Difficulty: Intermediate | Prerequisites: CHEM 210 (Organic Chemistry I), familiarity with Lewis structures, electronegativity, and basic bonding concepts.
This module is the bridge between CHEM 210 and CHEM 212. It revisits the foundational reaction logic that drives roughly 95% of the organic chemistry you will encounter across both courses: the interaction between nucleophiles and electrophiles. If you can identify which species is electron-rich and which is electron-poor, you can predict what reacts with what and why.
The material also covers molecular orbital (MO) theory at a practical level, focusing on HOMO and LUMO as tools for predicting reactivity rather than as abstract quantum concepts. Finally, it introduces the three core mechanistic steps (nucleophilic attack, loss of leaving group, proton transfer) that combine to explain nearly every reaction mechanism you will draw this semester.
If you are coming in cold, make sure you are comfortable with Lewis structures, formal charges, and electronegativity trends before diving in.
Organic reactions are, at their core, electron-rich species (nucleophiles) donating electrons to electron-poor species (electrophiles). You find them by spotting polar bonds and lone pairs. Molecular orbital theory tells you which orbitals interact (HOMO of the nucleophile meets LUMO of the electrophile), and curved arrows let you draw the electron flow step by step. Master the three elementary steps, nucleophilic attack, leaving group departure, and proton transfer, and you can piece together nearly any mechanism in this course.
Nucleophile
A species that is electron-rich (carries a partial or full negative charge) and donates a pair of electrons to form a new bond. In simple terms, the nucleophile is the "attacker" in a reaction, bringing electrons to share.
Electrophile
A species that is electron-poor (carries a partial or full positive charge) and accepts a pair of electrons to form a new bond. Think of it as the "target" that the nucleophile goes after.
Polar bond
A covalent bond in which electrons are shared unequally because the two atoms have different electronegativities. This unequal sharing creates partial charges (delta plus and delta minus) that let you identify nucleophilic and electrophilic sites. Common examples: O–H, O=C, C–Br, N–H, C–Cl, N≡C.
HOMO (Highest Occupied Molecular Orbital)
The highest-energy molecular orbital that contains electrons. This is the orbital the nucleophile uses to donate electrons. In simple terms, the HOMO is where the "available" electrons sit.
LUMO (Lowest Unoccupied Molecular Orbital)
The lowest-energy molecular orbital that is empty. This is the orbital the electrophile uses to accept electrons. Think of it as the empty parking space electrons are heading for.
Sigma bond (σ)
A bond formed by head-on overlap of orbitals along the internuclear axis (e.g. C–Br, C–H). Its antibonding counterpart is σ*.
Pi bond (π)
A bond formed by side-on overlap of p orbitals above and below the internuclear axis (e.g. the second bond in C=O). Its antibonding counterpart is π*.
Non-bonding orbital (n)
An orbital holding a lone pair that is not involved in bonding. It sits at an intermediate energy level. In simple terms, these are the lone pairs on atoms like oxygen or nitrogen.
Atomic orbital (a)
An unhybridised p orbital on an atom, sometimes relevant as a LUMO in certain electrophiles.
Curved arrow
A notation showing the movement of a pair of electrons in a mechanistic step. Arrows always start at a bond or a lone pair (the electron source) and point to where those electrons end up.
Nucleophilic attack (NA)
A mechanistic step in which the nucleophile's HOMO donates electrons into the electrophile's LUMO, forming a new bond. The orbital interaction is n → π* or n → σ*.
Loss of leaving group (LLG)
A mechanistic step in which a bond breaks and the leaving group departs with the bonding electrons, typically forming an anion.
Proton transfer (PT)
A mechanistic step in which a base removes a proton (H⁺) from an acid, or an acid donates a proton to a base. The orbital interaction is n → σ*.
S_N2 reaction
A one-step substitution combining nucleophilic attack and loss of leaving group simultaneously (NA + LLG). The nucleophile attacks from the back side as the leaving group departs.
E2 reaction
A one-step elimination combining proton transfer and loss of leaving group simultaneously (PT + LLG). A base removes a proton while the leaving group departs, forming a double bond.
Nucleophiles are electron-rich (δ⁻ or full negative charge). They have lone pairs or π electrons available to donate.
Electrophiles are electron-poor (δ⁺ or full positive charge). They have empty or low-energy orbitals ready to accept electrons.
Both are identified by looking for polar bonds in the molecule. Wherever a bond has unequal electron sharing, one end is nucleophilic and the other is electrophilic.
