Sapling Synthesis Problem

Chapter 7: Problem #27 Explained

Step 1: Reactant

Figure 1

For this step, you simply must know what acetylene is, which is not necessarily easily discernible from the name.  However, from our discussions in class and readings from the book, we know that acetylene has a structure of HCCH, with the central carbons connected by a triple bond as seen in Figure 1.

Step 2: Product

Figure 2

Now we need to have an idea of our desired product so that we can plan out our reaction pathway.  Trans-2-pentene will be a 5-carbon chain, with a double bond between carbon 2 and carbon 3 and the hydrogens on those two carbons should be on opposite sides from one another, as seen in Figure 2. (hydrogens other than the two necessary for the trans attribute are implied but not shown.)

         Step 3: Plan

Figure 3

The acetylene reactant has only two carbons but the desired product has five, so it is obvious that some carbons will need to be added along the way.  We also start with an alkyne and need to end with a central alkene.  We know how to add carbons to a carbon chain with an alkyne by bromination so that is a place to start.  Since the alkene needs to be in the middle, we will add to both ends of the chain.

Step 4: Carbon Addition

Figure 4

Creating carbon-carbon bonds is accomplished by the addition of NaNH₂ followed by the alkyl halide of desired length.  One end of the acetylene needs an added methyl group, so we treat the reactant with NaNH­₂ first, and then methyl bromide (shortened as MeBr), to get our first intermediate as shown in Figure 3.

  This process is then repeated, replacing the methyl bromide with an ethyl bromide (shortened to EtBr) to add two carbons to the remaining side of the alkene. This leaves us with a 5-carbon chain with the alkyne between carbons two and three.  This yields are second intermediate, as seen in Figure 4.

Step 5: Hydrogen Addition

  To reduce the alkyne to and alkene, we need to add hydrogen and for the proper trans product it needs to be a mechanism that allows for that.  The most commonly used H₂/Pd/C approach would yield a cis product and so may not be used.  Alternatively, using Na and NH₃ as the reagents will generate the trans product that we want.  The full reaction is laid out below in Figure 5.

 

 All figures were generated using ChemBioDraw Ultra.  The Problem was taken from Sapling Learning.