ORGANIC 2 DISCUSSION

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Your Name:

Alkynes have a very interesting structure as they consist of a sigma bond and two separate pi-

bonds to make up the overall triple bond characteristic of the alkyne functionality.

1. Consider the structures below and discuss the reason for the different pKa values being indicated

for the C-H bond of a terminal alkyne vs. vinyl C-H (alkene) vs. alkyl C-H bond. From these

values it is clear that the terminal alkyne is most acidic (lower pKa). Your discussion needs to

address the reason why rather than simply say “because the pKa is lower”. Why is the pKa lower?

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2. Terminal alkynes are a valuable carbon source due to their relatively more acidic C-H bond.

Furthermore, the C-C triple bond of an alkyne can be further reacted upon to form other useful

functional groups. Consider the transformation below and propose a synthesis for the product

being indicated starting from the acetylene (otherwise known as ethyne). You may use any other

reactions that have been covered in chapter 9 as well as those covered in previous chapters of the

text. Show all reagents needed and draw the product expected after each step in your proposed

synthesis. It may be helpful to draw out a retrosynthetic scheme first so that you have a workable

plan for your synthesis. Make sure that the reactions used are consistent with the relative

stereochemistry indicated in the product shown.

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3. Allenes have both similar and different structural properties compared to alkynes. Look up the

three-dimensional orbital diagram for both an alkyne and an allene. Then, redraw each structure

below showing the hybrid atomic orbitals clearly. Label the specific hybrid orbitals that overlap

to make the sigma-bonds and also label each of the p-orbitals clearly. It will be helpful to first

know what the hybridization of each carbon atom is when doing this so that you know what atomic

orbitals (hybridized and unhybridized) are available to hold the bonding and non-bonding

electrons. In your drawing, be sure to clearly illustrate the plane each of the pi-bonds are oriented

about and explain why different planes are required for each of the pi-bonds, respectively.

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4. Interestingly, although the alkyne and allene look similar in terms of the orbitals, the protons

attached to them have very different 1H NMR chemical shifts. The allene proton chemical shift

is approximately 4.5 ppm whereas the alkyne proton chemical shift is approximately 2.1 ppm.

Propose a reason for why these two protons have drastically different 1H NMR chemical shifts.

(Hint: look up the concept of ring currents). Draw a picture that illustrates this explanation clearly.