Stereochemistry involves the study of the arrangement and structure of atoms in molecules in the 3D space. This arrangements of atoms are responsible for the complex formations of the molecules. It plays a significant role in drug developments, bio chemistry, study of enzymatic catalysis etc.. This article will discuss stereochemistry, its types, methods of determination, applications and other facts related to it in detail.
What is Stereochemistry?
Stereochemistry is an important part of organic chemistry. It involves the studies and process of spatial arrangements of atoms in three dimensional space and understanding their structures in molecules. These atoms must be arranged in a scientific way, not randomly, as they have effects on the properties of molecules. It is also known as the chemistry of space. The term "stereochemistry" originated from the Greek word "stereos," which refers to the hardness, three-dimensionality, etc. of a compound. It is also indirectly related with inorganic, biological and physical chemistry. Stereoisomers are compounds of stereochemistry, with the same molecular formula but different arrangements of atoms.
Stereoisomers
Stereoisomers are molecules with same molecular formula but the structure and arrangement of atoms in space are different. The types of stereoisomer are discussed below:
- Enantiomers
- Diastereomers
- Cis-trans isomers
- Atropisomers
- Conformational isomers
Having knowledge of various types of stereoisomers is crucial in the fields of biochemistry, pharmaceutical sector, medical sector etc., as it helps to ascertain the molecular properties and behavior.
Enantiomers
Enantiomers are a type of isomers which are mirror images of each other or mirror-image twins in the molecular world. This type of isomerism is called enantiomerism.
- Enantiomers are non-superimposable mirror image of each other and can not be superimposed on its mirror image by any changes like rotation, translation etc.
- Enantiomers can not be aligned in space, as they won't overlay perfectly upon each other, such example is: 2-chlorobutane. In one enantiomer, the chlorine atom might be placed in the right, while in the other, it's placed in the right. Despite this difference, they share the same chemical formula.
- Each enantiomer is structured by a specific arrangement of atoms around its chiral center. This center determines the overall shape and orientation of a molecule by acting like a central point.
- Knowledge of enantiomer is important in drug development. It has influence on efficacy and safety of drugs.
Diastereomers
Unlike enantiomers, diastereomers are not mirror image twins. They have some similarities but not identical twins. Diastereomers shows different chemical and physical properties. They have multiple chiral centers but different in the arrangement of atoms in space.
- If a molecule have n numbers of asymetric carbon atoms, it can have upto 2n diastereomers.
- The prefixes ‘erythro’ and ‘threo’ are used to denote diastereomers, that have diastereomers with two similar groups on opposite side of the molecule and two similar groups on the same side of the molecule respectively.
- An example of diastereomers is tartaric acid. Tartaric acid contains two chiral centers and four stereoisomers.
- Another example of diastereomers is 2-dichloroethene. It contains cis-trans isomers, which are not mirror image of each other.
- Understanding diastereomers is important in the scientific fields like drug development. Slight changes in the structure of atoms can cause serious side effects.
Cis-Trans Isomers
In Cis-trans isomerism the same atoms are shared among cis-trans isomers. The atoms are joined to one another in the same way but they have a different configuration. This is generally observed in the case of alkenes and complexes.
- They are known as geometric isomers.
- The differences in arrangement of atoms in 3D space between cis and trans isomers might have significant effects on their physical and chemical properties. Such as 'cis' means 'this side of ' and cis isomers means functional groups are on the same side of a plane whereas ' trans' means ' that side of ' and trans isomers means groups are on the opposite side of a plane. Cis isomers have higher boiling points and lower melting points compared to their trans isomers.
- having knowledge of cis-trans isomerism is crucial in the scientific fields including materials science, organic chemistry, biochemistry etc..
What Is Chirality?
Chirality is the geometric property of chiral. If a molecule can not be superimposed on its mirror image by any changes like rotation, translation etc., is called chiral. It is a asymmetrical structure in molecules. A molecule is considered chiral if it cannot be superimposed on its mirror image, like your left and right hands has similarity but cannot be superimposed. Large organic molecules often have one or more chiral centers containing an atom bonded to four different groups. The two mirror images of a chiral molecule are called enantiomers. Chirality is important in chemistry, especially in the field of pharmaceuticals and biochemistry.
