Aldehydes & Ketones

Inroduction
Nomenclature of Aldehydes and Ketones Synthetic Preparation of Aldehydes and Ketones Reactions of Aldehydes and Ketones

Aldehydes & Ketones

The functional group on aldehydes and ketones is a carbonyl group (double bond oxygen to a carbon). In aldehydes, the carbonyl group is on the "end" of a carbon chain, while in ketones, it is in the "middle" of a carbon chain.

The double bond in the carbonyl group is very susceptible to chemical reactions.




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Nomenclature of Aldehydes and Ketone

Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O. The carbon atom of this group has two remaining bonds that may be occupied by hydrogen or alkyl or aryl substituents. If at least one of these substituents is hydrogen, the compound is an Aldehyde. If neither is hydrogen, the compound is a Ketone.

The names of aldehydes and ketones are simply derived by dropping "-e" from the root and adding "-al" or "-one" respectively. A position number is needed for ketones since the carbonyl group may be on any number of several carbons in the "middle" of a chain. The carbonyl on the aldehyde is always on the number one carbon so no position number is needed.

Examples of IUPAC names are provided (in blue) in the following diagram. Common names are in red, and derived names in black. In common names carbon atoms near the carbonyl group are often designated by Greek letters. The atom adjacent to the function is alpha, the next removed is beta and so on. Since ketones have two sets of neighboring atoms, one set is labled α, β etc., and the other α', β' etc.

































Common Aldehydes and Ketones



Propanone (Acetone)- Chief use is a solvent since it dissolves in both water and non-polar organics. May be present in the urine of diabetics. Also used in fingernail polish remover.

Methanal (Formaldehyde) - Used to synthesize methanol and many plastics such as Bakelite and Melmac. Has antiseptic properties. Used in embalming and preservation of biological specimens.

Ethanal (Acetaldehyde) - Used to synthesize many organics. If heated with acid it polymerizes into paraldehyde which is used as an hypnotic and sedative.

Butanal - Fragrance of fresh bread.

3-phenyl-2-propenal (Cinnamaldehyde) - Causes the odor and flavor of cinnimon.

4-hydroxy-3-methoxy-benzaldehyde (Vanillin) - Flavor from vanilla beans.



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Synthetic Preparation of Aldehydes and Ketones

Aldehydes and ketones are obtained as products from many reactions discussed below.........



























































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Reactions of Aldehydes and Ketones



The Grignard Reaction



The reaction of an alkyl, aryl or vinyl halide with magnesium metal in ether solvent, produces an organometallic complex of uncertain structure, but which behaves as if it has the structure R-Mg-X and is commonly referred to as a Grignard Reagent.













Hydration of Aldehydes & Ketones


The hydration of carbonyl compounds is an equilibrium process and the extent of that equilibrium generally parallels the reactivity of the parent aldehyde or ketone towards nucleophilic substitution; aldehydes are more reactive than ketones and are more highly hydrated at equilibrium.














Reaction with Amines



The reaction of carbonyl compounds with amines involves the formation of an intermediate carbinolamine which undergoes dehydration to form an immonium cation which can loose a proton to form the neutral imine.



















Preparation of Alcohols by Reduction of Aldehydes and Ketones

Reduction of simple aldehydes and ketones with BH4- yields the corresponding alcohol directly. The reaction works well for simple compounds, but reaction of BH4- with α-β-unsaturated aldehydes and ketones can result in significant reduction of the double bond.


















Wolff-Kishner Reduction


The imine formed from an aldehyde or ketone on reaction with hydrazine (NH2NH2) is unstable in base, and undergoes loss of N2 to give the corresponding hydrocarbon.



















Clemmensen Reduction


Carbonyl compounds can also be reduced by the Clemmensen reduction using zinc-mercury amalgam in the presence of acid; the mechanism most likely involves free radicals.


















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