Van der Waals Compression of Hydrogen Atoms by Geminal Alkyl Groups Mohammad lsmail aureshi Garyounis University. P.O. Box 9480, Benghazi, Libya

'H NMR spectroscopy is perhaps the most powerful tool available to organic chemists for the structure determination of organic compounds. The relationship between the structure of an organic compound and the chemical shifts of its hydro- gen atoms is therefore a matter of considerable general in- terest.

The chemical shifts of hvdrwen atoms in organic molecules are generally believed w t<det&mined by the combined effect of three factom: inductive effert of the neighboring atoms and groups, anisotropy of the neiphhoring atlms and honds, and intramolecular van der Wads compres5ion of hydrogen atoms by the neighhoring atoms and g&ps. The present paper suggests that the last named factor, intramolecular van der Waals compression, has a wider role in determining the chemical shifts of protons than hitherto believed; it proposes that hydrogen atoms in organic molecules suffer intramo- lecular van der Waals compression-and hence deshield- ing-from geminal' alkyl groups.

The evidence in favor of the proposed idea is based largely on the arguments involving prediction of intramolecular van der Waals compression by means of scale models that have been used earlier by Schaefer et al. (I) to support their sug- gestion that a - and 8-hydrogens in alkyl halides suffer in- tramolecular van der Waals compression-and hence de- shielding-from the halogen atom. Further support is derived from the fact that the proposed deshielding due to intramo- lecular van der Waals compression of hydrogen atoms by geminal alkyl groups seems to explain the generally known-but not yet fully understood-order of chemical shifts observed among the similarly situated methyl, meth- ylene, and methine protons, and that it also explains the general trend of chemical shifts of olefinic protons observed in alkenes.

Deshieldings Due to Intramolecular van der Waals Compression

Intramolecular van der Wads compression of hydrogen atoms is known to cause a downfield shift in the 'H NMR signal of the compressed hydrogen atom. This downfield shift is attributed (2) to the asymmetrical distortion of the electron cloud surrounding the hydrogen nucleus. Deshieldings caused by intramolecular van der Waals compression have been re- ported numerous times (1,3,4) to explain unexpectedly downfield chemical shifts of protons in organic molecules. Instances of such deshieldings have been observed mostly in sterically rigid compounds in which one or more hydrogen atoms are forced too close to other atoms because of the rigid geometry of the molecule. In such compounds deshieldings as large as 4 ppm have been observed (4). An example is pro- vided by the report (4) that 'H NMR signal of the circled hydrogen atom in I appears a t 8 3.55, which is about 2.4 ppm downfield from its expected position. This large downfield shift has been attributed to the steric compression that the circled hydrogen atom in (I) suffers from the neighboring -OH group.

1 The term nerninalnlkvl arouo. as used in this DaDer. refers to an ~ ~ ? , ~ ~

alkyl goup that is present on the same carbon atom that carries the hydrogen atom whose chemical shin is being considered.

Reports of downfield shift caused by intramolecular van der Waals compression in nonrrgid molecules arr co~nparativrlg few. However, a numher of Inltances of such deshieldlngs have been reported (1.3). The most interesting example, perhaps, is the suggestion of Schaefer et al. (1) that a- and 8-protons in alkyl chlorides, alkyl bromides, and alkyl iodides suffer desbielding because of intramolecular van der Waals com- pression by the halogen atom.

Schaefer et al. have supported their suggestion with the help of a scale model for ethyl halides in which circles are drawn using van der Waals radii to represent atoms. The extent of overlap between any two circles was taken to indicate roughly the extent of van der Wads compression between the re- spective atoms. Schaefer et al. concluded from their scale model that electrical interactions between the halogen atom and the a-hydrogen (or the &hydrogen) would he large enough to affect bond symmetries and hence cause a low-field shift.

