Presentation on theme: "Unit 2 The World of Carbon. Fuels -substances which burn releasing energy. Petrol – from fractional distillation of crude oil and reforming of naphtha."— Presentation transcript:
Unit 2 The World of Carbon
Fuels -substances which burn releasing energy. Petrol – from fractional distillation of crude oil and reforming of naphtha fraction. Reforming produces - branched chain alkanes, - cycloalkanes - aromatic compounds. - decreases ‘knocking’ from auto-ignition. Lead compounds improve combustion but no longer used. Winter petrol blends have more volatile hydrocarbons than summer blends.
Alternative fuels Ethanol - from fermentation of sugar cane; renewable. Methanol - from synthesis gas (from steam reforming of CH 4 ) Advantages: virtually complete combustion; no aromatic carcinogens; cheaper than petrol; less explosive than petrol Disadvantages: absorbs water making in corrosive to engine; toxic; increases greenhouse gases unless CH 4 from biogas Methane- renewable if formed by anaerobic fermentation of organic waste e.g. manure; main constituent of biogas. Hydrogen – can be produced by electrolysis of water using solar energy; clean burning, only produces water; difficult to store and distribute.
Hydrocarbons Compounds of hydrogen and carbon only Homologous series: families of compounds which share general formula have similar chemical properties (react the same) show regular changes in physical properties with increasing size (by CH 2 each time)
Hydrocarbons Alkanes C n H 2n+2 single C-C bonds, saturated slow, substitution reactions Cycloalkanes C n H 2n single C-C bonds, saturated cyclic compounds slow, substitution reactions Alkenes C n H 2n at least 1 double C=C bond, unsaturated fast, addition reactions
Hydrocarbons Alkynes C n H 2n-2 at least 1 triple C=C bonds, unsaturated fast, addition reactions Naming hydrocarbons Name indicates number of carbon atoms in molecule: meth-1, eth-2, prop-3, but-4, pent-5, hex-6, hept-7, oct-8 (monkeys eat peanut butter penguins hate hairy oranges) family e.g. -ane, -ene, -yne
Naming hydrocarbons Choose longest carbon chain, be sure to include functional group e.g. double bond Number carbons, if any branches are present keep numbers as low as possible (this might mean going from right to left!) e.g. Branches name indicates number of carbons -CH 3, methyl; -C 2 H 5, ethyl; -C 3 H 7, propyl etc Isomers – same molecular formula, different structures (moving double bond produces isomer)
Addition reactions Alkenes addition of a halogen (1) hydrogen (2) a hydrogen halide (3) water (4)
Addition reactions Alkynes two-stage process: alkyne→ alkene→ alkane possibility of isomers being produced. compared to an alkene, complete addition to an alkyne will require twice the quantity of halogen (1) hydrogen (2) hydrogen halide (3)
Aromatic compounds Benzene very important feedstock ‘mother’ of all aromatics C 6 H 6 does not decolourise bromine rapidly, hence saturated each carbon forms only 3 bonds, leaving a ‘spare’ electron the ‘spare’ electrons can move (delocalised) around the ring
Aromatic compounds basically benzene with at least 1 hydrogen substituted with another atom/group of atoms phenyl group, -C 6 H 5 benzene ring minus 1 hydrogen used in manufacture of, e.g., plastics (including Bakelite), explosives, drugs, dyes
Alcohols (Alkanols) contain the hydroxyl functional group, -OH names end in –ol e.g. methanol CH 3 OH; ethanol C 2 H 5 OH need to specify carbon with –OH from propanol onwards when naming, longest carbon chain must include –OH e.g. primary alcohols (with exception of methanol) have 1 C attached to –COH, secondary alcohols have 2C, tertiary have 3C attached to -COH
Reactions of alcohols made by catalytic hydration of alkenes or reduction of ketones and aldehydes dehydrated, using aluminium oxide as a catalyst, to alkenes undergo combustion producing carbon dioxide and water primary and secondary alcohols undergo partial oxidation with mild oxidising agents: - acidified potassium dichromate (H + /Cr 2 O 7 2-, orange to green) - acidified potassium permanganate (H + /MnO 4 -, purple to colourless) - hot copper (II) oxide (black to red)
Partial oxidation of alcohols primary alcohols partially oxidised to alkanals (aldehydes) which can be partially oxidised further to alkanoic acids secondary alcohols partially oxidised to alkanones (ketones), these cannot be further partially oxidised tertiary alcohols do no undergo partial oxidation
Alkanals (aldehydes) and alkanones (ketones) made by partial oxidation of primary and secondary alkanols (alcohols) (see previous slide) can be reduced to the alcohol from which they were made aldehydes can be further partially oxidised to alkanoic acids contain the carbonyl group, C=O carbonyl group polar as O greater electronegativity than C when naming, carbon chain must include carbonyl group number carbons from end closest to carbonyl group e.g.
Experimentally distinguishing aldehydes from ketones ketones are resistant to partial oxidation, because the carbonyl group is flanked by two C atoms alkanals (aldehydes) can be partially oxidised to the corresponding alkanoic acid