O–H, N–H, Cl–H: the hydrogen end is δ⁺ (electrophilic), the heteroatom end is δ⁻ (nucleophilic)
O=C: oxygen is δ⁻ (nucleophilic), carbon is δ⁺ (electrophilic)
C–Br, C–Cl: halogen is δ⁻, carbon is δ⁺ (electrophilic)
N≡C: nitrogen is δ⁻ (nucleophilic)
Some molecules can act as both nucleophile and electrophile depending on the site. For example, a molecule with a C=O group has an electrophilic carbon and a nucleophilic oxygen. When deciding what role a molecule plays, look at which site is reacting with the other species.
Pairing two electrophiles or two nucleophiles together does not produce a reaction. You need one of each. Similarly, if two negative charges are very close together on a potential product, the reaction is unlikely because the charges repel.
The CHEM 210 figure of nucleophile and electrophile rankings (arranged by HOMO and LUMO energy) is a practical tool. Strong nucleophiles have high-energy HOMOs (e.g. carbanions, alkoxides). Strong electrophiles have low-energy LUMOs (e.g. protonated carbonyls, alkyl halides). Weak nucleophiles (water, alcohols) and weak electrophiles (neutral carbonyls, esters) sit lower on the scale.
Under basic conditions, strong nucleophiles (anions) are available to react with electrophiles at the top of the electrophile scale.
Under acidic conditions, strong electrophiles (protonated species) are available to react with weaker nucleophiles lower on the nucleophile scale.
There are six types of molecular orbitals, ranked from lowest to highest energy:
σ (sigma bonding) – formed by constructive head-on overlap (e.g. C–Br)
π (pi bonding) – formed by constructive side-on overlap (e.g. C=O)
n (non-bonding) – lone pairs sitting in sp, sp², or sp³ hybrid orbitals
a (atomic) – an unhybridised p orbital on an atom
π* (pi antibonding) – formed by destructive side-on overlap
σ* (sigma antibonding) – formed by destructive head-on overlap
Bonding orbitals are lower in energy. Antibonding orbitals are higher. Non-bonding and atomic orbitals sit in the middle.
The HOMO is the orbital of the nucleophile. It is the highest-energy orbital that still has electrons in it. Those electrons are the ones available to donate.
The LUMO is the orbital of the electrophile. It is the lowest-energy empty orbital, ready to accept electrons.
HOMOs can be: σ, π, or n (occupied orbitals)
LUMOs can be: a, π*, or σ* (empty orbitals)
A higher-energy HOMO makes a compound more reactive as a nucleophile. The electrons are easier to donate because they are held less tightly.
A lower-energy LUMO makes a compound more reactive as an electrophile. The empty orbital is more "eager" to accept electrons.
When you look at a molecule, ask: what is the highest occupied orbital, and what is the lowest empty one?
H₃O⁺ (protonated water, compound A in the source): HOMO = n (the lone pair on oxygen), LUMO = σ* (the O–H antibonding orbital). This species is primarily an electrophile.
A simple alkene (compound B): HOMO = π (the C=C bonding orbital). It acts as a nucleophile.
Bromocyclohexane (compound C): LUMO = σ* (the C–Br antibonding orbital). The carbon bearing the bromine is the electrophilic site.
A carbonyl compound (compound D): HOMO = π or n, LUMO = π* (the C=O antibonding orbital). The carbonyl carbon is electrophilic; oxygen is nucleophilic.
An enolate or carboxylate (compound E, with a negative charge on oxygen): HOMO = n (the lone pair on oxygen), acting as a nucleophile.
Curved arrows must always start at a bond or a lone pair. They show the movement of a pair of electrons. The arrow tail is where the electrons come from; the arrow head is where they go.
1. Nucleophilic attack (NA)
The nucleophile donates a pair of electrons from its HOMO into the electrophile's LUMO, forming a new bond. The orbital interaction is typically n → π* (attack on a carbonyl) or n → σ* (attack on a saturated carbon).
Example: a hydroxide ion (⁻OH) attacks the electrophilic carbon of a carbonyl group. The arrow starts from the lone pair on oxygen and points to the carbon.
2. Loss of leaving group (LLG)
A bond breaks heterolytically: both electrons go with the departing fragment (the leaving group). The arrow starts at the bond being broken and points to the leaving group.
Example: C–Br bond breaks, with both electrons going to bromine. Br⁻ departs.
3. Proton transfer (PT)
A base removes a proton from an acid. The arrow goes from the base's lone pair to the proton (n → σ*), and a second arrow shows the bonding electrons in the X–H bond moving to X.
Example: a base attacks the H of X–H. Products are X⁻ and H–Base.
