close

Вход

Забыли?

вход по аккаунту

?

Observation of a Linear Relationship Between Thermal Stability and Strain Energy of Alkanes.

код для вставкиСкачать
Observation of a Linear Relationship Between Thermal
Stability and Strain Energy of Alkanesr']
.
\
70 -
191
By Christoph Ruchardt, Hans-Dieter Beckhaus, Goetz Hellmann, Siegried Weiner. and Rohert Winikery]
C--C
single bonds in hydrocarbons are cleaved
homolytically between room temperature"] and about 700°C
(ethane)['], depending on the structure of the groups bound
to the C atoms concerned.
--t .
6o
11'
1131
O
-
t
114t6
E
R'R'
I I
R~-c-c-R~
A 3A 3
-
500
-100
r
7
n
c
z.u
-
B
: \
I
R2'
c
c.'R3
R'
0
- 50-
0
tc
0
0
Q
L
O
1151.
1111.
This exceptionally high increase in thermolysis rate, corresponding to a factor of ca. lo3', has, as yet, been explained
only in an unsatisfactory qualitative manner'']. Attempts at
quantitative interpretation are almost completely lacking[3!
We have now found a linear relationship (r= 0.98) between
free activation energies AG* (300°C)[41of the thermolyses
of a large number of aliphatic hydrocarbon^[^] (see Tab. 1)
and their strain energiesE, estimated by force-field calculations
-u
131,
a1101
141
\I;,
40 -
30 -
- 300
t-
016'
h
171
\.
-*0°
1121
(I)
Fig. 1.Relationship between decomposition temperatures Tl ["C] for t l , 2 = 1 h
[4], AG* of thermolysis at 300°C and strain energy E. [6] of hydrocarbons
series; = non-symmetrimentioned in Table 1. = C,-C, series; o = C,-C,
cal bond cleavage.
This shows that the ground state strain energy of the alkanes
is the crucial factor determining their thermal stability.
Product analyses for compounds (1)-(12) show that bond
cleavage occurs mainly at the central C-€ bond; compounds
AG* = -0.60(+0.03)E,+65.6(+l.0)kcal/rnol
Table 1. Decomposition temperatures Tl (for tllZ = 1 h) [4], free activation enthalpies AG* of thermolysis at 300°C
and strain energies E , [a] of saturated aliphatic hydrocarbons R1R2R3C-CR1R2R3.
R'
R*
Me
Me
Me
Me
Me
Et
Et
Me
Me
t-Bu
t-Bu
t-Bu
2,2,4,4-Tetramethylpentane[c]
2,2,3,4,4-Pentamethylpentane [c]
2,2,3,3,4,4-Hexamethylpentane
[c]
R3
Me
Et
i-Pr
CH2CMe3
C6Hll
Et
C6Hll
t-Bu
H
H
H
H
(ti/2=f h)
AG* ( 3 0 0 ~ )
[kcal/mol]
E, [a]
[kcal/mol]
490
420
329
321
315
285
250
195
565 [b]
329
285
141
60.5
55.3
46.4
46.3
45.8
43.1
39.6
33.7
68 P I
46.7
42.6
29.6
7.8
14.9
26.3
27.8
32.1
42.4
44.3
51.8
2.0
32.6
38.5
62.7
502
415
350
63.9
55.8
48.8
6.4
15.1
24.9
TI ["C]
~
~~
[a] Enthalpy of formation AHr calculated using force fields according to [Sa], minus hypothetical enthalpy of formation
of strain-free molecule [6b].
[b] W Tsang, J. Chem. Phys. 43, 352 (1965).
[c] In order to calculate TI (tl/2=1 h) the experimental rate data kexp were subject to statistical correction, uiz
k , = k..,/2, as two equivalent bonds per molecule can suffer homolysis.
['I
Prof. Dr.Chr. Riichardt, Dr. H.-D. Beckhaus, Dr.G. Hellmann, Dip1.Chem. S. Weiner, Dip1.-Chem. R. Winiker
Cbemiscbes Laboratorium der Universitat
Albertstr. 21, D-7800Freiburg (Germany)
['*I Thermolabile Hydrocarbons, Part 5. This work was supported by the
Deutsche Forschungsgemeinschaft, the Fonds der Chemischen,Industrie and
the Otto Rohm Gedachtnisstiftung.-Part 4: H.-D. Beckhaus, G. Hellmann,
C . Riichardt, Chem. Ber., in press.
Angew. Chem. Int. Ed. Engl. 16 ( 1 9 7 7 ) N o . 12
