378
HERBERT C. BROWN
Vol. 67
two components exhibits a pressure of 4 mni.; at
properties with the highly hindered acids, triisopropyl- and tri-t-butylboron. The predictions of the steric strain hypothesis on the effect of tion. Cornpetition reactions were run between ani- F-strain on the relative strength of ammonia and Iiionia and trimethylainine with tri-1-butylboron trimethylamine are thus in complete accord with as the reference ncirl. Fmm X. 1 cc. of ammonia, experiment. triinethylamine and the boron acid was obtained Hefore closing, one further point niay be men.3:j.L' cc. of aiiiiiie \yhicli analyzed for ! , s ~ tri~ ~ ~ tioncd. There has been considerable discussion methylamine. I t should be pointed out that in as to whether a monotonic series of generalized these last eiperinieiits involving triisopropylborori acids and bases exists." The present investigaand tri-t-butylboron, the quantitative recovery of tion offers a clear-cut illustration of the dependexcess amine is a slow process- -apparently the ence of base strength upon the reference acid and addition compounds formed have a marked af- supports the viewpoint that such a monotonic series cannot be set up.12 finity for excess amine. Acknow1edginent.-The purchase aiid conDiscussion structioii of the apparatus used in this investiThe data 011 the relative strength of the bases ammonia and trimethylamine are summarized in gation was made possible by a grant from the Table 1. The letters W and S are used to indicate Penrose Fund of the American Philosophical the weaker and stronger of the two amines. The Society This assistance is gratefully acknowlfirst of the two figures in the parentheses following edged. Summary each letter gives the inole fraction of the amine which combined with the reference acid; the I . The relative strength of the bases amsecond gives the saturation pressure of the addi- monia and trimethylamine was determined using tion compound at 2 ; 7 O in mni. I t should be noted trimethyl-, triethyl-, triisopropyl- and tri-t-butylthat the order of base strength established by the boron as the reference acids. competition experiments is fully confirmed by the 2 . The order NH, < N(CH&, observed with saturation pressure data. the hyc!rogen acids and with trimethylboron, is reversed with triethylboron and the more hindered TABLE 1 acids. These observations are in accord with the RELATIVEBASESTRENGTH OF AMMONIA A N D TRIMETHYLpredicted effect of F-strain on the relative strength AMINE uf the two bases. Reference acid Ammonia Trimethylamine 3. In the course of this study iniproved proceTrimethylboron W (0.15, 6 . 4 ) S (0.85, 2 . 1 ) dures for the preparation and purification of triTriethylboron s ( .91,1.4) w ( . 0 9 , 1 2 . 1 ) inethylboron and triethylboron were developed; Triisopropylboron S ( .96, 2 . 3 ) U' ( .04, N. c.") and saturation pressures were measured for the Tri-t-butylboron S ( "98, 1 3 . 3 ) W ( .02, ru'. C.") addition compounds of triethyl-, triisopropyl-, a No compound formed. and tri-t-butylboron with ammonia and trimethylI t is evident that the order of base strength ob- amine. served with the reference acid trimethylboron, (11) Luder, Chcm. Rev., 97, 547 (1940). (1'2) Lewis, J . Franklrn I n s l . , 296, 293 (1838). NH3 < (CH3)3N,is reversed with triethylboron. ?vf oreover, trimethylamine exhibits no basic DETROIT,MICHIGAN RECEIVED SEPTEMBER 1, 1944 O o the pressure correspitls to complete dissocia-
[CONTRIBUTION FROM THE IIEPARTMENT OF CHEMISTKY OF W A Y N E UNIVERSITY]
Studies in Stereochemistry. VI. The Effect of F-Strain on the Relative Base Strengths of Ammonia and the Methylamines BY HERBERTC. BROWN I n the absence of steric complications, the strength of ammonia and the methylamines should increase regularly with the number of methyl groups as a result of the inductive effect (+I) of alkyl groups. However, this predicted order, NH3 < CH:&H2 < (CH&NH < (CH&N (Fig. 1, curve A ) , is not observed. B-Strain' slightly reduces the strength of dimethylamine and markedly reduces the strength of triniethylamine, so that the order of base strength estahi l ) R ~ w v , W t r t h d ~ ~ m ~: :~~n ~d .'r ~ V I C , ~ ,' I ' t r ~ Inrluv.4.L. 66, 4 3 5 :!q$z).
