Question

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

Thank you very much!

Answer 1

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.

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.

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.