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This reaction is typical of alkanes and alkyl-substituted aromatics. We use as an example the chlorination of methane, a reaction which has some industrial significance. We first need an initiation step to generate halogen atoms:
Cl2 → 2 Cl.
Once the halogen atoms are formed, the chain reaction can begin:
CH4 + Cl. → CH3. + HCl
CH3. + Cl2 → CH3Cl + Cl.
Notice how each step generates a reactive intermediate (CH3. or Cl.) which is used in the other step. In this way the chain reaction, once begun by the initiation step, is kept going. The net reaction is:
CH4 + Cl2 → CH3Cl + HCl.
In most hydrocarbons there are more than one possible product, depending on which hydrogen is replaced. Butane (CH3-CH2-CH2-CH3), for example, can be chlorinated at the "1" position to give 1-chlorobutane (CH3-CH2-CH2-CH2Cl) or at the "2" position to give 2-chlorobutane (CH3-CH2-CHCl-CH3). The product distribution depends on relative reaction rates: in this case the "2" position of butane reacts faster and 2-chlorobutane is the major product. Free radical halogenation generally proceeds in the following order:
Chlorination is generally less selective than bromination. Fluorination is not only even less selective than chlorination, but also highly exothermic and care must be taken to prevent an explosion or a runaway reaction. Free radical iodination is usually not possible because iodine is less reactive than the other halogens.
The positions next to the carbonyl group are easily halogenated, due to their ability to form enolate s in basis solution. A similar mechanism occurs in acid solution where the ketone can be enolized. An example is the bromination of acetoneIn chemistry, acetone (also known as dimethyl ketone 2-propanone propan-2-one and beta-ketopropane is the simplest representative of the ketones. Acetone is a colourless mobile flammable liquid with a pleasant, somewhat fruity odor, melting at -95. 4 ° C in basic solution:
CH3-CO-CH3 + OH- → CH3-CO-CH2- + H2O
CH3-CO-CH2- + Br2 → CH3-CO-CH2Br + Br-
This reaction has two steps, but is not a chain reaction. Multiple bromination can occur giving CH3-CO-CHBr2 etc.
This reaction is typical of aromatic compounds. An example is the chlorination of benzeneBenzene Chemical nameBenzene Chemical formulaCH Molecular mass78. 11 g/mol Density0. 8786 g/ml Melting point5. 5 °C Boiling point80. 1 °C Heat of vaporization44. 3 kJ/mol Heat of fusion9. 84 kJ/mol CAS number71-43-2 SMILESC1 CC CC C1 Benzene PhH or benzol:
(Discusstion of this reaction will require structural formulas) Overall: C6H6 + Cl2 → C6H5Cl + HCl
A few types of aromatic compounds, such as phenolPhenol or carbolic acid is a white crystalline solid, with a chemical formula of , a melting point of 43°C, and a boiling point of 182°C at the pressure of 1 atmosphere (or 101080 Pa). It is not a real alcohol, because the hydroxyl group is fixed on a uns, will react without a catalyst, but for typical benzene derivatives a Lewis acidIn chemistry, a Lewis acid is any acid that can accept a pair of electrons and form a coordinate covalent bond, after the American chemist Gilbert Lewis. The term acid is ambiguous; this is one of the possible interpretations. An electrophile is a Lewis a catalystEnthalpy profile for catalysed and uncatalysed reactions. A is the activation energy for an uncatalysed reaction, A is the reduced activation energy for the same reaction when catalysed. I represents the point at which a chemical intermediate has been for is required. Typical Lewis acid catalysts include AlCl3, FeCl3, FeBr3, and ZnCl2. These work by forming a highly electrophilic complex which attacks the benzene ring.
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