Alkenes

ALKENES

  • Alkenes are unsaturated hydrocarbons containing at least one double bond.
  • The general formula for alkenes is CnH2n, where n is the number of carbon atoms in the molecule.
  • If there is one double bond between two carbon atoms in alkenes, they must possess two hydrogen atoms less than alkanes.
  • Alkenes are also known as olefins, which means "oil forming".
  • The first member of the alkene series is ethylene or ethene (C2H4), which was found to form an oily liquid on reaction with chlorine. 

Structure of Double Bond

  • Alkenes contain at least one double bond and have the general formula of CnH2n.
  • The carbon-carbon double bond in alkenes consists of one strong sigma (σ) bond and one weaker pi (π) bond.
  • The pi (π) bond is formed by lateral or sideways overlapping of the two 2p orbitals of the two carbon atoms and is weaker than the sigma bond due to poor sideways overlapping.
  • The presence of the pi (π) bond makes alkenes behave as sources of loosely held mobile electrons, making them easily attacked by electrophilic reagents.
  • Alkenes are less stable than alkanes due to the weaker pi (π) bond and can be converted into single bond compounds by combining with electrophilic reagents.
  • The strength of the double bond is greater than that of a carbon-carbon single bond in ethane.
  • The double bond is shorter in bond length than the C-C single bond in alkanes.

 

Nomenclature of alkene

The IUPAC system is used for nomenclature of alkenes. The longest chain of carbon atoms containing the double bond is selected, and the chain is numbered from the end nearer to the double bond. The suffix ‘ene’ replaces ‘ane’ of alkanes. It is important to note that the first member of the alkene series is CH2 (replacing n by 1 in CnH2n), known as methene, but it has a very short life. The first stable member of the alkene series is C2H4, known as ethylene (common name) or ethene (IUPAC). Below are the IUPAC names of a few members of alkenes:

 

Isomerism of alkene

Alkenes can exhibit both structural isomerism and geometrical isomerism.

 

Structural isomerism

Structural isomers of alkenes have the same molecular formula but different arrangements of atoms in the molecule. For example, butene (C4H8) can exist in two structural isomeric forms: 1-butene and 2-butene, which differ in the location of the double bond in the molecule.

 

Geometrical isomerism

Geometrical isomers of alkenes have the same molecular formula and the same arrangement of atoms in the molecule, but differ in the orientation of the substituents around the double bond. This is due to restricted rotation around the double bond. The two common types of geometrical isomers of alkenes are cis and trans isomers. In the cis isomer, the substituents are on the same side of the double bond, while in the trans isomer, they are on the opposite side of the double bond. For example, 2-butene can exist in two geometrical isomeric forms: cis-2-butene and trans-2-butene.

 

preparation of alkene

From alkynes to alkenes

Alkynes on partial reduction with calculated amount of dihydrogen in the presence of palladised charcoal partially deactivated with poisons like sulphur compounds or quinoline give alkenes. Partially deactivated palladised charcoal is known as Lindlar’s catalyst. Alkenes thus obtained are having cis geometry. However, alkynes on reduction with sodium in liquid ammonia form trans alkenes.

From alkyl halide to Alkenes

Alkyl halides (R-X) on heating with alcoholic potash (potassium hydroxide dissolved in alcohol, say, ethanol) eliminate one molecule of halogen acid to form alkenes. This reaction is known as dehydrohalogenation i.e., removal of halogen acid. This is example of β-elimination reaction, since hydrogen atom is eliminated from the β carbon atom (carbon atom next to the carbon to which halogen is attached).

Nature of halogen atom and the alkyl group determine rate of the reaction. It is observed that for halogens, the rate is: iodine > bromine > chlorine, while for alkyl groups it is : tert > secondary > primary.

 

From vicinal dihalides to alkenes

Dihalides in which two halogen atoms are attached to two adjacent carbon atoms are known as vicinal dihalides. Vicinal dihalides on treatment with zinc metal lose a molecule of ZnX2 to form an alkene. This reaction is known as dehalogenation.

 

 

From alcohol by acidic dehydration to alkenes

Alcohols on heating with concentrated sulphuric acid form alkenes with the elimination of one water molecule. Since a water molecule is eliminated from the alcohol molecule in the presence of an acid, this reaction is known as acidic dehydration of alcohols. This reaction is also the example of β-elimination reaction since –OH group takes out one hydrogen atom from the β-carbon atom.

 

 

Physical properties

 Alkenes as a class resemble alkanes in physical properties, except in types of isomerism and difference in polar nature. The first three members are gases, the next fourteen are liquids and the higher ones are solids. Ethene is a colourless gas with a faint sweet smell. All other alkenes are colourless and odourless, insoluble in water but fairly soluble in non-polar solvents like benzene, petroleum ether. They show a regular increase in boiling point with increase in size i.e., every – CH2 group added increases boiling point by 20–30 K. Like alkanes, straight chain alkenes have higher boiling point than isomeric branched chain compounds.

 

Chemical properties of alkenes

Addition of dihydrogen

Alkenes add up one molecule of dihydrogen gas in the presence of finely divided nickel, palladium or platinum to form alkanes

 

Addition of halogens

Halogens like bromine or chlorine add up to alkene to form vicinal dihalides. However, iodine does not show addition reaction under normal conditions. The reddish orange colour of bromine solution in carbon tetrachloride is discharged when bromine adds up to an unsaturation site. This reaction is used as a test for unsaturation.