These elementary steps often occur in combination:
Sₙ₂ (NA + LLG): nucleophilic attack and loss of leaving group happen simultaneously. The nucleophile attacks the back side of the carbon bearing the leaving group, and the leaving group departs in the same step. One concerted step, two arrows.
E2 (PT + LLG): proton transfer and loss of leaving group happen simultaneously. A base removes a proton from a carbon adjacent to the leaving group, forming a π bond as the leaving group departs. One concerted step, products are B–H, an alkene (C=C), and X⁻.
Example a: alkene + H₃O⁺ (proton transfer)
The alkene (π bond, nucleophile) reacts with H₃O⁺ (electrophile). This is a proton transfer. The proton adds to the double bond, forming a carbocation. The major product has the positive charge on the more substituted carbon (Markovnikov's rule). Water is released as a byproduct.
Example b: ketone + H₃O⁺ (proton transfer)
The oxygen lone pair on the ketone (nucleophile) attacks a proton on H₃O⁺ (electrophile). This is a proton transfer. The product is a protonated ketone (oxocarbenium) and water.
Example c: carbonyl + hydroxide (nucleophilic attack)
Hydroxide (⁻OH, nucleophile) attacks the electrophilic carbonyl carbon. This is a nucleophilic attack. The π bond of C=O breaks, pushing electrons onto oxygen, forming an alkoxide intermediate. Alternatively, if an α-hydrogen is present, the base can perform a proton transfer to deprotonate at the α-position.
Example d: bromocyclohexane + alkoxide (two pathways)
Pathway 1 (PT): the alkoxide acts as a base, removing a proton adjacent to the C–Br bond, leading to elimination (E2). Products: alkene + alcohol + Br⁻.
Pathway 2 (NA + LLG): the alkoxide acts as a nucleophile, attacking the carbon bearing the bromine in an Sₙ₂2 reaction. Products: ether + Br⁻.
Example e: two electrophiles, no reaction
Bromocyclohexane (electrophilic at carbon) paired with an alkene (nucleophilic, but weak). If the alkene is acting as the nucleophile towards an electrophile that requires strong nucleophilic character, no productive reaction occurs. Not all pairings lead to a reaction.
σ < π < n < a < π* < σ*
This ranking matters because HOMO–LUMO interactions drive reactions. The smaller the energy gap between the nucleophile's HOMO and the electrophile's LUMO, the more favourable the reaction.
Nucleophilic attack on a carbonyl: n → π* (lone pair into the C=O antibonding orbital)
Nucleophilic attack on a saturated carbon (Sₙ₂2): n → σ* (lone pair into the C–X antibonding orbital)
Proton transfer: n → σ* (base's lone pair into the X–H antibonding orbital)
Nuc:⁻ + R–X → Nuc–R + X⁻ (NA + LLG, concerted)
B:⁻ + H–C–C–X → B–H + C=C + X⁻ (PT + LLG, concerted)
Nucleophile-electrophile chemistry is the basis of how drugs bind to enzyme active sites. Most enzyme inhibitors work by presenting a nucleophilic or electrophilic functional group that reacts with a complementary site on the protein. Understanding these interactions is what makes medicinal chemistry possible.
Proton transfer reactions are also central to buffer chemistry in biological systems. Every time your blood maintains its pH, it is running proton transfers between weak acids and their conjugate bases, governed by the same orbital logic covered here.
Students often think that the HOMO is always the electrophile's orbital. It is the opposite. The HOMO belongs to the nucleophile (the electron donor). The LUMO belongs to the electrophile (the electron acceptor).
Students confuse "high-energy HOMO = more reactive nucleophile" with "high-energy LUMO = more reactive electrophile." A higher HOMO does increase nucleophilic reactivity, but for electrophiles it is a lower LUMO that increases reactivity. The logic is inverted.
Curved arrows are sometimes drawn starting from the electrophile. Arrows must always start at the electron source (a lone pair or a bond), which is the nucleophile's side. Electrons flow from rich to poor.
Students sometimes assume any two species will react. Two nucleophiles together or two electrophiles together will not react productively. You need one of each.
⚠️ Identifying nucleophilic and electrophilic sites is tested constantly. If you can label which atom is δ⁺ and which is δ⁻ in a polar bond, you can predict which species attacks which.
⚠️ HOMO/LUMO assignments for specific molecules are a common exam question. Be able to name the HOMO and LUMO type (σ, π, n, a, π*, σ*) for simple molecules like water, alkenes, alkyl halides, and carbonyls.
⚠️ Curved arrow drawing is a core exam skill. Every arrow must start at a bond or lone pair, never at an atom or a charge. Getting this wrong typically costs full marks on mechanism questions.
