Editing Lewis acids and bases (section)

==Lewis acids==
[[File:NH3-BF3-adduct-bond-lengthening-2D.png|thumb|270px|left|Major structural changes accompany binding of the Lewis base to the coordinatively unsaturated, planar Lewis acid BF<sub>3</sub>]]
Lewis acids are diverse and the term is used loosely. Simplest are those that react directly with the Lewis base, such as boron trihalides and the pentahalides of phosphorus, arsenic, and antimony.

In the same vein, {{chem2|CH3+}} can be considered to be the Lewis acid in methylation reactions.  However, the methyl cation never occurs as a free species in the condensed phase, and methylation reactions by reagents like CH<sub>3</sub>I take place through the simultaneous formation of a bond from the nucleophile to the carbon and cleavage of the bond between carbon and iodine (S<sub>N</sub>2 reaction). Textbooks disagree on this point: some asserting that alkyl halides are electrophiles but not Lewis acids,<ref>{{Cite book|last=Vollhardt|first=K. Peter C. |title=Organic chemistry : structure and function|date=2018|others=Neil Eric Schore|isbn=978-1-319-07945-1|edition=8th|location=New York|page=73|oclc=1007924903}}</ref> while others describe alkyl halides (e.g. CH<sub>3</sub>Br) as a type of Lewis acid.<ref>{{Cite book|last=Carey|first=Francis A. |title=Organic chemistry|date=2003|publisher=McGraw-Hill|isbn=0-07-242458-3|edition=5th|location=Boston|page=46|oclc=48850987}}</ref> The [[IUPAC Gold Book|IUPAC]] states that Lewis acids and Lewis bases react to form Lewis adducts,<ref name="Gold Book Lewis acid L03508"/> and defines electrophile as Lewis acids.<ref>{{GoldBookRef |title=Electrophile (Electrophilic) |file=E02020 }}</ref>

===Simple Lewis acids===
Some of the most studied examples of such Lewis acids are the boron trihalides and [[organoboranes]]:<ref>{{cite journal |doi=10.1021/ic990713m|title=Ligand Close-Packing and the Lewis Acidity of BF3 and BCl3 |year=1999 |last1=Rowsell |first1=Bryan D. |last2=Gillespie |first2=Ronald J. |last3=Heard |first3=George L. |journal=Inorganic Chemistry |volume=38 |issue=21 |pages=4659–4662 |pmid=11671188 }}</ref>
:BF<sub>3</sub> + F<sup>−</sup> → {{chem2|BF4-}}
In this adduct, all four fluoride centres (or more accurately, [[ligand]]s) are equivalent.
:BF<sub>3</sub> + OMe<sub>2</sub> → BF<sub>3</sub>OMe<sub>2</sub>
Both BF<sub>4</sub><sup>−</sup> and BF<sub>3</sub>OMe<sub>2</sub> are Lewis base adducts of boron trifluoride.

Many adducts violate the [[octet rule]], such as the [[triiodide]] anion:
:I<sub>2</sub> + I<sup>−</sup> → {{chem2|I3-}}
The variability of the colors of iodine solutions reflects the variable abilities of the solvent to form adducts with the Lewis acid I<sub>2</sub>.

Some Lewis acids bind with two Lewis bases, a famous example being the formation of [[hexafluorosilicate]]:
:SiF<sub>4</sub> + 2 F<sup>−</sup> → {{chem2|SiF6(2-)}}

===Complex Lewis acids===
Most compounds considered to be Lewis acids require an activation step prior to formation of the adduct with the Lewis base. Complex compounds such as [[Ethylaluminium sesquichloride|Et<sub>3</sub>Al<sub>2</sub>Cl<sub>3</sub>]] and [[Aluminium chloride|AlCl<sub>3</sub>]] are treated as trigonal planar Lewis acids but exist as aggregates and polymers that must be degraded by the Lewis base.<ref>Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. {{ISBN|0-7506-3365-4}}.{{page needed|date=September 2021}}</ref> A simpler case is the formation of adducts of borane. Monomeric BH<sub>3</sub> does not exist appreciably, so the adducts of borane are generated by degradation of diborane:
:B<sub>2</sub>H<sub>6</sub> + 2 H<sup>−</sup> → 2 {{chem2|BH4-}}
In this case, an intermediate {{chem2|B2H7-}} can be isolated.

Many metal complexes serve as Lewis acids, but usually only after dissociating a more weakly bound Lewis base, often water.
:[Mg(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> + 6 NH<sub>3</sub> → [Mg(NH<sub>3</sub>)<sub>6</sub>]<sup>2+</sup> + 6 H<sub>2</sub>O

===H<sup>+</sup> as Lewis acid===
The [[proton]] (H<sup>+</sup>) <ref name="hydron"/> is one of the strongest but is also one of the most complicated Lewis acids. It is convention to ignore the fact that a proton is heavily solvated (bound to solvent). With this simplification in mind, acid-base reactions can be viewed as the formation of adducts:
*H<sup>+</sup> + NH<sub>3</sub> → {{chem2|NH4+}}
*H<sup>+</sup> + OH<sup>−</sup> → H<sub>2</sub>O

===Applications of Lewis acids===
A typical example of a Lewis acid in action is in the [[Friedel–Crafts alkylation]] reaction.<ref name=March>March, J. “Advanced Organic Chemistry” 4th Ed.  J. Wiley and Sons, 1992: New York.  {{ISBN|0-471-60180-2}}.{{page needed|date=September 2021}}</ref> The key step is the acceptance by AlCl<sub>3</sub> of a chloride ion lone-pair, forming {{chem2|AlCl4-}} and creating the strongly acidic, that is, [[electrophilic]], carbonium ion.
:RCl +AlCl<sub>3</sub> → R<sup>+</sup> + {{chem2|AlCl4-}}