Hydrocarbons, saturated, reactions with atoms

The saturated hydrocarbons are relatively inert except at high temperatures. For example, sodium metal is usually stored immersed in an alkane such as kerosene (8 to 14 carbon atoms) to protect it from reaction with water or oxygen. Combustion is almost the only important chemical reaction of the alkanes. That reaction, however, makes the hydrocarbons one of the most important energy sources of our modern technology. 

The analysis of published data on reactions of ozone with low molecular hydrocarbons shows that double bonds react with ozone more quickly than saturated bonds (12). Ozone reacts with saturated hydrocarbons in reactions in which hydrogen abstraction s followed by re-hydridization of the carbon atom form sp to sp state (43,44) ... 

In contrast to saturated hydrocarbons, the unsaturated hydrocarbons react with atomic fluorine by two pathways, i.e. (atomic fluorine addition at >C=C< double bond and hydrogen substitution by fluorine atoms. The reaction of fluorine with aromatic hydrocarbons proceeds with the formation of F-derivatives and hydrogen atoms break off ... 

Chlorine atoms react with aromatic hydrocarbons, but only at a significant rate with those having saturated side chains from which the chlorine atom can abstract a hydrogen or unsaturated side chains to which it can add. For example, the rate constant for the Cl atom reaction with benzene is 1.3 X 10"15 enr3 molecule-1 s-1 (Shi and Bernhard, 1997). On the other hand, the rate constants for the reactions with toluene and p-xylene are 0.59 X 10-10 and 1.5 X 10-l() enr3 molecule"1 s"1, respectively (Shi and Bernhard, 1997), and that for reaction with p-cymene is 2.1 X 10"10 cm3 molecule"1 s-1 (Finlayson-Pitts et al., 1999). Hence... 

It follows that oxygen atom reactions with saturated and unsaturated hydrocarbons proceed with scission of the C—C and C=C bonds, respectively. 

Some conclusions on the reaction mechanism may be drawn from the rate constants obtained. It was shown for hydroxyl reactions with saturated compounds (propane, for example) that the main reaction of OH was the hydrogen atom abstraction in the formation of water. This is an accepted point of view. However, another route is possible for reactions with unsaturated hydrocarbons, i.e., addition at the double bond. This is the case for the H atom with saturated compounds H reacts by abstraction, and with unsaturated ones by addition. 

Substantial production of Hg(3P0) atoms has been seen by Callear and McGurk (180). Continuous emissions that presumably originate from loose complexes between the Hg(3P0) atoms and the hydrocarbons have been observed near 2537 A (938). No HgH has been detected from the (3Pi) atom reactions with saturated hydrocarbons (177). 

Fig- 4. Summary of probable mechanisms for the reaction of atomic oxygen with saturated and unsaturated hydrocarbons in the gas phase. The reaction of 0( D) with an alkene is believed to form the expoxide directly, thus skipping the formation of a triplet biradical. 

Analysis of all the data on O-atom reactions with squalane and consideration of the various possible reactions of ground state 0( P) and electronically-excited 0( D) lead to a qualitative summary (Fig. 16) of the initial reactions between atomic oxygen and a saturated hydrocarbon surface. The first step leading to the production of volatile reaction products is direct H-atom abstraction by 0( P). The initial OH product may... 

Class 1. Characteristics (i) The decomposition in seasoned vessels exhibits first-order kinetics and there are no apparent induction periods (i7) the rate is unaffected by packing the reaction vessel or by the addition of known radical-chain inhibitors such as propene iii) the Arrhenius pre-exponential factor is of the order of 10 sec L This behaviour is consistent with a unimolecular mechanism for the decomposition. Among reactions in this class are included the dehydrohalogena-tion of monochlorinated saturated hydrocarbons containing /S-hydrogen atoms e.g., chloropropane 2-chloropropane f-butyl chloride) and of most of the secondary and tertiary monobrominated saturated hydrocarbons. 

Abstraction of a hydrogen atom together with a pair of electrons from a saturated hydrocarbon. This reaction has already been mentioned (p. 40) and will be discussed again under the Alkylation of Olefins (p. 143) and the Isomerization of Alkanes (p. 59). 

Atomic selenium was monitored in flashed CSe2 by kinetic absorption spectroscopy, and its rate of reaction with ethylene was measured in the temperature range 302-412 K. The rate of appearance of a new spectrum in the far ultraviolet (intense bands at 2259 and 2208 A) was symmetrical with atomic decay, and was assigned to ethylene selenide, i.e. selenirane (1) (Equations (la) and (lb)). Absorption spectra of flashed CSe2 and alkene mixtures show band systems in the 2000-2300 A region which do not occur in flashed saturated-hydrocarbon and CSe2 mixtures. The band centers obtained from their spectra are listed in Table 4. 

Based on elemental composition and relative molecular mass determinations, the formula of benzene was found to be C6H6. The saturated hydrocarbon hexane has the molecular formula C6H14 and therefore it was concluded that benzene was unsaturated. Kekule in 1865 proposed the cyclic structure 4 for benzene in which the carbon atoms were joined by alternate single and double bonds. Certain reactions of benzene, such as the catalytic hydrogenation to cyclohexane, which involves the addition of six hydrogen atoms, confirmed that benzene was a ring compound and that it contained three double bonds. However, since benzene did not undergo addition reactions with HC1 and HBr, it was concluded that these double bonds were different from those in ethene and other unsaturated aliphatic compounds. 

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