# Arrows used in chemical reactions

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:

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"?

The IUPAC Green Book (Quantities, Units and Symbols in Physical Chemistry, Third Edition, 2007) reads:

2.10.1 Other symbols and conventions in chemistry
[...]
(iv) Equations for chemical reactions

(a) On a microscopic level the reaction equation represents an elementary reaction [...] A single arrow is used to connect reactants and products in an elementary reaction. An equal sign is used for the "net" reaction, the result of a set of elementary reactions. [...]

$\ce{H + Br2 -> HBr + Br} \quad$ one elementary step in HBr formation

$\ce{H2 + Br2 = 2 HBr} \quad$ the sum of several such elementary steps

(b) On a macroscopic level, different symbols are used connecting the reactants and products in the reaction equation, with the following meanings:

$\ce{H2 + Br2 = 2 HBr}\quad$ stoichiometric equation

$\ce{H2 + Br2 -> 2 HBr}\quad$ net forward reaction

$\ce{H2 + Br2 <--> 2 HBr}\quad$ reaction, both directions

$\ce{H2 + Br2 <=> 2 HBr}\quad$ equilibrium

The two-sided arrow $\ce{<->}$ should not be used for reactions to avoid confusion with resonance structures [...]

2.12.1 Other symbols, terms, and conventions used in chemical kinetics
[...]
(ii) Composite mechanisms
A reaction that involves more than one elementary reaction is said to occur by a composite mechanism. The terms complex mechanism, indirect mechanism, and stepwise mechanism are also commonly used. Special types of mechanisms include chain-reaction mechanisms, catalytic reaction mechanisms, etc.

Examples
A simple mechanism is composed of forward and reverse reactions $\quad\quad\quad\quad\quad\quad\ce{A -> B + C}$
$\quad\quad\quad\quad\ce{B + C -> A}$
It is in this particular case conventional to write these in one line
$\quad\quad\quad\quad\quad\quad\ce{A <--> B + C}$

However, it is useful in kinetics to distinguish this from a net reaction, which is written either with two one-sided arrows or an "equal" sign
$\quad\quad\ce{A <=> B + C}$
$\quad\quad\ce{A = B + C}$

When one combines a composite mechanism to obtain a net reaction, one should not use the simple arrow in the resulting equation.
$\quad$Example \begin{align} \ce{A &\to B + C}\quad\text{ unimolecular elementary reaction} \\ \ce{B + C &\to D +E}\quad\text{ bimolecular elementary reaction} \\ \hline \ce{A &= D + E}\quad\text{net reaction (no elementary reaction,no molecularity)} \end{align}

Many kinds and combinations of arrows can be used. For example, two full arrows in opposite directions ($\ce{<-->}$) indicate a reaction that is proceeding in both directions. Two arrows with half heads in opposite directions ($\ce{<=>}$) indicate a reaction in equilibrium. A single arrow with heads on both sides ($\ce{<->}$) indicates resonance structures, not a reaction.

• PS. I realize that the arrow display degraded a little bit. I will improve that with the next release of MathJax/mhchem. – mhchem Dec 6 '17 at 8:41

The second one, $(2)$, $\ce{<->}$, indicates resonance structures.

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

What are the correct equilibrium arrows?

Loong basically says that $\ce{<-->}$, $(4)$, 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, $\ce{<=>}$, $(3)$, 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.