{"id":7768,"date":"2021-06-08T21:57:40","date_gmt":"2021-06-08T21:57:40","guid":{"rendered":"https:\/\/opentextbc.ca\/introductorychemistry\/chapter\/hydrocarbons\/"},"modified":"2021-10-13T16:04:53","modified_gmt":"2021-10-13T16:04:53","slug":"hydrocarbons","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/introductorychemistry\/chapter\/hydrocarbons\/","title":{"raw":"Hydrocarbons","rendered":"Hydrocarbons"},"content":{"raw":"[latexpage]\r\n
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Learning Objectives<\/p>\r\n\r\n<\/header>\r\n

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  1. Identify alkanes, alkenes, alkynes, and aromatic compounds.<\/li>\r\n \t
  2. List some properties of hydrocarbons.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\nThe simplest organic compounds are those composed of only two elements: carbon and hydrogen. These compounds are called hydrocarbons<\/strong>. Hydrocarbons themselves are separated into two types: aliphatic hydrocarbons and aromatic hydrocarbons. Aliphatic hydrocarbons<\/strong> are hydrocarbons based on chains of C atoms. There are three types of aliphatic hydrocarbons. Alkanes<\/strong>\u00a0are aliphatic hydrocarbons with only single covalent bonds. Alkenes<\/strong>\u00a0are aliphatic\u00a0hydrocarbons that contain at least one C\u2013C double bond, and alkynes<\/strong>\u00a0are aliphatic\u00a0hydrocarbons that contain a C\u2013C triple bond. Occasionally, we find an aliphatic hydrocarbon with a ring of C atoms; these hydrocarbons are called cycloalkanes<\/strong> (or cycloalkenes or cycloalkynes).\r\n\r\nAromatic hydrocarbons<\/strong>, such as benzene, are flat-ring systems that contain continuously overlapping p<\/em> orbitals. Electrons in the benzene ring have special energetic properties that give benzene physical and chemical properties that are markedly different from alkanes. Originally, the term aromatic<\/strong> was used to describe this class of compounds because they were particularly fragrant. However, in modern chemistry the term aromatic denotes the presence of a very stable ring that imparts different and unique properties to a molecule.\r\n\r\nThe simplest alkanes have their C atoms bonded in a straight chain; these are called normal alkanes<\/em>. They are named according to the number of C atoms in the chain. The smallest alkane is methane:\r\n

    [latex]\\chemfig{H-C(-[:90]H)(-[:-90]H)-H}[\/latex]<\/p>\r\n\r\n\r\n[caption id=\"attachment_3271\" align=\"aligncenter\" width=\"274\"]\"Four Figure 16.1 \"Three-Dimensional Representation of Methane.\" The methane molecule is three-dimensional, with the H atoms in the positions of the four corners of a tetrahedron.[\/caption]\r\n\r\nTo make four covalent bonds, the C atom bonds to four H atoms, making the molecular formula for methane CH4<\/sub>. The two-dimensional diagram for methane is misleading, however; the four covalent bonds that the C atom makes are oriented three-dimensionally toward the corners of a tetrahedron. A better representation of the methane molecule is shown in Figure 16.1 \"Three-Dimensional Representation of Methane.\"\r\n\r\nThe next-largest alkane has two C atoms that are covalently bonded to each other. For each C atom to make four covalent bonds, each C atom must be bonded to three H atoms. The resulting molecule, whose formula is C2<\/sub>H6<\/sub>, is ethane:\r\n

    [latex]\\chemfig{H-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}[\/latex]<\/p>\r\nPropane has a backbone of three C atoms surrounded by H atoms. You should be able to verify that the molecular formula for propane is C3<\/sub>H8<\/sub>:\r\n

