Alcohols and Ethers
Senior 5 · Chemistry
Alcohols and Ethers
Lesson 1 of 5
22 min
The World Health Organization publishes a recipe for hand rub that any pharmacy or hospital can make: ethanol at 80% by volume (or propan-2-ol at 75%), a little hydrogen peroxide to kill spores, and 1.45% glycerol to stop the skin drying out. Ethanol and propan-2-ol each have one –OH group; glycerol, propane-1,2,3-triol, has three. All three are alcohols, and the –OH group explains why they mix with water, why they evaporate slowly enough to work on the hands, and why glycerol is thick and sticky.
Definition
Alcohol: a compound in which an –OH (hydroxyl) group is bonded to an carbon atom. Alcohols with one –OH group have the general formula . Example: ethanol, . (When –OH is bonded directly to a benzene ring the compound is a phenol, with different properties: Senior 6.)
Primary, secondary and tertiary alcohols: the carbon carrying –OH is bonded to one, two or three other carbon atoms. Methanol counts as primary.
Diol, triol: an alcohol with two or three –OH groups, on different carbons. Example: ethane-1,2-diol, .
Hydrogen bond: a strong intermolecular attraction between a hydrogen atom bonded to N, O or F and a lone pair on an N, O or F atom of another molecule.
Replace the "-e" of the alkane with "-ol" and give the locant of the –OH: propan-1-ol, propan-2-ol. With several –OH groups, keep the "-e" and use "-diol", "-triol": propane-1,2,3-triol. Branches are prefixes: 2-methylpropan-2-ol, . A ring gives cyclohexanol. Alcohols show chain, position and optical isomerism, and are functional-group isomers of ethers (Lesson 5).
Figure 1.2: The eight alcohols C₅H₁₂O, with their classes. Three of them contain a chiral carbon.
The O–H bond is very polar (O 3.44, H 2.20 on the Pauling scale), and oxygen has two lone pairs. So alcohol molecules hydrogen-bond to each other.
Figure 1.1: Hydrogen bonding in ethanol. Each O–H hydrogen is attracted to a lone pair on the oxygen of a neighbouring molecule.
| Compound | Boiling point / °C | Solubility in water | |
|---|---|---|---|
| propane | 44 | −42 | insoluble |
| methoxymethane, | 46 | −24 | soluble |
| ethanol | 46 | 78 | mixes in all proportions |
| methanol | 32 | 65 | mixes in all proportions |
| propan-1-ol | 60 | 97 | mixes in all proportions |
| butan-1-ol | 74 | 118 | about 8 g per 100 g water |
| pentan-1-ol | 88 | 138 | about 2 g per 100 g water |
| ethane-1,2-diol | 62 | 197 | mixes in all proportions |
| propane-1,2,3-triol | 92 | 290 | mixes in all proportions |
Worked example
Problem. Name each alcohol and classify it: (a) ; (b) ; (c) ; (d) .
Solution. (a) butan-2-ol, secondary (and chiral). (b) 2-methylbutan-2-ol, tertiary. (c) 2-methylpropan-1-ol, primary: the –OH carbon has only one carbon neighbour. (d) A diol: propane-1,2-diol; one –OH is on a primary carbon and one on a secondary carbon.
Worked example
Problem. Ethanol and methoxymethane are isomers, . Ethanol boils at 78 °C and methoxymethane at −24 °C, yet both dissolve in water. Explain.
Solution. Both have the same mass and similar London forces. Ethanol has an O–H bond, so its molecules hydrogen-bond to each other, and much more energy is needed to separate them. Methoxymethane has no H on oxygen, so its molecules cannot hydrogen-bond to each other: only weaker dipole–dipole and London forces act. However, its oxygen still has lone pairs, which accept hydrogen bonds from water molecules, so it dissolves.
Warning
Do not draw the hydrogen bond to the H of the C–H bonds or along the O–H bond. It runs from the H of one O–H group to a lone pair on the O of another molecule, with O–H···O close to a straight line.
Tip
To compare boiling points, ask in turn: Can it hydrogen-bond? Then how many –OH groups? Then how long is the chain, and how branched?
Leah4sci: Physical Properties of Alcohol: Hydrogen Bonding, Solubility and Boiling Point (14 min). Watch how hydrogen bonding raises boiling points, and how chain length decides solubility in water.
Practice
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