Mercaptans dissociation

Mercaptan dissociation is low and corresponds to a dissociation constant of around 10"1. In FCC condensates, the major sulfur component is NH4HS along with small concentrations of HzS, In spent caustic, the main components are sodium mercaptides, RSNa, along with the NazS sulfide. 

Dissociation extraction is the process of using chemical reac tion to force a solute to transfer from one liquid phase to another. One example is the use of a sodium hydroxide solution to extract phenolics, acids, or mercaptans from a hydrocarbon stream. The opposite transfer can be forced by adding an acid to a sodium phenate stream to spring the phenolic back to a free phenol that can be extrac ted into an organic solvent. Similarly, primary, secondary, and tertiary amines can be protonated with a strong acid to transfer the amine into a water solution, for example, as an amine hydrochloride salt. Conversely, a strong base can be added to convert the amine salt back to free base, which can be extracted into a solvent. This procedure is quite common in pharmaceutical production. 

By contrast, derivatives formed by the action of certain nucleophilic reagents (e.g, H2O2, alcohols, mercaptans, CN, SOsH, Grig-nard compounds) can only exist in a cyclic form. The most that can occur here is an equilibrium between the nondissociated and the dissociated form (for example, pseudocyanide and true cyanide),-if the newly formed C—O, C—S, or —C bond is sufificiently polar. 

For less reactive ketones where an excess of mercaptan is needed to shift equilibrium (9 b) sufficiently to the right, the measurement of the anodic mercaptan wave is not advantageous, because its height changes only slightly. In the calculation of the equilibrium constant K, the dissociation of the mercaptan according to equilibrium (9 a) must be taken into consideration. 

They assumed that the primary cation radical of PMMA spontaneously and quickly dissociated to form carbocation, which then recombined with the liberated electron to form an excited radical with a ferr-alkyl structure. This excited radical was thought to be the precursor of the scission of the main chain. This reaction model interpreted well their observation that the G value for the scission of the side chain was close to that of the main chain and that the mercaptan added to scavenge electrons suppressed the main-chain scission efficiently without affecting the formation of volatile products from the ester side chain. The above reaction model motivated us to apply the ESE method to the study of radicals in irradiated PMMA. The model now seems inadequate, because it cannot accommodate some recent ESE results as mentioned later. 

Adsorptions of CS2 (102, 117), S02 (118), (CH3)2S (104, 119, 120), (C2H5)2S (105), n-propyl mercaptan (120, 121), thiophene (120), and isopropyl, n-butyl, isobutyl, and tert-butyl mercaptans (121) on Ni have also been investigated. It is speculated that CS2 adsorbs dissociatively on Ni surfaces at room temperature (117). The interaction of CS2 with Ni is limited to the surface at 193 K, but above 298 K bulk sulfidation is observed (102). Sulfur dioxide chemisorbs rapidly and irreversibly on Ni at 193 K, but extensive incorporation into the bulk is not observed below 373 K. 

The thermal dissociation of S-alkyl-N,N -diphenyhsothioureas into mercaptans and diphenylcarbodiimide was already discovered during the last century. " Schlack and Keil utilized this reaction to synthesize several dicycloalkyl andbis-carbodiimides 29 by heating the isothioureas 28 at 125-240 °C under vacuum (yields 40-90 %). 

Adsorption of methyl mercaptan in moist conditions was performed on numerous samples of activated cartons of various origins. Methyl mercaptan adsorption was tested by a dynamic method. The amount of products of surface reaction was evaluated using thermal analysis. The results revealed that the main product of oxidation, dimethyl disulfide, is adsorbed in pores smaller than SO A. There is apparent competition for adsorption sites between water (moist conditions) and dimethyl disulfi. The comp ition is won by the latter molecule due to its strong adsorption in the carbon pore system. Althou dimethyl disulfide has to compete with water for the adsorption sites it can not be formed in a significant quantity without water. Water facilitates dissociation of methyl mercaptan and thus ensures the efficient removal process. 

The liquid phase photolysis of dimethyl disulfide (2537 A) has been reported to produce dimethyl sulfide and dimethyl trisulfide in equal quantities, and traces of CH4 and methyl tetrasulflde. Ethyl disulfide again gave mono, tri- and tetra-sulfides with a trace of ethyl mercaptan. All irradiations were prolonged, of the order 24-48 hours. It was claimed that C-S fission in the primary photolytic dissociation... 

Apart from controlling the degree of dissociation of the analyte species, eluent pH changes are necessary to avoid unwanted side reactions in an acidic or alkaline medium. This applies, for example, to mercaptans which may react to form disulfides in alkaline medium. 

