2.8Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Organic Chemistry: Alcohols — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 10 parts Lesson 8 of 53 1 quiz
Curriculum Equation Key Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Lesson summary
2.8 Organic Chemistry: Alcohols — classification, reactions and oxidation — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Classify alcohols, describe combustion, halogenation and dehydration, and explain the oxidation of primary, secondary and tertiary alcohols and the tests for aldehyde…
- Fehling's solution
- Tollens' reagent
- Primary alcohol
- Oxidation (organic)
- Heating under reflux
- Functional group
Part 1 of 8Alcohols and their classification
Alcohols and their classification
Alcohols contain the hydroxyl group, -OH, with general formula CnH2n+1OH, named alkan + ol; a molecule with two -OH groups is a diol.
Primary
Classified by the number of carbons attached to the C-OH carbon.
- Primary
- one other carbon (or alkyl group), e.g. ethanol.
- Secondary
- two other carbons, e.g. propan-2-ol.
- Tertiary
- three other carbons, e.g. 2-methylpropan-2-ol.
The polar O-H group forms hydrogen bonds with other alcohol molecules and with water.
Alcohols have higher boiling points than alkanes of similar Mr,
- and short-chain alcohols are soluble in water; solubility falls as the chain lengthens.
Part 2 of 8Combustion of alcohols
Combustion of alcohols
Alcohols burn completely to carbon dioxide and water: alcohol + oxygen → CO2 + H2O.
C2H5OH + 3O2 → 2CO2 + 3H2O, with a clean blue flame.
Part 3 of 8Conversion to halogenoalkanes
1
Chlorination with PCl5
- Added to the alcohol, it reacts vigorously at room temperature, so no heating is needed.
- CH3CH2CH2OH + PCl5 → CH3CH2CH2Cl + POCl3 + HCl.
- Steamy fumes of HCl are a qualitative test for an -OH group.
2
Tertiary alcohols can be chlorinated by shaking with concentrated HCl at room temperature.
- (CH3)3COH + HCl → (CH3)3CCl + H2O.
- This does not work for primary or secondary alcohols.
3
Bromination with KBr and 50% concentrated H2SO4, warmed
- 2KBr + H2SO4 → K2SO4 + 2HBr.
- The HBr then reacts with the alcohol: CH3CH2CH2CH2OH + HBr → CH3CH2CH2CH2Br + H2O.
- More concentrated acid would oxidise bromide ions to bromine.
4
Iodination with red phosphorus and iodine, heated under reflux
- 2P + 3I2 → 2PI3.
- 3C2H5OH + PI3 → 3C2H5I + H3PO3.
Part 4 of 8Dehydration to alkenes
1
Heat the alcohol with concentrated phosphoric(V) acid.
2
The OH and a hydrogen on the adjacent carbon are removed, forming C=C: CH3CH2OH → CH2=CH2 + H2O.
3
The acid acts as a catalyst and is left out of the equation because the water formed dilutes it.
4
Unsymmetrical alcohols can give more than one alkene.
Part 5 of 8Oxidation of alcohols
1
Primary alcohols oxidise to aldehydes, then to carboxylic acids.
2
Secondary alcohols oxidise to ketones only.
3
Tertiary alcohols are not oxidised because there is no hydrogen on the C-OH carbon.
4
The oxidising agent is acidified potassium dichromate(VI), K2Cr2O7/H2SO4.
- Dichromate is reduced to green Cr3+; H+ from the acid is needed.
- Orange turns green if oxidation occurs; with a tertiary alcohol it stays orange.
| Alcohol | Conditions | Product | Colour change |
|---|---|---|---|
| Primary | Distil as it forms | Aldehyde | Orange to green |
| Primary | Excess oxidant, reflux | Carboxylic acid | Orange to green |
| Secondary | Reflux | Ketone | Orange to green |
| Tertiary | Heat | No reaction | Stays orange |
Oxidation of alcohols
CH3CH2OH + [O] → CH3CHO + H2O and CH3CH2OH + 2[O] → CH3COOH + H2O.
CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O.
Part 6 of 8Reflux and distillation with addition
Reflux and distillation with addition
Heating under reflux gives full oxidation.
- It produces a carboxylic acid from a primary alcohol or a ketone from a secondary alcohol.
- A vertical condenser returns vapours to the flask, so products stay in the reaction mixture.
Distillation with addition gives an aldehyde.
- Only the oxidising agent is heated while the alcohol is added slowly.
- The aldehyde has a much lower boiling point than the alcohol,
- because aldehyde molecules have no hydrogen bonding,
- so it distils off immediately and is not oxidised further.
A ketone does not need to be distilled off straight away because it cannot be oxidised further.
Part 7 of 8Testing for aldehydes
Fehling's solution
- Fehling's solution alkaline Cu2+; an aldehyde is oxidised to a carboxylate and Cu2+ is reduced to Cu+.
- The blue solution forms an opaque red precipitate of copper(I) oxide, Cu2O.
Tollens' reagent
- Tollens' reagent ammoniacal silver nitrate; an aldehyde is oxidised and Ag+ is reduced to Ag.
- A silver mirror forms on the inside of the tube.
Ketones cannot be oxidised, so they give a negative result with both reagents.
In alkaline conditions the carboxylic acid formed is neutralised to its carboxylate salt.
Part 8 of 8More from the specification
- Functional group
- The atom or group of atoms that determines the reactions of a series.
Key terms
| Term | Meaning |
|---|---|
| Alcohol | An organic compound containing the -OH group on a saturated carbon. |
| Primary alcohol | An alcohol whose C-OH carbon is attached to one other carbon. |
| Tertiary alcohol | An alcohol whose C-OH carbon is attached to three other carbons. |
| Oxidation (organic) | Gain of oxygen or loss of hydrogen, shown as [O]. |
| Dehydration | Elimination of water from an alcohol to form an alkene. |
| Heating under reflux | Heating with a vertical condenser so vapours return to the flask. |
| Fehling's solution | Alkaline Cu2+ solution that gives a red precipitate with aldehydes. |
Avoid these mistakes
1
Giving the aldehyde from a primary alcohol under reflux — use distillation, otherwise the carboxylic acid forms.
2
Saying tertiary alcohols are oxidised — they have no H on the C-OH carbon and do not react.
3
Saying PCl5 requires heating — the reaction is vigorous at room temperature.
4
Using more concentrated sulfuric acid for bromination — it oxidises bromide to bromine.
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