Chapter 3
Structure and Stereochemistry of Alkanes

03-00.01UNT01

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Title
Hydrocarbon Classification
Caption
Summary of Hydrocarbon Classification
Notes
Compounds that contain carbon and hydrogen only are called hydrocarbons. Alkanes contain C-H single bonds and are said to be saturated, alkenes contain C-C double bonds, and alkynes contain C-C triple bonds. The term aromatic hydrocarbon is commonly used to indicate the presence of a benzene ring.
Keywords
alkanes, alkenes, alkynes, aromatic, hydrocarbon, saturated
03-01

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Title
Molecular Formulas of Alkanes
Caption
Figure 3-1 Examples of the general alkane molecular formula,
Notes
Alkanes have the general molecular formula CnH2n+2. The term saturated is used to describe alkanes since they have the maximum number of bonded hydrogens, thus the term saturated hydrocarbons.
Keywords
molecular formula, alkane, saturated
03-01-01UN

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Title
Nomenclature of Butanes and Pentanes
Caption
If all alkanes had unbranched (straight-chain) structures, their nomenclature would be very simple. Most alkanes have structural isomers, however, and we need a way of naming all the different isomers. For example, there are two isomers of formula C4H10. The unbranched isomer is simply called butane (or n-butane, meaning “normal” butane), and the branched isomer is called isobutane, meaning an “isomer of butane.’’ The three isomers of C5H12 are called pentane (or n-pentane), isopentane, and neopentane.
Notes
Butane is an alkane with the molecular formula C4H10. There are two possible molecular structures or isomers for compounds with this formula: n-butane and isobutane. Pentanes have a molecular formula of C5H12. There are three possible isomers for pentane: n-pentane, isopentane, and neopentane. The more carbons present, the more isomers will be possible.
Keywords
alkanes, isomers, butane, pentane
03-01-03UN

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Title
Nomenclature: The Main Chain
Caption
For example, the longest chain of carbon atoms in the following compound contains six carbons, so the compound is named as a hexane derivative. The longest chain is rarely drawn in a straight line; look carefully to find it. The following compound contains two different seven-carbon chains and is named as a heptane. We choose the chain on the right as the main chain because it has more substituents (in red) attached to the chain.
Notes
The first step in naming a compound is finding the longest continuous chain of carbons in the molecule. When more than one chain with the same the number of carbons is possible, the chain with more substituents attached to it must be chosen.
Keywords
carbon chain, substituent
03-02

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Title
Nomenclature: Alkyl groups
Caption
Figure 3-2 Some common alkyl groups.
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'.
Keywords
alkyl, substituent
03-03

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Title
Boiling Points of Branched and Unbranched Alkanes
Caption
Figure 3-3 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.
Keywords
London dispersion forces, boiling point, induced dipole
03-04

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Title
Melting Points of Alkanes
Caption
Figure 3-4 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 even number of carbons pack better than those with odd number of carbons.
Keywords
melting point, packing, three dimensional structure
03-04-07UN

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Title
Cracking and Hydrocracking
Caption
As discussed in Section 3-5B, the catalytic cracking of large hydrocarbons at high temperatures produces smaller hydrocarbons. The cracking process is usually operated under conditions that give the maximum yields of gasoline. In hydrocracking, hydrogen is added to give saturated hydrocarbons; cracking without hydrogen gives mixtures of alkanes and alkenes.
Notes
Long chains can be broken into smaller chains in a process known as cracking.
Keywords
hydrocarbon, catalyst, alkanes, alkenes
03-04-08UN

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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 1.09 Ā.
Notes
Any carbon with four sigma bonds has an sp3 hybridization.
Keywords
methane, tetrahedral, hybrid orbitals
03-04-09UN

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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 1.54 A and there is free rotation along this bond.
Keywords
ethane, hybrid orbitals
03-06

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Title
Ethane Conformations
Caption
Figure 3-6 Ethane conformations. The eclipsed conformation has a dihedral angle u 5 0°, and the staggered conformation has u 5 60°. Any other conformation is called a skew conformation.
Notes
In the Newman projection the rotation about the C-C single bond produced two different conformations. The eclipsed conformation has all hydrogens of the first carbon parallel to the hydrogens on the second carbon. The staggered conformation has the hydrogens of the first carbon at 60° from the hydrogens on the second carbon. The dihedral angle of eclipsed conformations is 0° while the dihedral angle for staggered conformations is 60°.
Keywords
conformation, Newman projection, dihedral angle, eclipsed, staggered
03-07