    [latex]\\chemfig{H-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}[\/latex]<\/p>\r\nThe diagrams we have seen so far representing alkanes are fairly simple Lewis structures. However, as molecules get larger, the Lewis structures become more and more complex. One way around this is to use a condensed structure,<\/strong> which lists the formula of each C atom in the backbone of the molecule. For example, the condensed structure\u00a0for ethane is CH3<\/sub>CH3<\/sub>, while it is CH3<\/sub>CH2<\/sub>CH3<\/sub>\u00a0for propane.\u00a0Table 16.1 \"The First 10 Alkanes\" gives the molecular formulas, the condensed structural formulas, and the names of the first 10 alkanes.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
    Table 16.1 The First 10 Alkanes<\/caption>\r\n
    Molecular Formula<\/th>\r\nCondensed Structural Formula<\/th>\r\nName<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
    CH4<\/sub><\/td>\r\nCH4<\/sub><\/td>\r\nmethane<\/td>\r\n<\/tr>\r\n
    C2<\/sub>H6<\/sub><\/td>\r\nCH3<\/sub>CH3<\/sub><\/td>\r\nethane<\/td>\r\n<\/tr>\r\n
    C3<\/sub>H8<\/sub><\/td>\r\nCH3<\/sub>CH2<\/sub>CH3<\/sub><\/td>\r\npropane<\/td>\r\n<\/tr>\r\n
    C4<\/sub>H10<\/sub><\/td>\r\nCH3<\/sub>CH2<\/sub>CH2<\/sub>CH3<\/sub><\/td>\r\nbutane<\/td>\r\n<\/tr>\r\n
    C5<\/sub>H12<\/sub><\/td>\r\nCH3<\/sub>CH2<\/sub>CH2<\/sub>CH2<\/sub>CH3<\/sub><\/td>\r\npentane<\/td>\r\n<\/tr>\r\n
    C6<\/sub>H14<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)4<\/sub>CH3<\/sub><\/td>\r\nhexane<\/td>\r\n<\/tr>\r\n
    C7<\/sub>H16<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)5<\/sub>CH3<\/sub><\/td>\r\nheptane<\/td>\r\n<\/tr>\r\n
    C8<\/sub>H18<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)6<\/sub>CH3<\/sub><\/td>\r\noctane<\/td>\r\n<\/tr>\r\n
    C9<\/sub>H20<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)7<\/sub>CH3<\/sub><\/td>\r\nnonane<\/td>\r\n<\/tr>\r\n
    C10<\/sub>H22<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)8<\/sub>CH3<\/sub><\/td>\r\ndecane<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nBecause alkanes have the maximum number of H atoms possible according to the rules of covalent bonds, alkanes are also referred to as saturated hydrocarbons<\/strong>.\r\n\r\nAlkenes have a C\u2013C double bond. Because they have less than the maximum number of H atoms possible, they are called unsaturated hydrocarbons<\/strong>. The smallest alkene \u2014 ethene \u2014 has two C atoms and is also known by its common name, ethylene:\r\n

    [latex]\\chemfig{H-[:45]C(-[:135]H)=C(-[:45]H)(-[:-45]H)}[\/latex]<\/p>\r\nThe next largest alkene \u2014 propene \u2014 has three C atoms with a C\u2013C double bond between two of the C atoms. It is also known as propylene:\r\n

    [latex]\\chemfig{H-[:45]C(-[:135]H)=C(-[:-45]H)-[:45]C(-[:135]H)(-[:-45]H)-[:45]H}[\/latex]<\/p>\r\nWhat do you notice about the names of alkanes and alkenes? The names of alkenes are the same as their corresponding alkanes except that the suffix (ending) is -ene<\/em>, rather than -ane<\/em>. Using a stem known as the parent chain<\/em> to indicate the number of C atoms in a molecule and an ending to represent the type of organic compound is common in organic chemistry, as we shall see.\r\n\r\nWith the introduction of the next alkene, butene, we begin to see a major issue with organic molecules: choices. With four C atoms, the C\u2013C double bond can go between the first and second C atoms, like so:\r\n

    [latex]\\chemfig{C(-[:135]H)(-[:-135]H)=C(-[:90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}[\/latex]<\/p>\r\nOr, the double bond can go between the second and third C atoms, like so:\r\n

    [latex]\\chemfig{C(-[:-135]H)(-[:135]C(-[:45]H)(-[:135]H)(-[:225]H))=C(-[:-45]H)-[:45]C(-[:-45]H)(-[:45]H)(-[:135]H)}[\/latex]<\/p>\r\n(A double bond between the third and fourth C atoms is the same as having it between the first and second C atoms, only flipped over.)\r\n\r\nThe rules of naming in organic chemistry require that these two substances have different names. The first molecule is named 1-butene<\/em>, while the second molecule is named 2-butene<\/em>. The number between the parent-chain name and suffix is known as a locant<\/strong>, and indicates on which carbon the double bond originates. The lowest possible number is used to number a feature in a molecule; hence, calling the second molecule 3-butene<\/em> would be incorrect. Numbers are common parts of organic chemical names because they indicate which C atom in a chain contains a distinguishing feature. When the double bond (or other functional group) is located on the first carbon, it is common practice for some authors to leave out the locant. For example, if butene were written without a locant, you should assume it refers to 1-butene<\/em>, not 2-butene<\/em>.\r\n\r\nThe compounds 1-butene and 2-butene have different physical and chemical properties, even though they have the same molecular formula \u2014 C4<\/sub>H8<\/sub>. Different molecules with the same molecular formula are called isomers<\/strong>. Isomers are common in organic chemistry and contribute to its complexity.\r\n

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    Example 16.1<\/p>\r\n\r\n<\/header>\r\n

    \r\n\r\nBased on the names for the butene molecules, propose a name for this molecule.\r\n

    [latex]\\chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C(-[:-90]H)=C(-[:-90]H)-C(-[:-90]H)(-[:90]H)-C(-[:-90]H)(-[:90]H)-H}[\/latex]<\/p>\r\nSolution<\/em>\r\nWith five C atoms, we will use the pent<\/em>- parent name, and with a C\u2013C double bond, this is an alkene, so this molecule is a pentene. In numbering the C atoms, we use the number 2<\/em> because it is the lower possible label. So this molecule is named 2-pentene.\r\n\r\nTest Yourself<\/em>\r\nBased on the names for the butene molecules, propose a name for this molecule.\r\n

