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Chapter 18 Carbonyl Compounds II Reactions of Aldehydes and Ketones More Reactions of Carboxylic Acid Derivatives Reactions of a , b -Unsaturated Carbonyl Compounds. Organic Chemistry 6 th Edition Paula Yurkanis Bruice. Nomenclature of Aldehydes.
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Chapter 18 Carbonyl Compounds II Reactions of Aldehydes and Ketones More Reactions of Carboxylic Acid Derivatives Reactions of a,b-Unsaturated Carbonyl Compounds Organic Chemistry 6th Edition Paula Yurkanis Bruice
If a compound has two functional groups, the one with the lower priority is indicated by its prefix:
Nomenclature of Ketones The carbonyl is assumed to be at the 1-position in cyclic ketones:
If a ketone has a second functional group of higher priority… A few ketones have common names:
The partial positive charge on the carbonyl carbon causes that carbon to be attacked by nucleophiles: An aldehyde has a greater partial positive charge on its carbonyl carbon than does a ketone:
Aldehydes Are More Reactive Than Ketones • Steric factors contribute to the reactivity of an aldehyde. • The carbonyl carbon of an aldehyde is more accessible • to the nucleophile. • Ketones have greater steric crowding in their transition • states, so they have less stable transition states.
The reactivity of carbonyl compounds is also related to the basicity of Y–:
Carboxylic acid derivatives undergo nucleophilic acyl substitution reactions with nucleophiles:
Aldehydes and ketones undergo nucleophilic addition reactions with nucleophiles: This is an irreversible nucleophilic addition reaction if the nucleophile is a strong base
Formation of a New Carbon–Carbon Bond Using Grignard Reagents Grignard reagents react with aldehydes, ketones, and carboxylic acid derivatives
Mechanism for the reaction of an ester with a Grignard reagent:
+ + NH3 Na Reaction of Acetylide Ions with Carbonyl Compounds
Mechanism for the reaction of an acyl chloride with hydride ion:
Mechanism for the reaction of an ester with hydride ion: Esters and acyl chlorides undergo two successive reactions with hydride ion and Grignard reagents
The reduction of a carboxylic acid with LiAlH4 forms a single primary alcohol: Acyl chloride is also reduced by LiAlH4 to yield an alcohol
An amide is reduced by LiAlH4 to an amine Mechanism for the reaction of an N-substituted amide with hydride ion:
Hydrogen cyanide adds to aldehydes and ketones to form cyanohydrins:
Compared with Grignard reagents and hydride ion, CN– is a relatively weak base; therefore, in basic solution…
Aldehydes and ketones react with a primary amine to form an imine: This is a pH-dependent nucleophilic addition–elimination reaction
Dependence of the rate of the reaction of acetone with hydroxylamine on the pH of the reaction: a pH-rate profile Maximum rate is at pH = pKa of +NH3OH; at this pH, both [H+] and [NH2OH] have the highest values Decreasing rate: [H+] is decreasing Decreasing rate: [NH2OH] is decreasing Composition of the rate- determining step:
Aldehydes and ketones react with secondary amines to form enamines: An enamine undergoes an acid-catalyzed hydrolysis to form a carbonyl compound and a secondary amine
Deoxygenation of the Carbonyl Group Called the Wolff–Kishner reduction
The equilibrium constant for the reaction depends on the relative stabilities of the reactants and products:
Addition of an Alcohol to an Aldehyde or a Ketone
Utilization of Protecting Groups in Synthesis LiAlH4 will reduce the ester to yield an alcohol, but the keto group will also be reduced
The keto group is protected as a ketal in this synthesis: The more reactive aldehyde is protected with the diol before reaction with the Grignard reagent:
An OH group can be protected as a trimethylsilyl (TMS) ether:
Protection of an OH group in a carboxylic acid as the ester:
The synthetically useful aldehyde anion does not exist But its equivalent is accessible via the thioacetal: