The carbonyl is a moiety comprising a carbon double-bonded to an oxygen. It’s a component of functional groups often encountered in Organic Chemistry. They appear in Biology courses in plenty of molecules, including amino acids.
Structure of a Carbonyl:
The anatomy of a carbonyl is simple: a carbon and an oxygen joined together by a double bond. It’s not a functional group on its own, but it’s an integral part of many. Let’s take a look at the Lewis structure of a carbonyl:
Carbonyl Lewis structure
Lewis Structure:
The “R” generally stands for any group; again, the carbonyl is just the C=O that I’ve highlighted in purple. Both the carbonyl carbon and the oxygen are sp2-hybridized. If we were to assume that the R-groups in this example are carbons, this would be a ketone.
Functional Groups:
So, what happens when we take those R-groups and replace them with others? Let’s assume that the R-groups in the image below are carbon groups.
Functional groups with carbonyls
From left to right, we have: aldehyde (RCHO); ketone (RCOR); carboxylic acid (RCOOH); acyl halide (RCOX), where “X” is usually bromine or chlorine; amide; imide; and acid anhydride. That’s definitely not an exhaustive list, but it is a nice starting point.
Carbonyl Reactions:
Carbonyls can undergo tons of reactions addition, condensation, alkylation, reduction, and more. Are they polar or nonpolar? Polar! The oxygen is much more electronegative than the carbon it’s attached to, so this molecule is polar. A nucleophile can “take advantage” of this polarity and attack the electrophilic carbonyl carbon in an addition reaction.
Nucleophilic attack
Here we have a generic nucleophile attacking the electrophilic carbon and forming a tetrahedral intermediate.
Check out my post on condensation reactions to see how a base can pull of an acidic alpha-hydrogen to undergo an alkylation reaction!
Carbonyl Reduction:
Let’s say you’re tired of your carbonyl and want an alcohol instead. Well, you could use a specialized reagent that is really good at reducing carbonyls. Lithium aluminum hydride (LAH) is really good at reducing any kind of carbonyl compound to an alcohol (or amine if it’s an amide); sodium borohydride (NaBH4) is really good at reducing aldehydes and ketones, but it’s not good at reducing amides, esters, or carboxylic acids.
Chemoselective reduction of a beta-ketoester
IR Frequency
In real life, what’s a good way to tell if you’ve got a carbonyl in your molecule? Analytical techniques like infrared spectroscopy (IR spect.). In IR spect., different moieties each have their own unique IR peak or absorption pattern—almost like a signature. Carbonyls tend to appear at about 1700 cm-1.
So that’s it for this quick overview of carbonyls. Good luck studying!