lished by acids with a comparatively low F-strain factor is that represented by curve B, NH3 < (CHJSN < CHaNHz < (CH3)ZNH. I t is the object of the present investigation to examine the effect of F-strain on the order shown by these amines. The order represented by curve B is observed both with hydrogen ion and with trimethylboron as the reference acid.' Replacement of the methyl groups in trimethylboron by larger, bulkier alkyl groups should lead to an increase in F-strain and thus result in a decrease iii the apparent strength of the bases. The higher
llarch. 1945
F-STRAIN EFFECTON BASESTRENGTHS OF
methylated bases, however, should be affected to a greater extent than the lower.* Indeed, it would be anticipated that a change of this kind in the reference acid (as from trimethylboron to tri-tbutylboron) would increase F-strain in the order, NH3 < CH3NHz < (CH3)2NH < (CH3)3N,resulting in an apparent decrease in base strength in the same order. I t follows that a progressive increase in F-strain should alter the observed order of base strength t o those represented by curves C, D and E. Finally, a reference acid with a very high F-strain factor should yield the order, (CH&N < (CH3)ZNH < CH3NH2 < NH3 (curve F). I t is noteuorthy that this last sequence {s the exact opposite of the order which would be predicted if only the polar efect of the methyl group were taken into consideration. The importance of considering the steric factor as well as the polar factor in theorizing is clearly indicuted. In order to test these conclusions as to the effect of increased F-strain on the relative strength of ammonia and the methylamines, the strengths of these amines were compared using tri-t-butylboron as the reference acid with a high F-strain factor. Experimental Part Preparation and purification of compounds used in this investigation and the methods used in their study are either described or referred to in the preceding paper of this series2 Results Monomethylamine-tri-t-butylboron was prepared by combining 0.445g. (2.45 millimoles) of tri-f-butylboron with 55.3 cc. (2.47 millimoles) of monomethylamine. The properties of the addition compound thus formed are quite similar to those of the ammonia derivative previously prepared and described.2 Like the latter i t is liquid a t 0’ and solidifies to a glass a t -80”. All attempts a t crystallization failed. The saturation pressures of the compound are slightly lower than those of ammonia-tri-t-butylboron and exhibit the same dependence upon the proportion of the substance which is in the vapor phase.2 The data3 are listed in Table I.
AMMONIA AND LIETHYLAMINES
379
b F NHa (CHdNH: (CHdzNH (CHdrN
of B- and F-strain on the relative 5trength of ammonia and the methylamines.
Fig. 1.-Effect
amine, was also liquid a t 0’ and solidified t o a glass a t -80’. The saturation pressures3 exhibited by the substance are far higher than those of the corresponding ammonia and methylamine derivatives (Table 11).
TABLEI1 SATURATION PRESSURES OF DIMETHYLAMINE-TRI-~-BUTYLBORON
Temp., “C. Press., min.
-40 1.4
-30 3.9
-20
9.3
-10
0.0
22.8 44.5
Trimethylamine and tri-t-butylboron do not conibine to form an addition compound a t temperatures as low as -80°.2 Competition experiments were carried out to determine the relative strength of the four amines. From a reaction mixture of 42.5 cc. each TABLEI of ammonia, methylamine and tri-t-butylboron (0.345 g.) was obtained 39.8 cc. of uncombined SATURATION PRESSURES OF METHYLAMINE-TRI-~-BUTYLamines. The molecular weight of this amine BORON mixture was 21.3, indicating that it consisted of Temp., “C. 0 . 0 15.0 2 5 . 0 3 0 . 0 3 5 . 0 40.0 69(% ammonia and 31% monomethylamine. Press., mm. 1 . 5 4 . 7 9 . 1 14.5 18.4 2 5 . 0 Siinilarly, from 46.0 cc. each of ammonia, diDimethylamine-tri-t-butylboron, prepared methylamine and the boron acid (0.374 g.) was from 0.874 g. (2.05 millimo!es) of tri-t-butyl- obtained 43.6 cc. of amines, the average molecular boron with 45.9 cc. (2.05 millimoles) of dimethyl- weight of which was 41.5, corresponding to a composition of 9% ammonia and 91% dimethyl(2) Brown, TWSJOURNAL, 67, 374 (1045). amine. The relative strength of ammonia and (3) The pressure mensurernents on this compound and the corresponding dimethylamine derivative were made under conditions trimethylamine with tri-t-butylboron as the closely similar to those used for the ammonia derivative. The relareference acid had been determined previouslytive values of the ohserved pressures at a given temperature may the recovered amine analyzed for 2% ammonia lhrrrlixe he taken a+ a measure of the relative “volatility” of the and 98y0trimethylamine. addition compounds.
Discussion 'i'lie tl,Lta pertaiiiiiix to tlic relative streiixtlix of : i i i i i i i o i i i : i a i i t l t l i e t1irc.e iiietliylaiiiitics as tleteri i i i t i ~ lwitli t t i - / - l ) t i L ~ l l I o r o ! ;is i the refererice aciil re suIiiriiarizet1 i i i I'niilc I I I . 'Ihe iiunierals givy the relative ori1c.r ( ~t hie aiiiiiics, tlie strorigest k i n g iridicatctl 11). i , the ncst by 2 , etc. Tlic first oi tht. two figures iii v.acli parenthesis following the ~iuiiieralsgives the iiiole fraction of tlie
xiiiiiie which conihincxl with the reference acid the conipetitioir c.speriiiierits, the comparison being iriade against aninionia; the second figure gives the saturatiori pressure (if the additiori com[)ourid a t 33" i n n i i i i . I t is of interest that the correlation poi11 tetl o L ; t previously2 between the relative stability arid the magnitude of the saturation pressure values is further subsi.:rntiat et1 by the i:i
-rALjI,E
III
RELATIVE r3ASE STREXGTII iJF AMMONAAND T l i E hIET1fTI.AMISBS I V i T i i '1'RI ~-I{UTSI,BOROh
2 (l.C)O, 1 3 . 3 ' 1. (0.69,,9. li 3 ( .09, >20ot 4 ( .02, s. C.'
