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3. Organic Compounds: Alkanes and Their Stereochemistry Why this Chapter Alkanes are unreactive, but provide useful vehicle to introduce important ideas.

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Presentation on theme: "3. Organic Compounds: Alkanes and Their Stereochemistry Why this Chapter Alkanes are unreactive, but provide useful vehicle to introduce important ideas."— Presentation transcript:

1 3. Organic Compounds: Alkanes and Their Stereochemistry Why this Chapter Alkanes are unreactive, but provide useful vehicle to introduce important ideas about organic compounds Alkanes will be used to discuss basic approaches to naming organic compounds We will take an initial look at 3-D aspects of molecules

2 2 3.1 Functional Groups Functional group - collection of atoms at a site that have a characteristic behavior in all molecules where it occurs The group reacts in a typical way, generally independent of the rest of the molecule For example, the double bonds in simple and complex alkenes react with bromine in the same way

3 Functional Groups with: Multiple Carbon–Carbon Bonds Alkenes have a C-C double bond Alkynes have a C-C triple bond Arenes have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring

4 4 Alkenes and Alkynes Alkenes contain a double bond between adjacent carbon atoms. Alkynes contain a triple bond. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

5 Functional Groups with: Carbon Singly Bonded to an Electronegative Atom

6 Alcohols and Ethers An alcohol contains the hydroxyl (-OH) functional group. In an ether, an oxygen atom is bonded to two carbon atoms –C–O–C–. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

7 Functional Groups with: Carbon–Oxygen Double Bond (Carbonyl Groups)

8 8 Aldehydes and Ketones An aldehyde contains a carbonyl group (C=O), which is a carbon atom with a double bond to an oxygen atom, attached to at least one hydrogen. In a ketone, the carbon of the carbonyl group is attached to two other carbon atoms. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

9 9 Carboxylic Acids and Esters Carboxylic acids contain the carboxyl group, which is a carbonyl group attached to a hydroxyl group. O  — C—OH An ester contains the carboxyl group between carbon atoms. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

10 10 Amines and Amides In amines, the functional group is a nitrogen atom. | —N — In amides, the hydroxyl group of a carboxylic acid is replaced by a nitrogen group. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

11 11 Summary of Functional Groups

12 12 Survey of Functional Groups

13 13 Survey of Functional Groups

14 14 Methyl amine (fish) Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Functional Groups in Everyday Items

15 15 Learning Check Classify each of the following as alcohol, ether, aldehyde, ketone, carboxylic acid, ester, or amine. 1)CH 3 – CH 2 – CH 2 – OH 2) CH 3 – O – CH 2 – CH 3 3) CH 3 – CH 2 – NH 2 O O || || 4) CH 3 – C – OH 5) CH 3 – C – O – CH 3

16 16 Solution Classify each of the following as alcohol, ether, aldehyde, ketone, carboxylic acid, ester, or amine. 1)CH 3 ─ CH 2 ─ CH 2 ─ OH 2) CH 3 ─ O ─ CH 2 ─ CH 3 alcohol ether 3) CH 3 ─ CH 2 ─ NH 2 amine O O || || 4) CH 3 ─ C ─ OH 5) CH 3 ─ C ─ O ─ CH 3 carboxylic acid ester

17 What functional group is not present in the naturally occurring compound shown below? 1. carboxylic acid 2. amine 3. primary alcohol 4. secondary alcohol 5. methyl group Learning Check

18 What functional group is not present in the naturally occurring compound shown below? 1. carboxylic acid 2. amine 3. primary alcohol 4. secondary alcohol 5. methyl group Solution

19 Alkanes and Alkane Isomers Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups) Connecting carbons can lead to large or small molecules C n H 2n+2 The formula for an alkane with no rings in it must be C n H 2n+2 where the number of C’s is n saturated Alkanes are saturated with hydrogen (no more can be added They are also called aliphatic compounds

20 20

21 21 Condensed Structures of Alkanes We can represent an alkane in a brief form or in many types of extended form A condensed structure does not show bonds but lists atoms, such as ◦ CH 3 CH 2 CH 2 CH 3 (butane) ◦ CH 3 (CH 2 ) 2 CH 3 (butane)

22 22 Expanded and Condensed Structures

23 23 Some Structures for Butane Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

24 24 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.

25 25 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

26 26 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?

