Chapter 2 Alkanes

Hydrocarbons Compounds that contains only Carbons and Hydrogens Two types ◦ Saturated hydrocarbons  Carbon-Carbon single bond ◦ Unsaturated hydrocarbons  One or more Carbon-Carbon double bonds or triple bonds or benzene rings

2.1 Alkanes Saturated hydrocarbon Also referred as aliphatic hydrocarbonds General formula: CnH2n+2

2.2 How do we write structural formulas of 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.

Table 2.1 The first 10 alkanes with unbranched chains

2.3 What are Constitutional Isomers? Constitutional isomers: Constitutional isomers: Compounds that have the same molecular formula but different structural formulas (a different connectivity of their atoms). 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.

2.3Constitutional Isomers Problem: Problem: Do the structural formulas in each set represent the same compound or constitutional isomers?

2.3 Constitutional Isomers Problem: Problem: Do the line-angle formulas in each set represent the same compound or constitutional isomers?

2.3 Constitutional Isomerism Problem: Problem: Draw line-angle for the three constitutional isomers of molecular formula C 5 H 12.

2.4 How Do We Name Alkanes? 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. -ane (2) The suffix -ane: shows that the compound is a saturated hydrocarbon.

Alkyl Groups Alkyl group: Alkyl group: A substituent group derived from an alkane by removal of a hydrogen atom (Table 2.3). Commonly represented by the symbol R-. ane yl Named by dropping the -ane from the name of the parent alkane and adding the suffix -yl.

Alkyl groups Table 2.3

Rules for naming alkanes 1. Determine the longest chain of carbon-carbon bonding without disconnecting the chain (parent) then give its name that ends with –ane 2.Determine the number of substituents and identify them. (alkyl ends with –yl) 3. Determine the position(s) of the carbons that contains those subsituents. 4. If more than one of the same substituent occurred then indicate them with Latin prefixes (di-, tri-, tetra-, etc…) C2 and C4 4. 2,4-Dimethyl

Rules for naming alkanes 1. If more than two different substituents, list them in alphabetical order 6. Indicate all position of carbons that has a substituent even if they’re the same subsituents

Rules for naming alkanes 7. Name the alkanes in this order 1.Postition-substituent then parent 2.Give the substituent lowest position as possible 3.Use hyphen to separate each substituents 4.Use comma to separate position of carbons that has the same substituents

Examples Write the molecular formula and IUPAC name for these alkanes

Examples Draw a line-angle formula for these alkanes ◦ 2,3-dimethylbutane ◦ 4-isobutyl-2,5-dimethyloctane

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 C 4 H 10 are named butanes, all those with the molecular formula C 5 H 12 are named pentanes, etc. iso 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.

2.5 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.

Petroleum Figure 2.3 Fractional distillation of petroleum.

2.6 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 (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.

2.6 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.

Examples Give an IUPAC name for this molecule Draw its structure using line-angle formula ethylcyclohexane

2.7 Conformations of Alkanes Conformation: Conformation: Any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond. Figure 2.5 Three conformations for a butane molecule.

2.7 Cyclopentane Figure 2.5 The most stable conformation of a cyclopentane ring is an envelope conformation.

2.7 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, equatorial. six C-H bonds are equatorial. axial. 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

2.8 Cis/Trans Isomers Cis: Cis: on the same side of the ring. Trans: Trans: on the opposite side of the ring, In drawing cis-trans isomers of disubstituted cyclopentanes, we can view a cyclopentane ring edge- on.

2.8 Cis-Trans Isomers Alternatively, we can view the cyclopentane ring from above. Substituents are shown by solid wedges (above) or dashed wedges (below).

2.8 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. stereoisomers. Because cis-trans isomers differ in the orientation of their atoms in space, they are stereoisomers. Cis-trans isomers are one type of stereoisomer.

Examples Following is a chair conformation of cyclohexane with carbon atoms numbered 1 through 6. ◦ Draw methyl groups that are equatorial on carbon 1,2 and 4 ◦ Draw methyl group that is axial on C2, C3 and equatorial on C6

Examples Does the following cycloalkanes show cis-trans isomerism? Why? For each that does, draw both isomers ◦ 1,3-dimethylcyclopentane ◦ Ethylcyclopentane ◦ 1,3-dimethylcyclobutane

2.9 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.

Melting Points 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. More branching, the lower the boiling point

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 because they are unable to form hydrogen bonds with water. ◦ Liquid 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/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 both less dense and insoluble, 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).

2.10Reactions of Alkanes Reaction with halogens (halogenation) Halogenation of an alkane is a substitution reaction. Hydrogen from alkane carbon is replaced with a halogen

Reactions of Alkanes

IUPAC names of haloalkanes Halogen atoms act as subsituent ◦ Fluoro- ◦ Chloro- ◦ Bromo- ◦ Iodo- Apply the same rules as for alkyl substituents

Examples Predict all possible products Reaction of propane with bromine gives two products, each with molecular formula C 3 H 7 Br. Draw structural formulas for these two compounds and give each an IUPAC name

2.10 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.

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.