J. Phys. Chem. 1991,95, 10214-10214
10214
and
W = 0.849(0.8475)2/0.296 = at
2.060 g of H20/mmol of HPSS
i = 0.3783 TH+x
Wselected =
(5115)3
= 0.296 (0.6726)2
2.062 g O f H20 * W value returned = mmol of HPSS’ 2.060 QED
COMMENT TABLE I: Bond Dissociation Energy Data and Group Electronegativity‘
Effect of Chemical Envlronment on the Electronegatlvlty of a Group Sir: We support the opinion of Luo and Pacey that “the idea of a fixed group EN is controversial”’ (EN = electronegativity). They have correctly pointed out that the EN of a group in a molecule depends on its chemical environment. This we demonstrate below by evaluating the EN (xG’)of some 20 groups (G) by Pauling’s thermochemical method in CH3G molecules and then by comparing them with those (xG)obtained previously2in the HG molecules. The value of the constant K in eq 1, where D is the bond
D(CH3-G) = [D(CHS-CHS) D(G-G)]’12
+K
(AxG’)~
(1)
dissociation energy of the molecules given within the parentheses and lAxG’l = (XG’ - xCH,lr is found to be 35. The optimization of K follows the same method as described earlier? The xG’ values are given in Table I.f5 Comparison with the EN’S (xG)obtained with reference to H (Table I)2 reveals that, apart from the differences in the numerical values, there are certain differences in the trends observed in the two scales xG and xG’. For examples, unlike earlier,2 in terms of xG’, the relative order of the EN’S of CH,,i-C3H7,and t-C,H, is found to be correct; and the relative orders of the EN’S of COH, COCHS, and COC6H5in the two scales are opposite to each other. The various differences observed in these two scales can be, probably, attributed to the ability of the CH3 to form hyperconjugation with the group G. However, the xG‘ scale correlates less well with Inamoto’s i scale and the ‘J,(ortho-ipso) coupling constants in monosubstituted benzenes ( I ) Luo, Y.;Pacey, P. D. J . Phys. Chem. 1991, 95.6745. (2)Datta, D.;Singh, S. N. J . Phys. Chem. 1990, 94, 2187. (3)Of the 21 groups studied here, for CIHS, SH, and SCH3 AxG’ values
are found to be imaginary. Average error in the determination -of bond dissociation energy is 0+2 kcal/mol.‘ In case of C2HS,an increase of 1 kcal/mol in the D(CH3-C2Hs) value would yield a real value of Axc’. However, for the two thio groups an increase of ~ 3 . 5kcal/mol in the D(CH3-G) values is required for obtainin real values of Axc’. Though this aspect has been commented u n earlier, here we note that similar problems were encountered by Paulingp”also while calculating atomic EN’S for which he held bond dissociation energy data as partially responsible. (4) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33,
F
493. ( 5 ) Pauling, L. The Nafure of the Chemical Bond, 3rd ed.;IBH Publishing Co.:New Delhi, 1967;Chapter 3.
0022-3654/91/2095-10214$02.50/0
groupG I. H 2. CH, 3. CZHS 4. i-C,H, 5. t-C,H, 6.O H 7. OCH, 8.OCzHS 9.O-n-C3H, IO. O-i-C,H, 1 I . O-r-C,H9 12. OZCCH, 13. C H O 14.COCH, 15. COC,HS 16.S H 17. SCH3 18. NH2 19. NO1 20. C F j 21. SiH, 22. Si(CH&
D(CH,-G)
D(G-G)
90.4 85.8 85.7 84.1 92.3 83.3 82.7 82.0 82.8 83.1 83.0 82.5 81.2 81.9 74.0 77.2 84.9 61.0 101.6 88.2 89.4
90.4 82.2 79.0 71.2 51.0 37.6 37.9 37.1 37.1 38.0 30.4 68.4 67.4 66.4 66.0 72.0 65.8 13.6 98.7 74.0 80.5
xo’
xob
2.lb 2.68b c 2.50 2.35 3.51 3.53 3.51 3.51 3.52 3.52 3.61 2.35 2.38 2.32 c
2.1 2.68 2.59 2.54 2.65 3.46 3.40 3.39 3.39 3.41 3.41 3.51 2.44 2.42 2.50 2.65 2.54 3.10 3.38 2.58 1.76
c
3.15 3.54 2.23 2.25 2.34
OThe meanings of the symbols are same as in the text. The bond dissociation energy data are given in kcal/mol and taken from ref 4. From ref 2. e AxG’ comes out to be imaginary. See text.
than the xG scale. Anyway, it is demonstrated that the EN of a group G in an H-G molecule can be quite different from that in the corresponding methyl compound. Department of Inorganic Chemistry Indian Association for the Cultivation of Science Calcutta 700 032, India Department of Chemistry Presidency College Motbung, Manipur, India
S.Nabakishwar Singh
Received: June 24, 1991
0 1991 American Chemical Society
Dipankar Datta*
