Organic Chemistry
ByDr. Mehnaz KamalAssistant Professor,
Pharmaceutical ChemistryPrince Sattam Bin Abdulaziz University
1-Defintion of Isomers.2-How they could be Classified?3- Naming of different isomers.4-Some of their physical and chemical properties.
2D vs 3DX
Y
X
Y
Z
StereochemistryIt 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 isomerism2 .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
moleculs having the same molecular
formula but different arrangement
In space.
IsomerismIsomerism
This is a branched-chainalkane.
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
CH3CH2
CH2CH2
CH3 CH3C
CH2CH3 CH3
CCH3
CH3 CH3H CH3
2,2-dimethylpropanen-pentane 2-methylbutane
B. Position isomers
C5H12
C3H7BrBr
2-bromopropane
Br
1-bromopropane
C. Functional isomers
C2H6OCH3
OH3C
Dimethyl ether
OHCH3C
H
H
Ethanol
Constitutional isomers include:
Type of Isomerism….cont
1-Structural isomerism /Constitutional isomers..cont
Type of Isomerism….cont
1-Structural isomerism /Constitutional isomers..cont
Write all of the constitutional isomers having the molecular formula C3H6O2
O
O
HO
O
H
O
OH
OOO
O
1-Structural isomerism /Constitutional isomers..cont
Type of Isomerism….cont
Examples
C4H10 CH3CH2CH2CH3
CH3
CH3-CH-CH3
C3H8O CH3CH2CH2CH2CH3
CH3
CH3-CH-CH2CH3 (CH3)4C
C6H6O2
OHOH
Catechol
OH
Resorcinol
OH
OH
Hydroquinone
OH
2-StereoisomerismGreek, 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.
Type of Isomerism….cont
Isomers
Structural isomerism)Constitutional isomers(
Position of function groups
Skeleton or chain of carbon Functional group
Stereoisomerism
Optical isomers
)Diastereomers)Enantiomers
GeometricalCis & trans
ConformationalConfigurational
Type of IsomerismThere are two main classes of isomers:
compare
The atoms at either end of a single bond can rotate relative to one another.
Type of Isomerism….cont2-Stereoisomerism
Conformational isomers
Free rotation of carbon- carbon single bonds
• 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.
I-Diastereomers (Geometric isomerism)
Configuration IsomersDiffer only in arrangement of their atoms in space (3D geometry)
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)
Type of Isomerism….cont.2-Stereoisomerism…cont.
I-Diastereomers (Geometric isomerism)
Restricted rotation of carbon-carbon double bonds
Type of Isomerism….cont.
2-Stereoisomerism…cont.I-Geometric isomerism…cont
i- Geometric isomerism in alkenesThe requirement for this type of isomerism is that each carbon atom involved in the double bond has different substitution
C CE
A
B
D
D ≠ E A ≠ B
CH
Cl
H
Cl
C
CH
H
Cl
Cl
C
On same side cis On different sides
trans
Type of Isomerism….cont.
2-Stereoisomerism…cont.I-Geometric isomerism…cont
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
Hcis-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
HC 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
Type of Isomerism….cont.
2-Stereoisomerism…cont.I-Geometric isomerism…cont
Diastereoisomers are designed cis or trans according to the priority order or sequence rule [ atomic number]
53I > 35Br > 17Cl > 16S > 9F > 8O > 7N > 6C > 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
Type of Isomerism….cont.
2-Stereoisomerism…cont.I-Geometric isomerism…cont
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
transtrans
cis
Sequence Rule or Priority Order
I-Geometric isomerism…cont
2-Stereoisomerism…cont.
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> > > > > >
2-Stereoisomerism…cont.
I-Geometric isomerism…cont
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 Number2. 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 carbonare 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 EIf priority 1 groups on opposite sides of double bond:
E isomer = entgegen = opposite (in German)
Z isomer = zusammen = together (in German)
sameside opposite
sides
ASSIGNING PRIORITIES1. 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
2F > 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
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=HC COOH
OCH3
cis cis cis
C C
H3CC CCH2Cl
Cl
O O
ClH2C
C C
H3CC Cl
CCH2Cl
O
ClH2C
O
CisZ
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