Aliphatic hydrocarbons Alkanes Alkenes Alkynes

On the basis of structure, hydrocarbons are divided into two main classes, aliphatic and aromatic. Aliphatic hydrocarbons are further divided into families alkanes, alkenes, alkynes, and their cyclic analogs. Figure 1-6 shows the relationships between some of these classes and families of hydrocarbons. 

Open-chain aliphatic hydrocarbons constitute alkanes, alkenes, alkynes, and their isomers. Alkanes have the general formula C H2 +2, where n is the number of carbon atoms in the molecnles, snch as methane, propane, n-pentane, and isooctane. Alkenes or olefins are nnsaturated compounds, characterized by one or more double bonds between the carbon atoms. Their general formula is C H2 . Examples are ethylene, 1-butene, and... 

Saturated aliphatic hydrocarbons (alkanes) generally do not react with chlorosulfonic acid, although the more nucleophilic unsaturated hydrocarbons (alkenes and alkynes) are often reactive and may sometimes be directly sulfonated by the reagent. 

As shown in Figure 20.1 , we can classify hydrocarbons into four different types alkanes, alkenes, alkynes, and aromatic hydrocarbons. Alkanes, alkenes, and alkynes— also called aliphatic hydrocarbons—are differentiated based on the kinds of bonds between carbon atoms. (We discuss aromatic hydrocarbons in detail in Section 20.7.) As shown in Table 20.1, alkanes have only single bonds between carbon atoms, alkenes have a double bond, and alkynes have a triple bond. 

Aliphatic hydrocarbons include three major groups alkanes alkenes and alkynes Alkanes are hydrocarbons m which all the bonds are single bonds alkenes contain at least one carbon-carbon double bond and alkynes contain at least one carbon-carbon... 

We will return to the orbital hybridization model to discuss bonding m other aliphatic hydrocarbons—alkenes and alkynes—later m the chapter At this point how ever we 11 turn our attention to alkanes to examine them as a class m more detail... 

Monocyclic Aliphatic Hydrocarbons. Monocyclic aliphatic hydrocarbons (with no side chains) are named by prefixing cyclo- to the name of the corresponding open-chain hydrocarbon having the same number of carbon atoms as the ring. Radicals are formed as with the alkanes, alkenes, and alkynes. Examples ... 

Aliphatic (Section 3.2) A nonaromalic hydrocarbon such as a simple alkane, alkene, or alkyne. 

Because the types of carbon-carbon bonds present in the molecule tend to dominate its properties, an aliphatic hydrocarbon is first classified as an alkane, alkene, or alkyne. Then the longest chain of carbon atoms is used to form the root of the name. Other hydrocarbon groups attached to the longest chain are named as side chains. 

Scientists classify hydrocarbons as either aliphatic or aromatic. An aliphatic hydrocarbon contains carbon atoms that are bonded in one or more chains and rings. The carbon atoms have single, double, or triple bonds. Aliphatic hydrocarbons include straight chain and cyclic alkanes, alkenes, and alkynes. An aromatic hydrocarbon is a hydrocarbon based on the aromatic benzene group. You will encouter this group later in the section. Benzene is the simplest aromatic compound. Its bonding arrangement results in special molecular stability. 

Aromatic carbon-carbon bonds are intermediate in length (139 pm) between that of a C—C single bond (154 pm) and a C—C double bond (134 pm). An aromatic ring is characterized by a delocalized r system in contrast to the localized bonds present in aliphatic hydrocarbons. 18.65 The empirical formula is C2H5. The compound is an alkane the molecular formula might be C4H10, which matches the formula for alkanes (C H2n + 2). It is not an alkene or alkyne, because they both have (mole H) (mole C) ratios less than 2.5. 

Hydrocarbons are also divided into classes called aliphatic and aromatic. Aromatic hydrocarbons are related to benzene and are always cyclic. Aliphatic hydrocarbons may be open-chain or cyclic. Aliphatic cyclic hydrocarbons are called alicyclic. Aliphatic hydrocarbons are one of three types alkanes, alkenes, and alkynes. 

The name "aliphatic" comes from the Greek word aleiphatos, meaning "fat." Early chemists found these compounds to be less dense than water and insoluble in water, like fats. "Aliphatic" now refers to the classes of hydrocarbons called alkanes, alkenes, and alkynes. 

The hydrocarbons we have examined thus far— including the alkanes, alkenes, and alkynes, as well as the conjugated dienes and polyenes of Chapter 16— have been aliphatic hydrocarbons. In Chapter 17, we continue our study of conjugated systems with aromatic hydrocarbons. 

From our study so far vve know what kind of chemical properties to expect of an aliphatic hydrocarbon, that is, of an alkane, alkene, or alkyne. We know what kind of chemical behavior to expect of the parent aromatic hydrocarbon, benzene. Many important compounds are not just aliphatic or Just aromatic, however, but contain both aliphatic and aromatic units hydrocarbons of this kind are known collectively as arenes. Ethylbenzene for example, contains a benzene ring and an aliphatic side chain. 

Organic compounds that contain benzene rings as part of their structure are called aromatic compounds. The term aromatic was originally used because many of the benzene-related compounds known in the nineteenth century were found in pleasant-smelling oils that came from spices, fruits, and other plant parts. Hydrocarbons such as the alkanes, alkenes, and alkynes are called aliphatic compounds to distinguish them from aromatic compounds. The term aliphatic comes from the Greek word for fat, which is aleiphatos. Early chemists obtained aliphatic compounds by heating animal fats. 

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