Helpful Hints - Alkynes

1. Alkynes have pi bonds that are weakly nucleophilic, so the reactions of alkynes are generally VERY similar to reactions of alkenes.

2. Alkynes can be prepared from a double elimination of H-X from dihalides; either vicinal (1,2) or geminal (1,1) dihalides.

3. Alkynes add H-X via a vinylic carbocation to give mostly the product derived from anti addition, that can add another molecule of H-X to give geminal dihalides (Markovnikov regiochemistry for terminal alkynes).

4. X2 adds to alkynes to give trans dihalo alkenes, that add another molecule of X2 to give tetrahaloproducts.

5. Alkynes react with HgSO4 in water to give an enol, that rearranges to the ketone product. Markovnikov addition is observed with terminal alkynes.

6. Hydroboration gives the alternative, non-Markovnikov ketone product with terminal alkynes.

7. Normal catalytic reduction of alkynes using hydrogen goes all the way to an alkane. Using the Lindlar catalyst gives a cis alkene. Using Li/NH3 gives the trans alkene.

8. Alkynes are oxidized with KMnO4 to give carboxylic acids (internal alkynes) and/or CO2 (external alkyne).

9. Terminal alkynes are relatively acidic (pKa ~ 25), since the resulting anion has the negative charge on an sp hybridized carbon atom. The high percent (50%) of scharacter in an sp orbital is stabilizing (s orbital are lower in energy than p orbitals).

10. Deprotonated alkynes can react with methyl or primary alkyl halides to give new carbon-carbon bonds!!!!!!

Homework Problems 8.25-8.33 are VERY IMPORTANT problems, since they deal with organic synthesis.