ketone

Examples of ketone in the following topics:

• Diazo Ketone Reactions

  • The Arndt-Eistert reaction is a special case of a more general class of diazo ketone reactions.
  • If we assume that diazo ketones normally decompose to acyl carbenes, then numerous subsequent reactions can be imagined, and many have been realized.
  • The first diagram below outlines some of these transformations, originating from the diazo ketone formula in the center of the diagram.

• Occurrence of Aldehydes and Ketones

  • Aldehydes and ketones are widespread in nature, often combined with other functional groups.

• Aldehydes and Ketones

  • The ketone carbon is sp2 hybridized, and it adopts a trigonal planar geometry around the ketonic carbon.
  • Ketones are not usually hydrogen bond donors, and they tend not to exhibit intermolecular attractions with other ketones.
  • Ketones have alpha-hydrogens which participate in keto-enol tautomerism.
  • The keto form predominates at equilibrium for most ketones.
  • Both ketones and aldehydes can be identified by spectroscopic methods.

• Synthetic Preparation of Aldehydes and Ketones

  • Aldehydes and ketones are obtained as products from many reactions discussed in previous sections of this text.
  • In the following sections of this chapter we shall find that one of the most useful characteristics of aldehydes and ketones is their reactivity toward carbon nucleophiles, and the resulting elaboration of molecular structure that results.
  • In short, aldehydes and ketones are important intermediates for the assembly or synthesis of complex organic molecules.

• Nomenclature of Aldehydes and Ketones

  • Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O.
  • If neither is hydrogen, the compound is a ketone.
  • The IUPAC system of nomenclature assigns a characteristic suffix to these classes, al to aldehydes and one to ketones.
  • Since ketones have two sets of neighboring atoms, one set is labeled α, β etc., and the other α', β' etc.
  • Very simple ketones, such as propanone and phenylethanone (first two examples in the right column), do not require a locator number, since there is only one possible site for a ketone carbonyl function.

• Reactions at the α-Carbon

  • Indeed, treatment of this ketone reactant with acid or base alone serves to racemize it.
  • Not all carbonyl compounds exhibit these characteristics, the third ketone being an example.
  • Cyclohexanone (the first ketone) has two alpha-carbons and four potential substitutions (the alpha-hydrogens).
  • The second ketone confirms this fact, only the alpha-carbon undergoing substitution, despite the presence of many other sites.
  • This is demonstrated convincingly by the third ketone, which is structurally similar to the second but has no alpha-hydrogen.

• Properties of Aldehydes and Ketones

  • A comparison of the properties and reactivity of aldehydes and ketones with those of the alkenes is warranted, since both have a double bond functional group.
  • Because of the greater electronegativity of oxygen, the carbonyl group is polar, and aldehydes and ketones have larger molecular dipole moments (D) than do alkenes.
  • We expect, therefore, that aldehydes and ketones will have higher boiling points than similar sized alkenes.
  • Furthermore, the presence of oxygen with its non-bonding electron pairs makes aldehydes and ketones hydrogen-bond acceptors, and should increase their water solubility relative to hydrocarbons.
  • Consequently, with the exception of formaldehyde, the carbonyl function of aldehydes and ketones has a π-bond energy greater than that of the sigma-bond, in contrast to the pi-sigma relationship in C=C.

• The Arndt-Eistert Reaction

  • The starting acid, written on the left, is converted first to an acyl chloride derivative, and then to a diazomethyl ketone.
  • Acylation of the methylene carbon produces an equilibrium mixture of a diazonium species and the diazomethyl ketone plus hydrogen chloride (written in brackets).
  • If the HCl is not neutralized by a base, this mixture reacts further to give a chloromethyl ketone with loss of nitrogen.
  • However, if the HCl is neutralized as it is formed, the relatively stable diazo ketone is obtained and may be used in subsequent reactions.
  • The products are the diazo ketone and methyl chloride (a gas) from the reaction of diazomethane with HCl.

• Reversible Addition Reactions

  • It has been demonstrated (above) that water adds rapidly to the carbonyl function of aldehydes and ketones.
  • Similar reversible additions of alcohols to aldehydes and ketones take place.
  • Ketone derivatives of this kind were once called ketals, but modern usage has dropped that term.
  • Most aldehydes and ketones also react with 2º-amines to give products known as enamines.
  • Cyanohydrin formation is weakly exothermic, and is favored for aldehydes, and unhindered cyclic and methyl ketones.

• Reactions with Organometallic Reagents

  • The facile addition of alkyl lithium reagents and Grignard reagents to aldehydes and ketones has been described.
  • The aldehyde or ketone product of this elimination then adds a second equivalent of the reagent.
  • Since acyl chlorides are more reactive than esters, isolation of the ketone intermediate formed in their reactions with organometallic reagents becomes an attractive possibility.
  • Specific examples of ketone synthesis using these reagents are presented in the following diagram.
  • Grignard reagents add to nitriles, forming a relatively stable imino derivative which can be hydrolyzed to a ketone.