Ketones (as exemplified by propanone)

Introduction to Ketones

Welcome to the world of Ketones! If you have ever used nail polish remover, you have already encountered the most famous ketone: propanone (also known as acetone). In this chapter, we will focus on ketones as part of our journey through Organic Chemistry. We will learn how to identify them, how they are named, and the specific chemical reactions they undergo according to your H1 Chemistry syllabus.

Don't worry if Organic Chemistry feels like a lot of symbols at first. Think of functional groups like "personality traits" for molecules. Once you recognize the trait, you can predict how the molecule will behave!

1. What is a Ketone?

A ketone is an organic compound that contains a carbonyl group. A carbonyl group consists of a carbon atom double-bonded to an oxygen atom: \( C=O \).

The Secret to Identifying Ketones:
In a ketone, the \( C=O \) group is "sandwiched" between two other carbon atoms. It is always found inside the carbon chain, never at the very end. If the \( C=O \) was at the end of the chain, it would be an aldehyde!

Naming Ketones (Nomenclature)

To name a ketone, we use the suffix -one (pronounced like "own").
For your syllabus, our star example is propanone.

• Prop- means there are 3 carbon atoms in the chain.
• -an- means it is based on an alkane (single bonds between carbons).
• -one tells us it is a ketone.

Structural Formula of Propanone: \( CH_3COCH_3 \)
Displayed Formula: Imagine a central Carbon double-bonded to an Oxygen. This central Carbon is also single-bonded to two \( -CH_3 \) (methyl) groups, one on the left and one on the right.

Quick Review:
• Functional Group: Carbonyl group \( (C=O) \).
• General Formula: \( R-CO-R' \) (where R and R' are alkyl groups).
• Key Example: Propanone \( (CH_3COCH_3) \).

Key Takeaway: Ketones are defined by a \( C=O \) group located between two carbon atoms. Propanone is the simplest possible ketone.

2. Physical Properties and Shapes

Before we jump into reactions, let's look at how these molecules are built. Because the Oxygen atom is more electronegative than the Carbon atom, the \( C=O \) bond is polar. This means the Oxygen pulls electrons toward itself, becoming slightly negative (\( \delta- \)), while the Carbon becomes slightly positive (\( \delta+ \)).

Molecular Shape

The carbon atom in the carbonyl group is bonded to three other atoms (the oxygen and two neighboring carbons). This creates a trigonal planar shape around that carbon atom, with bond angles of approximately 120°.

Analogy: Think of the central carbon as a fidget spinner with three arms spreading out flat on a table.

Did you know?
Because propanone is polar, it is excellent at dissolving other polar substances, which is why it's such a great solvent for removing paint and glue!

3. Chemical Reactions: Reduction

In the H1 syllabus, the most important reaction for ketones you need to know is Reduction. Reduction in organic chemistry often means adding Hydrogen atoms to a molecule.

From Ketone to Secondary Alcohol

When you reduce a ketone, it turns into a secondary (2°) alcohol.
A secondary alcohol is one where the \( -OH \) group is attached to a carbon that is bonded to two other carbon atoms.

Reagents and Conditions

There are two ways to achieve this reduction:

Method 1: Using \( LiAlH_4 \)
• Reagent: Lithium aluminium hydride (\( LiAlH_4 \)).
• Condition: In dry ether. (This is important because \( LiAlH_4 \) reacts violently with water!).

Method 2: Using Hydrogen Gas
• Reagent: Hydrogen gas (\( H_2 \)).
• Condition: Nickel (Ni) catalyst and heat.

The Equation for Propanone Reduction

Using the symbol \( [H] \) to represent the reducing agent:
\( CH_3COCH_3 + 2[H] \rightarrow CH_3CH(OH)CH_3 \)

The product is propan-2-ol.

Step-by-Step Breakdown:
1. The double bond of the \( C=O \) breaks.
2. One Hydrogen atom attaches to the Oxygen (making it an \( -OH \) group).
3. One Hydrogen atom attaches to the Carbon atom.
4. You now have a secondary alcohol!

Common Mistake to Avoid:
Students often forget that reducing a ketone always gives a secondary alcohol. Reducing an aldehyde gives a primary alcohol. Keep them separate in your mind!

Key Takeaway: Ketones (like propanone) are reduced to secondary alcohols (like propan-2-ol) using either \( LiAlH_4 \) in dry ether or \( H_2/Ni \).

4. Ketones vs. Aldehydes: Oxidation

You might be wondering: "Can we oxidize ketones?"
Under the standard conditions in your syllabus (using acidified \( K_2Cr_2O_7 \) or \( KMnO_4 \)), ketones cannot be oxidized.

This is a very important "negative" result. Because aldehydes can be oxidized to carboxylic acids but ketones cannot, we can use this to tell them apart in a lab.

• Aldehyde + Acidified \( K_2Cr_2O_7 \): Color changes from orange to green (oxidation occurs).
• Ketone + Acidified \( K_2Cr_2O_7 \): Remains orange (no reaction).

Memory Aid: "Ketones are Alone." They don't have a Hydrogen atom attached to the carbonyl carbon, so they don't want to react with oxidising agents!

Key Takeaway: Unlike aldehydes, ketones are resistant to oxidation. This difference is a key way to distinguish between the two functional groups.

Summary Checklist

Before you move on, make sure you can answer these questions:
1. Can I draw the structural formula for propanone? (\( CH_3COCH_3 \))
2. Do I know the reagents for reducing propanone? (\( LiAlH_4 \) in dry ether OR \( H_2/Ni \))
3. Do I know the product of that reduction? (Propan-2-ol)
4. Do I understand why ketones don't react with acidified \( K_2Cr_2O_7 \)? (They lack the \( H \) atom on the carbonyl carbon required for oxidation in this context.)

Don't worry if this seems tricky at first! Organic chemistry is like learning a new language. The more you "speak" the names and "write" the reactions, the more natural it will feel. Keep practicing those equations!