An aldehyde functional group is a carbonyl carbon bonded to at least one hydrogen, written as R-CHO. In Organic Chemistry II, that structure explains its naming, oxidation, and nucleophilic addition reactions.
An aldehyde functional group is a terminal carbonyl, meaning the carbonyl carbon is bonded to one hydrogen and one carbon group, usually written as R-CHO. That hydrogen is what separates an aldehyde from a ketone, and it is a big reason aldehydes behave the way they do in Organic Chemistry II.
Because the carbonyl carbon is sp2-hybridized and strongly polarized, the C in C=O is electrophilic. The oxygen pulls electron density toward itself, so nucleophiles are attracted to the carbonyl carbon. In aldehydes, that carbonyl is at the end of a chain, which leaves it less sterically crowded than a ketone and usually more reactive in nucleophilic addition.
You will usually see aldehydes named by changing the alkane ending to -al, or by using common names for smaller molecules. For example, ethanal is the IUPAC name for acetaldehyde. When the aldehyde is attached to a ring, the formyl carbon is numbered as part of the functional group, and the naming rules become a little different from open-chain compounds.
Aldehydes are also easy to oxidize compared with many other organic functional groups. A mild oxidizing agent can convert an aldehyde into a carboxylic acid, which is why aldehydes often show up as intermediates in synthesis rather than final products. That same reactivity is why tests like chromic acid oxidation can distinguish aldehydes from ketones.
They also undergo addition reactions with nucleophiles. A Grignard reagent can add to an aldehyde to form an alcohol after workup, and sodium borohydride can reduce an aldehyde to a primary alcohol. So when you see an aldehyde in a mechanism problem, think about the carbonyl carbon as a reaction site that is ready for addition, oxidation, or reduction, depending on the reagent.
Aldehyde functional groups are one of the first places Organic Chemistry II asks you to connect structure to reactivity. If you can spot an aldehyde quickly, you can predict whether a molecule will oxidize, reduce, or undergo nucleophilic addition instead of guessing reaction products.
This term also sits right inside the larger carbonyl unit. Many mechanism questions in this course compare aldehydes with ketones, then ask you to explain why one reacts faster or gives different products. The extra hydrogen on the aldehyde carbonyl carbon matters, because it changes both steric access and oxidation behavior.
Aldehydes also show up in synthesis planning. If a problem asks how to make a primary alcohol, a carboxylic acid, or a carbon-carbon bond, an aldehyde can be the intermediate that gets carried through the sequence. That makes the functional group more than a label, it is a checkpoint for choosing reagents and predicting the next step.
You will also use aldehydes in lab-style identification problems. A positive 2,4-dinitrophenylhydrazine result tells you a carbonyl is present, but you still have to decide whether the compound is an aldehyde or ketone using oxidation tests, spectra, or reaction behavior.
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Visual cheatsheet
view gallerycarbonyl group
The aldehyde functional group is a specific kind of carbonyl. Every aldehyde has a C=O, but not every carbonyl compound is an aldehyde. In Organic Chemistry II, recognizing the carbonyl first helps you narrow the reaction type, then the attached substituents tell you whether you are dealing with an aldehyde, ketone, acid derivative, or something else.
ketone
Aldehydes and ketones are the pair most often compared in carbonyl chemistry. The difference is one hydrogen versus two carbon groups attached to the carbonyl carbon, and that changes reactivity. Aldehydes are generally more reactive in nucleophilic addition and easier to oxidize, which is why many mechanism questions separate the two.
nucleophilic addition
Most aldehyde reactions in this course are nucleophilic additions to the carbonyl carbon. The electrophilic carbonyl carbon accepts attack, the C=O π bond opens, and a new bond forms before protonation or workup. If you know the molecule is an aldehyde, you can predict that additions often happen more easily than with ketones.
Chromic Acid
Chromic acid is a classic reagent for testing and oxidizing aldehydes. Aldehydes usually oxidize to carboxylic acids under these conditions, while many ketones do not react the same way. That makes chromic acid useful in structure identification and in mechanism problems that ask you to classify oxidation outcomes.
A quiz question may show you a structure and ask whether it is an aldehyde or a ketone, so you need to check whether the carbonyl carbon is bonded to a hydrogen. A mechanism problem may then ask you to predict the product after sodium borohydride, a Grignard reagent, or chromic acid. In each case, the aldehyde tells you how the carbonyl will react and what functional group comes next.
You may also see an unknown compound in a lab writeup or spectroscopy question. If the compound has aldehyde behavior, you can use oxidation tests, addition reactions, or naming clues to support your answer. The main move is simple: identify the -CHO pattern, then connect that structure to the reaction or test being asked about.
Aldehydes and ketones both contain a carbonyl group, so they look similar at first glance. The difference is that an aldehyde has at least one hydrogen attached to the carbonyl carbon, while a ketone has two carbon groups. That small structural change affects naming, oxidation, and how reactive the molecule is in nucleophilic addition.
An aldehyde functional group is a terminal carbonyl written as R-CHO, with the carbonyl carbon bonded to at least one hydrogen.
In Organic Chemistry II, aldehydes are treated as reactive carbonyl compounds that often undergo nucleophilic addition, oxidation, and reduction.
Aldehydes are usually easier to oxidize than ketones, which makes them useful in oxidation tests and synthesis sequences.
If you can spot the -CHO group, you can predict how the molecule will be named and which reagents are likely to react with it.
Aldehydes and ketones look similar, but the hydrogen on an aldehyde carbonyl carbon changes the molecule's chemistry in a noticeable way.
It is a carbonyl group at the end of a carbon chain, where the carbonyl carbon is bonded to one hydrogen and one carbon group. That structure is written as R-CHO. In Organic Chemistry II, this setup matters because it makes aldehydes more reactive than many related carbonyl compounds.
Look at the carbonyl carbon. If it has at least one hydrogen attached, it is an aldehyde. If it is bonded to two carbon groups, it is a ketone. That structural difference is the fastest way to separate the two on naming or mechanism questions.
Aldehydes have a hydrogen attached to the carbonyl carbon, so oxidation can turn that carbon into a carboxylic acid. Ketones do not have that hydrogen, so they are much less likely to oxidize under the same mild conditions. This is why aldehydes often give positive oxidation tests that ketones do not.
Aldehydes commonly undergo nucleophilic addition, reduction to primary alcohols, and oxidation to carboxylic acids. Reagents like sodium borohydride and Grignard reagents are common in course problems, and chromic acid is often used to show how readily aldehydes oxidize.