IB DP Chemistry - R3.4.6 Lewis acid- Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – R3.4.6 Lewis acid – Study Notes – New Syllabus
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Reactivity 3.4.6 - Lewis Acids and Bases
Reactivity 3.4.6 – Lewis Acids and Bases
Definition
- Lewis acid: A species that accepts an electron pair to form a covalent bond.
- Lewis base: A species that donates an electron pair to form a covalent bond.
This theory is broader than the Brønsted–Lowry definition (which focuses on proton transfer), and it applies to both protonic and non-protonic systems.
Mechanism
- Lewis bases have a lone pair of electrons that they can donate.
- Lewis acids have an empty orbital that can accept an electron pair.
- When a Lewis base donates an electron pair to a Lewis acid, a dative (coordinate) covalent bond forms.
Visual representation of Lewis acid – base reaction:
\( \text{B:} + \text{A}^+ \rightarrow \text{B→A} \)
(Lewis base) (Lewis acid) (Lewis adduct)
Key Characteristics of Lewis Acids
- Electron-deficient species (e.g. \( \text{BF}_3 \), \( \text{AlCl}_3 \))
- Positive ions (e.g. \( \text{Fe}^{3+} \), \( \text{H}^+ \))
- Transition metal cations with vacant d orbitals
Key Characteristics of Lewis Bases
- Species with lone pairs (e.g. \( \text{NH}_3 \), \( \text{H}_2\text{O} \))
- Negatively charged ions (e.g. \( \text{OH}^- \), \( \text{Cl}^- \), \( \text{CN}^- \))
- Neutral molecules with lone pairs (e.g. ethers, amines, alcohols)
Example
Identify the Lewis acid and Lewis base in the following reaction:
\( \text{NH}_3 + \text{BF}_3 \rightarrow \text{H}_3\text{N→BF}_3 \)
▶️Answer/Explanation
\( \text{NH}_3 \) has a lone pair → it acts as a Lewis base.
\( \text{BF}_3 \) is electron deficient → it acts as a Lewis acid.
A coordinate bond forms between the nitrogen of ammonia and the boron atom.
Example
Classify the species in the reaction:
\( \text{Ag}^+ + 2\text{NH}_3 \rightarrow [\text{Ag(NH}_3)_2]^+ \)
▶️Answer/Explanation
\( \text{NH}_3 \): Lewis base (donates electron pair)
\( \text{Ag}^+ \): Lewis acid (accepts electron pair)
The product is a complex ion where both NH₃ molecules are bonded to Ag⁺ via dative bonds.
Common Misconceptions
- Not all Lewis acids are Brønsted acids – some do not donate protons (e.g. \( \text{BF}_3 \)).
- All Brønsted bases are Lewis bases, but the reverse isn’t always true.
Why Lewis Theory is Important
- It explains acid–base behaviour in non-aqueous systems.
- It extends the concept of acids and bases to cover metal complex formation and organic reactions like electrophilic and nucleophilic substitution.
Applying Lewis Acid – Base Theory in Inorganic and Organic Chemistry
Lewis acid–base theory defines acids as electron-pair acceptors and bases as electron-pair donors. This model allows us to analyze chemical behavior in systems where no proton transfer occurs — extending far beyond aqueous acid–base reactions into coordination chemistry and organic mechanisms.
1. Inorganic Chemistry Applications
a. Complex ion formation
Many transition metal ions act as Lewis acids by accepting electron pairs from ligands (Lewis bases) to form coordination compounds. These ligands donate lone pairs to the empty d-orbitals of metal ions, forming coordinate (dative covalent) bonds.
