Classes of Hydrocarbons

Hydrocarbons are compounds that contain only carbon and hydrogen and are divided into two main classes aliphatic hydrocarbons and aromatic hydrocarbons. This classification dates from the nineteenth century, when organic chemistry was almost exclusively devoted 

Aliphatic hydrocarbons include three major groups alkanes, alkenes, and alkynes. Alkanes are hydrocarbons in which all the bonds are single bonds, alkenes contain a carbon-carbon double bond, and alkynes contain a carbon-carbon triple bond. Examples of the three classes of aliphatic hydrocarbons are the two-carbon componnds ethane, ethylene, and acetylene. 

Bonding in ethane, ethylene, and acetylene was discussed in Sections 1.16-1.18. 

Another name for aromatic hydrocarbons is arenes. Arenes have properties that are much different from alkanes, alkenes, and alkynes. The most important aromatic hydrocarbon 

Many of the principles of organic chemistry can be developed by examining the series of hydrocarbons in the order alkanes, alkenes, alkynes, and arenes. Alkanes are introduced in this chapter, alkenes in Chapters 5 and 6, alkynes in Chapter 9, and arenes in Chapters 11 and 12. 

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Hydrocarbons are divided into two main classes aliphatic and aromatic. This classification dates from the nineteenth century, when organic chemistry was devoted almost entirely to the study of materials from natural sources, and terms were coined that reflected a substance s origin. Two sources were fats and oils, and the word aliphatic was derived from the Greek word aleiphar meaning fat. Aromatic hydrocarbons, irrespective of their own odor, were typically obtained by chemical treatment of pleasant-smelling plant extracts. 

De Broglie s and Schrddinger s contributions to our present understanding of electrons were described in Section 1.1. 

All of the forces in chemistry, except for nuclear chemistry, are electrical. Opposite charges attract like charges repel. This simple fact can take you a long way. 

Plot of potential energy versus distance for two hydrogen atoms. At long distances, there is a weak attractive force. As the distance decreases, the potential energy decreases, and the system becomes more stable because each electron now feels the attractive force of two protons rather than one. The lowest energy state corresponds to a separation of 74 pm, which is the normal bond distance in H2. At shorter distances, nucleus-nucleus and electron-electron repulsions are greater than electron-nucleus attractions, and the system becomes less stable. 

We have demonstrated that hydrocarbons and many related compounds are often used as fuels in our highly industrialized society but we have not looked closely at their structures and differences. At this point it is useful to take a closer look at the many types or classes of hydrocarbons and some related compounds as these have many uses beyond those as fuels. There are four classes of hydrocarbons the alkanes, which contain carbon—carbon single bonds the alkenes, which contain one or more carbon—carbon double bonds the alkynes, which contain carbon—carbon triple bonds and the aromatics, which consist of benzene, benzene derivatives, and fused benzene rings. 

Alkanes are referred to as saturated hydrocarbons because they contain the highest ratio of hydrogen to carbon possible. 

Rice fields in the Philippines. Decay of organic matter in rice fields is estimated to make up one-fifth of all methane emitted each year due to human activities. 

Alkanes Hydrocarbons with carbon-carbon single bonds 

The simplest alkane is methane (CH ), the principal component of natural gas. Alkanes are saturated hydrocarbons (Section 5.3) with the general formula (Table 12.5). Notice that all hydrocarbon formulas are tradi- 

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We will give two examples of analysis of these components, bearing in mind that the distinction between condensable and noncondensable hydrocarbons rarely holds in actual refining streams, most of them producing both classes of hydrocarbons simultaneously. 

Table 1 7 shows that hydrocarbons are extremely weak acids Among the classes of hydrocarbons acetylene is a stronger acid than methane ethane ethylene or benzene but even its K is 10 ° smaller than that of water... 

Among the various classes of hydrocarbons alkanes contain carbon m its most reduced state and alkynes contain carbon m its most oxidized state... 

The classes of hydrocarbons are alkanes, alkenes, alkynes, and arenes Alkanes are hydrocarbons in which all of the bonds are single bonds and are characterized by the molecular formula C H2 +2... 

Benzene is the parent of a class of hydrocarbons called arenes, or aro matic hydrocarbons... 

What about a substance wrth the molecular formula 71414 Thrs compound can not be an alkane but may be erther a cycloalkane or an alkene because both these classes of hydrocarbons correspond to the general molecular formula C H2 Any time a ring or a double bond is present in an organic molecule its molecular formula has two fewer hydrogen atoms than that of an alkane with the same number of carbons... 

Paraffins. Methane and ethane are simple asphyxiants, whereas the higher homologues are central nervous system depressants. Liquid paraffins can remove oil from exposed skin and cause dermatitis or pneumonia in lung tissue. Generally, paraffins are the least toxic class of hydrocarbons. 

