Alkenes show different kinds of reactions, those reaction kinds are defined according to the mechanisms of reactions. Reason for alkene's characteristic reactions is the double bond between carbon atoms. A strong σ and a weak Π bond include in double bond. Two electrons of Π bond are attached very lightly. Therefore these Π electrons are more movable than σ electrons. Due to this reason, Π electrons are easily polarized than σ electrons by reagents.
Alkenes show following types of reactions and we will discuss them in detail in this tutorial.
The area of double bond has a higher electron density. Therefore alkenes allure to react with electrophiles (positively charged or neutral species). Weak Π bond is readily broken and electrophiles are added through the carbon atoms where double bond was. This kind of reactions are defined as electrophilic additive reactions
Halogens (specially Cl2 and Br2) reacts with alkenes and produces dihalogen derivatives as shown below.
Addition of halogens to alkenes is an electrophilic addition reaction. Normally, halogen molecules( Cl2, Br2) are not electron deficiency compounds. But due to higher electrons density of double bond of alkenes, halogen molecule gets polarized. The most closed atom of halogen molecule to the double bond is positively charged and other halogen atom gets negatively charged.
The positively charged halogen atom (electron deficiency atom) react with double bond and make bonds with both carbon atoms of double bond. Other halogen molecule forms a halide ion.
Carbocation holds positive charges. Carbocations are formed as intermediate compounds during the reaction. If positive charge density is low, that carbocation show some stability. That positive charge density depends on number of other alkyl groups which are located around the positively charged Carbon atom and state of those alkyl groups. Alkyl groups (CH3CH2- , CH3- ) can release electrons.
This carbocation of stability of carbocation is useful when we learn some reactions of alkenes. Some examples are addition of halogen acids to alkenes, hydration of alkenes.
A cyclic brominium cation is given as the intermediate product. Next, one carbon atom in the brominium ion is attacked by bromide ion (Br -). Then a bond between Br- and that carbon atom forms and the existing bromine-carbon bond in the ring is broken. A dibromide forms as the final product.
Reaction is started through Brδ+ which has minor positive charge due to Van der Waals forces. Br δ+ behave as an electrophilic. Therefore, this is an addition reaction. Finally, we introduce this reaction as an Electrophilic Addition Reaction.
As an addition reaction, halogen atom and hydrogen atom join to carbon atoms which have the double bond.
One part of the halogen acid attaches to one carbon atom (in the double bond) and other part to other carbon atom in the double bond.
Negative part of the addendum (here Cl, Br,I) joins with carbon atom which carries the smaller number of hydrogen atoms and positive part (H) goes to the carbon atom which has more hydrogen atoms.
When HBr ( HF , HCl and HI does not show this effect ) is added to an unsymmetrical alkene in the presence of organic peroxides, the reaction takes place opposite to the markovnikov rule.
That means, Br joins with carbon atom which carries the higher number of hydrogen atoms and H goes to other carbon atom.
More examples of addition of halogens to alkenes
This reaction is completed in two steps and they are explained in detail by taking ethene as the example.
Step 1: First, electron deficiency part of halogen acid (H atom) , is added to the π bond of alkene. Meanwhile, H-Br bond is broken and both electrons in the bond, gets displaced towards bromine atom. This reaction slowly occurs.
Step 2: Formed carbocation is very reactive. The nucleophile, Br- anion can give two electrons to carbon atom in the carbocation. This is the second step in the reaction and it's reaction rate is much higher than the first step.
All alkenes react with dilute H2SO4 and give give alcohols. This is a hydration. A H2O molecule is added through double bond and give primary, secondary, tertiary alcohols according to the structure of alkene. Alkene hydration is also decided by Markovnikov rule as described in unsymmetrical alkane and halogen acids reaction.
Step 1: First, a hydrogen atom (proton) is added to the alkene to give carbocation as an intermediate product. Two carbocations are formed in this reaction. According to the stability of carbocation, most stable carbocation forms more than other carbocation.
Step 2: Therefore, tertiary carbocation forms more. Next, carbocation is attacked by a water molecule. Water
molecule can performed as nucleophile because oxygen atom in the water molecule has two lone pair
electrons. Therefore electrons density of oxygen is high. After making a bond between oxygen atom and carbon atom, oxygen atom
receives a positive charge.
Step 3: Finally, a proton is eliminated from the ion and give the alcohol as the product.
When consider total reaction, a proton is used in the initial step. But in final step a proton is released again. Therefore in this reaction, H2SO4 is used as a catalyst. When there is no acid to provide proton, reaction does not occur. Parts of water molecule (H and OH) is added to double bond. Therefore this reaction is a hydration reaction.
Alkenes can be oxidized by some oxidizing agents and we will study following two oxidation reactions.
When alkenes react with strong oxidizing reagents such as acidic potassium permanganate(H+/KMnO4), acidic potassium dichromate (H+/ K2Cr2O7), double bond between carbon atoms are broken. So, carbon atoms (in the double bond) will be oxidized as the result. Carboxylic acids or carbon dioxides are formed according to the position of double bond was in alkene. During the oxidation process, single bonds between carbon atoms in the alkene remain unchanged in the carbon chain.
When alkenes react with dilute alkaline potassium permanganate solution (OH- / KMnO4), two -OH groups are attached to the two carbon atoms of the double bond. Also manganese dioxide(MnO2)a brown colour precipitate forms. As an example ethylene(CH2=CH2) is converted onto ethylene glycole(HO-CH2-CH2-OH).
Hydrogen peroxide add to alkenes to form glycols.