5.6 Relative Stabilities of alkenes CH3 CH3 CH3 H CH IGURE 5.4 Heats of com- Alkene CH,=CHCH,CH3 C=CH mers plotted on a common CH3 CH3 scale. All energies are in kilo- joules per mole. (An energy cis-2-Butene equivalent to 0.7 kcal/mol; 7 kJ/mol is equivalent to 1.7 2710 2707 △HP° △H° △H° O 4c02+4H,0 Analogous data for a host of alkenes tell us that the most important factors gov erning alkene stability are 1. Degree of substitution(alkyl substituents stabilize a double bond) 2. Van der Waals strain( destabilizing when alkyl groups are cis to each other) Degree of substitution. We classify double bonds as monosubstituted, disubstituted, trisubstituted, or tetrasubstituted according to the number of carbon atoms that are directly attached to the C=C structural unit. Monosubstituted alkenes RCH=CH as in CH3 CH CH=CH, (1-butene (R and r may be the same or different) RCHECHR CH3 CH=CHCH3 (cis-or trans-2-butene) R (CH3) (2-methy lpr Trisubstituted alkenes (R,R, and r may be the same or different R R as in (CH3)C=CHCH,CH3 (2-methyl-2-pentene) Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Analogous data for a host of alkenes tell us that the most important factors governing alkene stability are: 1. Degree of substitution (alkyl substituents stabilize a double bond) 2. Van der Waals strain (destabilizing when alkyl groups are cis to each other) Degree of substitution. We classify double bonds as monosubstituted, disubstituted, trisubstituted, or tetrasubstituted according to the number of carbon atoms that are directly attached to the CœC structural unit. Monosubstituted alkenes: Disubstituted alkenes: (R and R may be the same or different) Trisubstituted alkenes: (R, R, and R may be the same or different) C R R H R C as in (CH3)2C CHCH2CH3 (2-methyl-2-pentene) RCH CHR as in CH3CH CHCH3 (cis- or trans-2-butene) C H R H R C as in (CH3)2C CH2 (2-methylpropene) RCHœCH2 CH3CH2CHœCH2 as in (1-butene) 5.6 Relative Stabilities of Alkenes 177 cis-2-Butene trans-2-Butene Alkene 4CO2 4H2O CH3 6O2 C C H CH3 H 1-Butene 2-Methylpropene 7 7 2710 3 2700 Energy ∆H ∆H ∆H ∆H CH2 CHCH2CH3 CH3 C C H CH3 H CH3 C CH3 CH2 2717 2707 FIGURE 5.4 Heats of combustion of C4H8 alkene isomers plotted on a common scale. All energies are in kilojoules per mole. (An energy difference of 3 kJ/mol is equivalent to 0.7 kcal/mol; 7 kJ/mol is equivalent to 1.7 kcal/mol.) Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions Tetrasubstituted alkenes (R,R,R", and r" may be the same or different) (1, 2-dimethylcyclohexene) CH3 In the example shown, each of the highlighted ring carbons counts as a separate sub- tituent on the double bond PROBLEM 5.7 Write structural formulas or build molecular models and give the UPAC names for all the alkenes of molecular formula CsH12 that contain a trisub- stituted double bond. ( Don't forget to include stereoisomers. From the heats of combustion of the CaH alkenes in Figure 5.5 we see that each of the disubstituted alkenes CH H CH CH3 H H trans-2-Butene cis-2-Butene is more stable than the monosubstituted alkene CH, CH3 H 1-Butene In general, alkenes with more highly substituted double bonds are more stable than iso- mers with less substituted double bonds PROBLEM 5.8 Give the structure or make a molecular model of the most stable CH12 alkene Like the sp2-hybridized carbons of carbocations and free radicals, the sp2. hybridized carbons of double bonds are electron attracting, and alkenes are stabilized by substituents that release electrons to these carbons. As we saw in the preceding section, alkyl groups are better electron-releasing substituents than hydrogen and are, therefore better able to stabilize an alkene An effect that results when two or more atoms or groups interact so as to alter the electron distribution in a system is called an electronic effect. The greater stability of more highly substituted alkenes is an example of an electronic effect. van der Waals strain. Alkenes are more stable when large substituents are trans to each other than when they are cis. As was seen in Figure 5.4, trans-2-butene has a lower heat of combustion and is more stable than cis-2-butene. The energy difference between he two is 3 k/mol (0.7 kcal/mol). The source of this energy difference is illustrated in Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Tetrasubstituted alkenes: (R, R, R, and R may be the same or different) In the example shown, each of the highlighted ring carbons counts as a separate substituent on the double bond. PROBLEM 5.7 Write structural formulas or build molecular models and give the IUPAC names for all the alkenes of molecular formula C6H12 that contain a trisubstituted double bond. (Don’t forget to include stereoisomers.) From the heats of combustion of the C4H8 alkenes in Figure 5.5 we see that each of the disubstituted alkenes is more stable than the monosubstituted alkene In general, alkenes with more