441
Refining Chemistry
Except for butane, the isomerization of paraffins is generally accompanied by side reactions
involving carbon–carbon bond scissions when catalysts of the aluminum halide type are used.
Products boiling both higher and lower than the starting material are formed, and the disproportionation reactions occur with the pentanes and higher paraffins (>C 5 ) are caused by unpromoted
aluminum halide. A substantial pressure of hydrogen tends to minimize these side reactions.
The ease of paraffin isomerization increases with molecular weight, but the extent of disproportionation reactions also increases. Conditions can be established under which isomerization takes
place only with the butanes, but this is difficult for the pentanes and higher hydrocarbons. At 27°C
(81°F) over aluminum bromide (AlBr 3 ), the equilibrium mixture of n-pentane and isopentane, contains over 70% of the branched isomer; at 0°C (32°F) approximately 90% of the branched isomer
is present. Higher and lower boiling hydrocarbon products, hexanes, heptanes, and isobutane are
also formed in side reactions even at 0°C (32°F) and in increased amounts when the temperature
is raised. Although the thermodynamic conditions are favorable, neo-pentane [C(CH 3 ) 4 ] does not
appear to isomerize under these conditions.
Olefins are readily isomerized; the reaction involves either movement of the position of the double bond (hydrogen-atom shift) or skeletal alteration (methyl group shift). The double-bond shift
may also include a reorientation of the groups around the double bond to bring about a cis–trans
isomerization. Thus, 1-butene is isomerized to a mixture of cis- and trans-2-butene. Cis (same side)
and trans (opposite side) refer to the spatial arrangement of the methyl groups with respect to the
double bond.
Olefins having a terminal double bond are the least stable. They isomerize more rapidly that
those in which the double bond carries the maximum number of alkyl groups.
Naphthenes can isomerize in various ways; for example, in the case of cyclopropane (C 3 H 6 )
and cyclobutane (C 4 H 8 ), ring scission can occur to produce an olefin. Carbon–carbon rupture may
also occur in any side chains to produce polymethyl derivatives, whereas cyclopentane (C 5 H 10 ) and
cyclohexane (C 6 H 12 ) rings may expand and contract, respectively.
The isomerization of alkylaromatics may involve changes in the side-chain configuration, disproportionation of the substituent groups, or their migration about the nucleus. The conditions
needed for isomerization within attached long side chains of alkylbenzenes and alkylnaphthalenes
are also those for the scission of such groups from the ring. Such isomerization, therefore, does not
take place unless the side chains are relatively short. The isomerization of ethylbenzene to xylenes,
and the reverse reaction, occurs readily.
Disproportionation of attached side chains is also a common occurrence; higher and lower alkyl
substitution products are formed. For example, xylenes disproportionate in the presence of hydrogen fluoride–boron trifluoride or aluminum chloride to form benzene, toluene, and higher alkylated products; ethylbenzene in the presence of boron trifluoride forms a mixture of benzene and
1,3-diethylbenzene.
16.5 ALKYLATION
Alkylation in the petroleum industry refers to a process for the production of high-octane motor
fuel components by the combination of olefins and paraffins. The reaction of isobutane with olefins,
using an aluminum chloride catalyst, is a typical alkylation reaction.
In acid-catalyzed alkylation reactions, only paraffins with tertiary carbon atoms, such as isobutane and isopentane react with the olefin. Ethylene is slower to react than the higher olefins. Olefins
higher than propene may complicate the products by engaging in hydrogen-exchange reactions.
Cycloparaffins, especially those containing tertiary carbon atoms, are alkylated with olefins in a
manner similar to the isoparaffins; the reaction is not as clean, and the yields are low because of the
several side reactions that take place.
Aromatic hydrocarbons are more easily alkylated than the isoparaffins by olefins. Cumene
(isopropylbenzene) is prepared by alkylating benzene with propene over an acid catalyst. The alkylating
Refining Chemistry
Except for butane, the isomerization of paraffins is generally accompanied by side reactions
involving carbon–carbon bond scissions when catalysts of the aluminum halide type are used.
