Alkene Addition Reactions

Learning Objectives: After this lesson, you should be able to…

• Predict the product of an alkene addition reaction
• Identify the type of carbocation based on substitution

Get excited!! Here is your first chance to apply the basic mechanistic principles we have been learning!

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Generally, alkenes react with strong acids to produce an addition product (Figure 1). We will focus on products that are alkyl halides and alcohols. Hydrogen adds to one carbon of the alkene and "X" (I, Br, Cl, or OH) adds to the other. We will limit our discussion in this PLA to symmetric alkenes. In symmetric alkenes, either way H and X add gives the same product.

Here is a specific example:

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Which reactant is the nucleophile and which is the electrophile? Draw the mechanism! (Reflect on PLA16)

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You should have found a carbocation intermediate in your mechanism. Carbocations have a formal positive charge. We will discuss the stability of various types of carbocations in detail during class. For now, I want you to practice identifying types of carbocations. We will classify carbocations based on the number of alkyl groups attached. Primary carbocations have one bond to another carbon. Secondary carbocations have two bonds to other carbons. Finally, tertiary carbocations have three carbon groups attached. (See Figure 3.) Carbocations on an $$sp^2$$ hybridized center (vinyl or aryl) are too high in energy and we will not consider them.

Head back to Canvas to practice predicting the products of alkene addition reactions and determining carbocation substitution! :)