Quick Summary
Here's the whole chapter in one scan before you dive into the detail:
What an alcohol is
CnH2n+1OH — a hydrocarbon chain plus a hydroxyl (–OH) group.
Primary, secondary, tertiary
Classified by how many carbons are attached to the C bonded to –OH.
Combustion
Alcohol + O₂ → CO₂ + H₂O (complete combustion).
Halogenation
–OH is swapped for –Cl, –Br or –I using different reagents for each.
Dehydration
Heat with concentrated phosphoric acid → alkene + water.
Oxidation
1° → aldehyde → carboxylic acid. 2° → ketone. 3° → no reaction.
1. Alcohols — Introduction
1 What actually makes something an "alcohol"?
Every alcohol contains a hydroxyl functional group, –OH, bonded to a carbon chain. That's
the whole definition. The general formula is CnH2n+1OH,
which is exactly the formula for an alkane (CnH2n+2) but with one hydrogen swapped out
for an –OH group. That's a useful way to think about it: alcohol = alkane skeleton + one H replaced by OH.
Naming follows the pattern alkane name + "ol" — so the 2-carbon alkane ethane becomes
ethanol, propane becomes propanol, and so on. If a molecule has
two –OH groups, it's called a diol (and with three, a triol).
| Alcohol | Structural formula | Carbons |
|---|
| Methanol | CH₃OH | 1 |
| Ethanol | CH₃CH₂OH | 2 |
| Propanol | CH₃CH₂CH₂OH | 3 |
| Butanol | CH₃CH₂CH₂CH₂OH | 4 |
Think of it like this
If alkanes are the "boring but stable" backbone of organic chemistry, the –OH group is like a little
handle bolted onto that backbone. Nearly every reaction in this chapter is really a reaction happening
at that handle — the rest of the carbon chain is basically just watching.
2 Primary, secondary, tertiary — and why it matters
This classification is one of the most important ideas in the whole chapter, because it controls
which reactions can happen later (especially oxidation, and the HCl chlorination shortcut). The rule
is simple: look at the carbon atom that the –OH is directly bonded to, and count how many other carbons
are attached to it.
- Primary (1°) alcohol — the OH-bearing carbon is attached to one other carbon (or none, as in methanol). Example: propan-1-ol, CH₃CH₂CH₂OH.
- Secondary (2°) alcohol — the OH-bearing carbon is attached to two other carbons. Example: propan-2-ol, CH₃CH(OH)CH₃.
- Tertiary (3°) alcohol — the OH-bearing carbon is attached to three other carbons. Example: 2-methylpropan-2-ol, (CH₃)₃COH.
2. Reactions of Alcohols
1 Combustion
3. Oxidation of Alcohols
1 The oxidation "ladder"
into
- Primary alcohols
aldehydes
carboxylic acids
- Secondary alcohols
ketones only
- Tertiary alcohols
do not oxidise at all