Is it legal?

[little boy]

Is it legal to simplify

a->b->c as a->c

a <=> b <=> c as a <=> c ?

[hypervalent_iodine]

Could you perhaps provide some more context? Are you asking this in terms of chemical reactions?

[little boy]

Could you perhaps provide some more context? Are you asking this in terms of chemical reactions?

 

yes, They are chemical reactions.

[hypervalent_iodine]

Ok, well in that case the simple answer is yes, but you have to notate your arrows correctly. If you don't include reagents above/below your arrows, then your ability to do that is dependant on the nature of what compound 'b' and 'c' are. If, for example, 'b' was a reactive intermediate or a transition state, then yes you can simply draw the reaction going from a to c. The way that you can and furthermore, the way you should draw reactions depends entirely on the reaction and the point you are trying to make (i.e. the context of the reaction). I'll try and give an over-view of it as best as I can. Since I don't know your level of chemistry education, I'll apologise in advance if some of the jargon I use is confusing or foreign to you.

 

I'll start with the following example of a Friedal-Crafts alkylation reacting benzene with t-butyl chloride in the presence of AlCl₃. We can draw the reaction in the simplest expression as this:

 

 

 

 

 

 

 

However, similar to your generalised example given in the OP, this is not a full representation of the structures that form during the course of the reaction. Thus, if we ignore arrow pushing, we can also draw it as this:

 

 

 

You'll notice the product here is not the product I drew in the first reaction, which brings me to my first point. Rather than drawing all that out again, we can draw multiple arrows to represent the multiple steps taken to forming the product:

 

 

 

 

Multiple arrows can be used in a number of contexts. For example, in your reaction, rather than drawing out product 'b', you could simply draw is as this:

 

 

 

As a word of caution though, I should point out that the magical multiple arrows aren't applicable in every situation. If the in-between steps are irrelevant to whatever point you are making or are a re-hash of steps you've already drawn (as in the Friedal-Crafts reaction, above), then you can use them. Their use does come with an obvious loss of information though, so if using them means you're sacrificing vital steps or it makes what you are saying convoluted, you shouldn't use them.

 

Another example using the multiple arrows representation comes from a reaction I overviewed in my own research proposal and also illustrates another way to draw reactions:

 

 

 

 

 

 

In this reaction I used the multiple arrows so that I could show the initial starting material and the final reaction in the total synthesis of the natural product, discorhabdin C. Doing this does two things: I didn't waste valuable page space with reactions that were not pertinent to the point of the text and the reader could clearly see which reaction I was pointing out.

 

The other thing I wanted to point out with this reaction is seen in the last arrow leading to discorhabdin C. You'll notice there are two sets of reagents listed, set 1.) and 2.). This represents another way by which multi-step reactions may be represented without having to draw the bits in between. If we consider your simplified scheme of 'a' forming product 'c' as being the result of two separate reactions, you could draw it like this:

 

 

 

Or simply like this:

 

 

 

 

The final case is if we consider transition states or reactive intermediates. Without going into too much detail about exactly how these form, what they are and the difference between the two, it is enough to state that they are structures or configurations that form during the course of a reaction but are generally high-energy and short lived. Typically they are drawn if you are trying to give mechanistic insight into a reaction are are usually shown with square brackets around them. Intermediates don't always have to abide by the square brackets rule (the cationic intermediate structures in the Friedal Crafts reaction I drew are an example of this), while transition states do. Transition states additionally have a 'double-dagger' drawn on the outer-top-right of the brackets. Again, the necessity to show these structures depends entirely on the context of your argument. If you are making comments about the mechanism of a reaction, then you draw them. If it's for an undergraduate assignment or exam, it's usually required that you draw them. If it's neither of the two, then you generally don't have to.

 

 

This only applies to unidirectional arrows. In the other part to your question, could you perhaps clarify do you mean this kind of arrow:

 

 

or this kind:

 

 

 

 

They both mean different things, but in either case you cannot apply the same principles as with the mono-directional arrow. In this case, you could have to include 'b' in your scheme.

 

 

Anyway, I hope that helps. If you need any clarification, feel free to ask more questions.

Edited September 13, 2011 by hypervalent_iodine

[little boy]

wow.. very detail