Chapter 11 Introduction to Organic Chemistry: Alkanes - CMS

Chapter 11 Introduction to Organic Chemistry: Alkanes 11.1 Organic Compounds

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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Organic Compounds An organic compound Is a compound made from carbon atoms. Has one or more C atoms. Has many H atoms. May also contain O, S, N, and halogens.

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Properties of Organic Compounds Typical organic compounds Contain carbon. Have covalent bonds. Have low melting points. Have low boiling points. Are flammable. Are soluble in nonpolar solvents. Are not soluble in water.

Oil (organic) and water (inorganic) Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 3

Organic vs. Inorganic Propane, C3H8, is an organic compound used as a fuel. NaCl, salt, is an inorganic compound composed of Na+ and Cl− ions. Why is propane an organic compound, but NaCl is not?


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Comparing Organic and Inorganic Compounds TABLE 11.1


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Learning Check Identify each characteristic as most typical of compounds that are (I) inorganic or (O) organic. 1. Has a high melting point 2. Is not soluble in water 3. Has a formula CH3─CH2─CH3 4. Has a formula MgCl2 5. Burns easily in air 6. Has covalent bonds

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Solution Identify each characteristic as most typical of compounds that are (I) inorganic or (O) organic. I

1. Has a high melting point

O

2. Is not soluble in water

O

3. Has a formula CH3─CH2─CH3

I

4. Has a formula MgCl2

O

5. Burns easily in air

O

6. Has covalent bonds

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Writing Formulas for Carbon Compounds In carbon compounds Carbon has 4 valence electrons and hydrogen has 1. • •C• •

H•

To achieve an octet, C forms four bonds. H H ••

H:C:H

H

C

H

••

H

H

CH4 , methane 8

Tetrahedral Structure of Carbon VSEPR theory predicts that a carbon atom with four single, covalent bonds has a tetrahedral shape.


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Organic Compounds with More Carbon Atoms In organic molecules with more carbon atoms, Valence electrons are shared. Covalent bonds form between carbon and carbon atoms. Covalent bonds form between carbon and hydrogen atoms. H H H H ••

••

H:C:C:H ••

••

H

H

H

C

C

H

H

H

ethane, CH3─CH3

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Tetrahedral Structure of Carbon In molecules with two or more carbon atoms, each carbon atom with four single bonds has a tetrahedral shape.

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 11

Chapter 11 Introduction to Organic Chemistry: Alkanes 11.2 Alkanes


Names of Alkanes The names of alkanes Are determined by the IUPAC (International Union of Pure and Applied Chemistry) system. Use a prefix to indicate the number of carbons in a chain. End in –ane.

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IUPAC Names for Alkanes TABLE 11.2


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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.

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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

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Structural Formulas Alkanes are written with structural formulas that are Expanded to show each bond. Condensed to show each carbon atom and its attached hydrogen atoms. Expanded Condensed H H

C

H

CH4 , methane

H

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Writing Structural Formulas Carbon atoms in a chain Maintain tetrahedral shape. Are connected in a zig-zag pattern. Are drawn as 2-dimensional. Can be written in several conformations.

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Expanded and Condensed Structures TABLE 11.3


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

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Conformations The groups attached to a C-C single bond Rotate around the bond, Give different relative arrangements called conformations.

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

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cumming 22

Some Structures for Butane TABLE 11.3


Hexane has Six Carbon Atoms Hexane Is an alkane with 6 carbon atoms in a continuous chain. Has a “zig-zag” look because each carbon atom is at the center of a tetrahedron. Is represented by a ball-and-stick model as


Learning Check A. Write the condensed formula for:

H

H

H

H

H

H

C

C

C

C

C

H

H

H

H

H

H

B. What is its molecular formula? C. What is its name? 25

Solution A. CH3CH2CH2CH2CH3 B. C5H12 C. pentane

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Learning Check Write the condensed structural formula for A. ethane

B. heptane

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Solution Write the condensed structural formula for A. ethane

CH3─CH3

B. heptane CH3─CH2─CH2─CH2─CH2─CH2─CH3

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Cycloalkanes Cycloalkanes Are cyclic alkanes. Have 2H fewer than the open chain. Are named by using the prefix cyclo- before the name of the alkane chain with the same number of carbon atoms.

