Arrows used in chemical reactions A point was raised about arrows used in a chemical reaction. I think we can all agree that $\ce{-&gt;}$ arrow in the reaction

$$\ce{A + B -&gt; C} \tag{1}$$

means that the reaction goes in one direction. But what of

$$\ce{A + B &lt;-&gt; C} \tag{2}$$

$$\ce{A + B &lt;=&gt; C} \tag{3}$$

$$\ce{A + B &lt;--&gt; C} \tag{4}$$

$$\ce{A + B &lt;=&gt;&gt; C} \tag{5}$$

$$\ce{A + B &lt;&lt;=&gt; C} \tag{6}$$

Is there any IUPAC or American Chemical Society guideline about how double arrows are supposed to be used?



Examples of usage:


  (1) Used for example:
  "Removal of Triphenylphosphine Oxide by Precipitation with Zinc Chloride in Polar Solvents",
  Donald C. Batesky, Matthew J. Goldfogel, and Daniel J. Weix,
  J. Org. Chem., 2017, 82 (19), pp 9931–9936
  
  "Cation Radical Accelerated Nucleophilic Aromatic Substitution via Organic Photoredox Catalysis",
  Nicholas E. S. Tay and David A. Nicewicz*
  Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
  J. Am. Chem. Soc., 2017, 139 (45), pp 16100–16104
  
  (4) Used for example:
  "Cation Radical Accelerated Nucleophilic Aromatic Substitution via Organic Photoredox Catalysis",
  Nicholas E. S. Tay and David A. Nicewicz*
  Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
  J. Am. Chem. Soc., 2017, 139 (45), pp 16100–16104




Previous questions in Chemistry:

In his answer user Chemgoat noted that this question had been asked before:

What are the correct equilibrium arrows?

A comment on that question points to yet another

What is the difference between &quot;reaction in both directions&quot; and &quot;equilibrium&quot;?
The second one, $(2)$, $\ce{&lt;-&gt;}$, indicates resonance structures. 

The difference between the second two is explained in another post - 

What are the correct equilibrium arrows?

Loong basically says that $\ce{&lt;--&gt;}$, $(4)$, indicates forward and reverse elementary steps happening in equilibrium, that is that those are the only two things actually reacting with each other. On the other hand, $\ce{&lt;=&gt;}$, $(3)$, indicates a net reaction, that is that there may be multiple steps in between, leading to this net equation. He does a more in depth explanation of this in his post.

Case $(5)$ indicates a net equilibrium where products are favored, and $(6)$ indicates a net equilibrium where reactants are favored.