Major Topics
Structural isomerism
Stereo isomerism
Structural isomerism is further classifed as
Chain or Nuclear Isomerism
Functional isomerism
Metamerism
Tautomerism
Sterio isomerism is further classfied as
Geometrical isomerism
Optical isomerism, enantiomerism or d,l isomerism
Geometrical isomerism - E and Z configuration
Optical isomerism has the follwing concenpts
Diastereomer
erthroenantiomers
Threoenantiomers
Meso compound
Racemic mixture
R and S configuration
Conformations
Showing posts with label Isomerism. Show all posts
Showing posts with label Isomerism. Show all posts
Wednesday, December 24, 2008
Monday, January 28, 2008
Revision Alkenes Isomerism
Isomerism in alkenes
1. Structural isomerism: Alkenes show chain isomerism and position isomerism.
a. Chain isomerism: C-4H-8 exists as two chain isomers, n-Butene and Isobutene.
b. Position isomerism: The isomers differ in the position of the double bonds. C-4H-8 can have the double bond as the terminal bond or it can be in the middle. The one with the terminal bond is n-Butene and the one with the double bond in the middle is but-2-ene. Thus butane with the structural formula C-4H-8 has three structural isomers.
2. Geometrical isomerism: Alkenes exhibit geometrical isomerism.
Molecules of the type C2A2B2 are available as geometrical isomers. The two atoms attached to the same carbon atom are different. In this symbols, A and B are different.
The molecule in which similar atons or groups lie on the same side of the double bond is called cis-isomer. Both As are attached to the carbon on one side of the double bond. Simiarly two Bs.
If they are attached on the opposite sides, it is a trans-isomer
Examples
CH3HC=CHCH3 exhibits geometrical isomerism.
As CH3 and H are different groups
Maleic acid and Fumaric acid are geometrical isomers.
HOOCHC=CHCOOH
Cis isomer is maleic acid and trans isomer is fumaric acid.
If they are attached on the opposite sides, it is a trans-isomer
1. Structural isomerism: Alkenes show chain isomerism and position isomerism.
a. Chain isomerism: C-4H-8 exists as two chain isomers, n-Butene and Isobutene.
b. Position isomerism: The isomers differ in the position of the double bonds. C-4H-8 can have the double bond as the terminal bond or it can be in the middle. The one with the terminal bond is n-Butene and the one with the double bond in the middle is but-2-ene. Thus butane with the structural formula C-4H-8 has three structural isomers.
2. Geometrical isomerism: Alkenes exhibit geometrical isomerism.
Molecules of the type C2A2B2 are available as geometrical isomers. The two atoms attached to the same carbon atom are different. In this symbols, A and B are different.
The molecule in which similar atons or groups lie on the same side of the double bond is called cis-isomer. Both As are attached to the carbon on one side of the double bond. Simiarly two Bs.
If they are attached on the opposite sides, it is a trans-isomer
Examples
CH3HC=CHCH3 exhibits geometrical isomerism.
As CH3 and H are different groups
Maleic acid and Fumaric acid are geometrical isomers.
HOOCHC=CHCOOH
Cis isomer is maleic acid and trans isomer is fumaric acid.
If they are attached on the opposite sides, it is a trans-isomer
Sunday, January 27, 2008
IIT JEE Revision Alkynes Isomerism
Alkynes exhibit and chain isomerism and position isomerism
Chain isomerism
The arrangement of the chain is different in these isomers
Ex: Pent-1-yne Linear chain pentyne with triple bond at the terminal carbon.
3-Methylbut-1-yne - branched isomer having a methyl group at the 3rd carbon and triple at the terminal carbon.
Position isomerism - position of triple bond changes. Alkynes having more than four carbon atoms exhibit this isomerism.
1-Alkyne 2 Alkyne
Alkynes do not have geometrical isomerism as the bond angle is 180°
Chain isomerism
The arrangement of the chain is different in these isomers
Ex: Pent-1-yne Linear chain pentyne with triple bond at the terminal carbon.
3-Methylbut-1-yne - branched isomer having a methyl group at the 3rd carbon and triple at the terminal carbon.
Position isomerism - position of triple bond changes. Alkynes having more than four carbon atoms exhibit this isomerism.
1-Alkyne 2 Alkyne
Alkynes do not have geometrical isomerism as the bond angle is 180°
Saturday, January 26, 2008
IIT JEE Revision - Amines - Isomerism
1. Chain
Aliphatic amines containing four or more carbon atoms show chain isomerism as branched and straight chain alkyl groups can be attached to the nitrogen atom.
Example: Butan-1-amine, 2-Methylpropan-1-amine, and 2 Methylpropan-2-amine are chain isomers.
2. Position Isomerism
The amino group can be bonded to different carbon atoms of the alkyl group.
