03 - Structure and Stereochemistry of Alkanes - Wade 7th

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Organic Chemistry, 7th Edition L. G. Wade, Jr …

Organic Chemistry, 7th Edition L. G. Wade, Jr

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  • <number>
    Figure: 03_01-04UN.jpg
    Title:
    Nomenclature: Same Length Chains
    Caption:
    When looking for the longest continuous chain, look to find all the different chains of that length. Often, the longest chain with the most substituents is not obvious.
    Notes:
    When there are two carbon chains of the same length, the one that has the most substituents must be chosen to name the compound.
  • <number>
    Figure: 03_02.jpg
    Title:
    Nomenclature of Alkyl Groups
    Caption:
    Some common alkyl groups and their names.
    Notes:
    Substituents on a carbon chain are called alkyl groups. They are named by replacing the -ane ending of the alkane with -yl. The “n” and “iso” groupings are used to describe an alkyl chain attached by a primary carbon. The name of an alkyl chain attached by a secondary carbon is “sec,” and that of chains attached by a tertiary carbons “tert.”
  • Copyright © 2006 Pearson Prentice Hall, Inc.
  • Copyright © 2006 Pearson Prentice Hall, Inc.
  • Copyright © 2006 Pearson Prentice Hall, Inc.
  • <number>
    Figure: 03_03.jpg
    Title:
    Boiling Points of Branched and Unbranched Alkanes
    Caption:
    Alkane boiling points. Comparison of the boiling points of the unbranched alkanes (blue) with those of some branched alkanes (red). Because of their smaller surface areas, branched alkanes have lower boiling points than unbranched alkanes.
    Notes:
    The only intermolecular force of nonpolar molecules are London dispersion forces which result from induced dipole attractions. Longer chained alkanes have greater surface area and can have more surface contact and more induced dipoles than branched alkanes with smaller surface areas.
  • <number>
    Figure: 03_04.jpg
    Title:
    Melting Points of Alkanes
    Caption:
    Alkane melting points. The melting point curve for n-alkanes with even numbers of carbon atoms is slightly higher than that for alkanes with odd numbers of carbons.
    Notes:
    In solids, the packing of the molecules into a three dimensional structure affects the melting point. When molecules can pack in neat order avoiding empty pockets the melting point will be higher than when the packing is not ordered. Alkanes with an even number of carbons pack better than those with an odd number of carbons.
  • <number>
    Figure: 03_04-08UN.jpg
    Title:
    Methane Representations
    Caption:
    The simplest alkane is methane, CH4. Methane is perfectly tetrahedral, with the 109.5° bond angles predicted for an sp3 hybrid carbon. The four hydrogen atoms are covalently bonded to the central carbon atom, with bond lengths of 1.09Å.
    Notes:
    Any carbon with four sigma bonds has an sp3 hybridization.
  • <number>
    Figure: 03_04-09UN.jpg
    Title:
    Ethane
    Caption:
    Ethane, the two-carbon alkane, is composed of two methyl groups with overlapping sp3 hybrid orbitals forming a sigma bond between them.
    Notes:
    Ethane has two sp3 carbons. The C-C bond distance is 1.54Å and there is free rotation along this bond.
  • <number>
    Figure: 03_04-10UN.jpg
    Title:
    Conformations of Ethane
    Caption:
    The different arrangement formed by rotations about a single bond are called conformations, and a specific is called conformer. Pure conformers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations.
    Notes:
    Some conformations can be more stable than others.
  • <number>
    Figure: 03_05.jpg
    Title:
    Newman Projections
    Caption:
    The Newman projection looks straight down the carbon-carbon bond.
    Notes:
    The Newman projection is the best way to judge the stability of the different conformations of a molecule.
  • <number>
    Figure: 03_07.jpg
    Title:
    Conformational Analysis of Ethane
    Caption:
    The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about 3.0 kcal/mol (12.6 kJ/mol) higher in energy. At room temperature, this barrier is easily overcome, and the molecules rotate constantly.
    Notes:
    The staggered conformations are lower in energy than the eclipsed conformation because the staggering allows the electron clouds of the C-H bonds to be as far apart as possible. The energy difference is only 3 kcals/mol which can be easily overcome at room temperature.
