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Reactivity of Aldehydes & Ketones

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  • Aldehydes are typically more reactive than ketones due to the following factors.

    1. Aldehydes are less hindered than ketones (a hydrogen atom is smaller than any other organic group).
    2. The carbonyl carbon in aldehydes generally has more partial positive charge than in ketones due to the electron-donating nature of alkyl groups. Aldehydes only have one e- donor group while ketones have two.

    • Addition-Elimination Reactions
      This page looks at the reaction of aldehydes and ketones with 2,4-dinitrophenylhydrazine (Brady's reagent) as a test for the carbon-oxygen double bond. It also looks briefly at some other similar reactions which are all known as addition-elimination (or condensation) reactions.
    • Addition of Alcohols to form Hemiacetals and Acetals
      In this organic chemistry topic, we shall see how alcohols (R-OH) add to carbonyl groups. Carbonyl groups are characterized by a carbon-oxygen double bond. The two main functional groups that consist of this carbon-oxygen double bond are Aldehydes and Ketones.
    • Addition of Secondary Amines to Form Enamines
      Most aldehydes and ketones react with 2º-amines to give products known as enamines. It should be noted that, like acetal formation, these are acid-catalyzed reversible reactions in which water is lost. Consequently, enamines are easily converted back to their carbonyl precursors by acid-catalyzed hydrolysis.
    • Addition of Water to form Hydrates (Gem-Diols)
      It has been demonstrated that water, in the presence of an acid or a base, adds rapidly to the carbonyl function of aldehydes and ketones establishing a reversible equilibrium with a hydrate (geminal-diol or gem-diol).
    • Alpha-carbon Reactions
      Many aldehydes and ketones undergo substitution reactions at alpha carbons. These reactions are acid or base catalyzed, but in the case of halogenation the reaction generates an acid as one of the products, and is therefore autocatalytic. If the alpha-carbon is a chiral center, as in the second example, the products of halogenation and isotopic exchange are racemic.
    • Carbonyl Group-Mechanisms of Addition
      The Carbonyl Group is a polar functional group that is made up a carbon and oxygen double bonded together. There are two simple classes of the carbonyl group: Aldehydes and Ketones. Aldehydes have the carbon atom of the carbonyl group is bound to a hydrogen and ketones have the carbon atom of the carbonyl group is bound to two other carbons. Since the carbonyl group is extremely polar across the carbon-oxygen double bond, this makes it susceptible to addition reactions like the ones that occur i
    • Carbonyl Group Reactions
      The metal hydride reductions and organometallic additions to aldehydes and ketones, described above, both decrease the carbonyl carbon's oxidation state, and may be classified as reductions. As noted, they proceed by attack of a strong nucleophilic species at the electrophilic carbon. Other useful reductions of carbonyl compounds, either to alcohols or to hydrocarbons, may take place by different mechanisms.
    • Clemmensen Reduction
      The reaction of aldehydes and ketones with zinc amalgam (Zn/Hg alloy) in concentrated hydrochloric acid, which reduces the aldehyde or ketone to a hydrocarbon, is called Clemmensen reduction.
    • Conjugate Addition Reactions
      One of the largest and most diverse classes of reactions is composed of nucleophilic additions to a carbonyl group. Conjugation of a double bond to a carbonyl group transmits the electrophilic character of the carbonyl carbon to the beta-carbon of the double bond. These conjugated carbonyl are called enones or α, β unsaturated carbonyls.
    • Cyanohydrins
      Cyanohydrins have the structural formula of R2C(OH)CN. The “R” on the formula represents an alkyl, aryl, or hydrogen. In order to form a cyanohydrin, a hydrogen cyanide adds reversibly to the carbonyl group of an organic compound thus forming a hydroxyalkanenitrile adducts (commonly known and called as cyanohydrins).
    • Irreversible Addition Reactions of Aldehydes and Ketones
      The distinction between reversible and irreversible carbonyl addition reactions may be clarified by considering the stability of alcohols.
