5 STERO CHEMISTRY OF CARBON COMPOUNDS

MOLECULAR REPRESENTATION:

1. Flying Wedge Formula : To write flying wedge formula, the following conventions are followed; a. A tetrahedral molecule is viewed in such a way that two groups are in the

plane of the paper and two groups below it.b. A bond coming towards US (viewer) out of the plane of the paper is

represented by a solid wedge c. A bond going away from us (viewer) behind the plane of the paper is

represented by a broken wedge d. A bond lying in the plane of the paper is represented by ordinary line.

Ex: Let us write the flying wedge structure of a general tetrahedral molecule cabed. Where groups ‘b’ and ‘c’ lie towards us and groups ‘a’ and ‘d’ lie away from us.

Ex: 1. Latic acid 2. Glyceraldehydes.

2. FISCHER PROJECTION FORMULAE:

Formula the carbon chain is projected vertically, the horizontal bonds attached to a carbon are considered to be above the plane of the paper and towards the viewer and the vertical bonds are considered to be below the plane of the paper and at the back of viewer.

The Fischer Formula, if two like groups are on the same side, the molecule is called ‘Erythro” and if two like groups are on opposite side it is called ‘threo’ .

Ex: 2,3 – bromobutane

3. SAWHORSE FORMULAE :

In this representation, the molecule is viewed slightly from above and form the right and then projected on the paper. The bond between the two carbon atoms is drawn diagonally and of a relatively greater length for the sake of clarity. The lower left hard carbon is taken as the front carbon and the upper right hand carbon as the back carbon .

The sawhorse presentation of Eclipsed and staggered conformations of Ethane are as follow.

All parallel bonds in sawhorse formula are Eclipsed and all anti parallel bonds are opposite or trans/anti to each other.

4. NEWMAN PROJETION FORMULAE:

Newman devised a very simple method of projecting three dimensional formula on paper which are known as Newman projections.

In these Formulae the molecule is viewed from the front. The carbon atom nearer to the eye is represented by a point and the three atoms or groups are shown attached to it by three lines at an angle of 1200 to each other.

Newman projections for Eclipsed and staggered conformation of Ethane are;

In Newman’s formula all parallel bonds are Eclipsed and all anti- parallel (or) opposite bonds are

STEREISOMERISM: Those compounds which have identical molecular and structural formulae as well as identical position of bonds but differ only in spatial arrangement of atoms in molecules are called stereoisomers and this phenomenon is known as stereoisomerism.

DEFINATION OF FOLLOWING TERMS:

1. Enantiomers : The optical isomers which are non-superimposable mirror images of each other are known as Enantiomers (or) optical antipodes. Such isomers are related to each other as an object to its mirror image. Their physical and chemical properties are same . Ex: 2- chlorobutane (CH3-CHCl-C2H5)

2. Diasteromers : The stereo isomers which are not mirror images of each other are called diastereoisomers. It is to be noted that unlike enantiomers, two diastereomers differ in Physical as well as chemical properties Ex: α, β – dibromocrinnamic acid.

Ex: 2,3 – dihydroxybutanoic acid

3. Conformational Isomerism : Different spatial arrangement of atoms arising in a molecule, by free rotation about c-c bond axis is called conformers (or) conformational isomer. The Phenomenon is call conformational isomers. The conformers differ in their energy due to which their stability also differs. Arise due to free rotation about c-c single bond.

4. Configurational Isomers: Configuration represent relative arrangement of atoms and groups of a molecule in the space which characterize its structure.

Result due to presence of one (or) more chiral carbon atoms (optional isomers) or due to restricted rotation about c=c (geometrical Isomers) Here spatial position of atoms (or) groups in a molecule remains fixed. Conversion of one configuration into another is possible only upon bond dissociation i.e. breaking of bonds and formation of new bonds

CONFORMATIONAL ISOMERISM OF ETHANE AND N-BUTAN.

I. Conformation of Ethane

a. Eclipsed form: In Eclipsed form the hydrogen atoms linked with one carbon atom are exactly parallel with the hydrogen atom linked with other carbon atom. Since in this structure the bonde electrons and the hydrogen atoms are close to one another, the repulsive forces are maximum. As a consequence, the energy of this form is more and stability is minimum. This structure is called Eclipsed form.

b. Staggered form : In staggered form the hydrogen atoms linked with one carbon atom are exactly in the middle position occupied by the hydrogen atoms are at a maximum distance. The bond pairs (C-H) of electrons are also for away. Hence the repulsive forces are minimum. Due to this the energy is minimum and its stability is maximum. This is called staggered form.

