# 14: Electrophilic Reactions

• 14.1: 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.
• 14.2: 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.
• 14.3: 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.
• 14.4: 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.
• 14.5: 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.
• 14.6: 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)