GUEST AUTHORS Kenneth 1. Marsi Long Beach State College Long Beach, California Samuel H. Wilen The Citv Colleae New York, New york
Textbook Errors, 44
I
Friedel-Crafts Alkylation
Friedel-crafts alkylation is one of the reactions examined in some detail in many undergraduate organic chemistry textbooks. Most of these textbooks show that such alkylations involve carbonium ions as intermediates and that the ions often rearrange in the alkylation processes. Some of the textbooks1 state that n-alkyl derivatives cannot be synthesized by the Friedel-Crafts reaction, or that they are formed in only minor amounts. That this is not always the case is evident from examination of the table. Thus, it has now been firmly established that in some alkylations with n-alkyl halides, n-alkylhenzenes are formed in significant amounts along with sec-alkylbenzenes. The mechanistic treatment of the Friedel-Crafts reaction on the undergraduate level is unfortunately greatly oversimplified. This has become increasingly clear within the last ten years in which newer instrumental analytical methods (infrared and Raman spectroscopy, mass spectrometry, and vapor phase chromatography) have been applied to the re-examination of older available data and in current investigations.
Since the intervention of primary carhonium ions in Friedel-Crafts reactions would be expected to yield rearranged products exclusively, the formation of appreciable quantities of normal products must he otherwise rationalized. Thus, it is now thought that alkylations, in particular those involving primary alkyl halides, are nucleophilic displacements of the type suggested (1):
acHsHZcH +
MXa
+
HBr
This view is supported by kinetic and other data (6). Formation of mixtures (table) may he due to simultaneous incidence of a more classical pathway (2), (3):
Suggestions of material suitable for this column and guest columns suitable for publication directly are eagerly solicited. They should be sent with as many details as possible, and particularly with reference to modern textbooks, to Karol J. Mysels, Department of Chemistry, University of Southern California, Los Angeles 7, California. Since the purpose of this column is to prevent the s p r e d and continuation of errors and not the evaluation of individual texts, the source of errors discussed will not be cited. The error must occur in at least two independent recent standard books to be presented.
Variants of (2) and (1) include isomerization without ionization:
Examoles of Friedel-Crafts Alkvlation Alkylating agent .
Substrate
Catalyst
Temperature, OC
Analytical methoe
1. n-PrCl
c&
AICL
-18 to 80
I R ; VPC
2. n-BuC1
C&
AICls
0
IR; VPC
3. n-BuC1
CsH,
AICls
80
I R ; VPC
Product 30-35yo n-Prg 65-70% iso-Prd 32-36% *Bug 64-68% see-Bug 19-24% n-BuQ ..-. ," ...
4. see-BuC1 5. see-BuC1
CsHe C&
AICla AICla
0 and 30 80
IR; VPC I R ; VPC
6. iso-BuC1 7. n-BuC1
CsHs m-xvlene
AICls AlCI,
-18 to 80 35
IR; VPC D: DER: I R
8. see-BuC1
m-xylene CaH,
AICls GaBra
35 35
DER; I R IR
10. n-PrCI
CaHs
AIClr
35
D: DER
11. n-PrOH 12. n-BuOH 13. neoPentOH
CsH. CsH, CnHe
BFI BFI BFa
60 60 60
MS; I R MS; I R MS: I R
9. n-PrBr
Reference (1) (1) (1)
--7
14-27% iso-Bug see-Bug 35y0 sseoBug 65y0 iso-Bud terdBug 63Yn n-BuAr" 37% 'ose-BuArb see-BuArb 28% n-Prg 72% iso-Prg 40% n-Prg 60% iso-Prd iso-Prm sec-Bug 9 6 4 7 % tert-amvlm
(1) (1) (1) (.8.)
(8 (3)
(. .4. ) (6) (5)
(6)
IR: Infrared spectroscopy; VPC: Vapor phase chromatography; D: Distillation; DER: Formation of solid derivatives; MS: Mass spectrometry. Vsomerization accompanied these alkylations yielding 1,3,5-isomers rather than the 1,2,4-isomers.
