Ankan 2.ppt

HOÁ HỌC HỮU CƠ
Organic Chemistry
CHƯƠNG 6 (t.t)
Alkanes
6.3. TÍNH CH?T C?A 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 O2 6 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


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CHỈ SỐ 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.

CHỈ SỐ 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.

CHỈ SỐ OCTAN ( OCTANE NUMBER (RON))
Isomerisation:
This uses a Pt catalyst followed by separation and recycling of unchanged material. Thus pentane (RON 62) can be converted to the branched-chain isomer 2-methylbutane with RON 93.

Reforming:
This uses a Pt/Re catalyst which can convert alkanes to cycloalkanes, and cycloalkanes to aromatics. Thus hexane (RON 25) can be converted to cyclo-hexane (RON 83), cyclo-hexane to benzene (RON 106), and methylcyclohexane (RON 70) to methylbenzene (toluene; RON 120).

Cracking:
Heavy oils (C30 – C40) are heated over a catalyst in a fluidised bed, which gives
alkane to branched alkane + branched alkene
alkane to smaller alkane + cycloalkane
cycloalkane to alkene + branched alkene
alkene to smaller alkene
The conditions and the nature of the catalyst are varied to give the desired products.
CRACKING
In petroleum geology and chemistry, cracking is the process whereby complex organic molecules (e.g. kerogens or heavy hydrocarbons) are converted to simpler molecules (e.g. light hydrocarbons) by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products are strongly dependent on the temperature and presence of any catalysts.
http://en.wikipedia.org/wiki/Cracking_(chemistry)
CRACKING
In an oil refinery cracking processes allow the production of "light" products (such as LPG and gasoline) from heavier crude oil distillation fractions (such as gas oils) and residues. Fluid Catalytic Cracking (FCC for short) produces a high yield of gasoline and LPG while hydrocracking is a major source of jet fuel, gasoline components and LPG. Thermal cracking is currently used to "upgrade" very heavy fractions ("upgrading", "visbreaking"), or to produce light fractions or distillates, burner fuel and/or petroleum coke. Two extremes of the thermal cracking in terms of product range are represented by the high-temperature process called steam cracking or pyrolysis (ca. 750 to 900 °C or more) which produces valuable ethylene and other feeds for the petrochemical industry, and the milder-temperature delayed coking (ca. 500 °C) which can produce, under the right conditions, valuable needle coke, a highly crystalline petroleum coke used in the production of electrodes for the steel and aluminum industries.