Addition reaction is a chemical reaction involving the addition of two or more molecules to form a single molecule. An addition reaction changes unsaturated compounds (with multiple bonds) into saturated compounds (single bond).
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Addition Reaction
In the case of organic compounds, an addition reaction involves the addition of Nucleophiles, Electrophiles, or free radicals to compounds like Alkenes, Alkynes, Aldehydes, and Ketones. The addition reaction is the inverse of the elimination reaction.
The main phenomenon that causes the addition reaction is called the Electromeric effect. No byproducts are formed in addition reaction.
Different Types of Addition Reaction
The different types of addition reactions are:
- Nucleophilic addition reaction
- Electrophilic addition reaction
- Free radical addition reaction
Nucleophilic addition reaction
Nucleophiles are the species that donate a pair of electrons to form a covalent bond, they are usually referred to as electron-rich species. Nucleophilic addition reactions usually occur in electrophilic unsaturated compounds.
Aldehydes and Ketones mainly undergo nucleophilic addition reactions. In organic compounds, the presence of bigger groups around the sp2 hybridized carbon will reduce the electrophilicity, and will create hindrance for the nucleophile and thereby reduce the chance of nucleophilic addition.
Example: Nucleophilic addition in Carbonyl Group
In the carbonyl compounds, the electron pairs in the pi bond are more attracted towards the Oxygen which in turn makes the Carbon electrophilic.
In the presence of a nucleophile, the pi bond is shifted towards the Oxygen atom and the carbon atom becomes the electrophilic centre. As a result of nucleophilic addition, an intermediate tetrahedral product is formed which on hydrogenation gives the end product.
Electrophilic addition reaction
Electrophiles are species that accept a pair of electrons to form a new covalent bond. They are usually referred to as the electron-deficient species.
In an electrophilic addition reaction, the electrophile gets attacked by the pi electrons of the unsaturated species and in the next step, the nucleophile attacks the electron-deficient species.
Alkenes and Alkynes are the main species that undergo electrophilic addition reactions.
Example: Addition of HX to alkenes
- In the first step, the pi electrons of the alkenes attack the electron-deficient hydrogen ion.
- After the electrophilic addition, an intermediate carbocation is formed. Carbocation formation is a slow step and the most stable carbocation will undergo nucleophilic addition
- In the second step, the most stable carbocation undergoes nucleophilic addition by the bromide ion.
The stability of intermediate carbocation is determined using Markovnikov’s rule.
Markovnikov’s rule states that in addition reaction to unsymmetrical alkenes, the electron-rich or negatively charged particle of the reagent attacks the carbon having less hydrogen bonded to it.
Free radical addition reaction
Free radical addition is also known as the Kharasch effect or Anti-Markovnikov addition reaction.
It is a very special addition reaction that occurs only when the reagent is HBr and the solvent is peroxide. No other species show free radical addition. It determines the addition of hydrogen-free radicals into the intermediate form.
Anti-Markovnikov’s rule states that when unsymmetrical alkynes or alkenes react with HX(especially HBr) in the presence of peroxide the free radical will be formed on the carbon with less hydrogen bonded to it.
Mechanism of Reaction:
Initiation: In the presence of sunlight the peroxide (solvent) molecules undergo homolytic fission and form free radicals. The reagent HBr reacts with the solvent radicals and forms Bromine radicals.
Propagation: The bromine radical generated in the initiation process will react with the alkene to attain stability. The radical is attracted by the pi electrons of the alkene. The stability of the intermediate radical formed will be determined by the Anti-Markovnikov’s rule.
Termination: The reaction ceases only when any of the two reactants is finished.
Things To Remember based on Addition Reaction
- Based on the first attack of reagent, Addition reactions are divided into Nucleophilic, Electrophilic, and Free- radical addition reactions.
- Nucleophilic addition reactions are generally found in Aldehydes and Ketones. The Presence of an electrophilic centre triggers the nucleophilic addition reaction.
- Electrophilic addition is accompanied by nucleophilic addition to form a stable product.
- The stability of intermediate carbocation determines the rate of reaction.
- Markovnikov’s rule is used to find the most stable carbocation.
