Preparation of Alkenes.pdf
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Alkenes Organic chemistry 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 hydro...
Alkenes Organic chemistry 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. The carbon-carbon double bond is shorter than the carboncarbon 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 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. Alkenes Cis, Trans Isomerism in Alkenes Because of restricted rotation about a carboncarbon double bond, any alkene in which each carbon of the double bond has two different groups bonded to it shows cis , trans isomerism. For example, 2-butene has two stereoisomers. In cis-2- butene, the two methyl groups are on one side of the double bond and the two hydrogens are on the other side. cis-2- butene Alkenes Cis, Trans Isomerism in Alkenes In trans-2-butene, the two methyl groups are on opposite sides of the double bond. These two compounds cannot be converted into one another at room temperature because of the restricted rotation about the double bond; they are different compounds (diastereomers), with different physical and chemical properties. trans-2- butene Preparation of alkenes 1- Dehydrohalogenation of alkyl halides Alkyl halides are converted into alkenes by dehydrohalogenation: elimination of the elements of hydrogen halide. Dehydrohalogenation involves removal of the halogen atom together with a hydrogen atom from a carbon adjacent to the one bearing the halogen. Preparation of alkenes As we can see, in some cases this reaction yields a single alkene and in other cases yields a mixture. For example: n-Butyl chloride can eliminate hydrogen only from C-2 and hence yields only 1butene. sec-Butyl chloride, on the other hand, can eliminate hydrogen from either C-l or C-3 and hence yields both 1-butene and 2-butene. Where the two alkenes can be formed, 2-butene is the chief product. Preparation of alkenes Mechanism of dehydrohalogenation The function of hydroxide ion is to pull a hydrogen ion away from carbon; jointly a halide ion separates and the double bond forms. We should notice that, in contrast to free radical reactions, the breaking of the C─H and C─X bonds occurs in an unsymmetrical fashion: hydrogen give up both electrons to carbon, and halogen retains both electrons. The electrons left behind by hydrogen are now available for formation of the second bond (the π bond) between the carbon atoms. Preparation of alkenes Preparation of alkenes 2- Dehydration of alcohol Alcohols are compounds of the general formula, ROH, where R is any alkyl group and the hydroxyl group OH, is characteristic of alcohols. Dehydration requires the presence of an acid and the application of heat. It is generally carried out in either of two ways: 1. Heating the alcohol with sulfuric or phosphoric acid 2. Passing the alcohol vapor over alumina, A12O3, alumina here serving as a Lewis acid. Preparation of alkenes Preparation of alkenes A Lewis acid is any species (molecule or ion) that can accept a pair of electrons, and a Lewis base is any species (molecule or ion) that can donate a pair of electrons. Preparation of alkenes Mechanism of dehydration of alcohols The generally accepted mechanism for the dehydration of alcohols is summarized in the following equations; Ethyl alcohol is used as the example. The alcohol unites (step 1) with a hydrogen ion to form the protonated alcohol, which dissociates (step 2) into water and a carbonium ion; the carbonium ion then loses (step 3) a hydrogen ion to form the alkene. The double bond is thus formed in two stages, OH being lost (as H2O) in step (2) and H being lost in step (3). Preparation of alkenes Preparation of alkenes Rearrangement of carbonium ion Indeed, the alkene is formed from a carbonium ion, it is not the same carbonium ion that is initially formed from the alcohol. A carbonium ion can rearrange to form a more stable carbonium ion N-Butyl alcohol, for example, yields the n-butyl cation; this rearranges to the sec-butyl cation, which loses a hydrogen ion to give (predominantly) 2-butene: Preparation of alkenes Preparation of alkenes In a similar way, the 2-methyl-l -butyl cation rearranges to the 2-methyl-2butyl cation. 3,3-dimethyl-2-butyl cation rearranges to the 2,3-dimethyl-2-butyl cation. We notice that in each case rearrangement occurs in the way that yields the more stable carbonium ion: primary to a secondary, primary to a tertiary, or secondary to a tertiary. Preparation of alkenes A migration of hydrogen with a pair of electrons is known as a hydride shift; A similar migration of an alkyl group is known as an alkyl shift. Preparation of alkenes If isomeric alkenes can be formed in this step, which, if any, will predominate? The examples we have already encountered give us the answer: Preparation of alkenes 3- dehalogenation of vicinal dihalides The dehalogenation of vicinal dihalides is a common method for the synthesis of alkenes. Vicinal dihalides, compounds that have halogens on adjacent carbons, are prepared by the reaction between a halogen and an alkene. Preparation of alkenes 4- reduction of alkynes Reduction of an alkyne to the double-bond stage can yield either a cis-alkene or a trans-alkene. Just which isomer predominates depends upon the choice of reducing agent, except when the triple bond is at the end of a chain.