27 27 Solution 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? CH 3  CH 2  CH 2  CH 2  CH 3 C 5 H 12 pentane

28 28 Learning Check Write the condensed structural formula for A. ethane B. heptane

29 29 Solution Write the condensed structural formula for A. ethane CH 3 ─ CH 3 B. heptane CH 3 ─ CH 2 ─ CH 2 ─ CH 2 ─ CH 2 ─ CH 2 ─ CH 3

30 30 Alkane Isomers CH 4 = methane, C 2 H 6 = ethane, C 3 H 8 = propane The molecular formula of an alkane with more than three carbons can give more than one structure ◦ C 4 (butane) = butane and isobutane ◦ C 5 (pentane) = pentane, 2-methylbutane, and 2,2- dimethylpropane Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes

31 31 Constitutional Isomers Isomers that differ in how their atoms are arranged in chains are called constitutional isomers Compounds other than alkanes can be constitutional isomers of one another They must have the same molecular formula to be isomers

32 32 Isomers of Butane Isomers Have the same molecular formula. Have different atom arrangements. Of butane (C 4 H 10 ) consist of a straight chain and a branched chain each with 4 carbon atoms. branch

33 33

34 What kind of carbon is not present in the molecule below? 1. primary 2. secondary 3. tertiary 4. quaternary 5. All 4 kinds of carbon are present Learning Check

35 What kind of carbon is not present in the molecule below? 1. primary 2. secondary 3. tertiary 4. quaternary 5. All 4 kinds of carbon are present Solution

36 How many secondary hydrogens are there in the following molecule? Learning Check

37 How many secondary hydrogens are there in the following molecule? Solution

38 Which of the following choices is not an alkane? 1. C 12 H C 10 H C 11 H C 16 H C 20 H 42 Learning Check

39 Which of the following choices is not an alkane? 1. C 12 H C 10 H C 11 H C 16 H C 20 H 42 Solution

40 A brilliant chemist determined the number of secondary carbons and secondary hydrogens for an unknown alkane. Well, actually, the chemist is having trouble deciding which numbers are correct. Which of these answers is possible for an alkane? 2° hydrogens2° carbons Learning Check

41 A brilliant chemist determined the number of secondary carbons and secondary hydrogens for an unknown alkane. Well, actually, the chemist is having trouble deciding which numbers are correct. Which of these answers is possible for an alkane? 2° hydrogens2° carbons Solution

42 Naming Alkanes Compounds are given systematic names by a process that uses Follows specific rules ◦ Find parent hydrocarbon chain ◦ Carbons in that main chain are numbered in sequence ◦ Substituents are identified numbered ◦ Write compound name is single word ◦ Name a complex substituents as though it were a compound itself See specific examples in text

43 43 IUPAC Names for Simple Alkanes Use a prefix to indicate the number of carbons in a chain. End in –ane.

44 44 Learning Check A. Give the name of each compound: 1) CH 3 —CH 3 2) CH 3 —CH 2 —CH 3 3) CH 3 —CH 2 —CH 2 —CH 2 —CH 2 —CH 3 B. Write the condensed structural formula of pentane.

45 45 Solution A. Give the name of each compound: 1) CH 3 —CH 3 ethane 2) CH 3 —CH 2 —CH 3 propane 3) CH 3 —CH 2 —CH 2 —CH 2 —CH 2 —CH 3 hexane B. Write the condensed structural formula of pentane. CH 3 —CH 2 —CH 2 —CH 2 —CH 3

46 Alkyl Groups Alkyl group – remove one H from an alkane (a part of a structure) General abbreviation “R” (for Radical, an incomplete species or the “rest” of the molecule) Name: replace -ane ending of alkane with -yl ending ◦  CH 3 is “methyl” (from methane) ◦  CH 2 CH 3 is “ethyl” from ethane

47 47

48 48 Types of Alkyl groups Classified by the connection site (See Figure 3.3) ◦ a carbon at the end of a chain (primary alkyl group) ◦ a carbon in the middle of a chain (secondary alkyl group) ◦ a carbon with three carbons attached to it (tertiary alkyl group)

49 49 Naming Substituents IUPACCommon methyl ethyl propyln-propyl 1-methylethylIsopropyl butyln-butyl 1-methylpropylsec-butyl 2-methylpropylIsobutyl 1,1-dimethylethyltert-butyl or t-butyl

50 IUPACCommon pentyln-pentyl or amyl 1-methylbutylsec-pentyl 3-methylbutyl isopentyl or isoamyl or i-amyl 1,1- dimethylpropyl tert-pentyl or tert-amyl or t-amyl 2,2- dimethylpropyl neopentyl 50 Naming Substituents

