Conformations of fused rings

1. PREPAREDBY:HINAISLAMCHISHTY
SUBMITTEDTO:
PROF.Dr.MUSHTAQUEALIJAKHRANI
ADVANCED STEREOCHEMISTRY
INSTITUTEOFCHEMISTRY
SHAH ABDULLATIF UNIVERSITY
KHAIRPURMIR`S

2. INTRODUCTION
• Molecules with multiple rings are very common in nature.A
prime example is the steroid structure, exemplified by the
common oral contraceptive ethinyl estradiol.
• Here, we are discussing two cyclohexane
rings are bonded to each other in
most common way- with the two
ring junctions (“bridgeheads”)on
adjacent carbons, a situation called fused rings. The parent
molecule is called decalin since there are ten carbon in total.

3. Conformation of fused rings

4. Conformation of fused rings:

5. Example: Synthetic Hormones

6. Example: pills

7. Stability of Fused Rings

8. Many Bicyclic Systems
• There are many important structures that result when one
ring is fused to another.
• Camphor, which you smelled the first day of class, and
camphene are fragrant natural products isolated from
evergreens.

9. Bicyclic Compounds
Nomenclature
The two molecules are not identical, therefore they cannot have
the same name.
Count the number of carbons connecting the
bridgeheads.

10. Bicyclic Compounds
Nomenclature
• Representing compounds with two fused rings.
• To name a bicyclic compound, include the prefix “bicyclo” in front of the total
carbon alkane name. For example, the compounds below could both be named,
bicycloheptane.

11. Conformation:
• Conformation cycloalkane fused ring is same as
that of cyclohexane. Let us consider cyclohaxane. If
the carbons of a cyclohexane ring were placed at
the corners of a regular planar hexagon, all the C-
C-C bond angles would have to be 120". Because
the expected normal C-C-C bond angle should be
near the tetrahedral value of 109.5", the suggested
planar configuration of cyclohexane would have
angle strain at each of the carbons, and would
correspond to less stable cyclohexane molecules
than those with more normal bond angles. The
actual normal value for the C-C-C bond angle of an
open-chain -CH2-CH,-CH2- unit appears to be
about 112.5", which is 3 times greater than the
tetrahedral value. From this we can conclude that
the angle strain at each carbon of a planar
cyclohexane would be (120" - 112.5") = 7.5". Angle
strain is not the whole story with regard to the
instability of the planar form, because in addition
to having C-C-C bond angles different from their
normal values, the planar structure also has its
carbons and hydrogens in the unfavorable eclipsed
arrangement.
Cyclohexane in the
strained planar
configuration showing
how the hydrogens
become eclipsed during
fused ring formation

12. Examples: Chair and Boat
form configurations.
• If the carbon valence angles are kept near the tetrahedral value,
you will find that you can construct ball-and-stick models of the
cyclohexane six-carbon ring with two quite different
conformations.
• These are known as the "chair" and "boat" conformations .
• It has not been possible to separate cyclohexane at room
temperature into pure isomeric forms that correspond to these
conformations, and actually the two forms appear to be rapidly
interconverted. The chair conformation is considerably more
stable and comprises more than 99.9% of the equilibrium mixture
at room temperature.
Chair (left) and boat (right) conformations of the six carbons of a cyclohexane ring
with normal C-C-C bond angles.

13. Doyouknow?
Whyistheboatformlessstablethanthechairform,if
bothhavenormalC-C-Cbondangles?
• The answer is that the boat form has unfavorable non-
bonded interactions between the hydrogen atoms around
the ring. If we make all of the bond angles normal and
orient the carbons to give the "extreme boat"
conformation ,a pair of 1,4 hydrogens (the so-called
"flagpole" hydrogens) have to be very close together (1.83
A). Hydrogens this close together would be on the rising
part of a repulsion potential energy curve, for hydrogen-
hydrogen non-bonded interactions. This steric hindrance
at an H-H distance of 1.83 A corresponds to a repulsion
energy of about 3 kcal mole-l. There is still another factor
that makes the extreme boat form unfavorable; namely,
that the eight hydrogens along the "sides" of the boat are
eclipsed, which brings them substantially closer together
than they would be in a staggered arrangement (about
2.27 A compared with 2.50 A). This is in striking contrast
with the chair form.

