Termapaper-optical and Geometrical Isomerism-chemistry
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Transcript of Termapaper-optical and Geometrical Isomerism-chemistry
LOVELY PROFESSIONAL UNIVERSITY
Phagwara(Punjab.)
TERM PAPER
SUB: -CHEMISTRY-101
TOPIC: - ISOMERISM(OPTICAL AND GEOMETRICAL).
SUBMITTED TO, SUBMITTED BY,
Mr. Rahul Mehta Omkar kumar jha
RH4901-A12
10902923
Mechanical Engg.. IV Sem(LEET)
INDEX
1. Acknowledgement
2. Introduction
3. Types (Isomerism)
4. STEREOISOMERISM
5. TYPES (STEREOISOMERISM)
6. OPTICAL ISOMERISM
7. GEOMETRICAL ISOMERISM
8. IN BRIEF
9. refrences
ACKNOWLEDGEMENT
I hereby submit my term paper given to me by my teacher
of the subject ‘CHE-101’ namely ‘Mr. Rahul Mehta’.I have
prepared this term paper under the guidance of my subject
teacher. I would thank my class teacher, my subject teacher
and my friends who have helped me to complete the term
paper. I am also highly thankful to all the staff and
executives of the esteemed university namely ‘LOVELY
PROFESSIONAL UNIVERSITY, PHAGWARA, JALANDHAR’.
Omkar kumar jha
RH-4901-A12
10902923
B-tech ME(IV Sem)
INTRODUCTION
The word is derived from the Greek ισομερης, isomerès; isos
= "equal", méros = "part".
And the the isomer’s are
Compounds having the same molecular formula, but
differing in physical and chemical properties are known as
isomers. This phenomenon is known as isomerism. The
isomers can be identified and distinguished from one
another because of difference in their physical and chemical
properties.
Or,
Some web pages includes as:-
Britannica Concise Encyclopedia: -
One of two or more substances with identical molecular formulas but
different configurations, differing only in the arrangement of their
component atoms. It usually refers to stereoisomer (rather than
constitutional isomers or tautomers; see isomerism, tautomerism), of
which there are two types. Optical isomers, or enantiomers, occur in
mirror-image pairs. Geometric isomers are often the result of rigidity
in the molecular structure; in organic compounds, this is usually due
to a double bond or a ring structure. In the case of a double bond
between two carbon atoms, if each has two other groups bonded to it
and all are rigidly in the same plane, the corresponding groups can be
on the same side of the C=C bond or across the C=C bond from each
other. An analogous distinction can be made for ring structures that
are all in a plane, between isomers whose substituent groups are on
the same side and isomers whose substituent groups are on both
sides of the plane. Diastereomers that are not enantiomers also fall
into this category. Most cis-trans isomers are organic compounds.
Food and Nutrition chemistry :-
Molecules containing the same atoms but differently arranged, so that
the chemical and biochemical properties differ. (1)In positional
isomers the functional groups are on different carbon atoms; e.g.
leucine and isoleucine.(2)D- and L-isomerism refers to the spatial
arrangement of four different chemical groups on the same carbon
atom (stereo-isomerism or optical isomerism). R- and S-isomerism is
the same, but determined by a set of systematic chemical rules. See D-
.(3)Cis- and trans-isomerism refers to the arrangement of groups
adjacent to a carbon-carbon double bond; in the cis-isomer the groups
are on the same side of the double bond, while in the trans-isomer
they are on opposite sides.
Columbia Encyclopedia: -
isomer (ī 'səmər), in chemistry, one of two or more compounds having
the same molecular formula but different structures (arrangements of
atoms in the molecule). Isomerism is the occurrence of such
compounds. Isomerism was first recognized by J. J. Berzelius in 1827.
Early work with stereoisomers was carried out by Louis Pasteur, who
separated racemic acid into its two optically active tartaric acid
components by crystallization (1848). Pasteur's results were given
theoretical basis by J. H. Van't Hoff and independently by J. A. le Bel
(1864).
+++++++++++++++++++++++++++++++++++++++++++
===========================================+++++++++++++++++++++++++++++++++++++++++++
A simple example of isomerism is given by propanol: it has the
formula C3
H8
O (or C3
H7
OH) and occurs as two isomers: propan-1-ol (n-
propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)
Note that the position of the oxygen atom differs between the two: it
is attached to an end carbon in the first isomer, and to the center
carbon in the second.
