There are different types of arrow notations that are frequently encountered in Chemistry, mainly Organic Chemistry. Each one has a specific purpose and cannot be used interchangeably.
Few of the most common ones are:
a) Chemical Reaction Arrow
The most common arrow type used to show the conversion of a reactant to a product is the chemical reaction arrow, also known as the forward arrow. The tail of the arrow lies towards the substrate and the head points to the products. It shows the direction in which the chemical reaction proceeds.
The chemical equation of all the basic reactions in chemistry (addition, substitution, displacement, decomposition, etc.) is expressed using the reaction arrow.
Mostly, reagents, catalysts are written above the arrow and the reaction conditions (time, temperature, solvent) written below it.
If a reaction takes place in more than one step, it can be shown using only one reaction arrow. The successive steps are numbered, the reagents for the reactions are mentioned and is placed above and below the chemical reaction arrow.
b) Reversible Reaction Arrow
In forward reaction, the reactants react together to give the products, but in the case of the reversible reaction, the backward or the reverse reaction takes place wherein the products formed, again react to give back the starting material. Both the reactions occur simultaneously, and there is no complete consumption of the reactants or the products. A reversible reaction arrow is, therefore, bidirectional and half headed. The tail end of a reversible reaction arrow lies near the reactants, and the arrowhead always points towards the products.
c) Equilibrium Reaction Arrow
In a reversible reaction, there comes a stage at which the rate of the forward and the reverse reaction is the same. This leads to an equilibrium state. In such a reaction, even on keeping the reaction for a longer time to progress, it does not lead to any change in the concentration of either the reactant or the product. The reaction continues, and the concentration remains unchanged. The equilibrium reaction arrow is like the reversible reaction arrow. If the concentration of both the reactant and the product is the same, the arrow is of the same length.
Altering the reaction conditions can change the direction of the reaction and the concentration of the substances present in the equilibrium mixture. If the forward reaction is favored more than the reverse reaction, it is indicated by a longer forward reaction arrow. Same is the case when the reverse reaction is dominant than the forward reaction.
d) Double Headed Curly Arrow
The electron transfer reactions or the reaction mechanisms in organic chemistry are shown using the curly arrows. These arrows have a spearhead (double headed). The movement of the two electrons is indicated using a curly arrow. The tail end of the arrow is at the electron-rich atom (nucleophilic center) and the arrowhead on the electron deficient atom (electrophilic center).
e) FishHook Curly Arrow
The single electron transfer reaction known as the free radical reaction are shown using the fishhook curly arrow. It has half-head (half of a spearhead) and looks like a fishhook. The fishhook curly arrow illustrates the movement of one electron that is picked up from the tail and ends at the arrowhead on the atom carrying the radical/electron. The fishhook arrow mostly appears in pairs.
For example, a homonuclear diatomic molecule Br2 is made of a two-electron covalent bond. On homolytic bond cleavage, shown by using the fishhook arrow, the electrons are shared equally between the two atoms generating two free radicals.
f) Dashed Arrow
The general reaction conversion ideas are expressed using a dashed arrow. It is only a proposition. The reaction must be verified experimentally and confirmed. If the reaction was successful, the arrow later changes from dashed to the chemical reaction arrow in the reaction scheme.
g) The Crossed or The Broken Arrow
The crossed arrow is used to show a failed chemical reaction. It means that the reaction did not yield the desired product/s. Therefore, a cross sign is drawn on the reaction arrow or the arrow is shown as broken.
h) Resonance Arrow
A two-headed arrow is used to show the presence of resonance/mesomerism in a molecular structure. The resonance is the movement of the electrons or the electron delocalization within the structure. The electron delocalization is shown as a set of structures with the resonance arrow separating each structure.
The molecule does not go back and forth between these structures but only exists as a resonance hybrid. The resonance structures are mainly used to show the stability of the molecule due to the electron delocalization that decreases the overall potential energy of the molecule.
In the resonance structures, the electron movement in a clockwise manner is shown with the curly arrows. The position from which the electrons are delocalized acquires a positive charge and the place where it is localized acquires a negative charge.
Nowadays, to avoid confusion, the term delocalization is used instead of resonance, and the resonance structures are called contributing structures. The set of structures is now separated by a comma instead of a double headed resonance arrow.
i) Retrosynthesis Arrow
The planning of the synthesis of a target molecule is done using the retrosynthesis arrow. The arrow looks like a block arrow. It is made of two straight line and has one single head.
