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Draw the Products of Alkene Addition Reactions

PostedJuly 30, 2022
UpdatedJuly 30, 2022
Bykshimizu

Introduction:

The next set of reactions that we learn (after the substitution and elimination reactions) are the addition reactions to alkenes.  We will also learn 8 to 10 different alkene reactions, which seems a bit overwhelming.  However, if you identify the common features of these reactions, then it is pretty easy to learn these reactions as a group.

The individual addition reactions may seem very different, but they all involve the breaking of the C=C pi-bond and the attachment of two new groups (A and B) via single bonds to each carbon of the C=C bond.  This could potentially lead to four different products.  Each addition reaction will give one or mixture of two of these four products.

The four products can be grouped into two categories based on:  

  1. Regiochemistry:  Is B (the more electronegative negative group) attached to the more or less substituted carbon of the C=C?
  2. Stereochemistry: Are the A and B groups on the same side (syn-product) or the opposite side (anti-product)?

So for each new addition reaction, you need to remember:  1) what are A and B, 2) what is the regiochemistry, and 3) what is the stereochemistry?  These are summarized in the table below below.

How to:

Follow the steps in the order below.

Step 1: Identify the alkene addition reaction based the reagents and the A and B groups.

The reagents are H2O and cat. H2SO4.  This matches up with the acid-catalyzed hydration reaction (from the Table), which adds an -H and -OH group to the alkene.

Step 2: Redraw the reactant on the product side and identify the alkene in the reactant by circling it.

Step 3: Identify the more and less substituted carbon of the alkene and erase the C=C double bond.

Step 4: Attach the A and B groups, paying attention to attaching A and B with the right regiochemistry.

In the acid-catalyzed hydration reaction, the -OH group is attached to the more substituted carbon (and the -H group is attached to the less substituted carbon).

Step 5: Apply the appropriate stereochemistry (whether the A and B groups are on the same or opposite sites or both).

In the acid-catalyzed hydration reaction, stereochemistry of the -OH and -H groups is a mixture of same and opposite sides.  (Note: When you have a mixture of same and opposite sides, it is best to switch the group on the more substituted carbon.)

Step 5: Clean up the product by removing any unnecessary hydrogens and checking if the two products are the same.

In these products, we can remove the -H’s because it would not matter if they were up or down.  The two products are also different (enantiomers) so we need to show both of them (or in this case we can just show the racemic product).

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