Acidity of Acetylene and Terminal Alkynes

How do bond distances and bond strengths change with electronegativity in the series NHs, H2O, and HF  

The property that most separates acetylene from ethane and ethylene is its acidity. It, too, can be explained on the basis of the greater electronegativity of p-hybridized carbon compared with sp and sp.  

The C—H bonds of hydrocarbons show little tendency to ionize, and alkanes, alkenes, and alkynes are all very weak acids. The acid-dissociation constant for methane, for example, is too small to be measured directly but is estimated to be about 10 (p 60). 

The conjugate base of a hydrocarbon is called a carbanion It is an anion in which the negative charge is borne by carbon. Because it is derived from a very weak acid, a carbanion such as CH3 is an exceptionally strong base. 

Using the relationship from the preceding section that the electronegativity of carbon increases with its character (sp sp sp), the order of hydrocarbon acidity is seen to increase with increasing s character of carbon. 

The C—H bonds of hydrocarbons show little tendency to ionize, and alkanes, alkenes. 

In general, the ability of an atom to bear a negative charge is related to its electronegativity. Both the electronegativity of an atom X and the acidity of H—X increase across a row in the periodic table. 

The acidity increases as carbon becomes more electronegative. Ionization of acetylene gives an anion in which the unshared electron pair occupies an orbital with 50% s character. 

Recall that a lower piT corresponds to a greater acidity. Therefore, acetylene (piT = 25) is significantly more acidic than ethane or ethylene. To be precise, acetylene is 19 orders of magnitude (10,000,000,000,000,000,000 times) more acidic than ethylene. The relative acidity of acetylene can be explained by exploring the stability of its conjugate base, called an acetylide ion (the suffix ide indicates the presence of a negative charge)  

The conjugate base of ethane exhibits a lone pair in an sp -hybridized orbital. The conjugate base of ethylene has the lone pair in an sp -hybridized orbital, and the conjugate base of acetylene has the lone pair in an sp-hybridized orbital. 

p-hybridized orbital, the electron density is closer to the positively charged nucleus and is therefore more stable. 

Now let s consider the equilibrium that is established when a strong base is used to deprot-onate acetylene. Recall that the equilibrium of an acid-base reaction will always favor formation of the weaker acid and weaker base. For example, consider the equilibrium established when an amide ion (H2N ) is used as a base to deprotonate acetylene  

Equilibrium favors formation of weaker acid and weaker base 

---

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

The acidity of acetylene and terminal alkynes permits them to be converted to their conjugate bases on treatment with sodium amide These anions are good nucleophiles and react with methyl and primary alkyl halides to form carbon-carbon bonds Secondary and tertiary alkyl halides cannot be used because they yield only elimination products under these conditions... 

---