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This site periodically attracts questions that essentially ask for a way to utilize the amassed synthetic knowledge and wisdom of the chemical profession without either 1) doing the hard work to get there, or (more commonly) 2) understanding that the question itself represents something akin to the total sum of all knowledge in the field.

Here are some examples:

These questions get closed as too broad which is probably the correct response. However, there can be a definitive correct answer to questions like this.

Proposal: Let's have one definitive question and answer about this topic. Then we close the new questions as duplicates.

The answers to this meta question are:

  • Yes, this is a great idea! It is so great, I want to go back into the past and do it three years ago! Anyway, let's do it now.
  • No, this is not a good idea. Let's just keep closing these questions as too broad.

Vote!

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    $\begingroup$ I say just do it. Answer, before it gets closed ;) $\endgroup$ – Jan May 4 '16 at 16:05
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    $\begingroup$ Some people seem to like the idea. It is done: chemistry.stackexchange.com/questions/50684/…. As much as I would appreciate any rep coming from this question, it should probably become a community wiki. $\endgroup$ – Ben Norris May 5 '16 at 1:03
  • $\begingroup$ @BenNorris: Yeh, that would be great to make it a canonical question; we at physics frequently do this in order to cope such sort of same or nearly same questions. $\endgroup$ – user5764 May 19 '16 at 5:09
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Yes, this is a great idea! It is so great, I want to go back into the past and do it three years ago! Anyway, let's do it now.

An example of such a definitive answer might be:

$${\Large\text{Can I predict the products of any chemical reaction?}}$$

In theory, yes!

Every substance has characteristic reactivity behavior. Likewise pairs and sets of substances have characteristic behavior. For example, the following combinations of substances only have one likely outcome each:

$$ \ce{HCl + NaOH -> NaCl + H2O} \\ \ce{CH3CH2CH2OH->[1.\ \ce{(COCl)2,\ (CH3)2SO}][2.\ \ce{Et3N}] CH3CH2CHO}$$

However, it is a not suited to brute force or exhaustive approaches

There are millions or perhaps billions of known or possible substances. Let's take the lower estimate of 1 million substances. There are $999,999,000,000$ possible pairwise combinations. Any brute force method (in other words a database that has an answer for all possible combinations) would be large and potentially resource intensive. Likewise you would not want to memorize the nearly 1 trillion combinations.

If more substances are given, the combination space gets bigger. In the second example reaction above, there are four substances combined: $\ce{CH3CH2CH2OH,\ (COCl)2,\ (CH3)2SO,\ \& \ Et3N}$. Pulling four substances at random from the substance space generates a reaction space on the order of $1\times 10^{24}$ possible combinations. And that does not factor in order of addition. In the second reaction above, there is an implied order of addtion:

$$\begin{align} &1.\ \ce{CH3CH2CH2OH}\\ &2.\ \ce{(COCl)2,\ (CH3)2SO}\\ &3.\ \ce{Et3N} \end{align}$$

However, there are $4!=24$ different orders of addition for four substances, some of which might not generate the same result. Our reaction space is up to $24\times 10^{24}$, a bewildering number of combinations. And this space does not include other variables, like time, temperature, irradiation, agitation, concentration, pressure, control of environment, etc.

In practice, in can be manageable!

Even though the reaction space is bewilderingly huge, chemistry is an orderly predictable business. Folks in the natural product total synthesis world do not resort to random combinations and alchemical mumbo jumbo. They can predict with some certainty what type of reactions do what to which substances and then act on that prediction.

When we learn chemistry, we are taught to recognize if a molecule belongs to a certain class with characteristic behavior. In the first example above, we can identify $\ce{HCl}$ as an acid and $\ce{NaOH}$ as a base, and then predict an outcome that is common to all acid-base reactions. In the second example above, we are taught to recognize $\ce{CH3CH2CH2OH}$ as a primary alcohol and the reagents given as an oxidant. The outcome is an aldehyde.

These examples are simple ones in which the molecules easily fit into one class predominantly. More complex molecules may belong to may categories. Organic chemistry calls these categories Functional Groups. The ability to predict synthetic outcomes then begins and ends with identifying functional groups within a compound's structure. For example, even though the following compound has a more complex structure, it contains a primary alcohol, which will be oxidized to an aldehyde using the same reagents presented above. We can also be reasonably confident that no unpleasant side reactions will occur.

swern oxidation example on complex structure

There are too many classes of compounds to list here. Likewise even one class, like primary alcohols (an OH group at the end of a hydrocarbon chain) has too many characteristic reactions to list here. Folks who learn how to analyze combinations of compounds spend years taking courses and reading books and research articles to accumulate the knowledge and wisdom necessary. It can be done. Computer programs can be (and have been) designed to do the same analysis, but they were designed by people who learned all of the characteristic combinations. There is no shortcut.

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  • $\begingroup$ +1 for the Swern oxidation, as well as for the idea. Since this already seems like a great starting point for an answer, my 2c: people who ask this kind of question probably do not appreciate that the main focus of the reaction is the oxidation of the organic substrate. So, you might very well need to tell them that the reaction also produces $\ce{CO}$, $\ce{CO2}$, $\ce{Me2S}$, as well as $\ce{Et3NH+Cl-}$. $\endgroup$ – orthocresol May 4 '16 at 13:03
  • $\begingroup$ If we want to make this a more comprehensive answer, I would advise adding info on reaction mechanisms for organic chemistry (basic nucleophile/electrophile stuff), as that is a way to predict products of reactions that you have seen before. Also, you could add stuff on reduction potentials and basic reaction types (addition, replacement, redox, etc), which probably relates to the information that most people who ask this question actually want to know. $\endgroup$ – Niels Kornerup May 8 '16 at 23:27
  • $\begingroup$ Apropos of the discussion on DMSO oxidations: ursula.chem.yale.edu/~chem220/chem220js/STUDYAIDS/oxidation/… $\endgroup$ – user55119 Apr 11 '18 at 15:45
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No, this is not a good idea. Let's just keep closing these questions as too broad.

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  • $\begingroup$ I don't think this would be a smart idea - people will continue to answer it, and it is best to at least give them some idea why it cannot just be answered. $\endgroup$ – Niels Kornerup May 23 '16 at 23:14

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