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3-1 © 2005 John Wiley & Sons, Inc. All rights reserved Chapter 3: Alkanes and Cycloalkanes.

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Presentation on theme: "3-1 © 2005 John Wiley & Sons, Inc. All rights reserved Chapter 3: Alkanes and Cycloalkanes."— Presentation transcript:

1 3-1 © 2005 John Wiley & Sons, Inc. All rights reserved Chapter 3: Alkanes and Cycloalkanes

2 3-2 © 2005 John Wiley & Sons, Inc. All rights reserved Organic Chemistry

3 3-3 © 2005 John Wiley & Sons, Inc. All rights reserved Structure Hydrocarbon: Hydrocarbon: a compound composed only of carbon and hydrogen Saturated hydrocarbon: Saturated hydrocarbon: a hydrocarbon containing only single bonds Alkane: Alkane: a saturated hydrocarbon whose carbons are arranged in a open chain Aliphatic hydrocarbon: Aliphatic hydrocarbon: another name for an alkane

4 3-4 © 2005 John Wiley & Sons, Inc. All rights reserved Structure Shape tetrahedral about carbon all bond angles are approximately 109.5°

5 3-5 © 2005 John Wiley & Sons, Inc. All rights reserved Representing Alkanes

6 3-6 © 2005 John Wiley & Sons, Inc. All rights reserved Nomenclature Alkanes have the general formula C n H 2n+2 names of unbranched chain alkanes

7 3-7 © 2005 John Wiley & Sons, Inc. All rights reserved Constitutional Isomers Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms there are two constitutional isomers with molecular formula C 4 H 10

8 3-8 © 2005 John Wiley & Sons, Inc. All rights reserved Constitutional Isomerism the potential for constitutional isomerism is enormous World population is about 6,000,000,000

9 3-9 © 2005 John Wiley & Sons, Inc. All rights reserved IUPAC Nomenclature anesuffix -ane specifies an alkane prefix tells the number of carbon atoms

10 3-10 © 2005 John Wiley & Sons, Inc. All rights reserved IUPAC Nomenclature Parent name: Parent name: the longest carbon chain Substituent Substituent: a group bonded to the parent chain alkyl group: R-alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R- CH 4 becomes CH 3 - (methyl) CH 3 CH 3 becomes CH 3 CH 2 - (ethyl)

11 3-11 © 2005 John Wiley & Sons, Inc. All rights reserved IUPAC Nomenclature Common alkyl groups

12 3-12 © 2005 John Wiley & Sons, Inc. All rights reserved IUPAC Nomenclature ane 1.The name of an alkane with an unbranched chain consists of a prefix and the suffix ane 2. For branched alkanes, the parent chain is the longest chain of carbon atoms 3. Each substituent is given a name and a number 4. If there is one substituent, number the chain from the end that gives it the lower number

13 3-13 © 2005 John Wiley & Sons, Inc. All rights reserved IUPAC Nomenclature 6. If there are two or more different substituents, list them in alphabetical order number from the end of the chain that gives the substituent encountered first the lower number

14 3-14 © 2005 John Wiley & Sons, Inc. All rights reserved IUPAC Nomenclature 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization

15 3-15 © 2005 John Wiley & Sons, Inc. All rights reserved Classification of Carbons Primary (1°):Primary (1°): a C bonded to one other carbon Secondary (2°):Secondary (2°): a C bonded to two other carbons Tertiary (3°):Tertiary (3°): a C bonded to three other carbons Quaternary (4°)Quaternary (4°): a C bonded to four other carbons

16 3-16 © 2005 John Wiley & Sons, Inc. All rights reserved Cycloalkanes C n H 2n General formula C n H 2n five- and six-membered rings are the most common Structure and nomenclature cyclo-,to name, prefix the name of the corresponding open- chain alkane with cyclo-, and name each substituent on the ring if only one substituent, no need to give it a number if two substituents, number from the substituent of lower alphabetical order if three or more substituents, number to give them the lowest set of numbers, and then list substituents in alphabetical order

17 3-17 © 2005 John Wiley & Sons, Inc. All rights reserved Cycloalkanes Commonly written as line-angle formulas examples:

18 3-18 © 2005 John Wiley & Sons, Inc. All rights reserved Bicycloalkanes and Spiroalkanes Bicycloalkane Alkane that contains two rings that share two carbon atoms Bridgehead atoms  shared C Bridges: carbon chains connecting cycles C n H 2n-2 Spriroalkane Two rings share only one atom Sprio carbon: single carbon atom shared by two rings

19 3-19 © 2005 John Wiley & Sons, Inc. All rights reserved Bicyclo [4.3.0] nonane Bicyclo [4.4.0] decane Bicyclo [2.2.1] heptane Spiro [4.4] nonane Draw the following structural formulas Bicyclo [4.2.0] octane Sprio [2.4] heptane

