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Why is Ethyne a Stronger Acid than Ethane or Ethene?

Why is Ethyne a Stronger Acid than Ethane or Ethene? If we compare acid strengths of the simple hydrocarbons, we find that ethyne is substantially more acidic than ethene or ethane in the gas phase or in solution. Why is this? The simplest explanation is that there is a direct connection between C—H acidity and the amount of s character associated with the σ-bonding carbon orbital. Other things being equal, acidity increases with increasing s character in the carbon orbital.

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Figure 11-4 Generalized valence bond (GVB) orbitals for one hydrogen of ethyne (left) and of ethane (right); see Section 6-7. The hydrogen and carbon nuclei are located in the X, Y plane of the coordinate system at the positions indicated by crosses, the hydrogen nucleus being on the left. The long dashes indicate locations of change in orbital phase. The dotted lines are contour lines of electron amplitude of opposite phase to the solid lines. Notice how the contours of the ethyne hydrogen orbital are distorted toward carbon compared to those of the ethane hydrogen orbital. (Drawing courtesy W. A. Goddard III.)

On the average the s electrons are closer to the carbon nucleus than are p electrons. Therefore, the more 5 character there is to the C—H bond, the closer the electrons of the bond are, on the average, to the carbon nucleus. This makes it easier to remove the hydrogens as protons. This displacement of the electrons is clearly shown by the GVB orbitals for the hydrogen-bonding orbitals of ethane and ethyne.

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