Electrophilic addition reactions examples

The reaction is an example of a polar reaction type known as an electrophilic addition reaction and can be understood using the general ideas discussed in the previous section. Let s begin by looking at the two reactants. 

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.5 that alkenes behave as nucleophiles (Lewis bases) in polar reactions. The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile (Lewis acid), for example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methylpropane. A careful study of this and similar reactions by Christopher Ingold and others in the 1930s led to the generally accepted mechanism shown in Figure 6.7 for electrophilic addition reactions. 

What evidence is there to support the carbocation mechanism proposed for the electrophilic addition reaction of alkenes One of the best pieces of evidence was discovered during the 1930s by F. C. Whitmore of the Pennsylvania State University, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HC1 with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the "expected" product, 2-chloro-3-methylbutane. 

The synthesis of polyhalide salts, R4NX , used in electrophilic substitution reactions, are described in Chapter 2 and H-bonded complexed salts with the free acid, R4NHX2, which are used for example in acid-catalysed cleavage reactions and in electrophilic addition reactions with alkenes, are often produced in situ [33], although the fluorides are obtained by modification of method I.I.I.B. [19, 34], The in situ formation of such salts can inhibit normal nucleophilic reactions [35, 36]. Quaternary ammonium chlorometallates have been synthesized from quaternary ammonium chlorides and transition metal chlorides, such as IrClj and PtCl4, and are highly efficient catalysts for phase-transfer reactions and for metal complex promoted reactions [37]. 

Quaternary ammonium tribromides can also be produced in situ from the quaternary ammonium bromide, sodium hypochlorite and sodium bromide and can be used, for example, in electrophilic addition reactions reaction with alkenes and alkynes. 

It is important to be able to look at a molecular structure and deduce the possible reactions it can undergo. Take an alkene, for example. It has a 7t bond that makes it electron-rich and able to attack electrophiles such as water, halogens and hydrogen halides in electrophilic addition reactions. Haloalkanes, on the other hand, contain polar carbon-halogen bonds because the halogen is more electronegative than carbon. This makes them susceptible to attack by nucleophiles, such as hydroxide, cyanide and alkoxide ions, in nucleophilic substitution reactions. 

The double bonds in 2,3-dihydro-l,4-dioxin, 2,3-dihydro-l,4-oxathiin and 2,3-dihydro-1,4-dithiin undergo standard electrophilic addition reactions. Under acid catalysis, methanol adds to 2,3-dihydro-l,4-oxathiin to give 2-methoxy-1,4-oxathiane (66HC(21-2)842). Various examples are available of reactions of the double bonds with carbenoids to give bicyclo[4.1.0]diheteroheptanes (77LA910,78ZC15), and with alkenes in [2 + 2] cycloadditions (78CB3624). 

In general, the mechanisms of nucleophilic additions to double bonds have not been as much studied or systemized as those of electrophilic addition. Reactions 7.51 and 7.52 are examples of the very useful Michael condensation, in which a carbanion adds to an a,/ -unsaturated carbonyl or nitrile compound. The usefulness of these reactions arises from the fact that the number of ways of building longer carbon chains from smaller ones is limited. 

Electrophilic addition. The examples available succeed by virtue of functionality placed near the reacting center. In the instances illustrated (Figure 2) the hydroxyl group of an allylic alcohol participates intimately with the electrophilic species to provide an ability to discriminate the faces of a double bond (24 2 5). The latter reaction was employed in a sequence eventuating in the sex pheromone of the gypsy moth, disparlure. 

Alkane functionalization by electrophilic addition reactions is also possible for example, the particularly stable tertiary adamaniyl cation must be involved in equation (34), a reaction which gives an excellent 75% yield of adduct In a similar way, a variety of alkenes and arenes can be alkylated by alkanes, or alkanes acylated by RCOCl/AlBra. ... 

The word mechanism will often be used loosely here. In contrast to the S l reaction of alkyl halides or the electrophilic addition reactions of alkenes, the details of some of the mechanisms presented in Chapter 12 are known with less certainty. For example, although the identity of a particular intermediate might be confirmed by experiment, other details of the mechanism are suggested by the structure or stereochemistry of the final product. 

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. For example, addition ofHBr to 1,3-butadiene yields a mixture of two producLs (not counting cis-ivans isomers). 3-Bromo- l-butene is the typical Markovnikov product of 1,2 addition, but l-bromo-2-butene appears unusual. The double bond in this product has moved to a position between carbons 2 and 3, and HBr has added to ceirbons 1 and 4, a result described as 1,4 addition. 

