Page 476 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

operating conditions, the catalysts incorporate sulfur, which modifies the catalytic 457
properties and tends to enhance rearrangement and aromatization. 166
There is rapid interconversion of alkenes and carbocations over acidic zeolite TOPIC 4.1
catalysts, and the carbocations permit skeletal rearrangements and hydride transfer The Role Carbocations
and Carbonium Ions in
reactions. These reactions proceed in the direction of formation of more stable Petroleum Processing
isomers. 161 The rearrangements probably proceed through protonated cyclopropanes
(see Section 4.4.4).
H
H H
C CH 3
+
CH CH CH CH C HCH 3 CH CH CH C CH 2 CH CH CH C +
3
2
2
2
2
2
3
3
2
2
CH
H 3
An important advance in understanding the mechanisms of reactions of alkanes
with acidic catalysts resulted from study of the reactions of alkanes with superacids.
This work demonstrated that both C−H and C−C bonds can be protonated, leading
to fragmentation and formation of alkanes and carbocations. 167
CH CH 3 + H + CH 3 + + CH4
3
CH CH CH 3 + H + CH CH 2 + + CH4
3
3
2
(CH ) CCH CH 3 + H + (CH ) C + + CH CH 3
3
3 3
2
3 3
The reactions of propane and isobutene with HF-SbF have been studied using C≡O
5
to trap the carbocation intermediates. 168
CH CH CH 3 + H + CH CH 2 + + CH4
2
3
3
–
2
CH 3 CH 2 + + : C O + CH CH C O +
3
and
+
CH CH 2 + + CH CH CH 3 CH 3 CH 3 + CH C HCH 3
2
3
3
3
+ – + (CH 3 ) 2 CHC O +
CH 3 C HCH 3 + : C O
The C−C bond of propane is more reactive than the C−H bonds, in accord with
its lower bond strength. With isobutane, however, the reaction occurs with both the
primary and tertiary C−H bonds, leading to hydrogen exchange.
DF
(CH ) CH (CH ) CD + DCH CH(CH )
2
3 3
3 3
3 2
SbF 5
The carbocation intermediates can also alkylate alkanes, leading to chain length-
ening.
CH CH 2 + + CH CH CH CH 3 CH CH CHCH CH 3 + H +
3
2
3
2
2
3
2
CH 3
166 P. G. Mennon and Z. Paal, Ind. Eng. Chem. Res., 36, 3282 (1997).
167 G. A. Olah, Y. Halpern, J. Shen, and Y. K. Mo, J. Am. Chem. Soc., 95, 4960 (1973).
168
J. Sommer and J. Bukala, Acc. Chem. Res., 26, 370 (1993).