Question

Using the Bronsted-Lowry concept of acids and bases, which is the Bronsted acid and base in the following reaction?
${\left( {C{H_3}} \right)_2}NH(g) + B{F_3}(g) \to {\left( {C{H_3}} \right)_2}NHB{F_3}(s)$

Answer 1

Acid and bases can be classified under three major classifications i.e., Arrhenius definition which states that an acid produces proton in a solution while a base gives hydroxide ion in the solution, Bronsted-Lowry explain the concept on the basis of proton transfer whereas Lewis theory explains the concept of acid and base on the basis of electron transfer.

Complete Step By Step Answer:
According to the Bronsted-Lowry concept, an acid is defined as a species which has a tendency to donate a proton while a base is a species which has a tendency to accept a proton from other substances. The proton transfer can be explained with the help of an example which is as follows:
$HCl + {H_2}O \rightleftharpoons {H_3}{O^ + } + C{l^ - }$
In the above reaction, $HCl$ donates its proton to water molecule, thus it acts as an acid i.e., proton donor while ${H_2}O$ accepts a proton from $HCl$ , so it acts as Bronsted base i.e., proton acceptor. If we consider the reverse reaction, then ${H_3}{O^ + }$ will donate a proton to chloride ion to form $HCl$ and thus will act as Bronsted acid and chloride ion will act as Bronsted base.
So, in general Bronsted-lowry acid base reaction can be represented as follows:
${\text{aci}}{{\text{d}}_1} + \;{\text{bas}}{{\text{e}}_2} \rightleftharpoons {\text{aci}}{{\text{d}}_2} + \;{\text{bas}}{{\text{e}}_1}$
Now, as per question the given reaction is as follows:
${\left( {C{H_3}} \right)_2}NH(g) + B{F_3}(g) \to {\left( {C{H_3}} \right)_2}NHB{F_3}(s)$
In this reaction, the Bronsted acid-base concept cannot be applied to determine the acid and base in the reaction because no proton transfer takes place in the reaction, instead electron transfer takes place which can be explained using the Lewis acid-base concept.
Now, as per the Lewis acid base concept, the species which donate electron pairs are known as Lewis bases and the species which accept electron pairs or are electron deficient are Lewis acids. Thus, we can conclude that $B{F_3}$ is a Lewis acid and ${\left( {C{H_3}} \right)_3}NH$ is a Lewis base.

Note:
Remember that Bronsted Lowry theory was not able to explain the reactions between acidic oxides like $S{O_3}$ , $C{O_2}$ , etc and the basic oxides like $CaO$ , $BaO$ , etc. Also, it wasn’t able to explain the reactions occurring in aprotic solvents like $S{O_2}$ , ${N_2}{O_4}$ . Thus, the broader concept of acid base which involves electron transfer was introduced by Lewis.