Chlorine atom+methane

Chlorine atom Methane Hydrogen chloride Methyl radical... 

Spin density surface for chlorine atom+methane transition state shows location of unpaired electron. 

Examine the structures of the two transition states (chlorine atom+methane and chlorine+methyI radical). For each, characterize the transition state as early (close to the geometry of the reactants) or as late (close to the geometry of the products) In Ught of the thermodynamics of the individual steps, are your results anticipated by the Hammond Postulate Explain. 

Kandel S A and Zare R N 1998 Reaction dynamics of atomic chlorine with methane importance of methane bending and tortional excitation in controlling reactivity J. Chem. Phys. 109 9719-27... 

Chlorine substitutes the hydrogen of methane giving successively the chlorides CH3CI, CH2CI2, CHCI3 and CCI4. It is to be noted that if a hydrocarbon is unsaturated, chlorine atoms will first add to the double or triple bond after which substitution may occur. 

The boiling points of the chlorinated derivatives of methane increase with the num ber of chlorine atoms because of an increase m the induced dipole/mduced dipole attrac tive forces... 

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... 

Step 2 Hydrogen atom abstraction from methane by a chlorine atom... 

Termination steps are m general less likely to occur than the propagation steps Each of the termination steps requires two free radicals to encounter each other m a medium that contains far greater quantities of other materials (methane and chlorine mol ecules) with which they can react Although some chloromethane undoubtedly arises via direct combination of methyl radicals with chlorine atoms most of it is formed by the propagation sequence shown m Figure 4 21... 

As in the chlorination of methane it is often difficult to limit the reaction to monochlo rmation and derivatives having more than one chlorine atom are also formed... 

This cycle is terrninated by the reaction of chlorine atoms with methane Cl + CH — HCl + CH3. The importance of this cycle depends on the avadabiLity of oxygen atoms and varies with altitude as well as the time of year it accounts for only 5% of the halogen-controUed loss at 15 km, but increases to 25% at 21 km. 

Chlorine atoms obtained from the dissociation of chlorine molecules by thermal, photochemical, or chemically initiated processes react with a methane molecule to form hydrogen chloride and a methyl-free radical. The methyl radical reacts with an undissociated chlorine molecule to give methyl chloride and a new chlorine radical necessary to continue the reaction. Other more highly chlorinated products are formed in a similar manner. Chain terrnination may proceed by way of several of the examples cited in equations 6, 7, and 8. The initial radical-producing catalytic process is inhibited by oxygen to an extent that only a few ppm of oxygen can drastically decrease the reaction rate. In some commercial processes, small amounts of air are dehberately added to inhibit chlorination beyond the monochloro stage. 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Bonds may also be broken symmetrically such that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired like all the other electrons of the atom, i.e. it does not have a partner of opposite spin. Atoms possessing odd unpaired electrons are termed free radicals and are indicated by a dot alongside the atomic or molecular structure. The chlorination of methane (see later) to produce methyl chloride (CH3CI) is a typical free-radical reaction ... 

The process is believed to initiate with formation of chlorine atom (either thermally or photochemically), which then abstracts a hydrogen from methane. 

Depending on the relative amounts of the two reactants and on the time allowed, a sequential substitution of the alkane hydrogen atoms by chlorine occurs, leading to a mixture of chlorinated products. Methane, for instance, reacts with CI2 to yield a mixture of CH3CI, CH2CI2, CHCI3, and CCI4. We ll look at this reaction in more detail in Section 5.3. 

Chloroform, CHCla, is an example of a polar molecule. It has the same bond angles as methane, CH4, and carbon tetrachloride, CCLi- Carbon, with sp3 bonding, forms four tetrahedrally oriented bonds (as in Figure 16-11). However, the cancellation of the electric dipoles of the four C—Cl bonds in CCL does not occur when one of the chlorine atoms is replaced by a hydrogen atom. There is, then, a molecular dipole remaining. The effects of such electric dipoles are important to chemists because they affect chemical properties. We shall examine one of these, solvent action. 

Chlorine atoms (each of which has one unpaired electron) are highly reactive they attack methane molecules and extract a hydrogen atom, leaving a methyl radical behind ... 

A chlorine atom abstracts a hydrogen This step produces a molecule of atom from a methane molucule. hydrogen chloride and a methyl radical. 

Figure 10.3 Potential energy diagrams (a) for the reaction of a chlorine atom with methane and (b) for the reaction of a bromine atom with methane.

Substitution reactions usually occur with saturated molecules. A typical case is the reaction of chlorine and methane in which the hydrogen atoms of methane are replaced by chlorine in sequence—for example,... 

Chlorination of methane results in the replacement of one, two, three or four atoms of hydrogen by chlorine. Data of product compositions obtained with various reaction times are tabulated (Gorin et al, Ind. Eng Chem 40 2135,... 

So, in summary, photo-initiated chlorination of methane requires UV light to generate hot atoms of molecular chlorine, which subsequently dissociate ... 

Figure 19.9 compares the observed and library spectra for dichloro-methane (retention time 3.45 min in the GC-MS run). The prominent chlorine isotope pattern for the two chlorine atoms in this spectrum makes it readily identifiable. The primary fragmentation is loss of a chlorine atom, producing the m/z 49 fragment. While this fragment clearly manifests a chlorine isotope pattern still, it reflects the fact that only one chlorine atom remains. Library search identifies this spectrum as dichloromethane with quality-of-fit measures of greater than 95%. 

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