When multiple bonds like a double or a triple bond get converted into other functional groups, an addition reaction is considered to have taken place. The reaction converts an unsaturated compound to more saturated and functionalized species. This module focuses on a number of examples of electrophilic addition to electron rich double bonds. When a double bond is activated by addition electron withdrawing groups, conjugated addition happens.
Electrophilic Addition Reaction/Markovnikov’ Addition
Markovnikov’s Rule
Hydrogen is added to an unsymmetrical olefin at those carbon atoms which have the maximum number of hydrogen atoms. (i.e.the carbon with least substitution). An electronegative group gets attached to a more substituted carbon atom. This kind of addition leads to the formation of a more stable carbocation. This type of reaction may form constitutional isomers but actually one of the products formed is always the major product.
If hydrogen is added at the terminal carbon better stabilization of carbocation happens and then the chances of stabilization become more with the increase in conjugation with an olefin. The terminal carbocation needs higher activation energy which is somehow unfavorable for the reaction and may lead to retardation of the reaction rate. However, the non-terminal carbocation is generated due to hyperconjugation stabilization that leads to the lower activation energy.
Reaction of Alkene
The trigonal planar geometry of olefin carbon atoms makes the addition reaction happen either on the same side (synperiplanar) or on the opposite sides (anti periplanar). Alkenes are mostly nucleophilic. The C=C double bond is responsible for a higher energy HOMO (highest occupied molecular orbitals). Electron donating groups enhance the rate for electrophilic attack as they help in carbocation and positive charge stabilization.
1.Halogenation Reaction
In this reaction, the pi bond of alkene and σ bond of haloacid gets broken resulting in the formation of two new σ bonds. The reaction generally happens following Markovnikov’s addition principle. The first step begins with the alkene pi bond acting as a Lewis base to add to an electrophile. In the second step, the halogen starts acting as a Lewis base to attack the carbocation that behaves as the Lewis acid. The reaction is exothermic in nature as reactant possesses higher energy bonds than the products. Bromination reaction can be considered under this category.
In this reaction, alkene interacts with bromine resulting in the formation of a three-membered cyclic bromonium ion intermediate. The bromide anion then attacks the cyclic bromonium ion to generate the product 1,2-dibromide.
Mechanism of Formation of Halohydrin
The reaction kicks off with the attack of π bond of alkene on σ* bond of Br2 that forms a three-membered cyclic bromonium ion. Water then attacks the bromonium ion through an SN2 transition state. Markovnikov’s rule is generally followed.
2.Addition of Water or Solvent to an Alkene (Oxymercuration, Demercuration, Solvomercuration )
Most of the alkenes do not undergo hydration if subjected to aqueous acid. Thus a better approach was identified to overcome the problem.
Oxymercuration:- involves the conversion of an alkene to organomercurial alcohol in the presence of an aqueous solvent.
Demercuration:- is the transformation of organomercurial alcohol to corresponding alcohol.
Solvomercuration:- is the change of organomercurial ether to product ether depending upon the solvent used.
General Reactions Involve:
Mechanism of these Reactions
Nucleophilic attack by double bond on +vely charged mercuric acetate species starts the reaction and the result is the formation of three-membered cyclic mercurinium ion. In this step, the formation of organomercurial species is due to SN2 attack by the solvent.
Demercuration involves substitution of hydrogen in place of the mercuric group to produce the final alcohol or ether. The reaction also happens following Markovnikov’s rule i.e. addition of hydrogen takes place at the least substituted end.
Solvomercuration happens in the presence of trifluoroacetate mercury (II) salt {Hg(OCOCF₃)₂}. The trifluoroacetate salt is used instead of acetate salt as it enhances electrophilicity. The conversion of 1-ethyl ethene to corresponding ether is an example of this reaction.
Cyclopropane Ring Formation Reaction
1. Simmon-Smith Reaction
The reaction of an alkene with di-iodo methane which is used for the synthesis of non-halogenated cyclopropane, in the presence of copper-zinc couple is called Simmon-Smith reaction. The addition of methylene group to a less sterically hindered face of alkene makes the reaction stereospecific.
The Simmons–Smith reaction is more preferable compared to other methods of cyclopropanation. However, it is comparatively expensive due to the high cost of diiodomethane. Modifications with cheaper alternatives have been now figured out, like dibromomethane or diazomethane and zinc iodide. The use of Furukawa modification can increase the reactivity of the system. This process involves the exchange of the zinc‑copper couple for diethylzinc.
