Constitution And Sequence Of Covalent Bonds Biology Essay

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The compound that have the same constitution and sequence of covalent bonds but differ in relative position of their atom or group in space are called stereoisomer.

Sterioisomer are two type:

(1)optical isomer

(2)geometrical isomer

optical isomers:the ability of some compound to rotate plane polarized light.

Diastereomers (diastereoisomers) are occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each otherWhen two diastereoisomers differ from each other at only one stereocenter they are epimers. Each stereocenter gives rise to two different configurations and thus to two different stereoisomers.

Diastereomers differ from enantiomers in that the latter are pairs of stereoisomers which differ in all stereocenters and are therefore mirror images of one another.Enantiomers o


(S)-(+)-lactic acid (left) and (R)-(-)-lactic acid (right) are nonsuperposable mirror images of each other An enantiomer is one of two stereoisomers that are mirror images of each other that are "nonsuperposable" (not identical), much as one's left and right hands are "the same" but opposite.

Enantiopure compounds refer to samples having, within the limits of detf a compound with more than one stereocenter are also diastereomers of the other stereoisomers of that compound that are not their mirror image. Diastereomers have different physical properties and different reactivity, unlike enantiomers.



ection, molecules of only one chirality.Enantiomers have, when present in a symmetric environment, identical chemical and physical properties except for their ability to rotate plane-polarized light (+/-) by equal amounts but in opposite directions. A mixture of equal parts of an optically active isomer and its enantiomer is termed racemic and has a net rotation of plane-polarized light of zero.

Enantiomers of each other often show different chemical reactions with other substances that are also enantiomers. Since many molecules in the body of living beings are enantiomers themselves, there is often a marked difference in the effects of two enantiomers on living beings.

(S)-Alanine (left) and (R)-alanine (right) are chiral molecule

A chiral molecule is a type of molecule that lacks an internal plane of symmetry and has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom.The term chiral that is non-superposable on its mirror image. Chiral (not chiral) objects are objects that are identical to their mirror image. chiral usually refers to molecules. Two mirror images of a chiral molecule are called enantiomers




In organic chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism or E-Z isomerism is a form of stereoisomerism describing the orientation of functional groups within a molecule. In general, such isomers contain double bonds, which cannot rotate, but they can also arise from ring structures, wherein the rotation of bonds is greatly restricted. Cis and trans isomers occur both in organic molecules and in inorganic coordination complexes.

conformational isomerism: is a form of stereoisomerism in which the isomers can be interconverted by rotations about formally single bonds.Such conformational isomers or conformers can rotate about one or more σ bonds. Rotamers are conformers that differ by rotation about only one single σ bond.Conformational isomers are distinct from other stereoisomers in which a bond has to be broken to obtain another one.

only one dihedral angle will be different between rotamers. Conformers can differ by rotations about many sigma bonds or just one . The rotational barrier, or barrier to rotation, is the activation energy required to convert from one roamer to another roamer.

Different confore existence of more than one conformation, usually with different energies, is due to hindered rotation about sp3 hybridized σ mers can inter convert by rotation around single bonds, without breaking chemical bonds. Thbonds. The comparative stabilities of different conformers of a molecule are usually explained through differences in a cChiral molecule


Racemic mixture

Chiral molecules have two forms (at each point of asymmetry) which differ in their optical characteristics: the levorotatory form (the (−)-form) will rotate the plane of polarization of a beam of light to the left, while the dextrorotatory form (the (+)-form) will rotate the plane of polarization of a beam of light to the right. The two forms, which are non-superimposable when rotated in 3 dimensional space, are said to be enantiomers.

A common misconception is that a point of asymmetry exists only if there are four different groups attached to the central atom (the chiral centre). Although this particular case always gives rise to chirality, other configurations may also make something chiral. When considering chemical stereochemistry it's important to recall that molecules exist in 3 dimensional space, and as such can have different arrangement of the atoms in that space. For eg allenes.


it examines the possible stereo chemical outcome of two general types of reaction:-

Addition reactions

Substitution reactions


An addition reaction in its simplest terms is an organic reaction where two or more molecules combine to form a larger one.