Products boiling both higher and lower than the starting material are formed, and the disproportionation reactions occur with the pentanes and higher paraffins (>C 5 ) are caused by unpromoted
aluminum halide. A substantial pressure of hydrogen tends to minimize these side reactions.
The ease of paraffin isomerization increases with molecular weight, but the extent of disproportionation reactions also increases. Conditions can be established under which isomerization takes
place only with the butanes, but this is difficult for the pentanes and higher hydrocarbons. At 27°C
(81°F) over aluminum bromide (AlBr 3 ), the equilibrium mixture of n-pentane and isopentane, contains over 70% of the branched isomer; at 0°C (32°F) approximately 90% of the branched isomer
is present. Higher and lower boiling hydrocarbon products, hexanes, heptanes, and isobutane are
also formed in side reactions even at 0°C (32°F) and in increased amounts when the temperature
is raised. Although the thermodynamic conditions are favorable, neo-pentane [C(CH 3 ) 4 ] does not
appear to isomerize under these conditions.
Olefins are readily isomerized; the reaction involves either movement of the position of the double bond (hydrogen-atom shift) or skeletal alteration (methyl group shift). The double-bond shift
may also include a reorientation of the groups around the double bond to bring about a cis–trans
isomerization. Thus, 1-butene is isomerized to a mixture of cis- and trans-2-butene. Cis (same side)
and trans (opposite side) refer to the spatial arrangement of the methyl groups with respect to the
double bond.
Olefins having a terminal double bond are the least stable. They isomerize more rapidly that
those in which the double bond carries the maximum number of alkyl groups.
Naphthenes can isomerize in various ways; for example, in the case of cyclopropane (C 3 H 6 )
and cyclobutane (C 4 H 8 ), ring scission can occur to produce an olefin. Carbon–carbon rupture may
also occur in any side chains to produce polymethyl derivatives, whereas cyclopentane (C 5 H 10 ) and
cyclohexane (C 6 H 12 ) rings may expand and contract, respectively.
The isomerization of alkylaromatics may involve changes in the side-chain configuration, disproportionation of the substituent groups, or their migration about the nucleus. The conditions
needed for isomerization within attached long side chains of alkylbenzenes and alkylnaphthalenes
are also those for the scission of such groups from the ring. Such isomerization, therefore, does not
take place unless the side chains are relatively short. The isomerization of ethylbenzene to xylenes,
and the reverse reaction, occurs readily.
Disproportionation of attached side chains is also a common occurrence; higher and lower alkyl
substitution products are formed. For example, xylenes disproportionate in the presence of hydrogen fluoride–boron trifluoride or aluminum chloride to form benzene, toluene, and higher alkylated products; ethylbenzene in the presence of boron trifluoride forms a mixture of benzene and
1,3-diethylbenzene.
16.5 ALKYLATION
Alkylation in the petroleum industry refers to a process for the production of high-octane motor
fuel components by the combination of olefins and paraffins. The reaction of isobutane with olefins,
using an aluminum chloride catalyst, is a typical alkylation reaction.
In acid-catalyzed alkylation reactions, only paraffins with tertiary carbon atoms, such as isobutane and isopentane react with the olefin. Ethylene is slower to react than the higher olefins. Olefins
higher than propene may complicate the products by engaging in hydrogen-exchange reactions.
Cycloparaffins, especially those containing tertiary carbon atoms, are alkylated with olefins in a
manner similar to the isoparaffins; the reaction is not as clean, and the yields are low because of the
several side reactions that take place.
Aromatic hydrocarbons are more easily alkylated than the isoparaffins by olefins. Cumene
(isopropylbenzene) is prepared by alkylating benzene with propene over an acid catalyst. The alkylating