2-how they could be classified? - psau.edu.sa...1-defintion of isomers. 2-how they could be...

http://www.google.com.sa/imgres?imgurl=http://image.slidesharecdn.com/extractionofplantcontituents-130122180846-phpapp02/95/slide-1-1024.jpg&imgrefurl=http://fr.slideshare.net/rahulbs89/extraction-of-plant-contituents&docid=dZaXOnJHMPihEM&tbnid=WuhHzXyM1cW1bM:&w=1024&h=768&ei=LosDUp6MDIOg0wWh84GoBQ&ved=0CAIQxiAwAA&iact=c

1-Defintion of Isomers.

2-How they could be Classified?

3- Naming of different isomers.

4-Some of their physical and

chemical properties.

http://www.google.com.sa/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=fF4W1GsOCZ4JGM&tbnid=g2GYvpcbsqA_ZM:&ved=0CAcQjRw&url=http://www.uic.edu/~kbruzik/text/phar331.htm&ei=_9ESVKqmOM7paLXxgOgO&psig=AFQjCNEX_CmI4qLhzQViroiCx4fAOG3sKQ&ust=1410605163595822

http://www.google.com.sa/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=lz3yJMaTvdsfUM&tbnid=OJdrFnRu1YLzbM:&ved=0CAcQjRw&url=http://home.nas.net/~dbc/cic_hamilton/stereo.html&ei=zNYSVKCcHIPPaN2egeAG&psig=AFQjCNHLzvt7kH00Rdz_W6ULEEUYriIe5w&ust=1410607145272467

http://www.google.com.sa/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=xhNwiV8F1qA-PM&tbnid=0xH9U_RgPyLkAM:&ved=0CAcQjRw&url=http://toolkit.smallbiz.nsw.gov.au/part/1/1/3&ei=XN4SVPrcO5LfavbggsAK&psig=AFQjCNH8P4goG9-wMipRQlOnnvzXXNsM9g&ust=1410609079066827




2D vs 3D

X

Y

X

Y

Z

Stereochemistry

It is that part of the science which deals with structure in three dimensions

Study of the arrangement of atoms in space (three-dimensional

shape “3D”) or molecules and how this affects their chemical and

physical properties

Isomerism

Structural Isomerism

1. Chain isomerism

2. Position isomerism

3. Functional isomerism

Stereoisomerism

1. Optical active isomers.

2. Geometrical isomers.

3. Conformational isomers.

Lecture Contents

When two or more organic

compounds have the same molecular

formula but differ in their physical or

chemical properties they are called

Isomers and the phenomenon called

isomerism.

A phenomenon resulting from

molecules having the same molecular

formula but different arrangement

In space.

Isomerism

Isomerism

Type of Isomerism….cont

You cannot convert one to the other without taking it apart and

rebuilding it ( i.e., without breaking and remaking bonds).

They are different molecules. They have the same

formula, but they are connected together differently

All the physical properties of constitutional isomers are different

different melting points and boiling points, different solubility

and densities, etc.

1-Structural isomerism /Constitutional isomers.

Thy include: A) Chain Isomerism (Or Skeletal Isomerism)

B) Positional Isomerism.

C) Functional Isomerism

A. Chain isomers

CH3CH2CH2CH2CH3 CH3

CCH2CH3 CH3

CCH3

CH3 CH3H CH3

2,2-dimethylpropanen-pentane 2-methylbutane

B. Position isomers

C5H12

C3H7Br

Br

2-bromopropane

Br

1-bromopropane

C. Functional isomers

C2H6O CH3

O

H3C

Dimethyl ether

OHCH3C

H

H

Ethanol

Constitutional isomers include:


1-Structural isomerism /Constitutional isomers..cont

Write all of the constitutional isomers having the molecular formula

C3H6O2

O

O

H

O

O

H

O

OH

OOO

O



Examples

C4H10 CH3CH2CH2CH3

CH3

CH3-CH-CH3

C3H8O CH3CH2CH2CH2CH3

CH3

CH3-CH-CH2CH3 (CH3)4C

C6H6O2

OH

OH

Catechol

OH

Resorcinol

OH

OH

Hydroquinone

OH

2-Stereoisomerism

Greek, Stereos=occupying space).

