Reaction of Alkane + Introduction to Alkene PDF

Reactions of Alkanes Organic chemistry Reactions of alkanes 1- Halogenation Reactivity X2: Cl2 > Br2 H2: 3O < 2O < 1O < CH3-H e.g. Reactions of alkanes Experiment has shown that on halogenation an alkane yields a mixture of all possible isomeric products, indicating that all hydrogen atoms are susce...

Reactions of Alkanes Organic chemistry Reactions of alkanes 1- Halogenation Reactivity X2: Cl2 > Br2 H2: 3O < 2O < 1O < CH3-H e.g. Reactions of alkanes Experiment has shown that on halogenation an alkane yields a mixture of all possible isomeric products, indicating that all hydrogen atoms are susceptible to replacement. For example, for chlorination: Reactions of alkanes Bromination gives the corresponding bromides but in different proportions: Reactions of alkanes Although both chlorination and bromination yield mixtures of isomers, the results given above show that the relative amounts of the various isomers differ markedly depending upon the halogen used. Chlorination gives mixtures in which no isomer greatly predominates; in bromination, by contrast, one isomer may predominate to such an extent as to be almost the only product, making up 97- 99% of the total mixture. In bromination, there is a high degree of selectivity as to which hydrogen atoms are to be replaced. Reactions of alkanes Mechanism of halogenation Halogenation of alkanes proceeds by the following mechanism: Reactions of alkanes A halogen atom abstracts hydrogen from the alkane (RH) to form an alkyl radical (R.). The radical in turn abstracts a halogen atom from a halogen molecule to yield the alkyl halide (RX). Which alkyl halide is obtained depends upon which alkyl radical is formed. Reactions of alkanes Reactions of alkanes How fast an alkyl halide is formed depends upon how fast the alkyl radical is formed. Here, of the two chain propagating steps, step (2) is more difficult than step (3), and hence controls the rate of overall reaction. Formation of the alkyl radical is difficult, but once formed the radical is readily converted into the alkyl halide. Reactions of alkanes Orientation of halgenation It is an important problem, because orientation determines what product we obtain. As an example let us take chlorination of propane. The relative amounts of n-propyl chloride and isopropyl chloride obtained depend upon the relative rates at which n-propyl radicals and isopropyl radicals are formed. Reactions of alkanes If, say, isopropyl radicals are formed faster, then isopropyl chloride will be formed faster, and will make up a larger fraction of the product. As we can see, n-propyl radicals are formed by abstraction of primary hydrogens, and isopropyl radicals by abstraction of secondary hydrogens. Reactions of alkanes Study of the chlorination of a great many alkanes has shown that these are typical results. After allowance is made for differences in the probability factor, the rate of abstraction of hydrogen atoms is always found to follow the sequence 3o > 2o > 1o. Reactions of alkanes 2- Combustion Combustion is a rapid oxidation that takes place at high temperatures, converting alkanes to carbon dioxide and water. Little control over the reaction is possible, except for moderating the temperature and controlling the fuel/air ratio to achieve efficient burning. Reactions of alkanes The reaction is extremely exothermic and yet requires a very high temperature, that of a flame, for its initiation. Combustion is the most common reaction of alkanes. Lightning initiated this fire in a tank containing 3 million gallons of gasoline Reactions of alkanes 3- Pyrolysis ( cracking) Decomposition of a compound by the action of heat alone is known as pyrolysis. chemists means "cleavage by heat"; compare hydrolysis, "cleavage by water." Alkane 400 – 600o; with or without catalysts H2 + smaller alkanes + alkenes Reactions of alkanes The pyrolysis of alkanes, particularly when petroleum is concerned, is known as cracking. In thermal cracking alkanes are simply passed through a chamber heated to a high temperature. Large alkanes are converted into smaller alkanes, alkenes, and some hydrogen. This process yields predominantly ethylene (C2H4) together with other small molecules Alkenes Alkenes The alkenes were described as being obtained from alkanes by loss of hydrogen in the cracking process. The general formula for this family is CnH2n. Since alkenes evidently contain less than the maximum quantity of hydrogen, they are referred to as unsaturated hydrocarbons. The simplest member of the alkene family is ethylene C2H4. Ethylene Alkenes The carbon-carbon double bond is the distinguishing feature of the alkene structure. The carbon-carbon "double bond" is thus made up of a strong σ bond and a weak π bond. Since the carbon atoms are held more tightly together, the C─C distance in ethylene is less than the C─C distance in ethane; that is to say, the carbon-carbon double bond is shorter than the carbon-carbon single bond. Names of alkenes Common names are seldom used except for three simple alkenes: ethylene, propylene, and isobutylene. Most alkenes are named by the IUPAC system. Names of alkenes The rules of the IUPAC system are: 1. Select as the parent structure the longest continuous chain that contains the carbon-carbon double bond. The parent structure is known as ethene, propene, butene, pentene, and so on, depending upon the number of carbon atoms; each name is derived by changing the ending -ane of the corresponding alkane name to -ene: Names of alkenes 2. Indicate by a number the position of the double bond in the parent chain. Although the double bond involves two carbon atoms, designate its position by the number of the first doubly-bonded carbon encountered when numbering from the end of the chain nearest the double bond. Names of alkenes 3. Carbon–carbon double bonds take precedence over alkyl groups and halogens in determining the main carbon chain and the direction in which it is numbered. Names of alkenes Physical properties The alkenes possess physical properties that are essentially the same as those of the alkanes. They are insoluble in water, but quite soluble in nonpolar solvents like benzene, ether, and chloroform, the boiling point rises with increasing carbon number; as with the alkanes.