  • <number>
    Figure: 03_08.jpg
    Title:
    The Newman Projection of Propane
    Caption:
    Propane is shown here as a perspective drawing and as a Newman projection looking down one of the carbon-carbon bonds.
    Notes:
  • <number>
    Figure: 03_09.jpg
    Title:
    Conformational Analysis of Propane
    Caption:
    Torsional energy of propane. When a bond of propane rotates, the torsional energy varies much like it does in ethane, but with 0.3 kcal/mol (1.2 kJ/mol) of additional torsional energy in the eclipsed conformation.
    Notes:
    Much like ethane the staggered conformations of propane is lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, the torsional strain will be 0.3 kcal/mol higher for propane than for ethane.
  • <number>
    Figure: 03_10.jpg
    Title:
    Newman Projections of Butane
    Caption:
    Butane conformations. Rotations about the center bond in butane give different molecular shapes. Three of these conformations are given specific names.
    Notes:
    For butane there will be two different staggered conformations: gauche and anti. The gauche conformation has a dihedral angle of 60° between the methyl groups while the anti conformation has a dihedral angle of 180° between the methyl groups. There distinct eclipsed conformation when the dihedral angle between the methyl groups is 0°, this conformation is referred to as totally eclipsed.
  • <number>
    Figure: 03_11.jpg
    Title:
    Conformational Analysis of Butane
    Caption:
    Torsional energy of butane. The anti conformation is lowest in energy, and the totally eclipsed conformation is highest in energy.
    Notes:
    The eclipsed conformations are higher in energy than the staggered conformations of butane, especially the totally eclipsed conformation. Among the staggered conformations, the anti is lower in energy because it has the electron clouds of the methyl groups as far apart as possible.
  • <number>
    Figure: 03_11-01UN.jpg
    Title:
    Totally Eclipsed Conformation of Butane
    Caption:
    The totally eclipsed conformation is about 1.4 kcal (5.9 kJ) higher in energy than the other eclipsed conformations, because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion. This kind of interference between two bulky groups is called steric strain or steric hindrance.
    Notes:
    The other eclipsed conformations are lower in energy than the totally eclipsed conformation but are still more unstable than the staggered conformations.
  • <number>
    Figure: 03_12.jpg
    Title:
    Cycloalkanes
    Caption:
    Structures of some cycloalkanes.
    Notes:
    The molecular formula of alkanes is CnH2n, two hydrogen less than an open chain alkane. Their physical properties resemble those of alkanes.
  • <number>
    Figure: 03_15.jpg
    Title:
    Angle Strain in Cyclopropane
    Caption:
    Angle strain in cyclopropane. The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms. This severe angle strain leads to nonlinear overlap of the sp3 orbitals and “bent bonds.”
    Notes:
    The angle compression of cyclopropane is 49.5°. The high reactivity of cyclopropanes is due to the non-linear overlap of the sp3 orbitals.
  • <number>
    Figure: 03_16.jpg
    Title:
    Conformations of Cyclopropane
    Caption:
    Torsional strain in cyclopropane. All the carbon-carbon bonds are eclipsed, generating torsional strain that contributes to the total ring strain.
    Notes:
    The angle strain and the torsional strain in cyclopropane make this ring size extremely reactive.
  • <number>
    Figure: 03_14.jpg
    Title:
    Ring Strain and Torsional Strain of Cyclobutane
    Caption:
    The ring strain of a planar cyclobutane results from two factors: Angle strain from the compressing of the bond angles to 90° rather than the tetrahedral angle of 109.5°, and torsional strain from eclipsing of the bonds.
    Notes:
    The angle compression for butane is 19.5°. Angle strain and torsional strain account for the high reactivity of 4-membered rings.
  • <number>
    Figure: 03_17.jpg
    Title:
    Conformations of Cyclobutane
    Caption:
    The conformation of cyclobutane is slightly folded. Folding gives partial relief from the eclipsing of bonds, as shown in the Newman projection. Compare this actual structure with the hypothetical planar structure in Figure 3-14.
    Notes:
    Cyclic compound with 4 carbons or more adopt non-planar conformations to relieve ring strain. Cyclobutane adopts the folded conformation to decrease the torsional strain caused by eclipsing hydrogens.
