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SKO 3013BASIC ORGANIC CHEMISTRY
NAME NO. MATRIC
NG SENG WEI D20091035105
TAY SIEW BEE D20081032270
Geometrical
isomerIntroduct
ion/ eliciting
ideas
Overview previous lesson
Learning outcomes
Structuring/
restructuring ideas
Cis-trans isomer
Structures
Activity
Physical
properties
Games
The E, Z rules
Applications of ideas
Problem solving 1
Reflection/
closure
Short quiz
Learning outcomes
homework
WHAT HAVE YOU LEARNED FROM THE PREVIOUS LESSON?
Isomerism
Constitutional Isomers
~Chain~Position~Functional group
Stereoisomers
Conformation Isomers
Configurational Isomers
Geometrical Isomers
Optical Isomers
Geometrical Isomers
LEARNING OUTCOMESLEARNING OUTCOMES
At the end of this subtopic, student should be able to
Distinguish geometrical isomers (C4). Identify cis-trans isomerism of a given
structural formula.Use sequence rules: The E, Z designation.
How geometric arise ??
What happen in molecules where is involving unrestricted rotation about carbon bonds - in other words where the carbon-carbon bonds are all single?
Two possible of configuration 1,2 dichoroethane
Two models exactly same
molecules and not isomer
• Organic compounds which have same
molecular formula but different non-
interchangable geometry due to
different orientation of atoms in space.
• Restricted rotation (often involving
carbon-carbon double bonds)
• Maximum overlap between the p
orbital of a pi bond.
GEOMETRICAL ISOMERS
CIS-TRANS ISOMERISM
PROBLEM 1
There are two steps to determine the cis or trans structure below:
First step
Draw an imaginary line collinear with that carbon-carbon double
bond
--------------------
Second steps
The structure that has the equivalent substituent on the same side of the imaginary is the cis stereoisomer and the
opposite side is trans stereoisomer.
-------------------- --------------------
Cis-1-chloro-1-propeneTrans-1-chloro-1-
propene
Cis isomer
Apply in daily item to remember!!
The two wheels on a motorcycle show the equivalent substituent on the same side of the imaginary. The body of the motorcycle shows the C=C double bond.
Trans isomer
Unbalanced seesaw
• Results from rigidity in molecules and occurs in
only two classes of compound:
i. Alkenes
ii. Cyclic compounds
1,2 -dichloroethene
These two molecules aren't the same. The
carbon-carbon double bond won’t rotate
and would have to take the models to
pieces in order to convert one structure into
other one. That is simple test for isomer.
i. Alkenes
Two chlorine atoms are locked on
opposite side of double
bond. Known as trans isomer.
Two chlorine atoms are locked on
same side of double bond.
Known as cis isomer.
EXAMPLE:
Because ring don’t rotate freely in space like linear chains two different possibilities exist. Cis Isomers are when two atoms are on the same side of a ring. Trans Isomers are when two atoms are on different side of a ring.
HH
H
HH
H
HH
Cis Isomers
Trans Isomers
ii. Cyclic compound
EXAMPLE:
H
CH3
H
CH3
CH3
CH3
H
H
cis-1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane
YOU WON’T HAVE GEOMETRIC ISOMERS IF THERE ARE TWO GROUPS THE SAME ON ONE END OF THE BOND. THE TWO PINK GROUPS ON THE LEFT HAND END.
YOU WON’T HAVE GEOMETRIC ISOMERS IF THERE ARE TWO GROUPS THE SAME ON ONE END OF THE BOND. THE TWO PINK GROUPS ON THE LEFT HAND END.