(13)-(15)
Elost likely lose a tert-butyl radical in the rate
determining step of the thermolysis.
It is remarkable that this relationship is evidently independent of whether homolysis occurs between two quaternary
C atoms [C,-C, series, (1)-(8)
and (Is)], between two
or between a
tertiary C atoms [C,-C, series, (9)-(12)]
quaternary C atom and a tertiary [(14)] or a secondary
875
C atom [(23)]. On the other hand, the deviation of the
thermal stability of ethane from this correlation does indicate
that the type of bond can be significant. The remarkable
difference in stability of diastereomers (10) and (II)[5cI also
results in quite different bond lengthsL81.
According to the above relationship, the C--C bond energy
of a hypothetical strain-free alkane is calculated as D z 76 kcal/
moIrg1,which is of the expected order of magnitude["].
The slope of the straight line in Figure 1 shows that only
60 % of the change in strain energy E, is reflected in activation
enthalpy differences AAG* and AAH*. It can be concluded
that about 40% of strain energy prevails at the transition
state of the thermolysis reaction, obviously largely independent
of structure. Therefore, for radical recombination, i. e. the reverse reaction of thermolysis, an activation energy corresponding to 40% of the ground state strain energy E, is required.
via the polar intermediates ( 3 ) in yields of ca. 40-50%
together with varying amounts of the oligomers ( 5 )
(rn=2, 3,4)[61.w-Hydroxyoctanoic acid (1 a ) did not yield any
monomeric lactone ( 4 a ) but only the oligomers ( 5 a )
(m=2,3,4; cf. Table 1).
Received: October 3, 1977 [Z 850 IE]
German version: Angew. Chem. 89,913 (1977)
__
K . Ziegler, Angew. Chem. 61, 168 (1949).
A. Burcat, G. B. Skinner, R. W Crossley, K . Schefler, Int. J. Chem.
Kinet. 5 , 345 (1973); and references cited therein.
Cf. H.-D. Beckhaus, C. Riichardt, Chem. Ber. 110, 878 (1977).
The temperature at which the halflife period of unimolecular decomposition is t h (see Fig. 1) is the more exact value, as it can he estimated
without extrapolation. A linear correlation (r=0.998) was found for
TI ["C] (tlI2= 1 h) against AG* (300°C).
Results from [3] and a) Diplomarbeit R. Winiker, Universitat Freihurg
1976; h) Diplomarheit S. Weiner, Universitat Freihurg 1977; c) Dissertation G. Heflmann, Universitat Freihurg 1977.
a) E. M . Engler, J . D. Andose, P . u. R. Schfeyer, J . Am. Chem. SOC.
95, 8005 (1973); h) P . D. R. Schleyer, J . E . Williams, K . R. Blanchard,
ibid. 92, 2377 (1970).-The relationship in Figure 1 is all the more
remarkable as the force field was parametrized for low-strained alkanes;
thermochemical measurements afforded good agreement between calculated and experimentally estimated strain energies [5c, 71 for a number
of compounds mentioned in Table 1.
Diplomarbeit G. Kratt, Universitat Freihurg 1976.
H . 4 . Lindner, B. Kitschke, unpublished results. Different P-P bond
lengths in diastereomers of diphosphane complexes were reported recently: G. Huttner, P . Friedrich, H. Willenberg, H.-D. Miiller, Angew. Chem.
89, 268 (1977); Angew. Chem. Int. Ed. Engl. 16, 260 (1977).
Calculated according to D = E.=AG* - TAS* + 2 R T with AS* = 15
cal/mol.K. By assuming an isentropical series the error is insignificant
15~1.
