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Addition reaction is when two or more reactants come together to form a single product through a chemical reaction. However, it should be noted that only those chemical compounds with multiple bond characters can undergo an addition reaction, because a double or triple bond is broken from the single bond required.
Decomposition reaction is a reaction in which a compound decomposes into one or more than one compound or elements. In contrast, addition reaction is the opposite or reverse of the decomposition reaction. An example of addition reaction can be Hydrochlorination of Propane (Propane is an Alkene). Alkenes can be used to give rise to products with various stereoselectivity and regioselectivity depending upon reaction conditions and on the reagent.
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Key Terms: Addition, Reaction, Bond, Reactant, Compound, Electrophilic Addition Reactions, Nucleophilic Addition Reactions
Classification of Addition Reactions
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The classification for polar addition reaction are as follows: -
- Electrophilic Addition Reactions
- Nucleophilic Addition Reactions
The classification for non-polar addition reaction are as follows: -
- Free Radical Addition Reaction
- Cycloadditions Reaction
- Addition Polymerization
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| Benzaldehyde | Haloform Reaction Mechanism | Difference between Aldose and Ketose |
Electrophilic Addition
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Electrophilic Addition is a type of addition reaction in which a reactant with multiple bonds which essentially means in a triple or a double bond undergoes its pie bond broken and two new Sigma bonds are formed.
Nucleophilic Addition
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It is a type of addition reaction where an electron-deficient chemical compound or any Electrophilic triple or double bond, a pie bond, reacts with an electron-rich reactant or in other words a nucleophile with the disappearance of the double bond to create two new single or Sigma bonds.
Non-Polar Addition Reaction
Non-Polar addition reactions have concerted mechanisms or radical intermediates which essentially means no intermediates in the chemical reaction. According to established studies there are three types of Non-polar addition reactions.
Free Radical Addition Reaction
This type of non-polar reaction has a radical intermediate in the reaction mechanism. In this reaction, the carbon-carbon double bond acts as a nucleophile and thereby donates one electron to the halogen radical to form a bond of carbon and halogen. Another radical is formed due to the breaking of a double bond in the first step itself. Secondly, hydrogen is plucked from hydrogen-halide, by the newly generated carbon radical, for a new carbon-hydrogen bond. Hence radical addition takes place.
Addition Polymerization
Addition polymerization is a free radical addition reaction. However, in this reaction a large amount of monomer gets added in a chain in the formation of polymer compounds. We can take the example of Polyvinyl Chloride (PVC) which is formed when Vinyl Chloride is synthesized.
Cycloaddition
Cycloaddition reactions are concerted reactions with no intermediates in the mechanism of the reaction.
Markovnikov’s Rule
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Hydrogen is added to an asymmetric olefin at the carbon atoms with the most hydrogen atoms. (That is, the least substituted carbon.) A more substituted carbon atom receives an electronegative group.
This addition results in the creation of a more stable carbocation. Constitutional isomers can occur in this sort of reaction, but the primary product is invariably one of the products generated. When hydrogen is added to the terminal carbon, the carbocation becomes more stable, and the odds of stabilization grow with increasing conjugation with an olefin.
The terminal carbocation demands more activation energy, which is unfavourable for the process and may cause it to slow down. The non-terminal carbocation, on the other hand, is produced as a result of hyperconjugation stabilization, resulting in reduced activation energy.
Reaction of Alkene
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The trigonal planar structure of olefin carbon atoms causes the addition reaction to occur on both the equal and opposite sides (synperiplanar) (anti periplanar). Alkenes are often nucleophilic. The C=C double bond is responsible for a higher power HOMO (maximum occupied molecular orbitals). Electron donating companies improve the cost of electrophilic assault by assisting in carbocation and high-quality rate stability.
Halogenation Reaction
The pi bond of alkene and the bond of haloacid are disrupted in this reaction, resulting in the production of new bonds. Normally, the reaction follows Markovnikov's addition principle. The first step begins with the alkene pi bond acting as a Lewis base to feature an electrophile. In the second phase, the halogen starts out as a Lewis base to attack the carbocation, which acts as a Lewis acid.
The reaction is exothermic because the reactant has stronger electrical bonds than the products. Underneath this category, bromination reactions can be considered. Alkene combines with bromine in this reaction, resulting in the creation of a 3-membered cyclic bromonium ion intermediate. The bromide anion then attacks the cyclic bromonium ion to generate the product 1,2-dibromide.
Addition of Water or Solvent to an Alkene (Oxymercuration, Demercuration, Solvomercuration)
Most alkenes do not undergo hydration if subjected to aqueous acid. Thus a better approach was identified to overcome the problem.
- Oxymercuration: It involves the conversion of an alkene to organomercurial alcohol in the presence of an aqueous solvent.
- Demercuration: It is the transformation of organomercurial alcohol to corresponding alcohol.
- Solvomercuration: It is the change of organomercurial ether to product ether depending upon the solvent used.
Some General Reactions Involve:
Mechanism of these Reactions
The reaction is initiated by a double bond's nucleophilic attack on a positively charged mercuric acetate species, and the outcome is the production of a three-membered cyclic mercurinium ion. The solvent's attack on SN2 is what causes the production of organomercurial species in this step.
To create the final alcohol or ether, demercuration involves replacing the mercuric group with hydrogen. The addition of hydrogen occurs at the end that is least substituted, in accordance with Markovnikov's rule, which is how the reaction also proceeds.
In the presence of the trifluoroacetate mercury (II) salt Hg(OCOCF3) 2, solvomercuration occurs. Because it improves electrophilicity, trifluoroacetate salt is used instead of acetate salt. An illustration of this reaction is the conversion of 1-ethyl ethene to the associated ether.
