Most molecules contain different types of protons (bonded hydrogens). However, when acting as acids, only the most acidic proton will participate in the acid-base reaction. Therefore, it is important to be able to identify the most acidic proton in a molecule.

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How to:

There are three general methods to estimate the acidity of a proton:

1) Use the table of common acids that we learned in class and is in your book (Table 3.1, inside front cover of the book), and try to either find the same acidic proton or similar one in the table.

2) Identify which functional group the proton is a part of like: alkane, alcohol, carboxylic acid, protonated amine, etc... 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.)


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3) Use the trends we learned in class to compare the stabilities of the conjugate bases (the acid with the proton taken away) of the acids.

The most stable conjugate base will be the strongest acid.


Examples:


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Method 1. These are two acids that are on the Table that we memorized. They are HCl (pKa = -7) and NH3 (pKa = 38). So, HCl is the stronger acid (lower pKa).

Method 2. HCl is a hydrogen halide with a pKa range of 3 to -10 and NH3 is most similar to a 1° amine (with R = H) that would have an approximate pKa of 35. So, HCl would be the stronger acid.

Method 3. For HCl, the proton is attached to a more electronegative atom (Cl) than in NH3 where the proton is attached to an N. Therefore, we would predict based on the EN trends that HCl would be more acidic.

ANSWER: Proton (a) is the most acidic.


Hint: For molecules with lots of different types of protons, I think method two is the easier to apply because it is the fastest.


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Method 2.

Proton (a) is part of a carboxylic acid so pka ≈ 5

Proton (b) is part of a phenol so pka ≈ 10

Protons (c) are bonded to an SP2 carbon so pka ≈ 45

ANSWER: Proton (a) is the most acidic.


Method 2.

Proton (a) is part of a protonated N so pka ≈ 9

Proton (b) is part of an alcohol so pka ≈ 16

Protons (c) are bonded to SP3 carbons so pka ≈ 50 **

**Note: this guess is a bit off for the four protons adjacent to the C=O. Their actual pKa"s are around 20 because their conjugate bases are stabilized by resonance with the C=O. But this will not change the overall answer of which proton is the most acidic.

ANSWER: Protons (a) are the most acidic.


Method 2.

Proton (a) are bonded to SP3 carbons so pka ≈ 50.

Proton (b) is bonded to a more electronegative atom (S). It is not one of the functional groups in the list above but there is a similar proton is Table of acids that we learned. H2S also has a proton attached to an S and has a pKa of 7.0, which is fairly acidic.

Protons (c) is bonded to an SP2 carbon which makes it a poor acid. However, this one is a particularly poor acid as the carbon is negatively charged. Taking away the proton to form the conjugate base would make a C-2, which is very unstable. So we would predict that this proton will have a pKa much higher than the normal alkenes (pKa >> 45).

Protons (d) are bonded to an SP2 carbon so pka ≈ 45.

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ANSWER: Proton (b) is the most acidic


Hint: Protons on carbon are rarely the most acidic because carbon is not very electronegative. Look at protons on more electronegative atoms first O, N, S etc first.