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Carboxyl Group: Definition & Structure

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  • 0:01 What's a Carboxyl Group?
  • 0:56 Carboxyl Group Formula
  • 2:36 Carboxyl vs. Carbonyl
  • 3:31 Lesson Summary
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Lesson Transcript
Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Without realizing, you may have already encountered a carboxyl group before when drawing his best friend, carboxylic acid. Learn more about the carboxyl group and its formula.

What's a Carboxyl Group?

A carboxyl group is a very common functional group seen in chemistry. A carboxyl group is defined as having a carbonyl and hydroxyl group both linked to a carbon atom. To refresh your memory, a carbonyl group is a carbon double-bonded to an oxygen, and a hydroxyl group is an OH group. As a side note, don't be fooled by the term carboxy group. This is just another way to say carboxyl group.

A great way to make sure you know you are dealing with carboxyl is to be on the lookout for two things: an OH and a carbon double-bonded to an oxygen. Better yet, if you look at the word carboxyl we can break it down into two parts: 'carb' and 'oxyl.' When you see 'carb' think carbon atom. When you see '-oxyl' think hydroxyl group. The molecular formula for a carboxyl group is -COOH.

Carboxyl Group Formula

When you hang out with carboxyl as an organic compound, you receive the benefit of joining the club of acidity. Carboxyl groups will make organic compounds acidic because of their own ionizing property. Let's dig a little deeper on this subject of carboxyl's ionizing property.

Carboxyl groups will ionize themselves by letting go of the hydrogen atom on the hydroxyl group. This process of ionizing themselves occurs often. When the hydrogen atom is free floating, it is now called a free proton. It is the release of this hydrogen atom that makes a carboxyl group acidic.

But what does that mean for the oxygen atom? Well, instead of remaining lonely and sad that hydrogen left, the oxygen atom will become negatively charged. As a comeback to losing his pal hydrogen, the oxygen atom will go as far as sharing this negative charge with the second oxygen atom present. By sharing a negative charge between both oxygen atoms, the carboxyl is able to remain stable while ionized.

Carboxyl remains stable when ionized.

Wait, so why is the carboxyl group so willing to let the hydrogen go? The answer goes back to stability. When hydrogen is present, carboxyl is linked by a single bond to the hydroxyl group and a double bond to the oxygen atom. These two different bonds make carboxyl feel uneasy as more energy is required to maintain its configuration. When hydrogen leaves, the double bond is broken, and now carboxyl is free to not only lower its energy state but also increase its stability. Hence, it makes perfect sense why carboxyl is so quick to let hydrogen go and turn into an acid.

Properties of carboxyl groups

Carboxyl vs. Carbonyl

Now I am sure you probably already thought, 'Hey carboxyl sounds similar to the term carbonyl.' There is actually some truth to that statement. Think of carbonyl and carboxyl groups as part of the same functional group family. The carbonyl does not have a hydroxyl group, like carboxyl does. So, although they may sound very similar, they are quite different. Here's a reference to make sure you can see the difference between carbonyl and carboxyl groups.

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