Which hydrogens are acidic

Therefore, it is important to be able to identify the most acidic proton in a molecule. There are three general methods to estimate the acidity of a proton:. Make sure that the proton in question is actually part of the functional group and is not simply attached to atoms that are attached to the functional group.

Here is a short list of acidities of common functional groups in organic chemistry Note: that the pKa's are an approximate value for that functional group. The most stable conjugate base will be the strongest acid. Method 1. How do we know which proton is the most acidic in a molecule such as acetic acid that contains more than one type of proton?

Remember that the higher the degree of positive character on the proton, the more acidic it is. Examination of a pKa table reveals some trends for acidic protons. The following guidelines can be used to predict acidity. Hydrogens directly attached to very electronegative atoms such as oxygen, sulphur, and the halogens carry a substantial degree of acidity. Hydrogens attached to a positively charged nitrogen, oxygen, or sulfur are acidic. The high electronegativity of these atoms makes them uncomfortable with the positive charge.

If you know these values for all of the acidic groups in your molecule, then the group with the lowest pK a contains the most acidic H. Case closed. If you do not recall pK a values for all of the acidic groups, a few general principles can guide you.

Negatively charged acids are rarely acidic. The formal charge rule applies even more strongly to NH acids. The following chart shows how each group of atoms activates an OH acid pK a values range from 16 to -2 :.

CH 3 is considered a spectator group wherever it appears in these molecules. A hydrogen atom in an acid that forms a positive ion when the acid dissociates.

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