Many similar hydrocarbon duids such as kerosene and other paraffinic and naphthenic mineral oils and vegetable oils such as linseed oil [8001-26-17, com oil, soybean oil [8001-22-7] peanut oil, tall oil [8000-26-4] and castor oil are used as defoamers. Liquid fatty alcohols, acids and esters from other sources and poly(alkylene oxide) derivatives of oils such as ethoxylated rosin oil [68140-17-0] are also used. Organic phosphates (6), such as tributyl phosphate, are valuable defoamers and have particular utiHty in latex paint appHcations. Another important class of hydrocarbon-based defoamer is the acetylenic glycols (7), such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol which are widely used in water-based coatings, agricultural chemicals, and other areas where excellent wetting is needed. 

The ketones are a group of compounds with the general formula R-C-R. The -C-functional group is known as the carbonyl group or carbonyl radical it appears in many different classes of hydrocarbon derivatives. There are only a few important ketones, and they are all extremely hazardous. 

Hydrocarbons are segmented into a variety of categories. Each category possesses a distinct molecular profile and, in turn, set of chemical and physical properties. Each class of hydrocarbons therefore has historically served different markets. Crude petroleum is composed of four major hydrocarbon groups paraffins, olefins, naphthenes, and aromatics. 

One large and structurally simple class of hydrocarbons includes those substances in which all the carbon-carbon bonds are single bonds. These are called saturated hydrocarbons, or alkanes. In the alkanes the carbon atoms are bonded to each other in chains, which may be long or short, straight or branched. 

Another class of hydrocarbon binders used in propints are the carboxy-terminated polybutadiene polymers which are cross-linked with either tris[l-(2-methyl)aziridinyl] phosphine oxide (MAPO) or combinations with phenyl bis [l -(2-methyl)aziridinyl] phosphine oxide (Phenyl MAPO). Phenyl MAPO is a difunctional counterpart of MAPO which makes possible chain extension of polymers with two carboxylic acid groups. A typical propint formulation with ballistic properties is in Table 11 (Ref 83) Another class of composites includes those using hydroxy-terminated polybutadienes cross-linked with toluene diisocyanate as binders. The following simplified equations illustrate typical reactions involved in binder formation... 

Wall Loss of Oxidation Products. It is known that some classes of hydrocarbons (the higher terpenes, for instance) are prolific aerosol formers when subjected to atmospheric oxidation. Other classes, aromatic hydrocarbons for instance, although they do not form large amounts of suspended aerosol, have been shown to lose (at least under some conditions) large amounts of oxidation products to the reaction vessel walls. The fate of these oxidation products in the open atmosphere remains open to question, as does the extent to which they continue to participate in gas-phase chemistry (187). 

Both classes of hydrocarbon occur naturally, notably in oil and coal deposits. Aromatic compounds are also products of incomplete combustion of organic compounds, and are released into the environment both by human activities, and by certain natural events, for example, forest tires and volcanic activity. 

Considering the long saga of hydrocarbon chemistry, it is surprising that two new classes of hydrocarbon - ionically dissociative hydrocarbons and hydrocarbon salts - have been discovered in the last decade. The syntheses of authentic samples as analytically pure solids have revealed the very existence of such novel hydrocarbons in an unquestionable way, but the investigation of their basic features is just in the inchoate stage. The search for such novel hydrocarbons depends primarily on the synthesis and examination of highly stabilized hydrocarbon cations and anions. As mentioned above, until now such elaboration has been concentrated on the carbocation side, and examination of the carbanion moiety has only just started. 

A class of hydrocarbons occurring in many essential oils of plants. They can be regarded as low-molecular weight polymers of isoprene (C5I I8)n. Dipentene is a terpene, while natural rubber, gutta percha and balata have been termed polyterpenes. 

Suggested half-life classes of hydrocarbons in various environmental compartments at 25°C... 

The upfield shift of signals of carbon atoms in 7-position to a newly introduced substituent was recognized very early. Grant and Paul (169) found a 7-parameter of —2.5 in linear and branched alkanes. Later this group studied various other classes of hydrocarbons (33,88,100,101,170-172) and developed an interpretation of the 7-effects in terms of a polarization of the bond between the carbon concerned and an adjacent, sterically perturbed hydrogen atom (33,88) that has come to be called the Grant-Cheney approach. ... 

Example The El mass spectmm of -decane is typical for this class of hydrocarbons (Fig. 6.18a). Branching of the aliphatic chain supports cleavage of the bonds adjacent to the branching point, because then secondary or tertiary carbenium ions and/or alkyl radicals are obtained (Fig. 6.18b,c). This allows for the identification of isomers to a certain degree. Unfortunately, hydrocarbon molecular ions may undergo skeletal rearrangements prior to dissociations, thereby obscuring structural information. 

