16.2 Structure and Bonding in Ethers and Epoxide 16.2 STRUCTURE AND BONDING IN ETHERS AND EPOXIDES Bonding in ethers is readily understood by comparing ethers with water and alcohols Van der Waals strain involving alkyl groups causes the bond angle at oxygen to be larger in ethers than alcohols, and larger in alcohols than in water. An extreme example is di tert-butyl ether, where steric hindrance between the tert-butyl groups is responsible for a dramatic increase in the C-o-c bond angle cH是cH,c1c是cc参a models of water ater Methanol Dimethyl ether Di-ferl-butyl ether ol, dimethyl ether, and (e typical carbon-oxygen bond distances in ethers are similar to those of alcohols their geometries, and examine pm)and are shorter than carbon-carbon bond distances in alkanes(153 pm). bond angle. Compare the C-o An ether oxygen affects the conformation of a molecule in much the same way bond distances in dimethyl ether that a CH2 unit does. The most stable conformation of diethyl ether is the all-staggered and di-tert-butyl ether. anti conformation. Tetrahydropyran is most stable in the chair conformation-a fact that has an important bearing on the structures of many carbohydrates Chair conformation of tetrahydropyraN Incorporating an oxygen atom into a three-membered ring requires its bond angle to be seriously distorted from the normal tetrahedral value In ethylene oxide, for exam- ple, the bond angle at oxygen is 61.5 H2C、CH2C-0- C angle615° C-C-O angle 59.20 Thus epoxides, like cyclopropanes, are strained. They tend to undergo reactions that open the three-membered ring by cleaving one of the carbon-oxygen bond PROBLEM 16.2 The heats of combustion of 1, 2-epoxybutane (2-ethyloxirane) and tetrahydrofuran have been measured: one is 2499 kJ/mol(597. 8 kcal/mol); the other is 2546 kJ/mol(609 1 kcal/mol). Match the heats of combustion with the respective compounds Ethers, like water and alcohols, are polar. Diethyl ether, for example, has a dipe moment of 1. 2 D Cyclic ethers have larger dipole moments; ethylene oxide and tetra drofuran have dipole moments in the 1.7-to 1. 8-D range--about the same as that of Back Forward Main MenuToc Study Guide ToC Student o MHHE Website16.2 STRUCTURE AND BONDING IN ETHERS AND EPOXIDES Bonding in ethers is readily understood by comparing ethers with water and alcohols. Van der Waals strain involving alkyl groups causes the bond angle at oxygen to be larger in ethers than alcohols, and larger in alcohols than in water. An extreme example is di￾tert-butyl ether, where steric hindrance between the tert-butyl groups is responsible for a dramatic increase in the C±O±C bond angle. Typical carbon–oxygen bond distances in ethers are similar to those of alcohols (142 pm) and are shorter than carbon–carbon bond distances in alkanes (153 pm). An ether oxygen affects the conformation of a molecule in much the same way that a CH2 unit does. The most stable conformation of diethyl ether is the all-staggered anti conformation. Tetrahydropyran is most stable in the chair conformation—a fact that has an important bearing on the structures of many carbohydrates. Incorporating an oxygen atom into a three-membered ring requires its bond angle to be seriously distorted from the normal tetrahedral value. In ethylene oxide, for exam￾ple, the bond angle at oxygen is 61.5°. Thus epoxides, like cyclopropanes, are strained. They tend to undergo reactions that open the three-membered ring by cleaving one of the carbon–oxygen bonds. PROBLEM 16.2 The heats of combustion of 1,2-epoxybutane (2-ethyloxirane) and tetrahydrofuran have been measured: one is 2499 kJ/mol (597.8 kcal/mol); the other is 2546 kJ/mol (609.1 kcal/mol). Match the heats of combustion with the respective compounds. Ethers, like water and alcohols, are polar. Diethyl ether, for example, has a dipole moment of 1.2 D. Cyclic ethers have larger dipole moments; ethylene oxide and tetrahy￾drofuran have dipole moments in the 1.7- to 1.8-D range—about the same as that of water. H2C O CH2 147 pm 144 pm C O C C C O angle 61.5° angle 59.2° Anti conformation of diethyl ether Chair conformation of tetrahydropyran H H O 105° Water H CH 108.5° 3 O Methanol CH3 112° CH3 O Dimethyl ether 132° O C(CH3)3 (CH3)3C Di-tert-butyl ether 16.2 Structure and Bonding in Ethers and Epoxides 621 Use Learning By Modeling to make models of water, methanol, dimethyl ether, and di-tert-butyl ether. Minimize their geometries, and examine what happens to the C±O±C bond angle. Compare the C±O bond distances in dimethyl ether and di-tert-butyl ether. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website