Taft equation Various equations are associated with
R.W. Taft, but the term is most often used to designate
the family of equations that emerged from Taft’s analy-
sis of the reactivities of aliphatic esters and that
involved the polar substituent constant σ* and the
steric substituent constant Es

lg k = lg ko+ ρ*σ* + δEs

or the one-parameter forms applicable when the role of
either the polar term or the steric term can be
neglected. Nowadays σ* is usually replaced by the
related constant σI.
See also HAMMETT EQUATION; RHO (ρ) VALUE;
SIGMA(σ) CONSTANT.

tautomeric effect SeeELECTROMERIC EFFECT.

tautomerism Isomerism of the general form

G–X–Y=Z X=Y–Z–G

where the isomers (called tautomers) are readily inter-
convertible; the atoms connecting the groups X,Y,Z are
typically any of C, H, O, or S, and G is a group that
becomes an ELECTROFUGEor NUCLEOFUGEduring iso-
merization. The most common case, when the elec-
trofuge is H+, is also known as “prototropy.”
Examples, written so as to illustrate the general pat-
tern given above, include: Keto-enol tautomerism, such as

H–O–C(CH 3 )CH–CO 2 Et (enol)

(CH 3 )C(O)–CH 2 –CO 2 Et (keto)

(G = H, X = O, Y = CCH 3 , Z = CHCO 2 Et)

ArCH 2 –NCHAr′ ArCHN–CH 2 Ar′

(G = H, X = CHAr, Y = N, Z = CHAr′)

The grouping Y may itself be a three-atom (or five-

atom) chain extending the conjugation, as in

The double bond between Y and Z may be

replaced by a ring, when the phenomenon is called

ring-chain tautomerism, as in







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