    [latex]\\chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C(-[:-90]H)(-[:90]H)-C(-[:-90]H)=C(-[:-90]H)-C(-[:-90]H)(-[:90]H)-C(-[:-90]H)(-[:90]H)-H}[\/latex]<\/p>\r\nAnswer<\/em>\r\n3-hexene\r\n\r\n<\/div>\r\n<\/div>\r\nAlkynes, with a C\u2013C triple bond, are named similarly to alkenes except their names end in -yne<\/em>. The smallest alkyne is ethyne, which is also known as acetylene:\r\n

    [latex]\\chemfig{H-C~C-H}[\/latex]<\/p>\r\nPropyne has this\u00a0structure:\r\n

    [latex]\\chemfig{H-C~C-C(-[:90]H)(-[:-90]H)-H}[\/latex]<\/p>\r\nWith butyne, we need to start numbering the position of the triple bond, just as we did with alkenes.\r\n\r\nThis is 1-butyne:\r\n

    [latex]\\chemfig{H-C~C-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}[\/latex]<\/p>\r\nAnd this is 2-butyne:\r\n

    [latex]\\chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C~C-C(-[:90]H)(-[:-90]H)-H}[\/latex]<\/p>\r\nBenzene is an aromatic compound composed of six C atoms in a ring, with alternating single and double C\u2013C bonds:\r\n

    [latex]\\chemfig{C*6((-H)-C(-H)=C(-H)-C(-H)=C(-H)-C(-H)=)}[\/latex]<\/p>\r\nThe alternating single and double C\u2013C bonds give the benzene ring a special stability, and it does not react like an alkene as might be expected.\r\n\r\nAs fundamental as hydrocarbons are to organic chemistry, their properties and chemical reactions are rather mundane. Most hydrocarbons are nonpolar because of the close electronegativities of C and H atoms. As such, they dissolve only sparingly in H2<\/sub>O and other polar solvents. Small hydrocarbons, such as methane and ethane, are gases at room temperature, while larger hydrocarbons, such as hexane and octane, are liquids. Even larger hydrocarbons, like hentriacontane (C31<\/sub>H64<\/sub>),\u00a0are solids at room temperature and have a soft, waxy consistency.\r\n\r\nHydrocarbons are rather unreactive, but they do participate in some classic chemical reactions. One common reaction is substitution with a halogen atom by combining a hydrocarbon with an elemental halogen. Light is sometimes used to promote the reaction, such as this one between methane and chlorine:\r\n

    [latex]\\ce{CH4}+\\ce{Cl2}\\xrightarrow{\\text{light}}\\ce{CH3Cl}+\\ce{HCl}[\/latex]<\/p>\r\nHalogens can also react with alkenes and alkynes, but the reaction is different. In these cases, the halogen molecules react with the C\u2013C double or triple bond and attach\u00a0onto each C atom involved in the multiple bonds. This reaction is called an addition reaction<\/strong>. One example is:\r\n

    [latex]\\chemfig{\\ce{CH2}=\\ce{CH2}}+\\ce{Cl2}\\rightarrow \\chemfig{H-C(-[:90]Cl)(-[:-90]H)-C(-[:90]Cl)(-[:-90]H)-H}[\/latex]<\/p>\r\nThe reaction conditions are usually mild; in many cases, the halogen reacts spontaneously with an alkene or an alkyne.\r\n\r\nHydrogen can also be added across a multiple bond; this reaction is called a hydrogenation reaction<\/strong>. In this case, however, the reaction conditions may not be mild; high pressures of H2<\/sub> gas may be necessary. A platinum or palladium catalyst is usually employed to get the reaction to proceed at a reasonable pace:\r\n

    [latex]\\chemfig{\\ce{CH2}=\\ce{CH2}}+\\ce{H2}\\xrightarrow{\\text{metal catalyst}}\\ce{CH3CH3}[\/latex]<\/p>\r\nBy far the most common reaction of hydrocarbons is combustion, which is the combination of a hydrocarbon with O2<\/sub> to make CO2<\/sub> and H2<\/sub>O. The combustion of hydrocarbons is accompanied by a release of energy and is a primary source of energy production in our society (Figure 16.2 \"Combustion\"). The combustion reaction for gasoline, for example, which can be represented by C8<\/sub>H18<\/sub>, is as follows:\r\n

    [latex]\\ce{2C8H18}+\\ce{25O2}\\rightarrow\\ce{16CO2}+\\ce{18H2O}+\\thicksim5060\\text{ kJ}[\/latex]<\/p>\r\n\r\n\r\n[caption id=\"attachment_3273\" align=\"aligncenter\" width=\"243\"]\"A Figure 16.2 \"Combustion.\" The combustion of hydrocarbons is a primary source of energy in our society. This image depicts the first gas from the Oselvar module on the Ula platform in Norway on April 14, 2012.[\/caption]\r\n\r\n

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    Key Takeaways<\/p>\r\n\r\n<\/header>\r\n

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