  • <number>
    Figure: 03_18.jpg
    Title:
    Conformations of Cyclopentane
    Caption:
    The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent CH2 groups.
    Notes:
    To relieve ring strain, cyclopentane adopts the envelope conformation.
  • <number>
    Figure: 03_19.jpg
    Title:
    Conformations of Cyclohexane
    Caption:
    The chair conformation of cyclohexane has one methylene group puckered upward and another puckered downward. Viewed from the Newman projection, the chair conformation has no eclipsing of the carbon-carbon bonds. The bond angles are 109.5°.
    Notes:
    Cyclohexane can adopt four non-planar conformations: chair, boat, twist boat, and half-chair. The most stable conformation is the chair because it has all the C-H bonds staggered.
  • <number>
    Figure: 03_20.jpg
    Title:
    Boat Conformation of Cyclohexane
    Caption:
    In the symmetrical boat conformation of cyclohexane, eclipsing of bonds results in torsional strain. In the actual molecule, the boat conformation is skewed to give the twist boat, a conformation with less eclipsing of bonds and less interference between the two flagpole hydrogens.
    Notes:
    In the boat conformation all bonds are staggered except for the “flagpole” hydrogens. There is steric hindrance between these hydrogens so the molecule twists a little producing the twist boat conformation which is 1.4 kcal (6 kJ) lower in energy than the boat.
  • <number>
    Figure: 03_21.jpg
    Title:
    Conformational Energy Diagram of Cyclohexane
    Caption:
    Conformational energy of cyclohexane. The chair conformation is most stable, followed by the twist boat. To convert between these two conformations, the molecule must pass through the unstable half-chair conformation.
    Notes:
    Interconversion between chair conformations require that cyclohexane go through its higher energy conformations.
  • <number>
    Figure: 03_22.jpg
    Title:
    Chair Conformation of Cyclohexane
    Caption:
    The axial bonds are directed vertically, parallel to the axis of the ring. The equatorial bonds are directed outward, toward the equator of the ring. As they are numbered here, the odd-numbered carbons have their upward bonds axial and their downward bonds equatorial. The even-numbered carbons have their downward bonds axial and their upward bonds equatorial.
    Notes:
    All the C-H bonds are staggered in the chair conformation. Axial hydrogens are pointed straight up or down, parallel to the axis of the ring. Equatorial hydrogens, like their name suggests, are pointed out of the ring along the “equator” of the molecule.
  • <number>
    Figure: 03_23.jpg
    Title:
    Chair-Chair Interconversion
    Caption:
    Chair-chair interconversion of methylcyclohexane. The methyl group is axial in one conformation, and equatorial in the other.
    Notes:
  • <number>
    Figure: 03_24.jpg
    Title:
    Newman Projection of Methylcyclohexane: Methyl Axial
    Caption:
    (a) When the methyl substituent is in an axial position on C1, it is gauche to C3. (b) The axial methyl group on C1 is also gauche to C5 of the ring.
    Notes:
    In the Newman projection it is easier to see the steric interaction between the methyl substituent and the hydrogens and carbons of the ring.
  • <number>
    Figure: 03_25.jpg
    Title:
    Newman Projection of Methylcyclohexane: Methyl Equatorial
    Caption:
    Looking down the C1-C2 bond of the equatorial conformation, we find that the methyl group is anti to C3.
    Notes:
    An equatorial methyl group will be anti to the C3. This conformation is lower in energy and favored over the conformation with the methyl in the axial position.
  • <number>
    Figure: 03_26.jpg
    Title:
    1,3-Diaxial Interaction
    Caption:
    The axial substituent interferes with the axial hydrogens on C3 and C5. This interference is called a 1,3-diaxial interaction.
    Notes:
  • <number>
    Figure: 03_26-01UN.jpg
    Title:
    Chair Conformations of cis-1,3-Dimethylcyclohexane
    Caption:
    Two chair conformations are possible for cis-1,3-dimethylcyclohexane. The unfavorable conformation has both methyl groups in axial positions, with a 1,3-diaxial interaction between them. The more stable conformation has both methyl groups in equatorial positions.