    • Oxidation of Aldehydes and Ketones
      This page looks at ways of distinguishing between aldehydes and ketones using oxidizing agents such as acidified potassium dichromate(VI) solution, Tollens' reagent, Fehling's solution and Benedict's solution.
    • Reactions with Grignard Reagents
      Reactions of aldehydes and ketones with Grignard reagents produce potentially quite complicated alcohols.
    • Reaction with Primary Amines to form Imines
      The reaction of aldehydes and ketones with ammonia or 1º-amines forms imine derivatives, also known as Schiff bases (compounds having a C=N function). Water is eliminated in the reaction, which is acid-catalyzed and reversible in the same sense as acetal formation. The pH for reactions which form imine compounds must be carefully controlled.
    • Reduction of Aldehydes and Ketones
      This page looks at the reduction of aldehydes and ketones by two similar reducing agents - lithium tetrahydridoaluminate(III) (also known as lithium aluminium hydride) and sodium tetrahydridoborate(III) (sodium borohydride).
    • Reduction of Carbonyls to Alcohols Using Metal Hydrides
      The most common sources of the hydride nucleophile are lithium aluminum hydride and sodium borohydride. In metal hydrides reductions the resulting alkoxide salts are insoluble and need to be hydrolyzed (with care) before the alcohol product can be isolated. In the sodium borohydride reduction the methanol solvent system achieves this hydrolysis automatically. In the lithium aluminum hydride reduction water is usually added in a second step.
    • Reductive Amination
      Aldehydes and ketones can be converted into 1°, 2° and 3° amines using reductive amination. The reaction takes place in two parts. The first step is the nucleophiic addition of the carbonyl group to form an imine. The second step is the reduction of the imine to an amine using an reducing agent.
    • Reversible Addition Reactions of Aldehydes and Ketones
      water adds rapidly to the carbonyl function of aldehydes and ketones. In most cases the resulting hydrate (a geminal-diol) is unstable relative to the reactants and cannot be isolated. Exceptions to this rule exist, one being formaldehyde (a gas in its pure monomeric state). Here the weaker pi-component of the carbonyl double bond, relative to other aldehydes or ketones, and the small size of the hydrogen substituents favor addition.
    • Simple Addition Reactions
      This page looks at the addition of hydrogen cyanide and sodium hydrogensulphite (sodium bisulphite) to aldehydes and ketones.
    • The Triiodomethane (Iodoform) Reaction
      This page looks at how the triiodomethane (iodoform) reaction can be used to identify the presence of a CH3CO group in aldehydes and ketones. There are two apparently quite different mixtures of reagents that can be used to do this reaction. They are, in fact, chemically equivalent.
    • The Wittig Reaction
      Organophosphorus ylides react with aldehydes or ketones to give substituted alkenes in a transformation called the Wittig reaction. This reaction is named for George Wittig who was awarded the Nobel prize for this work in 1979. A principal advantage of alkene synthesis by the Wittig reaction is that the location of the double bond is absolutely fixed, in contrast to the mixtures often produced by alcohol dehydration.
    • Tollens’ Test
      Tollens’ test, also known as silver-mirror test, is a qualitative laboratory test used to distinguish between an aldehyde and a ketone. It exploits the fact that aldehydes are readily oxidized, whereas ketones are not. Tollens’ test uses a reagent known as Tollens’ reagent, which is a colorless, basic, aqueous solution containing silver ions coordinated to ammonia
    • Wolff-Kishner Reduction
      Aldehydes and ketones can be converted to a hydrazine derivative by reaction with hydrazine. These "hydrazones" can be further converted to the corresponding alkane by reaction with base and heat. These two steps can be combined into one reaction called the Wolff-Kishner Reduction which represents a general method for converting aldehydes and ketones into alkanes. Typically a high boiling point solvent, such as ethylene glycol, is used to provide the high temperatures needed for this reaction.