Chapter 2 Alkanes and Cycloalkanes Hydrocarbons Hydrocarbon a

Chapter 2 Alkanes and Cycloalkanes

Hydrocarbons • Hydrocarbon: a compound composed of only carbon and hydrogen

Alkanes • Alkanes: hydrocarbons containing only carbon- carbon single bonds (saturated hydrocarbons) • the first two alkanes are methane and ethane

Alkanes • line-angle formula: • a line represents a carbon-carbon bond an angle represents a carbon atom • a line ending in space represents a -CH 3 group • hydrogen atoms are not shown in line-angle formulas

Alkanes • the first 10 alkanes with unbranched chains

Constitutional Isomerism • Constitutional isomers: compounds that have the same molecular formula but different structural formulas • CH 4, C 2 H 6, and C 3 H 8 only have one possible structural formula so they don’t have constitutional isomers • for the molecular formula C 4 H 10, two constitutional isomers are possible

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 • (2) the suffix -ane: -ane shows that the compound is a saturated hydrocarbon

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 • Alkyl group: a substituent group derived from an alkane by removal of a hydrogen atom • 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, 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

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 in 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

Common Names • Common names; in this older system • the number of carbon atoms determines the name • the first three alkanes are methane, 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. • for 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

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 • 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

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 - Alkanes • 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

Cyclopentane • 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

Cyclohexane • the more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial

Cis-Trans Isomers • Same molecular formula • Same connectivity of atoms • Different arrangement of atoms in space due to restricted rotation

Cis-Trans Isomers • Cis: on the same side • Trans: across from�� • we can view a cyclopentane ring edge-on

Cis-Trans Isomers • alternatively, we can view it from above

Cis-Trans Isomers • to determine cis-trans isomers if cis-trans isomers are possible, we can view a cyclohexane ring as a planar hexagon • because cis-trans isomers differ in the orientation of their atoms in space, they are stereoisomers • cis-trans isomers are one type of stereoisomers

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 2. 5 - 2. 1 = 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

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

Physical Properties

Physical Properties • alkanes that are constitutional isomers are different compounds and have different physical and chemical properties

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 11. 4 is about 0. 7 g/m. L; that of higher-molecular-weight alkanes is about 0. 8 g/m. L • all liquid and solid alkanes are less dense than water (1. 0 g/m. L) and, because they are insoluble in water, they float on water

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 • Reaction with halogens (halogenation) • halogenation of an alkane is a substitution reaction

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

Chapter 2 End Chapter 2