Animated Solution for Chemistry - Organic Chemistry: The total number of C-C sigma bond/s in mesityl oxide (${\text{C}}_6{\text{H}}_{10}\text{O}$) is ........ . (Round off to the nearest integer).

Have you ever wondered why some molecules have names that seem completely unrelated to their structure? Mesityl oxide is one of those classic examples in organic chemistry.

Despite its name, it doesn't contain a mesityl group, nor is it an oxide or an ether! It is actually an $\alpha ,\beta$-unsaturated ketone.

Its formal IUPAC name is 4-methylpent-3-en-2-one. Knowing this IUPAC name is the master key to unlocking its structure and solving this problem.

To count the bonds, we first need to visualize the molecule perfectly. Let's break down the IUPAC name: 4-methylpent-3-en-2-one.

The root word "pent" tells us there are five carbon atoms in the main chain.

The "2-one" indicates a ketone group (a carbonyl $\text{C=O}$ double bond) at the second carbon.

The "3-en" tells us there is a carbon-carbon double bond starting at the third carbon.

Finally, "4-methyl" means there is a methyl (${\text{-CH}}_3$) branch attached to the fourth carbon.

When we draw this out, we get a beautifully branched, conjugated system: ${\text{CH}}_3-\text{C}(=\text{O})-\text{CH}=\text{C}({\text{CH}}_3{)}_2$.

Now, the question asks us to count the C-C sigma ($\sigma$) bonds. Let's quickly recall the fundamental rules of chemical bonding.

Every single direct connection between two atoms is a sigma bond. It is the strongest type of covalent bond, formed by the head-on overlapping of atomic orbitals.

If two atoms are connected by a double bond, the first bond is a sigma bond, and the second is a pi ($\pi$) bond, formed by the lateral overlap of p-orbitals.

Therefore, whether we see a single bond or a double bond between two carbon atoms, it contributes exactly one sigma bond to our count.

Let's systematically scan our drawn structure from left to right and count the C-C connections.

First, the methyl group (${\text{C}}_1$) is attached to the carbonyl carbon (${\text{C}}_2$) via a single bond. That is our first $\sigma$ bond.

Next, the carbonyl carbon (${\text{C}}_2$) is attached to the alkene carbon (${\text{C}}_3$) via another single bond. That is our second $\sigma$ bond. (Remember, we ignore the $\text{C=O}$ bond because we only care about carbon-carbon bonds!)

Moving along, we encounter the double bond between ${\text{C}}_3$ and ${\text{C}}_4$. As we discussed, a double bond contains exactly one sigma bond. This gives us our third $\sigma$ bond.

Finally, we look at ${\text{C}}_4$. It is attached to two separate methyl groups (${\text{C}}_5$ and ${\text{C}}_6$) via single bonds. These two connections give us our fourth and fifth $\sigma$ bonds.

Summing them all up, we find that mesityl oxide contains a total of 5 carbon-carbon sigma bonds.

It is a straightforward counting exercise, but it beautifully tests your ability to translate a common name into a precise chemical structure!