 

Addition of hydrogen halides

Hydrogen halides (HCl, HBr,HI) add up to alkenes to form alkyl halides. The order of reactivity of the hydrogen halides is
HI > HBr > HCl. Like addition of halogens to alkenes, addition of hydrogen halides is also an example of electrophilic addition reaction. Let us illustrate this by taking addition of HBr to symmetrical and unsymmetrical
 alkenes

 

Addition reaction of HBr to symmetrical alkenes

 

Addition reactions of HBr to symmetrical alkenes (similar groups attached to double bond) take place by electrophilic addition mechanism.

Markovnikov rule

Mechanism

Hydrogen bromide provides an electrophile, H+, which attacks the double bond to form carbocation as shown below :



(a) less stable primary carbocation       (b) more stable secondary carbocation

 

(i) The secondary carbocation (b) is more stable than the primary carbocation (a), therefore, the former predominates because it is formed at a faster rate.

(ii) The carbocation (b) is attacked by Br ion to form the product as follows :

                                                                2-Bromopropane (major product)

Anti Markovnikov addition

 In the presence of peroxide, addition of HBr to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule. This happens only with HBr but not with HCl and Hl. This addition reaction was observed by M.S. Kharash and F.R. Mayo in 1933 at the University of Chicago. This reaction is known as peroxide or Kharash effect or addition reaction anti to Markovnikov rule.

(13.43)

 

Mechanism : Peroxide effect proceeds via free radical chain mechanism as given below:

(ii)

The secondary free radical obtained in the above mechanism (step iii) is more stable than the primary. This explains the formation of 1-bromopropane as the major product. It may be noted that the peroxide effect is not observed in addition of HCl and HI. This may be due to the fact that the H–Cl bond being stronger (430.5 kJ mol–1) than H–Br bond (363.7 kJ mol–1), is not cleaved by the free radical, whereas the H–I bond is weaker (296.8 kJ mol–1) and iodine free radicals combine to form iodine molecules instead of adding to the double bond.

 

Addition of sulphuric acid

Cold concentrated sulphuric acid adds to alkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction.

 

Addition of water to alkene

In the presence of a few drops of concentrated sulphuric acid alkenes react with water to form alcohols, in accordance with the Markovnikov rule.

 

Oxidation of alkene

Alkenes on reaction with cold, dilute, aqueous solution of potassium permanganate (Baeyer’s reagent) produce vicinal glycols. Decolorisation of KMnO4 solution is used as a test for unsaturation.

 

Acidic potassium permanganate or acidic potassium dichromate oxidises alkenes to ketones and/or acids depending upon the nature of the alkene and the experimental conditions

 

Ozonolysis of alkene

Ozonolysis of alkenes involves the addition of ozone molecule to alkene to form ozonide, and then cleavage of the ozonide by Zn-H2O to smaller molecules. This reaction is highly useful in detecting the position of the double bond in alkenes or other unsaturated compounds.

 

Polymerisation of alkene

Polythene is obtained by the combination of large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst. The large molecules thus obtained are called polymers. This reaction is known as polymerisation. The simple compounds from which polymers are made are called monomers. Other alkenes also undergo polymerisation.

Polymers are used for the manufacture of plastic bags, squeeze bottles, refrigerator dishes, toys, pipes, radio and T.V. cabinets etc. Polypropene is used for the manufacture of milk crates, plastic buckets and other moulded articles. Though these materials have now become common, excessive use of polythene and polypropylene is a matter of great concern for all of us.

 

Hydride and methyl shift reaction

 

Oxymercuration Demercuration Reduction

Alkoxymercuration Demercuration Reduction

Alkoxymercuration demercuration is a complicated phrase that describes many reactions. However, dividing this complex phrase into smaller sections makes it easier to understand:

  • Alk: This refers to an alkene. An alkene is a hydrocarbon with a carbon-carbon double bond (C=C).
  • Oxy: Oxy means the reaction of an alkene with an oxygen nucleophile. Water (H2O) and alcohols are both oxygen nucleophiles.
  • Mercuration: This refers to a reaction of an alkene with mercury(II) acetate. Mercuric acetate is primarily utilized for organic compound mercuration and alkene absorption. Mercuration means to combine or treat mercury
  • Demercuration: (De) means removal, referring to the removing of mercury from the anion of hydrogen that yields organic compounds containing the ether group. Ethers are represented by the generic formula R-O, where R indicates alkyl groups.

The reaction includes the reactant, catalyst, and product.

  • Reactant: Akene (CnH2n), mercury acetate (Hg(OAc)2), and oxygen nucleophile (alcohol)
  • Catalyst: Hydride ion (NaBH4)
  • Product: Ether

Thus, the reaction starts with alkene and ends with ether.

Syn and anti reaction

An addition reaction is a type of organic chemistry reaction in which atoms or chemicals are added across a double or triple bond. Energetically, addition reactions are generally exothermic because a weaker pi-bond is replaced with two stronger sigma (single) bonds.

There are two main categories of simple addition reactions: electrophilic and nucleophilic addition. Electrophilic addition starts with the double bond attaching to a partially positively charged atom such as hydrogen in an acid, and nucleophilic addition starts with a partially negatively charged atom attaching to the partially positively charged atom in the double or triple bond.