Welcome to "Colour by Design" - Organic Reactions

In this chapter, we are going to explore how organic chemistry is used to create the vibrant world of dyes and pigments. This is the "Colour by Design" (CD) storyline of your OCR Chemistry B course. We will look at why benzene is so special, how we can "swap" atoms on its ring to make new molecules, and how carbonyl compounds (like aldehydes and ketones) react to form the building blocks of many colorful substances.

Don't worry if organic mechanisms seem like a "spaghetti" of arrows at first! We will break them down into simple steps that make sense.

1. The Mystery of the Benzene Ring

In your earlier studies, you saw alkenes with double bonds. You might expect benzene (\(C_{6}H_{6}\)) to act like an alkene with three double bonds, but it is much more stable and "boring" in its reactivity. This stability is due to delocalisation.

What is Delocalisation?

In benzene, the electrons aren't stuck between two specific carbon atoms. Instead, they form a "cloud" or a "donut" of charge above and below the flat ring of carbons. We represent this with a circle inside a hexagon.

Evidence for the Benzene Structure

How do we know benzene isn't just a ring with three double bonds? We look at Enthalpy Changes of Hydrogenation:

  • If benzene had three double bonds, we would expect it to release about \(-360\text{ kJ mol}^{-1}\) when we add hydrogen.
  • In reality, it only releases \(-208\text{ kJ mol}^{-1}\).
  • The "Aha!" Moment: Benzene is \(152\text{ kJ mol}^{-1}\) more stable than expected. This is called the delocalisation energy.

Quick Review: Because benzene is so stable, it hates addition reactions (which would break the stable ring) and loves substitution reactions (where we swap a hydrogen for something else but keep the ring intact).

2. Electrophilic Substitution: Swapping on the Ring

An electrophile is an "electron-lover" (usually a positive ion). Because the benzene ring is a big cloud of negative electrons, it attracts these electrophiles.

Nitration (Making Nitrobenzene)

To put a nitro group (\(-NO_{2}\)) on benzene, we need a "nitrating mixture" of concentrated nitric acid (\(HNO_{3}\)) and concentrated sulfuric acid (\(H_{2}SO_{4}\)) at about \(50^{\circ}C\).

The sulfuric acid acts as a catalyst to produce the electrophile: the nitronium ion (\(NO_{2}^{+}\)).

Halogenation (Adding Bromine or Chlorine)

Unlike alkenes, benzene won't react with bromine water just by shaking it. It needs a halogen carrier (like \(FeBr_{3}\) or \(AlCl_{3}\)) to create a strong enough electrophile (\(Br^{+}\) or \(Cl^{+}\)).

Friedel-Crafts Alkylation and Acylation

These are fancy names for adding carbon chains to the benzene ring. They are vital for building complex dye molecules.
Example: Adding an "acyl" group (\(R-CO-\)) is called acylation. Again, you must use an \(AlCl_{3}\) catalyst.

Step-by-Step Mechanism:
1. The electron-rich ring "attacks" the positive electrophile (curly arrow from the circle to the electrophile).
2. A temporary, unstable intermediate forms (the ring "breaks" slightly, shown as a horseshoe shape with a + charge).
3. A hydrogen ion (\(H^{+}\)) leaves to restore the stable delocalised ring.

Key Takeaway: Benzene reactions = Electrophilic Substitution. Always look for a catalyst to "activate" the electrophile!

3. Making Dyes: Diazonium Compounds and Azo Dyes

Many of the colors in your clothes come from azo dyes. These contain the group \(-N=N-\).

Step 1: Making the Diazonium Ion

We start with phenylamine (benzene with an \(-NH_{2}\) group). We react it with nitrous acid (\(HNO_{2}\)).
Important Condition: This must be kept below \(5^{\circ}C\). If it gets too warm, the diazonium salt decomposes and you get bubbles of nitrogen gas instead of a dye!

Step 2: The Coupling Reaction

We take our diazonium salt and react it with a phenol or another aromatic amine. The two rings "couple" together via the nitrogen atoms.
Result: A brightly colored azo dye. The long system of delocalised electrons across both rings is what allows the molecule to absorb visible light.

Did you know? The more "spread out" (delocalised) the electrons are, the longer the wavelength of light absorbed, changing the color from yellow to orange to red!

4. Carbonyl Compounds: Aldehydes and Ketones

Carbonyls have a \(C=O\) bond. This bond is polar because Oxygen is much more electronegative than Carbon. This makes the Carbon slightly positive (\(\delta+\)).

Aldehyde vs. Ketone: How to remember?

  • Aldehydes: The \(C=O\) is at the end of the chain (it has at least one H attached). Think: "AL-de-hyde is at the edge."
  • Ketones: The \(C=O\) is in the middle of the chain. Think: "The KEY is in the middle of the lock."

Nucleophilic Addition with Cyanide (\(CN^{-}\))

Because the Carbon is \(\delta+\), it can be attacked by a nucleophile (a "nucleus-lover" with a lone pair of electrons).
When \(CN^{-}\) attacks a carbonyl, it forms a cyanohydrin.
Mechanism Hint: The arrow goes from the lone pair on the Carbon of the \(CN^{-}\) to the Carbon of the \(C=O\). The double bond then "breaks" and the electrons move to the Oxygen.

Telling them apart: Oxidation

Aldehydes are easy to oxidise into carboxylic acids, but ketones are stubborn and won't oxidise easily.

  • Tollens' Reagent: Aldehydes produce a "Silver Mirror" on the inside of the test tube. Ketones? No change.
  • Fehling’s Solution: Aldehydes change the blue solution to a brick-red precipitate. Ketones? Stay blue.
  • Acidified Dichromate: Aldehydes turn it from orange to green.

Quick Review: Aldehydes react to these tests because they can be oxidised. Ketones cannot.

5. Classifying Organic Reactions

The syllabus requires you to be able to label any reaction you see. Use this "cheat sheet":

  • Addition: Two molecules become one (e.g., Alkenes + Bromine).
  • Substitution: An atom/group is swapped for another (e.g., Benzene reactions).
  • Oxidation: Adding Oxygen or removing Hydrogen (e.g., Aldehyde \(\rightarrow\) Carboxylic Acid).
  • Reduction: Removing Oxygen or adding Hydrogen (e.g., Ketone \(\rightarrow\) Secondary Alcohol).
  • Condensation: Two molecules join and a small molecule (like \(H_{2}O\)) is kicked out (e.g., making azo dyes).
  • Hydrolysis: Using water to break a bond.

Key Takeaway: If you are stuck on a question asking for a "type of reaction," look at the "before and after" of the molecules to see if things were added, swapped, or if the molecule was broken apart!

Summary Checklist for your Exams:

1. Can you explain why benzene is more stable than its theoretical model? (Hydrogenation data!)
2. Do you know the reagents for nitrating benzene? (Conc \(HNO_{3}\) / Conc \(H_{2}SO_{4}\))
3. Can you draw the nucleophilic addition of \(CN^{-}\) to a carbonyl?
4. Do you know the color changes for Tollens' and Fehling's tests?

Don't worry if these mechanisms take a few tries to draw correctly. Practice makes perfect! Keep your arrows starting from a lone pair or a bond, and pointing exactly where the electrons are going.