4.stereochem1

1. Isomerism
Ms. Anjelyn del Rosario
Chemistry 31
UNIVERSITY OF THE PHILIPPINES MANILA
Padre Faura, Ermita, Manila
SS, 2009 –2010
1chem31 adr

2. Isomers
2chem31 adr
Compounds that have the same molecular formula but are
not identical
Isomerism

3. 3
•The two major classes of isomers are constitutional isomers and
stereoisomers.
Constitutional/structural isomers have different IUPAC names,
the same or different functional groups, different physical
properties and different chemical properties.
Stereoisomersdiffer only in the way the atoms are oriented in
space. They have identical IUPAC names (except for a prefix like
cisor trans). They always have the same functional group(s).
•A particular three-dimensional arrangement is called a
configuration. Stereoisomers differ in configuration.
The Two Major Classes of Isomers:
Isomerism

4. 4
Figure 5.3 A comparison of constitutional isomers
and geometric stereoisomers
Isomerism

5. Structural Isomerism
•Structural isomersare molecules
with the same chemical formulas
but different molecular structures
-different “connectivity”.
•They arise because of the many
ways to create branched
hydrocarbons.
•a.k.a. “Constitutional Isomers”CH
3
CH
2
CH
2
CH
2
CH
3 CH
3
CH
2
CH
CH
3
CH
3
n-pentane, C
5H
12
2-methylbutane, C
5H
12
Constitutional Isomers

6. The First 10 “Normal” Alkanes
Name Formula M.P.B.P.# Structural Isomers
•MethaneCH
4 -183-162 1
•EthaneC
2H
6 -172-89 1
•PropaneC
3H
8 -187-42 1
•ButaneC
4H
10 -1380 2
•PentaneC
5H
12 -13036 3
•HexaneC
6H
14 -9568 5
•HeptaneC
7H
16 -9198 9
•OctaneC
8H
18 -57126 18
•NonaneC
9H
20 -54151 35
•DecaneC
10H
22 -30174 75
C1 -C4 are Gases
at Room Temperature
C5 -C16 are Liquids
at Room Temperature

7. 1.Chain Isomers
have the same number of C and H atoms but different points
of attachmentCH
3
CH
2
CH
2
CH
2
CH
3 CH
3
CH
2
CH
CH
3
CH
3
n-pentane, C
5H
12
2-methylbutane, C
5H
12
Constitutional Isomers

8. 2. Position Isomers
Differ in the positions of substituents or multiple bonds
Constitutional Isomers

9. Constitutional Isomers
3. Functional Isomers
Differ in their functional groups.

10. 2. Stereochemistry
Stereochemistry
10chem31 adr
the study of the spatial arrangementof atoms in molecules.

11. Stereochemistry
Example of Stereoisomers:
Stereoisomersdiffer
only in the way the
atoms are oriented in
space. They have
identical IUPAC names
(except for a prefix like
cisor trans). They
always have the same
functional group(s).

12. Conformational Isomers
I. Rotation about
Single Bonds

13. Conformational Isomers
•Conformer:a specific conformation
1.Rotation about single bonds
2.Amine inversion
•Molecules constantly rotate through all the
possible conformations.
Conformation: any three-dimensional arrangement of atoms in
a molecule that results from rotation about a single bond

14. Conformational Isomers: Rotation about Single Bonds
Example: Ethane

15. •Staggered conformation: a conformation about a carbon-
carbon single bond in which the atoms or groups on one
carbon are as far apart as possiblefrom the atoms or
groups on an adjacent carbon H
H H
H H
H
Example: Ethane
Conformational Isomers: Rotation about Single Bonds

16. •Eclipsed conformation: a conformation about a carbon-
carbon single bond in which the atoms or groups of atoms
on one carbon are as close as possibleto the atoms or
groups of atoms on an adjacent carbonH
H H
H
H
H
Example: Ethane
Conformational Isomers: Rotation about Single Bonds

17. Conformational Isomers: Rotation about Single Bonds

18. •Torsional strain
–also calledeclipsed interaction strain
–strain that arises when nonbonded atoms separated by three
bonds are forced from a staggered conformation to an eclipsed
conformation
–the torsional strain between eclipsed and staggered ethane is
approximately 12.6 kJ (3.0 kcal)/mol +12.6 kJ/mol
Conformational Isomers: Rotation about Single Bonds

19. •Dihedral angle (Q):the angle created by two intersecting
planes
Conformational Isomers: Rotation about Single Bonds

20. Conformational Isomers: Rotation about Single Bonds

21. Example: Butane (C1-C2 bond)
Conformational Isomers: Rotation about Single Bonds

22. Example: Butane (C2-C3 bond)
(Anti is Greek for “opposite of ”; gauche is French for “left.”)
Conformational Isomers: Rotation about Single Bonds

23. Eclipsed Butane
–calculated energy difference between (a) the non-energy-
minimized and (b) the energy-minimized eclipsed
conformations is 5.6 kJ (0.86 kcal)/mol
Conformational Isomers: Rotation about Single Bonds

24. •anti conformation
–a conformation about a single bond in which the groups lie at
a dihedral angle of 180°CH
3
H H
H H
CH
3
Conformational Isomers: Rotation about Single Bonds

