Infrared Absorption Frequencies of the tert-Butoxy Group HORACE A. ORY Plasfics Division, Monsanto Chemical Co., Texas City, Tex.
b A study of the infrared absorption spectra of a number of compounds which contain the fert-butoxy group indicates that this group is associated with absorptions in the 7 2 0 - to 7 7 0 cm.-', 8 2 0 - to 920-cm.-', 1000- to 1040-cm.-', and 1 155- to 1200-cm.-' regions. In the 1000- to 1040-cm.-' ~ the and 1 155- to 1 2 0 0 - ~ m . -regions, bands of esters have lower frequencies than those of other compound types, while certain compounds are distinguished by high frequencies in the 820- to 920-cm.-' region. Electronic effects which originate in other parts of the molecules are responsible for these displacements.
T
HE application of infrared absorption spectroscopy to analysis and structure determination is largely dependent on recognition of certain absorption frequencies as characteristic of structural groups. V'hile sFectra-structure correlations have been compiled for a large number of structures, the infrared absorption frequencies of the tert-butoxy group have not been studied extensively. Philpoti s and Thain (8) studied the spectra of some tertiary peroxides and reported that the tert-butoxy group shows a strong absorption in the 800- to 920-cm.-' region. Zeiss and Tsutsui (14) determined that the C-0 absorptions of primary, secondary, and tertiary alcohols occurred in the 1050to 1085-cm.-', 1085- t o 1125-cm.-', and 1125- to 1 2 0 5 - ~ m . - ~regions, respectively. Other workers (1, 13) have discussed absorption bands which arise from primary and secondary alkoxy groups. I n this investigation the infrared absorption spectra of 17 tert-butoxy compounds were obtained and used, n i t h a number of other spectra available in the literature, to determine which regions of absorption could be associated with the tert-butoxy group. 'The locations of absorption bands m it hin these regions were examined to determine how the\were influenced by other parts of the molecules: some distinctive frequency displacements were observed. EXPERIMENTAL
Ethyl tert-butyl ether, n-propyl tert-
butyl ether, n-butyl tert-butyl ether, tert-butyl acetate, tert-butyl propionate, and tert-butyl benzoate were prepared according to the methods of Norris and Rigby (7'). The procedure of Bradley, Llehrota, and Wardlow ( 3 ) was used to prepare titanium tetratert-butoxide. The sample of purified p-tert-butoxypropionitrile was prepared by cyanoethylation of tert-butyl alcohol. The sample of purified 0,O-tertbutyl-0-ethyl monoperoxycarbonate was obtained from the reaction of tert-butyl hydroperoxide with ethyl chloroformate. The other compounds were obtained from commercial sources and purified b y distillation or chromatography, except di-tert-butyl diperphthalate, which was studied as a solution in dibutyl phthalate, as received from the Lucidol Division of Wallace and Tiernan, Inc. A Baird Associates, Inc., recording infrared spectrophotometer with sodium chloride optics was used to determine the spectra over the 665- to 5000-cm.-1 range. Except for di-tert-butyl cliperphthalate, spectra were recorded for pure liquid samples; in some cases the vapor spectra were obtained also. The spectrum of di-tert-butyl rliperphthalate was recorded from a solution in dibutyl phthalate; n o significant interference occurred in the regions discussed here. I n most cases a polystyrcne film spectrum was recorded on the same chart t o check frequency calibration. RESULTS
Frequencies. Examination of the infrared spectra obtained for tertbutoxy compounds, and of spectra available in t h e literature, disclosed four regions in which there occurred infrared absorptions common to the tert-butoxy compounds and assignable to the tert-butoxy group (CH, CHz, and CH, absorptions nere ignored). These spectral regions were 720 to 770 S20 to 920 em.-', 1000 to 1040 cm.-l, and 1155 t o 1200 em.-' While other compounds sometimes absorb in some of these regions, i t is felt that, taken together, they are generally characteristic of the tert-butoxy group and useful in structure analysis. The frequencies of these absorptions for each of the compounds included in this study are listed in Table I and include small scale corrections. Spectra obtained from the pure compounds are shown in Figure 1.
