[CONTRIEETION FROM THE DEPARTMENT O F CHEATISTRY,THEUNIVERSITY
Alkyl benzenes.
O F TEXAS]
VIII.I Rearrangements Accompanying Friedel-Craf ts Alkylations with Propyl and Butyl Chlorides BY ROYSTON M. ROBERTS AKD DEP SHIENCTHONG RECEIVED J U K E 29, 1959
T h e products from the alkylation of benzene in t h e presence of aluminum chloride with n-propyl, wbutyl, sec-butyl a n d isobutyl chlorides have been determined qualitatively and quantitatively by infrared spectrophotometry and/or vaporphase chromatography. T h e expectation t h a t sec-butyl chloride gives a mixture of sec-butylbenzene and isobutylbenzene was confirmed. No t-butylbenzene was produced, even a t 80". %-Butyl chloride gave a mixture of n-butylbenzene and (predominantly) sec-butylbenzene a t 0"; a t 80 a some isobutylbenzene was also produced. Isobutyl chloride gave only tbutylbenzene at temperatures from -18" t o 80'. A surprisingly small temperature effect on the proportion of n-propylbenzene and isopropylbenzene produced from n-propyl chloride was found in contrast t o earlier d a t a . The mechanism of Friedel-Crafts alkylations is discussed in the light of t h e fact t h a t extensive or complete rearrangement accompanies the reaction of even primary alkyl halides.
The production of both n-propylbenzeiic and Introduction The recent observation2 that scc-butylbenzene is isopropplbenzene by alkylation of benzene with 11,1 2 - 1 7 Some isomerized to isobutylbenzene by heating with n-propyl halides is w.cil docu~iicntcd.~~ itivestigators11.11,16 reportcd n-propylbenzene to be aluminum chloride led us to suspect that a mixture of sec-butylbenzene and isobutylbenzene may be the exclusive or principal product a t low teinperaproduced whenever alkylations are carried out with tures. This paper describes a reinvestigation of the a sec-butyl halide and aluminum chloride at elevated temperatures. The presence of the iso- alkylation of benzene with n-propyl, n-butyl, butylbenzene would have been unexpected in sec-butyl and isobutyl chlorides, using infrared earlier work, and it would have been difficult to spectrophotometry and/or vapor chromatography detect or separate from sec-butylbenzene due to for qualitative and quantitative analysis of the the dmost identical boiling points of the two alkylbenzenes produced. isomers. Experimental With the advent of infrared spectrophotometry, Analytical Procedures.--\ Baird double beam recording mass spectrometry and vapor phase chromatog- infrared spectrophotometer, model 4-55, mas used. Startraphy it has become possible to analyze mixtures ing materials and products were analyzed qualitatively as such as these very accurately and very conveniently. the pure liquids. Quantitative analysis of binary product mixtures was accomplished in 2,2,4-trimethylpentane Although these quantitative methods have becn (spectroscopic grade) solutions as described previously.2 applied in a number of recent investigations of T h e absorption peaks used in the optical density IPS.comFriedel-Crafts the conclusions re- position plots were: sec-h~itylbenzene (13.1 p)-isobutyl(13.5 A&); sec-hutylbenzene (13.1 p)-n-butylbengarding the occurrence and extent of rearrange- benzene (13.4 p ) ; isopropylbenzene (13.1 p)-v-propylbenzcne ments of n-propyl, n-butyl, sec-butyl and isobutyl zene 113.5 p ) . Quantitative analysis of ternary mixtures of ngroups during alkylations are based on earlier butylbenzene. scc-butylhenzene and isobutylbenzene was data, all of which are less quantitative and some of complicated by the overlap of t h e strongest characteristic absorption peaks of the three isomers. However, t h e prowhich are clearly open t o question. of n-butylbenzene could be determined b y vapor Alkylation of benzene by sec-butyl chloride and portion phase chromatography, and standard solutions containing aluminum amalgam a t 0"was found by E ~ t r e i c h e r , ~this proportion of n-butylbenzene and varied proportions in 1900, to produce sa-butylbenzene. Diuguid,6 of t h e other two isomers in 2,2,4-trimethylpentane were prein 1941, reported that the same treatment at 25" pared and compared with a solution of t h e reaction product mixture. gave t-butylbenzene as the principal product. For vapor phase chromatography, a Perkin-Elmer Vapor n-Butyl halides with aluminum chloride (or Fractometer, model l54-B, was used, equipped with a 2-m. amalgam) and benzene have been reported to Celite-packed column impregnated with di-2-ethylhexyl produce a mixture of n- and sec-butylben~ene,~-' sebacate. This column separated completely all of the alkyl chlorides and alkylbenzenes encountered in sec-butylbenzene6J and t-butylben~ene,~ depend- alcohols, this study, with the exception of sec-butylbenzene and isoing on the halide or the temperature. butylbenzene, which appeared as a single peak. (ForIsobutyl chloride has been reported to yield tunately mixtures of these two are easily resolved by infrared spectrophotometry.) only t-butylbenzenes~lO~ll even a t - 1 8 O . l ' Materials .-The alkyl chlorides were prepared from the (1) Preceding papers in this series: VI, R. M. Roberts a n d J. E. Douglass. Chemislvy b I d u s t r y , 926 (1959); V I I , I