- Free radical addition reaction mainly occurs when HBr is added to alkene or alkyne in the presence of peroxide.
- Stability of free radical – Tertiary> Secondary> Primary.
Sample Questions based on Addition Reaction
Ques. Why CH3CHO is more reactive than CH3COCH3 towards HCN? (2 marks)
Ans: The CH3COCH3 is having more hindrance towards the nucleophile and is less polar. Therefore it is less reactive than CH3CHO
Ques. Explain why Aldehydes are more reactive than ketones. (2 marks)
Ans: Aldehydes are more reactive than ketones due to the following two reasons :
- Due to the smaller +1 effect of one alkyl group in aldehydes, as compared to the larger +1 effect of two alkyl groups, the magnitude of positive charge on the carbonyl carbon is more in aldehydes than in ketones. As a result, nucleophilic addition reactions occur more readily in aldehydes than in ketones.
- Due to the presence of an H-atom on the carbonyl group, aldehydes can be more easily oxidised than ketones.
Ques. Arrange the following compounds in increasing order according to their reactivity towards nucleophilic addition. (2 marks)
CH3-CO-CH3, C6H5-CO-C6H5, CH3CHO
Ans: C6H5-CO-C6H5 < CH3-CO-CH3 < CH3CHO
Ques. Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+. (2 marks)
Toluene,p—H3C—C6H4—NO2, p—O2N—C6H4—NO2.
Ans: Here, the CH3 group is electron-donating but the NO2 group is electron-withdrawing. Therefore, the maximum electron density will be in toluene, followed by p-nitrotoluene followed by p-dinitrobenzene. Thus, the overall reactivity decreases in the order:
Toluene >p—H3C—C6H4—NO2, p—O2N—C6H4—NO2.
Ques. What are the characteristic features of the addition reaction? (2 marks)
Ans: The addition reaction converts unsaturated compounds into saturated compounds. It does not produce any by-products.
Ques. Predict the type of addition reaction which will occur in the given condition? (2 marks)
- CH3CHCH3 and HCl in the presence of peroxide
- CH3CHCH3 and HBr in the presence of peroxide
Ans: In the first condition, an electrophilic addition reaction will occur by the formation of an intermediate carbocation.
In the second case, the Free- radical addition reaction will occur due to the reaction of RO radical and HBr. The reaction will be in accordance with Anti-Markovnikov’s rule.
Ques. How do you account for the formation of ethane during the chlorination of methane? (3 marks)
Ans: Chlorination of methane is a free radical reaction that occurs by the following mechanism.
From the above mechanism, it is evident that during the propagation step, CH3 free radicals are produced which may undergo three reactions. In the chain termination step, the two CH3 free radicals combine to form ethane (CH3—CH3) molecules.
Ques. Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty? (2 marks)
Ans: Due to the presence of an electron cloud containing 6 n-electrons above and below the plane of the ring, benzene is a rich source of electrons. Consequently, it attracts the electrophiles (electron-deficient) reagents towards it and repels nucleophiles (electron-rich) reagents. As a result, benzene undergoes electrophilic substitution reactions easily and nucleophilic substitutions with difficulty.
Ques. Why are alkenes and alkynes attacked by the electrophiles? (2 marks)
Ans: Alkenes and alkynes are attacked by the electrophiles due to the presence of excess electrons in the pi bond.
Ques. Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+. (2 marks)
(a) Chlorobenzene, 2, 4-dinitrochlorobenzene, p-nitrochlorobenzene
Ans: The typical reactions of benzene are electrophilic substitution reactions. The higher the electron density in the benzene ring, the more reactive is the compound towards these reactions. Since NO2 is a more powerful electron-withdrawing group than Cl, therefore, the more the number of nitro groups, the less reactive the compound is. Thus, the overall reactivity decreases in the order:
Chlorobenzene > p-nitrochlorobenzene > 2, 4-dinitrochlorobenzene
Ques. Explain the mechanism addition of Grignard’s reagent to the carbonyl group of a compound forming an adduct followed by hydrolysis. (2 marks)
Ans: Carbonyl group undergoes nucleophilic addition reaction with Grignard reagent to form an adduct which undergoes hydrolysis to give alcohol in the following manner:
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