Cyclopropane Ring Formation Reaction
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Simmon-Smith Reaction - Simmon-Smith response is the reaction between an alkene and di-iodo methane utilized to create non-halogenated cyclopropane when copper and zinc are present. The response becomes stereospecific when methylene institution is added to an alkene face that is less sterically hindered.
Johnson-Corey-Chaykovsky Reaction - The answer concerns the synthesis of epoxides from aldehyde and ketone, aziridines from imines, and cyclopropanes from enones. The response is diastereoselective because it prefers trans substitution within the product regardless of the initial stereochemistry. The very last products are produced in situ as a result of the deprotonation of sulfonium halides with strong bases.
HydroBoration Reaction - The hydroboration-oxidation response occurs whilst water is brought to an alkene in the presence of a boron reagent. The addition is syn addition observed with the aid of using cis stereoselectivity. Unlike different addition reactions, wherein the hydroxyl institution is brought to the least substituted carbon. In this response peroxide additionally has a crucial function to play in figuring out the web website online of addition. Here, the reagent BH3 which includes electrophilic boron is used and the hydrogen atom will become the electron donor.
Addition Reaction of Alkynes
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Hydration Reaction - Hydration in alkynes happens in the presence of mercuric salts as a catalyst. A more stable carbocation is formed as per the rule of Markovnikov because of the additional reaction.
Hydrohalogenation Reaction - Addition reaction of haloacids happens in anti-fashion. Geminal Dihalide is formed due to the initial attack which forms haloalkene.
Hydrogenation - Gaseous Hydrogen at high pressure and Transition Metal Catalyst is required for the Hydrogenetion of alkyne. Only the Hydrogenation which has Linder's catalyst will produce alkene with no further reaction. Although, direct use of Pt or Ni is avoided as it will result in Hydrogenetion to an Alkene. Cis alkene and syn stereoselectivity is produced in the presence of Linder's catalyst.
Things to Remember
- Addition reaction can be defined as when two or more reactants come together to form a single product through a chemical reaction.
- Addition reaction involves Hydration reaction, Hydrohalogenation Reaction, Hydrogenation.
- Cycloaddition reactions can be expressed as concerted reactions with no intermediates in the mechanism of the reaction.
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Sample Questions
Ques. What causes addition reaction? [2 marks]
Ans. Addition reaction is a part of organic chemistry. In addition reaction, two or more molecules combine to form a bigger one. Molecules with carbon-hereto double bonds, such as imine (C=N) or carbonyl (C=O), can be added because they have double bond character.
Ques. What is the function of peroxides in the addition of HBr to the alkenes? [1 mark]
Ans. Here, peroxides perform the function of initiating the radical chain initiator.
Ques. Which reaction takes place when HCN reacts with a ketone? [2 marks]
Ans. The reaction is the nucleophilic addition reaction. The C=O double bond which is in the ketone, reacts with the HCN. In this reaction, the electrophilic carbon of the C=O bond is attacked by the CN- ion.
Ques. Is halogenation an addition reaction? [2 marks]
Ans. In this type of reaction, a halogen molecule is added to an Alkene functional group's carbon-carbon double bond. This type of reaction is generally a combination of Electrophilic Addition and halogenation.
Ques. Which intermediate has the highest energy? [2 marks]
Ans. Intermediate C has the highest energy. As the Ea required for the formation of C is much much higher than that in the case of E and the energy released as a result of formation of C is also very less.
Ques. What is the correct IUPAC name of the following compound? [1 mark]
Ans. Two Chiral Carbons with S configurations are contained in the given chemical compound. Both of these carbons have S-configuration. Hence, the IUPAC name of the given compound is (1S, 3S)-1-chloro-3-methylcyclohexane.
Ques. What happens when propene reacts with methanol in the presence of sulphuric acid? [1 mark]
Ans. Propene reacts with methanol in the presence of sulphuric acid to form 2-methoxypropane.
Ques. What is the significance of adding H2SO4 during the addition of H2O to an alkene? [2 marks]
Ans. Since pure water is not acidic. Water is not capable of itself producing a strong enough electrophile (H+ ions) to initiate the reaction with the alkenes. In order for the reaction to begin sulphuric acid is mixed with water to make it an acidic solution. Alcohol is formed when acidic water is added to an alkene.
Ques. What is photo halogenation reaction? [2 marks]
Ans. When a halogen reacts with an alkane in the presence of heat or ultraviolet radiation, a haloalkane is formed. One example can be chlorination of methane.
Ques. What is the main feature of addition reaction? [2 marks]
Ans. An addition reaction occurs when an atom is added to a combination with a double or triple bond. These type of reactions are liked to unsaturated molecules. They are hydrocarbons with double or triple bonds. After completion of an addition reaction no reactant residue is left.
Ques. Predict which of the following RX will undergo SN2 reaction most quickly? [2 marks]
(a) CH3CH2F(b) CH3CH2I
(c) CH3CH2Br
(d) CH3CH2Cl
Ans. (b.) CH3CH2I
Ques. The specific rotation due to (R)-2-butanol is -13.5o. If the specific rotation of the mixture is -5.4o, what percentage of the mixture of the given two enantiomers will be the (S)-2-butanol? [3 marks]
Ans. Due to the enantiomeric excess, the specific rotation is arising in the above case. The percentage of e.e. can be calculated as:
% e.e. = [(-5.4o) / (-13.5o)] x 100 % = 40 %
Hence, (100 – 40) % = 60% of the remaining solution remains as the racemic mixture.
Thus, the total % of (R)-2-butanol = (40 + 60/2) % = 70%.
% of (S)-2-butanol = 100 % – 70 % = 30%.
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