Chapter 17 General Organic Chemistry Part 3 by TEACHING CARE Online coaching and tuition classes

 

 

• E and Z system of nomenclature : ‘Cis’ and ‘Trans’ designations cannot be used if four different atoms or groups are attached to the carbon atoms of a double
  • C = C d
  • e

In such cases, E and Z system of nomenclature is used. This system is based on a priority system developed by Cahn, Ingold and Prelog.

In this system, the two atoms or groups attached to each of the doubly bonded carbon are put in order of precedence on the basis of sequence rules.

The symbol ‘E’ is assigned to an isomer in which the atoms or groups of higher precedence are on the opposite side (E from German word Entgegen = across or opposite).

The symbol ‘Z’ is assigned to an isomer in which the atoms or groups of higher precedence are on the same side (Z from German word, Zusammen = together).

2

 

C = C

1

E-isomer

C = C

Z -isomer

 

Note:®1 signifies higher precedence and 2 signifies lower precedence. In most of the cases ‘Z’ corresponds to cis-form and ‘E’ to trans-form. However, there are many exceptions.

The following rules are followed for deciding the precedence order of the atoms or groups;

• Higher priority is assigned to the atoms of higher atomic For example, the order of precedence in

 

the following atoms,

H,Cl, I, Br

is : I (at. no. 53)> Br (at. no. 35)>Cl (at. no. 17)>H (at. no. 1).

 

• If isotopes of the same element are attached, the isotope with higher mass number is given higher order of

precedence. For example, deuterium (2 D) is assigned higher priority in comparison to hydrogen (1 H ).

1                                                                                                                          1

• In the groups, the order of precedence is also decided on the basis of atomic number of fist atom of the For example, in the following set, – Cl,-OH,-COOH,- NH – CH₃,-SO₃H .

 

The order of the precedence is :

• Cl

• – SO₃H > –   OH

• – N HCH₃ > – COOH

 

¯

( at.no17)

¯

(at.no.16)

¯

(at.no.8)

¯

(at.no.7)

¯

(at.no. 6)

 

When the order of precedence of the groups cannot be settled on the first atom, the second atom or the

 

subsequent atoms in the groups are considered. For example, in the set

• CH2 – CH3 ,-CH3 ,-COOH,

the order

 

cannot be decided on the basis of first atom as it is same in all the groups. However, in

• CH2 – CH3 ,

the second

 

atom is carbon, in – CH₃, the second atom is hydrogen while in

• COOH, the second atom is Hence, the

 

order of precedence is : – COOH > –CH2 – CH3 > – CH3

 

¯

(at.no.8)

¯

(at.no.6)

¯

(at.no.1)

 

• A doubly or triply bonded atom is considered equivalent to two or three such For example,

N

|

 

The group

C = O

is equal to

C– O and the group – C º N

|

O

is equal to – C– N .

|

N

 

(4)  Number of geometrical isomers in polynes

• When compound has n double bonds and ends of a polyene are different, the number of geometrical isomers = 2ⁿ

C6 H5 – CH = CH – CH = CH – CH = CH – CH = CH – Cl

 

The given compound has four double bonds and the two ends are different (One is

Therefore, number of geometrical isomers = 2ⁿ = 2⁴ = 16 .

• When the ends of polyene are

C6 H5

and other is Cl).

 

 

 

Case I : When number of double bonds (=n) is even then the number of geometrical isomers = 2^n-1 + 2^{n / 2-1}

Cl – CH = CH – CH = CH – CH = CH – CH = CH – Cl

n=4, even

 

Number of geometrical isomers = 2^n-1 + 2^{(n / 2)-1}

= 2³ + 2¹

= 8 + 2 = 10 .

 

Case II : When number of double bonds (=n) is odd.

æ n+1 ö / -1

 

Number of geometrical isomers

= 2(n-1)

ç               ÷

+ 2è   2  ø

 

C6 H 6 – CH = CH – CH = CH – CH = CH – C6 H5

 

Number of geometrical isomers = 2² + 2^2-1

= 2² + 2¹

= 4 + 2 = 6 .

 

(5)  Geometrical Isomerism in nitrogen compounds

• Geometrical isomerism due to C = N –

 

The important class of compounds exhibiting geometrical isomerism due to nitrones, hydrazones and semicarbazones. But the most common compound is oxime.

C = N – bond are oximes,

 

Oximes : In aldoxime, when hydrogen and hydroxyl groups are on the same side, the isomer is known as syn. (analogous to cis) and when these groups are on the opposite side, the isomer is known as anti (analogous to

 

 

trans)

C6 H5  – C – H

||

N – OH

Syn-benzaldoxime

C6 H5  – C – H

||

HO – N

Anti-benzaldoxime

 

In ketoximes the prefixes syn and anti indicate which group of ketoxime is syn or anti to hydroxyl group. For example:

 

CH3  – C – C2 H5

||

N–OH

this compound will be named as;

 

• Syn-ethyl methyl ketoxime Þ HO and C2 H5

are syn or

 

• Anti-methyl ethyl ketoxime Þ HO

and C2 H3

are anti.