Addition reactions are limited to chemical compounds that have multiply-bonded atoms, i.e. molecules with carbon-carbon double bonds such as alkenes or with triple bonds such as alkynes. Also included are molecules containing carbon - hetero double bonds like those with carbonyl (C=O) groups or those with imine (C=N) groups.

There are two main types of polar addition reactions electrophilic addition and nucleophilic addition One non-polar addition reaction exists as well called free radical addition.

An addition reaction is the opposite of an elimination reaction. For instance the hydration reaction of an alkene and the dehydration of an alcohol are addition-elimination pairs. Addition reactions are also encountered in polymerizations and called addition polymerization.

in a substitution reaction, one group is replaced by another. In the following substitution reaction Br is replaced by O.

It can also be defined as a substitution reaction occurs when atoms or a collection of atoms are replaced with other atoms or another collection of atoms. This is common among the Alkanes.

Eg: The oxidation step of hydroboration - oxidation is also a substitution reaction in which the boron is replaced by an OH-group.

‾OH + CH3 OH + (CH3CH2)3B 3CH3CH2 OH + -B(OH)4

A substitution reaction can occur in two stereo-chemically different ways, called retention of configuration and inversion of configuration .

When a group X replaces another group X' with retention of configuration, then X & X' have the same relative stereo-chemical positions. X X'

Substitution with retention of configuration: Replaces X

X' Ethanol X

If X & X' have the same relative priorities in the R,S system, then the carbon that undergoes substitution will have the same configuration in the reactant and the product. Thus if this carbon has (for example) the R configuration in the starting material, it has the same, or R configuration in the product.

When the substitution occurs with inversion of configuration, then X and X' have different relative stereochemical positions. Thus if X is cis to Y in the starting material or reactant, X' is trans to Y in the product :

Substitution with inversion of configuration:

X X'

Replaces X

Y with X' X

Substitution with inversion also implies that if X and X' have the same relative priorities in the R, S system, then the carbon that undergoes substitution must have opposite-configurations in the reactant and the product. Thus, if this carbon has (for eg.) the R-configuration in the starting material, it is also possible that a reaction might occur so that both retention and inversion can occur at comparable rates in a substitution reaction. In such a case, stereo-isomeric product corresponding to both pathways will be formed.


During a reaction if no bond of stereocenter is broken the product will have the same general configuration of group around the stereocenter as the reactant is said to be retention

Both the previous reactions suggests that analysis of the stereochemistry of substitution requires that the carbon which undergoes substitution must be a stereocenter in both the reactants and the products. For example - in the following situation, the stereochemistry of substitution cannot be determined.

Not a stereo-centre Substitution by X' X'

X with retention


Substitution by X'

with inversion

Because the carbon that undergoes substitution is not a stereocentre, the same product is obtained from both retention and inversion modes of substitution.

A reaction in which particular isomer(s) of product are formed in excess of other(s) is said to be stereoselective reaction.

Thus, an addition that occurs only with anti-stereochemistry is a stereoselective reaction because only one pair of enantiomers is formed to the exclusion of a diastereomer of the product is formed to the exclusion of the other.

Stereoselectivity can occur to various degrees. For example - a reaction that gives considerable amounts of two possible stereoisomers (that may be a mixture of 52:48) is slightly stereoselective. A reaction that gives mostly one of the two possible stereoisomer (say 98:2 mixture) is highly stereoselective.combination of steric repulsion and electronic effects. A simplified example is that of a butane molecule viewed in the Newman projection shown (viewed down the central C2-C3 bond) with relative rotations of C1 and C4 illustrated. The only unique gauche conformer in case of butane has a dihedral angle of 60°, anti is 180°, and eclipsed is 0° in the CH3-CH2-CH2-CH3. In the case of 1-fluoropropane F-CH2-CH2-CH3, there is a gauche+ and a gauche- rotamer, with dihedral angles of +60 and -60 respectively. Butane does not have both because of symmetry.