There is, however, another isomer of C3
H8
O which has significantly
different properties: methoxyethane (methyl-ethyl-ether; III). Unlike
the isomers of propanol, methoxyethane has an oxygen connected to
two carbons rather than to one carbon and one hydrogen. This makes
it an ether, not an alcohol, as it lacks a hydroxyl group, and has
chemical properties more similar to other ethers than to either of the
above alcohol isomers.
Examples of isomers having different medical properties can be easily
found. For example, in the placement of methyl groups. In substituted
xanthines, Theobromine, found in chocolate, is a vasodilator with
some effects in common with caffeine, but if one of the two methyl
groups is moved to a different position on the two-ring core, the
isomer is theophylline, which has a variety of effects, including
bronchodilation and anti-inflammatory action. Another example of
this occurs in the phenethylamine-based stimulant drugs.
Phentermine is a non-chiral compound with a weaker effect than
amphetamine. It is used as an appetite reducing medication and has
mild or no stimulant properties. However, a different rearrangement
of the same atoms gives dextromethamphetamine, which is more
potent than amphetamine; it is a very strong stimulant.
Allene and propyne are examples of isomers containing different bond
types. Allene contains two double bonds, while propyne contains one
triple bond.
TYPES OF ISOMERISM
Isomerism may be classified into two types :-
1. Structural isomerism
2. Stereo isomerism
OR,
HERE WE WILL STUDY IN DEPTH ABOUT STEREOISOMERISM: -
STEREOISOMERISM
In stereoisomer the bond structure is the same, but the geometrical
positioning of atoms and functional groups in space differs. This class
includes enantiomers where different isomers are non-
superimposable mirror-images of each other, and diastereomers when
they are not.Diastereomerism is again subdivided into "cis-trans
isomers"", which have restricted rotation within the molecule
(typically isomers containing a double bond) and "conformational
isomers" (conformers), which can rotate about one or more single
bonds within the molecule. An obsolete term for "cis-trans isomerism"
is "geometric isomers".
For compounds with more than two substituent E-Z notation is used in
stead of cis and trans. If possible, E and Z (written in italic type) is
also preferred in compounds with two substituent. In octahedral
coordination compounds fac- (with facial legends) and mer- (with
meridional legends) isomers occur. Note that although conformers can
be referred to as diastereomers, they are not stable diastereomers,
since bonds in conformers can be rotated to make them mirror
images. While structural isomers typically have different chemical
properties stereoisomer behave identically in most chemical
reactions, except in their reaction with other stereoisomer. Enzymes
however can distinguish between different enantiomers of a
compound, and organisms often prefer one isomer over the other.
Some stereoisomer also differ in the way they rotate polarized light.
TYPES OF STEREOISOMERISM
1. OPTICAL ISOMERISM
2. GEOMETRICAL ISOMERISM
OPTICAL ISOMERISM
Why optical isomers?
Optical isomers are named like this because of their effect
on plane polarised light. Simple substances which show
optical isomerism exist as two isomers known as
enantiomers.
A solution of one enantiomer rotates the plane of
polarisation in a clockwise direction. This enantiomer
is known as the (+) form.
For example, one of the optical isomers (enantiomers)
of the amino acid alanine is known as (+)alanine.
A solution of the other enantiomer rotates the plane of
polarisation in an anti-clockwise direction. This
enantiomer is known as the (-) form. So the other
enantiomer of alanine is known as or (-)alanine.
If the solutions are equally concentrated the amount of
rotation caused by the two isomers is exactly the same
- but in opposite directions.
When optically active substances are made in the lab,
they often occur as a 50/50 mixture of the two
enantiomers. This is known as a racemic mixture or
racemate. It has no effect on plane polarised light.
How optical isomers arise
The examples of organic optical isomers required at A' level all
contain a carbon atom joined to four different groups. These two
models each have the same groups joined to the central carbon atom,
but still manage to be different:
Obviously as they are drawn, the orange and blue groups aren't
aligned the same way. Could you get them to align by rotating one of
the molecules? The next diagram shows what happens if you rotate
molecule B.
They still aren't the same - and there is no way that you can rotate
them so that they look exactly the same. These are isomers of each
other.
They are described as being non-superimposable in the sense that (if
you imagine molecule B being turned into a ghostly version of itself)
you couldn't slide one molecule exactly over the other one. Something
would always be pointing in the wrong direction.
What happens if two of the groups attached to the central carbon
atom are the same? The next diagram shows this possibility.
The two models are aligned exactly as before, but the orange group
has been replaced by another pink one.