highly substituted double bonds are more stable than isomers with less substituted double bonds. PROBLEM 5.8 Give the structure or make a molecular model of the most stable C6H12 alkene. Like the sp2 -hybridized carbons of carbocations and free radicals, the sp2 - hybridized carbons of double bonds are electron attracting, and alkenes are stabilized by substituents that release electrons to these carbons. As we saw in the preceding section, alkyl groups are better electron-releasing substituents than hydrogen and are, therefore, better able to stabilize an alkene. An effect that results when two or more atoms or groups interact so as to alter the electron distribution in a system is called an electronic effect. The greater stability of more highly substituted alkenes is an example of an electronic effect. van der Waals strain. Alkenes are more stable when large substituents are trans to each other than when they are cis. As was seen in Figure 5.4, trans-2-butene has a lower heat of combustion and is more stable than cis-2-butene. The energy difference between the two is 3 kJ/mol (0.7 kcal/mol). The source of this energy difference is illustrated in C CH2CH3 H H H C 1-Butene C CH3 H H CH3 C trans-2-Butene cis-2-Butene C H H CH3 CH3 C 2-Methylpropene C H CH3 H CH3 C C R R R R C as in CH3 CH3 (1,2-dimethylcyclohexene) 178 CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
5.6 Relative Stabilities of alkenes cis-2 Butene ans-2 Butene poke and space-filling mod ds of cis-and trans-2-but The space-filling mod tion between the crowded atoms. the combination of aals strain Figure 5.5, where it is seen that methyl groups approach each other very closely in cis- 2-butene. but the trans isomer is free of strain. An effect that results when two or more atoms are close enough in space that a repulsion occurs between them is one type of steric effect. The greater stability of trans alkenes compared with their cis counterparts similar steric effect is an example of a steric effect PROBLEM 5.9 Arrange the following alkenes in order of decreasing stability pentene;(E)-2-pentene:(Z)-2-pentene: 2-methyl-2-butene s-1, 2-dimethylcyclopropane The difference in stability between stereoisomeric alkenes is even more pronounced stereoisomer. with larger alkyl groups on the double bond. a particularly striking example compares cis-and trans-2, 2, 5, 5-tetramethyl-3-hexene, in which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10.5 kcal/mol) higher than that of the trans. The cis isomer is destabilized by the large van der Waals strain between the bulky tert-butyl groups on the same side of the double bond 2C、 CH3 CH3 CH H;C CH3 Energy difference alkenes are cis- and trans. 44 kJ/mol 3C CH (10.5 kcal/mol) cases such as this the com- H3C IUPAC cis-2,2, 5, 5-Tetramethyl-3-hexene trans-2, 2, 5.5-Tetramethyl-3-hexene are more readily associated with molecular structure Less stable More stable Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Figure 5.5, where it is seen that methyl groups approach each other very closely in cis- 2-butene, but the trans isomer is free of strain. An effect that results when two or more atoms are close enough in space that a repulsion occurs between them is one type of steric effect. The greater stability of trans alkenes compared with their cis counterparts is an example of a steric effect. PROBLEM 5.9 Arrange the following alkenes in order of decreasing stability: 1-pentene; (E)-2-pentene; (Z)-2-pentene; 2-methyl-2-butene. The difference in stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond. A particularly striking example compares cis- and trans-2,2,5,5-tetramethyl-3-hexene, in which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10.5 kcal/mol) higher than that of the trans. The cis isomer is destabilized by the large van der Waals strain between the bulky tert-butyl groups on the same side of the double bond. Energy difference 44 kJ/mol (10.5 kcal/mol) trans-2,2,5,5-Tetramethyl-3-hexene More stable H C C C CH3 H H3C H3C C CH3 CH3 H3C CH3 C C C C CH3 CH3 H H H3C CH3 H3C cis-2,2,5,5-Tetramethyl-3-hexene Less stable 5.6 Relative Stabilities of Alkenes 179 cis-2 Butene trans-2 Butene FIGURE 5.5 Ball-andspoke and space-filling models of cis- and trans-2-butene. The space-filling model shows the serious van der Waals strain between two of the hydrogens in cis-2- butene. The molecule adjusts by expanding those bond angles that increase the separation between the crowded atoms. The combination of angle strain and van der Waals strain makes cis-2 butene less stable than trans- 2-butene. A similar steric effect was seen in Section 3.12, where van der Waals strain between methyl groups on the same side of the ring made cis-1,2-dimethylcyclopropane less stable than its trans stereoisomer. The common names of these alkenes are cis- and transdi-tert-butylethylene. In cases such as this the common names are somewhat more convenient than the IUPAC names because they are more readily associated with molecular structure. