# Is the question about a theory that predicts pKa values based on structure really 'too broad'?

It has come to my attention that the following question was put on hold as being too broad: Is there a reliable chemical theory that predicts pKa based on structure?

While I initially commented that there are no rigorous and affordable theories or approximations available, I think that the question in itself is answerable with just that information. Furthermore, it is easy to summarise the current state of research, given in the paper referred to by theorist in a comment, as an answer.

I personally think this is a research level question, going far beyond the +/-I/M counting schemes that are employed in organic chemistry, even though it appears to be somewhat open ended. But we have allowed such kind of questions in the past, so why should we be changing our stance now?

• I probably was one of the people who VTC and did so without due diligence in regards to the comment thread. I agree with your assessment that it should be in our corpus and that persons with relevant and current knowledge of the state-of-the-field can answer. – Todd Minehardt Jun 27 '19 at 22:49
• I voted to close as well. I would like to have a specific question (the answer to the question in the title is "no"). Maybe what makes it difficult to predicts pKa values of organic molecules? Or how good are existing predictions of pKa values? Or what are the newest directions in trying to predict pKa values? If the paper cited in the comments is a good basis for an answer, maybe the OP should edit the question to indicate this. The examples in the paper are all made of just C, H, N, O. Is that enough? – Karsten Theis Jun 28 '19 at 18:00
• @KarstenTheis thank you for your insight. I understand more now. To be honest, I still disagree. One of our highest voted questions can be answered with a simple no. In any case, the reasons you have could have possibly also benefited the op. – Martin - マーチン Jun 28 '19 at 19:13
• Let’s hope for a great answer! – Karsten Theis Jun 28 '19 at 20:50

I thought the question was uneven, and could certainly be described as "too broad" (although it could well have fallen under "unclear what you are asking").

I have seen very similar questions (of the generic type "can I predict the properties of molecules from their structure") and perhaps this one was a little more concise than those.

Here is the question again:

Is there a reliable chemical theory that predicts pKa based on structure?

Ok, first off, a theory is by definition reliable if it predicts something properly.

I would have asked (for starters)

Is there a chemical theory that reliably predicts pKa based on structure?

Ok, now what, which type of molecules are we talking about? What solvent? etc etc...

I could have left a comment but from what you can see it was not difficult to reason that it was a winding road to nowhere.

I should add that you (the OP) provided a useful comment (suggesting you know the answer) but didn't bother to answer (ideally with supporting references, rather than rendering it opinion-based), suggesting you did not think much of the question $$^\dagger$$.

Edit:

Also, define "reliable". Naturally, this is a definition that could be worked into the answer, but it still makes the question a little "broad".

$$\dagger$$

As I mention in a comment: I am not unsusceptible to suggestion. Seeing that someone has voted to close a question does make me second guess its quality, seeing comments that represent a sufficient alternative to a full answer also, and I think this is acceptable

• I'd be very careful with this implication that leaving a comment but not an answer means that you think poorly of the question. I generally have very high standards for my own answers. If I do not have the time to do thorough research (for a question that requires it - obviously not all questions do), I will not post an answer. Judging from Martin's answers over the years, he likely has a similar outlook. So I think that paragraph of yours was unfounded and honestly unnecessary, considering that you weren't even sure that it was true ("suggesting"). – orthocresol Jun 29 '19 at 11:10
• @orthocresol I think one should be careful interpreting comments. They provide an idea of others' opinions of the quality of a question, sometimes subtly or not clearly at all, while often making it very distinct. They save time because they may flag problems with a question. They suggest why one might make a decision adn reinforce a reason to close. I attempt to gauge the quality of a question independently of other's opinions, sometimes opposing them. – Buck Thorn Jun 29 '19 at 13:09
• But I am not unsusceptible to suggestion. Seeing that someone has voted to close a question does make me second guess its quality, seeing comments that represent a sufficient alternative to a full answer also, and I think this is acceptable. – Buck Thorn Jun 29 '19 at 13:10
• Regarding research effort, I think the term "broad" sometimes reflects a failure to narrow a question such that an answer does not require extensive research. I felt this question did not suggest much effort, so ended up being broad/unclear. Providing a specific example, etc, could have made this a better question. – Buck Thorn Jun 29 '19 at 13:10
• In the same way that you won't post an answer unless you feel you can do the work, OPs are afaik expected to research a question properly before posting it. – Buck Thorn Jun 29 '19 at 13:12