12.8
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…
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.
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Q1: How does the strength of an oxidizing agent affect aldehyde formation from primary alcohols?
Mild oxidizing agents like pyridinium chlorochromate convert primary alcohols to aldehydes without over-oxidation. Strong oxidizing agents cause the aldehyde to be further oxidized to a carboxylic acid. Weaker oxidants such as Swern and Dess–Martin oxidations also effectively produce aldehydes from primary alcohols, making oxidizing agent selection critical for controlling product formation.
Q2: Why does secondary alcohol oxidation always produce ketones regardless of oxidizing agent strength?
Secondary alcohols form ketones through oxidation because the carbon bearing the hydroxyl group is bonded to two alkyl groups. This structural constraint prevents further oxidation beyond the ketone stage. Both mild and strong oxidants yield ketones from secondary alcohols, making the oxidizing agent strength irrelevant for this conversion.
Q3: What products result from ozonolysis of different alkene substitution patterns?
Monosubstituted alkenes yield formaldehyde and a substituted aldehyde through ozonolysis. Disubstituted alkenes produce either two aldehydes (1,2-disubstituted) or formaldehyde plus a ketone (1,1-disubstituted). Trisubstituted alkenes give mixtures of aldehydes and ketones, while tetrasubstituted alkenes produce only ketones. Reductive workup ensures clean product formation.
Q4: How do terminal and internal alkynes differ in hydroboration-oxidation reactions?
Terminal alkynes undergo hydroboration-oxidation via anti-Markovnikov addition to form aldehydes as the primary product. Internal alkynes follow the same anti-Markovnikov pathway but yield ketones instead. This regioselectivity difference makes hydroboration-oxidation a predictable method for synthesizing specific carbonyl compounds from alkynes.
Q5: What is the difference between acid-catalyzed hydration and hydroboration-oxidation for alkynes?
Acid-catalyzed hydration follows Markovnikov's addition rule, producing ketones from both terminal and internal alkynes. Hydroboration-oxidation follows anti-Markovnikov addition, yielding aldehydes from terminal alkynes and ketones from internal alkynes. The reaction conditions and mechanism determine which regioselectivity pathway dominates the transformation.
Q6: Why is ozonolysis a useful method for preparing aldehydes and ketones from alkenes?
Ozonolysis cleaves the carbon-carbon double bond, converting it into carbonyl groups with predictable products based on alkene substitution. The nucleophilic addition to the carbonyl group mechanism allows controlled synthesis of aldehydes and ketones. Reductive workup ensures clean product formation without further oxidation or unwanted side reactions.
Q7: Which alcohol class is most commonly used as a substrate for synthesizing aldehydes and ketones?
Primary alcohols are the most commonly used substrates because they readily form aldehydes under mild oxidation conditions. Secondary alcohols also serve as important substrates, consistently producing ketones regardless of oxidizing agent strength. The choice between primary and secondary alcohols depends on whether an aldehyde or ketone product is desired for downstream synthesis.