The arrow tells you that the product on the left (tail end) is made up of the starting materials on the right (facing the arrowhead). One arrow is used to go one step backward.
Therefore, during retrosynthesis, the final known product is the starting point, and its total synthesis must be planned. The target molecule is step-wise broken down into smaller fragments theoretically using chemistry rules and reasoning; to further analyze the set of smaller, simpler starting materials and series of reactions that was required for its complete synthesis.
j) Rearrangement Reaction arrow
A rearrangement reaction arrow looks like a chemical reaction arrow but with a loop in the middle. The arrow is used to show the rearrangement reaction in one single step. But the mechanism of the rearrangement reaction is shown in several steps using the curly arrows.
The rearrangement reactions involve bond breaking and the rearrangements occurring may be intramolecular or intermolecular.
Other Arrow Notation Types
1) Reflux Reaction Arrow
A solvent used in a chemical reaction is boiled at its boiling point and is condensed back using a water condenser such that its volume remains constant and there is no loss due to evaporation. Such a laboratory technique is known as reflux.
A reaction that requires a refluxing condition is denoted using two full headed arrow pointing in the upward and the downward direction. The arrows are written below the reaction arrow usually next to the solvent.
2) Orbital Electrons Arrow
The electron present in an atom that is a part of the shell and a sub-shell is indicated by using an upward or the downward arrow. One electron is one arrow, either upward or downward facing. The arrows may be drawn as a half-head or a full head.
When two electrons are part of the same sub-shell, they have opposite spins, and both the upward and the downward arrow are drawn side by side. Example, s-orbital electrons. The arrow indicates the position of the electrons in orbit.
The arrows are used to show the electronic configuration of an atom or in molecular orbital energy level diagrams.
3) Co-ordinate Covalent Bond Arrow
If a bond between two atoms in a molecule is a co-ordinate covalent bond where one atom is the donor and the other an acceptor, such a bond is then indicated using an arrow. The base of the arrow points to the electron-rich atom, and the arrowhead faces the electron deficient atom that accepts the electron pair.
4) Upward Arrow (Gas Evolution)
If the product or the byproduct of a chemical reaction is a gas and tends to escape from the reaction mixture, it is indicated by using an upward facing spearhead arrow. It is written next to the gaseous product in the chemical reaction equation on the right-hand side.
5) Downward Arrow (Precipitate)
If the product or the byproduct of a chemical reaction is an insoluble precipitate, its formation and deposition are indicated using a downward facing spearhead arrow. It is written next to the precipitated product on the right-hand side of the chemical reaction equation.
6) Clockwise and Anti-Clockwise/ Stereochemical Arrow
A spearhead curly arrow is drawn such that it shows clockwise rotation or an anti-clockwise rotation at a stereocenter is used to indicate the absolute configuration at the stereogenic center in the molecule.
If the rotation of the arrow is clockwise, the configuration is labeled ‘R’ (rectus/clockwise), and if the arrow rotates anti-clockwise, the configuration is labeled ‘S’ (sinister/anti-clockwise).
The direction of the rotation of the arrow is based on a set of rules named as Cahn-Ingold-Prelog naming system that assigns priority to the R- groups around the stereogenic center based on the atomic number (Z). Higher the atomic number, higher the priority.
7) Wavy Arrow
The wavy arrow is used to show an energy emission or a photon of light or an energy transfer.
The wavy arrow represents the wave-particle duality of light and electron. The ‘wave’ nature of the photon is shown as the long wiggling tail of the arrow, and the ‘particle’ part is the arrowhead carrying one energy unit.
The transition from a higher energy state to a lower energy state is shown by the arrow facing down. In photochemistry, it represents a non-radioactive decay.
8) Dipole Moment Arrow
If two atoms in a covalent bond have an electronegativity difference in the range of 0.5-2, a dipole is induced in the molecule. The more electronegative atom pulls the electron density in the bond towards itself thereby carrying a partial negative charge (δ -), and the less electronegative atom gets a partial positive charge (δ +).
The dipole is a vector quantity and directional in nature. The bond dipole arrow is written parallel to the covalent bond. The arrowhead points to the electronegative atom and the tail end have a built-in + sign that faces the less electronegative atom indicating the positive terminal of the dipole.
The resultant dipole moment of the molecule is also shown using the same arrow. The resultant dipole moment is the sum of all the individual bond moments in the molecule. If all the individual bond moments add up, the resultant Dipole Moment is higher. If few of the individual bond moment cancels out, the resultant Dipole moment of the molecule is lower.