Rotating molecule B this time shows that it is exactly the same as
molecule A. You only get optical isomers if all four groups attached to
the central carbon are different.
Some real examples of optical isomers
1. Butan-2-ol
The asymmetric carbon atom in a compound (the one with four
different groups attached) is often shown by a star.
It's extremely important to draw the isomers correctly. Draw one of
them using standard bond notation to show the 3-dimensional
arrangement around the asymmetric carbon atom. Then draw the
mirror to show the examiner that you know what you are doing, and
then the mirror image.
Notice that you don't literally draw the mirror images of all the letters
and numbers! It is, however, quite useful to reverse large groups -
look, for example, at the ethyl group at the top of the diagram.
It doesn't matter in the least in what order you draw the four groups
around the central carbon. As long as your mirror image is drawn
accurately, you will automatically have drawn the two isomers.
So which of these two isomers is (+)butan-2-ol and which is (-)butan-2-
ol? There is no simple way of telling that. For A'level purposes, you
can just ignore that problem - all you need to be able to do is to draw
the two isomers correctly.
2. aminopropanoic acid (alanine)
This is typical of naturally-occurring amino acids. Structurally, it is
just like the last example, except that the -OH group is replaced by -
NH2
The two enantiomers are:
Only one of these isomers occurs naturally: the (+) form. You can't tell
just by looking at the structures which this is.
It has, however, been possible to work out which of these structures is
which. Naturally occurring alanine is the right-hand structure, and the
way the groups are arranged around the central carbon atom is known
as an L- configuration. Notice the use of the capital L. The other
configuration is known as D-.
So you may well find alanine described as L-(+)alanine.
That means that it has this particular structure and rotates the plane
of polarisation clockwise.
Even if you know that a different compound has an arrangement of
groups similar to alanine, you still can't say which way it will rotate
the plane of polarisation.
The other amino acids, for example, have the same arrangement of
groups as alanine does (all that changes is the CH3
group), but some
are (+) forms and others are (-) forms.
It's quite common for natural systems to only work with one of the
enantiomers of an optically active substance. It isn't too difficult to
see why that might be. Because the molecules have different spatial
arrangements of their various groups, only one of them is likely to fit
properly into the active sites on the enzymes they work with.
In the lab, it is quite common to produce equal amounts of both forms
of a compound when it is synthesised. This happens just by chance,
and you tend to get racemic mixtures.
GEOMETRICAL ISOMERISM
What is it?
Geometrical isomerism is an example of stereo-isomerism. This occurs
when substances have the same molecular formula, but a different
arrangment of their atoms in space. There are three ways that this can
happen:
where there is a C=C bond in the molecule;
where a molecule has rings; or
where there is a >C=N bond.
In AS and A2 Chemistry, we only need to know about geometrical
isomerism caused by a C=C bond in the molecule.
If you are studying Biology, you will meet geometrical isomerism
caused by rings when you look at sugars such as glucose, fructose,
mannose and galactose.
What is here?
I have put models of the geometrical isomers of but-1-ene and but-2-
ene here. But-1-ene does not form geometrical isomers, even though it
has a C=C bond, because one of the double-bonded carbon atoms has
two identical groups on it (hydrogen atoms in this case). But-2-ene
does form geometrical isomers because each double-bonded carbon
atom has two different groups on it. You should be prepared to spot
geometrical isomers for simple organic compounds like these for your
examinations, and you also need to be able to name the them.
cis-but-2-ene trans-but-2-ene Where like groups are on the
same side of the double
bond, we call it a cis isomer;
where they are on opposite
sides we call it a trans
isomer.
but-1-ene Although but-1-ene contains a C=C bond,
it does not form geometrical isomers.
Take care - look for different groups
on the double-bonded carbon atoms!
GEOMETRICAL ISOMERISM
A web site includes that “Isomers are two molecules with the same
molecular formula, but differ in the way the atoms are arranged.
Geometrical isomers are one form of stereoisomers which have
identical molecular formulae, the atoms are bonded together in the
same order but the arrangement of atoms in space are different.
Optical isomers are another form of stereoisomers. Geometrical
isomerism is also called cis-trans isomerism.
Cis-trans isomers occur in organic compounds which contain a
carbon-carbon double or triple bond. An example is in but-2-ene.
The cis form is where the substituent groups are on the same side of
the double bond. The trans form is where they on opposite sides.
Geometric isomers are possible for both square planar and
octahedral complexes, but not tetrahedral.