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Cycloalkanes The structural formulas of cycloalkanes are usually represented by geometric figures, CH2

Cyclopropane CH2 Cyclobutane

CH2

CH2

CH2

CH2

CH2 30

More Cycloalkanes Cyclopentane

CH2 CH2

CH2

CH2

CH2

Cyclohexane CH2 CH2

CH2

CH2 CH2 CH2 31

Learning Check Name the following: A. CH3─CH2─CH2─CH3 B. C. CH3─CH2─CH2─CH2─CH2─CH2─CH2─CH3 D.

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Solution Name the following: A. CH3─CH2─CH2─CH3

butane

B.

cyclopropane

C. CH3─CH2─CH2─CH2─CH2─CH2─CH2─CH3 octane D. cyclohexane

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Chapter 11 Introduction to Organic Chemistry: Alkanes 11.4 Properties of Alkanes


Some Properties of Alkanes The properties of alkanes include being Nonpolar. Insoluble in water. Less dense than water Flammable in air. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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Alkanes with 1-4 Carbon Atoms Alkanes with 1-4 carbon atoms are Methane, ethane, propane, and butane. Gases at room temperature. Used as heating fuels.


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Alkanes with 5-17 Carbon Atoms Alkanes with 5-8 carbon atoms are Liquids at room temperature. Pentane, hexane, heptane, and octane. Very volatile. Used to make gasoline. Alkanes with 9-17 carbon atoms Are liquids at room temperature Have higher boiling points. Are found in kerosene, diesel, and jet fuels. 37

Alkanes with 18 or more Carbon Atoms Alkanes with 18 or more carbon atoms Have high molar masses. Are waxy solids at room temperature. Used in waxy coatings of fruits and vegetables.


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Boiling Points of Pentanes Boiling points of alkanes TABLE 11.6 Are lowest of organic compounds. Increase with larger molecules due to increased dispersion forces. Decrease for branched alkanes. Increase for cycloalkanes. 39

Learning Check For the following pairs of hydrocarbon, which one has the higher boiling point? Why? 1. butane or octane 2. hexane or 2,3-dimethylbutane

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Solution For the following pairs of hydrocarbon, which one has the higher boiling point? Why? 1. butane or octane

octane has more carbon atoms

2. hexane or 2,3-dimethylbutane hexane is not branched

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Crude Oil The hydrocarbons in crude oil are Separated by boiling points. Heated to higher temperatures to produce gases that can be removed and cooled. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 42

Classification of Carbon Atoms The classification of carbon atoms indicates the number of carbon atoms attached. A primary carbon (1°) bonds to one carbon atom. A secondary carbon (2°) bonds to two carbon atoms. A tertiary carbon (3°) bonds to three carbon atoms. CH3 | CH3 —CH2—CH2—CH3 CH3—CH—CH3 primary secondary tertiary

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Combustion of Alkanes Alkanes Undergo combustion by reacting with oxygen to produce carbon dioxide, water, and energy. Are typically not very reactive due to strong C-C single bonds. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

alkane + O2

CO2 + H2O + energy 44

Balancing A Combustion Equation Write the equation C5H12 + O2 Balance C C5H12 + O2 Balance H C5H12 + O2 Balance O with O2 C5H12 + 8O2

CO2 + H2O 5CO2 + H2O 5CO2 + 6H2O 5CO2 + 6H2O balanced

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Learning Check Propane is used to provide heat for cooking or warming a room. Write a balanced equation for the complete combustion of propane.


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Solution Unbalanced equation C3H8 + O2

CO2 + H2O

Balance C C3H8 + O2

3CO2 + H2O

Balance H C3H8 + O2

3CO2 + 4H2O

Balance O C3H8 + 5O2

3CO2 + 4H2O (Balanced) 47

Learning Check Write a balanced equation for the reaction for the complete combustion of heptane C7H16.

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Solution C7H16 + O2

CO2 + H2O

C7H16 + O2

7CO2 + 8H2O

C7H16 + O2

7CO2 + 8H2O

C7H16 +11O2

7CO2 + 8H2O

Balanced equation: C7H16 + 11O2

7CO2 + 8H2O 49

Learning Check Write the combustion reaction for ethane and balance.