Intramolecular van der Waals Compression of Hydrogen Atoms by Geminal Alkyl Groups

The suggestion of Schaefer et al. prompted the present author to look into the possibility of the existence of similar van der Waals compre&ion of hydrogen atoms in nonrigid organic molecules by atoms or groups other than halogens. Scale models-similar to that used by Schaefer et al.-of several simple organic compounds were inspected during the course of this study. hey appear to indicate that van der Waals compression should exist between a hydrogen atom, paraffinic or olefinic, and ageminal alkyl group.

Let us first consider the case of a paraffinic hydrogen atom in an organic molecule by examining the scale model shown in Figure 1. It is evident from this scale model that the van der ~ a a k comoression that a paraffinic hvdrogen atom in any . - organic compound suffers from a geminal methyl group is lareer than the van der Wads com~ression that an a-hydrogen

~ -

is alkyl halide suffers from even the largest halogen-io- dine. I t can he reasonably argued, therefore, that, if an a-hy- drogen in an alkyl halide can he deshielded due to van der Waals comoression hv the halogen atom, then similar van der . Waals compression of hydrogen atoms in any organic com- pound. bv eeminal alkvl groups should lead to an even larger . . . beshield& effect.

The situation can also be viewed from another angle: the scale model in Figure 1 shows that the van der Wads com- oression that a hydrogen atom experiences with a geminal methyl group is much larger than the van der Waals com- pression that i t experiences with ageminal hydrogen atom.

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Thus. for anv hvdroeen atom in an oreanic molecule, substi- . . - tution of a geminal hydrogen atom with a methyl group (or another alkyl group) has the effect of increasing van der Wads compression, and therefore causes deshielding.

The scale model in Fieure 2 indicates that the situation of olefinic hydrogen atomHis not much different. Here, again, it is anoarent. that the van der Waals com~ression that an

& .

olefinic hydrogen atom suffers from ageminal methyl group is much laraer than the van der Waals compression that i t suffers fromageminal hydrogen atom.

The scale models in Figures 1 and 2 were drawn by using the generally accepted values for bond lengths (5) (sp3C-sp3C, 1.54; sp3C-H, 1.11; sp3C-C1, 1.78; sp3C-Br, 1.94; sp3C-I, 2.14; sp3C_spZC, 1.50; sp2C=sp2C, 1.34; andspZC-H, 1.10 A) and van der Waals radii (6) (H, 1.2; Cl, 1.8; Br, 2.0; I, 2.2; and methyl group, 2.0 A). The bond angles were taken as 109.5O for an sp3 carbon and 120° for an sp2 carhon.

Chemical Shms of Similarly Situated Methyl, Methylene, and Methine Protons

It is a general observation in the 'H NMR spectra of organic compounds that in a given set of similarly situated methyl, methylene, and methine protons, the chemical shifts appear progressively downfield as one proceeds from methyl to methylene to methine group. An example is provided by the chemical shifts (7) of the circled hydrogen atoms in the series of compounds III-V.

A comparison of the observed (7) and calculated (in paren- theses) chemical shifts of olefinic protons in the following alkenes shows that this rule works quite well.

I t is obvious from this example that a proton suffers de- shielding each time a geminal hydrogen atom is replaced by an alkyl group; in other words, ageminal methyl group has a deshielding effect on paraffinic protons,i.e., on those pro- tons which are present on an sp3 carbon atom.

The deshielding effect shown by geminal alkyl groups is contrarv to what one would exDect on the basis of meater - electron-releasing inductive effect of alkyl groups compared to that of hvdroeen. This amarent anomalv has been resolved. , .. . . however, l)y tht. suggestion (8 ) that the dominant effect gov- erning rhe chemical s h i h in rhis narticular instance is not the indu&ive effect of the alkyl groip but the anisotropy of the

Figure 1. The shaded circle represents a hydrogen atom present on an sp3 carbon. The unshaded half-circles represent the geminal hydrogen, chlorine, bromine and iodine atoms, and the gemlnal methyl group. (Scale: I A = 3

carbon-carbon bond which operates in the opposite direction. The present paper suggests that deshielding due to van der Waals compression of hydrogen atoms by geminal alkyl groups, may also be contributing to the net deshielding effect of geminal alkyl groups.

