12.7:
Preparation of Aldehydes and Ketones from Alcohols, Alkenes, and Alkynes
Oxidation of different alcohols in the presence of suitable oxidizing agents gives aldehydes and ketones.
Partially oxidizing a primary alcohol using a mild oxidizing agent produces an aldehyde.
If, instead, a strong oxidizing agent is used, the aldehyde gets further oxidized to a carboxylic acid.
Irrespective of the strength of the oxidizing agent, oxidation of a secondary alcohol produces a ketone.
Aldehydes and ketones are also prepared from unsaturated hydrocarbons like alkenes and alkynes.
Gently cleaving a monosubstituted alkene using ozone, followed by a reductive workup, yields formaldehyde, and a substituted aldehyde.
Similarly, cleaving a 1,2-disubstituted alkene gives a mixture of aldehydes, while a 1,1-disubstituted alkene gives a mixture of formaldehyde and ketone.
Ozonolysis of a trisubstituted alkene produces a mixture of aldehydes and ketones, while a tetrasubstituted alkene gives only ketones.
Terminal alkynes on hydroboration-oxidation through anti-Markovnikov addition give aldehydes, while internal alkynes give ketones.
However, both terminal and internal alkynes favor Markonikov’s addition under acid-catalyzed hydration conditions to give ketones.
12.7:
Preparation of Aldehydes and Ketones from Alcohols, Alkenes, and Alkynes
Aldehydes and ketones are prepared from alcohols, alkenes, and alkynes via different reaction pathways. Alcohols are the most commonly used substrates for synthesizing aldehydes and ketones. The conversion of alcohol to aldehyde, which involves the oxidation process, depends on the class of the alcohol used and the strength of the oxidizing agent. For instance, primary alcohol will form an aldehyde when treated with a weak oxidizing agent; however, it gets over-oxidized to a carboxylic acid in the presence of a strong oxidizing agent. Hence, a mild oxidant like pyridinium chlorochromate is used to convert primary alcohols to aldehydes.
Similarly, Swern and Dess–Martin oxidations, which employ weaker oxidizing agents, convert primary alcohols to aldehydes. The strength of the oxidizing agent is irrelevant when converting secondary alcohol to a ketone. Both mild and strong oxidants give ketones from secondary alcohols.
Unsaturated hydrocarbons like alkenes undergo an ozonolysis reaction to give aldehydes and ketones. The product formed depends on the substitution present across the double bond in an alkene. A monosubstituted alkene forms formaldehyde and another aldehyde molecule. However, disubstitution gives rise to two cases. A 1,1-disubstituted alkene upon ozonolysis forms a mixture of formaldehyde and ketone, while a 1,2-disubstituted alkene yields a mixture of aldehydes. Both aldehydes and ketones are formed when trisubstituted alkenes undergo ozonolysis, while tetrasubstituted alkenes form ketones exclusively.
Alkynes also form aldehydes and ketones under hydroboration-oxidation and acid-catalyzed hydration reaction conditions. The hydroboration-oxidation reaction favors anti-Markovnikov’s addition. Hence terminal alkynes form aldehydes, and internal alkynes yield ketones. On the other hand, the acid-catalyzed hydration reaction follows Markonikov’s addition, and thus both terminal and internal alkynes generate ketones.