10.4. Methods of Preparation

Today, we’re discussing how to prepare haloalkanes from alcohols. The reaction uses hydrogen halides like HX along with a catalyst like ZnCl₂.

So the alcohol gets converted into a haloalkane? Can you explain how that works?

Absolutely! The general reaction is ROH + HX → RX + H₂O. The alcohol reacts with the hydrogen halide producing a haloalkane and water.

Is there a specific type of alcohol that works best for this reaction?

Generally, secondary and tertiary alcohols react more readily than primary ones due to sterics. Remember, primary alcohols might need stronger conditions!

Is the reaction considered nucleophilic substitution?

Yes! The reaction proceeds through nucleophilic substitution where the halide ion acts as a nucleophile. It's a key concept. Let's summarize: alcohols convert to haloalkanes using HX.

Next, we'll focus on alkanes. How do you think we can prepare haloalkanes from them?

Is it through a radical reaction using UV light?

Correct! The reaction CH₄ + Cl₂ under UV light produces CH₃Cl and HCl. This process involves free radical halogenation where radical species are generated.

What do we mean by free radical?

Free radicals are atoms or molecules with unpaired electrons. They're essential in this reaction for propagating the halogenation process. Just remember: radical = unpaired!

Can this happen with any alkane?

Yes, but reactivity differs; for example, tertiary alkanes react faster than secondary or primary due to stability. Let's summarize: alkanes can be halogenated using free radicals!

Now we’ll cover how alkenes can be transformed into haloalkanes. There are two main pathways; can anyone name them?

Addition of HX and halogenation?

Exactly! For HX addition, we follow Markovnikov's rule. For example, adding HBr to ethylene gives you CH₃CH₂Br.

And for halogenation, we directly add Br₂, right?

That's correct! The resulting product is a vicinal dibromide. Just remember the distinction: HX leads to haloalkanes while Br₂ gives you dihalides.

How do we know which addition proceeds?

Markovnikov's rule is your guiding principle for HX — it instructs which carbon atom receives the halogen. Always check the structure! To summarize, alkenes are versatile in reactions, producing haloalkanes and dihalides.

Let's conclude with preparing haloarenes. What do you know about aromatic compounds in this context?

They react through electrophilic substitution, don't they?

Correct! For example, benzene reacts with Cl₂ in the presence of FeCl₃ to form chlorobenzene. Does anyone know the significance of the catalyst?

I think it helps to form the electrophile, right?

Spot on! The catalyst iron(III) chloride generates the electrophile required for substitution. Remember, electrophilic substitution is key for aromatic chemistry. To summarize, aromatic compounds create haloarenes via electrophilic substitution.