51 51 Naming Substituents IUPAC Fluoro Chloro Bromo Iodo Hydroxy

52 52 Alkanes with Alkyl Groups methylpropane 2,4-dimethylpentane methyl groups

53 53 Write three isomers of C 5 H 12 and name each. Learning Check

54 54 Write three isomers of C 5 H 12 and name each. Solution A chain of 5 carbon atoms CH 3 ─ CH 2 ─ CH 2 ─ CH 2 ─ CH 3 pentane A chain of 4 carbon atoms and 1 carbon branch CH 3 │ CH 3 ─ CH ─ CH 2 ─ CH 3 2-methylbutane A chain of 3 carbon atoms and 2 carbon branches CH 3 │ CH 3 ─ C ─ CH 3 2,2-dimethylpropane │ CH 3

55 What is the best IUPAC name for the following compound? 1. 2-ethyl-3-isopropylhexane 2. 1-isopropyl-3-ethylhexane 3. 4-isopropyl-5-ethylhexane 4. 3-methyl-4-isopropylheptane 5. 4-isopropyl-3-methylheptane Learning Check

56 What is the best IUPAC name for the following compound? 1. 2-ethyl-3-isopropylhexane 2. 1-isopropyl-3-ethylhexane 3. 4-isopropyl-5-ethylhexane 4. 3-methyl-4-isopropylheptane 5. 4-isopropyl-3-methylheptane Solution

57 What is the systematic name of the alkane shown below? 1. 3,7-dimethyl-4-propyloctane 2. 2-methyl-5-(1-methylpropyl)octane 3. 2-ethyl-6-methyl-3-propylheptane 4. 3-methyl-4-(3-methylbutyl)heptane 5. 2,6-dimethyl-5-propyloctane Learning Check

58 What is the systematic name of the alkane shown below? 1. 3,7-dimethyl-4-propyloctane 2. 2-methyl-5-(1-methylpropyl)octane 3. 2-ethyl-6-methyl-3-propylheptane 4. 3-methyl-4-(3-methylbutyl)heptane 5. 2,6-dimethyl-5-propyloctane Solution

59 Which of the following is not an isomer of hexane? 1. 2,2-dimethylbutane 2. 2,2-dimethylpentane 3. 3-methylpentane 4. 2-methylpentane 5. 2,3-dimethylbutane Learning Check

60 Which of the following is not an isomer of hexane? 1. 2,2-dimethylbutane 2. 2,2-dimethylpentane 3. 3-methylpentane 4. 2-methylpentane 5. 2,3-dimethylbutane Solution

61 How many constitutional isomers of C 4 H 10 O are alcohols? Learning Check

62 How many constitutional isomers of C 4 H 10 O are alcohols? Solution

63 Properties of Alkanes Called paraffins (low affinity compounds) because they do not react as most chemicals They will burn in a flame, producing carbon dioxide, water, and heat

64 64 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

65 alkane + O 2 CO 2 + H 2 O + energy 65 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

66 66 Learning Check Propane is used to provide heat for cooking or warming a room. Write a balanced equation for the complete combustion of propane. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

67 67 Learning Check Propane is used to provide heat for cooking or warming a room. Write a balanced equation for the complete combustion of propane. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Solution C 3 H 8 + 5O 2 3CO 2 + 4H 2 O

68 68 Write a balanced equation for the reaction for the complete combustion of: heptane ethane Learning Check

69 69 Write a balanced equation for the reaction for the complete combustion of: heptane C 7 H 16. ethane Solution C 7 H O 2  7CO 2 + 8H 2 O 2C 2 H 6 + 7O 2  4CO 2 + 6H 2 O

70 70 Properties of Alkanes They react with Halogens like Cl 2 in the presence of light to replace H’s with Cl’s (not controlled)

71 71 Learning Check Give the structures and names of the possible monosubstituted products for the reaction of propane with bromine in the presence of light.