Both are same
molecule
There must be two
different groups on left and
right carbon to get
geometric isomer.
iii) H3C CH3
C=C
H3C H
Activity time
i) CH3CH2 H ii) H CH3
C=C C=C
H3C H Cl CH3
For these molecules above , give IUPAC name for the compounds and indicate if they are geometric isomers or neither.
i) CH3CH2 H ii) H CH3
C=C C=C
H3C H Cl CH3
2-methyl-1-butene 1-chloro-2-methylpropene
No cis – trans isomer
ANSWER:
No cis – trans isomer
iii) H3C CH3
C=C H3C HNo cis – trans isomer
2-methyl-2-butene
Geometrical Isomerism is
found in compound which
have
Planner Geometry
(real or virtual)
No interconvertibility
(presence of pi bond ,continuous
pi bond)
Different substituent on either side of
imaginary or real planner
geometry.
!!DON’T FORGET TO TAKE NOTE YA..
THE EFFECT OF GEOMETRIC ISOMERISM ON PHYSICAL PROPERTIES
ISOMER MELTING POINT (°C)
BOILING POINT(°C)
CIS -80 60
TRANS -50 48
Table shown the melting and boiling point of cis
and trans isomer of 1,2-dichloroethene
YOU WILL NOTICE THAT :
The trans isomer has the higher melting point.
The cis isomer has the lower boiling point.
Why is the boiling point
of the cis isomer higher?
Stronger intermolecular
forces.
Cis isomer is polar whereas trans is
not polar.
Higher dipole-dipole
interaction as well as
dispersion forces.
Alkyl groups attached to carbon-
carbon double bond tend to push
electrons away from themselves
and get polar molecules.
Why is the melting point of the cis isomer lower??
The ‘U’ shape of cis isomer does not pack as well as the straighter
shape of the trans isomer.
The poor packing in the cis isomer means that the intermolecular
forces weak and less energy is
needed to melt the molecule.
S P
I R L
H R
R
DOWN:When two identical substituent groups are oriented in the same site, it is called the ___(1)___ isomer.
Geometry isomerism is a form of ___(2)___.
The boiling point of the cis isomer is higher because it is a ___(3)___molecule.
CROSS:The trans isomer has the ___(4)___ melting point than cis isomer because the ___(5)___ forces are stronger.
___(6)___ isomer is the planar geometry when the two substituent groups are oriented in opposing site.
Crossword Puzzle Games
(1)
(2)
(3)
(4)
(5)
(6)
S T PC E O I N T E R M O L E C U L A RS E A O R
H I G H E R S O M E
T R A N S I S M
ANSWER
(1)
(2)
(3)
(6)
(4)
(5)
But even different things still have
geometric isomers.
Here the blue and
green groups are either on
the same side of the bond or the
apposite side.
Sequence rules: The E, Z
designation• Priority Rules for designating the precise isomer
configuration in stereoisomerism. • E and Z notations are introduced to designate geometrical
isomers of any type.
Cahn-Ingold-Prelog priority sequence rules
Geometrical isomer
Optical isomer
When dealing with a C-X bond grouping, the priority of X is given by ....
Sequence priority Rule 1:
The higher the atomic number of an atom the higher the priority it is assigned.
e.g. for typical non-metals ZX (Z = proton/atomic number) encountered in organic compounds ...
X = 53I > 35Br > 17Cl > 16S > 15P > 8O > 7N > 6C > 1H
Sequence priority Rule 2:
If the relative priority of two groups cannot be decided by Rule 1, it shall be determined by applying Rule 1 to the next atom or sequence of atoms in the group 'X'.e.g. for typing groupings in organic molecules where X is more than one atom ....
X = -CH2CH2CH3 > -CH2CH3 > -CH3 > -H (the longer the hydrocarbon carbon chain the
higher its priority)
Sequence priority Rule 3:
In groups containing multiple bonds, then for the purposes of determining priority the atom at the end of the bond is duplicated or triplicated as required.
PROBLEM 1
Determine the stereochemistry (E or Z ) below
First step
We assure ourselves that the carbon-carbon double bond is indeed stereogenic. We draw an imaginary line perpendicular to the carbon-carbon double bond
Second steps
Now rank the atoms directly bonded to the alkene carbons on each side of the imaginary line by atomic number high –to-low.
