Isomerism

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


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)


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

Isomerism

Documents

Transcript of Isomerism