Chiral Molecules
If a molecule can not be superimposed on its mirror image, it is called a chiral molecule. These molecules are a type of compounds that has identical composition or connectivity but different structures. They do not have center of symmetry i.e., consisting asymmetrical carbon center. Chiral carbon is also an optically active carbon.
The term Chiral is originated from the Greek word " Kheir ", which means " hand ". A chiral molecule must be an enantiomer, because of its non superimposable property. Chiral molecules consists of one or more carbon centers or chiral centers, which are always tetrahedral carbons (sp3-hybridized), attached with four different groups.
Some examples of chiral molecules are as follows:
- Glucose, some subunits of protein like amino acid, sugar, lactate, glyceraldehyde etc. are chiral molecules having non superimposable property. Glucose contains 6 chiral centers.
- Some Therapeutic drugs like Naproxen, Penicillamine are also examples of chiral molecules.
- DNA structure is spiral or helical, it is because of the chirality property of its building blocks. Thus DNA is also a chiral molecule.
Methods of Determining Stereochemistry
Determining stereochemistry, or the 3D arrangement of atoms in molecules, often requires a combination of experimental techniques and theoretical analysis. We can divide the methods used to determine stereochemistry as follows:
- Physical methods
- Chemical methods
- Spectroscopic method
Lets discuss these methods in details,
Physical Methods of Determining Stereochemistry
The physical methods of determining stereochemistry is discussed below:
- Polarimetry
Polarimetry is a process of measurement of the rotation of plane-polarized light. It passes through a sample containing chiral molecules. Chiral molecules rotate the plane of polarized light, and the magnitude and direction of rotation depend on several factors such as temperature, concentration etc. By comparing experimental measurements with known standards, the absolute configuration of chiral molecules can be determined.
- Optical Rotation Dispersion
Optical Rotation Dispersion is a process of stereochemical analysis that involves the measurement of variations in optical rotation of a chiral molecule to determine function of wavelength. The researchers and scientists examine how the optical rotation changes according to different wavelengths of light and from it they gain information regarding the electronic transitions and structural features of the molecule.
Chemical Methods of Determining Stereochemistry
- Reaction with Chiral Reagents
Chemical derivatization is a process to determine stereochemistry. It involves modification of the functional groups of a molecule to invent derivatives with distinct chemical or physical properties. Reaction with chiral reagents or resolving agents, is used to differentiate between enantiomers and diastereomers. This process is known as chiral derivatization technique. The resulting derivatives may exhibit differences in chromatographic behavior, their solubility and spectroscopic properties.
Spectroscopic Methods of Determining Stereochemistry
- X-ray Crystallography
X-ray crystallography is a spectroscopic method for determining the three-dimensional structure of atoms of molecules. When X-rays interact with a crystallized sample, diffraction of patterns are produced from the reaction. By analyzing this process the researchers can presume the positions of atoms within the molecule and determine its stereochemistry.
- Nuclear Magnetic Resonance (NMR)
NMR spectroscopy is a crucial analytical tool in organic chemistry. Proton NMR is one of the most widely used NMR methods in organic chemistry. This process provides valuable information about the connectivity and spatial arrangement of atoms in molecules. The Nuclear Overhauser Effect Spectroscopy technique can reveal the spatial proximity of different atoms i.e., the way group of atoms being close together in space, helping to explain the relative configuration of stereocenters in a molecule.
Stereoselectivity and Stereospecificity
Stereoselectivity and stereospecificity are important concepts of organic chemistry. These concepts describe the selectivity and specificity of reactions in stereochemistry. Stereospecific reaction leads to the stereoselective reaction of molecules to form stereoisomer. As the two terms quite similar, it becomes difficult for students to remember which one is of which concept.
- Stereoselectivity: Stereoselectivity is essential in reactions of stereochemistry. It helps to select a reaction towards the formation of one stereoisomer, which is favored over other possible stereoisomers. In a stereoselective reaction there is a possibility about the formation of multiple stereoisomeric products, but among them only one will predominate over other. This reaction is possibly influenced by several factors like steric hindrance, electronic effects etc.. The best example of this reaction is hydrogenation of alkenes.
