Chemical properties of alkene

Alkenes are more reactive than alkanes due to the presence of a double bond. The carbon-carbon double bond consists of a strong bond and a weak p bond. The typical reactions of alkenes involve the breaking of this weaker p bond, viz., and formation of two sigma (s) bonds.

formation of two sigma bonds

Such reactions are called addition reactions and are initiated by an electrophile, proceeding through ionic mechanism. However, some addition reactions proceed through free-radical mechanism.

Higher alkenes contain a long chain of carbon. That part of the chain that forms an alkane-like structure (consisting of C-C bonds), may undergo substitution reaction as also shown by alkanes. Some characteristic reactions shown by alkenes are described below:

Combustion

Alkenes, like alkanes, are highly combustible. Alkenes burn with a luminous flame to give carbon dioxide and water. The flame becomes luminous because of the higher carbon content of alkenes than alkanes. Their combustion reactions are exothermic.

combustion of alkenes

Due to the luminosity of the flame, the lower alkenes may be used as illuminants.

Addition reactions

The p electrons of the carbon-carbon-double bond are available to an electrophile (any species seeking electrons). Thus, the addition reactions shown by alkenes are in fact electrophilic addition reactions.

addition reactions shown by alkenes

 the addition reactions shown by alkenes
addition product

Some addition reactions proceed through free-radical mechanism.

Addition of hydrogen

Alkenes add hydrogen in the presence of platinum or nickel catalyst, to form alkanes. The reaction termed as hydrogenation, is an exothermic reaction.

CnH2n+ H2formation of alkanes by alkenes hydrogenation CnH2n+2 + heat

This is known as Sabatier-Senderens reduction.

CH2=CH2 + H2Sabatier-Senderens reduction reaction CH3-CH3 + 132.2 kJ

ethene ethane

Addition of halogens

Alkenes react with halogens to form dihaloalkanes. The order of reactivity is, chlorine > bromine > iodine. Simply mixing together the two reactants, usually in an inert solvent like carbon tetrachloride, best carries out the reaction.

Alkenes react with halogens to form dihaloalkanes

Alkene dihaloalkane

testing presence of double bond in the molecule

ethene 1,2-dibromoethane

testing presence of double bond in the molecule

propene 1,2-dibromopropane

Addition of bromine is useful for the detection of the carbon-carbon double bond. When a 5% solution of bromine in carbon tetrachloride is added to an alkene, it gets decolorized. This indicates the presence of a double bond in the molecule. This test is called 'bromine test'.

Mechanism of halogen addition

The addition of halogen across the double bond takes place through the following steps. Taking an example of ethene,

  • Ethene molecule undergoes electromeric effect

Ethene molecule undergoes electromeric effect

  • Due to its close proximity to the carbon-carbon double bond, the non-polar halogen molecule gets polarized

  • The polarized halogen molecule forms a transition-state complex with ethene molecule.

  • The X- ion attaches itself to the positively charged carbon.

Addition of halogen acids

Alkenes with concentrated aqueous solution of halogen acids give haloalkanes. The order of reactivity is,HI > HBr > HCl

For example:

formation of haloalkanes from alkene

Ethene gives

formation of haloethane from ethane

Ethane haloethane

2-butene with HBr gives

HBr with 2 butene

2-Butene 2-bromobutane

Thus, symmetrical alkenes give only one product, due to the equivalence of the two carbon atoms (the H and X may add to the molecule in any way).

In asymmetrical alkenes, the addition of a halogen acid takes place in a manner where by the halogen atom (the negative part of the molecule to be added) adds to the carbon atom, which has lesser number of hydrogen atoms on it. For example, in the case of propene, the product obtained is 2-iodopropane and not 1-iodopropane.

formation of iodopropane from propene

The I being negative part of the added molecule, goes to the carbon number 2 because it has only one H-atom on it. (lesser number of H-atom)

This rule of the addition of halogen acids to an asymmetrical alkene is known as Markownikoff's rule (1869).

Markownikoff's rule

This is an empirical rule but it may be explained theoretically on the basis that the addition occurs by a polar mechanism. For example, the addition of HI to propylene. Since a methyl group is electron-repelling, the propylene molecule is polarized as follows.

addition of HI to propylene
2-iodopropane

Hence, the proton of the hydroiodic acid gets attached to the negatively charged carbon and the iodide ion to the positive carbon.

Peroxide effect

The mode of addition of hydrogen bromide to unsymmetrical alkenes in the presence of oxygen and peroxides is contrary to Markownikoff rule. This addition of HBr to unsymmetrical alkenes against the Markownikoff's rule is known as peroxide effect, or anti-Markownikoff's rule.

For instance, the reaction of propene with HBr in the presence of peroxides, forms 1-bromopropane instead of 2-brompropane.

CH3-CH=CH2 + HBraddition of HBr to unsymmetrical alkenes CH3-CH2-CH2Br1 -bromopropane

The mode of addition of hydrogen chloride or hydrogen iodide is not affected by the presence of peroxides.

