Alkanes aliphatic compounds

ALKYLATION OF ALIPHATIC COMPOUNDS The first reported alkylation of branched-chain alkanes by ethylene, over aluminum chloride (69), made it possible to alkylate alkanes (except methane and ethane) with straight chain or branched alkenes. 

With comprehensive GC, we can now choose a rational set of columns that should be able to tune the separation. If we accept that each column has an approximate isovolatility property at the time when solutes are transferred from one column to the other, then separation on the second column will largely arise due to the selective phase interactions. We need only then select a second column that is able to resolve the compound classes of interest, such as a phase that separates aromatic from aliphatic compounds. If it can also separate normal and isoalkanes from cyclic alkanes, then we should be able to achieve second-dimension resolution of all major classes of compounds in petroleum samples. A useful column set is a low polarity 5 % phenyl polysiloxane first column, coupled to a higher phenyl-substituted polysiloxane, such as a 50 % phenyl-type phase. The latter column has the ability to selectively retain aromatic components. 

Clearly, whether or not ozone is formed depends also on the rate at which, for example, unsaturated hydrocarbons react with it. Rates of reactions of ozone with alkanes are, as noted above, much slower than for reaction with OH radicals, and reactions with ozone are of the greatest significance with unsaturated aliphatic compounds. The pathways plausibly follow those involved in chemical ozonization (Hudlicky 1990). 

All meteorite analyses are made more difficult because of the problem of contamination. Thus one group (Kvenholden, 1970) reported the presence of polycyclic aliphatic compounds, while a second (Studier, 1972) found straight-chain alkanes to be the dominant species. The latter result was often cited and taken as evidence that processes similar to the Fischer-Tropsch synthesis must have occurred in nebula regions of the cosmos. 

Fig. 16.32 Alkane degradation under aerobic conditions, showing incorporation of oxygen from molecular oxygen into the aliphatic compound, producing a fatty acid. Fatty acids are oxidized further by p-oxidation [H] indicates reducing equivalents that are either required or formed in the reaction step. (Bouwer and Zender 1993)...

Flexible aliphatic compounds are also selectively fluorinated. Such substrates may be alkanes, alcohols, carboxylic acid derivatives or ketones as long as the electron-withdrawing group is far enough from the reacting center (Table 2).44 There are differences in yields and reaction rates which are qualitatively easily understood and are directly related to the electron density of the reactive C —H bond. 

In principle, the acylation of aliphatic compounds is analogous with the Friedel-Crafts acylation of aromatics in the sense that a hydrogen of the reacting alkanes, alkenes, or alkynes is replaced by an acyl group to yield ketones, unsaturated ketones, or conjugated acetylenic ketones, respectively. As discussed subsequently, however, the reactions are more complex. The acylation of aliphatics is an important but less frequently used and studied process.11-13... 

Despite the well-known chemical stability of perfluorinated aliphatic compounds, pcr-fluorinated Decalin can be reacted w ith alkane- or arenethiols under very mild conditions yielding octakis(alkylsulfanyl)- or octakis(arylsulfanyl)naphthalenes 2.6 The reaction starts at the tertiary carbon atoms where the C — F bond is the weakest, since perfluorocyclohexane does not react under similar conditions, but perfluorocyclohexene does.6... 

PCBs > PAHs > chlorinated aliphatic alkanes > chlorinated aromatics > unsaturated chlorinated aliphatic compounds... 

A correlation for chlorinated alkanes was also established using a as a descriptor. As discussed earlier, cf can be used to describe the behavior of aliphatic compounds. Figure 5.23 demonstrates the Hammett correlation for chlorinated alkanes. 

The Hammett correlation for substituted carboxylic acids is demonstrated in Figure 5.26. The Hammett constant om fails for aliphatic compounds, and the derived constant o must be used to predict accurate Hammett correlations. The least-substituted carboxylic acid, formic acid, was used as the reference compound. The Hammett correlation for substituted carboxylic acids (CAs) demonstrates that the CAs substituted by electron-withdrawing substituents, such as Cl, oxidize the fastest. CAs substituted by electron-donating groups, such as CH3 and NH2, oxidize more slowly than those substituted by electron-withdrawing substituents. The reaction pathway for substituted carboxylic acids is shown in Figure 5.27. These trends are different for phenols and alkanes, because the reaction site is at the election pair located at the oxygen atom. 

Aliphatic compounds comprise hydrocarbons and their derivatives in which the molecular skeletons consist of tetrahedral carbon atoms connected by C-C single bonds. These tetrahedral carbon atoms can be arranged as chains, rings, or finite frameworks, and often with an array of functional groups as substituents on various sites. The alkanes CMH2n+2 and their derivatives are typical examples of aliphatic compounds. 

Remarkable are the relations between the chemical structure of aliphatic compounds and their odour threshold for the human olfactory system and hence for their importance in aroma chemistry. Thus, e.g. the odour threshold for alkanals decreases with increasing number of carbon atoms (C5 to C10), the introduction of a double bond generally lowering the threshold. In general, it can be assumed that alkenals... 

A large variety of aliphatic compounds exist in the resin as shown in Table 5-4. The amounts of alkanes and alcohols are relatively small, arachinol (C20), behenol (C22), and lignocerol (C24) representing the major alcohol components. Compounds of this type are very lipophilic and stable. 

Alkanes (and all other aliphatic compounds) have an important physical property. They are non-polar. As you know from Chapter 8, non-polar molecules have fairly weak intermolecular forces. As a result, hydrocarbons such as alkanes have relatively low boiling points. As the number of atoms in the hydrocarbon molecule increases, the boiling point increases. Because of this, alkanes exist in a range of states under standard conditions. 

The names of branched-chain alkanes (and most other aliphatic compounds) have the same general format, as shown in Figure 13.14. This format will become clearer as you learn and practise the rules for naming hydrocarbons. To start, read the steps on the next page to see how 2-methylpentane gets its name. 

The structure of an alkane can be much more complex than the structure of 2-methylpentane. For instance, there can be many branches bonded to the main chain, and the branches can be quite long. As a result, you need to know several other IUPAC rules for naming branched-chain alkanes and other aliphatic compounds. 

Branched-Chain Alkanes and Other Aliphatic Compounds... 

Unsaturated compounds have physical and chemical properties that differ from those of saturated compounds. For example, the boiling points of alkenes are usually slightly less than the boiling points of similar-sized alkanes (alkanes with the same number of carbon atoms). This difference reflects the fact that the forces between molecules are slightly less for alkenes than for alkanes. For example, the boiling point of ethane is -89°C, whereas the boiling point of ethene is -104°C. On the other hand, both alkenes and alkanes have a low solubility in water. Alkenes, like all aliphatic compounds, are non-polar. 

The main compounds in the noncondensable gaseous products were linear aUcenes and alkanes, ranging from Ci to C7 and accounted for almost 90% of the mixture, with the rest consisting mainly of cyclic aliphatic compounds. In terms of individual compounds, the gaseous mixture was composed of compounds similar to those found in the conventional pyrolysis of PE [72, 74, 92], with the difference however that negligible amounts of hydrogen were found [85],... 

The compounds we have looked at so far (alkanes, alkenes, and alkynes— open chain or cyclic) are called aliphatic compounds. 

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