Chemistry · Ch 8 — Ionic Equilibrium

Lewis Concept

8.1.3

Lewis Concept

In 1923, Gilbert N. Lewis proposed the most general acid-base concept of all, built on the electron pair rather than the proton: a Lewis acid is a species that accepts an electron pair, and a Lewis base is a species that donates an electron pair. A Lewis acid is typically a positive ion or an electron-deficient molecule, while a Lewis base is typically an anion or a neutral molecule carrying at least one lone pair. In the reaction between boron trifluoride and ammonia, boron has a vacant $2p$ orbital that accepts the lone pair donated by nitrogen to form a new coordinate covalent bond -- $BF_3$ is the Lewis acid (electron-pair acceptor) and $NH_3$ is the Lewis base (electron-pair donor), giving the adduct $F_3B \leftarrow NH_3$ . The same logic explains coordination compounds generally: the ligands act as Lewis bases and the central metal atom/ion, which accepts electron pairs from them, behaves as a Lewis acid.

Lewis acids and Lewis bases. A reference table classifying the common families of Lewis acids and Lewis bases with worked examples of each family, drawn together from the unit's discussion of electron-deficient species, metal ions/oxides, anions, polar-multiple-bond molecules, expandable-octet molecules and carbon-centred ions.

Lewis acids (electron-pair acceptors)	Examples	Lewis bases (electron-pair donors)	Examples
Electron-deficient molecules	$BF_3, AlCl_3, BeF_2$	Molecules with one or more lone pairs	$NH_3, H_2O, R{-}O{-}H, R{-}O{-}R, R{-}NH_2$
All metal ions	$Fe^{2+}, Fe^{3+}, Cr^{3+}, Cu^{2+}$	All anions	$F^-, SCN^-, SO_4^{2-}, Cl^-, CN^-$
Molecules with a polar double bond	$SO_2, CO_2, SO_3$	Molecules with a C–C multiple bond	$CH_2{=}CH_2, CH{\equiv}CH$
Molecules whose central atom can expand its octet (accessible empty d orbitals)	$SiF_4, SF_4, FeCl_3$	All metal oxides	$CaO, MgO, Na_2O$
Carbonium ion	$CH_3^+$ (as in $(CH_3)_3C^+$ )	Carbanion	$CH_3^-$

Worked example: hydration of Cr3+. Identifying the Lewis acid and Lewis base in $Cr^{3+} + 6H_2O \rightarrow [Cr(H_2O)_6]^{3+}$ : each of the six water molecules donates a lone pair of electrons to $Cr^{3+}$ to form the hydrated cation hexaaquachromium(III) ion. So the Lewis acid is $Cr^{3+}$ (electron-pair acceptor) and the Lewis base is $H_2O$ (electron-pair donor). …

Table 8.1.3-table-1Lewis acids and Lewis bases

Lewis acids (electron-pair acceptors)	Examples	Lewis bases (electron-pair donors)	Examples
Electron-deficient molecules	$BF_3, AlCl_3, BeF_2$	Molecules with one or more lone pairs	$NH_3, H_2O, R{-}O{-}H, R{-}O{-}R, R{-}NH_2$
All metal ions	$Fe^{2+}, Fe^{3+}, Cr^{3+}, Cu^{2+}$	All anions	$F^-, SCN^-, SO_4^{2-}, Cl^-, CN^-$
Molecules with a polar double bond	$SO_2, CO_2, SO_3$	Molecules with a C–C multiple bond	$CH_2{=}CH_2, CH{\equiv}CH$

Misc 8.1.3-exampleWorked example: hydration of Cr3+

Worked out. Identifying the Lewis acid and Lewis base in $Cr^{3+} + 6H_2O \rightarrow [Cr(H_2O)_6]^{3+}$ : each of the six water molecules donates a lone pair of electrons to $Cr^{3+}$ to form the hydrated cation hexaaquachromium(III) ion. So the Lewis acid is $Cr^{3+}$ (electron-pair acceptor) and the Lewis base is $H_2O$ (electron-pair …

Misc 8.1.3-eval3Evaluate yourself – 3: identifying Lewis acid and base

Worked out. A self-check box asking the student to identify the Lewis acid and Lewis base in two reactions: (i) $CaO + CO_2 \rightarrow CaCO_3$ -- the oxide ion of $CaO$ donates its lone pair to the electron-deficient carbon of $CO_2$ , so $CaO$ (via $O^{2-}$ ) is the Lewis base and $CO_2$ is the Lewis acid; (ii) trimethylaluminium reacting with dimethyl ether, $(CH_3)_2O + AlCl_3 \rightarrow (CH_3)_2O{\rightarrow}AlCl_3$ -- the ether oxygen's lone pair is donated to the electron-deficient aluminium, so the ether is the Lewis base and $A …

Misc 8.1.3-eval4Evaluate yourself – 4: nature of boric acid

Worked out. A self-check box: $H_3BO_3$ accepts a hydroxide ion from water, $H_3BO_3(aq) + H_2O(l) \rightleftharpoons B(OH)_4^-(aq) + H^+(aq)$ . Since boron in $H_3BO_3$ has an empty orbital and accepts an electron pair (via $OH^-$ ) rather than donating a proton itself, boric acid is predicted to be a Lewis acid, not a classical Bronsted acid -- it owes its acidity to accepting electrons, even though the net effect (release of $H^+$ ) looks Arrhen …

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