    Notes:
    Alkyl substituents on cyclohexane rings will tend to be equatorial to avoid 1,3-diaxial interactions. cis-1,3-dimethylcyclohexane can have both methyl groups on axial positions but the conformation with both methyls in equatorial positions is favored.
  • <number>
    Figure: 03_26-02UN.jpg
    Title:
    Chair Conformations of trans-1,3-Dimethylcyclohexane
    Caption:
    Either of the chair conformations of trans-1,3-dimethylcyclohexane has one methyl group in an axial position and one in an equatorial position. These conformations have equal energies, and they are present in equal amounts.
    Notes:
    Alkyl substituents on cyclohexane rings will tend to be equatorial to avoid 1,3-diaxial interactions. trans-1,3-dimethylcyclohexane has one methyl group axial and the other equatorial. Chair interconversion would still produce an axial and an equatorial methyl. In this case both chairs have the same energy, and they are present in equal amounts.
  • <number>
    Figure: 03_27.jpg
    Title:
    cis- and trans-Decalin
    Caption:
    cis-Decalin has a ring fusion where the second ring is attached by two cis bonds. trans-Decalin is fused using two trans bonds. The six-membered rings in cis- and trans-decalin assume chair conformations.
    Notes:
    The correct IUPAC name for decalin is bicyclo[3.3.0]decane but it is commonly known as decalin. There are two possible geometric isomers for decalin: cis and trans.
  • <number>
    Figure: 03_27-11UN.jpg
    Title:
    Conformations with Extremely Bulky Groups
    Caption:
    Some groups are so bulky that they are extremely hindered in axial positions. Cyclohexanes with tertiary-butyl substituents show that an axial t-butyl group is severely hindered. Regardless of the other groups present, the most stable conformation has a t-butyl group in an equatorial position. The following figure shows the severe steric interactions in a chair conformation with a t-butyl group axial.
    Notes:
    Alkyl substituents on cyclohexane rings will tend to be equatorial to avoid 1,3-diaxial interactions. Groups like tert-butyl are so bulky that it will force the chair conformation where it is in the equatorial position, regardless of other groups present.
  • <number>
    Figure: 03_27-12UN.jpg
    Title:
    Conformation of 1,4-di-t-butylcyclohexane
    Caption:
    The most stable conformation of cis-1,4-di-t-butylcyclohexane is a twist boat. Either of the chair conformations requires one of the bulky t-butyl groups to occupy an axial position.
    Notes:
    Since tert-butyl groups are most stable in the equatorial positions, when two t-butyl groups are present they will force the cyclohexane to interconvert to the twist boat conformation.
  • <number>
    Figure: 03_28-01UN.jpg
    Title:
    Bicyclic Compounds
    Caption:
    Two or more rings can be joined into bicyclic or polycyclic systems. There are three ways that two rings may be joined. Fused rings are most common, sharing two adjacent carbon atoms and the bond between them. Bridged rings are also common, sharing two nonadjacent carbon atoms (the bridgehead carbons) and one or more carbon atoms (the bridge) between them. Spirocyclic compounds, in which the two rings share only one carbon atom, are relatively rare.
    Notes:
    Three examples of bicyclic ring systems can be fused, bridged, or spirocyclic. Fused and bridged bicyclic rings are joined together by two carbons; in fused bicycles the two carbons are adjacent while in bridged bicycles the carbons are nonadjacent. Spirocycles are joined by only one carbon.
  • <number>
    Figure: 03_28-02UN.jpg
    Title:
    Nomenclature of Bicyclic Compounds
    Caption:
    The name of a bicyclic compound is based on the name of the alkane having the same number of carbons as there are in the ring system. This name follows the prefix bicyclo and a set of brackets enclosing three numbers. The following examples contain eight carbon atoms and are named bicyclo[4.2.0]octane and bicyclo[3.2.1]octane, respectively.
    Notes:
    When naming bicyclic rings, the alkane name used will denote the total amount of carbons in the compound. The prefix bicyclo is used followed by three numbers in brackets. These three numbers represent the number of carbons that bridge (connect) the two shared carbons. In the case of spirocycles, the prefix spiro is used instead of bicycle and only two numbers are written.
  • Copyright © 2006 Pearson Prentice Hall, Inc.
  • Copyright © 2006 Pearson Prentice Hall, Inc.
