Introduction to Alcohols
Welcome to Topic 6E: Alcohols! You might already know alcohols from everyday life (like the ethanol in hand sanitizers), but in organic chemistry, they are one of the most versatile "functional groups." Think of alcohols as the chemical "middle ground"—we can turn them into halogenoalkanes, alkenes, aldehydes, ketones, or carboxylic acids. In this chapter, we will look at how to transform alcohols and the practical skills you need to do it safely in a lab.
Note: If you need a refresher on naming alcohols or structural isomerism, check out Topic 6A.
1. Making Halogenoalkanes from Alcohols
Replacing the hydroxyl group (\(-OH\)) with a halogen atom is a classic reaction. The reagents we use depend on which halogen (\(Cl\), \(Br\), or \(I\)) we want to attach.
A. Chlorination (Adding Chlorine)
To make a chloroalkane, we react an alcohol with phosphorus(V) chloride (\(PCl_5\)). This reaction happens at room temperature and is quite vigorous!
The Equation:
\(ROH + PCl_5 \rightarrow RCl + POCl_3 + HCl\)
Why is this useful?
It produces misty fumes of hydrogen chloride (\(HCl\)) gas. Because of these visible fumes, \(PCl_5\) is often used as a chemical test to see if a mystery liquid contains an \(-OH\) group.
B. Bromination (Adding Bromine)
To make a bromoalkane, we usually make the reagent "in situ" (inside the reaction flask). We use a mixture of potassium bromide (\(KBr\)) and 50% concentrated sulfuric acid (\(H_2SO_4\)).
How it works:
1. The \(KBr\) and \(H_2SO_4\) react first to make \(HBr\).
2. The alcohol then reacts with that \(HBr\):
\(ROH + HBr \rightarrow RBr + H_2O\)
C. Iodination (Adding Iodine)
Making iodoalkanes is a bit different. We react the alcohol with red phosphorus and iodine (\(I_2\)). The reaction is heated under reflux.
How it works:
The phosphorus and iodine react first to create phosphorus(III) iodide (\(PI_3\)), which then reacts with the alcohol:
\(3ROH + PI_3 \rightarrow 3RI + H_3PO_3\)
Quick Review:
- Chlorine: Use \(PCl_5\).
- Bromine: Use \(KBr\) + \(50\% \ H_2SO_4\).
- Iodine: Use Red Phosphorus + \(I_2\).
2. Oxidation of Alcohols
Oxidation is where things get interesting! The product we get depends on the "class" of the alcohol (Primary, Secondary, or Tertiary). We use acidified potassium dichromate(VI) as the oxidizing agent. In equations, we represent this agent as \([O]\).
The Setup: \(K_2Cr_2O_7\) dissolved in dilute \(H_2SO_4\).
The Colour Change: The solution turns from Orange (Dichromate ion, \(Cr_2O_7^{2-}\)) to Green (Chromium(III) ion, \(Cr^{3+}\)).
Primary Alcohols (\(1^{\circ}\))
Primary alcohols can be oxidized in two stages:
- To an Aldehyde: Use distillation to remove the product as soon as it forms. This prevents further oxidation.
\(CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O\) - To a Carboxylic Acid: Use reflux and excess oxidizing agent to ensure the alcohol is fully oxidized.
\(CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O\)
Secondary Alcohols (\(2^{\circ}\))
Secondary alcohols oxidize to Ketones. Even if you keep heating them under reflux, they won't oxidize further.
\(CH_3CH(OH)CH_3 + [O] \rightarrow CH_3COCH_3 + H_2O\)
Tertiary Alcohols (\(3^{\circ}\))
Tertiary alcohols cannot be oxidized by acidified dichromate because they don't have a hydrogen atom on the carbon attached to the \(-OH\) group. The solution stays Orange.
Don't worry if this seems tricky: Just remember that Distillation = Aldehyde and Reflux = Carboxylic Acid for primary alcohols!
3. Elimination (Dehydration)
In an elimination reaction, we remove a molecule of water from the alcohol to create a carbon-carbon double bond (\(C=C\)), forming an alkene.
Reagent: Concentrated phosphoric acid (\(H_3PO_4\)).
Condition: Heat.
The Equation:
\(CH_3CH_2OH \xrightarrow{H_3PO_4} CH_2=CH_2 + H_2O\)
Did you know? This is the reverse of the reaction used to make ethanol from ethene and steam (Topic 6C)!
4. Practical Skills for Organic Chemistry
When you synthesize an organic liquid like an alcohol or a halogenoalkane, you need to know how to purify it. These techniques are vital for Paper 3.
A. Reflux
Reflux allows you to heat a reaction for a long time without losing your volatile reactants or products as vapor. The vapor rises, hits a cold condenser, turns back into a liquid, and drips back into the flask.
B. Distillation
Used to separate liquids with different boiling temperatures. It's especially useful for separating an aldehyde from a primary alcohol during oxidation.
C. Extraction (Using a Separating Funnel)
If you have a mixture of an organic liquid and water, you use a separating funnel. The two liquids form layers (immiscible). You open the tap to run off the bottom (denser) layer and keep the top layer.
D. Drying the Product
After extraction, your organic liquid might still contain traces of water. We add an anhydrous salt (like \(MgSO_4\) or \(CaCl_2\)) which acts as a drying agent. It clumps together as it absorbs water; you keep adding it until the liquid looks clear and the powder flows freely.
E. Boiling Temperature Determination
How do you know if your product is pure? You measure its boiling point using distillation apparatus. If the liquid boils over a narrow range at the temperature stated in data books, it is pure. If the range is wide or the temperature is wrong, it contains impurities.
Key Takeaway Box:
- Oxidation: \(1^{\circ} \rightarrow\) aldehyde or acid; \(2^{\circ} \rightarrow\) ketone; \(3^{\circ} \rightarrow\) no reaction.
- Dehydration: Alcohol \(\rightarrow\) Alkene + Water (using \(H_3PO_4\)).
- Purification: Separate (funnel) \(\rightarrow\) Dry (anhydrous salt) \(\rightarrow\) Distil (purity check).