Page 473 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

454 Topic 4.1. The Role Carbocations and Carbonium Ions in Petroleum
Processing
CHAPTER 4
Nucleophilic Substitution
Petroleum refining is a basic industry in the modern world. The industry provides
fuels for transportation, industrial energy, and residential heat, as well as petrochem-
icals for the manufacture of a wide range of products. The largest consumption of
petroleum is for transportation fuels. The fundamental technology of petroleum refining
involves distillation to remove nonvolatile materials and separate the hydrocarbons on
the basis of boiling range. The gasoline b.p. range is approximately 30–200 C, and the

fuel oil range is 200–300 C. There are also processes that modify the chemical compo-

sition, which include cracking, hydrocracking, and catalytic reforming. In cracking
and hydrocracking, larger hydrocarbons are converted to hydrocarbons in the gasoline
range; catalytic reforming involves isomerization to increase the fraction of branched
chain, cyclic, and aromatic hydrocarbons in the gasoline product. The objective of
catalytic reforming is to improve gasoline performance. One of the measures of
performance is the octane number, which is a measure of the degree of engine knocking
observed for a particular hydrocarbon or hydrocarbon mixture. The scale is calibrated
with n-heptane as 0.0 and 2,2,4-trimethylpentane as 100.0. Table 4.19 shows some
research octane numbers (RON) for the heptane and octane isomers. There is a second
scale, motor octane number that is also used. Note that chain branching leads to
improved octane numbers.
The chemical basis for engine performance is related to the rates of reaction of
the peroxy radicals involved in the combustion process. Components with high octane
numbers have relatively low rates of chain branching, which reduces the premature
ignition that causes poor engine performance. 156 Engine performance can also be
improved by gasoline additives. Tetraethyllead was used for this purpose for many
years before it became apparent that the accumulating lead in the environment had
many adverse consequences. Lead also interferes with the catalytic converters required

Table 4.19. Octane Numbers for Some Hydrocarbons
Heptanes RON Octanes RON
n-Heptane 0 0 n-Octane −19 0
2-Methylhexane 42 4 2-Methylheptane 21.7
3-Methylhexane 52 0 3-Methylheptane 36.8
3-Ethylpentane 65 0 4-Methylheptane 26.7
2,2-Dimethylpentane 92 8 3-Ethylhexane 33.5
2,3-Dimethylpentane 91 1 2,2-Dimethylhexane 72.5
2,4-Dimethylpentane 83 1 2,3-Dimethylhexane 71.5
3,3-Dimethylpentane 80 8 2,4-Dimethylhexane 65.2
2,2,3-Trimethylpentane 112 1 2,5-Dimethylhexane 55.5
3,3-Dimethylhexane 75.5
3,4-Dimethylhexane 76.3
2-Methyl-3-ethylpentane 87.3
3-Methyl-3-ethylpentane 80.8
2,2,3-Trimethylpentane 109.6
2,2,4-Trimethylpentane 100.0
2,3,3-Trimethylpentane 106.1
2,3,4-Trimethylpentane 102.7

156
C. Morley, Combust. Sci. Technol., 55, 115 (1987).