In Brønsted acids/bases, an acid gives up a proton and a base takes it, using one of its lone pairs. This page keeps the lone pair but takes the proton out of the definition. The result is a wider definition of acids and bases, and it covers many reactions that look nothing like acid–base chemistry.
An acid–base reaction with no proton in it
Start with boron trifluoride, BF₃. Boron brings only three valence electrons, so after its three B–F bonds it has six electrons around it, not eight. This is the incomplete octet you met in Lewis structures. Boron's leftover p orbital stays empty. It points straight up and down, at right angles to the flat molecule.
Now bring in ammonia, NH₃. Its nitrogen has three N–H bonds and one lone pair.
When the two molecules meet, nitrogen's lone pair moves into boron's empty orbital and becomes a new B–N bond. No proton moves anywhere, yet chemists still call this an acid–base reaction. BF₃ is the acid and NH₃ is the base.
The arrow. One curved arrow is enough. Its tail sits on nitrogen's lone pair and its head points at boron. The pair becomes the new B–N bond, and no existing bond breaks.
The charges. Count formal charges on the product, F₃B–NH₃. Nitrogen now has four bonds and no lone pair, so it is +1. Boron has four bonds and brings three valence electrons, so its charge is 3 − 0 − 4 = −1. The two charges cancel, so the product is neutral overall, but each atom still carries its own charge. Leaving them off is a common mistake.
The shape. Before the reaction, boron has three bonds and is trigonal planar, with 120° between them. Afterward it has four bonds and is tetrahedral, with about 109.5° between them. Boron now has a full octet.
The product is called an adduct: two molecules joined into one, with nothing lost. A bond whose two electrons both came from one atom is sometimes called a dative or coordinate bond. Once it has formed, it behaves like any other covalent bond; the name only records where its electrons came from.
The Lewis definition
The proton has dropped out of the definition, and that is the point. A Lewis acid needs no hydrogen at all, only room for an incoming pair of electrons.
Every Brønsted base is also a Lewis base. When ammonia takes a proton, it does so by donating its lone pair to the H⁺, just as it donated that pair to boron.
H⁺ itself is the smallest Lewis acid. A bare proton has no electrons and an empty 1s orbital, so every bond it forms is made from a pair that a base donates. A Brønsted acid such as HCl works by supplying this Lewis acid, H⁺. Textbooks differ on whether HCl itself should also be called a Lewis acid. Nothing chemical depends on the answer, and the arrows you draw are the same either way.
So the two definitions nest rather than compete. Lewis is the broader one, and Brønsted chemistry is the special case in which the pair goes to H⁺. Both are correct, and chemists use whichever suits the reaction in front of them.
Two ways to make room for a pair
BF₃ has room because of a real gap: six electrons and an empty p orbital. AlCl₃ is built the same way. Aluminum sits directly below boron in the periodic table, brings three valence electrons, and is left with six electrons and an empty orbital after its three bonds. A carbocation has the same gap, as a later section shows.
FeBr₃, ZnCl₂, TiCl₄ and SnCl₄ make room another way. Their central atoms are metals, and a metal can hold more than eight electrons around it, so it does not need a gap in an octet. Instead, it takes on one more ligand, an atom or group bonded to the metal. The number of ligands is the metal's coordination number. Accepting a pair this way raises it by one, which chemists call expanding the coordination number.
Take TiCl₄ and acetone, (CH₃)₂C=O. Titanium starts with four bonds, one to each chlorine, and can hold five or six. One lone pair on acetone's oxygen forms a bond to titanium, which makes titanium five-coordinate. Count the formal charges as you did for BF₃: oxygen ends up +1 and titanium −1.
From the donor's side, the two kinds of Lewis acid do the same job. Each takes a lone pair and forms a new bond to the donor, so both count as Lewis acids.
A π bond can be the donor
Ethene, CH₂=CH₂, has no lone pair anywhere, yet it is a Lewis base. Its C=C double bond includes a π bond, and Hybridization showed where that pair of electrons sits: in the sideways overlap of two p orbitals, above and below the line joining the carbons.
That sideways overlap is weaker than the head-on overlap of a σ bond, so the π pair is held more loosely and is easier to give away. In Curved arrows you already started an arrow's tail on a π bond.
Take ethene and HBr. As Brønsted acids/bases showed, a proton never floats free; the H⁺ comes from an acid. One arrow takes ethene's π pair to the H, making a new C–H bond on one carbon. A second arrow moves the H–Br bonding pair onto bromine, which leaves as Br⁻. The π bond is gone, and the other carbon, now with only three bonds, becomes a carbocation. The acceptor is the H⁺ that HBr hands over.
The definition says "electron-pair donor" and never says which kind of pair. A lone pair qualifies, and so does a π bond. Preview: this fact drives the addition reactions of Alkenes & Alkynes, most of which begin with the π pair reaching out to an electron-poor partner.
A carbocation is a Lewis acid
A carbocation is a carbon with three bonds and a positive charge, as Formal charge showed. Take the tert-butyl cation, (CH₃)₃C⁺, whose positive carbon carries three methyl groups. What happens when it meets water, and which partner is the acid?
