Britannica Sources used in this answer

Which substances can act as Lewis acids without containing hydrogen ions?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

According to the Lewis theory proposed by Gilbert N. Lewis, a Lewis acid is defined as any chemical species capable of accepting an electron pair to form a covalent bond. Because this definition focuses on electron-pair acceptance rather than proton donation, many substances that do not contain hydrogen ions can function as Lewis acids.

Examples of such substances include:

Metal ions: Many positively charged metal ions act as electron-pair acceptors.12
Nonmetallic oxides: The oxides of certain nonmetallic elements, such as sulfur trioxide (SO $_{3}$ ), carbon dioxide (CO $_{2}$ ), and nitrogen pentoxide (N $_{2}$ O $_{5}$ ), are included in this classification.12
Solid compounds: Various solid compounds, including aluminum chloride (AlCl $_{3}$ ), boron trifluoride (BF $_{3}$ ), silica, and alumina, are recognized as Lewis acids.12
Other halides: Compounds such as titanium tetrachloride (TiCl $_{4}$ ) also function as Lewis acids in chemical reactions.3