Vladimir Vasilyevich Markovnikov was a Russian chemist. He formulated Markovnikov's rule, which was a huge milestone in the history of organic chemistry. Markovnikov's rule helps us predict the products of electrophilic addition reactions of asymmetrical alkenes(alkenes which have two different alkyl groups flanked around the double bond).
Although this rule was first published in 1870, it took almost 60 years for the scientific community to accept its validity. There are several reasons behind this, but once everyone realised that Markovnikov's rule holds true, it was a game changer as it led to other wonderful discoveries in organic chemistry.
It is worth noting that Markovnikov is also well-known for his contribution to isomerism. He conducted several experiments on butyric and isobutyric acids, based on which, he concluded that both these compounds have different structures, but the same molecular formula (Isomers).
Fig. 1: Vladimir Markovnikov | Wikimeida commons shared under public domain
Remembering Markovnikov and appreciating his contribution to organic chemistry, particularly to Electrophilic additions, let us dive deeper into this topic- Products of Electrophilic Addition Reactions.
- This article is about electrophilic addition.
- We will first explore the difference between electrophilic addition reactions and electrophilic substitution reaction.
- We will then explore the different products of electrophilic addition reactions; this will include the electrophilic addition of halogens and hydrogen halides.
- Finally, we will go through some examples of electrophilic addition reactions.
Electrophilic Addition Reactions vs electrophilic substitution
Before digging into electrophilic additions, let us understand the key difference between electrophilic addition reactions and electrophilic substitution reactions, both of which involve an electrophile.
But wait, what is an electrophile? Have a quick read at the definition below:
An atom or group that is attracted to an electron-rich centre is called an Electrophile. Electrophiles tend to have empty orbitals and/or a lack of electrons.
Electrophiles might be positively charged or neutral. Few examples include:
\(Br_2\) , \(H^+\) , \(BF_3\)
Now, let us look at the differences between the two types of reactions involving electrophiles.
Electrophilic substitution is a reaction where an electrophile replaces a functional group in a molecule. This means that in the product of a reaction, one functional group is replaced by the electrophile that is attacking.
\(C_6H_6 + Cl_2 \rightarrow C_6H_5Cl + HCl\)
Observe how one of the hydrogens in benzene[ \(C_6H_6\) ] has been replaced by chlorine giving the product chlorobenzene [\(C_6H_5Cl\)].
On the other hand, in an electrophilic addition, the electrophile is added to the overall compound instead of a functional group being replaced. Therefore, in an electrophilic addition, no atoms/chemical moieties are lost.
Electrophilic additions generally happen in unsaturated compounds (alkenes and alkynes). In this article, our main agenda is to discuss the electrophilic additions in alkenes.
Electrophilic Addition Reactions of Alkenes
Alkenes are unsaturated aliphatic compounds containing a carbon-carbon double bond [ \( C=C\) ]. Alkenes are electron-rich species, and they are ready to donate that pair of pi electrons to electrophiles. Hence, we can say that alkenes tend to act as lewis bases.
During electrophilic addition, the carbon-to-carbon double bond (C=C) is broken, as a result, the carbons require the fourth bond, as carbons can make a total of four bonds (valency of carbon = 4), and this is where electrophiles come and take their place within the product. Also, a carbon can have a maximum of 4 bonds so if all of those places are taken no addition is possible (only substitution).
In the above example, notice how the pi bond is broken and new sigma bonds (in pink) are formed with A and B of the reagent AB.
Electrophilic Addition Reactions- Mechanism
Now that we have discussed the differences between the electrophilic substitution reaction and electrophilic addition, we will move further to understand how the addition reactions take place in alkenes.
A detailed step-by-step view of what happens when molecules interact, and how the bonds break while forming new ones will be discussed. This is nothing but the reaction mechanism of electrophilic addition reactions.
We will explore this electrophilic addition reaction mechanism with different types of electrophiles.
Electrophilic Additions of halogens
We will start off by exploring the electrophilic addition of halogen to alkenes.
