A point was raised about arrows used in a chemical reaction. I think we can all agree that $\ce{->}$ arrow in the reaction
$$\ce{A + B -> C} \tag{1}$$
means that the reaction goes in one direction. But what of
$$\ce{A + B <-> C} \tag{2}$$
$$\ce{A + B <=> C} \tag{3}$$
$$\ce{A + B <--> C} \tag{4}$$
$$\ce{A + B <=>> C} \tag{5}$$
$$\ce{A + B <<=> C} \tag{6}$$
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 "reaction in both directions" and "equilibrium"?