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER FIVE Structure and Preparation of Alkenes Elimination Reactions PROBLEM 5.10 Despite numerous attempts, the alkene 3, 4-di-tert-butyl-2, 2, 5,5- tetramethyl-3-hexene has never been synthesized. Can you explain why? Try mak- ing a space-filling model of this compound 5.7 CYCLOALKENES Double bonds are accommodated by rings of all sizes. The simplest cycloalkene, cyclo- propene, was first synthesized in 1922. A cyclopropene ring is present in sterculic acid, substance derived from one of the components of the oil present in the seeds of a tree (Sterculia foelida) that grows in the Philippines and Indonesia. Sterculic acid and related H CH3(CH2) (CH2)7CO,H an article in the July 1982 is. As we saw in Section 3.9, cyclopropane is destabilized by angle strain because its 60 bond angles are much smaller than the normal 109.5 angles associated with sp hybridized carbon. Cyclopropene is even more strained because the deviation of the bond angles at its doubly bonded carbons from the normal sp- hybridization value of 120o is greater still. Cyclobutene has, of course, less angle strain than cyclopropene, and the angle strain of cyclopentene, cyclohexene, and higher cycloalkenes is negligible So far we have represented cycloalkenes by structural formulas in which the dou ble bonds are of the cis configuration. If the ring is large enough, however, a trans stereoisomer is also possible. The smallest trans cycloalkene that is stable enough to be isolated and stored in a normal way is trans-cyclooctene Make molecular models neI gy 39 kJ/mol(9.2 kcal/mol) hedr are their H-C-C-Hdi. (E)-Cyclooctene (Z)-Cyclooctene (trans-cyclooctene) (cis-cyclooctene) Less stable More stable Ins-Cycloheptene has been prepared and studied at low temperature(-90oC)but is too reactive to be isolated and stored at room temperature. Evidence has also been presented for the fleeting existence of the even more strained trans-cyclohexene as a reactive intermediate in certain reactions PROBLEM 5.11 Place a double bond in the carbon skeleton shown so as to rep- a)(Z)-1-Methylcyclodecene (a)(E)-3-Methylcyclodecene (b)(E)-1-Methylcyclodecene (e)(Z)-5-Methylcyclodecene (c)(Z)-3-Methylcyclodecene (f)(E)-5-Methylcyclodecene CH Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
PROBLEM 5.10 Despite numerous attempts, the alkene 3,4-di-tert-butyl-2,2,5,5- tetramethyl-3-hexene has never been synthesized. Can you explain why? Try making a space-filling model of this compound. 5.7 CYCLOALKENES Double bonds are accommodated by rings of all sizes. The simplest cycloalkene, cyclopropene, was first synthesized in 1922. A cyclopropene ring is present in sterculic acid, a substance derived from one of the components of the oil present in the seeds of a tree (Sterculia foelida) that grows in the Philippines and Indonesia. As we saw in Section 3.9, cyclopropane is destabilized by angle strain because its 60° bond angles are much smaller than the normal 109.5° angles associated with sp3 - hybridized carbon. Cyclopropene is even more strained because the deviation of the bond angles at its doubly bonded carbons from the normal sp2 hybridization value of 120° is greater still. Cyclobutene has, of course, less angle strain than cyclopropene, and the angle strain of cyclopentene, cyclohexene, and higher cycloalkenes is negligible. So far we have represented cycloalkenes by structural formulas in which the double bonds are of the cis configuration. If the ring is large enough, however, a trans stereoisomer is also possible. The smallest trans cycloalkene that is stable enough to be isolated and stored in a normal way is trans-cyclooctene. trans-Cycloheptene has been prepared and studied at low temperature ( 90°C) but is too reactive to be isolated and stored at room temperature. Evidence has also been presented for the fleeting existence of the even more strained trans-cyclohexene as a reactive intermediate in certain reactions. PROBLEM 5.11 Place a double bond in the carbon skeleton shown so as to represent (a) (Z)-1-Methylcyclodecene (d) (E)-3-Methylcyclodecene (b) (E)-1-Methylcyclodecene (e) (Z)-5-Methylcyclodecene (c) (Z)-3-Methylcyclodecene (f) (E)-5-Methylcyclodecene CH3 H H (Z)-Cyclooctene (cis-cyclooctene) More stable H H (E)-Cyclooctene (trans-cyclooctene) Less stable Energy difference 39 kJ/mol (9.2 kcal/mol) (CH2) CH3(CH2)7 7CO2H H H Sterculic acid H H H H Cyclopropene 180 CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions Sterculic acid and related substances are the subject of an article in the July 1982 issue of Journal of Chemical Education (pp. 539–543). Make molecular models of (E) and (Z)-cyclooctene and compare their H±CœC±H dihedral angles. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website