Download Functional Group Isomerism

Isomerism
Dean Martin
City and Islington College
Topic
Chemistry – Isomerism
Aims


To help introduce Isomerism
Aimed at 16-19 Year olds
Level
Level 3
Method
PowerPoint slides, ALL hand-outs (apart from slide 2)
Equipment




Projector
Laptop
Hand-Outs
Pens
Duration
>30 Minutes
Learning Outcomes
• What are isomers?
• Types of isomerism
• Explain each type of isomerism
What are Isomers?
Isomers are molecules that have the same
molecular formula, but have a different
arrangement of the atoms in space. That
excludes any different arrangements which
are simply due to the molecule rotating as a
whole, or rotating about particular bonds
Types of Isomerism
CHAIN ISOMERISM
STRUCTURAL ISOMERISM
Same molecular formula but
different structural formulae
POSITION ISOMERISM
FUNCTIONAL GROUP
ISOMERISM
GEOMETRICAL ISOMERISM
STEREOISOMERISM
Same molecular
formula but atoms
occupy different
positions in space.
Occurs due to the restricted
rotation of C=C double bonds...
two forms… E and Z (CIS and
TRANS)
OPTICAL ISOMERISM
Occurs when molecules have a
chiral centre. Get two nonsuperimposable mirror images.
What are Structural
Isomers?
Structural isomers have different structural
formulae because their atoms are linked
together in different ways.
Structural Isomerism
Introduction
Chain
different arrangements of the carbon skeleton
similar chemical properties
slightly different physical properties
more branching = lower boiling point
Positional
same carbon skeleton
same functional group
functional group is in a different position
similar chemical properties - slightly different physical properties
Functional Group
different functional group
different chemical properties
different physical properties
Sometimes more than one type of isomerism occurs in the same molecule.
The more carbon atoms there are, the greater the number of possible isomers
Chain Isomerism
These isomers arise because of the possibility of
branching in carbon chains.
butane, C4H10
pentane, C5H12
Structural
Isomerism - Chain
Chemical
DIFFERENCES BETWEEN CHAIN ISOMERS
Isomers show similar chemical properties because
the same functional group is present.
Physical
Properties such as density and boiling point show trends
according to the of the degree of branching
Boiling Point
“straight” chain isomers have higher values than branched
ones. The greater the degree of branching the lower the
boiling point branching decreases the effectiveness of
intermolecular forces less energy has to be put in to separate the
molecules
- 0.5°C
straight chain
- 11.7°C
branched
greater branching
= lower boiling point
Position Isomerism
In position isomerism, the basic carbon skeleton remains
unchanged, but important groups are moved around on
that skeleton.
Functional
Group Isomerism
In this variety of structural isomerism, the isomers contain
different functional groups – that is, they belong to different
families of compounds (different homologous series).
molecular formula C3H6O
molecular formula C3H6O2
Functional
Group Isomerism
molecules have same molecular formula
molecules have different functional groups
molecules have different chemical properties
molecules have different physical properties
ALCOHOLS and ETHERS
ALDEHYDES and KETONES
ACIDS and ESTERS
Functional
Group Isomerism
ALCOHOLS and ETHERS
Name
ETHANOL
METHOXYMETHANE
Classification
ALCOHOL
ETHER
Functional Group
R-OH
R-O-R
Physical properties
polar O-H bond gives rise
to hydrogen bonding.
get higher boiling point
and solubility in water
No hydrogen bonding
low boiling point
insoluble in water
Chemical properties
Lewis base
Wide range of reactions
Inert
Functional
group Isomerism
ALDEHYDES and KETONES
Name
PROPANAL
PROPANONE
Classification
ALDEHYDE
KETONE
Functional Group
R-CHO
R-CO-R
Physical properties
polar C=O bond gives
dipole-dipole interaction
polar C=O bond gives
dipole-dipole interaction
Chemical properties
easily oxidised to acids of
same number of carbons
undergo oxidation under
extreme conditions only
reduced to 1° alcohols
reduced to 2° alcohols
Functional
group Isomerism
CARBOXYLIC ACIDS and ESTERS
Name
PROPANOIC ACID
METHYL ETHANOATE
Classification
CARBOXYLIC ACID
ESTER
Functional Group
R-COOH
R-COOR
Physical properties
O-H bond gives rise
to hydrogen bonding.
get higher boiling point
and solubility in water
No hydrogen bonding
insoluble in water
Chemical properties
acidic
react with alcohols
fairly unreactive
hydrolysed to acids
What are
Stereoisomers?
Stereoisomers have the same structure and bond
order but their atoms and groups of atoms are
arranged differently in space. They have different
spatial arrangements and their molecules are not
super imposable.
Geometric
Isomerism
Involves a double bond, usually C=C, that does not allow free
rotation about the double bond (unlike a C Csingle bond). They
are not superimposable.
e.g. cis-but-2-ene and trans-but-2-ene
Stereoisomers may possess quite different physical properties, such as
melting point, density and solubility in water. Ring structures and other steric
factors also result in geometric isomerism.
Geometric
Isomerism
To get geometric isomers you must have:
1. Restricted rotation (often involving a carbon double
bond for introductory purposes);
2. Two different groups on the left-hand end of the bond
and two different groups on the right hand end. It
doesn't matter whether the left hand groups are the
same as the right-hand ones or not.
Geometric
Isomerism
•
•
•
•
an example of stereoisomerism
found in some, but not all, alkenes
occurs due to the RESTRICTED ROTATION OF C=C bonds
get two forms...
CIS (Z)
Groups/atoms are on the
SAME SIDE of the double
bond
TRANS (E)
Groups/atoms are on
OPPOSITE SIDES across the
double bond
Geometric
Isomerism
RESTRICTED ROTATION OF C=C BONDS
Single covalent bonds can easily rotate. What appears to be a different
structure is not. It looks like it but, due to the way structures are written out,
they are the same.
ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C
BONDS HAVE ‘FREE’ ROTATION
Geometric
Isomerism
RESTRICTED ROTATION OF C=C BONDS
C=C bonds have restricted rotation so the groups on either end of the bond are
‘frozen’ in one position; it isn’t easy to flip between the two.
This produces two possibilities. The two structures cannot interchange
easily so the atoms in the two molecules occupy different positions in space.
Geometric
Isomerism
E/Z or CIS-TRANS
E/Z
Z (zusammen)
higher priority groups / atoms on
the SAME side of C=C bond
E (entgegen)
higher priority groups / atoms on
OPPOSITE sides of C=C bond
To determine priority, the Cahn, Ingold and Prelog convention is used.
eg
C2H5 > CH3 > H
E
and
Z
I > Br > Cl > F > C > H
Z
E
Geometric
Isomerism
E/Z or CIS-TRANS
CIS /
TRANS
Should only be used when there are two H’s and two
non-hydrogen groups attached to each carbon.
cis
non-hydrogen groups / atoms on the
SAME side of C=C bond
trans
non-hydrogen groups / atoms on
OPPOSITE sides of C=C bond
cis
trans
cis
trans
Geometric
Isomerism
Isomerism in Butene
There are 3 structural isomers of C4H8 that are alkenes. Of these ONLY
ONE exhibits geometrical isomerism.
but-1-ene
cis but-2ene
(Z) but-2ene
trans but-2-ene
(E) but-2-ene
2methylpropene
Geometric
Isomerism
How to tell if it exists
Two different
atoms/groups
attached
Two different
atoms/groups
attached