Addition of sulphuric acid

In accordance to Markownikoff's rule alkenes readily add concentrated sulphuric acid to form alkyl hydrogen sulphates. For example,

Ethene gives,

alkenes with concentrated sulphuric acid  form alkyl hydrogen sulphates

ethane sulphuric acid ethyl hydrogen sulphate

Propene gives,

alkenes with concentrated sulphuric acid  form alkyl hydrogen sulphates

Iso-propyl hydrogen sulphate

An alkyl hydrogen sulphate on boiling with water gives the alcohol and sulphuric acid. Alcohols are prepared from alkenes obtained from the cracking of petroleum. For example,

alkenes with concentrated sulphuric acid  form alkyl hydrogen sulphates

ethyl hydrogen sulphate ethanol

Addition of hypohalous acids

Hypohalous acid (HOX) in accordance with the Markownikoff's rule, add to the molecule of an alkene at the double bond. For example,

Addition of hypohalous acids

ethene hypochlorous acid 2-chloroethanol(ethylene chlorohydrin)

Addition of hypohalous acids

propene 1-chloro-2-propanol(propylene chlorohydrin)

Addition of water (Hydration of alkenes)

Water molecule adds to an alkene molecule across the double bond in the presence of dilute acids and a catalyst. For example, ethane gives ethanol when a mixture of ethene and steam is passed over phosphoric acid and silica under a pressure 65 atm, and at 300C.

Hydration of alkenes

ethene ethanol

Addition of oxygen

Lower alkenes are mixed with air and passed under pressure over a silver catalyst at 200-400°C. This gives epoxides by adding one atom of oxygen across the double bond. The epoxides so obtained are used in detergents.

Addition of oxygen with lower alkenes

ethene ethene epoxide

Addition of ozone

Ozonides are formed when alkenes add a molecule of ozone across the double bond. For example, ethene gives ethene ozonide.

action of alkenes with ozone

ethene ethene ozonide

Ozonides on hydrolysis with water in the presence of a reducing agent give aldehydes.

Ozonides on hydrolysis with water give aldehydes

The oxidation of alkenes by ozone followed by decomposition of the formed ozonide with water, is termed as 'ozonolysis'. The nature of the products (aldehydes and ketones) formed due to ozonolysis depends upon the location of the double bond in the parent alkene. Therefore, this reaction provides a very convenient way of locating the position of the double bond in any molecule.

As in the above example, the only product formed upon the hydrolysis of ethene ozonide is formaldehyde (containing one carbon unit each) hence the double bond has only one carbon unit on either side.

In the following example,

formation of acetone and acetaldehyde by ozonolysis

The products of ozonolysis are, acetone (3 carbon unit) and acetaldehyde (2 carbon unit).

It implies the location of the double bond being between two carbon chains of 2 carbon and 3 carbon atoms.

Oxidation

Alkenes can be readily oxidized, but the nature of the products depends upon the oxidizing agent used.

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With cold, alkaline KMnO4

When alkenes are oxidised with cold, alkaline KMnO4, dihydroxy compounds (diols or glycols) are formed. The KMnO4 gets decolorized. This reaction is therefore, used as Bayer's test for unsaturation (the presence of double or triple bonds) in any molecule.

Ethene gives ethane-1,2-diol.

Bayer s test is to check the presence of double or triple bonds

ethane ethane-1,2-diol

4-Or-K2Cr2O7">

With acidified KMnO4 Or K2Cr2O7


Acidified potassium permanganate (or potassium dichromate) oxidises the dihydroxy compound so produced in reaction to ketone and/or carboxylic acid. For example,

carboxylic acid formation from alkene

ketone formation from alkene

formation of acetic and formic acid from propene

Substitution reactions

At elevated temperatures (500°C), higher alkenes give substitution products with chlorine. For example,

CH3-CH=CH2 + Cl2higher alkenes give substitution products with chlorine ClCH2-CH=CH2 +HCl

propene 3-chloropropene

Branched-chain alkenes give substitution reaction easily. For example isobutene gives substitution product with chlorine even at room temperature.

 isobutene gives substitution product with chlorine

Polymerization

Addition polymerization is a process by which a large number of molecules of the same species join together (without the elimination of simple molecules like HX, H2O, etc.,) to form a giant molecule, called a polymer. Alkenes undergo addition polymerization when heated under pressure, in the presence of suitable catalysts. When ethene is heated to 1000C under 1000 atm pressure in presence of oxygen, we get polyethene

Alkenes undergo addition polymerization to form polythene

Similarly, when vinyl chloride is polymerized in the presence of peroxide catalyst, it forms polyvinyl chloride (PVC)

production of PVC

formation of teflon

Isomerization

Alkenes when heated alone at high temperatures (500-700°C) or at lower temperatures (200-300°C) isomerizes in the presence of catalyst, such as Al2(SO4)3. Alkenes isomerism due to,

  • The shifting of the double bond which tends to move towards the center of chain, e.g., pentene-1 isomerizes to pentene-2.

CH3-CH2-CH2-CH=CH2illustration of alkenes isomerism due to  double bond shiftCH3-CH2-CH=CH-CH3

pentene-1 pentene-2

  • The migration of a methyl group, e.g., butene-1 isomerizes to 2-methylpropene (iso-butene).
illustration of alkenes isomerism due to  methyl group migration