounds (blue balls represent H atoms; red balls are C atoms). Write the condensed structural formula and give the name for each. Answers unsaturated unsaturated C 6 H 10 C 4 H 8 Add bromine solution (reddish-brown) to each. Pentane will not react, and the reddish-brown color persists; 1-pentene will react, leaving a colorless solution. 2C 6 H 6 + 15O 2 → 12CO 2 + 6H 2 O; 132 g carcinogenic, flammable number not needed can’t have two groups on one carbon atom on a benzene ring can’t have a substituent on the same carbon atom as the nitro group CH 3 CH=CHCH 2 CH 2 CH 3 ; 2-hexene