72 72 Solution Give the structures and names of the possible monosubstituted products for the reaction of propane with bromine in the presence of light. CH 3 —CH 2 —CH 2 — Br 1-bromopropane; propyl bromide Br | CH 3 —CH—CH 3 2-bromopropane; isopropyl bromide

73 73 Physical Properties Boiling points and melting points increase as size of alkane increases Dispersion forces increase as molecule size increases, resulting in higher melting and boiling points

74 Conformations of Ethane Stereochemistry concerned with the 3-D aspects of molecules  bonds are cylindrically symmetrical Rotation is possible around C-C bonds in open-chain molecules

75 75 Conformers Conformation- Different arrangement of atoms resulting from bond rotation Conformations can be represented in 2 ways:

76 76 Torsional Strain We do not observe perfectly free rotation There is a barrier to rotation, and some conformers are more stable than others Staggered- most stable: all 6 C-H bonds are as far away as possible Eclipsed- least stable: all 6 C-H bonds are as close as possible to each other

77 77

78 Conformations of Other Alkanes The eclipsed conformer of propane has 3 interactions: two ethane-type H-H interactions, and one H-CH 3 interaction

79 79 Conformational situation is more complex for larger alkanes Not all staggered conformations has same energy, and not all eclipsed conformations have same energy Conformations of Other Alkanes

80 80 Conformations of Butane Anti conformation- methyl groups are 180˚ apart Gauche conformation- methyl groups are 60˚ apart Which is the most energetically stable?

81 81

82 82 Steric Strain Steric strain- repulsive interaction occurring between atoms that are forced closer together than their atomic radii allow

83 What is the correct order of stability, starting with the most strained, for the conformations shown below? 1. A B, C 2. A, C, B 3. B, A, C 4. C, A, B 5. C, B, A Learning Check

84 What is the correct order of stability, starting with the most strained, for the conformations shown below? 1. A B, C 2. A, C, B 3. B, A, C 4. C, A, B 5. C, B, A Solution

85 C 6 H 14 has 5 isomers. How many of these isomers have at least one tertiary carbon? Learning Check

86 C 6 H 14 has 5 isomers. How many of these isomers have at least one tertiary carbon? Solution

87 Which statement about bond rotations around C-C bonds in ethane and ethylene is correct? 1. Rotation around σ bond is easy because the orbital overlap does not change with rotation. 2. Rotation around π bond is easy because there are p orbitals available in all directions of space. 3. Rotation around σ bond is very hard because the electron pairs from the adjacent bonds must pass by each other. 4. Rotation around π bond is hard because the electrons are delocalized. Learning Check

88 Which statement about bond rotations around C-C bonds in ethane and ethylene is correct? 1. Rotation around σ bond is easy because the orbital overlap does not change with rotation. 2. Rotation around π bond is easy because there are p orbitals available in all directions of space. 3. Rotation around σ bond is very hard because the electron pairs from the adjacent bonds must pass by each other. 4. Rotation around π bond is hard because the electrons are delocalized. Solution

89 Which of the above represents the Newman projection of 2-bromo-3-chlorobutane? 1. a 2. b 3. c 4. d 5. e Learning Check

90 Which of the above represents the Newman projection of 2-bromo-3-chlorobutane? 1. a 2. b 3. c 4. d 5. e Solution

91 The lowest energy conformation of decane resembles a straight rod. It is a fully stretched “zig-zag” conformation with all bonds staggered and all large substituents anti to each other. Imagine we bend this chain at carbon 5. What is the minimum strain that such a once-bent conformation (bent rod) must have? kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol Learning Check

92 The lowest energy conformation of decane resembles a straight rod. It is a fully stretched “zig-zag” conformation with all bonds staggered and all large substituents anti to each other. Imagine we bend this chain at carbon 5. What is the minimum strain that such a once-bent conformation (bent rod) must have? kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol Solution

93 What is the energy difference between the two lowest- energy conformations of 2,3-dimethylbutane? 1. the energy of one gauche interaction 2. the energy of two gauche interactions 3. the energy of two H ↔ CH 3 eclipsing interactions 4. the energy of three H ↔ CH 3 eclipsing interactions 5. the energy of two CH 3 ↔ CH 3 eclipsing interactions Learning Check

94 What is the energy difference between the two lowest- energy conformations of 2,3-dimethylbutane? 1. the energy of one gauche interaction 2. the energy of two gauche interactions 3. the energy of two H ↔ CH 3 eclipsing interactions 4. the energy of three H ↔ CH 3 eclipsing interactions 5. the energy of two CH 3 ↔ CH 3 eclipsing interactions Solution

95 The highest energy conformation of ethane has 12 kJ/mol of strain. The two highest conformations of butane have 16 and 19 kJ/mol of strain, respectively. How much strain does the highest energy conformation of 2,3-dimethylbutane have? kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol Learning Check

96 The highest energy conformation of ethane has 12 kJ/mol of strain. The two highest conformations of butane have 16 and 19 kJ/mol of strain, respectively. How much strain does the highest energy conformation of 2,3-dimethylbutane have? kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol Solution


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