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Isomerism
Isomerism of organic compound
What are isomers?
Isomers are molecules with the same
molecular formula, but different
arrangements of atoms. There are different
types of isomers, shown by the diagram.
Isomerism
../chain.htm../isomers.htm#structural../positional.htm../functional.htm../geometrical.htm../isomers.htm#stereo../optical.htm
Structural Isomerism
Structural isomerism occurs when two or more organic compounds have the same molecular formulae, but different structures. These differences tend to give the molecules different chemical and physical properties. There are three types of structural isomerism that you need to be aware of at course: chain isomerism, positional isomerism and functional isomerism. There is a fourth type, known as tautomerism (where there are two isomers are known as the keto and enol isomers).
http://www.creative-chemistry.org.uk/molecules/chain.htmhttp://www.creative-chemistry.org.uk/molecules/positional.htmhttp://www.creative-chemistry.org.uk/molecules/functional.htmhttp://www.creative-chemistry.org.uk/molecules/functional.htm
Chain Isomerism
Chain isomerism occurs when the way carbon atoms are linked together is different from compound to compound. It is an example of structural isomerism, and is also called nuclear isomerism.
see the isomers of pentane pentane 2-methylbutane 2,2-dimethylpropane
C5H12
http://www.creative-chemistry.org.uk/molecules/hexane.htm
Positional Isomerism
Positional isomerism, an example of
structural isomerism, occurs when
functional groups are in different positions
on the same carbon chain. You usually
meet positional isomers of alcohols and
alkenes.
butan-1-ol butan-2-ol
but-1-ene but-2-ene
Note: this is cis-
but-2-ene, which
has a geometric
isomer called
trans-but-2-ene
(select here to find
out more)
2-methylphenol 3-methylphenol 4-methylphenol
http://www.creative-chemistry.org.uk/molecules/geometrical.htm
Functional Isomerism
Functional isomerism, an example of
structural isomerism, occurs substances
have the same molecular formula but
different functional groups. This means that
functional isomers belong to different
homologous series. There are two functional
group isomers of which you need to be
aware:
alcohols and ethers
aldehydes and ketones
Ethanol methoxymethane C2H6O Alcohols have the
hydroxyl group,
–OH.
Ethers have the
functional group
R–O–R'.
Propanal propanone (acetone) C3H6O
Aldehydes and ketones both have the carbonyl group C=O. In ketones this is attached to two carbon atoms; in aldehydes it is attached to 1 or 2 hydrogen atoms.
Stereo-isomerism
Stereo-isomerism occurs when the atoms in a molecule can have different arrangements in space. There are two types of stereo-isomerism: geometrical isomerism and optical isomerism. Geometrical isomers can have very different physical properties, such as different melting points, but they tend to have the same chemical properties. Optical isomers have the same chemical and physical properties, except that one structure rotates the plane of polarised light to the right and the other rotates it to the left.
http://www.creative-chemistry.org.uk/molecules/geometrical.htmhttp://www.creative-chemistry.org.uk/molecules/optical.htm
Geometrical Isomerism
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.
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!
C C
CH3
H
CH3
H
C C
H
H
CH3
CH3
C C
H
H
H
CH2 CH3
Optical Isomerism
Optical isomerism is an example of stereoisomerism. It occurs when substances have the same molecular and structural formulae, but one cannot be superimposed on the other. Put simply, they are mirror images of each other (see the diagram).
No matter how hard you try, the molecule on the left will not turn into the molecule on the right – unless you break and make some bonds! Molecules like this are said to be chiral (pronounced ky-ral), and the different forms are called enantiomers
Optical isomers can occur when there is an asymmetric carbon atom.
An asymmetric carbon atom is one which is bonded to four different
groups. The groups can be something hideously complex, or
something comfortably simple like a hydrogen or chlorine atom.
Remember:
* there must be four groups, and
* they must be different.
Tro, Chemistry: A Molecular
Approach
15
Nonsuperimposable Mirror
Images
mirror image cannot be rotated so all its atoms align
with the same atoms of the original molecule
Tro, Chemistry: A Molecular
Approach
16
Chirality
any molecule with a nonsuperimposable mirror image is said to be chiral
any carbon with 4 different substituents will be a chiral center
a pair of nonsuperimposable mirror images are called a pair of enantiomers
Tro, Chemistry: A Molecular
Approach
17
Optical Isomers of 3-methylhexane
Tro, Chemistry: A Molecular
Approach
18
Plane Polarized Light
light that has been filtered so that only
those waves traveling in a single plane are
allowed through
Tro, Chemistry: A Molecular
Approach
19
Optical Activity
a pair of enantiomers have all the same physical properties except one – the direction they rotate the plane of plane polarized light
each will rotate the plane the same amount, but in opposite directions
dextrorotatory = rotate to the right
levorotatory = rotate to the left
an equimolar mixture of the pair is called a racemic mixture
rotations cancel, so no net rotation
20
Chemical Behavior of
Enantiomers
a pair of enantiomers will have the same chemical
reactivity in a non-chiral environment
but in a chiral environment they may exhibit different
behaviors
enzyme selection of one enantiomer of a pair
Optical isomers can rotate the plane of polarisation of plane-polarised light:
one enantiomer rotates the polarised light clockwise (to the right) and is the (+) enantiomer;
the other rotates the polarised light anticlockwise (to the left) and is called the (–) enantiomer.
