Aldehydes and Ketones Help
Introduction and Nomenclature
Aldehydes and ketones have only H, R, or Ar groups attached to the carbonyl group. Aldehydes have at least one hydrogen bonded to the carbonyl group; ketones have only R's or Ar's.
Aldehyde names are based on the longest continuous chain including the carbon of the aldehyde group (the carbonyl carbon). The -e of the alkane name is replaced by the suffix -al. The carbon of –CHO is number 1. Common names for aldehydes replace the suffix -ic (-oic or -oxylic) and the word acid of the corresponding carboxylic acids by -aldehyde. The compound is named as an aldehyde whenever –CHO is attached to a ring.
Common names for ketones use the names of R or Ar as separate words, along with the word ketone. The IUPAC system replaces the -e of the name of the longest chain by the suffix –one, along with a number to indicate the position of the carbonyl group.
The electron-releasing alkyl groups (R) diminish the electrophilicity of the carbonyl C, lessening the chemical reactivity of ketones. Furthermore, the R's, especially large bulky ones, make approach of reactants to the C more difficult. For these reasons, ketones are less reactive than aldehydes.
Preparation of Aldehydes and Ketones
Ozonolysis of alkenes and cleavage of 1,2-diols afford carbonyl compounds. Ketones and aldehydes can also be produced by hydration of alkynes. Friedel-Crafts acylations of arenes with RCOCl in the presence of AlCl3 give good yields of ketones.
Alcohols are the most important precursors in the synthesis of carbonyl compounds, being readily available. Ordinarily, H2SO4/CrO3 (Jones' reagent) is used to oxidize 2° R2CHOH to R2CO. However, oxidizing 1° RCH2OH to RCHO without allowing the overoxidation of RCHO to RCO2H requires special reagents. The most common of these is pyridinium chlorochromate, PCC.
Acid chlorides, esters (RCO2R'), and nitriles (RCN) are reduced with lithium tri-t-butoxyaluminum hydride, LiAlH[OC(CH3)3] or with DIBAL (diisobutyl aluminum hydride, AlH[CH2CH(CH3)2]2), at very low temperatures, followed by H2O workup. The net reaction is a displacement of X with H.
Reaction of Aldehydes and Ketones
Oxidation. Aldehydes undergo oxidation and ketones are unreactive under normal chromic acid oxidation conditions. A more mild oxidant is Tollens' reagent, Ag(NH3)2 + (from Ag+ and NH3). Formation of the shiny Ag mirror in this test is a positive test for aldehydes. The RCHO must be soluble in aqueous alcohol. This mild oxidant permits –CHO to be oxidized in a molecule having groups more difficult to oxidize, such as 1° or 2° OH's.
Reduction. Aldehydes and ketones are readily reduced to 1° and 2° alcohols, respectively, with NaBH4 or LiAlH4
Addition Reactions of Nucleophiles to C = O
The carbon of the carbonyl group is electrophilic, due to a partial positive charge on the less electronegative carbon. This is easiest to see in the resonance hybrid of a carbonyl group. A variety of different nucleophiles will add to carbonyl groups, including Grignard reagents, which are discussed in Chapter 9.
Acetal Formation. Alcohols add to aldehydes and ketones in an acidcatalyzed reaction to produce hemiacetals and acetals. In H3O+, RCHO is regenerated because acetals undergo acid-catalyzed cleavage much more easily than do ethers. Since acetals are stable in neutral or basic media, they are used to protect the –CH=O group.
If the following methylation reaction were carried out directly, the acetylide ion would react as a nucleophile, attacking the carbonyl group. To prevent this side reaction, the carbonyl is protected by acetal formation before the carbanion is formed using the strongly basic sodium hydride (NaH). The acetal is stable under the basic conditions of the methylation reactions. The aldehyde is later unmasked by acidcatalyzed hydrolysis.
Ylides. A carbon anion can form a π bond with an adjacent phosphorus or sulfur. The resulting charge delocalization is especially effective if phosphorus or sulfur, furnishing the empty d orbital, also has a + charge. Carbon anions with these characteristics are called ylides.
The Wittig reaction uses phosphorus ylides to change the carbonyl group to an alkene. The ylide is prepared in two steps from RX.
The ylide adds to the carbonyl group of the aldehyde (or ketone), and leads to an alkene and triphenylphosphine oxide. The cis-trans geometry of the alkene is influenced by the nature of the substituents, solvent, and dissolved salts. Polar protic or aprotic solvents favor the cis isomer.
Practice problems for these concepts can be found at: Aldehydes and Ketones Practice Problems
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