Gauche form (or) Skew form: Any other conformed lying in b/w the eclipsed form and staggered form is known as skew form. All these forms are continuously charging from one form to the other. The skew forms have less energy than the eclipsed form and more energy than staggered form.

Energy relativity : The energy difference b/w the eclipsed form and staggered is only 3 k.cal/mole. Due to this, little difference of energy one form changes to other by collisions b/w the molecules. Hence they cannot be separated.

CONFORMATIIONAL ISOMERISM n- Butane.

CH3 – CH2 –CH2 –CH3

For convenience sake, it can be considered as 1,2-dimethyl ethane. By rotating C2

(or) C3 carbon atom around C2 =C3 sigma bond, different conformers are obtained the important conformers are represented as follows;

E= Trans (or) staggered

F= Eclipsed (partial)

G= Skew (or) gauche

H= Fully Eclipsed (or) cis

Of all these form, staggered form has least repulsive forces because the methyl groups are far away from one another. Hence its stability is maximum. Due to this more molecules exist in this confirmation.

The fully eclipsed form (or) cis form has more repulsive forces because the two methyl groups are close to one another. Hence its energy is more than al the forms and its stability is minimum due to this, less number of molecules are present in this conformation.

The partially eclipsed form has less stability than, anti-conformation. But it has more stability than the fully eclipsed form. Energy dig at last?

ENANTIOMERS

Optical Activity :

The stereoisomers which are not superimposable on each other are called optical isomers (or) optical antipodes and this phenomenon is known as optical isomerism.

Various terms used in the study of optical isomers.

Plane Polarized light: When an ordinary rays of light vibraties in a plane perpendicular to its path of pagation. It such a light is passed through a Nicol Prism (which is a pecial type of prism composed of doubly refracting substance like calcite i.e., CaCo3 ) its vibrations in all planes, except one, are either rejected (or) obsorbed. Thus, light emerging through a Nicol Prism vibrates in one plane only.

Such a light vibrating in one plane only is known as plane polarized light.

Optical activity : If plane polarized light is passed through certain substances or their solutions, the latter have a remarkable property of turning the plane of polarization either towards right (clockwise) towards left (anti clock wise)., This phenomenon of turning (or) rotating plane polarized light is known as optical activity. The substance exhibiting this property are called optically active.

Ex: Some optically active compounds are glucose, lactic acid, tartaric acid etc.

Optical activity of a com-pound can be measured by an instrument called polarimeter.

3. Rotation of plane polarized light.

4. Specific rotation: The rotatory power of an optically active substance is measured by a term called specific rotation. It is defined as; the number of degrees of rotation observed when plane polarized light is passed through one decimeter length of its solution having concentration 1gm/milliliter. It is expressed as;

[α]tcD = өLxC Observedangel of rotationlength∈dm xconc

Where; ө = Observed angle of rotation L = length of sol in decimet C = 9m of the substance in 1ml sol

Usually sodium light (D) is used and the temperature t0 is 250 C. Rotatory power of a given substance in soln has been found to depend on the following factors; 1. Nature of the compound2. Nature of the solvent used3. Concentration of the solution in g/ml 4. Length of solution, in decimeter (1dm=10cm) 5. Temperature of the solution 6. Wave length of the light used. Specific rotation value is generally expressed with (+) and (-) sign indicates levo (1) compound.

CHIRAL MOLECULE : A chiral molecule (or an object) is defined as the one that is not superimposable on its mirror image B∶B , C∶C, E∶∍ Objects and molecule which are superimposable on their mirror images are termed as ‘achiral’. A∶A , M∶M,O ∶O

Criteria for Chirality : It has been found that only those structure (crystalline (or) molecular) which are not superimposable on their mirror images are optically active. This is necessary condition for optical activity in a compound and are considered as chiral molecules. Ex: Optical isomers of lactic acid (‘d’ and ‘l’ forms)

In case of compounds containing two or more chiral centres, it may not be easy to as certain whether the mirror image is superimpossable over the original (or) not. In such cases symmetry elements are to be taken into consideration.