Example: Propan-1-amine and Propan-2-amine
3. Functional
Primary, secondary and tertiary amines having the same molecular formula are isomers. This isomerism is functional isomerism because different alkyl groups (functional groups) are present in the molecules and give rise to isomerism
4. Metamerism
In amines metamerism, a type of isomerism in which different alkyl groups are attached to the Nitrogen atom of the amino group exists(molecular formula for isomers is same).
Example: Diethylamine, Methyl-n-propylamine, and Isopropylmethylamine are metamers.
5. Chiral amines
Nitrogen atom undergoes sp3 hybridisation. Hence secondary and tertiary amines have a chiral nitrogen atom. But chiral amines cannot be resolved. This is because the two enantiomeric forms rapidly interconvert and it is very difficult to isolate a pure sample of either enantiomer.
Aliphatic amines containing four or more carbon atoms show chain isomerism as branched and straight chain alkyl groups can be attached to the nitrogen atom.
Example: Butan-1-amine, 2-Methylpropan-1-amine, and 2 Methylpropan-2-amine are chain isomers.
2. Position Isomerism
The amino group can be bonded to different carbon atoms of the alkyl group.
Example: Propan-1-amine and Propan-2-amine
3. Functional
Primary, secondary and tertiary amines having the same molecular formula are isomers. This isomerism is functional isomerism because different alkyl groups (functional groups) are present in the molecules and give rise to isomerism
4. Metamerism
In amines metamerism, a type of isomerism in which different alkyl groups are attached to the Nitrogen atom of the amino group exists(molecular formula for isomers is same).
Example: Diethylamine, Methyl-n-propylamine, and Isopropylmethylamine are metamers.
5. Chiral amines
Nitrogen atom undergoes sp3 hybridisation. Hence secondary and tertiary amines have a chiral nitrogen atom. But chiral amines cannot be resolved. This is because the two enantiomeric forms rapidly interconvert and it is very difficult to isolate a pure sample of either enantiomer.
Sunday, December 23, 2007
Ch.18A Isomerism
Optical Activity
Optical activity describes the phenomenon by which chiral molecules are observed to rotate polarized light in either a clockwise or counterclockwise direction. This rotation is a result of the properties inherent in the interaction between light and the individual molecules through which it passes. Material that is either achiral or equal mixtures of each chiral configuration (called a racemic mixture) do not rotate polarized light, but when a majority of a substance has a certain chiral configuration the plane can be rotated in either direction.
What Is Plane Polarized Light?
Polarized light consists of waves of electromagnetic energy in the visible light spectrum where all of the waves are oscillating in the same direction simultaneously. Put simply, imagine a ray of light as a water wave, with crests and peaks. All the peaks of a water wave point in the same direction (up, against gravity) pretty much at the same time. Light is not usually this way - it's peaks and troughs are often in random array, so one ray of lights peaks might point in a direction 90o opposite of another ray. When all of the rays have their peaks pointing in the same direction - like all the waves in the ocean have peaks pointing up - then those rays of light are said to be polarized to one another.
Why Polarized Light Is Affected
So why do chiral molecules affect only polarized light, and not unpolarized? Well, they do affect unpolarized light, but since the rays have no particular orientation to one another, the effect can not be observed or measured. We observe the polarized light rays being rotated because we knew their orientation before passing through the chiral substance, and so we can measure the degree of change afterwards.
What happens is this; when light passes through matter, e.g. a solution containing either chiral or achiral molecules, the light is actually interacting with each molecule's electron cloud, and these very interactions can result in the rotation of the plane of oscillation for a ray of light. The direction and magnitude of rotation depends on the nature of the electron cloud, so it stands to reason that two identical molecules possessing identical electron clouds will rotate light in the exact same manner. This is why achiral molecules do not exhibit optical activity.
In a chiral solution that is not a racemic mixture, however, the chiral molecules present in greater numbers are configurationally equivalent to each other, and therefore each possesses identical electron clouds to its molecular twins. As such, each interaction between light and one of these 'majority' molecule's electron clouds will result in rotations of identical magnitude and direction. When these billions of billions of interactions are summed together into one cohesive number, they do not cancel one another as racemic and achiral solutions tend to do - rather, the chiral solution as a whole is observed to rotate polarized light in one particular direction due to its molecular properties.
Enantiomers
It is just such specificity that accounts for the optical isomerism of enantiomeric compounds. Enantiomers possess identical chemical structures (i.e. their atoms are the same and connected in the same order), but are mirror images of one another. Therefore, their electron clouds are also identical but actually mirror images of one another and not superimposable. For this reason, enantiomeric pairs rotate light by the same magnitude (number of degrees), but they each rotate plane polarized light in opposite directions. If one chiral version has the property of rotating polarized light to the right (clockwise), it only makes sense that the molecule's chiral mirror image would rotate light to the left (counterclockwise).
