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12.1 Alkanes

Chapter 12 Alkanes

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Alkanes

Alkanes:Contain only C and H.Have only single C—C bonds.Have a general formula of CnH2n +2

.

C Atoms H Atoms Formula1 2(1) + 2 = 4 CH43 2(3) + 2 = 8 C3H86 2(6) + 2 = 14 C6H14

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Structural formulas show the arrangement ofatoms 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

Structural Formulas

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Line-Bond FormulasBecause 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.

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Summary of Formulas

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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.

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Names of Continuous-Chain Alkanes

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Learning Check

A. Give the name of each compound:1) CH3—CH32) CH3—CH2—CH33) CH3—CH2—CH2—CH2—CH2—CH3

B. Write the condensed structural formula of pentane.

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SolutionA. Give the name of each compound:

1) CH3—CH3 ethane2) CH3—CH2—CH3 propane3) CH3—CH2—CH2—CH2—CH2—CH3

hexaneB. Write the condensed structural formula of

pentane.CH3—CH2—CH2—CH2—CH3

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A. What is the condensed formula forH H H H | | | |

H —C—C—C—C—H| | | |

H H H H

B. What is its molecular formula?C. What is its name?

Learning Check

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A. CH3—CH2—CH2—CH3

B. C4H10

C. butane

Solution

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

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Conformations of ButaneRotation of the end CH3- provides different conformations for butane.

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Drawing Structures for Butane

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Learning Check

For propane, write the:1) Molecular formula2) Expanded structural formula3) Condensed structural formula

(two conformations)4) Line-bond formula

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Solution1) Molecular formula C3H82) Expanded structural formula

H H H| | |

H —C—C—C—H| | |

H H H3) Condensed structural formula (two conformations)

CH3—CH2—CH3 CH3|

CH3—CH24) Line-bond formula or

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12.2 IUPAC Naming System for Alkanes12.3 Drawing Structural Formulas

Chapter 12 Alkanes

CH3 CH3 | |

CH3—CH—CH2—CH—CH3

2,4-dimethylpentane

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Branched-Chain Alkanes

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

branch

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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—CH3secondary primary tertiary

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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

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Alkyl Groups

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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 Branched-Chain Alkanes

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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.

Naming Summary

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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

Learning Check

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A. CH3 CH3 | |

CH3—CH—CH2—CH—CH32,4-dimethylpentane

B. CH2CH3 CH3| |

CH3—CH2—CH—CH2—C—CH2—CH3 |

CH3 5-ethyl-3,3-dimethylheptane

Solution

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What Is In A Name?

The structural formula is drawn from the IUPAC name.

2, 4-dimethylhexane

2, 4 dimethyl hexaneLocation of Two CH3- groups 6 carbon main chainbranches on attached with single C-C bondsmain chain

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Drawing Structural FormulasDraw the condensed formula of 2-methylpentane1. Draw the main chain of 5 carbon atoms.

C—C—C—C—C2. Add a CH3 branch to C-2 on the main chain.

CH3 |

C—C—C—C—C3. Add hydrogen atoms to give 4 bonds to each C.

CH3|

CH3—CH—CH2—CH2—CH3

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Draw the condensed structural formula forA. 2, 3-dimethylbutane

B. 2, 3, 4-trimethylpentane

Learning Check

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Write the condensed structural formula forA. 2, 3-dimethylbutane

CH3 CH3| |

CH3—CH—CH—CH3

B. 2, 3, 4-trimethylpentaneCH3 CH3 CH3| | |

CH3—CH—CH—CH—CH3

Solution

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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—CH3Butane 2-Methylpropane

Writing Isomers

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Write 3 constitutional isomers of C5H12 and name each.

Learning Check

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CH3—CH2—CH2—CH2—CH3pentane

CH3|

CH3—CH—CH2—CH3 2-methylbutane

CH3|

CH3—C—CH3 2,2-dimethylpropane|

CH3

Solution

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12.4 Haloalkanes

Chapter 12 Alkanes

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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.

CH3Brbromomethane (methyl bromide)Cl|

CH3—CH—CH2—CH3 2-chlorobutane(sec-butyl chloride)

Haloalkanes

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Two or more substituents are named in alphabetical order.