X o close correlations betm-een different absorption regions are readily apparent, although in the two higher frequency regions esters have frequencies distinctly lower than those of the other compounds, and in the 820- to 920-cm.-' region certain compounds, in which the tert-butoxy group is bonded to strongly electrophilic atoms, have frequencies distinctly higher than those of the other compounds. Titanium tetra-tert-butoxide is exceptional in that it does not show the same absorptions which are observed from all the other compounds. Titanium tetra-tert-butoxide shows its strongest absorption a t 1004 cm.+ and a very strong band a t 797 cm.-', but shows none in the 720- to i'iO-cm.-l or the 820- to 920-cm.-' regions. Because the tert-butoxy group is bonded to a transition metal atom and electronic considerations would suggest dissimilar behavior, this observation is not considered to invalidate the conclusions based on the spectra of many normally bonded compounds. Intensities. T h e intensities of t h e bands vary considerably from one molecule t o another, and from one region t o another. I n Table I t h e absorption intensities are indicated as very strong, strong, medium, weak, or very weak. The intensities of the 720to 770-cm.-1 absorptions range from medium to very neak. For tert-butyl alcohol and the series of ethers a progression from medium t o very weak intensity is clearly shown as the fraction of the molecule represented by the tert-butoxy group decreases. Similar intensity variations from very strong to weak occur for the 820- to 920-cm.-' absorptions. More intensity variation occurs among the 1000- to 1040-cm.-' absorptions, which are often very weak, IT hile the intensities of the 1155- to 1200em.-' absorptions are almost always very strong, sometimes the strongest in the infrared region. DISCUSSION
Among tert-butoxy compounds, absorption in the 720- to 770-cm.-' region probably arises primarily from the symmetrical skeletal stretching vibration of the tert-butyl group (IO),and is shifted to slightly higher frequencies and enhanced VOL. 32, NO. 4, APRIL 1960
509
2500
5000
1500
11000 ___
WAVE NUMBERS IN CM-' 700 2000 1400 -4000 ~---
900
IO00
BOO
625
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I
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.
I
,-BUTYL
I
dl +-BUTYL PEROXIDE
om
2
3
4
5
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6
7
8
9
Figure 1 .
18
11
12
13
14
15 1 6 2
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6
7
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8
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9
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PROPIONATE
niAwn
m a f WTOXIDE
,lbm
11
12
13
.
14
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lS-i6
Infrared absorption spectra of pure liquid tert-butoxy compounds
The two small bands marked in the tert-butyl peracetate spectrum probably arise from a small amount of benzene impurity
in intensity, as compared with hydrocarbons, through perturbation by the adjacent oxygen atom. Although early studies of some tertbutyl peroxides suggested that the 820t o 920-cm.-l absorption might arise from the peroxide linkage ( I I ) , more 510
0
ANALYTICAL CHEMISTRY
recent investigation has sh0n.n that the absorption originates from a skeletal vibration of the tert-butoxy group (8), as is indicated by the data presented here, The absorptions from most of the compounds occur between 820 and 885 cm.-1 tert-Butyl alcohol, tri-tert-
butyl borate, aluminum tri-tert-butoxide, and di-tert-butoxydiaminosilane differ sharply by absorbing between 895 and 920 em.-', although such compounds as tetra-tert-butoxysilane and tert-butyl hypochlorite absorb a t 841 and 842 em.-', respectively. Electron
attraction by the hi-o amino groups of di-tert-butoxydiaminosilane Fould be expected t o render the silicon atom rather positive. I n the other compounds which absorb at relatively high frequencies the tert-butoxy group is bonded to atoms noted for their electron-accrpting ability. The more electrophilic character of the atom t o which the tert-butoxy group is bonded thus appears responsible for the higher frequencies in this region, Absorption observed in the 1000- t o 1040-cm.-' region is attributed to C-C vibration within the tert-butoxy group. Among the compounds studied, the esters absorb between 1000 and 1018 em.-', while the other compounds studied absorb a t higher frequencies from 1020 to 1040 em.-', except n-propyl Brt-butyl ether, which has a peak a t 1007 cm.-l and a shoulder at 1021 cm.-' Possibly the shoulder represents the appropriate band and the ion-er frequency peak arises from another vibration. A decreased frequency for esters, relative to the other compounds, is also observed in the 1150- t o 1200-cm.-1 region. The vibration primarily responsible for the very strong 1155- to 1200-cm.-' absorption is that of the t e r t - b u t y l C - 0 bond. While the esters absorb b e h e e n 1155 and 1175 em.-', the other compounds absorb betwem 1178 and 1200 em.-' -4 displacement toward lower frequencies observed for esters probably results from diminished electron density on the a l k j l oxygen atom, and consequrnt weakening of the alkyl C-0 bond, resulting from resonance such as: CH,
I
C",-C-0-C-R
Table 1.
