How do you identify aldehydes in skeletal structures?

Sometimes, when I try to name an aldehyde, I see it as a ketone and completely misinterpret the molecule. For example, in the molecule below, I see the double-bonded oxygen as a ketone, which would make sense normally. However, after I see that it is named as an aldehyde, I realize that the "methyl group" I thought was attached to the oxygen was actually CH, which would definitely be an aldehyde. THank
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Thank you very much!

1 Answer
May 3, 2018

Here's what I get.

Explanation:

A bond-line structure hides all #"H"# atoms directly attached to carbon atoms.

Each carbon atom must have four bonds, so any missing ones are #"C-H"# bonds.

In your structure, the carbon on the far right has only one bond, so there must be three #"C-H"# bonds.That carbon is a methyl group.

bonds

All the carbons at the angles of the zig-zag line show two bonds. They must also have two #"C-H"# bonds, so they are #"CH"_2# group.

Now, we examine the #"=O"# group. It is not attached to a #"CH"#, but to a #"C"#.

The group is a #"C=O"# carbonyl group. It does not become an aldehyde or ketone until you identify the groups attached to it.

C=O

In the first structure, the carbonyl carbon has three bonds.

The fourth bond must be a #"C-H"# bond, as in the middle structure, so the compound must be an aldehyde.

A ketone must have a #"C-C"# on both sides of the carbonyl carbon, as in the third structure.