Curly arrows represent transformations in organic chemistry, rationalizing what happens in a chemical reaction. It shows the movement of two and one electrons while writing the organic reaction mechanism.
The movement of two electrons is shown using a double-headed arrow. For single electron movement, a single head/ fishhook arrow is used.
Post the curly arrow mechanism; the bonds are reorganized, indicating a new bond formation, old bond breaking, or movement of bonds (electron delocalization) within the molecule as seen in the resonance structures.
The tail of the arrow is always pointed at the electron-rich centre- lone pair (shown as two dots ..), radical (shown as one dot .), charges (shown as a negative sign -), or bond (shown as a dash ___ ).
The arrowhead points to the electron-deficient centre that receives the electrons. The electron-deficient centre is shown as δ+ (partially positive), + (fully positive), a radical (as a dot . on an atom) or as an empty p-orbital (dumbbell-shaped ∞).
Fun Learning Tip for curly arrows-
The TAIL is RICH, but the HEAD is NOT !
The arrow direction is clockwise or anticlockwise, like a writing progression, always going in one direction. The arrows must not converge or diverge but represent the sequential electron flow.
Two-head/Double head Curved arrow
The two-headed curly arrow always represent the flow of two electrons originating from a lone pair, charge, or bond.
Examples of curved arrows involving Lone pair
A lone pair of electrons are the electrons of an atom that didn’t participate in the bond formation process; therefore, also called nonbonding electrons. These electrons are shown as two dots (..) over the atom’s symbol.
The atoms carrying the lone pair are electron rich and tend to donate the electrons to an electron-deficient atom (shown as δ+), forming a new bond with it (shown in red). This entire mechanism of electron donation and forming of new bond is represented using the double headed curly arrows.
Visually understanding the reaction mechanism using curly arrows– The electron-rich nitrogen of the dimethylamine (CH3-NH-CH3) donates its electrons (arrow tail) to the electron-deficient carbon (arrowhead) of the ethyl bromide (CH3-δ+CH2-Br) and forms a new bond with it.
The carbon atom that receives the lone pair of electrons must maintain its tetravalency. Therefore, the Bromo leaves as a leaving group.
Since the Nitrogen atom exhausted its lone pair by forming a new bond, the formal charge on it becomes positive.
Do note that the lone pair containing atoms are always electronegative, so the maximum positive charge it can carry is always one in organic reactions. For example, an oxygen atom with two lone pairs can only donate one and attain a formal charge of one.
Examples of curved arrows involving negative charge
The excess of electrons is denoted with a negative charge.
The atoms develop a negative charge post a heterolytic (unequal bond cleavage). In heterolytic bond breaking, one of the atoms, the most electronegative one, keeps with it both the bond electrons and these excess two electrons (in addition to any lone pairs, if any) is denoted with a negative charge.
Visually reading the reaction mechanism using curly arrows– The electron-rich base (arrow tail) donates its electrons to the electron-deficient Hydrogen (shown as δ+, arrowhead) and forms a bond with it. The Hydrogen atom with a valency of one can only form one bond with the base. Therefore, the bond breaks unequally so that the bond electrons reside with the Oxygen giving it an excess of electrons, thereby developing a negative charge.
The negatively charged oxygen atom is electron dense and reactive (indicated with the arrow tail). It can donate its electron density to an electron-deficient center, the second carbon of the CH3-δ+CH2-δ-X (an electrophile due to inductive effect, shown as δ+ or + sign), and form a covalent bond with it ( shown in green). In the process, the oxygen atom loses its negative charge and becomes neutral and more stable by fulfilling its valency of two.
Carbon, on the other hand, can only form four bonds, so the halogen leaves as a leaving group, taking the two bond electrons along with it shown using the curly arrow. The arrow base is on the source of the electrons, the bond, and the arrowhead points to the carrier halogen atom.
Examples of curved arrows involving Bond
A bond is a source of two electrons. Chemical reactions often use the covalent bond as a nucleophile that transfers electrons to the electron-deficient electrophile, and the mechanism is represented using curly arrows.
Visually reading the reaction mechanism using curly arrows–
The electron-rich pie bond of alkene gives its two electrons to the electrophilic terminal of the Bromine and forms a bond with it. Since Bromine is a monovalent halogen, the covalent bond between the two Bromine atoms breaks, and the two electrons rest with the other Bromine atom.
The pie bond was held between two carbon atoms fulfilling its valency, but since only one carbon atom formed a bond with the bromine (shown in red), the other carbon atom is two-electron deficient (shown with a + sign). The electron-rich bromine (Br-) donates its electrons to the electron-deficient carbon and forms a new bond with it (shown in green).
Curved Arrows in Resonance
The various canonical/resonance structures show electron delocalization in the molecular structure using curved arrows. The movement of electrons starts from the following sources- lone pairs, bonds, or charges going in clockwise or anticlockwise direction.
Single head/Fishhook Curved arrow
A fishhook arrow represents lone electron movement, as seen in radical reactions.
The radicals are formed by the equal division of a covalent bond so that the electrons are shared equally between two atoms, known as a homolytic bond cleavage.
Since the radicals have only one electron (shown as a dot . on the atom), they are electron deficient, and therefore, considered very reactive species. The atoms carrying the radical will try to partner up with similar or dissimilar species by forming a new covalent bond, known as a dimerization reaction (shown in red).
The joining of two atoms, breaking of two atoms to produce radicals, or stabilizing the free radicals is shown using a curved fishhook arrow. Here, the arrows can be shown to converge or diverge.
When the arrows diverge, it means that the covalent bond is breaking to give two new radicals. The convergence of arrows indicates a new bond formation.
When arrows are shown to move in one direction, there is always a hidden arrow representing the breaking of a bond which must always be considered.
The arrows are moved, keeping the atom’s valency, the electron count, and the charges constant.
Related Reading- Types of Arrows in Chemistry, Difference between base and nucleophile, Inductive effect, Electrophile, Nucleophile.