It is a molecule has either 1. A place (or) symmetry2. A centre of symmetry3. An alternating axis of symmetry. Then it will have a superimposable mirror image.If a molecule has a superimposable mirror image, it does not exhibit optical activity. There fore it is clear that for a molecule to exhibit optical activity it should not have any of the above mentioned elements of symmetry. Molecules which do not possess any elements of symmetry are chiral.

(I) plane of symmetry : An imaginary plane which divides an object or a molecule into two identical halves is called a plane of symmetry.Eg: A circle can be divided into two identical halves by a plane passing through its diameter. A hand has no plane of symmetry.

A molecule which has a 1 lane of symmetry does not exhibit optical activity even though it contains symmetric carbon atoms.

Eg: meso-tartaric acid (optically inactive)

Here; mesotargaric acid optically inactive even though it contain two a symmetric carbon atoms. This is because the molecules can be divided into two identical halves by an imaginary plane i,e, due to internal compensation.

Absence of centre of symmetry:- A molecule is considered to have a centre of symmetry; It all straight lines that can be drawn through the centre of the molecule meet identical atoms at equal distance form the centre.

Alternating axis of symmetry (Sn) : A molecule has a n-fold alternating axis of symmetry (Sn) if the molecule when rotated 360

n about an axis followed by reflection in a plane perpendicular to that axis, another identical structure results.S1 (one fold alternating axis of symmetry) is same as plane of symmetry ( ) and S2 (two fold alternating axis of symmetry) is same as centre of symmetry.

A Symmetric Molecule: A carbon atom attached to four different atoms (or) groups is called an a symmetric carbon atom. It is generally denoted by an asterisk (*) it is also knows as a chiral centre. A molecule which contains only one a symmetric carbon atom is always chiral and hence optically active. The term a symmetric denotes absence of any symmetry (I, Cn or Sn) . A molecule containing one asymmetric carbon atom is always a symmetric and it possesses no elements of symmetry. Some examples of asymmetric molecules are given below;

Glyceraldehyde, lactic acid and alarine contain an asymmetric carbon atom (denoted by *).Each molecules and its mirror image are not super imposable. Such a pair (a molecule and its mirror image) of two optical isomers are known as ‘Enantimoers’.

Disymmetric Molecules : All asymmetric molecules are chiral. But a chiral molecule may not be necessary asymmetric. Some symmetric molecules whose symmetry is limited to their rotation axis (Cn) are also chiral and hence are optically active when the chiral. Molecle has a simple axis of symmetry (usually C2 axis) and no other symmetry element is present in it, it is then termed dissymmetric.

Eg: (i) (+) tartaric acid has only C2 acis of symmetry and no other symmetry. Element is present in it. So, it is a dissymmetric molecule (chiral molecule) and exhibits optical activity.

Eg: Trans 1, 2, - dichloro cyclopropane.

CHIRAL CENTR : A chiral molecule is the oen that is non superimposable on its mirror image. Most chiral compounds have a chiral centre which is a carbon atom bonded to four different atoms (or) groups (asymmetric carbon atom).

MOLECULES WITH SIMILAR CHIRAL CARBON (Tartaric acid)Compounds which have similar chemical and physical properties and differ only in their optical activity are known as optical isomers and the phenomenon is known as optical isomerism.

Optical isomerism in tartaric acid: Tartaric acid contains two similar asymmetric carbon atoms i.e., it is of the type cabe-cabe. This compound contains two similar

asymmetric carbon atoms and hence the number f stereoisomers will be less than four.

Definition of Mesomers : The molecule which can be divided into two identical halves by an imaginary plane is called Mesomers.

Ex: Compound of the type;

Ex: Tartaric acid exists in following four isomeric forms.

Case iii. Compounds containing two dissimilae chiral centres. Ex: 3- bromo-2-butanol.

No. of optically active isomers = 2n=22=4

No. of meso isomers = ∴ Total No. of optical isomers = 4+0=4

Number of Enantiomers and mesomers – Calculation:

If the number of chiral carbon atoms in a compound is n, then total number of optical isomers is 2n . It is called vant-H- Formula.

