How do you identify aldehydes in skeletal structures?

Question

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

Thank you very much!

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Answer 1 · 2018-05-03T21:09:07

Here's what I get.

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

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

In your structure, the carbon on the far right has only one bond, so there must be three $C-H$ $"C-H"$ 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 $C-H$ $"C-H"$ bonds, so they are ${CH}_{2}$ $"CH"_2$ group.

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

The group is a $C=O$ $"C=O"$ 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 $C-H$ $"C-H"$ bond, as in the middle structure, so the compound must be an aldehyde.

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