⚠️ You will be asked to classify mechanistic steps as NA, LLG, PT, or combinations (Sₙ₂2 = NA + LLG; E2 = PT + LLG). Know the orbital interaction for each step.
⚠️ The energy ranking of molecular orbitals (σ < π < n < a < π* < σ*) and the relationship between orbital energy and reactivity (higher HOMO = better nucleophile; lower LUMO = better electrophile) are frequently tested.
True or false: the HOMO is the orbital of the electrophile.
Fill in the blank: curved arrows must start at a ______ or a ______.
True or false: a lower-energy LUMO makes a compound more reactive as an electrophile.
Fill in the blank: in an Sₙ₂2 reaction, the two combined mechanistic steps are ______ and ______.
True or false: two nucleophiles paired together will produce a reaction.
Answers: 1. False (the HOMO is the nucleophile's orbital). 2. Bond, lone pair. 3. True. 4. Nucleophilic attack (NA) and loss of leaving group (LLG). 5. False (you need a nucleophile and an electrophile).
Q: What is the difference between a nucleophile and an electrophile?
A: A nucleophile is electron-rich and donates electrons to form a bond. An electrophile is electron-poor and accepts electrons. They are identified by looking for polar bonds in the molecule.
Q: List the six types of molecular orbitals in order of increasing energy.
A: σ (sigma bonding), π (pi bonding), n (non-bonding), a (atomic), π* (pi antibonding), σ* (sigma antibonding).
Q: For a molecule with a C=O group, what is the LUMO, and why does this make the carbon electrophilic?
A: The LUMO is π* (the C=O antibonding orbital). It is empty and low in energy, making the carbonyl carbon a good acceptor of electron density from nucleophiles.
Q: Draw the curved arrows for an Sₙ₂2 reaction between hydroxide (⁻OH) and bromomethane (CH₃Br). What are the products?
A: Arrow 1 goes from the lone pair on ⁻OH to the carbon of CH₃Br. Arrow 2 goes from the C–Br bond to Br. Products: CH₃OH + Br⁻. This is a combined NA + LLG step.
Q: An alkene is mixed with an alkyl bromide (R–Br). Identify the nucleophile, the electrophile, and predict whether a reaction occurs under typical conditions.
A: The alkene (π electrons) is the nucleophile. R–Br is the electrophile (σ* C–Br is the LUMO). However, alkenes are relatively weak nucleophiles and typically do not react with neutral alkyl halides under standard conditions. A stronger nucleophile or activation (e.g. acid to generate a carbocation) would be needed.
Q: Why does a higher-energy HOMO make a better nucleophile, while a lower-energy LUMO makes a better electrophile?
A: A higher-energy HOMO means the electrons are less tightly held, making them easier to donate. A lower-energy LUMO means the empty orbital is more accessible, creating a stronger "pull" on incoming electrons. In both cases, the HOMO–LUMO energy gap shrinks, which lowers the activation energy for the reaction.
Q: A student draws a curved arrow starting from the electrophilic carbon of a carbonyl. Explain the error.
A: Curved arrows must start at the electron source, which is a lone pair or a bond. The electrophilic carbon is electron-poor and has no electrons to donate. The arrow should start from the nucleophile (e.g. the lone pair on the attacking species) and point towards the electrophilic carbon.
This material connects directly to substitution and elimination reactions (Sₙ₂1, Sₙ₂2, E1, E2) covered later in CHEM 212. The decision of whether a species acts as a nucleophile or a base determines whether you get substitution or elimination, so the nucleophile/electrophile framework here is the foundation for those chapters.
MO theory and HOMO/LUMO concepts also feed into understanding of conjugation, aromaticity, and pericyclic reactions. When you get to Diels–Alder reactions, you will be matching the HOMO of a diene with the LUMO of a dienophile, which is the same logic applied at a larger scale.
The reactivity ranking chart (strong vs weak nucleophiles and electrophiles) will reappear when you study carbonyl chemistry. Knowing where a species sits on that scale tells you whether it reacts under acidic or basic conditions.
Nucleophile, electrophile, HOMO, LUMO, molecular orbital theory, curved arrow mechanism, electron flow, nucleophilic attack, leaving group, proton transfer, Sₙ₂2, E2, polar bond, sigma bond, pi bond, antibonding orbital, non-bonding orbital, lone pair, delta plus, delta minus, electron-rich, electron-poor, Markovnikov, carbocation, alkoxide, hydroxide, carbonyl electrophile, CHEM 210 review, CHEM 212 Module 1, organic reaction mechanisms, Penn State CHEM 212