An OH group makes alcohols hydrogen-bond, so they boil higher and mix with water. It also gives them a menu of reactions: with sodium, with halogenating agents, dehydration to alkenes and esterification with acids.
Alcohols contain the –OH group. Hydrogen bonding between molecules gives them much higher boiling points than alkanes of similar Mr, and lets short-chain alcohols mix with water. They are made by hydrating alkenes, hydrolysing halogenoalkanes, reducing carbonyl compounds or hydrolysing esters. They react with sodium to give an alkoxide and hydrogen (more slowly than water, as alcohols are weaker acids); with HX, PCl5 or SOCl2 to give halogenoalkanes; with hot concentrated acid or Al2O3 by dehydration to alkenes; and with carboxylic acids, using a concentrated H2SO4 catalyst, to form esters.
Sort alcohols into primary, secondary and tertiary in the simulator, then switch to the hydrogen bonding view and compare boiling points with alkanes.
| Compound | Mr | b.p. / °C | Main intermolecular force |
|---|---|---|---|
| propane | 44 | −42 | id-id |
| ethanol | 46 | 78 | hydrogen bonding |
| butane | 58 | −1 | id-id |
| propan-1-ol | 60 | 97 | hydrogen bonding |
Run each reaction of ethanol in the simulator and note the observations.
| Reagent | Equation | Observation |
|---|---|---|
| sodium | 2C2H5OH + 2Na → 2C2H5ONa + H2 | steady fizzing, gentler than with water |
| PCl5 | C2H5OH + PCl5 → C2H5Cl + POCl3 + HCl | steamy white fumes of HCl (a test for –OH) |
| hot conc. H2SO4 or Al2O3 | C2H5OH → C2H4 + H2O | gas that decolourises bromine water |
| CH3COOH, conc. H2SO4, heat | CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O | fruity smell of ethyl ethanoate |
Alcohols react with sodium more slowly than water does, because they are weaker acids: the ethyl group pushes electron density towards O, making O–H harder to break. In esterification, the concentrated H2SO4 is a catalyst and the reaction is reversible, so yields are well below 100%.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Why does ethanol (Mr 46) boil at 78 °C but propane (Mr 44) at −42 °C?
What gas is produced when sodium is added to ethanol?
What is observed when PCl5 is added to an alcohol?
Which catalyst is used in esterification?
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
How many different alkenes, including stereoisomers, form when butan-2-ol is dehydrated?
6.00 g of ethanoic acid (Mr 60.0) reacts with excess ethanol to give 5.28 g of ethyl ethanoate (Mr 88.0). What is the percentage yield?
Why does sodium react less vigorously with ethanol than with water?
What volume of hydrogen at room conditions is released when 2.30 g of ethanol (Mr 46.0) reacts with excess sodium?
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