214 / Journol of Chemical Education
and synchronous alkylation and isomerization. These mechanisms are not mutually exclusive. Friedel-Crafts alkylation with tertiary alkyl halides very likely involves the formation of a carbonium ion as an intermediate, while alkylation with a primary alkyl halide might take place either via (1) or (2 or 4) and (3). That portion of alkyl halide reacting by (2 or 4) and (3) would yield rearranged products while the remainder would yield normal products. For some alkyl halides, e.g., see-hutyl chloride (table, Nos. 4 and 5), formation of rearranged products may reflect an isomerization which follows alkylation (1). Isomerization of alkyl halides by aluminum chloride or other Lewis acids is known to take place inthe absence of arenes (7). It has been suggested that the presence of rearranged products in Friedel-Crafts alkylation with primary alkyl halides reflects c a m e n t or prim isomerizatibn of the alkyl halide (1, 7). Data on relative rates of substitution and isomerization are not yet on hand t o confirm this view, although some predictions based upon the available data suggest that substitution with little rearrangement (isomerization) would be obtained by using as substrates more nucleophilic arenes (5)%or arenes in which an alkyl cation has ready access to the aromatic component (8). Nightingale and Shackelford's work (table, nos. 7 and 8) partially confirms this (9). Older data which contradict some of the results listed in the table are still quoted in the literature with consequent confusion. It bears repeated emphasis: composition of mixtures determined on the basis only of boiling points, indexes of refraction and/or densities often are not valid (10). Even isolation of solid derivatives does not lead to unambiguous results; many cases are known in which a liquid mixture gives rise to but one solid derivative (I I ) . An isomer or other component present in low concentration might be over-looked. It is well worthwhile pointing out t o students that use of an instrumental method of analysis is of itself no guarantee of a reliable analysis. Individual techniques have substantial limitations such that most often a combination of techniques must be relied upon. Additionally, most instrumental methods of analysis still require the availability of comparison standards which depend ultimately upon synthesis. Other facts have come to light which explain some of the contradictory earlier data. For example, Roberts and Shiengthong have shown that part of the secbutylbenzene formed in the alkylation of benzene with n-butyl chloride is due to the presence of see-butyl
chloride as a contaminant (as much as 17%) in the former compound (1). This is now readily understood as being due t o intervention of the SN1mechanism in the conversion of n-hutyl alcohol to chloride. The data on alkylation indicate that in addition to the nature of the alkylating agent, the basicity of the hydrocarbon, the nature of the solvent, the nature of the catalyst (including the presence of traces of water), and to some extent the temperature appear to influence the proportion of normal to rearranged product formed (see the table). It is to be noted that disproportionations, transalkylations, and alkyl group migrations of primary alkylbenzenes-reactions which have features in common with alkylations-occur with little rearrangement, implying that carbonium ions do not intervene in all such reactions (8, 12, IS). I n a t least one case, it has been shown, using a 14C tracer, that a superficially simple retention of skeletal homogeneity covers up a most interesting scrambling of the carbon atoms in an alkyl halide undergoing substitution (14), an "invisible" rearrangement as it were. The displacement mechanism has been invoked to rationalize some of these disproportionations and transalkylations. Recently, however, Streitwieser and co-workers have shown that another interpretation is necessary to explain some of these reactions (15, 16). I n conclusion, the formation of rearranged alkylbenzenes in Friedel-Crafts alkylation, in particular with n-alkyl halides, is not as universal or as extensive as many organic textbooks imply. Although formation of rearranged products certainly is videspread, conditions and specific cases may favor the formation of the normal isomer in significant or preponderant amounts even if, thus far, no cases have come to light which are of synthetic utility. Literalure Cited
(6)
Thus, yields .of wpropyl derivatives of toluene, p-xylene, m-xylene, mesitylene, and pentamethylbenzene have been predicted as 49%, 54%, 75% 91% and 96%, respectively. NOTE ADDEDIN PROOF: Since the submission of this paper there has appeared one very significant and careful study which confirms the fitot that appreciable percentages of n.-alkylaromstics are formed in aluminum bromide-catalyzed alkylations (up to 50% for benzene andup to 887, for polymethylbeneenen) (SHARMAN, S. H. J., A m . Chem. Sac., 84, 2945 (1962)).
. .
\
(7) (8) . . (9) (10) (11)
2
ROBERTS, R. M., AND SHIENGTHONG, D., J. A m . Chenz. Soe., 82, 732 (1960). J. %I., J . A m . NIGHTINGALE, D . V., AND SHACKELFORD, Chem. Soc., 76, 5767 (1954). SMOOT,C. R., AND BROWN,H. C., J . 9 m . Chenz. Sac., 78, 6249 (1956). V. N., PINES,H., A N D SCHMERLING, L., J. OTB. IPAT~EFF, C h m . , 5, 253 (1940). A,, JR.,STEVENSON, D. P., AND SCHAEFFER, STREITWIESER, W. D . . J . A m . Chem. Soc.. 81. 1110 (1959). JUNGK,H., SMOOT, C. R., AND BROW-N, H. C.. J . A m . C h m . Sor 218.5 " ,. .., 78. .. ,. . .119.56i ....,. BROWN,H. C., AND WALLACE, W. J., J. A m . C h a . Soc., M., 7 5 , 6279 (1953); BROWN,H. C. .AND GRAYSON, 3 . A m . C h m . Sac., 7 5 , 6285 (1953). BADDELEY, . G...~Quart. Reviews, 8, 35.5 (19X). NIGHTINGALE, D. V., A N D SHACKELFORD, J. M . , J. Am. Chem. Soe., 7 8 , 1225 (1956). GILMAN,H., A N D MEALS,R. K.,J. Org. Chem., 8, 126 (19431. . . SCHLATTER, M. J., AND CLARK,R. D., J . Ant. Chem. Soc., 75, 3411 (1953). D. A.. AND LIEN,A. P.. J . Am. C h m . S m . . 7 5 . MCCAULAY.
(121
,
,
(13) KINNEY,R. E., AND HAMILTON, L. A,, J . Am. Chem. Soc., 76, 786 (1954). R. M., AND BIDLNDENBERGER, S.G., J.dm Chem. (14) ROBERTS, Soe., 7 9 , 5484 (1957). A,, JR., AND REIF, L., J. A m . Chem. Sm., (15) STREITWIESEH, 82. 5003 (1960). . . (16) STREIWIESER, A,, JR., A N D DOWNS,W. J., J. 0 1 g . Chem., 27, 625 (1062). Volume 40, Number
4,April 1963
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