Reactions and production of alcohols

Four reactions of alcohols

Dehydration: alcohol → alkene + water

Heating an alcohol with concentrated sulfuric or phosphoric acid (about 170–180 °C for ethanol) removes the OH and an H from the next carbon, forming a C=C. This is an elimination reaction, the reverse of hydration. The acid is a catalyst and also absorbs the water.

CH₃CH₂OH → CH₂=CH₂ + H₂O   (conc. H₂SO₄, heat)

If the OH is on an inner carbon, the H can come from either side, so butan-2-ol gives a mixture of but-1-ene and but-2-ene. Tertiary alcohols dehydrate most easily (at much lower temperatures) and primary alcohols least easily.

Substitution with hydrogen halides: alcohol → haloalkane

The OH group is replaced by a halogen atom:

CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O

Tertiary alcohols react fastest (2-methylpropan-2-ol reacts with concentrated HCl in minutes at room temperature, turning cloudy as the insoluble haloalkane forms), while primary alcohols need heating, often with HBr made in the flask from NaBr and H₂SO₄. HI is the most reactive hydrogen halide and HCl the least, because the H–I bond is the weakest.

Oxidation

Strong oxidising agents remove hydrogen from the C–OH carbon. The two used in the HSC are acidified potassium dichromate (Cr₂O₇²⁻, orange, reduced to green Cr³⁺) and acidified potassium permanganate (MnO₄⁻, purple, reduced to almost colourless Mn²⁺). The colour change is the observation that shows a reaction has happened.

• Primary alcohols are oxidised first to an aldehyde, then to a carboxylic acid. To stop at the aldehyde, distil it off as it forms (aldehydes boil lower than the alcohol because they can't hydrogen bond). To go all the way, heat under reflux with excess oxidant.
• Secondary alcohols are oxidised to a ketone, which resists further oxidation.
• Tertiary alcohols have no H on the C–OH carbon, so they are not oxidised under these conditions. The dichromate stays orange.

For full equations, combine half-equations. The organic half-equation is balanced with H₂O and H⁺:

C₂H₅OH + H₂O → CH₃COOH + 4H⁺ + 4e⁻
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

Making alcohols

• Hydration of alkenes (7E): CH₂=CH₂ + H₂O → CH₃CH₂OH, with steam over a phosphoric acid catalyst at about 300 °C and 6–7 MPa. Fast and continuous, but uses ethene from crude oil or gas.
• Substitution of haloalkanes: warming a haloalkane with aqueous sodium hydroxide swaps the halogen for OH: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. C–I bonds react fastest; C–F bonds are too strong to react.
• Fermentation: yeast enzymes convert sugars to ethanol and carbon dioxide in the absence of oxygen:

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)   (yeast, anaerobic, about 30–35 °C)

Conditions matter. Oxygen must be kept out, otherwise yeast respire aerobically to CO₂ and water, and bacteria can oxidise the ethanol to ethanoic acid (vinegar). Too hot and the enzymes denature; too cold and the reaction is slow. The yeast is poisoned once the ethanol reaches about 14–15%, so fuel ethanol is concentrated by fractional distillation. Starch and cellulose must first be hydrolysed to glucose.

Fossil fuels and biofuels

Fossil fuels (petrol, diesel, LPG, natural gas and coal) come from organisms buried millions of years ago; the syllabus calls them fuels from "inorganic sources" because they are dug out of the ground rather than grown. Biofuels are made from recently living biomass: bioethanol by fermenting sugar cane, grain or crop waste; biodiesel by reacting vegetable oils or animal fats with methanol (transesterification); and biogas, mostly methane, from anaerobic digestion of manure, sewage or landfill waste.

Both are hydrocarbons or hydrocarbon derivatives, both burn to CO₂ and water, and both can use the same pumps and engines when blended. NSW requires large fuel retailers to sell ethanol-blended fuel, and E10 (up to 10% ethanol) is sold at many service stations. Biofuels made from waste, such as biodiesel from used cooking oil or biogas from sewage, avoid the food-versus-fuel problem and are among the most promising.