Aldehydes are a class of organic compound. Like all organic compounds, aldehydes contain a carbon atom. The carbon atom in aldehydes is double-bonded to an oxygen atom, singly bonded to a hydrogen atom, and singly bonded to another atom or compound. These fragrant compounds have a wide variety of industrial and biological applications. Glucose, the most common sugar in the body, is an aldehyde. Does organic chemistry baffle you? There’s no need to hyde your head in shame! We’ll get you started on aldehydes.
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One of the most important features of an aldehyde is the presence of a carbonyl group. Carbonyls are carbon atoms double-bonded to oxygen atoms. Aldehydes and ketones are among the compounds that contain carbonyl groups. The carbon in a carbonyl group is electrophilic, meaning it attracts electrons. Among carbonyl compounds, aldehydes are the most electrophilic.
Nitriles are an organic compound containing a carbon triple-bonded to a nitrogen. Nitriles are used to help synthesize aldehydes in a process known as the Stephen Synthesis. The Stephen Synthesis reacts nitriles with tin (II) chloride, hydrochloric acid, and water to reduce the compounds, forming an aldehyde and ammonium chloride.
Ketones and aldehydes are similar organic compounds. Tollens’ Test is an effective method for distinguishing one from the other. In Tollens’ Test a reagent called, fittingly, Tollens’ Reagent, is added to the compound being examined. Tollens’ reagent is an aqueous solution of silver ions coordinated to ammonia. The reagent oxidizes aldehydes and is itself reduced in the process, turning the reagent’s silver ions in solution into metallic silver. This reduction will form a mirror on the test tube, indicating the presence of aldehydes. Ketones, conversely, will not be oxidized.
Jones’ Reagent is a very acidic, aqueous solution of chromic acid. The reagent reacts with primary and secondary alcohols. Primary alcohols are alcohols in which the hydroxy group (-OH) is attached to a single carbon atom. Jones’ Reagent oxidizes primary alcohols into aldehydes and then immediately oxidizes the aldehyde into carboxylic acid. This solution can be an effective and inexpensive method for synthesizing carboxylic acid.
Fehling’s Solution is used in the Fehling Test as a further means of differentiating between aldehydes and ketones. Deep blue in color and highly alkaline, Fehling’s Solution oxidizes aldehydes. This oxidation results in the formation of a reddish-brown precipitate. If the solution maintains its blue color (a negative result) we can confirm that our solution does not contain aldehydes.
Quizbowl is about learning, not rote memorization, so we encourage you to use this as a springboard for further reading rather than as an endpoint. Here are a few things to check out:
There are many kinds of aldehydes, but the best-known one may be formaldehyde. Learn about the history of this preservative compound in this podcast.
Ketones and aldehydes are very similar. If you want to go a bit deeper, here are some ways to tell them apart.
Watch this video to see an in-depth example of the Tollens’ Test.
If you can’t get enough of lab experiments, here’s a good video of Fehling’s Test that also goes further into the chemistry than we can here.
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