The properties of mercaptans and alcohols are quite different, although they appear to be similar in nature. The bond dissociation energy of the S-H bond is over 10/kcal/mol lower than the corresponding 0-H bond. The ease of free-radical hydrogen abstraction from a mercaptan supports this fact and permits these compounds to be included in preparative free-radical chemistry. The above fact, along with the different boiling points observed for the mercaptans compared to the corresponding alcohols and the differences in their acidities, helps explain why the chemistry of these two classes of compounds differs in so many ways. 

White and co-workers [589] have deposited Mo nanoparticles onto an Au(lll) surface. AES and TPD studies showed that bare Mo nanoparticles were very reactive and could cause complete dissociation of hydrogen sulphide, methyl mercaptan, and thiophene. However, the presence of An atoms on the Mo nanoparticles modified their reactivity. In the case of H2S and CH3SH, the overall activity for desulphurisation was unaffected by An encapsulation but the selectivity to form methane from CH3SH increased from 20% on bare Mo particles to 60% on Au-covered Mo particles. In contrast, Au-encapsulated Mo nanoparticles are relatively inert towards the dissociation of thiophene. 

When polymethylmethacrylate is irradiated in the presence of ethyl mercaptan, it is reasonable to consider that radiation ejected electrons in the matrix are scavenged by the additive. Indeed, in aqueous solution the efficiency of capture of solvated electrons by ethyl mercaptan [158] is about 500 times higher than by methyl pivalate, a low molecular weight model of polymethylmethacrylate [158]. Dissociative electron attachment has been observed by mass spectrometry [159]... 

We must also consider the relative reactivities of the bonds. In the absence of direct data, we can compare bond dissociation energies as a measure of relative reactivity. Table 11-3 shows some pertinent data collected from various sources [18, 19, 20, 21]. The carbon-sulfur dissociation energies for n-alkyl monosulfides and n-alkyl mercaptans lie within a narrow range. Whether the other sulfur linkage is to carbon, as in n-alkyl monosulfides, or to the hydrogen of a mercaptan has no major influence. There is a small but perceptible trend toward weaker bonding with secondary carbon (isopropyl) and with tertiary carbon (t-butyl) in the unsymmetrical methyl aryl monosulfides, a trend that is not firmly evident in the in the corresponding alkyl mercaptans. The vinyl-sulfur... 

A well-known example of a complex catalytic reaction that takes place on the surface of carbon is the oxidation of hydrogen sulfide [329,330], When water is present on the carbon surface and the surface has the basic pH required for dissociation of H2S, oxidation of the HS ions by active oxygen occurs either to elemental sulfur or sulfuric acid. The latter is formed when the reaction takes place in very small pores, where only sulfur radicals very susceptible for further oxidation to SO3 are formed. Catalytic oxidation also occurs in the case of methyl mercaptan adsorption [331], where on basic carbon, thiolate ions formed as a result of dissociation are further oxidized to dimethyldisulfide strongly adsorbed in the pore system. In the case of desulfurization, inorganic constituents of carbon such as iron and calcium also play a crucial role. Those elements, present even in small amounts, contribute significantly to the oxidation reactions as catalysts [332,333],... 

Catalytic oxidation of hydrogen sulfide or methyl mercaptan is also enhanced in the presence of nitrogen-containing functionalities [57], Besides providing the basic pH needed for effective dissociation of HS , they were proposed to activate oxygen via formation of superoxide ion, which participates in the oxidation of thiolate ions to sulfur and sulfuric acid [95],... 

CHjCHjSH. Previous experimental studies indicate that the primary UV dissociation of ethyl mercaptan (CH3CH2SH) also involves the scission of the C-S and S-H bonds [17,24,104-109]. The preference of S-H over C-S cleavage has been observed in CH3CH2SH photolysis at 254 nm [17,109]. Recently, Nourbakhsh et al, [43] performed a 193-nm photodissociation TOF mass spectrometric study of CH3CH2SH and obtained the kinetic... 

As for nitrogen, biogenic redox processes are essential to convert sulfur from its largest oxidation state +VI (sulfate) to its lowest -II (sulfide). Sulfate is the most stable compound in the atmosphere once it has been produced there is no abiotic reduction possible in the climate system. Organisms need sulfur in the form of thiols RSH (simple thiols are called mercaptanes) and sulfides R2S. Thiols are (similar to alcohol ROH) weak acids (but much stronger). Thiols are easy oxidized into sulfides (this reaction is the main function in biological chemistry Fig. 5.25), thereby thiols provide hydrogen for the reduction of other molecules (the dissociated form RS" acts as an electron donor) ... 

---