Chapter 12 Alkanes

Chapter 12 Alkanes 12.1 Alkanes

Alkanes Alkanes: • Contain only C and H. • Have only single C—C bonds. • Have a general formula of CnH2n +2. C Atoms H Atoms Formula 1 2(1) + 2 = 4 CH4 3 2(3) + 2 = 8 C3H8 6 2(6) + 2 = 14 C6H14

Structural Formulas Structural formulas show the arrangement of atoms in an organic compound. • In expanded structural formulas, all the individual bonds are drawn. • In condensed structural formulas, each carbon is written with the H atoms connected to it. H H | | H—C— = CH3— —C— = —CH2 — | | H H

Line-Bond Formulas • Because each C atom has a tetrahedral arrangement, the geometrical arrangement of carbon atoms is not a straight line. • A line-bond formula abbreviates the carbon chain and shows only the zigzag pattern of bonds from carbon atom to carbon atom.

Summary of Formulas

IUPAC System of Naming Alkanes • The IUPAC (International Union of Pure and Applied Chemistry) establishes the rules for naming organic compounds. • Alkanes are named with –ane endings. • The first four alkanes are methane, ethane, propane, and butane. • The names of longer carbon chains use Greek prefixes such as pent- for 5 C atoms and hex- for 6 C atoms.

Names of Continuous-Chain Alkanes

Learning Check A. Give the name of each compound: 1) CH3—CH3 2) CH3—CH2—CH3 3) CH3—CH2—CH2—CH2—CH2—CH3 B. Write the condensed structural formula of pentane.

Solution A. Give the name of each compound: 1) CH3—CH3 ethane 2) CH3—CH2—CH3 propane 3) CH3—CH2—CH2—CH2—CH2—CH3 hexane B. Write the condensed structural formula of pentane. CH3—CH2—CH2—CH2—CH3

Learning Check A. What is the condensed formula for H H H H | | | | H —C—C—C—C—H | | | | H H H H B. What is its molecular formula? C. What is its name?

Solution A. CH3—CH2—CH2—CH3 B. C4H10 C. butane

Conformations • As the groups attached to a C-C single bond rotate around the bond, their relative arrangements has different conformations.

Conformations of Butane • Rotation of the end CH3- provides different conformations for butane.

Drawing Structures for Butane

Learning Check For propane, write the: 1) Molecular formula 2) Expanded structural formula 3) Condensed structural formula (two conformations) 4) Line-bond formula

Solution 1) Molecular formula C3H8 2) Expanded structural formula H H H | | | H —C—C—C—H | | | H H H 3) Condensed structural formula (two conformations) CH3—CH2—CH3 CH3 | CH3—CH2 4) Line-bond formula or

Chapter 12 Alkanes 12.2 IUPAC Naming System for Alkanes 12.3 Drawing Structural Formulas CH3CH3 | | CH3—CH—CH2—CH—CH3 2,4-dimethylpentane

Branched-Chain Alkanes • In a branched-chain alkane, a side group called a branch or a substituent is attached to a carbon chain. branch

Classification of Carbon Atoms Carbon atoms are classified according to the number of attached carbon atoms. • Primary (1°) bonds to one carbon atom. • Secondary(2°) bonds to two carbon atoms. • Tertiary(3°) bonds to three carbon atoms. CH3 | CH3 —CH2—CH2—CH3 CH3—CH—CH3 secondaryprimary tertiary

Alkyl Groups An alkyl group: • Is composed of one or more carbon atoms attached to a carbon chain. • Is derived from the corresponding alkane by removing one hydrogen. • Is named by replacing the –aneending of the corresponding alkane with –yl. • Derived from methane is methyl and from ethane is ethyl.

Alkyl Groups

Naming Branched-Chain Alkanes The names of branched-chain alkanes indicate the groups bonded to the longest carbon chain. CH3 methyl group on C-3 | CH3—CH2—CH2—CH—CH2—CH3 hexane 6 5 4 3 2 1 3-Methylhexane

Naming Summary In the IUPAC system: • The longest chain is named as the main chain. • Any carbon branches use their alkyl names. • Each branch is numbered by counting the main chain from the end nearest the first side group. • Branches are listed in alphabetical order.