A mixture containing equal concentrations of the (+) and (–) enantiomers is not optically active (it will not rotate the plane of polarisation). It is called a racemic mixture or racemate.
(+)-lactic acid (–)-lactic acid
Lactic acid (2-hydroxypropanoic acid) is a fairly
common and simple example of optical isomerism. The
(+) enantiomer of lactic acid is found in muscle. Sour
milk contains a racemic mixture of the two enantiomers.
D-glyceraldehyde L-glyceraldehyde
2,3-dihydroxypropanal (glyceraldehyde) is a fairly common and simple example of optical isomerism, too. This is because glyceraldehyde is used as a standard by which other chiral molecules are compared. There are two enantiomers of glyceraldehyde, depending on the position of the –OH (hydroxyl) group on the molecule. These are known as D-glyceraldehyde and L-glyceraldehyde.
The little capital letters D and L are deliberate. The positions of the hydroxyl groups on other chiral molecules can be compared with glyceraldehyde to see if they are the D-enantiomer or the L-enantiomer. This is very common in Biology (natural sugars are D-enantiomers and amino acids are L-enantiomers). However, knowing whether a molecule is the D or L-enantiomer does not tell us whether it is the (+) or (–) enantiomer – so be careful!
MOLECULES WITH TWO OR MORE
STEREOCENTERS
To generalize, for a molecule with n stereocenters, the
maximum number of stereoisomers possible is 2n2.
For a molecule with one srereocenter, 21 = 2
stereoisomers are possible, for a molecule with two
srereocenters, 22 = 4 stereoisomers are possible.
Enantiomers and Diastereoisomers
C
C
CHO
OHH
H OH
CH2OH
C
C
CHO
HHO
HO H
CH2OH
C
C
COH
HHO
H OH
CH2OH
C
C
CHO
OHH
HO H
CH2OH
(a) (b)
One pair of enantiomers(Erythrose)
A second pair of enantiomers(Erythrose)
(d)(c)
Stereoisomers (a) and (b) are nonsuperposable mirror images and are,
therefore, a pair of enantiomers. Stereoisomers (c) and (d) are also
nonsuperposable mirror images and are a second pair of enantiomers.
Diastereoisomers are stereoisomers that are not mirror images.
Compound (a) and (c), (a) and (d), (b) and (c), (b) and (d) are
diastereisomers.
26
Figure 5.8 Summary: The four
stereoisomers of 2,3-
dibromopentane
Meso Compounds
C
C
COOH
OHH
H OH
COOH
C
C
COOH
HHO
HO H
COOH
C
C
COOH
HHO
H OH
COOH
C
C
COOH
OHH
HO H
COOH
(a) (b)
A pair of enantiomers A meso compound
(d)(c)
A plane of symmetry
In tartaric acid, carbon 2 and 3 are stereocenters, and, using the 2n rule, the maximum number of stereoisomers possible is
22 = 4. Structures (a) and (b) are nonsuperposable mirror images and are, therefore, a pair of enantiomers. Structures (c)
and (d) are also mirror images, but they are superposable. Therefore, (c) and (d) are not different molecules; they are the
same molecule, just oriented differently. Because (c) and its mirror image are superposable, (c) is achiral.
Another way to determine that (c) is achiral is to see that it has a plane of symmetry that bisects the molecule in such a way
that the top half is the reflection of the bottom half. Thus, even though (c) has two stereoisomers, it is achiral . The
stereoisomer of tartaric acid represented by (c) or (d) is called a meso compound. A meso compound is an achiral
compound that contains two or more stereocenters.
28
Figure 5.9 Summary: The three
stereoisomers 2,3-
dibromobutane
• Larger organic molecules can have two, three or
even hundreds of stereogenic centers.
30
How many stereogenic centers does each molecule have?
a)
Br
Br
b)
31
c)
H2N
HN
NH
OH
CO2H
O
SH
O
O
Only carbons attached to four different groups.
• Stereogenic centers may also occur at carbon
atoms that are part of a ring.
• To find stereogenic centers on ring carbons, always
draw the rings as flat polygons, and look for
tetrahedral carbons that are bonded to four different
groups.
33
Locate the stereogenic center in the following:a)
and b)
No stereogenic
centers.
b)
O
a)