Case : Compound containing one chiral centre. Ex: Latic acid

Number of optically active (d and l) isomer = 2n =2 =2 . Number of optically inactive Isomer =0 Total number of optical Isomers = 2+2=2.

Case ii: Compound containing two identical chiral centre.

Ex: Tartaric acid

CONFIGURATION.

The actual three dimensional arrangement of atom or groups around an asymmetric carbon atom (chiral centre) is known as the configuration.

Two systems have been developed to study the configuration of organic compounds.

1. Relative configuration (d, l-configuration) : Prior to 1951, there was n method available for determine 1951, there was no method available for determine the absolute configuration- (actual arrangement of atoms or groups around an asymmetric carbon) of a compound. So, configurations relative to that of (+) glyceraldehydes were determined. This system of configuration is known as D, t nomendation (or) D, l- configuration.

2. Absolute configuration (R, S- configuration): D, h system of configuration has some drawbacks. Therefore, a new system called R,S- system of configuration was developed by catin, Ingold and Preloc,

D,-L-system of configuration (relative configuration, In 1951, the absolute configuration of (+) glyceraldeyde was determined using x-ray diffraction technique. This configuraition of (+) glyceraldehydes and (-) glyceraldehydes are as shown below.

(+) glyceraldehydes having the –OH group on the right side and the hydrogen atom on the left was arbitrarily given the configurational symbol D(-) glyceraldehydes having the –OH group on the left side and the hydrogen atom on the right was assigned the configurational symbol ‘l’ IN both ‘D’ and ‘L’ compounds –CHO and –CH2OH groups are at the top and bottom respectively.

Any compound that can be prepared from covertic D(+) glyaraldehyde is considered to belong to the D-series similarly any compound that can be prepared from converting l(-) glyceraldehydes is considered to belong to the l-series.

For eg: D-glyceraldehyde can be converted to glyceric acid by oxidation and thus glyceric acid also belongs to D-series.

Similarly D(+) glyceraldehydes may be converted into D(-) lactic acid as shown below.

In the above conversions the D-configuration acid is retained in the final produce (+) and (-) sings indicate rotation of the plane polarized light.

All naturally occurring amino acids belong to t series in which the –NH2 group is on the left side and H on the right side.

R,S configuration (Absolute configuration); The D,t-system of configuration has some dracobacki. To over come these defects another system known as ‘R’ and ‘S’ system was developed by catin, Ingole and Prelog.

In this system the four groups attached to the asymmetric carbon are numbered 1,2,3 and 4 by following a set of sequence rules. The group with highest priority is given number 1 and the group with lowest priority is given number 4.

The sequence rules for assigning priority order to all the four groups attached to an asymmetric carbon atom (as1,2,3 and 4) are given below.

Rule (1) : If the four atoms attached to the asymmetric carbon are different priority is determined with respect to their atomic number. The atom with higher atomic number gets higher priority. If the two atoms are isotopes, atoms of higher mass number is given higher priority.

Rule (2) : If two atoms attached to the chiral carbon atoms are the same rule 1 cannot decide the priority. In such cases the atoms attached to each of these first atoms to be compared.

The relative priority of CH3 and C2Hs groups is decided as follows.

In Ch3 , the second atom are H, H, H and in C2 Hs they C,H,H. since carbon has as higher atomic number than hydrogen C2Hs has the higher priority over CH3. Hence the priority order is sec. butyl chloride is cl, C2Hs, CH3, H.

Rule (3) : A doubly for triply bonded atom A(A=C, O, N etc0 Is considered as equivalent to two (or) three as respectively.

AEx: = A is equivalent to and

A A= A is equivalent to A

A A [If the molecule contain both real

A A

Linkage and = A linkage, real A

Linkage will have higher priority over = A]

CHO is given higher priority over CH2OH since the former is considered as Equivalent to ;

By applying these rules to some common substitutes, we get the following priority sequence (group of highest priority first);

I, Br, Cl, SO3 H, SH, F, OCOR, OR, OH, NO2, NHCOR, NF, NHR, NH2, CCl3, COCl, COOR, COOH, CONH2, COR, CHO, CH2OH, CR3, C6H5, CHR2, Ch2R, CH3, D, H.After assigning the priority order to the four groups attached to the asymmetric carbon atom (by following the sequence rules), configuration symbol (R or S) is assigned. Ex: 1. Chloro bormoiodo methane (CHClBrI).

