Upload
others
View
6
Download
0
Embed Size (px)
Citation preview
4. Organic Compounds:
Cycloalkanes and their
Stereochemistry
Fall, 2014 by Dr.Isam Al Naser
Based on McMurry’s Organic Chemistry, 7th edition
2
Why this chapter?
Because cyclic molecules are commonly encountered in all classes of bimoleculars:
- Proteins.,Lipids.,Carbohydrates.,Nucleic acids…
3
4.1 Naming Cycloalkanes
Cycloalkanes are saturated cyclic hydrocarbons
Have the general formula (CnH2n)
4
Naming Cycloalkanes
Find the parent. of carbons in the ring.
Number the substituents
5
6
Give IUPAC names for the following cycloalkanes?
Draw structures corrsponding to the following
IUPAC names.
a)1,1-dimethylcyclooctane b) 3-cyclobutylhexane
c) 1,2-dichlorocyclopentane c)1,3-dibromo-5-
methylcyclohexane. 7
8
4.2 Cis-Trans Isomerism in
Cycloalkanes
Cycloalkanes are less flexible than open-chain
alkanes
Much less conformational freedom in
cycloalkanes
9
Because of their cyclic structure, cycloalkanes have 2 faces as
viewed edge-on
“top” face “bottom” face
- Therefore, isomerism is possible in substituted cycloalkanes
- There are two different 1,2-dimethyl-cyclopropane isomers
- Both isomers are stable compund, and neither can be
converted into the other without breaking chemical bonds
10
Stereoisomerism or chemical isomers
Compounds which have their atoms connected in the
same order but differ in 3-D orientation of the atoms.
Butane and isobutan.1,2-dimethylcyclopropanes
Name the following substances, including the
cis-or trans-prefix:
Draw the structures of the following molecules:
a) trans-1-bromo-3-methylcyclohexane
b) Cis-1,2-dimethylcyclobutane
c) Trans-1-tert-butyl-2-ethylcyclohexane.
11
12
4.3 Stability of Cycloalkanes:
Ring Strain Rings larger than 3 atoms are not flat
Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering
Larger rings have many more possible conformations than smaller rings and are more difficult to analyze Baeyer (1885): since carbon prefers to have bond angles of approximately 109°.
13
Stability of Cycloalkanes: The
Baeyer Strain TheoryThe strain compound equal total energy of the compound subtract the energy of a
strain-free reference compound.
Cycloalkane strain energies, calculated by taking the difference between
cycloalkane heat of combustion pre CH2 and acyclic alkane heat of combustion
Pre CH2,and multiplying by the number of CH2 units in a ring.
Cyclohexane rings are strain –free.
14
Summary: Types of Strain
Angle strain - expansion or compression of bond
angles away from most stable
Torsional strain - eclipsing of bonds on neighboring
atoms
Steric strain - repulsive interactions between
nonbonded atoms in close proximity
15
4.4 Conformations of Cycloalkanes
Cyclopropane 3-membered ring must have planar structure
Symmetrical with C–C–C bond angles of 60°
Requires that sp3 based bonds are bent (and weakened)
All C-H bonds are eclipsed
16
Bent Bonds of Cyclopropane
In cyclopropane, the C-C bond is displaced outward from
internuclear axis
Cyclopropane bonds are weaker and more reactive than typical
alkane bonds.
17
Cyclobutane
Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens
The total strain for cyclobutane 110 kJ/mol and 115kJ/mol for cyclopropane.
Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above
18
Cyclopentane
Planar cyclopentane would have no angle strain but very high torsional strain, total strain energy of 26 kJ/mol
Actual conformations of cyclopentane are nonplanar, reducing torsional strain
Four carbon atoms are in a plane
The fifth carbon atom is above or below the plane –looks like an envelope
19
4.5 Conformations of Cyclohexane
Substituted cyclohexanes occur widely in nature
The cyclohexane ring is free of angle strain and torsional strain
The conformation is has alternating atoms in a common plane and tetrahedral angles between all carbons
This is called a chair conformation
20
How to Draw Cyclohexane
Twist-boat conformation cyclohexane
21
22
4.6 Axial and Equatorial Bonds in
Cyclohexane The chair conformation
has two kinds of positions for substituents on the ring: axialpositions and equatorialpositions
Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorialhydrogens near the plane of the ring
23
Drawing the Axial and Equatorial
Hydrogens
24
Conformational Mobility of Cyclohexane
Chair conformations readily interconvert,
resulting in the exchange of axial and
equatorial positions by a ring-flip
Axial bromocyclohexane becaomes
equatorial bromocyclohexane after ring-flip.
Sins the different energy to chair-chair
interconversion is only about 45KJ/mol
25
26
Draw two different chair conformation of
cyclohexanol (hydroxycyclohexanol).Identify each
position as axial or equatorial.
Draw the two different chair conformation of trans-
1,4-dimethylcyclohexane,and label all positions as
axial or equatorial.
Draw the two chair conformation of menthol, and
tell which is more stable.
27
4.7 Conformations of
Monosubstituted Cyclohexanes Cyclohexane ring rapidly flips between chair
conformations at room temp.
Two conformations of monosubstituted cyclohexane aren’t equally stable.
The equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol
28
1,3-Diaxial Interactions
Difference between axial and equatorial conformers is due to
steric strain caused by 1,3-diaxial interactions
Hydrogen atoms of the axial methyl group on C1 are too close to
the axial hydrogens three carbons away on C3 and C5, resulting
in 7.6 kJ/mol of steric strain
29
Relationship to Gauche Butane
Interactions Gauche butane is less stable than anti butane by 3.8 kJ/mol
because of steric interference between hydrogen atoms on the two methyl groups
The four-carbon fragment of axial methylcyclohexane and gauche butane have the same steric interaction
In general, equatorial positions give more stable isomer
1,3-Diaxial Interactions
30
•For other monosubstitiuted
cyclhexanes Is more stable in
an equatorial position than in
an axial position
•It is Depends on the nature
and size of substituent.
•The values must be doubled
to arrive at the amount of
strain in a mono substitiuted
cyclohexane
31
4.8 Conformational Analysis of
Disubstituted Cyclohexanes
In disubstituted cyclohexanes the steric effects of both substituents must be taken into account in both conformations
There are two isomers of 1,2-dimethylcyclohexane. cis and trans In the cis isomer, both methyl groups are on the same face of the
ring, and compound can exist in two chair conformations
Consider the sum of all interactions
In cis-1,2, both conformations are equal in energy
32
Trans-1,2-Dimethylcyclohexane
Methyl groups are on opposite faces of the ring
One trans conformation has both methyl groups equatorial and only a gauche butane interaction between methyls (3.8 kJ/mol) and no 1,3-diaxial interactions
The ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactions
Steric strain of 4 3.8 kJ/mol = 15.2 kJ/mol makes the diaxial conformation 11.4 kJ/mol less favorable than the diequatorial conformation
trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial conformation
Conformation of cis-1tert-butyl-4-chlorocyclohexane
33
These conformations aren’t of equal energy because an axial tert- butyl
substitiutent and an axial chloro subsitituent produce different amounts
of steric strain.
1,3-diaxial interaction between a hydrogen and tert- butyl group cost 11
kJ/mol, the interaction between a hydrogen and chlorine only 1kJ/mol .
The stable conformation with the chlorine axial and the tert-butyl
equatorial.
34
4.9 Conformations of Polycyclic
Molecules Decalin consists of two cyclohexane rings joined to share two
carbon atoms (the bridgehead carbons, C1 and C6) and a common bond
Two isomeric forms of decalin: trans fused or cis fused
In cis-decalin hydrogen atoms at the bridgehead carbons are on the same face of the rings
In trans-decalin, the bridgehead hydrogens are on opposite faces
Both compounds can be represented using chair cyclohexane conformations
Flips and rotations do not interconvert cis and trans
Although steroids look complicated compared with cyclohexane or decalin.
35