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6.4.5: In the boron trifluoride affinity scale, the enthalphy change on formation of an adduct between the base and boron trifluoride is taken as a measure of Lewis basicity.

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    The physical meaning of Lewis acid and base affinities are defined as the negative of the enthalphy for formation of an adduct with a reference base and acid, respectively:

    \[B~+~A_{reference}~ ⇌ ~B-A~~~~~~~- \Delta H~=~\sf{Lewis~basicity~of~B~towards~A} \nonumber \]

    \[B_{reference}~+~A~ ⇌ ~B-A~~~~~~- \Delta H~=~\sf{Lewis~acidity~of~A~towards~B} \nonumber \]

    Two common scales of Lewis base affinity involve the use of SbCl5 and BF3 as the reference base. Specifically,

    • In the Guttman donor-acceptor scale, Lewis bases' donor numbers are equal to the negative enthalpy change for formation of the base's SBCl5 adduct in dilute 1,2-dichloroethane (EDC*):

    \[Base(EDC~solution) ~+~SbCl_5(EDC~solution)~\rightarrow~Base-SbCl_5(EDC~solution)~~~~~~- \Delta H~=~\sf{Guttman~Donor~Number} \nonumber \]

    • The BF3 affinity scale has largely supplanted the Guttman scale owing to the ease with which its results may be correlated with computational data (it is easier to calculate energies for adducts of BF3 than of SbF5). In the BF3 affinity scale, Lewis bases' BF3 affinities are equal to the negative enthalpy change for formation of a solution phase adduct between the base and gaseous BF3. The reaction is:

    \[Base(solution) ~+~BF_3(g)~\rightarrow~Base-BF_3(solution)~~~~~~- \Delta H~=~\sf{BF_3 ~Affinity}~ \nonumber \]

    BF3 affinity values for a wide range of organic bases were compiled by Laurence, Graton, and Gal in reference 2 and salient trends in this data will be discussed in subsequent sections.


    1. Laurence, C.; Graton, J.; Gal, J.-F., An Overview of Lewis Basicity and Affinity Scales. Journal of Chemical Education 2011, 88 (12), 1651-1657.
    2. Laurence, C.; Gal, J.-F. o., Lewis basicity and affinity scales: data and measurement. John Wiley: Chichester, West Sussex, U.K., 2010.


    * EDC stands for ethylene dichloride, the common name of 1,2-dichloroethane.