Light alkanes

In the laboratory it is more convenient to use light either visible or ultraviolet as the source of energy to initiate the reaction Reactions that occur when light energy IS absorbed by a molecule are called photochemical reactions Photochemical techniques permit the reaction of alkanes with chlorine to be performed at room temperature... 

The butane-containing streams in petroleum refineries come from a variety of different process units consequently, varying amounts of butanes in mixtures containing other light alkanes and alkenes are obtained. The most common recovery techniques for these streams are lean oil absorption and fractionation. A typical scheme involves feeding the light hydrocarbon stream to an absorber-stripper where methane is separated from the other hydrocarbons. The heavier fraction is then debutanized, depropanized, and de-ethanized by distillation to produce C, C, and C2 streams, respectively. Most often the stream contains butylenes and other unsaturates which must be removed by additional separation techniques if pure butanes are desired. 

Orga.nic Chemistry. The organic chemistry of sulfur dioxide, particularly as it relates to food appHcations, has been discussed (246). Although no reaction takes place with saturated hydrocarbons at moderate temperatures, the simultaneous passage of sulfur dioxide and oxygen into an alkane in the presence of a free-radical initiator or ultraviolet light affords a sulfonic acid such as hexanesulfonic acid [13595-73-8]. This is the so-called sulfoxidation reaction (247) ... 

Figure 4.12 continued) (c) An expansion of the inset region from (b), with the normal alkanes shown as (a-e). Other unidentified components (f-i) are presented to locate specific peaks for comparison purposes, (d) A light gas oil analysed under the same conditions as for the cycle oil, showing the same expanded region. In this case, the oil has not been ti eated in the same manner as the cycle oil, so it retains the components that were absent from the cycle oil. Peaks (a-i) are the same as those seen in (c). 

Methane is the most difficult alkane to chlorinate. The reaction is initiated by chlorine free radicals obtained via the application of heat (thermal) or light (hv). Thermal chlorination (more widely used industrially) occurs at approximately 350-370°C and atmospheric pressure. A typical product distribution for a CH4/CI2 feed ratio of 1.7 is mono- (58.7%), di-(29.3%) tri- (9.7%) and tetra- (2.3%) chloromethanes. 

The reaction of an alkane with Cl2 occurs when a mixture of the two is irradiated with ultraviolet light (denoted hv, where v is the Greek letter nu). 

I Substitution reactions occur when two reactants exchange parts to give two new products. An example is the reaction of an alkane with Cl2 in the presence of ultraviolet light to yield an alkyl chloride. A Cl atom from Cl2 substitutes for an H atom of the alkane, and two new products result. 

Ethylene, propylene, and butene are synthesized industrially by thermal cracking of light (C2-Cg) alkanes. 

Structurally simple alJkyl halides can sometimes be prepared by reaction of an alkane with Cl2 or Br2 through a radical chain-reaction pathway (Section 5.3). Although inert to most reagents, alkanes react readily with Cl2 or Br2 in the presence of light to give alkyl halide substitution products. The reaction occurs by the radical mechanism shown in Figure 10.1 for chlorination. 

Alkyl halides can be reduced to alkanes by a radical reaction with tributyltin hydride, (C4H9)3SnH, in the presence of light (hv). Propose a radical chain mechanism by which the reaction might occur. The initiation step is the light-induced homolytic cleavage of the Sn— H bond to yield a tributyltin radical. 

Chain reaction (Section 5.3) A reaction that., once initiated, sustains itself in an endlessly repeating cycle of propagation steps. The radical chlorination of alkanes is an example of a chain reaction that is initiated by irradiation with light and then continues in a series of propagation steps. 

Initiator (Section 5.3) A substance with an easily broken bond that is used to initiate a radical chain reaction. For example, radical chlorination of alkanes is initiated when light energy breaks the weak Cl-Cl bond to form Cl-radicals. 

A vapor phase process for deparaffmization of light gas oils performed by the BP works in this way The gas oil, boiling range 230-320°C, is passed over a 5-A molecular sieve at 320°C and a pressure of 3.6 bar. The space velocity is 0.63 vol liquid gas oil per vol molecular sieve and per hour, [liquid hourly space velocity (lhsv) = 0.63] the adsorption period takes 6 min. Together with the gas oil vapor 120 vol N2 per vol liquid gas oil is led over the sieve as carrier and purge gas. After the adsorption period the loaded molecular sieve is purged at the same temperature with pure N2 for 6 min. Subsequently, the adsorbed /z-alkanes are desorbed by 1 vol liquid /z-pentane per vol molecular sieve and per hour. The /z-pentane is recovered from the /z-alkane//z-pentane mixture by simple distillation [15]. The IsoSiv process of the Union Carbide Corporation works in a similar way [16]. The purity of the isolated /z-alkanes is >98%. 

Some other processes are known for sulfoxidation but have no technical importance. The acetic anhydride process has attracted some interest because it does not need exposure to light and enables conversion rates up to 15% of paraffin feedstock. Once started by peroxide or UV light initiation, it propagates without further radical-forming initiation steps. The addition of some 2.5% acetic anhydride to the reacting alkane is crucial to form a mixed anhydride of par-... 

Alkanes can be nitrosated photochemically, by treatment with NOCl and UV light. For nitration at an activated carbon, see 14-12. 

We have summarized below recent results concerning spectroscopic / flow reactor investigations of hydrocarbons partial and total oxidation on different transition metal oxide catalysts. The aim of this study is to have more information on the mechanisms of the catalytic activity of transition metal oxides, to better establish selective and total oxidation ways at the catalyst surface, and to search for partial oxidation products from light alkane conversion. 

Degassed water under ambient conditions has a relaxation time (T and T2) of about 4 s at 30 °C [11,12], However, air-saturated brines may have a relaxation time of about 2-3 s. Light hydrocarbons are even more sensitive to dissolved oxygen [10], For example, the relaxation time of deoxygenated pentane is 14 s while air-saturated pentane is about 3 s. The correlation for degassed alkanes between the relaxation time (Ti), viscosity (q) and temperature (T) is given by Eq. (3.6.1) [13]. 

The relationship between Ti and T2 was examined for a number of liquid alkanes and crude oils [15]. It was concluded that there is no difference for light oils, apparently because light oils satisfy the fast-motion condition (the correlation time is less than the Larmor period). However, viscous oils do not satisfy this condition as the departure between Tx and T2 correlates with an increasing viscosity and Larmor frequency. 

Detectability may be a significant problem with homologous series of unsaturated compounds, particularly //-alkanes. For these compounds, refractive index detection or evaporative light-scattering, both of which are described elsewhere in the book, may be of use. Indirect photometry is a useful detection scheme for compounds that do not absorb in the UV. Acetone, methylethyl ketone, methyl propyl ketone, methyl isopropyl ketone, methyl isobutyl ketone, and acetophenone are added to an acetonitrile/water mobile phase, generating a negative vacancy peak when the nonchro-mophoric analyte emerges and a positive peak if the ketone is adsorbed and displaced.70 Dodecyl, tetradecyl, cetyl, and stearyl alcohols also have been derivatized with 2-(4-carboxyphenyl)-5,6-dimethylbenzimidazole and the derivatives separated on Zorbax ODS in a mobile phase of methanol and 2-propanol.71... 

An intermediate organic nitroso compound RNO/ is formed, leading to N2 during its decomposition [1-5]. The mechanistic studies by Sachtler and co-workers [1 1] for the reduction of NOx by light alkanes over Fe/ZSM-5 involved adsorbed RNOx species which further react with gas-phase NOx to produce N2, through the decomposition of diazo compounds [2,4],... 

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