Atiunoiiia Monornetliylaniirie IXmethylamirlt Trimethyianiiue 'I
8
S o con~pountifornieii
I t is evident that with tri-t-butylboron as the reference acid, nioriornethylamine is a very slightly stronger base than ammonia, whereas dimethyl- and trimethylamine are considerably weaker. The order observed, (CH&IK < (CH& NH < NHa < C?H3NH2,is therefore that represented by curve 1': of Fig. 1. I t follows that to obtain the order of base strength represented by curve F, it will be necessary to use for the reference acid an even more highly hindered acid than tri-t-butylboron.4 However, it is apparent that the change in the relative strength of the inethylamines observed with tri-t-butylboron is in close agreement with tlie predicted effect of F-strain on the order of the four bases (Fig. 1). I t is of interest to make soinc estimates of the magnitude of these strains iroiii the a\railable data, crude though such estimates must be. At 100' the dissociation constants for aniiiioiii:i - TABLE IV r>ISSOCIATION IIATA OF
TRIMETHYLBORON ADDITION CUM 0 A N D 100"
POUNDS AT
Compound
SHa :B (CH3)j
CHzNHz:R(CHa)r (CH3)zXH:B(CHa)r (CH,),N :D (CH313
KIWO
3.6 0.0350 ,0214 ,472
KO 0
5.2 X 5 . 8 X lo-' 1 . 5 X lo-' 7.8x
AFoo
2840 6530 7250 5120
triinetiiylburon aiid n ~ o ~ ~ o ~ i i e ~ l i y I ~ c i ~ i i r ~ e - - t r i r r l e t h y i boron differ by a factor of 1 ;30; a t 0" the disparity between the two constants is even greater-they differ by a factor of I000 (Table IV). Frorii the results o f tlie competition experjnieiits on these amines, it inny be estiniated that a t 0" the ratio uf the dissociation constants of the corrcy)oiiding atltlition conipouritls of tri-t-butylboron has been greatly reduced to a figure in the neighborhood of 4. This nieans that tlie F-straiii between tri-~-bUty~btlr~iti and the methyl group in 11io1101iie thylainiiie is sutlicieiitly great to reduce the apparent strength of tlie ariiiiie to a point where it is cornparable to that of aiiinionia and has decreased the work required to separate the imine from the boron compound by nearly 3700 calories per mole. It is reasonable to assume that two methyl groups will have a t least twice, and three methyl groups a t least three times this effect on the free energy of dissociation. If tri-t-butylboron were as strong an acid as triniethylboron, the free energy of dissociation of diniethylatnine-tri-fbutylboron could be estimated as - 100 to -200 cal. per mole, and the corresponding triniethylamine derivative as -3000 cal. per mole.5 € 1 0 ~ ever, since both the inductive effect aiid B-strain will reduce the strength of tri-l-butylboron as an acid relative to triinethylboron, the true values of the free energies of dissociation must be considerably lower than the figures given. In this way the low stability of tlirnethylamine-tri-t-butylboron aiid tlie non-existence of the triimthylaiiiitie derivative may be accounted for readily. Acknowledgment.-The purchase and construction of the apparatus used in this investigation was made possible by. a grant from the Penrose Fund of the American Philosophical Society. 'This assistance is gratefully acknowledged.
Summary 1. The relative order of base strength, NH3 < (CI€&iK < CHaSH2 < i,CI€&NH, observed with hydrogen acids and with triinethylboron is altered to (CH&N < (CH3)2N€I< N H y < CH3NH2 with t,ri-t-butylboroii ;is the reference :tcitl. 2. l'his change in the order is in accord with tliv predicted effect o f incre:tsed F-strain o n the relative base strengths of aniiuonia ant1 the. methylamines. 3 . In the course of this work, the additiori coiiipounds monomethylamine-tri-t-butylboroii and diiiiethylamine-tri-t-butylboroii were prepared and characterized. DETROIT,MICHIGAN
RECEIVED NOVEMBER 16, 1944
__I___
[,A) \Vork on this phase of the problem is under way. However, i t may be noted a t this time t h a t preliminary results indicate t h i t t h e order represented by curve F is obtained hy using certain highly hindered triarylborons a s the reference acids.
( 5 ) These estimates a r c based on the assamptions t h a t the dissoci:ation constants a t 0" c a n be calculated from the dissociation d a t a iit higher temperatures and t h e ratios of t h e dissociation constants :ire not ~ r e a t l yaltered hv t h e c l i v n r e lrom t h e easeoil? t o the solid p h : i \ r