The Lewis acid is the cation. Its central carbon has three bonds and six valence electrons. It is sp² and trigonal planar, with an empty p orbital perpendicular to that plane: the same kind of gap as boron in BF₃. Nothing about it involves a proton.
The Lewis base is water. Either of oxygen's two lone pairs can do the job.
One arrow runs from a lone pair on water to the positive carbon. No bond breaks. The new C–O bond is made entirely from oxygen's pair, so it is a dative bond, which then behaves like any other.
The charges. Before the step, carbon is +1 and oxygen is 0. Afterward, carbon has four bonds and is neutral, while oxygen has three bonds and one lone pair, so it is +1. The product is an oxonium ion, an oxygen with three bonds and a positive charge. The charge did not vanish; it moved to the atom that supplied the electrons. Students often draw this product neutral on both atoms, and a formal-charge count catches the mistake.
The next step is a Brønsted one. A second water molecule removes a proton from the oxonium ion. That gives the neutral alcohol, tert-butyl alcohol, plus hydronium, H₃O⁺. So one reaction uses both definitions, one per step, and that is common.
Where carbocations come from, and what else they do, belongs to How Reactions Happen.
Two names for the same partners
Curved arrows gave the two partners in a step like these their mechanism names. The electron-rich partner that supplies the pair is the nucleophile. The electron-poor atom that receives it is the electrophile.
These are the roles the Lewis definition names. A nucleophile is a Lewis base, and an electrophile is a Lewis acid. Ammonia is the nucleophile toward BF₃, water toward the tert-butyl cation, and ethene toward the H of HBr.
The two vocabularies grew up in different branches of chemistry; they do not describe different events. In practice, "acid" and "base" tend to be used when the question is how far a reaction goes, and "electrophile" and "nucleophile" when the question is how fast it goes. How Reactions Happen explains why those two rankings do not always agree.
Preview: seen this way, nearly every electron-pair step in the course is a Lewis acid–base reaction under another name. That includes SN2 substitution, where one group on a carbon is swapped for another (Substitution & Elimination); addition to a C=O group (Carbonyl Chemistry); enolate alkylation, which builds a new C–C bond next to a C=O group (Enolate Chemistry); and substitution on a benzene ring (Aromatic Chemistry).
A preview: Lewis acids in later chapters
Nothing earlier on this page depends on this section. It shows two jobs that Lewis acids do in later chapters, each taught in full there.
Making a stronger electrophile. In Friedel–Crafts acylation (Aromatic Chemistry), AlCl₃ takes a lone pair from the chlorine of an acid chloride such as acetyl chloride, CH₃COCl (see Functional groups). That weakens the C–Cl bond enough for it to break. The chlorine keeps the bonding pair and leaves as part of AlCl₄⁻. At the same time a lone pair on oxygen moves in to make a third C–O bond. What remains is a free acylium ion, CH₃C≡O⁺, a carbon triple-bonded to a positive oxygen. The acylium ion is electron-poor enough for benzene's π electrons to attack it, which the acid chloride on its own is not. So the Lewis acid does not supply the electrophile; it manufactures one.
AlCl₃ is often called the catalyst in this reaction. In acylation, though, a full equivalent is needed, because the ketone product binds the AlCl₃ and holds on to it.
Activating a C=O group. In the TiCl₄–acetone complex drawn above, the oxygen carries a positive charge. It pulls electron density away from the C=O carbon, which makes that carbon a stronger electrophile. Adding H⁺ to the same oxygen does the same thing. A Lewis acid such as ZnCl₂ or TiCl₄ does it without any strong acid, so a molecule that strong acid would destroy can survive (Carbonyl Chemistry).
Hard and soft, briefly
Lewis acids and bases are often sorted into two kinds. Hard ones are small, with their charge packed tightly, and are not very polarizable, meaning their electron cloud is hard to distort. Soft ones are large and diffuse, and their electron clouds distort easily.
| Role | Hard | Soft |
|---|---|---|
| Lewis acids | H⁺, Li⁺, BF₃ | Ag⁺, Cu⁺ |
| Lewis bases | F⁻, RO⁻ (an alkoxide) | I⁻, RS⁻ (a thiolate, the anion of a thiol) |
The useful rule is that hard prefers hard and soft prefers soft.
Preview: you do not need the full theory yet. It appears here because How Reactions Happen reaches a result that looks wrong without it. A thiolate, RS⁻, bonds to the carbon of an alkyl halide faster than an alkoxide, RO⁻, does, even though the thiolate is by far the weaker base. Hard and soft explain why. A soft, polarizable donor pairs well with a soft acceptor such as that carbon, whatever their acid–base strengths, so basicity and reaction speed can point in opposite directions.
What carries forward
Besides the two jobs in the preview above, Lewis acids help open epoxides, three-membered rings that contain an oxygen (Alcohols, Ethers & Related Chemistry). More broadly, once nucleophilic attack is seen as Lewis acid–base chemistry, the course has fewer separate ideas than its long list of named reactions suggests.