14.0: Prelude to Electrophilic Reactions In this chapter, we will learn about a class of organic reaction that is central to the biosynthesis of ergot alkaloids in Claviceps. The key first step in the biosynthetic pathway is a reaction unlike any we have yet seen. 14.1: Electrophilic Addition to Alkenes The simplest type of electrophilic reaction to visualize is the addition of a haloacid such as \(HBr\) to an isolated alkene. It is not a biological reaction, but nonetheless can serve as a convenient model to introduce some of the most important ideas about electrophilic reactions. 14.2: Elimination by the E1 Mechanism The reverse of electrophilic addition is called E1 elimination. We will begin by looking at some non-biochemical E1 reactions, as the E1 mechanisms is actually somewhat unusual in biochemical pathways. 14.3: Electrophilic Isomerization Electrophilic reactions in biochemistry are not limited to addition to alkene double bonds. The position of a double bond in an alkene can also be shifted through an electrophilic, carbocation-intermediate reaction. An electrophilic alkene isomerization occurs when an initial \(\pi \) bond protonation event (step 1 below) is followed by deprotonation of an adjacent carbon to re-form the \(\pi \) bond in a different location. 14.4: Electrophilic Substitution Until now, have already been introduced to electrophilic addition and electrophilic isomerization - now, let's move to the third variation on the electrophilic theme, that of electrophilic substitution. In an electrophilic substitution reaction, a pair of \(\pi\)-bonded electrons first attacks an electrophile - usually a carbocation species - and a proton is then abstracted from an adjacent carbon to reestablish the double bond, either in the original position or with isomerization. 14.5: Carbocation Rearrangements Earlier in this chapter we introduced the so-called 'Markovnikov rule', which can be used to predict the favored regiochemical outcome of electrophilic additions to asymmetric alkenes. According to what we have learned, addition of HBr to 3-methyl-1-butene should result in a secondary bromoalkane. 14.E: Electrophilic Reactions (Exercises) 14.S: Electrophilic Reactions (Summary) 14.6: Cycloaddition reactions Cylcoaddition reactions are addition reactions that form a ring. The best-known cycloaddition reaction is the Diels-Alder reaction. It typically occurs between a conjugated diene and another alkene known as the dienophile. This reaction is very useful in synthesis because it forms two new C-C bonds at the same time.