Example 1 — naming a branched alcohol
Give the correct name of .
- Longest chain containing the carbon has carbons: pentane base.
- Number to give the lowest locant: on carbon .
- A methyl branch sits on carbon .
Answer: 4-methylpentan-2-ol
Definition
Fermentation: sugar (glucose) from a renewable source is converted to ethanol and carbon dioxide by yeast enzymes: . Best conditions: warm temperature (around –C, warm enough for the enzymes to work well but not so hot they're destroyed), and anaerobic conditions (absence of oxygen, since oxygen would let the yeast respire aerobically instead of fermenting, and could also oxidize the ethanol to ethanoic acid/vinegar).
Ethanol is obtained as a by-product in the sugar industry from the leftover molasses (a sugar-rich residue left after sugar crystals have been extracted from sugarcane juice), which still contains enough sugar to ferment into ethanol.
Definition
Catalytic hydration of ethene: ethene reacts directly with steam, using a catalyst (commonly phosphoric acid, ; another suitable catalyst is a solid acid catalyst / silicon dioxide with phosphoric acid), at a high temperature and high pressure: .
This method uses a non-renewable starting material (ethene, derived from petroleum, via cracking), unlike fermentation which uses a renewable sugar source.
Definition
Comparing the two methods: catalytic hydration of ethene is a faster, continuous process giving purer ethanol product, but needs a non-renewable raw material and high energy input (high temperature/pressure) plus specialised equipment. Fermentation is slower (a batch process) and gives a relatively dilute, impure ethanol (needing further purification by distillation, since the reaction mixture also contains water, yeast, and other by-products), but uses a cheap, renewable raw material and needs only mild conditions.
Remember
We get relatively impure alcohol from fermentation mainly because the reaction mixture also contains water, unreacted sugar, dead yeast cells, and other fermentation by-products all mixed together — the ethanol must be separated out afterwards, typically by distillation, to reach a higher purity.
Two disadvantages of fermentation: it's a slow, batch process (can't run continuously the way catalytic hydration can), and it produces a dilute mixture that needs extra purification (distillation) before use.
Definition
Ethanol's properties suit the cosmetic industry: it's a good solvent (dissolves many oils, fragrances, and other cosmetic ingredients), evaporates quickly (leaving skin feeling dry rather than greasy), and has an antiseptic effect — these combined properties make it useful in perfumes, lotions, and other cosmetic products.
Ethanol is used to clean wounds specifically because of its antiseptic (germ-killing) property, which helps prevent infection.
Ethanol is important for the pharmaceutical industry as a solvent for many drugs and medicinal preparations (since many active ingredients dissolve well in it but not in water), and as an antiseptic/disinfectant itself.
Ethanol (as an additive to petrol, and also as a feedstock for making vinegar/ethanoic acid and many other organic compounds) is added to petrol to improve combustion — it's the alcohol referenced when petrol contains an ethanol blend, boosting the octane rating and burning more cleanly and completely than petrol alone.
Definition
The heat evolved during combustion of alcohols increases going from lower (shorter-chain) to higher (longer-chain) alcohols, because a longer carbon chain has more C–C and C–H bonds to break and reform during combustion, releasing correspondingly more energy overall — this is also exactly why the alcohol with the maximum heat of combustion is the one with the longest carbon chain among those compared.
During combustion of alcohols, C–C, C–H, and O–H bonds in the alcohol (and O=O bonds in oxygen) break, while new C=O bonds (in ) and O–H bonds (in ) form — the reaction releases energy overall because the new bonds formed are stronger (more stable) than the bonds broken.
Bioalcohols (like ethanol from fermenting plant sugars) are considered environmentally friendly and are often called 'carbon neutral' because the released when they're burned is roughly balanced by the the source plants originally absorbed from the atmosphere during their growth (via photosynthesis) — so burning them doesn't add new, previously-locked-away carbon to the atmosphere the way burning fossil fuels does.
Definition
Two advantages of burning alcohols over fossil fuels: they're renewable (can be continually regrown/fermented, unlike finite fossil fuels), and they burn more cleanly, producing less soot/particulate pollution and being closer to carbon-neutral overall.
Two disadvantages of using alcohols as fuel: they have a lower energy content (heat of combustion) per unit volume/mass compared to petrol/diesel, meaning more fuel is needed for the same energy output; and producing them (especially by fermentation) can compete with food crop production for agricultural land.
Quick Recap
- ●Naming: longest chain through carbon lowest locant for (e.g. 4-methylpentan-2-ol)
- ●Fermentation: ; warm, anaerobic conditions; renewable, slow, impure product
- ●Catalytic hydration: ; high T/P; non-renewable, fast, purer product
- ●Ethanol uses: solvent quick-evaporating antiseptic cosmetics/pharma/wound cleaning; petrol additive/vinegar feedstock
- ●Longer-chain alcohols release more combustion heat (more bonds to break/form); alcohol combustion breaks C–C/C–H/O–H/O=O, forms C=O/O–H
- ●Bioalcohols are roughly carbon-neutral (absorbed CO released CO); pros: renewable, cleaner; cons: lower energy density, competes with food crops for land