  • Copyright © 2006 Pearson Prentice Hall, Inc.
  • Copyright © 2006 Pearson Prentice Hall, Inc.
  • Copyright © 2006 Pearson Prentice Hall, Inc.

Transcript

  • 1. Chapter 3 1 Chapter 3 Organic Chemistry, 7th Edition L. G. Wade, Jr. © 2010, Prentice Hall Structure and Stereochemistry of Alkanes
  • 2. Chapter 3 2 Hydrocarbons are molecules that are made of carbon and hydrogen ONLY. Hydrocarbons
  • 3. Chapter 3 3 Alkanes • General formula: CnH2n+2 • Found in everything from natural gas to petroleum. • The smaller alkanes have very low boiling points (b.p.) therefore they are gases. CH4 C2H6 C3H8 b.p. -160o C -89o C -42o C
  • 4. Chapter 3 4 Alkane Examples
  • 5. Chapter 3 5 Small Alkanes (CnH2n+2) • Methane • Ethane • Propane CH3 CH3 CH3 CH2 CH3 CH4
  • 6. Chapter 3 6 Butane: C4H10 Constitutional isomers are compounds with the same molecular formula but the carbons are connected differently. CH3CH2CH2CH3 n-butane CH3CHCH3 iso-butane CH3
  • 7. Chapter 3 7 Pentanes: C5H12 n-pentane iso-pentane neo-pentane CH3CH2CH2CH2CH3 CH3CHCH2CH3 CH3CCH3 CH3 CH3 CH3
  • 8. Chapter 3 8 IUPAC • International Union of Pure and Applied Chemistry • Common names kept: methane, ethane, propane, butane. • Alkanes: suffix “-ane” will be used after the number of carbons. – Example: An alkane with 5 carbons is “penta” for five and the suffix “-ane”: pentane
  • 9. Chapter 3 9 IUPAC Rules • Rule 1: Find the longest continuous chain of carbon atoms, and use the name of this chain as the base name of the compound. • Rule 2: Number the longest chain, beginning with the end of the chain nearest a substituent. • Rule 3: Name the groups attached to the longest chain as alkyl groups. Give the location of each alkyl group by the number of the main chain carbon atom to which it is attached. • Write the alkyl groups in alphabetical order regardless of their position on the chain.
  • 10. Chapter 3 10 Rule 1: Find the Longest Chain of Consecutive Carbons. The longest chain is six carbons: hexane
  • 11. Chapter 3 11 Main Chain When there are two longest chains of equal length, use the chain with the greatest number of substituents.
  • 12. Chapter 3 12 Common Alkyl Groups
  • 13. Chapter 3 13 Rule 2: Number the Longest Chain. Methyl is closest to this end of the main chain. Number the longest chain, beginning with the end of the chain nearest a substituent. 3-methylehexane
  • 14. Chapter 3 14 Rule 3: Alkyl Substituents CH3CHCH2CHCH2CH3 CH3 CH2CH3 • Name the groups attached to the longest chain as alkyl groups. • Give the location of each alkyl group by the number of the main chain carbon atom to which it is attached. • Write the alkyl groups in alphabetical order regardless of their position on the chain. 1 2 3 4 5 6 2-methyl 4-ethyl 4-ethyl-2-methylhexane
  • 15. Chapter 3 15 Organizing Multiple Groups • When two or more of the same substituents are present, use the prefixes di-, tri-, tetra-, etc. to avoid having to name the alkyl group twice. CH3CH2CH2CHCH2CHCH2CH2CHCH3 CH3 CH3 CH3 10 9 8 7 6 5 4 3 2 1 Three methyl groups at positions 2, 5, and 7 2,5,7-trimethyldecane
  • 16. Chapter 3 16 Solution: 4-Isopropyloctane has a chain of eight carbons, with an isopropyl group on the fourth carbon. 5-t-Butyldecane has a chain of ten carbons, with a t-butyl group on the fifth. Solved Problem 3-1 Give the structures of 4-isopropyloctane and 5-t-butyldecane.
  • 17. Chapter 3 17 Solved Problem 3-2 Give a systematic (IUPAC) name for the following compound.
