Difficulty: Intermediate | Prerequisites: Lewis structures, formal charge, electronegativity (Chapter 1.1–1.3)
Big Picture
Curved arrow formalism is the language organic chemists use to describe how electrons move during a reaction. If Lewis structures are the alphabet, curved arrows are the grammar. Every mechanism you draw for the rest of organic chemistry depends on getting this right. You should already be comfortable drawing Lewis structures and identifying lone pairs before tackling this section.
Curved arrows track where electron pairs go during a chemical reaction. Every arrow starts at an electron source (a lone pair or a bond) and points to an electron sink (an atom or a bond). Master the rules for drawing these arrows and you can describe any organic reaction mechanism.
Curved arrow (electron-pushing arrow)
A notation showing the movement of an electron pair from a source to a sink. The tail begins where the electrons are; the head points to where they go. In simple terms, it is a bookkeeping device that tells you which bonds break and which bonds form.
Nucleophile ("nucleus-loving")
A species that donates an electron pair to form a new bond. It is electron-rich, carrying either a lone pair or a pi bond it can share. Think of it as the electron giver in any reaction step.
Electrophile ("electron-loving")
A species that accepts an electron pair to form a new bond. It is electron-poor, often bearing a positive charge or a polarised bond. Think of it as the electron receiver.
Lewis base
An electron-pair donor. In organic chemistry, "Lewis base" and "nucleophile" overlap almost entirely, though Lewis base is the broader term from general chemistry.
Lewis acid
An electron-pair acceptor. Equivalent to electrophile in most organic contexts.
Intermolecular reaction
A reaction in which the electron-pair donor and acceptor are on two separate molecules, as opposed to an intramolecular reaction where both sites are on the same molecule.
"First arrow"
The curved arrow drawn first in a mechanism step. In an intermolecular reaction it originates from the nucleophile and points toward the electrophile. The first arrow always starts from the electron-rich species.
Lone pair
A pair of valence electrons on an atom that is not involved in bonding. Lone pairs are a common source (tail) of a curved arrow.
Bonding pair / sigma bond / pi bond
Electron pairs that hold atoms together. A sigma bond is a single bond along the internuclear axis; a pi bond is the side-on overlap found in double and triple bonds. Either type can be the source or the destination of a curved arrow.
Every curved arrow represents the movement of two electrons (a full pair). A single-barbed "fishhook" arrow represents one electron, but that belongs to radical chemistry, not this section.
The tail of the arrow sits on the electron source: a lone pair, a sigma bond, or a pi bond.
The head of the arrow points to the electron sink: usually an atom that will gain the electrons, or a bond that will shift.
Arrows flow from electron-rich to electron-poor, without exception. If you find yourself drawing an arrow from a positive centre to a negative centre, something has gone wrong.
Each arrow must respect the octet rule (or duet rule for hydrogen). You cannot place more than eight electrons around a second-row atom. If a new bond forms to an atom that already has a full octet, another bond to that atom must break simultaneously, shown by a second arrow.
In an intermolecular reaction, two separate molecules interact. One supplies the nucleophile (electron-pair donor), the other supplies the electrophile (electron-pair acceptor).
The first arrow always begins at the nucleophile and ends at the electrophile, forming a new bond between them.
If the electrophilic atom already satisfies the octet rule before the new bond forms, a second arrow must show a bond breaking away from that atom at the same time.
Question 1.29 (a), (b), and (c) each present an intermolecular reaction with a different nucleophile providing the first arrow.
The exercise asks you to swap the nucleophile from one reaction into the other two, producing six new combinations.
The method for each swap:
Identify the nucleophile (the species whose lone pair or bond is the tail of the first arrow) in reaction (a), (b), and (c).
Take the nucleophile from (a) and pair it with the electrophile from (b). Draw the first arrow from (a)'s nucleophile to (b)'s electrophile. Complete any additional arrows needed to avoid octet violations.
Repeat: nucleophile from (a) with electrophile from (c).
Then nucleophile from (b) with electrophiles from (a) and (c).
Then nucleophile from (c) with electrophiles from (a) and (b).
In each case, ask yourself: does the electrophilic atom already have a full octet? If yes, you need a leaving arrow as well.
Look for a formal negative charge, a lone pair on an electronegative atom, or a pi bond.
Between two candidates, the one with the higher electron density is generally the nucleophile.
In a drawn mechanism, the nucleophile is always the origin of the first curved arrow.
General pattern for an intermolecular nucleophilic attack:
Nucleophile (with lone pair) + Electrophile → New bond formed between the two
If the electrophilic atom already has a full octet, one of its existing bonds must break simultaneously:
Nuc: → E–LG becomes Nuc–E + :LG
where LG is the leaving group that departs with the bonding electrons.
