Initiation steps chlorination

Each chlorine atom formed m the initiation step has seven valence electrons and IS very reactive Once formed a chlorine atom abstracts a hydrogen atom from methane as shown m step 2 m Figure 4 21 Hydrogen chloride one of the isolated products from... 

Figure 10.1 Mechanism of the radical chlorination of methane. Initiation step Three kinds of steps are required initiation, propagation, and termination. The propagation steps are a repeating cycle, with Cl- a reactant in step 1 and a product in...

The results of kinetic studies suggest that alkane substitution reactions typically proceed by a radical chain mechanism (Section 13.9). The initiation step in the chlorination of methane is the dissociation of chlorine ... 

The mechanism for reaction 13 is not well established yet, but it is likely to proceed through ionic intermediates (11) the Cl atom in chlorine nitrate is slightly electropositive, so that it readily combines with negative chloride ions to produce CI2 the HQ on ice is expected to be at least partially ionized. We have found that HCl has a very high mobility on the ice surface, so that even small amounts of HCl will enable reaction 13 to occur. It is also possible for this reaction to proceed in two steps the initial step is the reaction of chlorine nitrate with ice it is followed by the reaction of the product HOCl with HCl on the ice substrate ... 

A. Synthetic Methods.—Electrophilic addition of P compounds to olefinic compounds is a well-established route to phosphonic acids, although yields are often disappointing. With phosphorus pentachloride it has been found that yields are greatly improved when phosphorus trichloride is added to the reaction mixture. Since the orientation of the addition implies that electrophilic addition to phosphorus rather than chlorine is the initial step, it seems likely that the trihalide participates by decreasing the free concentration of chlorine rather than by a more active role. This... 

In the chain-initiating step only the bond between two chlorine atoms is broken, and no bonds are formed. 

Upon treatment with nickeltetracarbonyl, dimethyl 3-vinyl-l,2-dichlorocy-clobutane-l,2-dicarboxylate 169 is rearranged, to dimethyl 1,4-cyclohexadiene-1,2-dicarboxylate 171 with concomitant loss of the chlorine atoms [87], Reduction to dimethyl 3-vinylcyclobutene-l,2-dicarboxylate 170 is involved in the initial step. (Scheme 64)... 

In CfE Higher Chemistry, you came across free radicals when we considered the mechanism of the substitution reaction between methane and chlorine In the presence of ultraviolet light. You will recall that the initiation step In the mechanism Is the homolytic fission of chlorine molecules to generate chlorine free radicals. 

The initiation step is the light-induced formation of chlorine atoms as the radicals. Only a few chlorine molecules will suffer this fate, but these highly reactive radicals then rapidly interact with the predominant molecules in the system, namely cyclohexane. 

Free-radical mechanisms obviously involve free radicals. A free radical is a species with an unpaired electron. In these mechanisms, single-headed curved arrows eire the norm. In Organic Chemistry 1, these free radicals first appear when excimining the chlorination of an alkane such as methane. The process begins with an initiation step as shown in Figure 2-14. (All initiation steps increase the number of free radicals.)... 

The initiation step is homoljdic bond cleavc e where each of the chlorine atoms receives one of the two electrons originally present in the bond and two chlorine free radicals form. The chlorine free radicals, like all free radicals, are very reactive. 

Chlorine combines with hydrogen forming hydrogen chloride, HCl. The reaction occurs rapidly when exposed to hght, involving a photochemical chain initiation step. 

Chlorination (and bromination) of alkanes such as methane, CH., has a radical-chain mechanism, as follows INITIATION STEP... 

The oxidation of other rubbers has been studied by FT-IR including polychloro-prenes >. These results suggest that the thermal oxidation of polychloroprenes involves the 1,2 and 3,4-structural irregularities in the initial stage. In particular, it is felt that the initial step is the abstraction of a tertiary allylic chlorine or hydrogen from the 1,2 or 3,4 units yielding a tertiary carbon radical. 

Atomic chlorine lowers the energy barrier of this reaction by providing an alternate pathway involving intermediate reactions, each having a lower activation energy than the uncatalyzed reaction. This alternate pathway involves two steps. Initially, the chlorine reacts with the ozone to form chlorine monoxide and oxygen ... 

A free radical chain reaction proceeds through a succession of free radicals. In the photochemical chlorination of an alkane, the initiating step is the homolytic lission of chlorine molecules to produce chloroalkanc molecules and chlorine free radicals. These two reactions constitute the propagating step. However, the chlorine free radicals may also combine to form chlorine molecules or react with the alkane free radicals to form chloroalkane molecules. Both of these reactions constitute terminating steps of the chain reaction. Il should be noted, however, that the foregoing sequence cannot take place in the dark. Exposure to light allows the series of reactions then to proceed rather violently. 

The initiation step is dissociation of chlorine to two chlorine atoms. 

Chloroprene is of high industrial importance for manufacture of synthetic rubbers. For a long time the synthesis was based on acetylene. More recent processes are based on butadiene as a feedstock, which is substantially cheaper [29]. The initial step is a gas-phase free-radical chlorination at 250 °C and temperature control is ensured by use of excess butadiene (molar ratio of Cl2 to butadiene 1 5 to 1 50) [44]. To limit side reactions, short contact time reactors operating at higher temperatures and residence times below one second are also known [45], Good mix-... 

Typical radical reactions are substitution and addition reactions as shown below (Scheme b). A typical substitution reaction is the halogenation of methane with chlorine gas under photolytic conditions, and generally available chlorohydrocarbons are prepared by this method. The chlorination reaction proceeds through a chain pathway via the initiation step, propagation step, and termination step as shown below (Scheme 1.1). 

You should already be familiar with the mechanism for the thermal chlorination of methane. We will use Figure 1.17 to review briefly the net equation, the initiation step, and the propagation steps of the monochlorination of methane. Figure 1.18 shows the energy profile of the propagation steps of this reaction. 

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