The most common example of electrophilic substitution at a trigonal planar center is electrophilic aromatic substitution, which will serve as our archetype. Aromaticity was covered in Section 1.9, and the reactivity trends of aromatics will be covered in detail in Chapter 5. Chapter 8 has additional reaction examples. Aromatic rings are usually poor nucleophiles therefore excellent electrophiles are needed for the reaction to proceed. 

Such structure reactions have existed in organic chemistry since long. Wilson gives an example of classification of electrophilic addition reactions of alkenes and alkynes. Patterns in organometalhc chemistry with applications in organic synthesis have been discussed by Schwartz and Labinger. 

This reactivity makes alkenes an important class of organic compounds because they can be used to synthesize a wide variety of other compounds. For example, alkyl halides, alcohols, ethers, and alkanes all can be synthesized from alkenes by electrophilic addition reactions. The particular product obtained depends only on the electrophile and the nucleophile used in the addition reaction. 

When an alkene that does not have the same substituents on its sp carbons undergoes an electrophilic addition reaction, the electrophile can add to two different sp carbons. We have just seen that the major product of the reaction is the one obtained by adding the electrophile to the sp carbon that results in the formation of the more stable carbocation (Section 4.3). For example, when propene reacts with HCl, the proton can add to the number-1 carbon (C-1) to form a secondary carbocation, or it can add to the number-2 carbon (C-2) to form a primary carbocation. The secondary carbocation is formed more rapidly because it is more stable than the primary carbocation. (Primary carbocations are so unstable that they form only with great difficulty.) The product of the reaction, therefore, is 2-chloropropane. 

Using the rule that the electrophile adds to the sp carbon bonded to the greater number of hydrogens is simply a quick way to determine the relative stabilities of the intermediates that could be formed in the rate-determining step. You will get the same answer, whether you identify the major product of an electrophilic addition reaction by using the rule or whether you identify it by determining relative carbocation stabilities. In the following reaction for example, H is the electrophile ... 

Some electrophilic addition reactions give products that are clearly not the result of the addition of an electrophile to the sp carbon bonded to the greater number of hydrogens and the addition of a nucleophile to the other sp carbon. For example, the addition of HBr to 3-methyl-1-butene forms 2-bromo-3-methylbutane (minor product) and 2-bromo-2-methylbutane (major product). 2-Bromo-3-methylbutane is the product you would expect from the addition of H to the sp carbon bonded to the greater number of hydrogens and Br to the other sp carbon. 2-Bromo-2-methylbutane is an unexpected product, even though it is the major product of the reaction. 

When a reactant that does not have an asymmetric carbon undergoes a reaction that forms a product with one asymmetric carbon, the product will be a racemic mixture. For example, the reaction of 1-butene with HBr forms identical amounts of (/ )-2-bromobutane and (5)-2-bromobutane. Thus, an electrophilic addition reaction that forms a compound with one asymmetric carbon from a reactant without any asymmetric carbons is not stereoselective because it does not select for a particular stereoisomer. Why is this so ... 

AS you continue your study of organic chemistry, you will notice that the concept of having delocalized electrons is invoked frequently to explain the behavior of organic compounds. For example, in Chapter 8 you will see that having delocalized electrons causes certain dienes to form products that would not be expected on the basis of what you have learned about electrophilic addition reactions in Chapters 3-6. Electron delocalization is such an important concept that this entire chapter is devoted to it. 

The electrophilic addition reactions of A -unsaturated steroids and other rigid cyclohexenes are controlled mainly by the conformational preference for diaxia) addition HOBr, for example, gives mainly a 5a-bromo-6)5-alcohol. A study of similar reactions with B-nor-A -unsaturated steroids suggests that the reaction of a cyclopentene is under electronic rather than conformational control.A variety of reagents (HOBr, BrF, Brj, BrOMe, and BrOAc) gave mainly 6a-bromo-5)S-substituted derivatives (155), indicating that the initial product, a 5a.6a-bromonium ion (154). reacts further according to Markovnikoff, with attack of the anion at the tertiary 5 -position. 

Contrary to some reports, electrophilic addition reactions may occur in other multiple-bond systems. In many of the reactions of aldehydes and ketones the first stage involves the addition of some entity across the carbon-oxygen bond, e.g., the formation of oximes, semicarbazones, hydrazones, hydrates (1,1-diols) and their ethers, and the aldol condensation. Most of these reactions entail a subsequent loss (elimination) of a small molecule e.g. water, ammonia, ethanol) and, while one must be careful to determine whether the rate-determining stage involves attack on the carbonyl compound or elimination from the adduct , there are some systems in which it is evident that electrophilic attack is involved in the slow stage of the reaction sequence. Examples of such reactions are the acid-catalysed formation of oximes of aliphatic - and aromatic carbonyl compounds, of furfural semi-carbazone , and of 1,1-diols from aldehydes or ketones . 

---