25. Angle strain
the strain induced in a molecule when the bond angles are
different from the ideal tetrahedral bond angle of 109.5°.
Torsional strain
the strain caused by repulsion between the bonding electrons of
one substituent and the bonding electrons of a nearby
substituent.
Steric strain
straincaused by atoms or groups of atoms approaching each
other too closely.
Conformational Isomers: Rotation about Single Bonds

26. Conformational Isomers: Rotation about Single Bonds

27. 27
•Large ring strain due to angle compression
•Very reactive, weak bonds
•Torsional strain because of eclipsed hydrogens
Conformational Isomers: Cycloalkanes
Cycloalkanes: Ring Strain
1. Cyclopropane

28. –puckering from planar cyclobutane reduces torsional strain
but increases angle strain
–the conformation of minimum energy is a puckered
“butterfly” conformation
–strain energy is about 110 kJ (26.3 kcal)/mol
Conformational Isomers: Cycloalkanes
Cycloalkanes: Ring Strain
2. Cyclobutane

29. –puckering from planar cyclopentane reduces torsional
strain, but increases angle stain
–the conformation of minimum energy is a puckered
“envelope” conformation
–strain energy is about 42 kJ (6.5 kcal)/mol
Conformational Isomers: Cycloalkanes
Cycloalkanes: Ring Strain
3. Cyclopentane

30. •Chair conformation: the most stable puckered conformation
of a cyclohexane ring
–all bond C-C-C bond angles are 110.9°
–all bonds on adjacent carbons are staggered
Conformational Isomers: Cycloalkanes
Cycloalkanes: Ring Strain
2. Cyclohexane

31. Axial and equatorial
Conformational Isomers: Cycloalkanes
Cycloalkanes: Ring Strain
2. Cyclohexane
ring flip

32. •Boat conformation: carbons 1 and 4 are bent toward each
other
–there are four sets of eclipsed C-H interactions and one flagpole interaction
–a boat conformation is less stable than a chair conformation by 27 kJ (6.5
kcal)/mol
Conformational Isomers: Cycloalkanes
Cycloalkanes: Ring Strain
2. Cyclohexane

33. •Twist-boat conformation
–approximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation
–approximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation
Conformational Isomers: Cycloalkanes
Cycloalkanes: Ring Strain
2. Cyclohexane

34. •Half-chair conformation
Conformational Isomers: Cycloalkanes
Cycloalkanes: Ring Strain
2. Cyclohexane

35. Chapter 3 35
=>
Conformational Energy

36. Conformational Isomers
II. Amine Inversion

37. •The lone-pair electrons on nitrogen allow an amine to turn
“inside out” rapidly at room temperature.
•The lone pair is required for inversion: Quaternary ammonium
ions—ions with four bonds to nitrogen and hence no lone
pair—do not invert.
•amine inversion takes place through a transition state in which
the sp
3
nitrogen becomes an sp
2
nitrogen.
Conformational Isomers
Amine Inversion

38. Configurational Isomers
I. Cis-Trans Isomers

39. Configurational Isomers
I. Cis-Trans Isomers
Alkenes and cyclic alkanes

40. I. Cis-Trans Isomers
Alkenes
Configurational Isomers

41. I. Cis-Trans Isomers
cis isomer
the isomer with the hydrogens on the same side of the double bond
trans isomer
the isomer with the hydrogens on opposite sides of the double bond
Configurational Isomers

42. I. Cis-Trans Isomers
Configurational Isomers

43. Exercise
Configurational Isomers

44. The E,Z System of Nomenclature
“If the hydrogens are on the same side of the double bond, it is the cis isomer;
if they are on opposite sides of the double bond, it is the trans isomer.”
The Z isomer has the high-priority groups on the SAME side.
The E isomer has the high-priority groups on the OPPOSITE side.
Configurational Isomers

45. The E,Z System of Nomenclature
Rule 1. The relative priorities of the two groups depend on the
atomic numbers of the atoms that are bonded directly to the sp
2
carbon. The greater the atomic number, the higher is the priority.
Configurational Isomers

46. Rule 2. If the two substituents bonded to an carbon start with the
same atom (there is a tie), you must move outward from the point
of attachment and consider the atomic numbers of the atoms that
are attached to the “tied” atoms.
The E,Z System of Nomenclature
Configurational Isomers

47. Rule 3. If an atom is doubly bonded to another atom, the priority
system treats it as if it were singly bonded to two of those atoms. If
an atom is triply bonded to another atom, the priority system
treats it as if it were singly bonded to three of those atoms.
The E,Z System of Nomenclature
Configurational Isomers

48. Rule 4. In the case of isotopes (atoms with the same atomic
number, but different mass numbers), the mass number is used to
determine the relative priorities.
The E,Z System of Nomenclature
Configurational Isomers

49. Exercise:
Draw and label the E and Z isomers for each of the following
compounds:
Configurational Isomers

50. I. Cis-Trans Isomers
cyclic alkanes
The cis isomer has its substituents on the same side of the ring.
The trans isomer has its substituents on opposite sides of the ring.
Configurational Isomers

51. I. Cis-Trans Isomers
cyclic alkanes
Configurational Isomers

52. Exercise:
Determine whether each of the following compounds is a cis isomer
or a trans isomer:
Configurational Isomers