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Alkenes are unsaturated hydrocarbons, which means that each molecule has at least one double bond. They demonstrate addition reactions in which an electrophile attacks the carbon-carbon double bond to create additional products due to the presence of pi electrons. These reactions are known as electrophilic addition reactions of alkenes. These addition reactions can also follow a free radical mechanism. Alkenes undergo a variety of reactions, including oxidation and ozonolysis.
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Electrophilic Addition Reaction
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There are various types of elements in chemistry, such as alkenes, alkynes, and so on. Each kind has its own behaviour and set of properties. An addition reaction is one in which two or more molecules combine to generate a strong molecule. The resultant molecule is called the adduct. In organic chemistry, there are two types of addition reactions. An electrophilic addition reaction and a nucleophilic addition reaction are the two types of addition reactions.
Check Important Notes for Interhalogen Compounds
Electrophilic Addition Reaction Mechanism
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Alkenes are a class of hydrocarbons with at least one double bond in each molecule. The alkenes undergo an addition reaction as a result of this double bond. An electrophilic addition reaction of alkenes happens when an electrophile attacks the double bond of carbon atoms with the help of pi electrons present in the alkenes. The mechanism of the electrophilic addition process is described below. At times, it also follows a free radical mechanism.
Electrophilic Addition Reaction
Alkenes, as we've seen, have a lot of additional reactions. The addition of hydrogen halides in hydrogen bromide and hydrogen chloride was the most basic example for learning the electrophilic addition reaction process. We call protons electrophiles and halides nucleophiles because hydrogen halides have both protons and halides. The first stage in the electrophilic addition is to assault a carbon-carbon double bond with an electrophile, which exerts a set of electrons. The deprotonation step is what it's called. As a result, the freed electrons became linked to the molecule. It now only contains a single positive-charged carbon-carbon bond. The process of the carbocation is what we call it. The halide will then be linked to the next step.
Generally, we can represent the hydrogen halides as HI > HBr > HCl.
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Markovnikov Rule
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A prediction rule had been proposed by a scientist named Markovnikov. The Markovnikov rule is the name for this rule. The end result of the reaction may be predicted using this rule. In most reactions, the resulting molecule will process fewer hydrogen atoms if the negative part of the reaction is connected to the carbon atom. It only describes the negative part of the adding molecule since it has both a negative and a positive part. It is simpler to predict the final result for symmetrical alkenes using this approach than it is for unsymmetrical alkenes. This implies that here Ethane is the symmetrical alkene, whereas propane is the unsymmetrical alkene.
Markovnikov's Rule
With the help of hydrogen bromide, we can observe the electrophilic process. The hydrogen bromide attacks the carbon-carbon double bond during the formation of the carbocation reaction. It produces H+, which has a positive charge. The bromide ion attacks the carbocation atoms, resulting in the formation of alkyl halides, as we already know the secondary carbocation is more stable than the initial carbocation.
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Electrophilic Addition Reactions for Ketone and Alcohol Formation
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If the electrophilic addition processes occur in the oxidising state, Ketones and Alcohols will develop. Both ketones and alcohol may be made with potassium permanganate. Let us have a look at them. After oxidation, the alkenes generate ketones if the potassium permanganate is in an acidic state. The alkenes will be oxidised with the potassium permanganate in a cooling aqueous condition, resulting in vicinal glycols.
Formation of Krones and Alcohol from Alkenes
Ketones and alcohols are formed when alkenes are oxidised. For example, Alkenes oxidise to vicinal glycols in the presence of a cold, aqueous potassium permanganate solution and to ketones oracids in the presence of acidic potassium permanganate.
As a result, alkenes may be used in a variety of chemical processes, including electrophilic addition reactions. When a group of elements is subjected to a variety of reactions in various states, it will react in a unique way. It may change in various states with other chemicals at different temperatures, as we saw with oxidisation. As a result, before completing any reaction, it is necessary to comprehend the notion and significance of that reaction.
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Things to Remember
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- Electrophiles are engaged in electrophilic substitution and addition processes.
- Addition reactions are classified as Nucleophilic, Electrophilic, or Free-radical depending on the first reagent assault.
- Aldehydes and ketones are known to undergo nucleophilic addition reactions. The nucleophilic addition process is triggered by the presence of an electrophilic centre.
- To generate a stable product, electrophilic addition is followed by nucleophilic addition.
- The most stable carbocation is found using Markovnikov's rule.
Sample Questions
Ques. Is hydrogenation electrophilic addition? [2 marks]
Ans. Another alkene reaction worth noting is hydrogenation, which is unrelated to the mechanism of electrophilic addition. Hydrogenation is the process of adding molecular hydrogen (H2) to an alkene double bond. It converts a basic alkene to an alkane.
Ques. Differentiate between electrophilic addition and nucleophilic addition? [2 marks]
Ans. An electron pair is welcomed by the group being introduced in electrophilic addition, whereas an electron pair is donated by the added group in nucleophilic addition.
Ques. What causes electrophilic addition reactions in alkenes? [2 marks]
Ans. The electrons in the side-to-side overlap of p orbitals that permits the pi bond may be transferred to an electrophile, such as electrophilic addition, by the electrons in the doubly bound and sp2 hybridised alkenes.
Ques. Why can't alkanes undergo addition reactions? [2 marks]
Ans. Alkanes do not undergo this reaction, because they now only have single bonds, so they do not get more structurally stable or stronger—they have already reached their maximum strength—and can only change things about in replacement reactions.
Ques. What is an addition reaction? [2 marks]
Ans. An external reaction in organic chemistry is a simple chemical reaction in which two or more molecules interact to generate a stronger one (the adduct). Electrophilic and nucleophilic addition are the two main types of polar addition processes.
Ques. Write structures of all the alkenes which on hydrogenation give 2-methylbutane. [2 marks]
Ans. The basic skeleton of 2-methylbutane is shown below:
On the basis of this structure, various alkenes that will give 2-methylbutane on hydrogenation are:
Ques. As per the Markovnikov’s rule, in which Carbon will the Electrophile Attack? [2 marks]
Ans. The electrophile will attack the less substituted carbon molecule whereas the most substituted one is taken over by a nucleophile.
Ques. List out some of the Electrophilic Addition Reactions with Alkene Reagents. [2 marks]
Ans. The Prins reaction, Hydroboration-oxidation reactions (using Diborane), Oxymercuration reactions to form water and mercuric acetate, Hydration reactions (H2O), Hydrogenation reactions, Hydrohalogenations (HX) and di-halo addition reactions (X2) are some of the common electrophilic addition reactions of alkene reagents.
Ques. Is an Electrophile Positive in Ion Charge? [2 marks]
Ans. Considering the aspect of an electrophile getting attracted to negatively charged ion, that is a nucleophile, an electrophile can be regarded to have a positive charge completely or at least slight the amount for achieving the opposite charge attraction.
Ques. Why is the Additional Reaction of Hydrogen and Fluorine Said to be the Slowest? [2 marks]
Ans. The addition reaction of hydrogen and fluorine is said to be the slowest because of the property of its toughness in the H-F bond. The bond between Hydrogen (H) and Fluorine (F) is difficult to break as compared to other halogens in the periodic table. This makes the complete addition reaction of H-F slower than the rest.
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