Animated Solution for Chemistry - Organic Chemistry: The IUPAC name of ${\text{CH}}_3\text{COCH}({\text{CH}}_3{)}_2$ is

Decoding Ketones

Organic chemistry nomenclature can sometimes feel like learning a new language. But just like any language, once you understand the grammar and the rules, it becomes incredibly logical and satisfying. Let's break down the process of naming the compound given by the condensed formula ${\text{CH}}_3\text{COCH}({\text{CH}}_3{)}_2$.

The first step in naming any organic compound is to visualize its actual structure. Condensed formulas are great for saving space, but they hide the true geometry of the molecule.

Let's unpack ${\text{CH}}_3\text{COCH}({\text{CH}}_3{)}_2$. We start with a methyl group (${\text{CH}}_3$) attached to a carbon atom. This carbon is written as $\text{CO}$, which in the middle of a chain signifies a carbonyl group—a carbon double-bonded to an oxygen ($-\text{C}(=\text{O})-$). This carbonyl carbon is then attached to a $\text{CH}$ group. Finally, the $({\text{CH}}_3{)}_2$ tells us that there are two methyl groups attached to that $\text{CH}$ carbon.

Drawing this out, we see a clear, branched structure. Visualizing the molecule is half the battle won!

The Functional Group

In the world of IUPAC nomenclature, functional groups are the VIPs. They dictate the suffix of the name and control the numbering of the carbon chain.

Looking at our expanded structure, the most prominent feature is the carbonyl group ($-\text{C}(=\text{O})-$) flanked by carbon atoms on both sides. This specific arrangement is the hallmark of a ketone. Because it's a ketone, we know our final IUPAC name will end with the suffix -one.

Next, we need to find the parent hydrocarbon. This is the longest continuous chain of carbon atoms that contains the principal functional group.

If we trace horizontally across the molecule, we count four carbon atoms. If we trace from the left, go to the $\text{CH}$ group, and then bend downwards to the bottom methyl group, we also count four carbon atoms. Both paths give us a butane parent chain. For simplicity, we usually select the straight horizontal chain.

Now comes the crucial part: numbering the chain. The golden rule of IUPAC numbering is to give the principal functional group the lowest possible locant (number).

- If we number from left to right, the ketone carbon gets position 2. - If we number from right to left, the ketone carbon gets position 3.

Since 2 is lower than 3, we must number the chain from left to right. This is a common trap where students make silly mistakes, so always double-check your numbering direction!

With our chain numbered 1 through 4 from left to right, we can identify any substituents—groups attached to the main chain but not part of it.

At carbon number 3, we have a methyl group ($-{\text{CH}}_3$) hanging off. This is our substituent, and it will be written as a prefix: 3-methyl.

Now, let's put all the pieces together like a puzzle: 1. Prefix: 3-methyl 2. Parent Chain: butan (from butane, dropping the 'e' because the suffix starts with a vowel) 3. Suffix: -2-one (indicating the ketone is at position 2)

Combining these gives us 3-methylbutan-2-one.

In older, but still widely accepted IUPAC conventions, the number for the functional group is often placed before the parent name, giving us 3-methyl-2-butanone. Both are correct, and looking at our options, this perfectly matches option (d).

And there you have it! By systematically breaking down the structure, identifying the functional group, and applying the numbering rules, we've successfully decoded the name.