Carbon Acidity of Strained Hydrocarbons

1 Carbon Acidity of Strained Hydrocarbons The difference in the formal hybridization at carbon accounts for the relative acidities of alkanes versus alkenes versus akynes. The greater the s-character of the carbon of Ihe C-H bond of interest, the more acidic is that proton. Of interest here is extension of this idea to strained compounds, which may employ greater or lesser s-character to accommodate the unusual geometries. Will these compounds have acidities that reflect their hybridization Are strained hydrocarbons more acidic than their unstrained analogs  

The general answer to these questions is yes. In 1989, DePuy and coworkers measured the gas-phase DPEs of a number of small alkanes using the reaction of hydroxide with alkyltrimethylsilanes in a flowing afterglow-selected ion flow tube. Their experimental DPEs for some simple alkanes are listed in Table 3.6. The DPE of cyclopropane, 411.5 kcal mol is much lower than that of a typical acyclic alkane (415-420 kcal mol ). Cyclobutane is less acidic than cyclopropane, reflecting the diminished s-character of its C-H bonds. Also listed in Table 3.6 are 

TABLE 3.6 DPEs (Aff, kcal mol ) of Some Simple Alkanes 

7 kcal mol . This is again in excellent accord with the computed values 103.3 (B3LYP) and 107.2 (G3) kcal mol . This bond dissociation energy is much more similar to that of methane (104.9 kcal mol ) than acetylene (132.8 kcal mol ). It appears that the radical 2r is more stable than expected. 

The last example is the acidity of the ethynyl-expanded cubane 3. Inspired by the synthesis of the methoxy-substituted buta-l,3-diynediyl expanded cubane 4, Bachrach examined some of the properties of its smaller homolog 3. Since the s-character of the C-H bonds of 3 should be large and its conjugate base 3cb can potentially delocalize the anion into three neighboring alkynyl units, 3 might be quite acidic. 

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