Reactions of Alkenes, Chemistry tutorial

Reactions of Alkenes:

The double bond consists of a strong σ bond and a weak π bond; so most of the reactions of alkenes would involve the breaking of this weaker bond.

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In addition reactions of alkene, the π -bond is broken and the electron pair comprising it is utilized in the formation of 2 new σ bonds. Thus, two sp2hybsridised carbon atoms are rehybridised to sp3carbons. Compounds containing π bonds are generally of higher energy than those having σ bonds. As a result, addition reactions are generally exothermic procedures.

In the region of the double bond, there is a cloud of electrons above and below the plane of bonded atoms. The π electrons are loosely held via the nuclei and are therefore easily available to electron-seeking reagent. These reagents are termed electrophilic reagents or electrophiles and the typical reaction of an alkene is the electrophilic addition. Several electrophilic significant reactions of alkenes are given in Table and discussed below:

Table: Reaction of Alkenes

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Addition of Halogens:

Halogens are fairly reactive towards alkenes. Treatment of alkens by halogens provides 1, 2-dihalogenated alkenes.

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Bromine and chlorine are mainly effective electrophilic addition reagents. Fluorine tends to be as well reactive and difficult to control for most laboratory procedures and iodine doesn't react with alkenes.

Mechanism:

Even though bromoine is non-polar, it is nevertheless highly polarisable and, in the vicinity of the nucleophilic double bond, the bromine molecule becomes polarised and therefore a partial positive charge (∂+) develops on 1 bromine atom and a partial negative charge (∂-) on another. The π electrons of alkene attack the positive end of the polarised bromine molecule, displacing bromide ion and forming a cyclic bromonium ion.

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The cyclic structure shields one side of the molecule and, for this reason, Br attacks from the opposite side of the erstwhile double bond to provide trans product. This process is recognized as transudation. This steric course of the reaction is significant in case of alkenes that can offer grow to different isomeric products. Addition of bromine is very useful for detection of carbon-carbon double bond. Quick decolourisaion of bromine solution serves as a test for the presence of the carbon-carbon double bond in a compound.

Hydrohalogenation:

An alkene is transferred via 4 hydrogen halide (halogen acid) into the analogous alkyl halide,

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As long as the alkene is symmetrical, we get only one product. In case of unsymmetrical alkene, the position of attachment of nucleophile is governed via the nature of substituents. Addition of HBr to propene should provide 2 products, for example, 1-bromopropane and 2-bromopropane.

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However, only one product, 2-bromopropane, is produed. These reactions are termed regiospecific reactions. Regiospecific: Only one of the 2 directions of addition is observed. To illustrate the exclusive formation of the product, the Russian chemist Markownikoff formulated a rule distinguish after him as Markownikoffs rule, which states that addition of a hydrogen halide to an unsymmetrical alkene occurs in such a way that the negative part of the reagent goes to that carbon atom of the alkene which carries the lesser number of hydrogen atoms. Markownikoffs rule can be clarified on the basis of the relative stabilities of carbocations which are of the order of tertiary, > secondary > primary. Consequently, the more substituted carbocation is shaped as an intermediate in preference to the less substituted one. For instance, in the addition of H+to propane, there exists the possibility of the formation of either a primary or a secondary carbocation. Because, the secondary carbocation is more stable, addition of H+ provides exclusively 2-bromopropane via the more stable intermediate.

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Peroxide effect:

we might be under the impression that additions to alkene always provide Markownikoffs product. But it is not so. After an extensive learn of the mechanism of addition of HBr to alkene, kharasch and Mayo found that in the presence of peroxide the product obtained was not the one predicted by Markownikoffs rule but it was contrary to the Markownikoffs rule. Such additions are sometimes referred to as anti-Markownikoff additions. Since the reversal of the addition reaction is brought about in the presence of peroxide, it is recognized as the proxide effect. For instance, the addition of hydrogen bromide to propene in the presence of peroxides provides 1-bromopropane rather than 2-bromopropane.

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The reaction in between in such additions is a free radical rather than a carbocation. The mechanism is somewhat alike to that of halogenation of an alkane that will be dealt in the "Organic Reaction Mechanism" course.

Addition of Water:

Addition of H2O to alkene is known as hydration of alkene. Hydration reaction occur when H2O, adds to alkenes in the presence of an acid catalyst to yield an alcohol,

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Like hydrogenation, addition of H2O to unsymmetrical alkene follows Markownikoffs rule.

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Another way utilized to accomplish Markownikoffs hydration of an alkene is oxymercuration-demercuration. Alkene reacts by mercuric acetate in the presence of water to give hydroxyl-mercurial compounds which on reduction accomplishes demercuration and produces an alcohol. The product of oxymercuration is usually decreased by sodium borohydride (NaBH4). Oxymercuration-demercuration reaction generally gives better yield of alcohols than the addition of water with H2SO4.