14. Do you know?
Differencebetweenchairand boat
conformation
• The chair structure is quite rigid, and rotation does
not occur around the C-C bonds with interconversion
to the boat structure. In contrast, the boat form is
quite flexible. Rotation about the C-C bonds permits
the ring to twist one way or the other from the extreme
boat conformation to considerably more stable, equal-
energy conformations, in which the flagpole hydrogens
move farther apart and the eight hydrogens along the
sides become largely but not completely staggered.
These arrangements are called the twist-boat
(sometimes skew-boat) con- formations and are
believed to be about 5 kcal mole-l less stable than the
chair form.

15. The twist-boatconformationsof cyclohexane:
(DreidingModels)Case1:

16. ConformationalEquilibriaand Equilibration
for Cyclohexanefusedrings
• Case 2:There are two distinct kinds of hydrogen in the
chair form of cyclohexane- six that are close to the
"average" plane of the ring (called equatorial hydrogens)
and three above and three below this average plane
(called axial hydrogens). This raises interesting questions
in connection with substituted cyclohexane. For example,
is the methyl group in methyl- cyclohexane equatorial or
axial. Since only one methyl cyclohexane is known, the
methyl group must be exclusively equatorial, exclusively
axial, or the two forms must be interconverted so rapidly
that they cannot be separated into isomeric forms. It
appears that the latter circumstance prevails, with the
ring changing rapidly from one chair form to another by
flipping one end of the chair up and the other end down.
This process is called ring inversion and it is process is
called ring frequency.

17. • Case 3: With cyclohexane, inversion is so fast at room
temperature t, on the average, the molecules flip about
100,000 times per second, over an energy barrier of
about 11 kcal mole-l. You will understand this flipping
process if you make a model of a cyclohexane ring
carrying a single substituent. By manipulating the
model you can discover some of the different ways the
process can occur. The simplest route is simply to flip
up one corner of the ring to convert the chair into a
boat and then flip down the opposite carbon:
• Because of the flexibility of the boat conformation, it is
possible to transform it to other boat conformations whereby
carbons other than the one indicated flip down and complete
the interconversion. At
Because of the flexibility of the boat conformation, it is possible to
transform it to other boat conformations whereby carbons other than
the one indicated flip down and complete the interconversion.

18. • Case 4: At room temperature the conformation of methyl
cyclohexane with the methyl equatorial is more stable than
the one with the methyl axial by 1.7 kcal molep1. The same is
true of all mono substituted cyclohexanes to a greater or
lesser degree. Reasons for this can be seen from space-filling
models (Figure 12-8), which show that a substituent group
has more room when the substituent is equatorial than when
it is axial. In the axial position the substituent is
considerably closer to the two axial hydrogens on the same
side of the ring than to other hydrogens, even hydrogens on
adjacent carbons when the substituent is in the equatorial
position.
equatorial
axial
Space-filling models of equatorial and axial chair conformations of
cyclohexyl bromide. Significant non bonded interactions are indicated for
the sawhorse formulas by dashed lines; these inter- actions are more severe
in the axial than the equatorial conformation.

19. Importanceof conformationof fusedrings in
organic synthesis:
• It`s gives information about stereochemistry of cyclohaxane
and their derivatives.
• It`s tells about difference spatial rearrangement of atoms
during passage of chemical reaction.
• It`s is useful in organic synthesis of macromolecules.

20. Reference:1. Seventh edition, Organic Chemistry by Robert Thornton Morrison, Robert Neilson Boyd, Saibal
Kanti Bhattacharjee.
2. Cycloalkanes, Cycloalkanes, and Cycloalkanes, Cyclohexane Conformations , Conformational
Equilibria and Equilibration for Cyclohexane Derivatives.
3. Google, Scifinder.
4. Journals of American chemical society.
5. Journals of Royal chemical society.
6. International Journal of Organic Chemistry.
7. https://www.researchgate.net/publication.
8. Major reference material Hill & Kolb, CHEMISTRY FOR CHANGING TIMES, 9th Ed, pp566-
573.
9. Grant, Ellen Dr. “ THE BITTER PILL” 1985 & “SEXUAL CHEMISTRY”1996.
10. Marschall, L., “The Sciences” p.1111, Nov/Dec1995. (This reference has the quote from Sanger
concerning reason for “pill” development).