What Is In A Name? The structural formula is drawn from the IUPAC name. 2, 4-dimethylhexane 2, 4 dimethyl hexane Location of Two CH3- groups 6 carbon main chain branches on attached with single C-C bonds main chain

Drawing Structural Formulas Draw the condensed formula of 2-methylpentane 1. Draw the main chain of 5 carbon atoms. C—C—C—C—C 2. Add a CH3 branch to C-2 on the main chain. CH3 | C—C—C—C—C 3. Add hydrogen atoms to give 4 bonds to each C. CH3 | CH3—CH—CH2—CH2—CH3

Learning Check Draw the condensed structural formula for A. 2, 3-dimethylbutane B. 2, 3, 4-trimethylpentane

Solution Write the condensed structural formula for A. 2, 3-dimethylbutane CH3 CH3 | | CH3—CH—CH—CH3 B. 2, 3, 4-trimethylpentane CH3 CH3 CH3 | | | CH3—CH—CH—CH—CH3

Writing Isomers The constitutional isomers for C4H10 can be written by first writing the continuous chain. Then remove one CH3 and attach it as a branch. CH3 | CH3—CH2—CH2—CH3 CH3—CH—CH3 Butane 2-Methylpropane

Learning Check Write 3 constitutional isomers of C5H12 and name each.

Solution CH3—CH2—CH2—CH2—CH3 pentane CH3 | CH3—CH—CH2—CH3 2-methylbutane CH3 | CH3—C—CH3 2,2-dimethylpropane | CH3

Chapter 12 Alkanes 12.4 Haloalkanes

Haloalkanes • In a haloalkane, one or more H atoms in an alkane is replaced by a halogen atom. • Haloalkanes containing one halogen are named alkyl halides. • In IUPAC names, a halogen is named as fluoro, chloro, bromo, or iodo. CH3Br bromomethane (methyl bromide) Cl | CH3—CH—CH2—CH3 2-chlorobutane (sec-butyl chloride)

Substituents • Two or more substituents are named in alphabetical order. Cl Br | | CH3—CH—CH2—CH—CH2—CH3 4-bromo-2-chlorohexane

Naming of Haloalkanes • Methane compounds with two or more chlorine atoms are sometimes named by common names that do not reflect their structures.

Learning Check The name of this compound is: Cl CH3 | | CH3—CH2—CH—CH2—CH—CH3 1) 2,4-dimethylhexane 2) 3-chloro-5-methylhexane 3) 4-chloro-2-methylhexane

Solution The name of this compound is: Cl CH3 | | CH3—CH2—CH—CH2—CH—CH3 3) 4-chloro-2-methylhexane

Learning Check Give the IUPAC name for each. A. CH3CH2—F Br Cl | | B. CH3—CH—CH—CH3

Solution Give the IUPAC name for each. A. CH3CH2—F fluoroethane Br Cl | | B. CH3—CH—CH—CH3 2-bromo-3-chlorobutane

Learning Check Halothane is widely used as an anesthetic, which is a compound that decreases the ability of the nerve cells to conduct pain. What is the IUPAC name of halothane? F Br | | F—C—C—Cl | | F H

Solution The IUPAC name is 2-bromo-2-chloro-1,1,1-trifluoroethane. F Br | | F—C—C—Cl | | F H

Chlorofluorocarbons (CFCs) • The ozone (O3) layer in the atmosphere absorbs most of the sun’s harmful radiation. • Chlorofluorocarbons (CFCs) cause reactions that destroy the ozone layer. • CFCs such as Freon-12 (CF2Cl2) have been used in refrigeration, air conditioning, and foam insulation. • The use of CFCs in spray cans is no longer allowed.

Chlorofluorocarbons and Ozone • In the stratosphere, the CFCs are cleaved by the high-energy UV radiation from the sun. UV light CF2Cl2 CF2Cl + Cl The Cl reacts with ozone (O3). The ClO produced destroys another O3 forming additional Cl. Thus one Cl can destroy many O3 molecules. Cl + O3 ClO + O2 ClO + O3 Cl + 2O2

Impact of Loss of Ozone • According to the National Academy of Sciences, each 1% loss of ozone (O3) increases the amount of UV radiation reaching the earth by 2%. • More UV radiation means more skin cancer and cataracts in humans, more intense photochemical smog, and lower crop yields.

Chapter 12 Alkanes 12.5 Cycloalkanes

Cycloalkanes Cycloalkanes: • Are rings of carbons that can be drawn as geometric figures. • Have a general formula of CnH2n or 2 H less than the alkane. propane C3H8 cyclopropane C3H6.butane C4H10 cyclobutane C4H8. • Are named with the prefix cyclo- in front of the corresponding alkane name.

Formulas of Cycloalkanes

Naming Cycloalkanes A cycloalkane with: • One substitutuent is named by placing the name of the substituent in front of the cycloalkane name. • Two or more substitutuents is named by numbering the ring in the direction that gives the lower numbers to the substituents.

Cycloalkanes with Side Groups

Chapter 12 Alkanes