Equal amounts of each enantiomer results in no rotation. Mixtures of this type are called racemic mixtures, and they behave much as achiral molecules do.
History
Via a magneto-optic effect, the (-)-form of an optical isomer rotates the plane of polarization of a beam of polarized light that passes through a quantity of the material in solution counterclockwise , the (+)-form clockwise. It is due to this property that it was discovered and from which it derives the name optical activity. The property was first observed by J.-B. Biot in 1815, and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals.
Louis Pasteur deduced in 1848 that the handedness of molecular structure is responsible for optical activity. He sorted the chiral crystals of tartaric acid salts into left-handed and right-handed forms, and discovered that the solutions showed equal and opposite optical activity.
Artificial composite materials displaying the analog of optical activity but in the microwave regime were introduced by J.C. Bose in 1898, and gained considerable attention from the mid-1980s.
-----------
Web sites
http://www.goiit.com/chapters/tutorial/chemistry/isomerism.htm
Optical activity describes the phenomenon by which chiral molecules are observed to rotate polarized light in either a clockwise or counterclockwise direction. This rotation is a result of the properties inherent in the interaction between light and the individual molecules through which it passes. Material that is either achiral or equal mixtures of each chiral configuration (called a racemic mixture) do not rotate polarized light, but when a majority of a substance has a certain chiral configuration the plane can be rotated in either direction.
What Is Plane Polarized Light?
Polarized light consists of waves of electromagnetic energy in the visible light spectrum where all of the waves are oscillating in the same direction simultaneously. Put simply, imagine a ray of light as a water wave, with crests and peaks. All the peaks of a water wave point in the same direction (up, against gravity) pretty much at the same time. Light is not usually this way - it's peaks and troughs are often in random array, so one ray of lights peaks might point in a direction 90o opposite of another ray. When all of the rays have their peaks pointing in the same direction - like all the waves in the ocean have peaks pointing up - then those rays of light are said to be polarized to one another.
Why Polarized Light Is Affected
So why do chiral molecules affect only polarized light, and not unpolarized? Well, they do affect unpolarized light, but since the rays have no particular orientation to one another, the effect can not be observed or measured. We observe the polarized light rays being rotated because we knew their orientation before passing through the chiral substance, and so we can measure the degree of change afterwards.
What happens is this; when light passes through matter, e.g. a solution containing either chiral or achiral molecules, the light is actually interacting with each molecule's electron cloud, and these very interactions can result in the rotation of the plane of oscillation for a ray of light. The direction and magnitude of rotation depends on the nature of the electron cloud, so it stands to reason that two identical molecules possessing identical electron clouds will rotate light in the exact same manner. This is why achiral molecules do not exhibit optical activity.
In a chiral solution that is not a racemic mixture, however, the chiral molecules present in greater numbers are configurationally equivalent to each other, and therefore each possesses identical electron clouds to its molecular twins. As such, each interaction between light and one of these 'majority' molecule's electron clouds will result in rotations of identical magnitude and direction. When these billions of billions of interactions are summed together into one cohesive number, they do not cancel one another as racemic and achiral solutions tend to do - rather, the chiral solution as a whole is observed to rotate polarized light in one particular direction due to its molecular properties.
Enantiomers
It is just such specificity that accounts for the optical isomerism of enantiomeric compounds. Enantiomers possess identical chemical structures (i.e. their atoms are the same and connected in the same order), but are mirror images of one another. Therefore, their electron clouds are also identical but actually mirror images of one another and not superimposable. For this reason, enantiomeric pairs rotate light by the same magnitude (number of degrees), but they each rotate plane polarized light in opposite directions. If one chiral version has the property of rotating polarized light to the right (clockwise), it only makes sense that the molecule's chiral mirror image would rotate light to the left (counterclockwise).
Equal amounts of each enantiomer results in no rotation. Mixtures of this type are called racemic mixtures, and they behave much as achiral molecules do.
History
Via a magneto-optic effect, the (-)-form of an optical isomer rotates the plane of polarization of a beam of polarized light that passes through a quantity of the material in solution counterclockwise , the (+)-form clockwise. It is due to this property that it was discovered and from which it derives the name optical activity. The property was first observed by J.-B. Biot in 1815, and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals.
Louis Pasteur deduced in 1848 that the handedness of molecular structure is responsible for optical activity. He sorted the chiral crystals of tartaric acid salts into left-handed and right-handed forms, and discovered that the solutions showed equal and opposite optical activity.
Artificial composite materials displaying the analog of optical activity but in the microwave regime were introduced by J.C. Bose in 1898, and gained considerable attention from the mid-1980s.
-----------
Web sites
http://www.goiit.com/chapters/tutorial/chemistry/isomerism.htm
Subscribe to:
Posts (Atom)