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Alcohols and Ethers Alcohols and Ethers Section 2B Coursepack, p. 165-204 1 Alcohols (cont.) • Alcohols are structurally similar to water, and, like water, can engage in hydrogen bonding: 2 Alcohols (cont.) • The presence of intermolecular hydrogen bonding leads to relatively high boiling points for alcohols 3 Acid and Base Properties of Alcohols • An alcohol can act as both a Brønsted acid and a Brønsted base: 4 Acid and Base Properties of Alcohols (cont.) • The lower a compound’s pKa, the stronger an acid it is. The table places the acidity of alcohols and their conjugate acids in context with other familiar acids… 5 Hydration • Hydrohalogenation is not the only type of addition reaction involving alkenes • Catalytic acid is necessary to create the carbocation intermediate; water is not a strong enough acid on its own Hydration (cont.) • A hydration reaction involving addition of water to an alkene to make an alcohol is possible, but requires an acid catalyst: Hydroboration • Consider borane (BH3), a compound you may have learned about in general chemistry as being an ‘exception’ to the octet rule because in it, B has only 6 valence electrons: Hydroboration (cont.) • Because of the empty p orbital on the B, borane is a good Lewis acid. • We have seen alkenes act as Lewis bases. What happens when borane and an alkene undergo a Lewis acid-base reaction? Hydroboration (cont.) • The reaction proceeds via a concerted pathway: Hydroboration (cont.) • This reaction is regiospecific, with the B adding to the less substituted side of the double bond (H is more electronegative than B): Hydroboration (cont.) • The greater stability of positive charge on the more substituted side helps rationalize this observation: Hydroboration (cont.) • Because addition of H and BH2 fragments is concerted, the reaction proceeds stereoselectively: Hydroboration (cont.) • The proposed concerted mechanism accounts for this: Hydroboration in Alcohol Synthesis • Once alkyl boranes are formed, the boron can be replaced with other groups, such as an OH to make an alcohol: Hydroboration in Alcohol Synthesis (cont.) • The net result of the mechanism of this oxidation is that the OH ends up where the boron was attached to carbon: Hydroboration in Alcohol Synthesis (cont.) • The net result of the hydroboration / oxidation is the formation of the “anti-Markovnikov” alcohol: Hydration via Oxymercuration • Alcohols can be made from alkenes without using a protic acid if a mercury compound such – as Hg(OAc)2 is used ( OAc = acetate anion). Hydration via Oxymercuration (cont.) • The mechanism involves a mercurinium ion, somewhat similar to the halonium ion: Hydration via Oxymercuration (cont.) • Once the OH group is added, the Hg-containing group is removed by reduction to give the final alcohol: Acid and Base Properties of Alcohols (cont.) 21 Which compound is most likely to accept an electron pair? 1) HBr 2) CH3CO2H 3) H2O 4) CH4 pKa= –9 4.8 16 50 Acid and Base Properties of Alcohols (cont.) • Because hydroxide is such a poor leaving group, an alcohol often needs to be “activated” by protonation before it can react further: 23 Acid and Base Properties of Alcohols (cont.) • We will see examples of this in the next chapter. • When deprotonated, alcohols form alkoxides that are stronger bases than hydroxide: 24 Draw the conjugate base of each of the following acids. O OH CH3OH H3C O H 1) 2) 3) Which conjugate base(s) is/are resonance stabilized? (Enter as many numbers as needed) On class white board: Alcohols reacting with HX (p. 177 -180) Alcohols reaction with PBr3 or SOCl2 (p. 180-181) 26 SN2 on Alcohols: LG Activation • A hydroxyl group can also be changed to a good leaving group by a two-step route with a sulfonyl chloride: 27 SN2 on Alcohols: LG Activation (cont.) • Or with thionyl chloride: • Reagents such as PBr3, PCl5, or PPh3/CCl4 also work. 28 Organic Synthesis (cont.) • One especially important substitution reaction is the Williamson Ether Synthesis: 29 Properties of Ethers • Unlike alcohols, ethers do not have the ability to hydrogen bond to one another. As a result, ethers have boiling points much more like alkanes of similar weight: 30 Base Properties of Ethers • Ethers lack any H atoms that can be easily removed, but the lone pairs on the O atom allow ethers to act as bases with strengths comparable to those of alcohols. • Ether as a Brønsted base: 31 Base Properties of Ethers (cont.) • Ether as a Lewis base: 32 Oxiranes (Epoxides) • The formation of three-membered ring compounds (as final products) from alkenes is also possible. • The formation of an epoxide (or oxirane) from alkenes occurs via a concerted mechanism by reaction with a peroxyacid (RC(O)OOH): Oxiranes (Epoxides) (cont.) • Because the reaction is concerted, there is no rearrangement of alkene substituents relative to one another… Oxiranes (Epoxides) (cont.) Which of the following show retention of alkene geometry? O O O O H H RCO3H R R R R R R R H H H H R R R 1) 2) 3) 4) Organic Chemistry In Our Everyday Life Professor Michael J. Krische WhereReactions Do Drugs Come From ?of 61% ofOxiranes the 877 small molecules introduced (Epoxides) as drugs worldwide from 1981 - 2002 were inspired by “Natural Products” (J. Nat. Prod. 2003, 1022). Hence, the discovery, biological profiling (SAR), and preparation of natural products is of paramount importance in terms of public health. HO O • Once made, oxiranes can undergo usefulO CH CH 3 The Structures of Natural 3 reactions underH both basic and acidicH conditions: O Products are Shaped by O OH Selective Pressure. CO2H They Evolve. CH3 H H CH3 CH3 Hirsutic Acid-C Coriolin (Anti-Biotic Properties) (Anti-Tumor Properties) Reactions of Oxiranes (Epoxides) (cont.) • In both cases, protonation and SN2 steps are required; the order depends on whether basic or acidic conditions are used: Reactions of Oxiranes (Epoxides) (cont.) • The difference in mechanism also leads to differences in regiochemistry: Reactions of Oxiranes (Epoxides) (cont.) • Under basic conditions, the nucleophile attacks the less sterically hindered side, as expected from an SN2 reaction: Reactions of Oxiranes (Epoxides) (cont.) • Under acidic conditions, the presence of a positive charge leads to nucleophilic attack at the more positive (more substituted) site: Reactions of Oxiranes (Epoxides) (cont.) • The organometallic reagents discussed in Chapter 6 (strongly basic) can be used to add R groups to oxiranes: Reactions of Oxiranes (Epoxides) (cont.) • Addition to the less sterically hindered side is favored in substituted oxiranes: Reactions of Oxiranes (Epoxides) (cont.) • Hydride can be added from LiAlH4: Reactions of Oxiranes (Epoxides) (cont.) • Because of side reactions with organolithium and Grignard reagents, organocuprates are sometimes used: FIRST NUMBER: What is the product of the following reaction? (Choose 1 or 2) OH OH O H3C 1. NaCN H C H C 3 H 3C 3 2. H2O H3C H3C CN 1) CN 2) SECOND NUMBER: What is the regiochemistry predominately controlled by? (Choose 3 or 4) 3) Steric effects 4) Electronic effects .
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