Two similar
atoms/groups
attached
Two similar
atoms/groups
attached

Two similar
atoms/groups
attached
Two different
atoms/groups
attached

Two different
atoms/groups
attached
Two different
atoms/groups
attached

GEOMETRICAL
ISOMERISM
Once you get two similar
atoms/groups attached to
one end of a C=C, you
cannot have geometrical
isomerism
GEOMETRICAL
ISOMERISM
The effect of
Geometric Isomerism on
Physical Properties
cis and trans isomers of 1,2-dichloroethene
You will notice that:
the trans isomer has the higher melting point;
the cis isomer has the higher boiling point.
Why is the boiling
point of the cis
Isomers higher?
• There must be stronger intermolecular forces between the
molecules of the cis isomers than between trans isomers.
• The difference between the two is that the cis isomer is a polar
molecule whereas the trans isomer is non-polar.
Both molecules contain polar chlorine-carbon
bonds, but in the cis isomer they are both on the
same side of the molecule. That means that one
side of the molecule will have a slight negative
charge while the other is slightly positive. The
molecule is therefore polar.
Because of this, there will be dipole-dipole
interactions as well as dispersion forces - needing
extra energy to break. That will raise the boiling
point.
Why is the melting
point of the cis
Isomers lower?
•
In order for the intermolecular forces to work well, the molecules must
be able to pack together efficiently in the solid.
•
Trans isomers pack better than cis isomers. The "U" shape of the cis
isomer doesn't pack as well as the straighter shape of the trans
isomer.
•
The poorer packing in the cis isomers means that the intermolecular
forces aren't as effective as they should be and so less energy is
needed to melt the molecule - a lower melting point.
trans
cis
Optical Isomerism
Involves an atom, usually carbon, bonded to four different atoms
or groups of atoms. They exist in pairs, in which one isomer is the
mirror image of the other.
These isomers are referred to as enantiomers. The central carbon atom to which
four different atoms or groups are attached, is called an asymmetrical carbon atom.
Enantiomers have identical physical constants, such as melting points and boiling
points, but are said to be optically active since they can be distinguished from each
other by their ability to rotate the plane of polarised light in opposite directions. A
mixture of enantiomers in equal proportions is optically inactive, and is called a
racemic mixture.
Chiral &
Achiral Molecules
A molecule which has no plane of symmetry is described as chiral. The carbon atom
with the four different groups attached which causes this lack of symmetry is
described as a chiral centre or as an asymmetric carbon atom.
The molecule on the left above (with a plane of symmetry) is described as achiral.
Only chiral molecules have optical isomers.
Detecting
Stereoisomers
http://www.youtube.com/watch?v=HuHphmJw-fA&feature=related
Some real
examples of
Optical Isomers
The asymmetric carbon atom in a
compound (the one with four
different groups attached) is often
shown by a star.
The two enantiomers are:
Some real
examples of
Optical Isomers
2-hydroxypropanoic acid (lactic acid)
The two enantiomers are:
Some real
examples of
Optical Isomers
2-aminopropanoic acid (alanine)
The two enantiomers are:
Naturally occurring alanine
Glycine
Optical
Isomers of Sugars
Fischer Projections
A Fischer projection
• is used to represent carbohydrates
• places the most oxidized group at the top
• shows chiral carbons as the intersection of vertical and horizontal
lines
D and L Notations
In a Fischer projection, the OH group on the
• chiral carbon farthest from the carbonyl group determines an L or
D isomer.
• left is assigned the letter L for the L form
• right is assigned the letter D for the D form
Examples of D and
L Isomers of
Monosaccharides
Difference between
Starch & Cellulose
1.
Cellulose is a polymer of glucose whose units can be rotated around the axis of a
backbone of polymer chains of glucose units while starch is a polymer of glucose wherein
all the repeat units are directed in one direction.
2.
