Reaction

A reaction captures a chemical transformation: reactants, an arrow type, products, and optional conditions rendered above and below the arrow per IUPAC.

Fields

Field	Type	Source	Example
reactants	Array of Molecule	left of arrow	2H_2 + O_2
products	Array of Molecule	right of arrow	2H_2O
arrow	Symbol	arrow token	:forward, :equilibrium, …
conditions	Conditions struct	[above][below]	Conditions(above: "Fe", below: "400°C")

Arrow kinds

Symbol	ASCII	Unicode	Use
:forward	->	→	Net forward
:reverse	<-	←	Net reverse
:equilibrium	<=>	⇌	Equilibrium (Haber, acid/base)
:resonance	<->	↔	Resonance structures

Diagram

            ┌───────────────┐
            │   Reaction    │
            │               │
   reactants[0] ─┐          │
   reactants[1] ─┤── left   │       above
   ...           ─┘         │       ▲
                  ──────────►│ ──── ⇌ ────
                            │       ▼
   products[0]  ─┐          │       below
   products[1]  ─┤── right  │
   ...           ─┘         │
                            └───────────────┘

Conditions

Conditions sit in bracketed groups immediately after the arrow. The first bracket is the above condition (catalyst), the second is the below condition (temperature / pressure / solvent).

N_2 + 3H_2 <=>[Fe][400°C] 2NH_3

${\mathrm{N}}_2+3{\mathrm{H}}_2\underset{\mathrm{Fe}}{\overset{\text{400°C}}{\rightleftharpoons }}2\mathrm{N}{\mathrm{H}}_3$

produces Conditions(above: "Fe", below: "400°C").

Grammar production

reaction := terms arrow terms
terms    := molecule ("+" molecule)*
arrow    := "<=>" | "<->" | "->" | "<-"
           (with optional [above] and [below] conditions)

Formatter behaviour

Formatter	Arrow rendering	Conditions
MathML	<mo>⇌</mo> (or <mover>/<munderover> with conditions)	<mtext>
Text	ASCII spelling	[above][below]
HTML	Unicode arrow	<sup> / <sub> around the arrow
LaTeX	mhchem arrow	[above][below] (mhchem native)
SVG	ASCII arrow	[above][below]

Round-trip

AsciiChem.parse("2H_2 + O_2 -> 2H_2O").to_text                       # => "2H_2 + O_2 -> 2H_2O"
AsciiChem.parse("N_2 + 3H_2 <=>[Fe][400°C] 2NH_3").to_text           # => same
AsciiChem.parse("HAc <=> Ac^- + H^+").to_text                        # => same

Linter checks

• BalanceCheck — atom counts must match on both sides, respecting coefficients and group multiplicities.

Cascade promotion

Single-step reactions stay as Reaction. Two or more reactions chained (A -> B -> C) promote to a ReactionCascade.