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 — In methane, CH4 the four valence electrons of carbon are shared with the single electrons of four hydrogen (H) atoms. Each pair of electrons is a single bond, which can be drawn as a line. When a structure is drawn to show each bond, it is called a complete structural formula.

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 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 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 CH3 CH3 | | 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 secondary primary 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 –ane ending 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 A branched-chain alkane is named by indicating the attached groups on the longest carbon chain. In this example, the longest carbon chain has 6 carbon atoms.

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.

Learning Check Give the IUPAC name for each alkane. A. CH3 CH3 | | | | CH3—CH—CH2—CH—CH3 B. CH2CH3 CH3 | | CH3—CH2—CH—CH2—C—CH2—CH3 | CH3

Solution A. CH3 CH3 | | CH3—CH—CH2—CH—CH3 2,4-dimethylpentane | | CH3—CH—CH2—CH—CH3 2,4-dimethylpentane B. CH2CH3 CH3 CH3—CH2—CH—CH2—C—CH2—CH3 | CH3 5-ethyl-3,3-dimethylheptane

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 | 3. Add hydrogen atoms to give 4 bonds to each C. 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—C—CH3 2,2-dimethylpropane

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

Learning Check Name each of the following cycloalkanes.

Solution Name each of the following cycloalkanes. 1,2-dimethylcyclobutane chlorocyclopentane 1-chloro-3-methylcyclohexane

Constitutional Isomers The rigid ring in a cyclic structure prevents rotation of substitutuents bonded to the ring. A cyclic structure with two substituents has two isomers called cis-trans isomers. The cis isomer, has two substituents on the same side of the ring. The trans isomer, has two substituents on opposite sides of the ring.

Cis-Trans Isomers The compound 1,2-dichlorocyclopropane has cis-trans isomers. Cl atoms on Cl atoms on same side opposite sides

Learning Check Name each as a cis or trans isomer.

Solution Name each as a cis or trans isomer. trans-1,2-dibromocyclopropane cis-1,3-dichlorocyclobutane trans-1-bromo-3-chlorocyclopentane

Chapter 12 Alkanes 12.6 Physical Properties of Alkanes and Cycloalkanes 12.7 Chemical Properties of Alkanes and

Physical Properties of Alkanes Alkanes, including cycloalkanes: Are nonpolar and insoluble in water. Have densities between 0.65 – 0.70 g/mL; they float on water. Have low melting and boiling points compared to other families of organic compounds.

Boiling Points of Alkanes Increase in larger alkane molecules due to increased dispersion forces. Are lower for branched alkanes because they have fewer contact points. Are higher for cycloalkanes because their rigid structures allow more contact and attraction between molecules.

Boiling Points of Pentanes

Uses of Alkanes Methane, ethane, propane, and butane are gases at room temperature and used as fuels. Alkanes with 5-15 carbon atoms are liquids and found in gasoline, diesel, and jet fuels. Alkanes with 16 or more carbon atoms are solids and found in the waxy coatings of fruits and vegetables.

Crude Oil The differences in boiling points are used to separate hydrocarbons in crude oil. Heating to higher temperatures produces gases that are removed and cooled.

Chemical Properties of Alkanes Alkanes are typically not very reactive due to strong C-C single bonds. The most typical reaction is combustion, where an alkane reacts with oxygen to produce carbon dioxide, water, and energy. alkane + O2 CO2 + H2O + energy

Combustion A fuel such as propane reacts with oxygen and burns, producing CO2 and H2O. Propane is burned to obtain energy and heat for cooking or warming a room. C3H8 + 5O2 3CO2 + 4H2O

Steps in Balancing a Combustion Equation Step 1 Write the reactants and products. C3H8 + O2 CO2 + H2O Step 2 Balance C. C3H8 + O2 3CO2 + H2O Step 3 Balance H. C3H8 + O2 3CO2 + 4H2O Step 4 Balance O with O2. C3H8 + 5O2 3 CO2 + 4H2O

Learning Check Complete and balance the reaction for the complete combustion of C7H16.

Solution Step 1 C7H16 + O2 CO2 + H2O Step 2 and 3

Learning Check Write the combustion reaction for ethane and balance.

Solution Write the combustion reaction for ethane and balance. ethane CH3—CH3 = C2H6 C2H6 + 7/2 O2 2CO2 + 3H2O Multiply through by 2 to clear 7/2 2C2H6 + 7O2 4CO2 + 6H2O

Halogenation of Alkanes (Substitution) When alkanes react with halogens, a mixture of halogenated products is produced. In a reaction called substitution, one or more H atoms are replaced with a halogen usually Cl or Br. The equation for the reaction of ethane and chlorine to give the monosubstituted product is CH3—CH3 + Cl2 CH3—CH2—Cl + HCl

Learning Check Give the structures and names of the monosubstituted products for the reaction of propane with bromine in the presence of light.

Solution Give the structures and names of the monosubstituted products for the reaction of propane with bromine in the presence of light. CH3—CH2—CH2— Br 1-bromopropane; propyl bromide Br | CH3—CH—CH3 2-bromopropane; isopropyl bromide