Cl Br| |

CH3—CH—CH2—CH—CH2—CH3

4-bromo-2-chlorohexane

Substituents

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Naming of Haloalkanes

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

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The name of this compound is:Cl CH3| |

CH3—CH2—CH—CH2—CH—CH3

1) 2,4-dimethylhexane2) 3-chloro-5-methylhexane3) 4-chloro-2-methylhexane

Learning Check

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The name of this compound is:Cl CH3| |

CH3—CH2—CH—CH2—CH—CH3

3) 4-chloro-2-methylhexane

Solution

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Give the IUPAC name for each.

A. CH3CH2—F

Br Cl| |

B. CH3—CH—CH—CH3

Learning Check

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Give the IUPAC name for each.A. CH3CH2—F fluoroethane

Br Cl| |

B. CH3—CH—CH—CH3

2-bromo-3-chlorobutane

Solution

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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

Learning Check

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The IUPAC name is 2-bromo-2-chloro-1,1,1-trifluoroethane.

F Br| |

F—C—C—Cl| |

F H

Solution

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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 (CFCs)

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In the stratosphere, the CFCs are cleaved by the high-energy UV radiation from the sun.

UV lightCF2Cl2 CF2Cl + Cl

The Cl reacts with ozone (O3). The ClOproduced destroys another O3 forming additional Cl. Thus one Cl can destroy many O3 molecules.

Cl + O3 ClO + O2ClO + O3 Cl + 2O2

Chlorofluorocarbons and Ozone

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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.

Impact of Loss of Ozone

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12.5 Cycloalkanes

Chapter 12 Alkanes

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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.

Cycloalkanes

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Formulas of Cycloalkanes

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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.

Naming Cycloalkanes

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Cycloalkanes with Side Groups

CH3

CH3

CH3

CH3

CH3

CH3

methylcyclopentane

1,2-dimethylcyclopentane

1,2,4-trimethylcyclohexane

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Learning CheckName each of the following cycloalkanes.

CH3

CH3

Cl

CH3

Cl

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SolutionName each of the following cycloalkanes.

1,2-dimethylcyclobutane

chlorocyclopentane

1-chloro-3-methylcyclohexane

CH3

CH3

Cl

CH3

Cl

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Constitutional IsomersThe 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.

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Cis-Trans Isomers

The compound 1,2-dichlorocyclopropane has cis-trans isomers.Cl atoms on Cl atoms onsame side opposite sides

Cl Cl Cl

Clcis-1,2-dichlorocyclopropane ` trans-1,2-dichlorocyclopropane

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Learning CheckName each as a cis or trans isomer.

Br

Br

ClCl

Br

Cl

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SolutionName each as a cis or trans isomer.

trans-1,2-dibromocyclopropane

cis-1,3-dichlorocyclobutane

trans-1-bromo-3-chlorocyclopentane

Br

Br

ClCl

Br

Cl

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Chapter 12 Alkanes

12.6 Physical Properties of Alkanes andCycloalkanes

12.7 Chemical Properties of Alkanes andCycloalkanes

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Physical Properties of AlkanesAlkanes, 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.

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Boiling Points of AlkanesBoiling points:

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.

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Boiling Points of Pentanes

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Uses of AlkanesMethane, ethane, propane, and butane are gases at room temperature and used as fuels.Alkanes with 5-17 carbon atoms are liquids and found in gasoline, diesel, and jet fuels.Alkanes with 18 or more carbon atoms are solids and found in the waxy coatings of fruits and vegetables.

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

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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

Chemical Properties of Alkanes

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CombustionA 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

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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

Steps in Balancing a Combustion Equation

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

Learning Check

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Step 1

C7H16 + O2 CO2 + H2O

Step 2 and 3

C7H16 + O2 7CO2 + 8H2O

Step 4

C7H16 + 11O2 7CO2 + 8H2O

Solution

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Write the combustion reaction for ethane and balance.

Learning Check

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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

Solution

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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

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

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SolutionGive 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 bromideBr|

CH3—CH—CH32-bromopropane; isopropyl bromide