The contribution of the various classes of hydrocarbons to the formation of particulate organic compounds is a complex function of their relative ambient concentrations, gas-phase reactivity, and ability to form products whose physical properties, especially vapor pressures, are of prime importance in the physical mechanisms controlling the gas-to-aerosol conversion process. In view of the results discussed previously, cyclic olefins appear to be the most important class of organic aerosol precursors. This is due to their high gas-phase reactivity and their ability... 

The GPC of a local crude (Bryan, Texas) sample spiked with a known mixture of n-alkanes and aromatics is shown in Figure 5 and the GPC of the crude is shown in Figure 6. The hydrocarbon mixture is used to calibrate the length of the species which separates as a function of retention volume. Ttie molecular length is expressed as n-alkane carboa units although n-alkanes represent only a fraction of the hydrocarbons in the crude. In addition to n-alkanes, petroleum crude is composed of major classes of hydrocarbons such as branched and cyclic alkanes, branched and cyclic olefins and various aromatics and nonvolatiles namely asphaltenes. Almost all of the known aromatics without side chains elute after n-hexane (Cg). If the aromatics have long side chains, the linear molecular size increases and the retention volume is reduced. Cyclic alkanes have retention volumes similar to those of aromatics. GPC separates crude on the basis of linear molecular size and the species are spread over 10 to 20 ml retention volume range and almost all of the species are smaller than the polystyrene standard (37A). In other words, the crude has very little asphaltenes. The linear... 

There is a variation in the differences between the enthalpies of formation of RMgBr(soln) and RLi, depending on the class of hydrocarbon substituent. For the sole example, or the average of multiple examples, the enthalpy of formation differences between the organohthium and the corresponding solution-phase Grignard are methyl (259), primary (268), secondary/tertiary (280) and aryl (256)kJmoP. Generally, the... 

Silicon forms a series of hydrides known as silanes, formula SinH2n+2, where n is the number of silicon atoms in the molecule. This general formula for the silicon hydrides is similar to the CnH2n+2 for the alkane class of hydrocarbons. The names, synonyms, CAS Registry numbers, formulas, and molecular weights of the first four hydrides are given below ... 

Alkenes A class of hydrocarbons referred to as olefins. These are usually liquids, but can also be gases at room temperature conditions. These are generally more toxic than the alkanes, but less toxic than aromatics. 

Naphtha Various volatile and often flammable liquid hydrocarbon mixtures used as solvents and diluents consists mainly of hydrocarbons with higher boiling point than gasolines and lower boiling point than kerosene principal component of chemical dispersants used prior to 1970. Naphthenes Class of hydrocarbons with similar physical and chemical properties to alkanes insoluble in water, generally boil at 10-20°C higher than corresponding carbon number alkanes. Narcosis Stupor or unconsciousness produced by chemical substances. 

The catalytic cracking of four major classes of hydrocarbons is surveyed in terms of gas composition to provide a basic pattern of mode of decomposition. This pattern is correlated with the acid-catalyzed low temperature reverse reactions of olefin polymerization and aromatic alkylation. The Whitmore carbonium ion mechanism is introduced and supported by thermochemical data, and is then applied to provide a common basis for the primary and secondary reactions encountered in catalytic cracking and for acid-catalyzed polymerization and alkylation reactions. Experimental work on the acidity of the cracking catalyst and the nature of carbonium ions is cited. The formation of liquid products in catalytic cracking is reviewed briefly and the properties of the gasoline are correlated with the over-all reaction mechanics. 

A whole lubricating oil fraction consists of four major classes of hydrocarbons—namely (a) asphalts and resins, (b) aromatics, (c) naphthenes and branched paraffins, and (d) paraffin wax. Sulfuric acid is remarkably effective for removing undesirable constituents a and b by a combination of reaction and extraction and has little or no effect on wax (which must be removed by other means) or the naphthenic-type materials which comprise a good lubricating oil. 

In this work it was found that while the relative amounts of the main classes of hydrocarbons in the gasoline fraction varied from petroleum to petroleum, the relative amounts of the individual compounds within a given class were of the same magnitude for the different petroleums. 

The gasoline fraction of different crudes may be characterized by specifying the relative amounts of the following five main classes of hydrocarbons n-paraffins, branched paraffins, alkyl cyclopentanes, alkyl cyclohexanes, and alkyl benzenes. 

The gasoline fractions of different crudes are composed of the same hydrocarbons, the differences from one crude to another being essentially in the relative amounts of the foregoing five classes of hydrocarbons. 

With adequate data on one representative petroleum, it appears possible to predict the order of magnitude of the amounts of the individual hydrocarbons in an appropriate fraction of a given naphtha when the relative amounts of the foregoing five classes of hydrocarbons are known, or alternatively for each class, the amount of one of the main components of that class is known. 

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