20 3-20 © 2005 John Wiley & Sons, Inc. All rights reserved IUPAC- A General System prefix-infix-suffix prefixprefix tells the number of carbon atoms in the parent infixinfix tells the nature of the carbon-carbon bonds suffixsuffix tells the class of compound

21 3-21 © 2005 John Wiley & Sons, Inc. All rights reserved IUPAC - a general system ene prop-en-e = propene anol eth-an-ol = ethanol anone but-an-one = butanone anal but-an-al = butanal anoic acid pent-an-oic acid = pentanoic acid anol cyclohex-an-ol = cyclohexanol yne eth-yn-e = ethyne anamine eth-an-amine = ethanamine

22 3-22 © 2005 John Wiley & Sons, Inc. All rights reserved Conformation Conformation Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond Newman projection: shorthand representation staggered conformationstaggered conformation: a conformation about a carbon-carbon single bond where the atoms on one carbon are as far apart as possible from the atoms on an adjacent carbon

23 3-23 © 2005 John Wiley & Sons, Inc. All rights reserved Conformation Eclipsed conformation:Eclipsed conformation: a conformation about a carbon- carbon single bond in which the atoms on one carbon are as close as possible to the atoms on an adjacent carbon the lowest energy conformation of an alkane is a fully staggered conformation

24 3-24 © 2005 John Wiley & Sons, Inc. All rights reserved Conformations Torsional strain: Torsional strain: strain that arises when atoms separated by 3 bonds are forced from a staggered conformation to an eclipsed conformation eclipsed interaction strainalso called eclipsed interaction strain the torsional strain between staggered and eclipsed ethane is approximately 3.0 kcal (12.6 kJ)/ mol

25 3-25 © 2005 John Wiley & Sons, Inc. All rights reserved

26 3-26 © 2005 John Wiley & Sons, Inc. All rights reserved Cycloalkanes Cyclopentane in planar cyclopentane, all C-C-C bond angles are 108°, which differ only slightly from the tetrahedral angle of 109.5° consequently there is little angle strain angle strain:angle strain: strain that arises when a bond angle is either compressed or expanded compared with its optimal value

27 3-27 © 2005 John Wiley & Sons, Inc. All rights reserved Cycloalkanes Cyclopentane (cont’d) in planar cyclopentane, there are 10 fully eclipsed C-H bonds, which create torsional strain puckering to an “envelope” conformation relieves part of this strain in an envelope conformation, eclipsed interactions are reduced but angle strain is increased slightly (105°)

28 3-28 © 2005 John Wiley & Sons, Inc. All rights reserved Cycloalkanes Cyclohexane chairthe most stable conformation is a puckered chair conformation in a chair conformation, all bond angles are approx °, and all bonds on adjacent carbons are staggered

29 3-29 © 2005 John Wiley & Sons, Inc. All rights reserved Cycloalkanes Chair cyclohexane six H are equatorial and six H are axial an axial bond extends from the ring parallel to the imaginary axis of the ring equatorial bonds are roughly perpendicular to the imaginary axis of the ring

30 3-30 © 2005 John Wiley & Sons, Inc. All rights reserved Cyclohexane chair cyclohexane (cont’d) there are two equivalent chair conformations all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa

31 3-31 © 2005 John Wiley & Sons, Inc. All rights reserved Cyclohexane Boat conformation: Boat conformation: a puckered conformation in which carbons 1 and 4 are bent toward each other a boat conformation is less stable than a chair conformation torsional strain is created by four sets of eclipsed hydrogen interactions steric strainsteric strain (nonbonded interaction strain) is created by one set of flagpole interactions

32 3-32 © 2005 John Wiley & Sons, Inc. All rights reserved Cyclohexane the alternative chair conformations interconvert via a boat conformation

33 3-33 © 2005 John Wiley & Sons, Inc. All rights reserved Cyclohexane a group equatorial in one chair is axial in the alternative chair the two chairs are no longer of equal stability

34 3-34 © 2005 John Wiley & Sons, Inc. All rights reserved Practice Exercise in the lab

35 3-35 © 2005 John Wiley & Sons, Inc. All rights reserved How to draw the chair conformation of Cyclohexane 1. Draw the chair so at so place the C2 and C3 atoms to the right of C5 and C6 with apex 1 pointing downward of the left and apex 4 pointing upward on the right

36 3-36 © 2005 John Wiley & Sons, Inc. All rights reserved 2. Add all the axial bonds as vertical lines, pointing downward at C1, C3, and C5 and upward on C2, C4, and C

37 3-37 © 2005 John Wiley & Sons, Inc. All rights reserved 3. Draw the two equatorial bonds at C1 and C4 at a slight angle from horizontal, pointing upward at C1and downward at C4, parallel to the bond between C2 and C3 (or C5 and C6)

38 3-38 © 2005 John Wiley & Sons, Inc. All rights reserved Add the remaining equatorial bonds at C2, C3, C5 and C6 aligning them parallel to the C—C bond “once removed”

