Group

37O EFFECTS OF STRUCTURE ON REACTIVITY

TABLE 9.4 The o, o+, and o- Yalues for Some Common Groups

CHAPTER 9

For the unsubstituted case,

At

The Hammett equation can be rew

so that

logi

-a(L3N,

and

-AG=

For a given reaction under a given setso that o is linear with AG.

The Hammett equation is not the ocorrelate structural changes in reacti(see p. 452) conelates changes in srrelates acidity to catalysis. The Taft rcorrelates only field effects.ar

Taft, following Ingold,a2 assumedsteric, and resonance effects will be thacid or base (see the discussion of esRate differences would therefore be cRCO.!R/. This is presumably a gooctransition state for acid_catalyzed tydrhence destabilized by _ I andsrabiiiz(

6+9Hz

R-c_oR,

OH

l-while the transition state for base-catacharge than the starting ester. Field e.determined by.measuring the rates of acXCH2COOR',43 where R, i, h"ld.oortcould be determined by the equationaa

opomon

o-NMe2NHzOHOMeCMe3MeHPhcooFCIBrIN:NPh8

COOH,cooRCOMeCF:NHiCN'SOzMeNozNMefNi

- 0.81"- 0.63-0.57

0.38'-0.28'- 0.15-0.14

00.05/0.11"0.150.240.260.28f0.340.440.440.470.530.60'0.700.730.810.8/r.g3k

-0.47"10. r0

-0.090.13"0.10

- 0.09- 0.06

00.050.02"0.340.3'70.370.340.280.350.350.360.460.86b0.620.640.710.88r1.65r

t czb

- 1.7- l . - f

_0.92'l-0.78

0.26- 0.31

0- 0.18-0.41b-0.07

0.110.150.140.170.420.48

0.66

0.790.411.g8t

- 715b

-0.16

0.05- 0.06- 0.1 0"

00"r

- 0. l0 '0.350.400.410.36

0.320.37

0.51"

0.56

0.13"0.36

0.130.680.87

1.00

LZl

3t

"See Ref. 24.

'See Ref. 25.

'See Ref. 26.dSee Ref. 27.

'See Ref. 28./See Ref. 29.sSee Ref. 30.

'See Ref.3l .rsee Ref.32.rsee Ref. 33.tsee Ref.34.rsee Ref. 35.

ring systems such as naphthalene3T and heterocyclic rings,38 and for ethylenic and

acetylenic systems.3eThe Hammett equation \s a linear free energy relationship (LFER). This can be

demonstrated as follows for the case of equilibrium constants (for rate constants a

similar demonstration can be made with AG+ instead of AG). For each reaction,

where X is any group,

AG: -RZlnK or :0.1S1 los/L " \