Answer:
Interesting question, and the answer is yes, a substance can be a Lewis acid but not a Bronsted-Lowrey acid. To see this, let’s take a look at the definitions of each.
Explanation:
Bronsted-Lowrey acid:
A compound that is a hydrogen ion (proton) donor. When dissolved in the solvent in question, these compounds lose a proton to the solution. The concentration of these protons in solution is referred to as acidity, and is measured on the pH scale.
Lewis acid:
A substance that is an electron pair receiver. In solution, free electron pairs will form bonds with the substance, either ionic or covalent. In this definition, a proton is itself an acid, rather than a part of an acid.
A key thing to note here is that, in the Bronsted-Lowrey definition, there must be a proton. That means, all Bronsted-Lowrey acids are of the form HXn→H++Xn− , showing the dissociation in solution. However, a Lewis acid needs only to have the ability to accept an electron pair, which means that H+ is a Lewis acid, instead of what makes a compound an acid. Additionally, that means that the number of compounds that qualify as a Lewis acid are expanded. A favorite example of mine is boron trifluoride, or BF3 . It is a common reagent in organic synthesis, it is a Lewis acid, but does not have any hydrogen, so it cannot be a Bronsted-Lowrey acid