- Stereospecificity: In stereospecific reaction, the stereochemistry of the reactants, determines the stereochemistry of the products. In this reaction, the arrangement of atoms in the reactants indicates the arrangement of atoms in the products. When you hear "stereospecificity" you should think about the " syn " and " anti " addition reactions of organic chemistry, just like halogenation of alkenes is an anti-addition reaction.
Stereoselective Reactions
- The best example of stereoselective reaction is Hydrogenation of alkenes. In this reaction hydrogen atom is added across the C=C i.e., carbon carbon double bond to form alkane. Palladium, platinum etc. are used as catalyst in this reaction.
- The Enantioselective reaction produce enantiomer. This reaction is stereoselective. Asymmetric hydrogenation is an example of this reaction.
Stereospecific Reactions
- SN2 Reaction: The SN2 reaction is a Bimolecular Nucleophilic Substitution reaction. In this reaction a nucleophile displaces a leaving group in a single step. It is highly stereoselective. In this reaction, reactants which are stereoisomers of each other are converted into products which are also stereoisomers of each other. This mechanism is known as stereospecificity
- The Isomerization process of cis-2-butene to trans-2-butene is stereospecific reaction. In this reaction, a metallic catalyst such as palladium is used.
- E1cB Elimination Reaction: In the Elimination Unimolecular Conjugate Base mechanism, a proton from a ?-carbon atom along with a leaving group is eliminated to form an alkene. The stereochemistry of the reactants determine the stereochemistry of alkene. Thus the reaction is stereospecific.
Applications of Stereochemistry
Stereochemistry is an important part of chemistry. It has wide range of applications in the fields of organic chemistry, bio chemistry, researches, pharmaceutical sector. It is particularly important in the study of the structure of molecules in space and its impact on various scientific fields.
- Stereochemistry encompasses various techniques such as X-ray crystallography, chiroptical spectroscopy, and NMR or nuclear magnetic resonance, which are used to determine the formation of natural products.
- stereochemistry is vital for the design and execution of enantioselective and stereoselective reactions of organic chemistry.
- It is used to check the purity of sugars. It also helps us to observe the process of sugar production as well.
- The knowledge of stereochemistry is essential for environmental study also. It helps us in understanding the environmental impact of pollutants and designing more safer chemicals
- In the pharmaceutical sector, it holds a very crucial role. Enantiomers are molecules with same atom but different in arrangement. It results in different effects in biological system. Thalidomide is a prime example of this effect, where one enantiomer caused birth effects and the other had therapeutic effects. Hence, the proper structure of a specific drug molecule is very important. Stereochemistry helps in determining the desired structure and shape of the drug molecule, required for a specific purpose or for treatment.
Conclusion
In the simplest terms of organic chemistry, the stereochemistry deals with the spatial arrangements of atoms in 3D space. It also involves the study of structures of these atoms in molecules. Stereochemistry can be determined by several methods such as polarimetry, optical rotation, X-ray crystallography etc. In modern chemistry, the stereochemistry remains of paramount importance across various fields, with its applications spanning from drug development to bio chemistry, from environmental chemistry to nanotechnology. Its importance can be determined by its ability to provide a clear understanding of molecular structure and function, and thus helping the researchers to discover solutions of wide range of sociochemical challenges.
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Sample Questions on Stereochemistry
Q1. Compounds with different atomic configurations in space but the same atoms are bonded to each other are said to as having
a) stereoisomerism
b) functional group isomerism
c) chain isomerism
d) position isomerism
Answer: a) stereoisomerism
Q2. Which of the following alkanes have the ability to show optical activity?
a) Neopentane
b) Isopentane
c) 3-methylpentane
d) 3-methylhexane
Answer: d) 3-methylhexane
Q3. What is meant by Chelate effect?
Answer:
Chelate effect is a process in which a bidentate or polydentate ligand containing donor atoms, positioned in such a way that they coordinate with central metal ion. As a result a five or six membered ring is formed and the complex becomes more stable. For example- the complex of Ni2+ with positive ion is more stable than NH3 .
Q4. Who is the father of stereochemistry?
Answer:
J.H. Van't Hoff is called the father of stereochemistry.