The glucose units in starch are connected by alpha linkages while the glucose units of
cellulose are connected by beta linkages.
3.
Starch is fit for human consumption while cellulose is not.
4.
Starch is soluble in water while cellulose cannot be dissolved in water.
5.
Cellulose is stronger than starch.
6.
Cellulose is more crystalline than starch.
7.
The main function of starch is as food and supplying the body with energy and helps in its
proper metabolism while cellulose has a more significant use in the clothing industry and
in the production of important materials like cellophane and rayon.
Difference between
Starch & Cellulose
Chiral
• Chiral molecules often react differently with other chiral
molecules.
• This is like the idea that a right hand does not fit a left
handed glove – the molecule must be the correct shape
to fit the molecule it is reacting with.
• Many natural molecules are chiral and most natural
reactions are affected by optical isomerism.
Continue…..
• For example, most amino acids (and so proteins) are
chiral, along with many other molecules.
• In nature, only one optical isomer occurs (e.g. all natural
amino acids are rotate polarised right to the left).
• Many drugs are optically active, with one enantiomer
only having the beneficial effect.
• In the case of some drugs, the other enantiomer can
even be harmful, e.g. Thalidomide.
Continue…..
O
O
NH
O
O
H2C
NH
O
C
C
N
CH2
H
O
O
N
H2C
CH2
H
O
S thalidomide (effective drug)
R thalidomide (dangerous drug)
The body racemises each
enantiomer, so even pure S is
dangerous as it converts to R in
the body.
Continue…..
•
Thalidomide was banned worldwide when the effects
were discovered.
•
However, it is starting to be used again to treat leprosy
and HIV.
•
Its use is restricted though and patients have to have a
pregnancy test first (women!) and use two forms of
contraception (if sexually active).
Continue…..
CH3
CH3
O
O
H
C
CH2
H3C
S carvone (caraway seed)
Caraway Seed has a warm, pungent,
slightly bitter flavour with aniseed overtones.
H2C
C
H
CH3
R carvone (spearmint)
Continue…..
CH3
CH2
C
CH3
H
CH3
H
C
CH2
H3C
S limonene (lemons)
R limonene (oranges)
Why is Isomerism
important?
The reason that isomers are so important in drug design is that
normally, only one particular isomer is effective in treating the
condition.
Other isomers are less effective or even harmful!
The classic example is thalidomide, where one enantiomer helped
women overcome morning sickness associated with the early weeks of
pregnancy, and the opposite enantiomer caused the horrendous birth
defects (lack of arms and legs) seen in what are now known as the
"thalidomide babies."
It was developed by a German pharmaceutical company and sold
between 1957 to 1961 in 50 countries.
Importance of
Isomers in Fuels
A common fuel used today is octane, which has the formula
C8H18 or CH3(CH2)6CH3. It is the primary component of
gasoline. This particular fuel has 18 isomers, each of which is
useful for different applications. For most applications, stable
burning is the goal in creating fuels. Certain isomers of octane
are more useful in achieving this result
Importance of
Isomers in Fuels
Useful Isomers
In fuel design, the most useful octane isomers are those which have "branched"
structures. These isomers are distinguished from those that have more linear
structures. A linear isomer has all its elements connected in a line, whereas a
branched may resemble a tree in structure, with each element not necessarily
touching other branches. This branched structure causes the octane to burn more
slowly and evenly, therefore, the most commonly used octane isomers in fuels are
those with branched structures.
Octane Rating
When gassing your car, you may notice the "octane rating" of the fuel you are using.
This rating is a measure of how fast the fuel will burn. The rating is determined both
by the isomers used in the octane, as well as the percentage of fuel which is not
octane due to the addition of an additive such as ethanol. The octane rating is largely
a measure of the isomers in a given fuel.
For further information please contact The STEM Alliance
[email protected] or visit www.STEMalliance.uk