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Section 16.6 REACTIONS OF ALCOHOLS: A REVIEW AND A PREVIEW Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Reaction with Hydrogen Halides • We have previously seen the reaction of ROH with hydrogen halides HX to give alkyl halides • Alcohols can be transformed into a variety of other functional groups Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Functional Group Transformations • SOCl2 and PBr3 also convert ROH to alkyl halides. Oxygen acts as a nucleophile in these reactions Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Functional Group Transformations • Oxygen also acts as a nucleophile in the conversion of ROH to sulfonates Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Elimination • Dehydration of alcohols using strong, non-nucleophilic acid yields alkenes • Rearrangements are sometimes observed; reactivity trend is 3º > 2º > 1º Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 16.7 CONVERSION OF ALCOHOLS TO ETHERS Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Synthesis of Ethers • The oxygen in a protonated alcohol ROH2+ is electrophilic • The oxygen of an alcohol ROH is nucleophilic • When ROH is heated with an acid catalyst, nucleophilic O in ROH attacks the electrophilic carbon, displacing water • The product is a symmetric ether • Only primary alcohols can be used; 2º and 3º alcohols undergo elimination (dehydration) Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Synthesis of Ethers • The oxygen in a protonated alcohol ROH2+ is electrophilic • The oxygen of an alcohol ROH is nucleophilic • When ROH is heated with an acid catalyst, nucleophilic O in ROH attacks the electrophilic carbon, displacing water • The product is a symmetric ether • Diols can react intramolecularly to give cyclic ethers Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 16.8 ESTERIFICATION Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Fischer Esterification • Under acidic conditions, alcohols and carboxylic acids form esters reversibly • Adding an excess of one reactant can drive the reaction forward Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Fischer Esterification • Under acidic conditions, alcohols and carboxylic acids form esters reversibly • Removing a product (H2O) also drives the reaction forward The mechanism will be discussed in a later chapter; for now, note that the alcohol C–O bond remains intact. Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Acid Chlorides • Acid chlorides are strongly electrophilic at the carbonyl carbon; they react with nucleophilic alcohols (usually with some base) to form esters • Anhydrides can also be used Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 16.9 OXIDATION OF ALCOHOLS Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Oxidation of Alcohols • Just as carbonyls can be reduced to alcohols, alcohols can be oxidized to carbonyls • Oxidation can be stopped at the aldehyde stage or carried through to the carboxylic acid (depending on the oxidant used) • Chromic acid (H2CrO4) is a strong oxidant used to form carboxylic acids from primary alcohols Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Mild Oxidation to Form Aldehydes • More mildly acidic chromate salts can be used to oxidize primary alcohols to aldehydes – Pyridinium chlorochromate (PCC) – Pyridinium dichromate (PDC) Secondary alcohols form ketones, but tertiary alcohols do not react or form complicated mixtures of products. Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Mechanism of Chromate Oxidation • The general mechanism of alcohol oxidation involves two stages: – The alcohol hydrogen (–OH) is transformed into a good leaving group (–OLG) – b-Elimination across C–O eliminates H–LG and forms C=O Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Mechanism of Chromate Oxidation • The general mechanism of alcohol oxidation involves two stages: – The alcohol hydrogen (–OH) is transformed into a good leaving group (–OLG) – b-Elimination across C–O eliminates H–LG and forms C=O Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Other Oxidants: Swern Oxidation • Other ROH oxidants are based on these same principles • (CH3)2S + (COCl)2 forms the electrophilic (CH3)2SCl+ ion, which promotes oxidation of ROH • This reaction is called the Swern oxidation Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 16.10 BIOLOGICAL OXIDATION OF ALCOHOLS Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Biological Oxidation of ROH • Oxidation of alcohols and reduction of carbonyls are important biological processes • For example, ethanol is oxidized to acetaldehyde by the enzyme alcohol dehydrogenase • Chromium-based oxidants are far too strong for a biological context! The nicotinamide group is “nature’s oxidant.” Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Mechanism of NAD+ Oxidation • The alcohols transfers hydride (C–H) to NAD+, forming NADH • Deprotonation of O–H by a basic group in the enzyme occurs simultaneously Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.