CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions dipoles. With alkenes the basic question concerns the alkyl groups attached to C-C Does an alkyl group donate electrons to or withdraw electrons from a double bond? This question can be approached by comparing the effect of an alkyl group, methyl for exam- H C=C trans-1-Chloropropene 0.3D 17D Ethylene, of course, has no dipole moment. Replacing one of its hydrogens by chlorine gives chloroethene, which has a dipole moment of 1.4 D. The effect is much smaller when one of the hydrogens is replaced by methyl; CH3CH=CH2 has a dipole moment of only 0.3 D. Now place CH3 and Cl trans to each other on the double bond. If methyl releases electrons better than H, then the dipole moment of trans-CH3 CH=CHCI should be larger than that of CH,=CHCl, because the effects of CH3 and CI reinforce each other. If methyl is electron attracting, the opposite should occur, and the dipole moment of trans-CH3CH-CHCI will be smaller than 1. 4 D. In fact, the dipole moment of trans- CH3CH=CHCI is larger than that of CH2=CHCl, indicating that a methyl group is an electron-donating substituent on the double bond. A methyl group releases electrons to a double bond in much the same way that it hby an inductive effe and by hyperconjugation(Figure 5.3). Other alkyl groups behave similarly and, as we go along, we'll see several ways in which the electron-releasing effects of alkyl substituents influence the properties of alkenes. The first is described in the following section. 5.6 RELATIVE STABILITIES OF ALKENES Earlier(Sections 2.15, 3.12)we saw how to use heats of combustion to compare the sta- bilities of isomeric alkanes. We can do the same thing with isomeric alkenes, Consider the heats of combustion of the four isomeric alkenes of molecular formula C4H. All C4H8+602→4CO2+4H2O When the heats of combustion of the isomers are plotted on a common scale as in Fig ure 5.4, we see that the isomer of highest energy(the least stable one)is 1-butene, CH2= CH3. The isomer of lowest energy (most stable) is 2-methylpropene (CH3)2C=CH carbon and are stabilized by electron-donating substituesdtive than sp-hybridized FIGURE donate ituent than hydrogen. Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitedipoles. With alkenes the basic question concerns the alkyl groups attached to CœC. Does an alkyl group donate electrons to or withdraw electrons from a double bond? This question can be approached by comparing the effect of an alkyl group, methyl for exam￾ple, with other substituents. Ethylene, of course, has no dipole moment. Replacing one of its hydrogens by chlorine gives chloroethene, which has a dipole moment of 1.4 D. The effect is much smaller when one of the hydrogens is replaced by methyl; CH3CHœCH2 has a dipole moment of only 0.3 D. Now place CH3 and Cl trans to each other on the double bond. If methyl releases electrons better than H, then the dipole moment of trans-CH3CHœCHCl should be larger than that of CH2œCHCl, because the effects of CH3 and Cl reinforce each other. If methyl is electron attracting, the opposite should occur, and the dipole moment of trans-CH3CHœCHCl will be smaller than 1.4 D. In fact, the dipole moment of trans￾CH3CHœCHCl is larger than that of CH2œCHCl, indicating that a methyl group is an electron-donating substituent on the double bond. A methyl group releases electrons to a double bond in much the same way that it releases electrons to the positively charged carbon of a carbocation—by an inductive effect and by hyperconjugation (Figure 5.3). Other alkyl groups behave similarly and, as we go along, we’ll see several ways in which the electron-releasing effects of alkyl substituents influence the properties of alkenes. The first is described in the following section. 5.6 RELATIVE STABILITIES OF ALKENES Earlier (Sections 2.15, 3.12) we saw how to use heats of combustion to compare the sta￾bilities of isomeric alkanes. We can do the same thing with isomeric alkenes. Consider the heats of combustion of the four isomeric alkenes of molecular formula C4H8. All undergo combustion according to the equation C4H8 6O2 ±£ 4CO2 4H2O When the heats of combustion of the isomers are plotted on a common scale as in Fig￾ure 5.4, we see that the isomer of highest energy (the least stable one) is 1-butene, CH2œCHCH2CH3. The isomer of lowest energy (most stable) is 2-methylpropene (CH3)2CœCH2. C H H H H C Ethylene   0 D H Cl C H H C Chloroethene   1.4 D H H H CH3 C C Propene   0.3 D Cl H H CH3 C C trans-1-Chloropropene   1.7 D 176 CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions sp2-hybridized carbons of an alkene are more electronegative than sp3-hybridized carbon and are stabilized by electron-donating substituents. C C H CH3 Methyl group is a better electron-donating substituent than hydrogen. FIGURE 5.3 Alkyl groups donate electrons to sp2 - hybridized carbons of an alkene. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website