Chapter 16 Outline PDF

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Chapter 16 Outline...

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Thursday, January 19, 2017

Chapter 16 Outline Section 16.1 | Structure and Bonding - An aldehyde contains a carbonyl group bonded to a hydrogen atom and a carbon atom - A ketone contains a carbonyl group bonded to two carbon atoms

Section 16.2 | Nomenclature - An aldehyde is named by chaining -e of the parent alkane to -al - A ketone is named by changing -e of the parent alkane to -one and using a number to locate the carbonyl group - In naming compounds that contain more than one functional group, the IUPAC system has established an order of precedence functions

• A selection of functional groups ranked from highest to lowest in order of precedence: carboxylic acids (highest), aldehyde, ketone, alcohol, amine, thiol (lowest)

• If the carbonyl group of an aldehyde or ketone is lower in precedence than other functional groups in the molecule, it is indicated by the infix -oxo-

Section 16.3 | Physical Properties - Aldehydes and ketones are polar compounds that engage in dipole-dipole interactions in pure liquid • They have higher boiling points than non polar compounds of comparable molecule weight

Section 16.4 | Reactions - An important structural feature of a carbonyl group is the strong dipole moment in which there is a partial negative charge on oxygen and a partial positive charge on carbon

• Lewis acids such as protons react with carbonyl groups at the oxygen atom • Nucleophiles react with carbonyl groups at the carbon atom • One of the most common reaction themes of aldehydes and ketones is addition of a nucleophile to the carbonyl carbon to form a tetrahedral carbonyl addition compound

• Often, a new chiral center is created by this reaction • When none of the starting materials is chiral, a racemic mixture is formed • Many of these reactions form new carbon-carbon bonds, making this a very important class of reactions in organic synthesis

Section 16.5 | Addition of Carbon Nucleophiles - Grignard reagents add to formaldehyde, aldehydes, and ketones to give primary, secondary, and tertiary alcohols - Organolithium and terminal alkyne anions add to the carbonyl group of aldehydes and ketones to give alcohols according to a mechanism similar to addition of Grignard reagents

- Hydrogen cyanide (HCN) adds to aldehydes and ketones to give cyanohydrins

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Thursday, January 19, 2017 Section 16.6 | The Wittig Reaction - Wittig reactions involve addition of phosphonium ylides (deporotonated phosphonium salts) with aldehyde and ketone carbonyl groups to give alkanes

• E and Z products are generally both formed • Wittig reagents with anion-stabilizing groups (such as carbonyls) adjacent to the negatively charged carbon are more E selective

Section 16.7 | Addition of Oxygen Nucleophiles - Water adds to aldehydes and ketones to give hydrates, which are geminal diols • The reaction is only favorable for simple aldehydes, especially formaldehyde - Alcohols add to aldehydes to give hemiacetals (one alcohol added) then acetals (two alcohols added) • Hemiacetals are only stable when five- and six-membered rings are formed from a carbonyl and OH group on the same molecule

• Hemiacetal formation can be catalyzed by either acid or base • Acetal formation can be catalyzed only by acid • The overall process of acetal formation form alcohols and an aldehyde or a ketone is acid catalyzed and reversible

• The relative ratio of alcohol to water in the reaction determines the ratio of carbonyl to acetal species present at equilibrium

• Water is removed from reactions to favor acetal formation using a Deak-Stark trap - Carbohydrates are predominantly found in the cyclic hemiacetal form • The anomeric carbon is the carbon bonded to two oxygen atoms • The anomeric carbon can be formed as either α or β anomer - Acetals, usually as five- or six-membered rings, are often used as carbonyl protecting groups • A protecting group reversibly masks the reactivity of a molecule so that unwanted side reactions are prevented • Grignard reagents can be prepared from molecules containing carbonyl groups as long as the carbonyl group is protected as a cynic acetal

• Cyclic acetal protecting groups are removed by adding excess aqueous acid • Tetrahydropyranyl ethers, which are cyclic hemiacetals, can be used as a protecting group for alcohols

Section 16.8 | Addition of Nitrogen Nucleophiles - Ammonia and primary amines add to aldehydes and ketones to give imines, sometimes called Schiff bases, that have carbon-nitrogen bonds

- Secondary amines, especially cyclic secondary amines such as piperidine, form enamines with aldehydes and ketones

• Enamines have a carbon-nitrogen single bond with an adjacent carbon-carbon double bond

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Thursday, January 19, 2017 - Hydrazine and related compounds react with aldehydes and ketones to give analogous products with carbonnitrogen bonds

Section 16.9 | Keto-Enol Tautomerism - The carbon atom adjacent to a carbonyl group is called an α-carbon and a hydrogen bonded to it is called an αhydrogen

• The pKa of an α-hydrogen of an aldehyde or a ketone is approximately 20, which makes it less acidic than alcohols but more acidic than terminal alkynes

• The α-hydrogen is acidic because the deprotonated enolate anion is stabilized by delocalization of charge through resonance

- Aldehydes and ketones equilibrate between keto and enol forms. Keto-enol equilibrium is catalyzed by both acid and base

• Base catalyzes of keto-enol equilibration involves an enolate anion intermediate • The keto form is favored for most aldehydes or ketones • Molecules that favor the enol form at equilibrium generally have conjugated eons

Section 16.10 | Oxidation - Aldehydes are easily oxidized to carboxylic acids using a variety of reagents, including chromic acid, silver salts, peroxides, and molecule oxygen, O2. Aldehydes are also selectively oxidized, even in the presence of alcohols to carboxylic acids by treatment with sodium chlorite (NaClO2) and NaH2PO4 with added 2-methyl-2-butene

- Ketones are not easily oxidized, required extremely strong oxidizing agents as well as heat

Section 16.11 | Reduction - Aldehydes and ketones are reduced to primary and secondary alcohols • Metal hydride reducing agents such as LiAlH4 and NaBH4 are effective for reducing aldehydes and ketones • Metal hydride reducing agents do not reduce carbon-carbon double bonds • Hydrogenation using H2 and a transition metal catalyst can be used to reduce aldehyde or ketone carbonyls, although carbon-carbon double bonds in a molecule may also be reduced

• Carbon-carbon double bonds are easier to reduce than carbonyls using hydrogenation, so conditions can often be found in which only a carbon-carbon double bond is reduced in the presence of an aldehyde or ketone carbonyl group

• Conversely, by using metal hydride reagents, carbonyls can be reduced without reducing carbon-carbon double bonds

- In reductive amination, ketones or aldehydes react with amines in the presence of an appropriate reducing agent such as NaBH3CN to give substituted amines

- Aldehyde or ketone carbonyl groups can be reduced to methyl or methylene groups using two different complementary reactions

• The Clemmensen reduction uses amalgamated zinc, Zn(Hg), in strong acid • The Wolff-Kishner reduction uses hydrazine and base

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Thursday, January 19, 2017 • The Clemmensen reduction is used when the molecule is stable to strong acid, and the Wolff-Kishner reduction is used when the molecule is stable to strong base

Section 16.12 | Reactions at an α-Carbon - The α-carbon of aldehydes and ketones has special reactivity derived from the keto-enol equilibrium (referred to as tautomerization) that occurs in acid or base

• α-Carbon chiral centers of aldehydes and ketones racemize in the presence of acid or base through formation of an achiral enol during keto-enol tautomerization

• Deuterium can be exchanged for α-hydrogens catalyzed by acid or base through formation of the enol during keto-enol tautomerization

- Aldehydes or ketones with at least one α-hydrogen react with halogens in acid or base to form α-haloaldehydes and α-haloketones

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