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 Hydroboration

Whenever an alkene reacts by borane, addition to the carbon-carbon double bond takes place to yield an organoborane - a compound with a carbon-boron bond. The reaction is known as hydroboration. Borane (BH3) itself is unknown but its dimmer, diborane (B2H6) behaves as if it were the hypothetical monomer (BH3)2. This reaction is very facile and requires only few seconds for completion at 272 K and gives organoboranes in very high yield.

 CH2 = CH2 + (BH3)2   → CH3 CH2 - BH2

Since BH3 has three hydrogen, addition occurs three times to produce triakylborane product e.g.

CH2 = CH2 + CH3CH2 - BH2    → (CH3CH2)2BH

CH2 = CH2 + (CH3CH2)2BH2    → (CH3CH2)3B

Hydroboration reaction is illustrated as anti-Markownikoffs addition. This is true only in literal sense, since hydrogen is the electronegative portion of the molecule instead of the electropositive portion.

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As given above the hydrogen (as a hydride ion, H-) goes to more substituted carbon. The effect appears to be anti-Markownikoffs addition. Organoborane are usually not isolated but are instead utilized directly as reactive intermediates for further synthetic reaction. For instance, oxidation of organoborane by alkaline H2O2 provides analogous alcohol. Treatment of organoboranes by a carboxylic acid leads to alkane,  

Ozonolysis:

In all the reactions of alkenes learnt so far, the carbon skeleton of the starting material was left intact. We have seen the adaptation of the carbon-carbon double bond into new functional groups (halide, alcohol, etc) via adding different reagents, but the carbon skeleton was not broken or rearranged. Ozonolysis is a cleavage reaction, for example a reaction in which the double bond is completely broken and alkene molecule is converted into 2 smaller molecules. Ozonolysis consists of two separate reactions, the first is oxidation of alkene via ozone to give an ozonide; and the second is reduction of the ozonide to yield cleavage product. Several examples of ozonolysis are following below:

Hydroxylation:

Alkenes are readily hydroxylated (addition of hydroxyl groups) to form a dihydroxy compound (diol) recognized as glycols. The  most popular reagent used to convert an alkene to diol is cold alkaline aqueous of potassium permanganate or osmium tetroxide.

Epoxidation:

The double bond in alkene is converted into epoxide via means of peracids. Perbenzoic acid (C6H5COO2H), monoperphthalic acid (HO2CC6H4COO2H) and p-nitroperbenzoic acid have been utilized, for example:

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Addition to Conjugated Diene:

Alkadienes by conjugated system of double bonds undergo abnormal addition reactions, for example, whenever 1, 3-butadience is treated through bromine, and 2 dibromo derivatives are attained. One of such is 3, 4-dibromo - 1-butene (due to 1:2 addition) and the other is 1, 4-dibromo-2-butene (due to 1:4 addition), a main product.

                                                         Br   Br                      Br               Br

                                                         |      |                        |                |

CH2 = CHCH = CH2      →           CH2CHCH=CH2      +         CH2CH=CHCH2

1, 3-butadine                                3, 4-dibromo-1-butane            1, 4-dibromo-2-butene

                                                               (minor)                             (major)

Through excess of bromine, the 1, 4 addition as well as the 1, 2-addition products would yield similar 1, 2, 3, 4-tetrabromonutane.

Mechanism:

The mechanism of halogenation of 1, 3-butadiene is illustrated below:

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Bromine might attach itself to either C1 or C2. The addition of the bromine atom at C2 would provide rise to an unstable primary localized carbocation. But the bromine addition at C1 consequences in the formation of resonance stabilized allylic cation. This as well illustrates the enhanced reactivity of dienes over isolated ethylenic double bonds. Whenever the allylic, carbocation is attacked via bromine ion (Br) to complete the electrophic addition reaction, the attack can occur at either C1 or C3, because both share the positive charge. The consequence is a mixture of 1, 2- and 1, 4-addition products, the latter formed in excess because it has the more highly substituted double bond and is therefore more stable.

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Diels-alder reaction:

In Diels-Alder reaction, a conjugated diene is treated through an unsaturated compound termed as the dienophile (diene-lover) to yield, a cyclic system. This reaction is symbolized after the German chemists, Diel and Alder. It is a extremely useful reaction for synthesizing cyclic systems. The simplest Diels-Alder reaction is the reaction of 1, 3-butadiene via ethane to yield cyclohexene. The resulting product (here cyclohexene) is termed as the adduct.

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This is an extremely slow reaction and it takes place only under conditions of heat and pressure. Diels-Alder addition take place most speedily and provide the highest yield if the alkene component has electron withdrawing groups or the diene has electron donating groups. The reaction has wide scope since triple bonded systems as well might be utilized as dienophiles. Various significant examples of Diels-Alder reaction are following below:

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