39 3-39 © 2005 John Wiley & Sons, Inc. All rights reserved Very Important!!! Conformational flipping interconverts axial and equatorial hydrogens

40 3-40 © 2005 John Wiley & Sons, Inc. All rights reserved Continue class

41 3-41 © 2005 John Wiley & Sons, Inc. All rights reserved Cis-trans Isomerism Cis-trans isomers have the same molecular formula the same connectivity of their atoms an arrangement of atoms in space that cannot be interconverted by rotation about sigma bonds

42 3-42 © 2005 John Wiley & Sons, Inc. All rights reserved Cis-trans isomerism the ring is commonly viewed through an edge or from above

43 3-43 © 2005 John Wiley & Sons, Inc. All rights reserved Cis-trans isomerism cyclohexanes may be viewed as planar hexagons

44 3-44 © 2005 John Wiley & Sons, Inc. All rights reserved Cis-trans isomerism or we can represent them as chair conformations in viewing chair conformations, remember that groups equatorial in one chair are axial in the alternative chair for trans-1,4-dimethylcyclohexane, the diequatorial chair is more stable than the diaxial chair

45 3-45 © 2005 John Wiley & Sons, Inc. All rights reserved Cis-trans isomerism for cis-1,4-dimethylcyclohexane, the alternative chairs are of equal stability

46 3-46 © 2005 John Wiley & Sons, Inc. All rights reserved 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

47 3-47 © 2005 John Wiley & Sons, Inc. All rights reserved 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. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

48 3-48 © 2005 John Wiley & Sons, Inc. All rights reserved 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.

49 3-49 © 2005 John Wiley & Sons, Inc. All rights reserved 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. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

50 3-50 © 2005 John Wiley & Sons, Inc. All rights reserved Physical Properties Constitutional isomers are different compounds and have different physical properties

51 3-51 © 2005 John Wiley & Sons, Inc. All rights reserved Boiling Points of Pentanes Boiling points of alkanes Are lowest of organic compounds. Increase with larger molecules Decrease for branched alkanes. Increase for cycloalkanes. TABLE 11.6

52 3-52 © 2005 John Wiley & Sons, Inc. All rights reserved 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. alkane + O 2 CO 2 + H 2 O + energy Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

53 3-53 © 2005 John Wiley & Sons, Inc. All rights reserved Write the equation C 5 H 12 + O 2 CO 2 + H 2 O Balance C C 5 H 12 + O 2 5CO 2 + H 2 O Balance H C 5 H 12 + O 2 5CO 2 + 6H 2 O Balance O with O 2 C 5 H O 2 5CO 2 + 6H 2 O balanced Balancing A Combustion Equation

54 3-54 © 2005 John Wiley & Sons, Inc. All rights reserved Testing your knowledge

55 3-55 © 2005 John Wiley & Sons, Inc. All rights reserved 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

56 3-56 © 2005 John Wiley & Sons, Inc. All rights reserved Solution Unbalanced equation C 3 H 8 + O 2 CO 2 + H 2 O Balance C C 3 H 8 + O 2 3CO 2 + H 2 O Balance H C 3 H 8 + O 2 3CO 2 + 4H 2 O Balance O C 3 H 8 + 5O 2 3CO 2 + 4H 2 O (Balanced)

57 3-57 © 2005 John Wiley & Sons, Inc. All rights reserved Write a balanced equation for the reaction for the complete combustion of heptane C 7 H 16. Learning Check

58 3-58 © 2005 John Wiley & Sons, Inc. All rights reserved C 7 H 16 + O 2 CO 2 + H 2 O C 7 H 16 + O 2 7CO 2 + 8H 2 O C 7 H O 2 7CO 2 + 8H 2 O Balanced equation: C 7 H O 2 7CO 2 + 8H 2 O Solution

59 3-59 © 2005 John Wiley & Sons, Inc. All rights reserved Write the combustion reaction for ethane and balance. Learning Check

60 3-60 © 2005 John Wiley & Sons, Inc. All rights reserved Write the combustion reaction for ethane and balance. ethane CH 3 CH 3 = C 2 H 6 C 2 H 6 + O 2 2CO 2 + 3H 2 O C 2 H O 2 2CO 2 + 3H 2 O C 2 H 6 + 7/2 O 2 2CO 2 + 3H 2 O Multiply through by 2 to clear 2C 2 H 6 + 7O 2 4CO 2 + 6H 2 O balanced Solution

61 3-61 © 2005 John Wiley & Sons, Inc. All rights reserved 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 CH 3 —CH 3 + Cl 2 CH 3 —CH 2 —Cl

62 3-62 © 2005 John Wiley & Sons, Inc. All rights reserved Learning Check Give the structures and names of the possible monosubstituted products for the reaction of propane with bromine in the presence of light.

63 3-63 © 2005 John Wiley & Sons, Inc. All rights reserved 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


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