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Solution Write the combustion reaction for ethane and balance. ethane CH3CH3 = C2H6 C2H6 + O2 2CO2 + 3H2O C2H6

+ 7 O2

2CO2 + 3H2O

C2H6

+ 7/2 O2

2CO2 + 3H2O

Multiply through by 2 to clear 2C2H6

+ 7O2

4CO2 + 6H2O balanced 51

Halogenation of Alkanes (Substitution) When alkanes react with halogens, The reaction is called substitution, one or more H atoms are replaced with a halogen usually Cl or Br. Light or heat is required. A mixture of halogenated products result, but we write the equation with the monosubstituted product. Example: ethane and chloride light

CH3—CH3 + Cl2

CH3—CH2 —Cl 52

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

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Solution Give the structures and names of the possible 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 54

Chapter 11 Introduction to Organic Chemistry: Alkanes 11.3 Alkanes with Substituents


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Isomers of Butane Isomers Have the same molecular formula. Have different atom arrangements. Of butane (C4H10) consist of a straight chain and a branched chain each with 4 carbon atoms.

branch


Alkyl groups Alkyl groups are Alkanes that are missing one H. Substituents attached to carbon chains. Named with a –yl ending. H H

H

C

methyl

CH3

H H

H

C

C

H

H

CH3

CH2

ethyl 57

Naming Substituents In the IUPAC system, A carbon branch is named as an alkyl group. Halogen atoms are named as halo.

TABLE 11.5


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Alkanes with Alkyl Groups CH3 CH3 CH CH3

methylpropane methyl groups

CH3

CH3

CH3 CH CH2 CH CH3

2,4-dimethylpentane

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Naming Alkanes with Substituents


Guide to Naming Alkanes Give the name of

CH3 CH3   CH3─CH─CH─CH3

STEP 1

Longest chain is butane.

STEP 2

Number chain.

CH3 CH3   CH3─CH─CH─CH3 1 2 3 4 STEP 3 Locate substituents and name. 2,3-dimethylbutane 61

Learning Check Write the IUPAC name for Cl CH3   CH3─CH2─CH─CH─CH3

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Solution STEP 1

Longest chain is pentane.

STEP 2

Number chain from end nearest substituent. Cl CH3 (nearest end)   CH3─CH2─CH─CH─CH3 5 4 3 2 1

STEP 3 Locate substituents and name alphabetically. 3-chloro-2-methylpentane 63

Learning Check Give the IUPAC name for each of the following: A.

CH3 CH3 | | CH3─CH─CH2 ─CH─CH2─CH3

B.

Cl CH3 | | CH3─CH2─CH─CH2─C─CH2─CH3 | Cl 64

Solution A.

CH3 CH3 | | CH3─CH─CH2 ─CH─CH2─CH3 2,4-dimethylhexane 1

B.

2

3

4

5

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Cl CH3 more substituents on C3 | | CH3─CH2─CH─CH2─C─CH2─CH3 | Cl 7

6

5

4

3

2

1

3,5-dichloro-3-methylheptane 65

Drawing Alkane Structures


What is In A Name? The structural formula is drawn from the IUPAC name. 2, 4-dimethylhexane 2, 4 dimethyl Location of Two CH3- groups branches on attached main chain

hexane 6 carbon main chain with single C-C bonds

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Learning Check Draw the condensed structural formula for 2-bromo-4-chlorobutane.

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Solution STEP 1

Longest chain has 4 carbon atoms. C─C─C─C STEP 2 Number chain and add substituents. Br  C ─ C ─ C ─ C ─ Cl 1 2 3 4 STEP 3 Add hydrogen to complete 4 bonds to each C. Br  CH3─CH─CH2─CH2─Cl 69

Naming Cycloalkanes with Substituents The name of a substituent is placed in front of the cycloalkane name. CH3 methylcyclobutane Number ring with two substituents 1-bromo-2-chlorocyclopentane Br Cl 70

Learning Check Name each of the following: 1.

CH3

CH2─CH3 2. Cl

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Solution Name each of the following: 1. methylcyclopropane

2. 3-chloro-1-ethylcyclohexane

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Learning Check Write three isomers of C5H12 and name each.

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Solution A chain of 5 carbon atoms CH3─CH2─CH2─CH2─CH3 pentane A chain of 4 carbon atoms and 1 carbon branch CH3 │ CH3─CH─CH2─CH3 2-methylbutane A chain of 3 carbon atoms and 2 carbon branches CH3 │ CH3─CH─CH3 2,2-dimethylpropane │ CH3

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