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Questions and Answers
What is the product of the reaction between hydroxylamine and ketones or aldehydes?
What is the product of the reaction between hydrazine and ketones or aldehydes?
What is the product of the reaction between semicarbazide and ketones or aldehydes?
What is the purpose of excluding air from aldehyde containers?
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What is the oxidizing agent used in the Tollens test?
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What is the purpose of the Tollens test?
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What is the most common reagent used to reduce ketones and aldehydes?
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When is the suffix carbaldehyde used in naming?
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What are the products of reducing ketones and aldehydes with sodium borohydride?
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How is a ketone or aldehyde group named as a substituent on a molecule with a higher priority functional group?
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What is the common name of the ketone with the formula (CH3)2CO?
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How are substituent locations given in common names of ketones and aldehydes?
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How are common names of aldehydes derived?
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What makes aldehydes and ketones polar compounds?
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Why do aldehydes and ketones have higher boiling points than non-polar compounds of comparable molecular weight?
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What is the term given to the carbon atom next to the carbonyl group in common names of aldehydes?
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What is the primary function of sodium borohydride in organic chemistry?
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What is the advantage of using sodium borohydride over catalytic hydrogenation in a laboratory setting?
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What is the catalyst commonly used in catalytic hydrogenation of ketones and aldehydes?
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Why is lithium aluminum hydride not preferred for simple reductions of ketones and aldehydes?
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What is the product of reducing a ketone with sodium borohydride?
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What is Raney nickel?
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Why is catalytic hydrogenation still widely used in industry?
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What is the function of sodium triacetoxyborohydride?
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What is the result of treating a nickel-aluminum alloy with a strong sodium hydroxide solution?
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What is the disadvantage of using catalytic hydrogenation in a laboratory setting?
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What is the purpose of deoxygenation of ketones and aldehydes?
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What is the number of electrons involved in a deoxygenation reaction?
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What is the role of H2 in deoxygenation reactions?
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What is the name of the reduction reaction used for deoxygenation under acidic conditions?
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What is the purpose of amalgamated zinc in the Clemmensen reduction?
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What type of compounds can be deoxygenated using the Wolff-Kishner reduction?
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What is the purpose of using ethylene glycol or diethylene glycol in the aldol condensation reaction?
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What is the result of the nucleophilic addition of an enolate ion to another carbonyl group?
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What is the product of an aldol condensation reaction?
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What is the result of the dehydration of an aldol product?
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What is a crossed aldol condensation?
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What is the importance of selecting the compounds carefully in a crossed aldol condensation reaction?
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What happens when the enolate of one aldehyde or ketone adds to the carbonyl group of a different aldehyde or ketone?
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What is the purpose of converting a ketone or aldehyde to its hydrazone?
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Study Notes
Nomenclature of Ketones and Aldehydes
- Aldehyde groups can be named as substituents on a molecule with a higher priority functional group as its root.
- The suffix "carbaldehyde" is used when the aldehyde group is a substituent of a large unit (usually a ring).
- Ketone or aldehyde carbonyl is named by the prefix "oxo-" if it is included as part of the longest chain in the root name.
- When an aldehyde CHO group is a substituent and not part of the longest chain, it is named by the prefix "formyl".
Common Names of Ketones and Aldehydes
- Ketones and aldehydes are often called by common names instead of their systematic IUPAC names.
- Ketone common names are formed by naming the two alkyl groups bonded to the carbonyl group.
- Substituent locations are given using Greek letters, beginning with the carbon next to the carbonyl group.
- Some ketones have historical common names, such as dimethyl ketone (acetone) and alkyl phenyl ketones (acyl group followed by the suffix "-phenone").
- Common names of aldehydes are derived from the common names of the corresponding carboxylic acids.
- Greek letters are used with common names of aldehydes to give the locations of substituents.
Physical Properties of Ketones and Aldehydes
- The polar carbonyl group makes aldehydes and ketones polar compounds, and hence they have higher boiling points than non-polar compounds or comparable molecular weight.
Condensations with Hydroxylamine and Hydrazines
- Ketones and aldehydes condense with other ammonia derivatives, such as hydroxyl amine and substituted hydrazines, to give imine derivatives.
- The equilibrium constants for these reactions are usually more favorable than for reactions with simple amines.
- Hydroxylamine reacts with ketones and aldehydes to form oximes; hydrazine and its derivatives react to form hydrazones; and semicarbazide reacts to form semicarbazones.
Oxidation of Aldehydes
- Aldehydes are easily oxidized to carboxylic acids by common oxidants such as bleach (sodium hypochlorite), chromic acid, permanganate, and peroxy acids.
- Aldehydes oxidize so easily that air must be excluded from their containers to avoid slow oxidation by atmospheric oxygen.
- Mild reagents such as Ag2O can oxidize aldehydes selectively in the presence of other oxidizable functional groups.
- The Tollens test involves adding a solution of silver-ammonia complex (the Tollens reagent) to the unknown compound, and if an aldehyde is present, its oxidation reduces silver ion to metallic silver.
Reductions of Ketones and Aldehydes
- Ketones and aldehydes are most commonly reduced by sodium borohydride.
- Sodium borohydride (NaBH4) reduces ketones to secondary alcohols and aldehydes to primary alcohols.
- Lithium aluminum hydride (LiAlH4) also accomplishes these reductions, but it is a more powerful reducing agent and is much more difficult to work with.
- Sodium triacetoxyborohydride (NaBH(OAc)3) is less reactive than NaBH4, and it selectively reduces aldehydes even in the presence of ketones.
Catalytic Hydrogenation
- Like alkene double bonds, carbonyl double bonds can be reduced by catalytic hydrogenation.
- Catalytic hydrogenation is slower with carbonyl groups than with olefinic double bonds, however.
- The most common catalyst for catalytic hydrogenation of ketones and aldehydes is Raney nickel.
Deoxygenation of Ketones and Aldehydes
- A deoxygenation replaces the carbonyl oxygen atom of a ketone or aldehyde with two hydrogen atoms, reducing the carbonyl group past the alcohol stage all the way to a methylene group.
- Deoxygenation can be accomplished by either the Clemmensen reduction (under acidic conditions) or the Wolff-Kishner reduction (under basic conditions).
- The Clemmensen reduction commonly converts acylbenzenes (from Friedel-Crafts acylation) to alkylbenzenes, but it also works with other ketones and aldehydes that are not sensitive to acid.
- The Wolff-Kishner reduction is used for compounds that cannot survive treatment with hot acid.
The Aldol Condensation of Ketones and Aldehydes
- Condensations are some of the most important enolate reactions of carbonyl compounds.
- The aldol condensation involves the nucleophilic addition of an enolate ion to another carbonyl group under basic conditions.
- The product, β-hydroxy a ketone or aldehyde, is called an aldol because it contains both an aldehyde group and the hydroxy group of an alcohol.
- The aldol product may dehydrate to an α,β-unsaturated carbonyl compound.
Crossed Aldol Condensations
- When the enolate of one aldehyde (or ketone) adds to the carbonyl group of a different aldehyde or ketone, the result is called a crossed aldol condensation.
- The compounds used in the reaction must be selected carefully, or a mixture of several products will be formed.
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Description
Learn about Raney nickel, a finely divided nickel powder saturated with hydrogen, and its application in hydrogenation reactions of ketones and aldehydes.