We know that halogens exist as homo-diatomic molecules \(Br_2 , Cl_2\) etc., Although both the atoms of a homo-diatomic molecule are of the same electronegativity. So, how is an electrophile of positive charge generated from a diatomic molecule whose atoms have the same electronegativities as bromine?
To understand how bromine can be polarised, read through the following mechanism.
Step 1
When a pi-bond-containing molecule like ethene, an alkene, approaches bromine, the electrons in bromine are repelled by the pi-electrons of ethene(like repels like). As a result, a temporary dipole is introduced in bromine which makes one of the bromine atoms partially positive (\( \delta^ + \) ) and the other will be partially negative (\( \delta^-\) ). The partially positive bromine acts as an electrophile, attacking the electron-rich carbon-generating a carbocation.
A carbocation is an intermediate formed in a reaction whose carbon atom is positively charged and is bonded to three other groups or atoms.
Carbocations are sp2 hybridised. So, what is its shape? That is for you to think.
Carbocations are of two types: a trivalent intermediate such as the below example is called a carbenium ion while a pentavalent intermediate is called a carbonium ion. But, for your A-level exams, it is just enough if you mention it as a carbocation.
Fig. 2: A bromoethyl carbocation.
Step-2
Now the electron-rich, partially negative bromine attacks the carbocation intermediate (the carbon with positive charge-see mechanism) forming a sigma bond with it, giving rise to the final product- an addition product. The product formed will be a saturated compound. Now, you cannot add anything else to the final product, but you can substitute one of the atoms in it with some other atom.
Fig. 3: Mechanism of electrophillic addition of bromine to ethene | Anonymouse197, CC BY-SA 3.0, via Wikimedia Commons [1]
This reaction is a test to identify if a given organic compound is saturated or unsaturated. When an unknown aliphatic organic compound is given, it is added to bromine water (reddish brown) and if the organic compound decolourises the bromine, then it must be an alkene or an alkyne. If the given compound is an alkane, it doesn't change the colour of bromine water-meaning no addition reaction takes place in the alkane.
Electrophilic Additions of hydrogen halides
As we have seen the mechanism of alkene addition to halogens, let us explore another mechanism, a very important one, the electrophilic addition of hydrogen halides. We will first look into how this happens in symmetrical alkenes, and we will later look at asymmetrical alkenes.
Hydrogen halides are inorganic compounds with a general formula of HX.
- H = Hydrogen.
- X = Halogens like F, Cl, Br, I.
- Aqueous forms of Hydrogen halides are called hydrohalic acid (Example: Hydrogen chloride(HCl) = Hydrochloric acid)
Let us see the reaction mechanism of the reaction between hydrogen bromide and alkene. Hydrogen halides are polar because of a difference in their electronegativities. As Bromine is more electronegative than hydrogen, it pulls the electrons away from hydrogen towards itself. Thus, a partially positive hydrogen( \( \delta^+\) ) and a partially negative bromine ( \(Br^-\) ) are formed.
The partially positive hydrogen acts as an electrophile and accepts the pi electron pair from the alkene. Thus, it forms a bond with the electron-rich carbon, generating a carbocation. This is the first step. Now, the carbocation accepts electrons from the partially negative Bromine, forming the final product- an alkene addition product which is a saturated compound.
Electrophilic addition major and minor products
As far as the electrophilic addition involves symmetrical alkenes, there will not be any confusion as to where the electrophile goes. Why? Take a look at ethene, all the four hydrogens surrounding the double bond are equivalent. No matter where you place your electrophile, it results in the same product.
But, propene is different. It is an unsymmetrical alkene where at least one of the groups surrounding the double bond is different. If the electrophile (\(H^+\) )sits on carbon-1, you get 2-bromopropane, but if it sits on carbon-2, you get 1-bromopropane. Both products are formed, but which one is highly likely to form? which product is the major product? These questions have been answered by Vladimir Markovnikov through his rule- the Markovnikov's rule.
Markovnikov's rule: In the addition reactions of halogen halide and alkene, the halogen will bond to the carbon that is most substituted.
