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HOAÙ HOÏC HÖÕU CÔ. CHÖÔNG 6 (t.t) Alkanes. Organic Chemistry. 6.3. T ÍNH CHẤT CỦA ALKANES. 6.3.1. Ph ản ứng thế 6.3.2. Ph ản ứng oxy hóa 6.3.3. Ph ản ứng đồng phân hóa 6.3.4. PHẢN Ứng CRACKING. Oxidation of Alkanes. Combustion
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HOAÙ HOÏC HÖÕU CÔ CHÖÔNG 6 (t.t) Alkanes Organic Chemistry
6.3. TÍNH CHẤT CỦA ALKANES 6.3.1. Phản ứng thế 6.3.2. Phản ứng oxy hóa 6.3.3. Phản ứng đồng phân hóa 6.3.4. PHẢN Ứng CRACKING
Oxidation of Alkanes • Combustion • Antoine L. Lavoisier (1743-1794) was the first to demonstrate the true nature of combustion as an oxidation reaction and to give oxygen its modern name. • “In all combustion, pure air in which the combustion takes place is destroyed or decomposed and the burning body increases in weight exactly in proportion to the quantity of air destroyed or decomposed”
Stoechiometry • from Greek stoikheion element + -metry • proportions in which elements are combined in compounds and the quantitative relationships between reactants and products in chemical reactions CH4 + 2 O2 CO2 + 2 H2O C6H6 + 15/2 O26 CO2 + 3 H2O
Halogenation reaction • Alkanes undergo substitution reactions with halogens such as fluorine, bromine and chlorine in the presence of heat or light
Mechanism of Radical Reactions • The reaction mechanism has three distinct aspects: Chain initiation, chain propagation and chain termination • Initiation • Homolytic cleavage
Mechanism of Radical Reactions • Initiation and Propagation
Mechanism of Radical Reactions • Chain termination • Occasionally the reactive radical intermediates are quenched by reaction pathways that do not generate new radicals • The reaction of chlorine with methane requires constant irradiation to replace radicals quenched in chain-terminating steps
Mechanism of Radical Reactions • The order of reactivity of methane substitution with halogens is: Fluorine > chlorine > bromine > iodine • Monochlorination of alkanes proceeds to give some selectivity Tertiary hydrogens are somewhat more reactive than secondary hydrogens which are more reactive than primary hydrogens Tertiary, secondary and primary hydrogens
CHÆ SOÁ OCTAN ( OCTANE NUMBER (RON)) • Auto-ignition or pre-ignition. • the fuel-air mixture is compressed rapidly in the cylinder and gets hot; the compression ratio (the ratio of maximum minimum gas volumes in the cylinder) in a petrol engine is between 8:1 and 10:1 • the mixture may catch fire because of the heat generated by this adiabatic compression; or because of residual heat in the cylinder which is greater if solid products of incomplete combustion are present in the cylinder; • this auto-ignition is intentional in a diesel engine, but in a petrol engine leads to ignition before the spark: pre-ignition, knocking or pinking • this reduces engine performance and causes damage to the engine, because the piston is still travelling upwards when the early explosion occurs; • diesel engines are built to withstand the extra stresses arising from the higher compression ratio - that's why they're noisier.
CHÆ SOÁ OCTAN ( OCTANE NUMBER (RON)) • Solving pre-ignition: • by the use of additives; the commonest is tetraethyl lead, Pb(C2H5)4 ( or MTBE), a colourless, covalent liquid; this is thought to prevent the radical reactions which lead to pre-ignition; • however the products from the exhaust are toxic, and waste lead; lead destroys the catalysts designed to reduce nitrogen oxide and carbon dioxide emissions; • the favoured route is to blend the fuel with aromatic and branched-chain hydrocarbons as an alternative since these have higher resistance to pre-ignition - RON 120 for methylbenzene; • some unleaded fuels may be as much as 40% aromatics; • alternatively alcohols can be used, for example methanol or ethanol, which also have the effect of reducing the inlet temperature; • residual cylinder temperature can be reduced by using a better thermal conductor for the cylinder block - aluminium, for example.