The number of geometric isomers expected for common
stereochemistries are as follows:
Square Planar:
CompoundNo. type of isomers
Ma2
b2
2(cis-andtrans-)
Mabcd 3(usecis-and rans- relations)
here a, b, c, and d refer to monodentate ligands.
A number of examples of these types have been isolated and
characterised and they show very different chemical and biological
properties. Thus for example, cis-PtCl2
(NH3
)2
is an anti-cancer agent
(cisplatin) whereas the trans- isomer is inactive against cancer (it is
toxic), and so not useful in Chemotherapy. cis- and trans- refer to the
position of 2 groups relative to each other. In the cis- isomer they are
"next to each other" i.e. at 90 degrees in relation to the central metal
ion, whereas in the trans- isomer they are "opposite each other", i.e. at
180 degrees relative to the central metal ion.
a
|
|
M ----b a----M----b
cis- trans-
The first report of the three geometric isomers being isolated and
characterised for complexes of the type [Mabcd] was by Il'ya
Chernyaev in 1928. The example above was reported by Anna Gel'man
in 1948.
IN BRIEF
We can now return to compounds that differ only in their 3-
dimensional structures. Geometric isomers have the same structural
formulas but differ in the arrangement of groups at a single atom, at
double bonds, or in rings. Cis- and trans-platin (see Figure 37) are
examples of geometric isomers based on the different arrangement of
groups at a single atom. Cis- and trans-2-butene differ in the
arrangement of the methyl groups about the double bonds.
Although geometric isomers have completely different physical and
chemical properties (for example, cis- and trans-2-butene have
different boiling points and densities), optical isomers (also called
enantiomers) differ in only one characteristic--their interaction with
plane polarized light. When a beam of light is passed through a
certain type of filter, all of the waves except those in one plane are
removed. Figure 39 shows this plane-polarized light impinging upon
and being rotated by two optical isomers. One of the optical isomers
rotates the light in one direction, the other rotates the light in the
opposite direction but by the same amount. In every other way, such
as boiling point, density, refractive index, viscosity, etc., the two
optical isomers are identical.
Figure 39. The interaction of optical isomers with plane-polarized
light.
What are these optical isomers? Optical isomers are mirror images
that are not superimposable. Your hands, assuming that we do not
examine them too closely for scratches and other imperfections, are
nonsuperimposable mirror images. Imagine that you approach a
mirror and raise your right hand in greeting so that you see the palm
of your right hand in the mirror. What you see in the mirror is the
mirror image of your right hand and it looks exactly like the palm of
your left hand. So your left hand is a mirror image of your right hand.
Now, try to superimpose your two hands by laying one atop the other
with the palms facing in the same direction. Notice that no matter how
you twist them, they cannot be oriented so that the right thumb is on
top of the left thumb, the right little finger on top of the left little
finger, etc.
If you now perform the same exercise with a pencil, you will notice
that the mirror image of a pencil is superimposable on the pencil.
Clearly, there must be something about the symmetry of your hands
that cause them to be nonsuperimposable.
On the molecular scale, most optical isomers are organic compounds,
and they have non superimposable mirror images when there are four
different groups on a carbon (the carbon is said to be chiral, from the
Greek meaning "handed"). Amino acids are good examples of
molecules that have nonsuperimposable mirror images. Alanine has
the structural formula
and the two mirror images shown in Figure 40. The peptides and
proteins that make up much of our bodies are constructed from
combinations of amino acids like alanine. One of the fascinating
aspects of chemical evolution is the fact that only one optical isomer
is used by natural organisms in producing biological materials. It is
also frequently the case that optical isomers differ in their biological
activity. We have already seen this in part I during our discussion of
levorphane and dextrophane.
Reference: 1. www.chemguide.co.uk/basicorg/isomerism/structural.html 2. www.britannica.com/EBchecked/topic/296365/isomerism 3. www.chem.uwimona.edu.jm/courses/IC10Kiso.html 4. www.goiit.com/.../0/content-isomerism-804382.htm? 5. www.chem.purdue.edu/gchelp/cchem/ 6. BOOK WRITTEN BY FORIGN WRITER OVER
WWW.BOOKS.GOOGLE.COM 7. IMAGES ARE SEARCHED OVER THE PARENT SITES AND OVER
THE GOOGLE IMAGES. 8. WWW.en.wikipedia.org/wiki/Isomer 9. www.chemguide.co.uk/basicorg/isomerism/optical.html