The Simmons–Smith reaction is generally subjected to steric effects, and thus cyclopropanation usually occurs on the less hindered face. However, when a hydroxy substituent is there in the substrate with close proximity to the double bond, the zinc interacts with the hydroxy substituent, thus pushing cyclopropanation cis to the hydroxyl group.
Mechanism Involved in this Reaction
The mechanism involves the transfer of carbene assisted by Zinc catalyst. The stereoselectivity varies depending on the face on which the addition occurs&what type of different groups is present on the substrate. In substrates like allylic alcohols, the formation of the cyclopropane ring will occur on the same side of the –OH group due to its weak bonding.
2. Johnson-Corey-Chaykovsky Reaction
The reaction is about the synthesis of epoxide generating from aldehyde & ketone, aziridines from imines, cyclopropanes from enones. The reaction is diastereoselective as it favors trans substitution in the product irrespective of the initial stereochemistry. The final products are generated in situ as a result of the deprotonation of sulfonium halides with strong bases.
Mechanism Involved
In the reaction starts with the attack of the nucleophilic sulfur ylide on the carbonyl or imide substrate. The negative charge gets then transferred from anionic carbon of ylide to electronegative group of the substrate. This happens because sulfonium cation is expelled and cyclopropane or epoxide ring is formed.
3. Hydro Boration Reaction
The hydroboration-oxidation reaction happens when water is added to an alkene in the presence of boron reagent. The addition is syn addition accompanied by cis stereoselectivity. Unlike other addition reactions, where the hydroxyl group is added to the least substituted carbon. In this reaction peroxide also has an important role to play in deciding the site of addition. Here, the reagent BH3 consisting of electrophilic boron is used and the hydrogen atom becomes the electron donor. There is no carbocation intermediate involved in these reactions. This suggests that a concerted addition has happened. BH3, if used as
reagent then it hydroborates three alkene units. The number of alkenes undergoing hydroboration and the no. of hydrogens attached directly to boron in the borane reagent are always equal. The products obtained at the end are a racemic mixture. Use of chiral borane reagents increases the stereoselectivity.
Mechanism
In the first step nucleophilic alkene attacks on electrophilic Boron. In the Second step, Peroxide acting as nucleophile attacks the electrophilic boron. This step is followed by migration of C-B bond to generate C-O bond. Alcohol is formed due to hydrolysis.
Nucleophilic Addition Reactions of Olefins
The electron rich HOMO favors the reaction of olefins with electrophiles. For nucleophilic addition to happen the alkene should have an electron withdrawing group attached to it. This group can withdraw electron density from the pi-bonds of alkenes. These electron-withdrawing groups make the LUMO become more stabilized. This helps to enhance the interaction with the incoming nucleophile. This class of reactions includes Conjugate addition reactions and Hydroamination reaction.
The simplest example of an addition reaction can be defined as the chemical transformation of a carbon-carbon double bond. Many reagents, both inorganic and organic, add to this functional group. Most of these reactions are exothermic in nature. It is due to the fact that the C-C pi-bond is relatively weak (ca. 63 kcal/mole) as compared to the sigma-bonds, that are formed to the atoms or groups of the reagent. It is important to remember that the bond energies of a molecule are the energies needed to break (homolytically) all the covalent bonds present in the molecule. Consequently, if the bond energies of the product molecules are more than that of the bond energies of the reactants, the reaction will be an exothermic one.
Addition Reaction of Alkynes
1. Hydration Reaction
Hydration in alkynes happens in the presence of mercuric salts as catalyst. The addition results in the formation of more stable carbocation as per the Markovnikov’ s rule
2. Hydrohalogenation Reaction
Addition reaction of haloacids happens in anti-fashion i.e. the more substituted carbon forms the carbocation to which halogen is attached. The Initial attack forms haloalkene which again reacts to produce the geminal dihalide.
3. Hydrogenation
Hydrogenation of alkyne requires gaseous hydrogen at high pressure and transition metal catalyst (Pt, Pd, Ni ). Hydrogenation with Lindlar’s catalyst (prepared by deactivation of palladium catalyst when treated with lead acetate and quinoline) produces alkene with no further reaction. Although, direct use of Pt or Ni is avoided as it will result in hydrogenation to an alkane. Reaction in the presence of the above-mentioned catalyst produces cis alkene with syn stereoselectivity.