1
12
2
The two groups of higher priority
are label as 1 whereas the
other two groups with lower
priority are label as 2.
Third steps
Finally we connect the winning group
1
2
2
1
In this case, we cross the double bond to connect
the higher ranking groups. Thus alkene has the E stereochemistry and the systematic name of the molecule is (1E)-1-
bromo-2-fluoro-1-propene where “1” in “1 E” refer to the location of the
carbon-carbon double bond.
Fourth steps
1
2
2
1
The structure above is(1Z)-1-bromo-2-fluoro-propene
All the four steps were same in order to determine stereochemistry (E or Z ) for the structure below.
The Z isomer(Zusammen)
• Both highest priority groups are on the same side of the double bond (includes all cis configurations of the old convention).
• Example:
The E isomer (Entgegen)
• Two highest priority atoms/groups are diagonally opposite each other on differentsides of the plane of the double bond system (includes all trans isomers of the old convention).
• Example:
Z-hep-2-ene
E-hep-2-ene
cis/trans 2-heptene
Rank the set of substituents below in order of priority according to the Cahn-Ingold-Prelog sequence rules.1 = highest priority.
a) -COOHb) -CNc) -C=Od) -CH2Br
Explain why certain ones were ranked higher than others?
Problem
SOLUTION:a) -COOH-----2
b) -CN----4c) -C=O-----3d) -CH2Br ----1
To establish priority, you look at the atoms' atomic numbers and whichever one is greater takes priority. If they are equal, then you look at the next atom that it is bonded to and compare their atomic numbers. Now, if you encounter a double bond, you count that atom twice (so C=O is counted as C,O,O).
summaryStudents divide into two groups• Each group choose one item below for
your quizzes.
1. What is the difference between structural isomer and geometric isomer?
Structural isomerism is the same emperical formula but constructed differently and different structural formula.
Geometric isomerisim is where you have different isomers due to differences arrising from cis and trans arrangements of atoms.
Answers:
1,1-dibromocyclobutane, is a positional isomer of C4H6Br2 and cannot exhibit geometrical isomerism because the two bromine atoms are attached to the same carbon.
2. Can the compound below exhibit geometrical isomerism? Why?
Answers:
1,1-dibromocyclobutane
3. Determine the stereochemistry (E or Z) of the double bonds in this molecule:
Answers:
E-3-methylhex-3-ene
4. Give two examples of cis-trans isomerism in cyclic compounds.
Answers:
Z-1,2-dichlorocyclopropane (cis)E-1,2-dichlorocyclopropane (trans)
Z/cis E/trans
LEARNING OUTCOMESLEARNING OUTCOMES
At the end of this subtopic, students are able to
Distinguish geometrical isomers (C4). Identify cis-trans isomerism of a given
structural formula.Use sequence rules: The E, Z designation.
HOMEWORK
Text book Study problems page 59
No 4.3 i, ii, iii and iv. No 4.4
Worksheets (2 members per group).
THE END
Next lesson: Optical isomers
REFERENCES
1. Saripah Salbiah Syed Abdul Azziz, Azmi Mohamed, Mohamad Saufi Rosmi, Nurul Aini Binti Bakar dan Mimi Marzziana Hamzah. Basic Organic Chemistry. Sultan Idris Education University.
2. Stereoisomerism Geometric Isomerism. Retrieved on 15 March 2011 at
http://www.chemguide.co.uk/basicorg/isomerism/geometric.html
3. Organic Chemistry. Retrieved on 15 March 2011 at people.bu.edu/birubio/ch203ampmpdfs/l13.pdf
4. Cahn-Ingold-Prelog Priority Rule. Retrieved on 16 March 2011 at http://askville.amazon.com/Cahn-Ingold-Prelog-Priority-Rule/AnswerViewer.do?requestId=4776615
5. Organic Stereoisomerism-E/Z isomerism. Retrieved on 16 March 2011 at http://www.docbrown.info/page07/isomerism2.htm