The stereoisomers have the same structural formulas but differ in

their arrangement of atoms in space but have the same bonding

pattern.

In other words, stereoisomerism is exhibited by such

compounds which have identical molecular structure

but different configurations.

Stereoisomers are the same constitutional isomer, but have a

difference in the way they are arranged in three-dimensional space at

one or more of their atoms.


Isomers

Structural isomerism

(Constitutional isomers)

Position of function groups

Skeleton or chain of carbon

Functional group

Stereoisomerism

Optical isomers

(Diastereomers

)Enantiomers

Geometrical

Cis & trans

Conformational

Configurational

Type of Isomerism

There are two main

classes of isomers:

compare

[ animation ]

The atoms at either end of a single bond can rotate relative to one

another.

Configuration Isomers Differ only in arrangement of their atoms in space

(3D geometry)


2-Stereoisomerism Conformational isomers

• Compounds which have the same molecular formula

but differ in arrangement of atoms or groups in space

different configuration

• Diastereoisomers have different physical and chemical

properties, as solubility and boiling points

thus:

- if liquids……. Fractional distillation

- if solids …….. Fractional crystallization

Type of Isomerism….cont.

2-Stereoisomerism…cont.

Geometric isomerism)Diastereomers (-I

An isomerism resulting from rigidity in molecules and occurs

only in two class of compounds

alkenes and cyclic compounds

Rigidity ≠ Flexibility

σ – bond free rotation ( flexibility )

- bond C = C no free rotation ( rigidity )

σ – bond restricted rotation ( semi-flexible)



Geometric isomerism)Diastereomers (-I

Free rotation of carbon-

carbon single bonds

Restricted rotation of carbon-

carbon double bonds



I-Geometric isomerism…cont

http://www.chm.bris.ac.uk/motm/thalidomide/cis_dce.mol

http://www.chm.bris.ac.uk/motm/thalidomide/trans_dce.mol

i- Geometric isomerism in alkenes

The requirement for this type of isomerism is that each carbon

atom involved in the double bond has different substitution

C C

E

A

B

D

D ≠ E A ≠ B

C

H

Cl

H

Cl

C

C

H

H

Cl

Cl

C

On same side

cis On different sides

trans




Geometrical Isomerism

If a ≠ c and b ≠ d then there are 2 diastereomeric forms:

C C

b

d

a

c

cis/trans isomers

cis, Latin on this side trans, Latin across

C C

H

Cl

Cl

H

C C

Cl

H

Cl

H

cis-1,2-Dichloroethene trans-1,2-Dichloroethene

C C

R

H

R

H

C C

H

R

R

H

RR

R

R

COMPARE cis / trans ISOMERS IN RING COMPOUNDS

cis trans

In alkenes cis / trans isomers used to be called geometric isomers,

a term generally not used for cyclic systems (rings). You may find the

term geometric isomers used in older books or papers.

C C

Cl

Cl

Cl

H

C C

Cl

Cl

H

H

1,1-dichloroethene 1,1,2-trichloroethene

cis-trans isomerism is not possible if one carbon atom of

the double bond bears 2 identical groups.

Which of the following alkenes can exist as cis-trans isomers?

Draw their structures?

H2C CHCH2CH3 H2C C(CH3)2

H3CHC CHCH3 H3CH2CHC CHCl

• Cis-trans isomers differ from one another in the way the atoms/ groups are positioned in space

– Cis –same, trans –across

– They have different physical and chemical properties

– They interact differently with enzymes/ receptor sites

– They cannot be interconverted by rotation around C-C bonds

• Rotation is restricted by double bond or cyclic

structure

Summary




Diastereoisomers are designed cis or trans according

to the priority order or sequence rule

[ atomic number = atomic weight / 2 ]