Ex. 2. (+) glyceraldehydes

The group with lowest priority (H) is brought to the bottom of the vertical line by carrying out two successive interchanges of groups.

Definition of a. Recemic Mixture b. Recemisation

c. Resolution.

a. Recemic mixture : The equimolecular mixture of the two enantiomers of a compound is known as the racemic mixture. The racemic mixture of reacemic medication shows no optical activity. If one enantiomer (+) rotates the plane of polarized light to the right, the others enantiomer (-) rotates the plane of polarized light to the left to the same extent. As a result the net rotation will be zero. Thus recemic mixture is optically inactive due to external compensation. The racemic mixture is represented by dl (-) or ±. The individual Enantiomer can be separated from the racemic mixture by using some special methods.

b. Racemisation : Racemisation is the process of converting an optically active (d (or) l) compound into the racemic modification. Racemic modification is an Equimolecular mixture of pair of Enantiomers i.e, (+) and (-) forms. It is denoted by dl (or) ± Racemisation may be achieved by the application of heat, light (or) chemical reagents. Ex: Lactic acid gives the racemic mixture quickly in the presence of a base. Racemisation takes place through the formation of an unstable enolic intermediate in which chirenlity disappears.

c. Resolution: When an optically active compound is synthesized in the laboratory, it is found to be optically inactive. The optical inactivity is due to

the presence of two possible Enantiomers of the compound in equal amount. The rotation produced by one Enartiomer is exactly neutralized by that due to other in opposite compounds as (+) and (-) forms is known as resolution. Resolution can be done by mectianical separation, biochemical method (or) chemical method, chemical method is the most widely used and the best method. Ex: Racemic latic acid can be separated using an optically active base such as(-) brucine.

Methods of Resolving a Racemic Mixture : 1. Mechanical Separation: This method is applicable only in a few cases where

the crystal shapes of the two optical isomers are Enantiomorphic having different shapes. If the Enantiomers of a substance exist in well defined crystalline form, the separation can be done by “hand picking” with the help of a magnifying lens and a pair of tweezers.Ex: Enantiomers of Sodium ammonium tartrate can be separated by this method.

2. Biochemical Separation: This method is based on the fact that some microorganism (bactria, yeast, moulds etc) selectively destroy one Enantiomer in the racemic mixture leaving the other one. Ex: Penicillium glaucum preferentially destroys the (+) isomer of racemic ammonium tartrate leaving (-) isomer in the solution.

3. By the formation of diastereoisomers : In this method the enantiomers of racemic mixture are converted into diastereo isomers by adding a pure Enantiomer of another compound. The diastereo isomers thus formed, can be separate easily into it two components by fractional crystallization, then the two isolated diasterioisomers can be converted into original compounds by a suitable reaction. Eg: Racemic modification of acids can be separated by using bases. Alkaloids like brucine, stryclinine, quinine are used as bases.

Racemic modification of bases can be separated by using acids. Acids like tartaric acidomalic acid, camphor, β – sulphonic acid is used.

E,Z-configuration.

Geometrical isomers are designated as cis (or) trans depending or whether the similar groups lie on the same side (or) opposite side of the double bond. This is possible when only two different types of groups (a and b) are attached to the doubly bonded carbon atoms.This system (as and trans nomenclature) become ambiguovs (or) cannot be applied when the alkeni is trisubstituted (having a, b and d groups) or tetrasubstitued (having a,b, d and e groups).

We cannot assign cis (or) trans configuration for the above compounds. A new system of nomenclature called E,Z – nomenclature was developed to assign configuration to geometrical isomers.

This system involves the following two steps; Step (1) following the set of sequence rules given under ‘R’ and ‘S’ configuration, a sequence of priority (1 and 2) is assigned to the two group attached to each of the doubly bonded carbon atoms.

Step (2) The configuration in which the groups having similar priority order lie one the same side is labeled as ‘A’ (z means Zusammen, German word meaning together). Similarly the configuration in which groups having similar priority order lie on the opposite side is labeled as ‘E’ (E means Entgegern, German word meaning opposite).