D = 78.2 kcal/mol was estimated for the central bond of 2,3-dimethylbutane: K . W Egger, A . 7: Cocks, Helv. Chim. Acta 56, 1516, 1537 (1973).
A New Lactone Synthesis
By Helmut Vorbriiggen and Konrad Krolikiewicz "1
Dedicated to Professor Robert B. Woodward on the occasion
of his 60th birthday
In the recently published cyclizations of o-hydroxycarboxylic acids to medium or large lactone rings, activation of the
carboxylicr'J or of the hydroxy groupd21 gives rise to polar
intermediates in which the ends of the chains are attracted
by opposite charges and thus react with each otherf3].
Since the esterification of carboxylic acids with amide acegives rise also to such a polar intermediate by activation of the alcoholic hydroxyl group, we have added slowly
and simultaneously toluene solutions of w-hydroxy acids[']
and N,N-dimethylformamide dineopentylacetal (2)r41 to boiling toluene. During this reaction the lactones ( 4 ) were formed
p] Prof. Dr. H. Vorbriiggen, K. Krolikiewicz
Research Laboratories of Schering AG, Berlin/Bergkamen
Postfach 6503 11, D-1000 Berlin 65 (Germany)
876
Table 1. Lactones ( 4 ) and ( 5 ) from o-hydroxycarhoxylic acids ( I ) .
~~
(1)
(a)
(b)
(c)
n
6
10
14
R
H
n-C6HI3
H
Reaction
conditions
(4)
16h/llO"C
19 h/llO"C [a] 48.3
15 h/llO"C
40.0
~~~~
Yield [%]
(5)
m=2 m =3
14.3
11.8
16.6
10.8
4.6
3.9
m= 4
9.2
1.9
1.4
[a] A concentrated solution of (26) and (3) in methylene chloride gave
on standing for 1 week at 24°C some unreacted ( I b ) and a small amount
of ( 4 b ) as well as a large amount of ( 5 6 ) (m=2).
As to be expected from intermediates of type (3), this
lactonization occurs via a Walden inversion of a chiral center
at the hydroxyl-carrying carbon atom. Optically active 12-hydroxystearic acid (1 b ) [n = 10, R =n-C6HI3] thus afforded the
Iactone ( 4 b ) [CD (dioxane);t =218 nrn, BE= + 0.589] in 48 %
yield. Saponification of ( 4 b ) with methanolic KOH and a
second lactonization with (2) gave the lactone ( 4 b ) [CD
(dioxane) h=218nm, AE= -0.6811 with opposite configuration at C-12.
On using boiling 1,2-dichloroethane instead of toluene as
solvent, the reaction of (1 a ) with ( 2 ) afforded the 2-chloroethyl ester ( 6 ) [m/e=161, 135, 122, 971 as major product.
The compound (6) is probably formed via attack of the
carboxylate anion on 1,2-dichloroethane with formation of
the P-chloroethyl ester and subsequent attack of the newly
formed C1- on C-8 which is activated by reaction with (2)['].
Procedure:
2.4 g (8mmol) 12-hydroxystearic acid and 2.08 g (9 mmol)
N,N-dimethylformamide dineopentylacetal were each dissolved in IOOml anhydrous toluene and, using two special
addition funnels (Normag No. 8056), added with stirring during 19 h to 100ml boiling toluene. After two further hours'
stirring at 130°C the mixture was evaporated and the residue
(3.0g) chromatographed on 150g AlzO, (neutral, Woelm A
111). After discarding the first 400ml eluate, the next 1.51
hexane afforded 3.092g (48.3 %) ( 4 b ) as colorless oil.
Received: October 17, 1977 [Z 863 IE]
German version: Angew. Chem. 89, 914 (1977)
Anqew. Chem. lnt. Ed. Engl. 16 (1977) No. 12
Документ
Категория
Без категории
Просмотров
2
Размер файла
203 Кб
Теги
thermal, strait, observations, energy, alkane, relationships, stability, linear
1/--страниц
Пожаловаться на содержимое документа