126I > 80Br > 35Cl > 32S > 19F > 16O > 14N > 12C > 1H

Sequence Rule or Priority Order

1- If the two atoms in question are different, the atom of

highest atomic number receives the highest priority

I > Br > Cl > ……

C C

H

H2C=HC

H

CH3

C C

H

H3C

Br

Cl

C C

H

H3C

CH2CH2CH3

OH

cis

trans

cis




2- If the two atoms are identical, the atomic numbers of next

atoms are used, until the priority is determined at the first

point of difference along the chain

C C

H

H3C

CH2OH

CH2NH2

C C

H

H3C

CH2CH2CL

CH2CH2Br

C C

H

H3C

CH2CH2CH3

CH2CH3

trans trans

cis

Sequence Rule or Priority Order



3- Atoms attached to double or triple bonds are given

single-bond equivalencies, each doubly bonded atom is

duplicated (or triplicated for triple bonds)

R-C-R

O

R-C-R

O

R-C-OH

O

R-C-OH

O

R-C-H

O

R-C

N N

N

NCR

O

O

OHCR

O

O

RCR

RCH2OH RCN -CH=CR2 > > > > > >



Sequence Rules or Priority Order

E / Z SYSTEM OF NOMENCLATURE

Diastereoisomers can also be designated by:

1- E instead of trans

2- Z instead of cis

IC C

Cl

F

Br

cis or trans?

E and Z Based on Priority:

Cahn- Ingold-Prelog:

1. Atomic Number

2. Atomic weight

3. Atomic number of the

next atom

----------------

35

53 1

2 17

9 2

1

Higher priority at the opposite side of pi bond (E)

Higher priority at the same side of pi bond (Z)

(E)-1-bromo-2-chloro-

2-fluoro-1-iodoethene

In this system the two groups attached to each carbon

are assigned a priority ( 1 or 2 ).

If priority 1 groups are both on same side of double bond:

11

2 2

21

2 1

Z E

If priority 1 groups on opposite sides of double bond:

E isomer = entgegen = opposite (in German)

Z isomer = zusammen = together (in German)

same

side opposite

sides

ASSIGNING PRIORITIES

1. Look at the atoms attached to each carbon of the double bond.

2. The atom of higher atomic number has higher (1)

priority.

example IF

H Br

1

2

1

2

F > H I > Br

Since the 1’s are on the same side, this compound is Z

(Z)-1-bromo-2-fluoro-1-iodoethene

notice use of parentheses

Br

Cl

ClH2CH2C

H3C

3 4dc

On carbon c, the chloroethyl substituent is of higher priority than methyl

On carbon d, the bromine substituent is of higher priority than chlorine

IUPAC name is (Z)-1-bromo-1,4-dichloro-2-methylbut-1-ene

Extended Comparison

• If atomic numbers are the same, compare at next connection point

at same distance

• Compare until something has higher atomic number

• Do not combine – always compare

CH

CR2 C N CH2OH

O

H

O

R

O

OH

Increasing priority

BrCl

CO2HH2N

CO2HCl

BrH2N

Z E

CO2HH

H

CO2H

E Z

cis/trans Geometry of Alicyclic Compounds

The cyclic systems use the traditional cis/trans nomenclature.

trans-1,2-dimethylcyclobutane cis-1,2-dimethylcyclobutane

trans-1,2-dimethylcyclohexane cis-1,2-dimethylcyclohexane

C C

H

H3C

CHO

COOH

C C

H

H3C

CH2NH2

CN

C C

H2C=HCCOOH

OCH3

cis cis cis

C C

H3CCCCH2Cl

Cl

OO

ClH2C

C C

H3CCCl

CCH2Cl

O

ClH2C

O

Cis

Z

Trans

E

Sequence Rules or Priority Order

Examples

http://www.chemeddl.org/resources/stereochem/definitions1.htm

http://orgchem.chem.uconn.edu/courses/243f97-rs-

practice.html

Useful resources

2-how they could be classified? - psau.edu.sa...1-defintion of isomers. 2-how they could be...

Documents