Structure and Stereochemistry of Alkanes Organic Chemistry, 8th Edition L. G. Wade, Jr. Chapter 3 Lecture Structure and Stereochemistry of Alkanes

Lecture 8: Overview Halogenation Conformers of Alkanes (Propane & Butane) Cycloalkanes Chair Conformations of Cyclohexane Bicyclic Systems

Halogenation Halogenation Reaction Heat or Light needed to initiate reaction F2 way too fast (too reactive) Cl2, Br2 moderate reaction rate (easily controlled) I2 very slow, may not react at all

Methane Representations File Name: AAAKPLH0 Figure: 03_04-09un.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. Tetrahedral. sp3 hybrid carbon with angles of 109.5º. Chapter 3

Conformers of Alkanes Structures resulting from the free rotation of a C-C single bond. May differ in energy. The lowest-energy conformer is most prevalent. Molecules constantly rotate through all the possible conformations.

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. File Name: AAAKPLI0 Figure: 03_04-10un.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. Chapter 3

Conformations of Ethane File Name: AAAKPLJ0 Figure: 03_04-11un.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. Pure conformers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations. Chapter 3

Newman Projections File Name: AAAKPLK0 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. The Newman projection is the best way to judge the stability of the different conformations of a molecule. Chapter 3

Ethane Conformations File Name: AAAKPLM0 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. 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. Chapter 3

Propane Conformations File Name: AAAKPLN0 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: Propane is shown here as a perspective drawing and as a Newman projection looking down the C1—C2 bond. Chapter 3

Propane Conformations File Name: AAAKPLO0 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. 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. Chapter 3

Butane Conformations File Name: AAAKPLP0 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. 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. Chapter 3

Conformational Analysis of Butane File Name: AAAKPLQ0 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. Chapter 3

Steric Strain in Butane 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. File Name: AAAKPLR0 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. Chapter 3

Higher Alkanes Anti conformation is lowest in energy “Straight chain” is actually zigzag pattern. Chapter 3

Cycloalkanes Ring of carbon atoms (-CH2- groups) General Formula: CnH2n Nonpolar insoluble in water Compact shape Melting and boiling points similar to branched alkanes with same number of carbons

Cycloalkanes contain rings of carbon atoms. Cycloalkanes: CnH2n File Name: AAAKPLT0 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. Cycloalkanes contain rings of carbon atoms. Chapter 3

Physical Properties of Alkanes Nonpolar. Relatively inert. Boiling point and melting point depend on the molecular weight. Table: 03-T04 Chapter 3

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. ethylcyclopentane Chapter 3

Cycloalkane Nomenclature If there are two or more substituents, number the main chain to give all substituents the lowest possible number. 1 1 3 3 1,3-dimethylcyclohexane 3-ethyl-1,1-dimethylcyclohexane Chapter 3

Students accidentally draw cyclic Hint Students accidentally draw cyclic structures when acyclic structures are intended, and vice versa. Always verify whether the name contains the prefix cyclo-. Chapter 3

Cycloalkanes as Substituents The cycloalkane becomes a substituent when the cyclic portion of the molecule contains fewer carbons than the acyclic part or when there is a more important functional group in the molecule. File Name: AAAKPLW0 Figure: 03-12-03un Chapter 3

Geometric Isomers 1 Same side: cis- 2 2 Opposite side: trans- 1 cis-1,2-dimethylcyclohexane 2 2 Opposite side: trans- 1 trans-1-ethyl-2-methylcyclohexane Chapter 3

Stabilities of Cycloalkanes Five- and six-membered rings: most stable in nature. Carbons of cycloalkanes are sp3 hybridized and thus require an angle of 109.5º. When a cycloalkane carbon 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 phenomenon. Torsional strain arises when all the bonds are eclipsed. Chapter 3

Angle Strain in Cyclopropane File Name: AAAKPMG0 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. 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.” Chapter 3

Torsional Strain in Cyclopropane File Name: AAAKPMH0 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. All the C—C bonds are eclipsed, generating torsional strain that contributes to the total ring strain. Chapter 3

Cyclobutane: C4H8 File Name: AAAKPME0 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. 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. Chapter 3

Nonplanar Cyclobutane Cyclic compound with four carbons or more adopt nonplanar conformations to relieve ring strain. Cyclobutane adopts the folded conformation (“envelope”) to decrease the torsional strain caused by eclipsing hydrogens. File Name: AAAKPMI0 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. Chapter 3

Cyclopentane: C5H10 File Name: AAAKPMJ0 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. The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent methylene (CH2) groups. Chapter 3

Chair Conformation of Cyclohexane File Name: AAAKPMK0 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. Chapter 3

Chair Conformation The chair is the most stable conformational isomer of cyclohexane. The chair has no eclipsing interactions. Bond angles in the chair conformation are 109.5°. Chapter 3

Boat Conformation of Cyclohexane File Name: AAAKPML0 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. Chapter 3

Boat and Twisted Boat Conformation Eclipsing bonds result in torsional strain. The twist boat conformation has fewer eclipsing bond interactions and less interference between the flagpole hydrogens. Chapter 3

Conformational Energy Diagram of Cyclohexane File Name: AAAKPMM0 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. Chapter 3

Axial and Equatorial Positions Axial bonds (red) are directed vertically parallel to the axis of the ring. Equatorial bonds (green) are directed outward toward the equator of the molecule. File Name: AAAKPMN0 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. Chapter 3

Chair–Chair Interconversion File Name: AAAKPMX0 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: The most important result in chair conversion is that any substituent that is axial in the original conformation becomes equatorial in the new conformation. Chapter 3

Axial Methyl in Methylcyclohexane File Name: AAAKPMY0 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. Chapter 3

Equatorial Methyl Group File Name: AAAKPNA0 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. Chapter 3

1,3-Diaxial Interaction File Name: AAAKPNB0 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: The axial substituent interferes with the axial hydrogens on C3 and C5. This interference is called a 1,3-diaxial interaction. Chapter 3

tert-Butylcyclohexane File Name: AAAKPNP0 Figure: 03_26-14un.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. Substituents are less crowded in the equatorial positions. Chapter 3

Cis-1,3-dimethylcyclohexane File Name: AAAKPND0 Figure: 03_26-02un.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. 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. Be able to explain why this is true. Chapter 3

Trans-1,3-dimethylcyclohexane File Name: AAAKPNE0 Figure: 03_26-03un.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. Both conformations have one axial and one equatorial methyl group so they have the same energy. Chapter 3

Cis-1,4-ditertbutylcyclohexane The most stable conformation of cis-1,4-di-tert-butylcyclo hexane is the twist boat. Both chair conformations require one of the bulky t-butyl groups to occupy an axial position. File Name: AAAKPNQ0 Figure: 03_26-15un.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. Chapter 3

Bicyclic Systems Fused rings share two adjacent carbon atoms and the bond between them. Bridged rings share two nonadjacent carbon atoms and one or more carbon atoms (the bridge) between them. Spirocyclic compounds are rare; the two rings share only one carbon. ile Name: AAAKPNR0 Figure: 03_26-16un.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. Chapter 3

Nomenclature of Bicyclic Systems File Name: AAAKPNS0 Figure: 03_26-18un.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. Bicyclo [#.#.#]alkane where the #s are the numbers of carbons on the bridges (in decreasing order) and the alkane name includes all the carbons in the compound. Chapter 3

Decalin File Name: AAAKPNX0 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. 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. Chapter 3