16.7 Reactions of ethers a review and a preview The alternative synthetic route using the sodium salt of benzyl alcohol and an isopropyl halide would be much less effective, because of increased competition from elimination as the alkyl halide becomes more sterically hindered. PROBLEM 16.7 Only one combination of alkyl halide and alkoxide is appropri ate for the preparation of each of the following ethers by the Williamson ether synthesis. What is the correct combination in each case? (c)(CH3)3COCH2 C6H5 CHCH.O (b)CH2=CHCH2 OCH(CH3)2 SAMPLE SOLUTION (a) The ether linkage of cyclopentyl ethyl ether involves a rimary carbon and a secondary one. Choose the alkyl halide corresponding to he primary alkyl group, leaving the secondary alkyl group to arise from the alkox ide nucleophile ONa CHa CH, Br Sodium cyclopentanolate Ethyl bromide Cyclopentyl ethyl ether The alternative combination, cyclopentyl bromide and sodium ethoxide is not ppropriate, since elimination will be the major reaction CHach2oNa+ CH3CH2OH+ Bromocyclopentane Ethanol Cyclopentene (major products) Both reactants in the Williamson ether synthesis usually originate in alcohol pre- cursors. Sodium and potassium alkoxides are prepared by reaction of an alcohol with the appropriate metal, and alkyl halides are most commonly made from alcohols by reaction with a hydrogen halide(Section 4.8), thionyl chloride(Section 4. 14), or phosphorus tri- bromide(Section 4. 14). Alternatively, alkyl p-toluenesulfonates may be used in place of alkyl halides; alkyl p-toluenesulfonates are also prepared from alcohols as their imme- diate precursors(Section 8. 14) 16.7 REACTIONS OF ETHERS: A REVIEW AND A PREVIEW Up to this point, we havent seen any reactions of dialkyl ethers. Indeed, ethers are one of the least reactive of the functional groups we shall study. It is this low level of reac- tivity, along with an ability to dissolve nonpolar substances, that makes ethers so often used as solvents when carrying out organic reactions. Nevertheless, most ethers are haz ardous materials, and precautions must be taken when using them. Diethyl ether is extremely flammable and because of its high volatility can form explosive mixtures in air relatively quickly. Open flames must never be present in laboratories where diethyl ether is being used. Other low-molecular-weight ethers must also be treated as fire hazards PROBLEM 16.8 Combustion in air is, of course, a chemical property of ethers that is shared by many other organic compounds. Write a balanced chemical equa tion for the complete combustion(in air) of diethyl ether Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteThe alternative synthetic route using the sodium salt of benzyl alcohol and an isopropyl halide would be much less effective, because of increased competition from elimination as the alkyl halide becomes more sterically hindered. PROBLEM 16.7 Only one combination of alkyl halide and alkoxide is appropri￾ate for the preparation of each of the following ethers by the Williamson ether synthesis. What is the correct combination in each case? (a) (c) (CH3)3COCH2C6H5 (b) CH2œCHCH2OCH(CH3)2 SAMPLE SOLUTION (a) The ether linkage of cyclopentyl ethyl ether involves a primary carbon and a secondary one. Choose the alkyl halide corresponding to the primary alkyl group, leaving the secondary alkyl group to arise from the alkox￾ide nucleophile. The alternative combination, cyclopentyl bromide and sodium ethoxide, is not appropriate, since elimination will be the major reaction: Both reactants in the Williamson ether synthesis usually originate in alcohol pre￾cursors. Sodium and potassium alkoxides are prepared by reaction of an alcohol with the appropriate metal, and alkyl halides are most commonly made from alcohols by reaction with a hydrogen halide (Section 4.8), thionyl chloride (Section 4.14), or phosphorus tri￾bromide (Section 4.14). Alternatively, alkyl p-toluenesulfonates may be used in place of alkyl halides; alkyl p-toluenesulfonates are also prepared from alcohols as their imme￾diate precursors (Section 8.14). 16.7 REACTIONS OF ETHERS: A REVIEW AND A PREVIEW Up to this point, we haven’t seen any reactions of dialkyl ethers. Indeed, ethers are one of the least reactive of the functional groups we shall study. It is this low level of reac￾tivity, along with an ability to dissolve nonpolar substances, that makes ethers so often used as solvents when carrying out organic reactions. Nevertheless, most ethers are haz￾ardous materials, and precautions must be taken when using them. Diethyl ether is extremely flammable and because of its high volatility can form explosive mixtures in air relatively quickly. Open flames must never be present in laboratories where diethyl ether is being used. Other low-molecular-weight ethers must also be treated as fire hazards. PROBLEM 16.8 Combustion in air is, of course, a chemical property of ethers that is shared by many other organic compounds. Write a balanced chemical equa￾tion for the complete combustion (in air) of diethyl ether. E2 CH3CH2ONa Sodium ethoxide Br Bromocyclopentane (major products) CH3CH2OH Ethanol Cyclopentene SN2 ONa Sodium cyclopentanolate CH3CH2Br Ethyl bromide OCH2CH3 Cyclopentyl ethyl ether CH3CH2O 16.7 Reactions of Ethers: A Review and a Preview 627 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website