Hydrocarbons Hydrocarbon: A compound composed only of carbon and hydrogen. Alkanes Alkanes: Hydrocarbons that contain only carbon-carbon single bonds. • The first two alkanes are methane and ethane. Alkanes Line-angle formula • A line represents a carbon-carbon bond. • A vertex and a line terminus represent a carbon atom. • Hydrogen atoms are not shown in line-angle formulas. Alkanes Table 11.1 The first 10 alkanes with unbranched chains Constitutional Isomerism Constitutional isomers: Compounds that have the same molecular formula but different structural formulas (a different connectivity of their atoms). • For the molecular formulas CH4, C2H6, and C3H8, only one structural formula is possible. There are no constitutional isomers for these molecular formulas. • For the molecular formula C4H10, two constitutional isomers are possible. Constitutional Isomers Problem: Do the structural formulas in each set represent the same compound or constitutional isomers? Constitutional Isomerism Solution: (a) They represent the same compound. Solution: (b) They represent constitutional isomers. Constitutional Isomerism Problem: Draw structural formulas for the five constitutional isomers of molecular formula C6H14. Constitutional Isomerism Problem: Draw structural formulas for the five constitutional isomers of molecular formula C6H14. Solution: 2 1 3 4 6 5 Six carbons in an unbranched chain 4 1 2 3 2 5 1 4 3 5 Five carbons in a chain; one carbon as a branch 1 2 3 4 2 1 4 3 Four carbons in a chain; two carbons as branches IUPAC Names The name of an alkane with a branched chain of carbon atom consists of: • a parent name: the longest chain of carbon atoms. • substituent names: the groups bonded to the parent chain. IUPAC Names The IUPAC name of an alkane with an unbranched chain of carbon atoms consists of two parts: (1) A prefix shows the number of carbon atoms in the chain. (2) The suffix -ane: shows that the compound is a saturated hydrocarbon. Alkyl Groups Alkyl group: A substituent group derived from an alkane by removal of a hydrogen atom (Table 11.3). • Commonly represented by the symbol R-. • Named by dropping the -ane from the name of the parent alkane and adding the suffix -yl. IUPAC Names 1. The name for an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms and the ending -ane. 2. For branched-chain alkanes, the longest chain of carbon atoms is the parent chain and its name is the root name. 3. Give each substituent on the parent chain a name and a number. Use a hyphen to connect the number to the name. IUPAC Names 4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number. IUPAC Names 5. If the same substituent occurs more than once: • Number the parent chain from the end that gives the lower number to the substituent encountered first. • Indicate the number of times the substituent occurs by a prefix di-, tri-, tetra-, penta-, hexa-, and so on. • Use a comma to separate position numbers. IUPAC Names 6. If there are two or more different substituents: • List them in alphabetical order. • Number the chain from the end that gives the lower number to the substituent encountered first. • If there are different substituents at equivalent positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number. IUPAC Names 7. Do not include the prefixes di-, tri-, tetra-, and so on or the hyphenated prefixes sec- and tert- in alphabetizing; • Alphabetize the names of substituents first, and then insert these prefixes. In the following example, the alphabetizing parts are ethyl and methyl, not ethyl and dimethyl. Common Names Common names; an older system • The number of carbon atoms determines the name. • The first three alkanes are methane, ethane, and propane. • All alkanes with the molecular formula C4H10 are named butanes, all those with the molecular formula C5H12 are named pentanes, etc. • For alkanes beyond propane, iso shows that one end of an otherwise unbranched chain terminates in (CH3)2CH- For more complex alkanes, use the IUPAC system. Source of Hydrocarbons: Petroleum Figure 11.3 Fractional distillation of petroleum. Cycloalkanes Cyclic hydrocarbon: A hydrocarbon that contains carbon atoms joined to form a ring. Cycloalkane: A cyclic hydrocarbon in which all carbons of the ring are saturated (have only carbon-carbon single bonds). Cycloalkanes with ring sizes of from 3 to over 30 carbon atoms are found in nature. • Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant in nature. Cycloalkanes Nomenclature • To name a cycloalkane, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring. • If there is only one substituent on the ring, there is no need to give it a location number. • If there are two substituents, number the ring beginning with the substituent of lower alphabetical order. Conformations of Alkanes Conformation: Any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond. • Figure 11.4 Three conformations for a butane molecule. Cyclopentane Figure 11.5 The most stable conformation of a cyclopentane ring is an envelope conformation. Cyclohexane The most stable conformation of a cyclohexane ring is the chair conformation. • All bond angles are approximately 109.5°. Cyclohexane In a chair conformation, • six C-H bonds are equatorial. • six C-H bonds are axial. • Figure 11.6 Chair conformation of cyclohexane Cyclohexane The more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial. Figure 11.7 Methylcyclohexane Cis/Trans Isomers Cis: on the same side of the ring. Trans: on the opposite side of the ring, • In drawing cis-trans isomers of disubstituted cyclopentanes, we can view a cyclopentane ring edgeon. Cis-Trans Isomers • Alternatively, we can view the cyclopentane ring from above. Substituents are shown by solid wedges (above) or dashed wedges (below). Cis-Trans Isomerism • To determine cis-trans isomers in disubstituted cyclohexanes, we can view a cyclohexane ring either as a planar hexagon or viewed from above. • Because cis-trans isomers differ in the orientation of their atoms in space, they are stereoisomers. • Cis-trans isomers are one type of stereoisomer. • In Ch 15, we study another type called enantiomers. Physical Properties Physical Properties • Alkanes that are constitutional isomers are different compounds and have different physical and chemical properties. Reactions Oxidation (combustion) • Oxidation of hydrocarbons, including alkanes and cycloalkanes, is the basis for their use as energy sources for heat [natural gas, liquefied petroleum gas (LPG), and fuel oil] and power (gasoline, diesel fuel, and aviation fuel). Reactions of Alkanes Reaction with halogens (halogenation) • Halogenation of an alkane is a substitution reaction. The Chlorofluorocarbons Chlorofluorocarbons (CFCs) • Manufactured under the trade name Freon. • CFCs are nontoxic, nonflammable, odorless, and noncorrosive. • Among the CFCs most widely used were CCl3F (Freon11) and CCl2F2 (Freon-12). CFCs were used as; • Heat-transfer agents in refrigeration systems. • Industrial cleaning solvents to prepare surfaces for coatings and to remove cutting oils from millings. • Propellants for aerosol sprays. CFC Replacements Chlorofluorocarbons (CFCs) cause destruction of the Earth’s stratospheric ozone layer. The most prominent replacements are the hydrofluorocarbons (HFCs) and the hydrochlorofluorocarbons (HCFCs). • These compounds are chemically more reactive than CFCs and are destroyed before they reach the stratosphere.