Infrared Absorption Frequencies of the fert-Butoxy Group
Compound tert-Butyl alcohol Liquid Gas Methyl tert-butyl ether Liquid Gas Ethyl tert-butyl ether (liquid) n-Propyl tert-butyl ether (liquid) n-Butyl tert-butyl ether (liquid) B-tert-Butoxypropionitrile (liquid) tert-Butyl hydroperoxide Liquid Gas Di-tert-butyl peroxide Liquid Gas tert-Butyl acetate (liquid) tert-Butyl propionate (liquid) tert-Butyl benzoate (liquid) tert-Butyl peracetate (liquid) tert-Butyl perbenzoate (liquid) Di-tmt-butyl diperphthalate (solution) 0,O-tert-Butyl-0-ethyl monoperoxycarbonate (liquid) tert-Butyl hypochlorite (liquid) Titanium tetra-tert-butoxide (liquid) Di-tert-butyl terephthalate (9) tert-Butyl percaprylate (9) 4-tert-Butoxy-2,6-tert-butylphenol(6) 4-tert-Butoxy-2,6-tert-butyl phenoxide ion
Frequency, Cm. -1 1194 (vs) 1024 ( w ) 1210 1012
918 (vs) 753 (m) 918 i51
1200 (vs) 1206 1200 (vs) 1199 (vs) 1198 (s) 1192 (vs)
852 (vs) 727 (m)
1191 (vs) 1024 (vw) 1021 11i4 (vs) 1015 (vs) 1157 fvsJ 1001 ( w ) 1166 (vSj 1014 (vw) 1179 (vs) 1038 (w) 1185 (vs) 1023 (vs) 1188 (vs) 1024 (vs) 1195
(6)
a
i27
745 (n) 881 (m) 733 ( v w ) 871 in) 738 ( v w ) 866 (w) 740 (F-)
754 ( w ) 752 i62 1m) i60 (w) c-/sa ( w )
741 (m)
1020
1183
1024 1022 1025 1025 1028 1029
855 841 822 833 831 832
746 727 750 756 754 755
1183-90
1022-8
833
742
1183-90 1183-90 1183-90 1181
1022-8 1022-8 1022-8 1038
835 827 897 912
746
1188
1022 (T) 1028 (vw) 1004(vs) 1016 1040
882 (vs) 882 843 (s) 853 (m) 849 (vs) 852 (s) 864 (n-) 842 (w)
846 ( w ) 754 (vw) 842 (m) 766 (w) . 797 (V6) i51 811 765. 858 763 840
1183 (vs) 1179 (vs) 1179 (vs) 1161 1198
(6)
Hexa-tert-butoxycyclotrisilazane (6) Di-tert-butoxydiaminosilane ( 6 ) Tri-tert-butyl borate (9) Aluminum tri-tert-butoxide ( 2 )
857 850 (m)
1190 (vs) 1028 ( v w ) 848 (m) 750 (m) 1198 1022 881 750
Tetra-tert-butoxysilane ( 4 ) Di-tert-butoxydiethoxysilane(9) Diphenoxydi-tert-butoxysilane( 9 ) Di-tert-butoxyethoxy-o-toloxysilane(9) Di-tert-butoxydi-o-toloxysilane(9) Di-tert-butoxybis-( 2-chloroethoxy )silane (6) Di-tert-butoxybis-( 2-cyanoethoxy )silane
1020 ( w ) 1013 1025 (vw) 1021 (w) 1038 (w1 1026 ( v 6 )
a
D
0 0
770
901
(1
Not mentioned.
0
II
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+
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1
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CH3
To n lesser degree, this leads to a decrcascd electron density on the tertiary carbon atom, and thutj a relative lowering of C-C frequencies in the 1000- to 1040-cm.-' region for esters. Absorption associated with vibration of the C-0 bond contiguous t o the carbonyl group in esters has been reported (12) a t somewhat higher frequencies than those reported here for the tcrt-butyl C-0 bond in esters. This relationship would be expected from thc resonance effects discussed above, because the force constant of the C-0 bond contiguous to the carbonyl group would be increased relative to that of the alkyl C-0 bond. The spectral regions in which these tert-butoxy absorptions occur overlap
many other regions used to characterize structural groups. While little confidence can be placed in characterizations based on only one band in these regions, the observation of four bands with appropriate intensities in the specified regions is good evidence for the presence of the tert-butoxy group in the molecule under investigation. ACKNOWLEDGMENT
K. 11. Taylor provided the sample of P-tert-butoxypropionitrile; W. 31.Padgett and R. P. Arganbright provided the sample of 0,O-tert-butyl-0-ethyl monoperoxycarbonate. Helpful discussions n i t h G. W. Daues and G. L. Roberts, and the assistance of J. 31. Talbert in recording some of the spectra discussed here, are greatly appreciated. Permission to report these results was kindly granted by the llonsanto Chemical Co.
H., Goldenson, J., ANAL. CHEM.25, 1720 (1953).
(2) Bellamy, L. J., "Infrared Spectra of
Complex Molecules," p. 106, RIethuen & Co., London, 1954. (3) Bradley, D. C., Rlehrota, R. C., Wardlow, IT., J . Chem. Sac. 1952, 4204 ~~. -
(4) Breederveld, H., Waterman, H. I., Rec. trav. chim. 73, 871 (1954). (5) George, P. D.. Sommer, L. H., \Thitmore, E.'C., J . i l m . Chem. $or. 75, 6308 11953'1. (6) ili~iller,~ E . ,Ley, I