  • 18. Chapter 3 18 The longest carbon chain contains eight carbon atoms, so this compound is named as an octane. Numbering from left to right gives the first branch on C2; numbering from right to left gives the first branch on C3, so we number from left to right. Solved Problem 3-2: Solution 4-isopropyl-2,2,3,6-tetramethyloctane
  • 19. Chapter 3 19 Boiling Points of Alkanes As the number of carbons in an alkane increases, the boiling point will increase due to the larger surface area and the increased van der Waals attractions.
  • 20. Chapter 3 20 Melting Points of Alkanes • Melting points increase as the carbon chain increases. • Alkanes with an even number of carbons have higher melting points than those with an odd number of carbons. • Branched alkanes have higher melting points than unbranched alkanes.
  • 21. Chapter 3 21 Cracking and Hydrocracking
  • 22. Chapter 3 22 Methane Representations • Tetrahedral • sp3 hybrid carbon with angles of 109.5º.
  • 23. Chapter 3 23 Ethane Representations • Two sp3 hybrid carbons. • Rotation about the C—C sigma bond. • Conformations are different arrangements of atoms caused by rotation about a single bond.
  • 24. Chapter 3 24 Conformations of Ethane Pure conformers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations.
  • 25. Chapter 3 25 Newman Projections The Newman projection is the best way to judge the stability of the different conformations of a molecule.
  • 26. Chapter 3 26 The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about 3.0 kcal/mol (12.6 kJ/mol) higher in energy. At room temperature, this barrier is easily overcome, and the molecules rotate constantly. Ethane Conformations
  • 27. Chapter 3 27 Propane Conformations Propane is shown here as a perspective drawing and as a Newman projection looking down the C1—C2 bond.
  • 28. Chapter 3 28 The staggered conformations of propane is lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, the torsional strain will be 0.3 kcal/mol higher for propane than for ethane. Propane Conformations
  • 29. Chapter 3 29 Butane Conformations • Butane has two different staggered conformations: gauche (60° between the methyl groups) and anti (180° between the methyl groups). • The eclipsed conformation where the dihedral angle between the methyl groups is 0° is referred to as totally eclipsed.
  • 30. Chapter 3 30
  • 31. Chapter 3 31 Steric Strain • The totally eclipsed conformation is higher in energy because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion. • This kind of interference between two bulky groups is called steric strain or steric hindrance.
  • 32. Chapter 3 32 Cycloalkanes: CnH2n
  • 33. Chapter 3 33 Physical Properties • Non-polar • Relatively inert • Boiling point and melting point depends on the molecular weight.
  • 34. Chapter 3 34 Cycloalkane Nomenclature • Cycloalkane is the main chain: alkyl groups attached to the cycloalkane will be named as alkyl groups. • If only one alkyl group is present, then no number is necessary. CH2CH3 ethylcyclopentane
  • 35. Chapter 3 35 Cycloalkane Nomenclature • If there are two or more substituents, number the main chain to give all substituents the lowest possible number. CH3 CH3 H3C CH3 CH2CH3 1,3-dimethylcyclohexane 3-ethyl-1,1-dimethylcyclohexane 1 3 1 3
  • 36. Chapter 3 36 Geometric Isomers CH3 CH3 CH3 CH2CH3 Same side:Same side: cis-cis- Opposite side:Opposite side: trans-trans- 1 2 cis-1,2-dimethylcyclohexane 1 2 trans-1-ethyl-2-methylcyclohexane
  • 37. Chapter 3 37 Stabilities of Cycloalkanes • Five- and six-membered rings are the most common in nature. • Carbons of cycloalkanes are sp3 hybridized and thus require an angle of 109.5º. • When a cycloalkane has an angle other than 109.5º, there will not be optimum overlap and the compound will have angle strain. • Angle strain is sometimes called Baeyer strain in honor of Adolf von Baeyer who first explained this phenomenon. • Torsional strain arises when all the bonds are eclipsed.
  • 38. Chapter 3 38 Cyclopropane: C3H6 • The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms. • This severe angle strain leads to nonlinear overlap of the sp3 orbitals and “bent bonds”.
  • 39. Chapter 3 39 Torsional Strain All the C—C bonds are eclipsed, generating torsional strain that contributes to the total ring strain.