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Title
Conformational Analysis of Ethane
Caption
Figure 3-7 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.
Keywords
staggered conformation, eclipsed conformation, Newman projection, ethane
03-09

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Title
Conformational Analysis of Propane
Caption
Figure 3-9 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.
Keywords
staggered conformation, eclipsed conformation, torsional strain
03-10

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Title
Newman Projections of Butane
Caption
Figure 3-10 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.
Keywords
staggered conformation, gauche, anti, dihedral angle, totally eclipsed
03-11

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Title
Conformational Analysis of Butane
Caption
Figure 3-11 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.
Keywords
eclipsed conformation, staggered conformation, totally eclipsed, gauche, anti, dihedral angle
03-11-01UN

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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.
Keywords
totally eclipsed, conformation, repulsion, bulky groups, steric strain, steric hindrance
03-12

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Title
Cycloalkanes
Caption
Figure 3-12 Structures of some cycloalkanes.
Notes
The molecular formula of alkanes is CnH2n, two hydrogen less than an open chain alkane. Their physical properties resemple those of alkanes.
Keywords
alkanes, cycloalkanes
03-13

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Title
Cis-Trans Isomerism in Alkanes
Caption
Figure 3-13 Cis-trans isomerism in cycloalkanes. Like alkenes, cycloalkane rings are restricted from free rotation. Two substituents on a cycloalkane must be either on the same side (cis) or on opposite sides (trans) of the ring.
Notes
Unlike open chain alkanes there is no free rotation around the C-C single bond of cycloalkanes. Cis-trans isomerism is also used to describe the geometry of alkenes around the double bond.
Keywords
free rotation, cycloalkanes, cis, trans, substituent
03-14

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Title
Ring Strain of Cyclobutane
Caption
Figure 3-14 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.
Keywords
ring strain, angle strain, torsional strain
03-15

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Title
Angle Strain in Cyclopropane
Caption
Figure 3-15 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 cyclorpopanes is due to the non-linear overlap of the sp3 orbitals.
Keywords
ring strain, angle strain, torsional strain
03-16

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Title
Conformations of Cyclopropane
Caption
Figure 3-16 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.
Keywords
angle strain, torsional strain
03-171-3

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Title
Conformations of Cyclobutane
Caption
Figure 3-17 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.
Keywords
eclipsed conformation, torsional strain
03-18

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Title
Conformations of Cyclopentane
Caption
Figure 3-18 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.
Keywords
ring strain, cyclopentane, conformation, envelope
03-19

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Title
Conformations of Cyclohexane
Caption
Figure 3-19 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.
Keywords
conformations, chair, boat, twist boat, half chair
03-20

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Title
Boat Conformation of Cyclohexane
Caption
Figure 3-20 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.
Keywords
boat, steric hindrance, twist boat
03-21

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Title
Conformational Energy Diagram of Cyclohexane
Caption
Figure 3-21 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.
Keywords
cyclohexane, chair conformation, twist boat, half-chair
03-22

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Title
Chair Conformation of Cyclohexane
Caption
Figure 3-22 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.
Keywords
chair conformation, axial, equatorial
03-24

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Title
Newman Projection of Methylcyclohexane: Methyl Axial
Caption
Figure 3-24 (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.
Keywords
axial, equatorial, gauche, Newman projection
03-25

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Title
Newman Projection of Methylcyclohexane: Methyl Equatorial
Caption
Figure 3-25 Looking down the 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.
Keywords
axial, equatorial, anti, Newman projection
03-27

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Title
Chair Conformations of cis-1,3-Dimethylcyclohexane
Caption
Figure 3-27 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.
Keywords
1,3-diaxial interaction, steric hindrance, axial, equatorial
03-27-01UN

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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.
Keywords
1,3-diaxial interaction, steric hindrance, axial, equatorial
03-27-12UN

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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.
Keywords
butyl, axial, 1,3-diaxial interaction, steric interaction
03-28

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Title
Conformation of 1,4-di-t-butylcyclohexane
Caption
Figure 3-28 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.
Keywords
chair, boat, steric hindrance, steric interaction,
03-28-01UN

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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.
Keywords
bicyclic, polycyclic, bridged rings, spirocyclic compounds
03-28-02UN

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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.
Keywords
bicyclic compound, polycyclic compound, fused bicycle, bridged bicycle, spirocycle
03-29

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Title
Cis- and Trans-decalin
Caption
Figure 3-29 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.
Keywords
bicyclis compounds, decalin

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