Identify Lewis Acid–Base Sites and Track Electron Pairs

The Lewis model describes bond formation by following an electron pair from its donor to its acceptor. It is most useful when applied to a particular atom and reaction partner, rather than to a molecule's charge alone.

Locate a donor and an acceptor

A donor needs an available pair of electrons. An acceptor needs an accessible orbital that can receive the pair. A filled lone-pair orbital on oxygen or nitrogen often supplies the donor site; electron-deficient centers can supply the acceptor. The two orbitals must be able to interact.

Follow a new adduct example

Consider diethyl ether and boron trifluoride:

$\mathrm{Et_2O: + BF_3 \rightleftharpoons Et_2O\!\rightarrow\!BF_3}.$

Here Et means $\mathrm{CH_3CH_2}$. Ether oxygen begins with two lone pairs. One pair forms a bond to boron, so oxygen is the donor site and boron the acceptor site. A curved electron-flow arrow starts at the oxygen lone pair and ends at boron; it must not point from the electron-deficient boron toward the oxygen.

In an ordinary single-bond Lewis drawing of the adduct, oxygen has three bonds, one remaining lone pair and formal charge +1. Boron has four bonds and formal charge −1. The adduct is neutral overall: $+1+(-1)=0$. Those formal charges are bookkeeping assignments, not a claim that oxygen becomes a free cation or boron a free anion.

Separate electron flow from atom movement

The curved arrow represents a pair of electrons being used in a bond. It is not an arrow showing the path of an atom. Once formed, a coordinate bond is a covalent bond; “coordinate” records where the pair came from.

This also explains why positive charge is an unreliable shortcut. The neutral $\mathrm{BF_3}$ molecule can accept a pair, while a neutral ether can donate one. Identify the actual orbital interaction first, then check valence, formal charges and overall charge conservation.

The Lewis model gives a way to identify the bond-forming roles. It does not, by itself, provide an equilibrium constant, reaction rate or universal ranking of acid strength.