Example
Deduce the Lewis acid and base in the following:
\( \text{Fe}^{3+} + 6\text{CN}^- \rightarrow [\text{Fe(CN)}_6]^{3-} \)
▶️Answer/Explanation
\( \text{Fe}^{3+} \): Lewis acid — electron pair acceptor
\( \text{CN}^- \): Lewis base — donates lone pairs
A complex ion forms via 6 dative bonds between cyanide ions and Fe³⁺.
b. Reaction of acidic/basic oxides
Acidic oxides (non-metal oxides) act as Lewis acids by accepting electron pairs from water or hydroxide ions. Basic oxides (metal oxides) can act as Lewis bases by donating lone pairs to acidic species.
Example
Apply Lewis theory to this reaction:
\( \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \)
▶️Answer/Explanation
\( \text{SO}_3 \): Lewis acid — accepts a lone pair from water
\( \text{H}_2\text{O} \): Lewis base — donates lone pair
A coordinate bond forms between the oxygen of water and sulfur in \( \text{SO}_3 \).
c. Lewis acid-base adducts in halide systems
Molecules like \( \text{AlCl}_3 \) and \( \text{BF}_3 \) are electron-deficient and readily accept lone pairs, forming stable adducts with bases such as ammonia or halide ions.
Example
Explain the formation of this adduct:
\( \text{BF}_3 + \text{F}^- \rightarrow \text{BF}_4^- \)
▶️Answer/Explanation
\( \text{BF}_3 \): Lewis acid (accepts a lone pair into an empty p orbital)
\( \text{F}^- \): Lewis base (donates an electron pair)
This leads to the tetrahedral anion \( \text{BF}_4^- \) via coordinate bonding.
2. Organic Chemistry Applications
a. Nucleophilic substitution reactions (SN1 and SN2)
In these reactions, the nucleophile acts as a Lewis base by donating a pair of electrons to an electrophilic carbon atom (the Lewis acid), usually bonded to a leaving group.
Example
Classify the role of each species in:
\( \text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Br}^- \)
▶️Answer/Explanation
\( \text{OH}^- \): Lewis base — donates a lone pair to the electrophilic carbon
\( \text{CH}_3\text{CH}_2\text{Br} \): Lewis acid — accepts electrons at the δ⁺ carbon
The result is substitution of Br by OH via a nucleophilic attack.
b. Electrophilic addition to alkenes
The π bond in alkenes is electron-rich and behaves as a Lewis base. It donates a pair of electrons to an electrophilic species (Lewis acid), often a proton or halogen.
Example
Identify acid–base roles:
\( \text{CH}_2=CH_2 + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br} \)
▶️Answer/Explanation
Alkene π electrons → Lewis base
\( \text{H}^+ \) from HBr → Lewis acid (accepts π electrons)
The π bond donates electrons to H⁺, forming a carbocation that is then attacked by \( \text{Br}^- \).
c. Carbonyl chemistry
The carbon in the C=O group is electrophilic (Lewis acid) because of the polar bond; nucleophiles (Lewis bases) can attack this carbon, donating electrons into the vacant π* orbital.
Example
Classify the species in the nucleophilic addition:
\( \text{CH}_3\text{CHO} + \text{H}^- \rightarrow \text{CH}_3\text{CH}_2\text{O}^- \)
▶️Answer/Explanation
\( \text{H}^- \): Lewis base (donates electron pair)
\( \text{CH}_3\text{CHO} \): Lewis acid (electrophilic carbon accepts electron pair)
A new C-H bond forms via nucleophilic attack on the carbonyl group.
3. Identifying Lewis Acids and Bases
- Lewis Acid — Electron-pair acceptor:
- Electron-deficient atoms (e.g. B, Al, transition metals)
- Carbocations or partially positive carbon atoms
- π-accepting molecules (e.g. \( \text{CO}_2 \), \( \text{SO}_3 \))
- Lewis Base — Electron-pair donor:
- Lone-pair bearing atoms (e.g. N, O, S, halide anions)
- Negatively charged ions (e.g. \( \text{OH}^- \), \( \text{CN}^- \), \( \text{NH}_2^- \))
- π-electron sources like alkenes and aromatics in some cases