Arrow convention summary:
Tail on electrons (lone pair or bond)
Head on the atom or bond receiving electrons
Double-barbed arrow = two electrons moving
Single-barbed (fishhook) arrow = one electron moving (radical reactions, not covered here)
Curved arrow formalism is how medicinal chemists design drug molecules: they trace exactly which bonds form and break to predict side products and optimise yields. Every enzyme mechanism in biochemistry, from how aspirin blocks COX-2 to how your body metabolises caffeine, is drawn using the same curved arrow notation you are learning here.
Students often draw the arrow starting at the electrophile and pointing toward the nucleophile. This is backwards. Arrows always flow from electron-rich to electron-poor.
Students sometimes draw a curved arrow to or from a single atom without specifying whether the electrons come from a lone pair or a bond. Be explicit: the tail must sit directly on the lone pair or on the bond line.
A common error is forming a new bond to an atom that already has eight electrons without simultaneously breaking another bond. The octet cannot be exceeded for second-row elements (C, N, O, F).
Students frequently confuse double-barbed arrows (two electrons, ionic mechanisms) with single-barbed fishhook arrows (one electron, radical mechanisms). In Section 1.4, you are working exclusively with double-barbed arrows.
⚠️ Curved arrows appear on virtually every exam in Organic Chemistry I and II. If you cannot draw them correctly, you cannot answer mechanism questions, which typically account for a large share of exam marks.
⚠️ Expect questions that give you a nucleophile and an electrophile and ask you to draw the complete curved arrow mechanism and products. The POD 3 exercise (swapping first-arrow reagents) is exactly this format.
⚠️ The direction of the arrow (electron-rich to electron-poor) is a favourite point of deduction. Reversing it costs full marks on that arrow.
⚠️ Octet violations are another common deduction. Always check that no second-row atom ends up with more than eight electrons after you draw your arrows.
True or False: The tail of a curved arrow is placed on the atom that receives the electrons.
Answer: False. The tail is placed on the electron source (a lone pair or bond). The head points to the receiver.
Fill in the blank: A nucleophile is an electron-pair ______, and an electrophile is an electron-pair ______.
Answer: donor; acceptor.
True or False: You can form a new bond to carbon without breaking an existing bond, even if carbon already has four bonds.
Answer: False. Carbon in the second row cannot exceed eight electrons (four bonds). One bond must break if a new one forms.
Fill in the blank: In an intermolecular mechanism, the first curved arrow always starts at the ______ and points toward the ______.
Answer: nucleophile; electrophile.
True or False: A double-barbed curved arrow represents the movement of a single electron.
Answer: False. A double-barbed arrow represents two electrons. A single-barbed (fishhook) arrow represents one electron.
Q: Given a nucleophile with a lone pair on oxygen and an electrophile that is a carbocation (positively charged carbon), draw the curved arrow for the first step and state what new bond forms.
A: The curved arrow starts at the oxygen lone pair (tail on oxygen's lone pair) and the head points to the positively charged carbon. A new C–O bond forms. No bond needs to break because the carbocation has only three bonds (six electrons), so it can accept another pair without violating the octet.
Q: Hydroxide ion (HO⁻) acts as a nucleophile toward the carbon of CH₃Cl. Why must a second arrow be drawn, and what does it show?
A: Carbon in CH₃Cl already has four bonds (a full octet). When HO⁻ donates a pair to carbon, a second arrow must show the C–Cl bond breaking, with the bonding electrons moving onto chlorine. This produces CH₃OH and Cl⁻.
Q: In the POD 3 exercise, you are asked to swap the "first arrow" reagents among three reactions. What does "first arrow reagent" mean, and what should you check each time you make a swap?
A: The first arrow reagent is the nucleophile, the species whose electrons initiate the mechanism. Each time you swap it into a different reaction, check whether the electrophilic atom in that reaction already has a full octet. If it does, you need a second arrow showing a bond breaking. If it does not (e.g. a carbocation), the first arrow alone completes the step.
Q: A student draws a curved arrow from a positively charged nitrogen toward a negatively charged oxygen. What error have they made?
A: They have reversed the arrow. Curved arrows flow from electron-rich to electron-poor. The arrow should start at the negatively charged oxygen (the nucleophile) and point toward the positively charged nitrogen (the electrophile).
Q: Explain the difference between an intramolecular and an intermolecular curved arrow step.
A: In an intermolecular step, the nucleophile and electrophile are on different molecules, so two separate species come together. In an intramolecular step, both the electron-pair donor and the acceptor are on the same molecule, and the arrow curves from one part of the molecule to another, often forming a ring.
This material connects directly to acid-base chemistry (Chapter 3), because every Brønsted acid-base reaction can be described with curved arrows, and recognising nucleophiles and electrophiles is the same skill as identifying Lewis bases and Lewis acids. It also underpins all of substitution and elimination (SN1, SN2, E1, E2) later in the course, where you will draw multi-step mechanisms using exactly these conventions. If you are comfortable with Section 1.4, those chapters become a matter of applying what you already know to new substrates.
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