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OCR A Level Chemistry A H432 — Complete Chapter Practice

1. Development of Practical Skills in Chemistry

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2. Foundations in Chemistry

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3. Periodic Table and Energy

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4. Core Organic Chemistry

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5. Physical Chemistry and Transition Elements

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6. Organic Chemistry and Analysis

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Text lesson

4.4 Alcohols

4.4 Alcohols — OCR A LEVEL CHEMISTRY A H432 — COMPLETE CHAPTER PRACTICE notes: Explain the properties and classification of alcohols and describe their combustion, oxidation, elimination and substitution reactions.

Structure and hydrogen bonding
Alcohols
Alcohols compounds containing at least one hydroxy (–OH) group, with general formula CnH2n+1OH
Hydrogen bonding
  • Hydrogen bonding it forms between the δ+ H of one molecule and a lone pair on the oxygen of another
  • The O–H bonding electrons are pulled towards the very electronegative oxygen, making H slightly positive.
Boiling points
  • Boiling points alcohols boil higher than the corresponding alkanes
  • Alkanes have only temporary induced dipole–dipole forces, and hydrogen bonds are much stronger.
  • Propane boils at −42 °C and propanol at 97 °C.
Volatility
  • Volatility alcohols have lower volatility than alkanes of similar molecular mass because the stronger hydrogen bonds need more energy to overcome
Chain length
  • Chain length boiling points rise as the carbon chain lengthens
  • More electrons give larger temporary induced dipole–dipole forces.
Solubility and classification
Solubility
  • Solubility small alcohols are completely soluble in water and form one solution
  • Solubility falls as the hydrocarbon chain lengthens.
  • The –OH end forms new hydrogen bonds with water, but the hydrocarbon chain cannot and breaks hydrogen bonds between water molecules.
Primary alcohol
  • Primary alcohol the carbon bonded to –OH is attached to one other carbon (or alkyl group)
Secondary alcohol
  • Secondary alcohol the carbon bonded to –OH is attached to two other carbons
    • (or alkyl groups)
Tertiary alcohol
  • Tertiary alcohol the carbon bonded to –OH is attached to three other carbons
    • (or alkyl groups)
1Complete combustion
  • Complete combustion alcohol + oxygen → carbon dioxide + water
  • Lower alcohols burn with an almost invisible flame and make good fuels.
2Ethanol fuel
  • Ethanol fuel it can be produced sustainably by fermenting sugars
  • Energy density is the energy released in kJ per kg of fuel, and ethanol’s is lower than gasoline’s.
  • Ethanol-fuelled cars therefore need larger tanks or more frequent refuelling.
3Blending with petrol or diesel
  • Blending with petrol or diesel raises energy density and makes flames more visible in fires, improving safety
4Socio-economic concern
  • Socio-economic concern using farm land to grow fuel crops competes with land used for food
Alcohol Oxidation product
Primary Aldehyde, then carboxylic acid on further oxidation
Secondary Ketone only
Tertiary No oxidation
  • Oxidising agents
    • Acidified potassium dichromate(VI), K2Cr2O7, is orange and is reduced to green Cr3+.
    • Acidified potassium manganate(VII), KMnO4, is purple and is reduced to colourless Mn2+.
    • The colour changes show the oxidising agent has been reduced while the alcohol is oxidised.
Aldehyde
  • Aldehyde gently heat a primary alcohol with the oxidising agent
  • The aldehyde has a lower boiling point than the alcohol, so it is distilled off as it forms.
Carboxylic acid
  • Carboxylic acid if the aldehyde is not distilled off, reflux with excess oxidising agent
  • Further oxidation of the aldehyde gives the carboxylic acid.
Ketone
  • Ketone gently heat a secondary alcohol with the oxidising agent
  • Propan-2-ol gives propanone with acidified dichromate.
  • Ketones cannot be oxidised further, so the product need not be distilled off.
Reaction
  • Reaction an alcohol loses water to form an alkene, an elimination reaction
  • The water forms from the –OH group and a hydrogen on the adjacent carbon.
  • The products are the alkene and water as a small molecule by-product.
Conditions
  • Pass the alcohol vapour over hot aluminium oxide (Al2O3) powder or porous pot.
  • Alternatively heat the alcohol with excess hot concentrated sulfuric or phosphoric acid.
Collection
  • Collection ethanol over hot aluminium oxide gives ethene gas, which is collected over water
Reaction
  • Reaction the –OH group of the alcohol is replaced by a halogen to form a haloalkane
In situ reagent
  • In situ reagent the hydrogen halide can be made during the reaction
  • KBr with H2SO4 or H3PO4 produces HBr.
  • The HBr reacts with the alcohol to form the bromoalkane.
Term Meaning
Alcohol A compound containing the –OH functional group
Primary alcohol An alcohol whose –OH carbon is attached to one other carbon
Hydrogen bond Attraction between δ+ H on O, N or F and a lone pair on O, N or F
Reflux Heating with a condenser so vapour returns to the flask
Elimination reaction A reaction in which a small molecule is removed from a larger molecule
Energy density Energy released per kg of fuel
1

Saying alcohols have lower boiling points than alkanes — hydrogen bonding makes them higher
2

Saying tertiary alcohols oxidise to ketones — they do not oxidise
3

Distilling instead of refluxing to make a carboxylic acid — reflux with excess oxidant is needed
4

Saying the dichromate turns blue — orange Cr2O72− turns green Cr3+
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4.4 Alcohols

OCR A Level Chemistry A H432 — Complete Chapter Practice / 4. Core Organic Chemistry

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