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Chapter 18 Aldehydes and Ketones Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Aldehydes and Ketones • Aldehydes and ketones are extremely common examples of carbonyl compounds • In this chapter: – – – – Structure and bonding of the carbonyl group Nucleophilic addition as a paradigmatic reaction of carbonyls Examples of reactivity Spectroscopic signatures of aldehydes and ketones Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 18.1 NOMENCLATURE Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Nomenclature of Aldehydes • Aldehydes contain the formyl group (–CHO). They are named by replacing –e in the name of the parent alkane with –al • Rings with a formyl substituent are named as carbaldehydes Note that the formyl group does not need a locant. Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Nomenclature of Aldehydes • A few aldehydes have common names acceptable to IUPAC • Aldehydes take precedence over alcohols (ranking is done by oxidation state) Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Nomenclature of Ketones • To name ketones, –e in the name of the parent alkane is replaced with –one • The carbonyl carbon is given a locant (smallest possible); C–1 in cyclic ketones • Aldehydes take precedence over ketones • Ketones can also be named using functional class nomenclature as alkyl alkyl’ ketones Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Common Ketones • A few ketones have acceptable common names (all of which end in –one) Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 18.2 STRUCTURE AND BONDING: THE CARBONYL GROUP Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Structure and Hybridization • The carbonyl carbon is sp2 hybridized, with trigonal planar geometry and (approximately) 120º bond angles Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Bonding and Polarity • The two bonds in C=O are a s bond and a π bond, analogous to the C=C bonds in alkenes Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Bonding and Polarity • The two bonds in C=O are a s bond and a π bond, analogous to the C=C bonds in alkenes • However, the C=O bonds are strongly polarized toward oxygen, leaving carbon with partial positive charge Nucleophiles (anions, lone pairs) bond to the carbonyl carbon; electrophiles (proton, Lewis acids) bond to the carbonyl oxygen. Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 18.3 PHYSICAL PROPERTIES Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Physical Properties • Aldehydes and ketones experience strong dipole-dipole forces • Boiling and melting points are generally higher than alkenes of comparable molecular weight • Boiling and melting points are lower than related alcohols, however (why?) Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 18.4 SOURCES OF ALDEHYDES AND KETONES Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Natural Aldehydes and Ketones • Nature produces aldehydes and ketones usually via oxidation of alcohols Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Laboratory Synthesis • Redox reactions of alcohols and carbonyls are extremely versatile—aldehydes and ketones can be accessed from almost any C–O oxidation level • For example, a reduce-then-oxidize method is useful for aldehyde synthesis from RCO2H Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Laboratory Synthesis • Ketones can be prepared from aldehydes via Grignard addition followed by oxidation – Grignard addition forms an intermediate secondary alcohol – Chromic acid oxidation yields the ketone Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Other Reactions Yielding Carbonyls Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Other Reactions Yielding Carbonyls Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Other Reactions Yielding Carbonyls Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 18.5 REACTIONS OF ALDEHYDES AND KETONES: 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. Past Reactions of Carbonyls • We have seen several reactions of ketones and aldehydes previously: – Reduction to alkanes: Wolff-Kishner or Clemmensen – Reduction to alcohols: catalytic hydrogenation or metal hydride reduction – Nucleophilic additions: RLi or RMgX • These last two reactions in particular illustrate that carbonyls engage in nucleophilic addition at carbon • We will examine the reactivity of other nucleophiles toward C=O throughout this chapter Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.