  • 40. Chapter 3 40 Cyclobutane: C4H8 The ring strain of a planar cyclobutane results from two factors: angle strain from the compressing of the bond angles to 90° rather than the tetrahedral angle of 109.5° and torsional strain from eclipsing of the bonds.
  • 41. Chapter 3 41 Non-Planar Cyclobutane • Cyclic compound with four carbons or more adopt non- planar conformations to relieve ring strain. • Cyclobutane adopts the folded conformation (“envelope”) to decrease the torsional strain caused by eclipsing hydrogens.
  • 42. Chapter 3 42 Cyclopentane: C5H10 The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent methylene (CH2) groups.
  • 43. Chapter 3 43 Chair Conformation of Cyclohexane
  • 44. Chapter 3 44 Boat Conformation of Cyclohexane
  • 45. Chapter 3 45 Conformational Energy Diagram of Cyclohexane
  • 46. Chapter 3 46 Axial and Equatorial Positions
  • 47. Chapter 3 47 Chair–Chair Interconversion The most important result in chair conversion is that any substituent that is axial in the original conformation becomes equatorial in the new conformation.
  • 48. Chapter 3 48 Axial Methyl in Methylcyclohexane
  • 49. Chapter 3 49 Equatorial Methyl Group
  • 50. Chapter 3 50 1,3-Diaxial Interaction The axial substituent interferes with the axial hydrogens on C3 and C5. This interference is called a 1,3-diaxial interaction.
  • 51. Chapter 3 51 Cis-1,3-dimethylcyclohexane • Cis-1,3-dimethylcyclohexane can have both methyl groups in axial positions or both in equatorial positions. • The conformation with both methyl groups being equatorial is more stable.
  • 52. Chapter 3 52 Trans-1,3-dimethylcyclohexane Both conformations have one axial and one equatorial methyl group so they have the same energy.
  • 53. Chapter 3 53 Decalin Cis-decalin has a ring fusion where the second ring is attached by two cis bonds. Trans-decalin is fused using two trans bonds. Trans-decalin is more stable because the alkyl groups are equatorial.
  • 54. Chapter 3 54 Tert-butylcyclohexane Substituents are less crowded in the equatorial positions.
  • 55. Chapter 3 55 Cis-1,4-ditertbutylcyclohexane The most stable conformation of cis-1,4-di-tertbutylcyclohexane is the twist boat. Both chair conformations require one of the bulky t-butyl groups to occupy an axial position.
  • 56. Chapter 3 56 Bicyclic Systems
  • 57. Chapter 3 57 Nomenclature of Bicyclic Systems Bicyclo [#.#.#]alkane Where # are the number of carbons on the bridges (in decreasing order) and the alkane name includes all the carbons in the compound.
  • 58. Chapter 3 58 Solved Problem 3-3 (a) Draw both chair conformations of cis-1,2- dimethylcyclohexane, and determine which conformer is more stable. (b) Repeat for the trans isomer. (c) Predict which isomer (cis or trans) is more stable.
  • 59. Chapter 3 59 Solved Problem 3-3: Solution (a) a) There are two possible chair conformations for the cis isomer, and these two conformations interconvert at room temperature. Each of these conformations places one methyl group axial and one equatorial, giving them the same energy.
  • 60. Chapter 3 60 Solved Problem 3-3 : Solution (b) (b) There are two chair conformations of the trans isomer that interconvert at room temperature. Both methyl groups are axial in one, and both are equatorial in the other. The diequatorial conformation is more stable because neither methyl group occupies the more hindered axial position.
  • 61. Chapter 3 61 Solved Problem 3-3 : Solution (c) (c) The trans isomer is more stable. The most stable conformation of the trans isomer is diequatorial and therefore about 7.6 kJ/mol (1.8 kcal/mol) lower in energy than either conformation of the cis isomer, each having one methyl axial and one equatorial. Remember that cis and trans are distinct isomers and cannot interconvert.
  • 62. Chapter 3 62 Solution: First, we draw the two conformations. The ethyl group is bulkier than the methyl group, so the conformation with the ethyl group equatorial is more stable. Solved Problem 3-1 Draw the most stable conformation of trans-1-ethyl-3-methylcyclohexane.