&lt;number&gt; 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.
&lt;number&gt; 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.”
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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:
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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:
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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:
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
&lt;number&gt; 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.
Chapter 3 2
Hydrocarbons are molecules that are made of carbon
and hydrogen ONLY.
Chapter 3 3
• General formula: CnH2n+2
• Found in everything from natural gas to
• The smaller alkanes have very low boiling
points (b.p.) therefore they are gases.
CH4 C2H6 C3H8
Chapter 3 8
• International Union of Pure and Applied
• Common names kept: methane, ethane,
• 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”:
Chapter 3 9
• 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
• 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
• Write the alkyl groups in alphabetical order
regardless of their position on the chain.
Chapter 3 10
Rule 1: Find the Longest Chain of
The longest chain is
Chapter 3 11
When there are two longest chains of equal length, use the
chain with the greatest number of substituents.
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.
Chapter 3 14
Rule 3: Alkyl Substituents
• Name the groups attached to the longest chain as
• 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
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.
10 9 8 7 6 5 4 3 2 1
Three methyl groups at positions 2, 5, and 7
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
Chapter 3 17
Solved Problem 3-2
Give a systematic (IUPAC) name for the following
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
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.
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
• Branched alkanes have
higher melting points than
Chapter 3 22
hybrid carbon with angles of 109.5º.
Chapter 3 23
• Two sp3
• Rotation about the C—C sigma bond.
• Conformations are different arrangements of
atoms caused by rotation about a single bond.
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.
Chapter 3 25
The Newman projection is the best way to judge the
stability of the different conformations of a molecule.
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
Chapter 3 27
Propane is shown here as a perspective drawing and as
a Newman projection looking down the C1—C2 bond.
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.
Chapter 3 29
• 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 31
• 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
• This kind of interference between two bulky groups is
called steric strain or steric hindrance.
Chapter 3 33
• Relatively inert
• Boiling point and melting point depends on
the molecular weight.
Chapter 3 34
• 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.
Chapter 3 35
• If there are two or more substituents, number the
main chain to give all substituents the lowest
Chapter 3 37
Stabilities of Cycloalkanes
• Five- and six-membered rings are the most common in
• 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
• Torsional strain arises when all the bonds are eclipsed.
Chapter 3 38
• 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
orbitals and “bent bonds”.
Chapter 3 39
All the C—C bonds are eclipsed, generating torsional
strain that contributes to the total ring strain.
Chapter 3 40
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 41
• 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
Chapter 3 42
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 43
Chair Conformation of Cyclohexane
Chapter 3 47
The most important result in chair conversion is that any substituent
that is axial in the original conformation becomes equatorial in the
Chapter 3 48
Axial Methyl in Methylcyclohexane
Chapter 3 50
The axial substituent interferes with the axial hydrogens on C3
and C5. This interference is called a 1,3-diaxial interaction.
Chapter 3 51
• 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.
Chapter 3 52
Both conformations have one axial and one equatorial
methyl group so they have the same energy.
Chapter 3 53
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 54
Substituents are less crowded in the equatorial
Chapter 3 55
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.
Chapter 3 57
Nomenclature of Bicyclic Systems
Where # are the number of carbons on the bridges
(in decreasing order) and the alkane name
includes all the carbons in the compound.
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.
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.
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.
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.
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