Deshielding Effect of Geminal Alkyl Groups on Olefinic Protons The effect of alkyl substituents on the chemical shifts of ole- finic protons is so consistent that empirical rules (9,lO) have been formulated that enable one to make fairly accurate predictions of the chemical shifts of olefinic protons in a given alkene. One of these rules (9) is reproduced below (in part):

aH = 5.25 + Z,,, + Z,, + Zt,,

where, for hydrogen,

and for alkyl groups,

Figwe 2. The shaded circle represents a hydrogen atom present on an sp2 carbon. The unshaded half-circles reprerent the geminal hydrogen atom and the geminal melhyl group. (Scale: 1 A = Bun).

862 Journal of Chemical Education

Literature Cited

11) Sehaofer,T.,Reynold~, W.P.,andYonunofo,T.,Cen. J. Chem. 41,2969(1963). (21 Wdlims.0. H.,and Fleming. 1,"Sprctrostopie Methodsin Organic Chamisfry." 2nd

ed.,McGraw-Hill.UK. 1973.p. 82. (3) Gil, V. M. S., and Gibbons, W. A , Mol. Phyr., 8,199 (19641. 1 0 Winatein. S.. Carter. P., Anot. F. A. L., and Bourn, A. J. R.. J A m m Cham. Soc.. 67,

5247 (19651. (5) Mereh,J., "Advanced Organic Chemistr-Reactions, Mmhaniams, and Structure:

It is apparent from the above rule that, whereas dkyl groups 2nd ed., Mdirdiw-Hill, Near York, 1977. P. 24. (6 ) Godd, E. S.,"Meehaniam and Structure in Organic Chemiatry:Holt, Rinehart, and

at cis and trans positions shield the olefinic protons, pre- winsfon, N ~ W york, 1 % ~ . P. 51.

sumablv due to their electron-releasine inductive effect. the 17) BOVW . ~ F. A , "N.M.R. ~ a t a ~ a b ~ e s hr organic Com~unds," ~ d ~ b f e f e ~ f e f e f e , ~ e w

effect 02 geminal alkyl groups is one oFdeshielding. he ob- Yark, 1967. 181 Silverstein. R. M.. Bmler. G. C.. and M o d . T. C.."Soedrometrieldentiflealioo of

served deshielding effect of guminal alkyl groups is in complete orzsntr c~mpnundr. ' 4th ?d ..I. hn \v~lcy. NW ~ , r k , 19LI.p 181

accord with the supgesl i~n made in this paper that paraffinic 191 U.ll8.m~. D H. and Hrnun&. I.. ' amtr tmlpa Methndp 1nOr*m1c(:hcm~atni."2nd c d . Llctirsu-H8.I. Fngland. 1971.1~. I {-,and Malt-r I' E .l'arrual.C .Prnsol.E

ns well as olelinic prur<,ni s u f f e r i n t r a m o l e c t ~ l a r v a n der \\'ads 1,. \ . .A . i m m I! a l .d . i t rmhc~l .~ . rc!ronron " . 25, h'li IW

comprpssi(,,,-and hence dPshielding -fr,,,,, geminal alkyl $101 *hmrn~nn . F ' 6 0 ~ r , .\u I " ~ C . ~ , C ! O L ~ L ~ S I ~ . ~ O ' ~ P # C M~rlvu ls for th~ I d m . ficatlnn d O r 6 1 r i ~ ~ o r n p ~ und.' \'I I I. 12er:rmm Prr.7 Oxford. 1970.19 M41.

groups. s l . l ~ P ~ ~ N ~ I . ~ . M ~ ~ ~ ~ . . I . . ~ ~ ~ ilrncn. !v .H.. I c h . ~ .A.-,V 19. 163 1166.

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