According to Markovnikov's rule, when an unsymmetrical alkene reacts with a hydrogen halide, the hydrogen attaches itself to the double bonded carbon which has greater number of hydrogens.
Let us understand this concept through an example.
Electrophilic Addition Reaction- Examples
Consider propene, an unsymmetrical alkene. In propene, there is a double bond between C1 and C2 . Carbon-1 has two hydrogens while carbon-1 has one hydrogen. So, when hydrogen bromide attacks the double bond, it prefers to attach itself to carbon-1 which has two hydrogens instead of carbon-2. As a result of this attack, a secondary carbocation is formed which is more stable.
On the other hand, if the hydrogen attacks the carbon-2 (lesser number of hydrogens), a primary carbocation is generated which is less stable. Hence, although a product is formed from the primary carbocation, the %yield will be very low. Thus, the product that is formed through the Secondary carbocation intermediate is the major product.
Electrophilic Addition Reactions- Conditions
The reaction conditions, such as the catalysts used will dictate the products formed. Contrary to Markovnikov's rule, Propane reacts with HBr in the presence of peroxides to produce 1-bromopropane as the major product.
Have a look at the following Anti-Markovnikov addition to understand why this happens.
Fig. 6: Anti-Markovnikov's free radical addition | Created using images from Wikimedia Commons |Lpr6a2, CC BY-SA 4.0, via Wikimedia Commons
In the presence of peroxides, bromine undergoes homolytic fission generating a bromine free radical. This bromine free radical attaches to the carbon with less number of hydrogens, generating a secondary carbon free radical intermediate.
Remember that when peroxides are present, the bromine radical's attack goes first followed by the addition of hydrogen, but in Markovnikov's addition, the attack of hydrogen goes first. Thus, two different conditions produce two different major products.
Anti-Markovnikov's addition is also referred to as the Kharasch peroxide effect.
Electrophilic Addition Reactions of Benzene
Interestingly, even though benzene has double bonds, just like alkenes, benzene doesn't undergo addition reactions. The reason is the delocalisation of electrons in benzene, meaning the double bonds are not fixed in their positions, but rather the electron cloud is evenly distributed among all the carbons of benzene. Delocalisation of electrons is the unique character of aromatic compounds like benzene which makes them very stable.
So, if benzene was to undergo addition reaction, this delocalisation has to be disrupted, which is not favoured. Hence, benzene prefers substitution to electrophilic reactions. Benzene is an electron-rich compound and thus attracts the electrophiles, but only the hydrogens of the benzene are replaced with the incoming electrophile and the double bonds stay intact.
Fig. 7: Electrophilic substitution reactions in benzene | Y is an electrophile with a positive charge that kicks the hydrogen and takes its place | Wikimedia commons public domain
This is the reason why benzene doesn't give bromine discolouration reaction despite possessing double bonds.
Now, you would have understood what an electrophile is and how unsaturated compounds like alkenes react with halogens and hydrogen halides. Alternatively, we have also seen how Markovnikov's rule helps us in predicting the major and minor products in unsymmetrical alkenes.
Also, benzene is different when it comes to addition reactions despite having double bonds.
Be like benzene-stable, yet different and no electrophile can break its stability!
Electrophilic Addition - Key takeaways
- In electrophilic addition, an electrophile is added to a compound whilst in electrophilic substitution, an electrophile replaces a group in a compound.
- When an electrophile is a hydrogen halide, the hydrogen first accepts electrons from the double carbon =carbon bond, causing the bond between hydrogen and bromide to break and a positive carbocation to be formed, where the halide then bonds to.
- In asymmetrical alkenes, we use Markovnikov's rule to determine the position of the hydrogen and halide within a molecule.
- Benzene is a stable molecule with delocalised pi bonds which makes it resistant to electrophilic addition reactions. However, benzene undergoes electrophilic substitution reactions.
References
- https://creativecommons.org/licenses/by-sa/3.0/
- https://creativecommons.org/licenses/by-sa/4.0/
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