7552
Communications to the Editor Relative Stabilities of Carbonium Ions in the Gas Phase and Solution. A Comparison of Cyclic and Acyclic Alkylcarbonium Ions, Acyl Cations, and Cyclic Halonium Ions
determined. Carbonium ion stabilities can thus be assessed by comparing their R-X heterolytic bond dissociation energies as defined by eq 2 (AHi = bromide affinity when X= Br-). We wish to report preliminary data for the relative stabilities, using the reference base Br-, of a series of ions R f , including cyclic and acyclic alkylcarbonium ions, acyl cations, and cyclic bromonium ions. These data are of particular interest since heats of ionization in fluorosulfuric acid are available for several of the bromides.l1-I3 C o n bined with these data our results yield interesting insights into the effect of solvation on relative carbonium ion stabilities. Experiments were performed as previously described for the reference base F-.l0 Data for two systems of interest are shown in Figure 1. In a mixture of 1,2-dibromoethane and 1,4-dibromobutane, reaction 3 is observed to proceed entirely to the right (Figure la). In reaction 4, 1-adamantyl
Sir:
A thorough and fundamental understanding of chemical transformations in solution demands an assessment of solvent effects on ion stabilities. Ionic heats of solvation can be determined by combining gas phase and solution data in appropriate thermochemical cycles. Such studies have been carried out for protonation of amines in aqueous solut i o ~ ~ ,protonation ’-~ of amines and phosphines in fluorosulfuric acid (HS03F),4-6and deprotonation of alcohols in dimethyl ~ u l f o x i d e . ~ ~ ~ Recently we described ion cyclotron resonance techniques8-I0 which permit determination of carbonium ion stabilities by examining the preferred direction and equilibrium in reaction I , where the relative binding energy of the reference base X- to two carbonium ions (Lewis acids) is
+ %+ * [RIX&+] m R,+ + R2X RX R+ + X-; AH,=D(R+-X-)
R,X
-
6
+ Br(CH?),Br
(1)
-
Br(CHJ,Br
+
6
(3)
bromide is observed to transfer Br- to CH3CH2CO+ (Figure 1 b). For a series of carbonium ions R+, similar experi-
(‘4
Table I. Estimated Relative Enthalpies of Solution of R + in H S O s P R+ CH,COt
AHib
AHi‘
(9)
(S)
153.3 f 1 169.9 =t7 141.313 159.21 1 148.7 k 1 146.8 12
6-
~RAH,~ (R+)
-190.1 13
- 20
.o
-178.6 18
-8
-28.0-1 5
.-177.6 f 6
-7
-172.2 11
-2
+1
-3.3 f 0 . 5 -14.5 12 -8.4 10 . 9
.14.3 10 . 2
-162.5
>8
-160.3 i 3
>-159.8
1Oh
>11
All data in kcal/mol at 298 K. Gas phase bromide afinities D (R+-Br-) were determined by combining experimental results for bromide transfer reactions with calculated values for reference compounds; corrections were made for entropy terms arising from symmetry numbers, ring closure, and mixing (see text). Heats of ionization for RBr in HSOsF are from ref 11-13, 22, 23. Heats of solvation for R+ Br- in HSOaF were calculated via a thermochemical cycle similar to that described in ref 1. 6 Heats of vaporization used in the cycle are from J. D. Cox and G. Pilcher, “Thermochemistry of Organic and Organometallic Compounds,” Academic Press, New York, N.Y., 1970, or were estimated from boiling point data. f The heat of sublimation of adamantyl bromide (a solid at room temperature) used in the cycle was estimated using data from P. v. R. Schleyer, J. E. Williams, and K. R. Blanchard, J. Amer. Chem. Soc., 92, 2377 (1970), as described in A. Bondi, J . Chem. Eng. Data, 8, 371 (1963). g Heats of solvation of R+ relative to that of (CH&C+. See footnote 23.
+
Journal ofthe American Chemical Society / 96:24 / November 27, I974
7553 Br
I
(a)
+
+ CH3CH2CO+
4
CH3CH,COBr
+ t
ments indicate the following order for relative free energies of RBr heterolytic bond cleavage:14
a Br+
CHT (217.7) > C2H2 (181.9) > (CH3)2CH+(162.9)>
6
Br'
2
> I
(16i.7) 2
2
0
100 Time ( m s e c )
Ti,
Br+
c-C,H,+(l58.6) > CH,C0+(153.3)>
_v
C6HjCH2+(143.8)2 (CHd3Cf(148.7)> CH3CH&0+(141.3)> +
Br+
CH3CH2C0+(141.3)>
Br'
0,
