CH acidity
In organic chemistry, CH acidity is the tendency of a compound to release hydrogen atoms bound to a carbon atom as protons and thus to act formally as an acid . Since carbon itself is not much more electronegative than hydrogen, the CH bond, in contrast to NH and OH bonds, is usually not very polar and the willingness to form a carbanion is accordingly low. This leads to very high pK S values for unsubstituted alkanes such. B. pK S 50 for Ethan . However, if the carbon atom is bound to strongly electron-withdrawing groups such as carbonyls (in an ester , ketone or aldehyde ), sulfones , nitriles , trifluoromethyl or nitro groups ( α-position to these groups), the pronounced negative inductive effect ensures that the CH bond on the α-carbon atom is more polarized and the proton can be split off more easily.
This is used in practice in reactions with enolates , for example the Knoevenagel condensation . When using derivatives of malonic acid (pK S ≈ 13) or acetylacetone (pK S ≈ 9), which as β- dicarbonyls can be deprotonated quite well, these reactions can be carried out with comparatively mild bases such as ethanoates or triethylamine .
Similarly, carbon-carbon multiple bonds provide through approximation of the carbon atoms for greater polarization so that ethyne with a pK S of about 25 relative CH-acidic and acetylides , z. B. silver acetylide can form.
The formation of an aromatic system can also increase the CH acidity. Cyclopentadiene , for example, has a pK s value of 16. It is relatively strong CH-acid because the resulting cyclopentadienide anion is an aromatic resonance-stabilized , and for example, as a stable complex ligand in the metallocene can occur compounds.
meaning
The pK S value of CH-acidic compounds is of great importance in various chemical processes and processes. For example, by deprotonating the very weakly acidic alkanes, very strong bases , such as. B. in butyllithium . On the other hand, one uses the high acid strength of activated compounds (see above) to carry out CC linkage. Even if the CH acidity is very low, it can play a decisive role in chemical processes, e.g. B. in the racemization of hyoscyamine to atropine .
