Chapter 9, Problem 42DSP

Thinking Mechanistically About Alkynes

The preparation and properties of alkynes extend some topics explored in earlier chapters:

* Alkynes can be prepared by elimination reactions related to the $\text{E2}$ dehydrohalogenation of alkyl halides used to prepare alkenes.

* Alkynes can be prepared by ${\text{S}}_{\text{N}}\text{2}$ reactions in which a nucleophile of the type $\text{RC}\equiv {\text{C:}}^{-}$ reacts with a primary alkyl halide.

* Alkynes undergo addition reactions, especially electrophilic addition, with many of the same compounds that add to alkenes.

The greater s character of sp hybrid orbitals compared with ${\text{sp}}^{\text{3}}$ and ${\text{sp}}^{\text{2}}$ gives alkynes certain properties beyond those seen in alkanes and alkenes. It is convenient to think of sp-hybridized carbon as more electronegative than its ${\text{sp}}^{\text{2}}$ or ${\text{sp}}^{\text{3}}$ counterparts.

* The $\equiv \text{C-H}$ unit of an alkyne is more acidic than a $\text{C-H}$ unit of an alkene or alkane, allowing acetylene and terminal alkynes to be converted to their conjugate bases $\equiv {\text{C:}}^{-}$ by ${\text{NaNH}}_2\text{.}$ *

* Unlike alkenes, alkynes are reduced by metals, especially $\text{Li, Na, and K}$.

* Unlike alkenes, alkynes can undergo nucleophilic as well as electrophilic addition.

Problems $9.38–9.42$ emphasize mechanistic reasoning. By thinking mechanistically youreduce the need to memorize facts while increasing your ability to analyze and understand newmaterial. Nucleophilic addition to alkynes, for example, is not covered in this chapter but is the focus of Problem $9.42$, which can be solved by thinking mechanistically.

Nucleophilic addition can occur with alkynes that bear strong electron-attractingsubstituents such as ${\text{CF}}_3$ on the triple bond. Predict the product of nucleophilic additionof ${\text{CH}}_3\text{OD}$ to $\text{3,3,3-trifluoropropyne}$. The stereochemistry of addition is anti, and the first step in the mechanism is bond formation between ${\text{CH}}_3{\text{O}}^{\text{-}}$ and one of the carbons of the triple bond.