2 2-1 © 2006 Thomson Learning, Inc. All rights reserved Chapter 2 Alkanes and Cycloalkanes Alkanes and Cycloalkanes

2 2-2 © 2006 Thomson Learning, Inc. All rights reserved Hydrocarbons Hydrocarbon: Hydrocarbon: a compound composed of only carbon and hydrogen.

2 2-3 © 2006 Thomson Learning, Inc. All rights reserved Alkanes Alkanes: Alkanes: hydrocarbons containing only carbon- carbon single bonds. The first two alkanes are methane and ethane.

2 2-4 © 2006 Thomson Learning, Inc. All rights reserved Alkanes line-angle formula: line-angle formula: A line represents a carbon-carbon bond and a vertex and a line terminus represent a carbon atom. Hydrogen atoms are not shown in line-angle formulas.

2 2-5 © 2006 Thomson Learning, Inc. All rights reserved Alkanes Table 2.1 The first 10 alkanes with unbranched chains

2 2-6 © 2006 Thomson Learning, Inc. All rights reserved Constitutional Isomerism Constitutional isomers: Constitutional isomers: compounds that have the same molecular formula but different structural formulas (different connectivity). For the molecular formulas CH 4, C 2 H 6, and C 3 H 8, only one structural formula is possible; there are no constitutional isomers for these molecular formulas. For the molecular formula C 4 H 10, two constitutional isomers are possible.

2 2-7 © 2006 Thomson Learning, Inc. All rights reserved Constitutional Isomerism Problem:Problem: do the structural formulas in each set represent the same compound or constitutional isomers?

2 2-8 © 2006 Thomson Learning, Inc. All rights reserved Constitutional Isomerism Solution:Solution: (a) They represent the same compound. Solution:Solution: (b) They represent constitutional isomers.

2 2-9 © 2006 Thomson Learning, Inc. All rights reserved Constitutional Isomerism Problem:Problem: draw structural formulas for the five constitutional isomers of molecular formula C 6 H 14 Solution:Solution:

© 2006 Thomson Learning, Inc. All rights reserved IUPAC Names The IUPAC name of an alkane with an unbranched chain of carbon atoms consists of two parts: (1) a prefix: the number of carbon atoms in the chain. -ane(2) the suffix -ane: shows that the compound is a saturated hydrocarbon.

© 2006 Thomson Learning, Inc. All rights reserved 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

© 2006 Thomson Learning, Inc. All rights reserved IUPAC Names Alkyl group: Alkyl group: a substituent group derived from an alkane by removal of a hydrogen atom. commonly represented by the symbol R-. ane ylnamed by dropping the -ane from the name of the parent alkane and adding the suffix -yl.

© 2006 Thomson Learning, Inc. All rights reserved IUPAC Names ane. 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. Name and number each substituent on the parent chain; use a hyphen to connect the number to the name.

© 2006 Thomson Learning, Inc. All rights reserved IUPAC Names 4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number.

© 2006 Thomson Learning, Inc. All rights reserved 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.

© 2006 Thomson Learning, Inc. All rights reserved 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 in equivalent positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number.

© 2006 Thomson Learning, Inc. All rights reserved 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

© 2006 Thomson Learning, Inc. All rights reserved Common Names Common names; in this older system, The number of carbon atoms determines the name. The first three alkanes are methane, ethane, and propane. All alkanes of formula C 4 H 10 are called butanes, all those of formula C 5 H 12 are called pentanes, etc. isoFor alkanes beyond propane, iso shows that one end of an otherwise unbranched chain terminates in (CH 3 ) 2 CH- For more complex alkanes, use the IUPAC system.

© 2006 Thomson Learning, Inc. All rights reserved Cycloalkanes Cyclic hydrocarbon: Cyclic hydrocarbon: a hydrocarbon that contains carbon atoms joined to form a ring. Cycloalkane: Cycloalkane: a cyclic hydrocarbon in which all carbons of the ring are saturated. Cycloalkanes of ring sizes ranging from 3 to over 30 carbon atoms are found in nature. Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant in nature.

© 2006 Thomson Learning, Inc. All rights reserved Cycloalkanes Nomenclature cyclo-,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.

© 2006 Thomson Learning, Inc. All rights reserved Conformations - Alkanes Conformation: Conformation: any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond. following are three conformations for a butane molecule.

© 2006 Thomson Learning, Inc. All rights reserved Physical Properties The most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity. The electronegativity difference between carbon and hydrogen is = 0.4 on the Pauling scale. Given this small difference, we classify a C-H bond as nonpolar covalent. Alkanes are nonpolar compounds and the only interaction between their molecules are the very weak London dispersion forces.

© 2006 Thomson Learning, Inc. All rights reserved Physical Properties Melting and boiling points Boiling points of alkanes are lower than those of almost any other type of compound of the same molecular weight. In general, both boiling and melting points of alkanes increase with increasing molecular weight.

© 2006 Thomson Learning, Inc. All rights reserved Physical Properties

© 2006 Thomson Learning, Inc. All rights reserved Physical Properties Alkanes that are constitutional isomers are different compounds and have different physical and chemical properties.

© 2006 Thomson Learning, Inc. All rights reserved Physical Properties Solubility: a case of “like dissolves like”. Alkanes are not soluble in water; they are unable to form hydrogen bonds with water. Alkanes are soluble in each other. Alkanes are also soluble in other nonpolar organic compounds, such as toluene and diethyl ether. Density The average density of the liquid alkanes listed in Table 2.4 is about 0.7 g/mL; that of higher-molecular-weight alkanes is about 0.8 g/mL. All liquid and solid alkanes are less dense than water (1.0 g/mL) and, because they are insoluble in water, they float on water.

© 2006 Thomson Learning, Inc. All rights reserved 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).

© 2006 Thomson Learning, Inc. All rights reserved Reactions Reaction with halogens (halogenation) Halogenation of an alkane is a substitution reaction.

© 2006 Thomson Learning, Inc. All rights reserved The Chlorofluorocarbons Chlorofluorocarbons (CFCs) manufactured under the trade name Freon CFCs are nontoxic, nonflammable, odorless, and noncorrosive. Among the CFCs most widely used were CCl 3 F (Freon- 11) and CCl 2 F 2 (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.

© 2006 Thomson Learning, Inc. All rights reserved CFC Replacements Chlorofluorocarbons (CFCs) cause destruction of the Earth’s stratospheric ozone layer. The most prominent replacements are the hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). These compounds are chemically more reactive than CFCs and are destroyed before they reach the stratosphere.

© 2006 Thomson Learning, Inc. All rights reserved Sources of Alkanes Natural gas 90 to 95 percent methane 5 to 10 percent ethane, and a mixture of other relatively low-boiling alkanes, chiefly propane, butane, and 2-methylpropane. Petroleum A thick, viscous liquid mixture of thousands of compounds, most of them hydrocarbons formed from the decomposition of marine plants and animals.

© 2006 Thomson Learning, Inc. All rights reserved End Chapter 2