aldehyde

Examples of aldehyde in the following topics:

• Occurrence of Aldehydes and Ketones

  • Aldehydes and ketones are widespread in nature, often combined with other functional groups.
  • Note that the aldehyde function is often written as –CHO in condensed or complex formulas.

• 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.

• Aldehydes and Ketones

  • Aldehydes and ketones are classes of organic compounds that contain a carbonyl (C=O) group.
  • Like ketones, aldehydes are sp2 hybridized and can exist in the keto or enol tautomer.
  • Aldehydes are named by dropping the suffix of the parent molecule, and adding the suffix "-al."
  • For instance, a three-carbon chain with an aldehyde group on a terminal carbon would be propanal.
  • Both ketones and aldehydes can be identified by spectroscopic methods.

• Nomenclature of Aldehydes and Ketones

  • Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O.
  • If at least one of these substituents is hydrogen, the compound is an aldehyde.
  • The IUPAC system of nomenclature assigns a characteristic suffix to these classes, al to aldehydes and one to ketones.
  • In all cases the aldehyde function has a higher status than either an alcohol, alkene or ketone and provides the nomenclature suffix.
  • Because ketones are just below aldehydes in nomenclature suffix priority, the "oxo" substituent terminology is seldom needed.

• 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.

• Oxidation

  • The most common and characteristic oxidation reaction is the conversion of aldehydes to carboxylic acids.
  • In discussing the oxidations of 1º and 2º-alcohols, we noted that Jones' reagent (aqueous chromic acid) converts aldehydes to carboxylic acids, presumably via the hydrate.
  • Even the oxygen in air will slowly oxidize aldehydes to acids or peracids, most likely by a radical mechanism.
  • Useful tests for aldehydes, Tollens' test, Benedict's test & Fehling's test, take advantage of this ease of oxidation by using Ag(+) and Cu(2+) as oxidizing agents (oxidants).
  • Saturated ketones are generally inert to oxidation conditions that convert aldehydes to carboxylic acids.

• 1:3-Diastereoselection in Reactions with Chiral Aldehydes

  • Two examples of 1:3-diastereoselection in reactions of β-substituted aldehydes are shown in the following diagram.
  • It should be noted that the anti-configuration of substituents in aldehyde I mutually reinforce Felkin-Ahn control, especially with the E-enolate.
  • Corresponding reactions of aldehydes I and II (above) with comparable E-enolborinates and Z-chlorotitanium enolates are shown below.
  • Felkin-Ahn control is only strong in the reaction of the mutually reinforced anti-substituted aldehyde I with the E-borinate.
  • Reactions of aldehydes I and II with comparable E-enolborinates and Z-chlorotitanium enolates

• 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.
  • Acetals are geminal-diether derivatives of aldehydes or ketones, formed by reaction with two equivalents of an alcohol and elimination of water.
  • 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.

• Supporting and Conflicting Substituent Effects

  • The ketone enolate is a single enantiomer; the aldehyde reactants in reactions A and D are enantiomers, as are the aldehydes in reactions B and C.
  • Again, the aldehyde reactants in reactions A and D are enantiomers, as are the aldehydes in reactions B and C; however, they have changed their location.
  • In reaction A the α and β-stereogenic centers of the aldehyde and the α'-center of the enolate have matching influences on product diastereoselectivity.
  • In reaction B the β-polar substituent of the aldehyde is mismatched in this respect, but has little effect on the overall diastereoselectivity.
  • In reaction C the α-substituent of the aldehyde is mismatched, leading to dominance of the anti-Felkin-Ahn product.

• Enantioselective Aldol Reactions

  • A Z-enolborinate is prepared in the usual way, and this reacts with a number of achiral aldehydes with very high enantioselectivity.
  • In this example the re-face of the enolate bonds to the si-face of the aldehyde.
  • In the upper equation a chelated Z-titanium enolate is initially formed and then reacted with an aldehyde.
  • Interestingly, when the large Lewis acid (C2H5)2AlCl was used to activate the aldehyde, the anti-enantiomer A1 was formed in 90% de, accompanied by S2.
  • Steric hindrance of the Lewis acid with the Z-methyl group changes the facial selectivity of the aldehyde from re to si.