ALKENE
General Characteristic The alkenes have at least one double bond consisting of a sigma and a pi-bond between two adjacent carbons in the molecule. The two carbons are sp2 hybridized. The geometrical shape of the two sp2 carbons is ‗trigonal planer‘. The Pi bond is much weaker than the sigma bond. The alkenes are more reactive than alkanes. In a molecule that has pi-bonds and sigma bond, the pibond breaks without disturbing the sigma bond, during chemical reactions. The general formula for alkenes with only one double bond in the molecule is CnH2n where n is the number of carbon atoms in the molecule. The alkenes are more reactive than alkanes. The pi-bond is electron deficient hence alkenes are more acidic in nature. Structures of Alkenes Few structure of alkene are shown below:
Physical Properties They follow the same trend as alkanes. Physical state depends on molecular mass. The ethene to butene are gases and pentene onwards are liquids. The simplest alkenes, ethylene, propylene and butylene are gases.
Linear alkenes of approximately five to sixteen carbons are liquids. The higher alkenes are waxy solids. The boiling point increases with molecular mass. The branching reduces the boiling point. The alkenes are slightly soluble in water. Their density is lower than water. Physical properties of Alkynes Alkynes are compounds containing at least one carbon to carbon triple bond. The trend in physical properties is similar to alkanes and alkenes. They are slightly more reactive, more polar, more acidic and more soluble than alkenes. Stability of alkenes The order of stability is opposite to the order of reactivity. It means that ―less stable is more reactive‖ and vice versa. The stability pattern is related to the number of alkyl groups attached to carbon of double bond. Higher the number of attached alkyl groups greater is the stability of alkenes. It means that the more highly substituted the carbon atom of the double bond greater is the stability of alkenes. The following trends are noticed.
However the rule of ‗less stability more reactivity ‗is not followed in certain addition reactions of alkenes.(Discussed in later section) The most stable alkenes are most reactive when mixed with electrophile. The above order is nearly reversed when alkenes reacts with an electrophile.
Synthesis of Alkenes The preparation of alkenes is dominated by elimination reactions. The two most common alkene formation reactions are dehydrohalogenation of alkyl halide and dehydration of alcohols. Some of these reactions follow Saytzeff rule. Elimination reaction Elimination reaction is the reaction in which one or two function groups are removed and the electrons left behind combined to form a double bond. Saytzeff rule The rule states that in dehydrohalogenation the preferred product is the alkenes that has the greater number of alkyl group attached to the doubly bonded carbon atoms. Dehydrohalogenation of Alkyl halide The reaction is classified as elimination reaction. It may follow either E1 or E2 mechanism. E1 mechanism is a two step process and no strong base is needed. E2 mechanism requires a strong and bulky base and completes in one step. Dehydrohalogenation: E1 Mechanism The reaction is completed in two steps The first step is a slow and rate determining step. Conversion of tert-butyl chloride to 2-Methylpropene is explained as an example.
tert-butyl chloride is treated with 80% aqueous ethanol at 250C to give the product Over all reaction is
Initial step is formation of tert butyl cation. This is a slow step
1) Aided by polar solvent, chlorine leaves carbon with its bonded electron pair. 2) The step also produces stable 30 carbocation and a chlorine ion. 3) The ions are solvated and stabilized by surrounding water molecules. 4) The reaction is unimolecular.
5) In the 2nd step solvent molecule acts as base and pulls a Bhydrogen as proton.
The electron pair moves in to form a double bond between α and β carbon. The step produces the alkenes and a hydronium ion. Dehydrohalogenation E2 mechanism
It is a single step reaction. It is bimolecular reaction. It needs a strong base. Concentration of reactant is an important rate determining factor. Synthesis of propene from isopropyl bromide follows E2 mechanism and is taken as example to explain the mechanism. Isopropyl bromide is heated with sodium ethoxide in ethanol to obtain propene.
Sodium ethoxide is prepared by using sodium hydride with excess of alcohol which becomes solvent for the reaction.
Basic ethoxide ion starts removing a proton (by forming a bond) from β-carbon It uses its electron pair to form a bond to proton. At the same time the electron pair of β-carbon and hydrogen starts moving.
The bond which starts moving becomes pi-bond of the double bond. The bromine begins to depart with bonded electron. In transition state partial bond exists between oxygen and β-hydrogen. Another partial bond exists between α-carbon and bromine. At this stage carbon – carbon bond is developing double bond character. The double bond of alkenes is fully formed. The other products are ethanol and bromide ion. It has been noticed that if the base is very bulky the reaction does not follow Saytzeff rule and a least substituted alkene is formed. Synthesis of alkenes by dehydration of alcohol Dehydration means elimination of a water molecule. In this reaction a water molecules is removed from alcohol The relative ease off dehydration of alcohol is 30 > 20 > 10 > methyl. The dehydration is done using a strong acid at high temperature. The temperature and acid concentration depends upon type of alcohol. ( primary secondary or tertiary ) The mechanism of reaction is Elimination type E1. The rate of reaction depends upon the concentration of alcohol. The re