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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
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.



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





+

+ NH3

Na

Reaction of Acetylide Ions with Carbonyl Compounds




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:





Compared with Grignard reagents and hydride ion, CN cyanohydrins:– is

a relatively weak base; therefore, in basic solution…


Aldehydes and ketones react with a primary amine to cyanohydrins:

form an imine:

This is a pH-dependent nucleophilic addition–elimination reaction


Dependence of the rate of the reaction of acetone with cyanohydrins:

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 cyanohydrins:

form enamines:

An enamine undergoes an acid-catalyzed hydrolysis to

form a carbonyl compound and a secondary amine


Enamine reactions
Enamine Reactions cyanohydrins:




Reductive Amination cyanohydrins:


Deoxygenation of the Carbonyl Group cyanohydrins:

Called the Wolff–Kishner reduction





The equilibrium constant for the reaction depends on the cyanohydrins:

relative stabilities of the reactants and products:


Addition of an Alcohol to cyanohydrins:

an Aldehyde or a Ketone



Utilization of Protecting Groups cyanohydrins:

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: cyanohydrins:

The more reactive aldehyde is protected with the diol

before reaction with the Grignard reagent:







The synthetically useful aldehyde anion does not exist cyanohydrins:

But its equivalent is accessible via the thioacetal:


Formation of Alkenes: cyanohydrins:

The Wittig Reaction


Preparation of the Phosphonium Ylide cyanohydrins:

The phosphonium ylide should be prepared from

sterically hindered alkyl halide:

Synthetic target:

Preferred synthetic approach:


  • This reaction is the best way to make a terminal alkene.

  • Stable ylides form primarily E isomers, and unstabilized

  • ylides form primarily Z isomers.

  • Stable ylides have a group (C=O) that can share the

  • carbanion’s negative charge.

Example:


Stereochemistry of cyanohydrins:

Nucleophilic Addition Reaction


Disconnections, Synthons, and cyanohydrins:

Synthetic Equivalents


A synthetic equivalent is the reagent that is actually used cyanohydrins:

as the source of a synthon


Nucleophilic Addition to cyanohydrins:a,b-Unsaturated Aldehydes and Ketones


  • Nucleophiles that form stable addition products can

  • form direct addition products or conjugate addition

  • products.

  • If the rate of direct addition is slowed down by steric

  • hindrance, a Grignard reagent will form the conjugate

  • addition product.


Strong bases form direct addition products with reactive cyanohydrins:

carbonyl groups and conjugate addition products with

less reactive carbonyl groups:



Nucleophilic Addition to cyanohydrins:a,b-Unsaturated

Carboxylic Acid Derivatives


Enzyme-Catalyzed Additions to cyanohydrins:a,b-Unsaturated Carbonyl Compounds


Addition reactions to a b unsaturated carbonyls
Addition Reactions to cyanohydrins:a,b-Unsaturated Carbonyls

  • Michael addition nucleophiles:

  • Cyanide

  • Sulfide

  • Organocuprate

  • Amine

  • Halides

  • Direct addition nucleophiles:

  • Grignard

  • LAH

  • Organolithiums



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