 

 

Similarly consider the following structure

C2H5 –C–CH3

||

N–OH

Syn-methyl ethyl ketoxime

or Anti-ethyl methyl ketoxime

 

• Geometrical isomerism due to N = N

 

C6 H5 – N

||

C6 H5 – N

||

 

C H  – N

6     5

N–C6H5

Anti-azobenzene

 

Syn-azobenzene

• Geometrical isomerism show by cumulatrienes : Cumulatrienes (Trienes with three adjacent double

 

bonds) show only geometric isomerism. This is because their molecule is planar, as such the terminal

• CH3

groups

 

and H- atoms lie in the same plane. Therefore, in this case their planar structure can exist in two diastereoisomeric forms, cis- and trans- but no enantiomeric forms are possible.

 

H3 C H

C = C = C = C

cis-Hexa-2,3,4-triene

CH3

H

H3 C H

C = C = C = C

trans-Hexa-2,3,4-triene

H CH3

 

• Geometrical isomerism in cycloalkanes : Disubstituted cycloalkanes show geometrical

 

 

 

CH₃

OH

 

H                                                                                                                H

Cis-1,2-dimethylcyclopropane     Cis-1,2-cyclohexanediol                           Trnas-1,2-cyclopentanediol

Note : ®Certain compounds show geometrical as well optical isomerism. Such type of isomerism is know as

geometrical enantiomerism.

• Compounds having similar physical and chemical properties but they have the ability to rotate the plane of polarised light either to the right (Clockwise) or to the left (Anticlockwise) are termed as optically active or optical isomer and the properly is called optical activity or optical

The optical activity was first observed in organic substances like quartz, rock-crystals and crystals of potassium chlorate (KClO₃) , potassium bromate (KBrO₃) and sodium periodate (NaIO₄ ) .

Biot (In 1815) suggested that optical activity of an organic compound was a molecular phenomenon, i.e., it was due to constitution of an organic compound rather than its crystalline nature.

(2)  Measurement of optical activity : The measurement of optical activity is done in terms of specific rotation which is defined as the rotation produced by a solution of length of 10 centimetres (One decimetre)   and   unit   concentration   (1   g/mL)   for   the   given   wavelength   of   the   light   at   the   given temperature.

 

Specific rotation, [a ]t °C

= aobs

wavelength

l ´ C

 

Where a _obc

is the rotaion observed, l is the length of the solution in decimeters and C is the number of grams

 

D

in 1mL of solution. The specific rotation of the sucrose at 20°C using sodium light (D-line, l=5893Å) is +66.5°C and is denoted as: [a ]20°C   = +66.5°C(C = 0.02 g / mL water)

+ sign indicates the rotation in clockwise direction.

• On the basis of the study of optical activity, the various organic compounds were divided into four types :
  • The optical isomer which rotates the plane of the polarised light to the rigth (Clockwise) is known as dextrorotatory isomer (Latin: dextero = right) or d-form or indicated by +ve

 

 

• The optical isomer which rotates the plane of the polarised light to the left (Anticlockwise) is known as laevorotatory isomer (Latin; laevo = left) or l-form or indicated by –ve
• The optical powers of the above two isomers are equal in magnitude but opposite in An equimolar mixture of the two forms, therefore, will be optically inactive due to external compensation. This mixture is termed racemic mixture or dl-form or (±) mixture.
• Optical isomer with a plane of symmetry is called meso It is optically inactive due to internal compensation, i.e., the rotation caused by upper half part of molecule is neutralised by lower half part of molecule.

(4)  Chirality

• Definition : A molecule (or an object) is said to be chiral or dissymmetric, if it is does not possess any element of symmetry and not superimposable on its mirror image and this property of the molecule to show non- superimposability is called

On the other hand, a molecule (or an object) which is superimposable on its mirror image is called achiral (non-dissymmetric or symmetric).

To understand the term chiral and achiral let us consider the alphabet letters ‘P’ and ‘A’ whereas ‘P’ is chiral, ‘A’ is achiral as shown in fig.

 

Mirror

Mirror

 

 

 

 

Non-superimposable (Chiral or dissymmetric)

Superimposable (Achiral or non-dissymmetric)

 

• Elements of symmetry : There are three elements of symmetry,

• Plane of symmetry : It may be defined as a plane which divides a molecule in two equal parts that are related to each other as an object and mirror e.g.,

COOH

|

H – C– OH

      |             Plane of symmetry

H – C – OH

|

COOH

• Centre of symmetry : It may be defined as a point in the molecule through which if a line is drawn in one direction and extended to equal distance in opposite direction, it meets another similar group or atom,

 

 

 

CH 3

|

C

|

H

 

NH – CO CO – NH

CH3

|

C

|

H

 

 

and

CH 3

|

C

|

H

 

NH – CO

CO – NH

Centre of symmetry

H

|

C

|

CH3

 

cis -Dimethyl diketo piperazine                                      trans -Dimethyl diketo piperazine

Since trans form contains a centre of symmetry, it is optically inactive.

 

 

 

• Alternating axis of symmetry : A molecule is said to possess an alternating axis of symmetry if an oriention indistinguishable from the original is obtained when molecule is rotated Q degree around an axis passing through the molecule and the rotated molecule is reflected in a mirror that is perpendicular to the axis of rotation in step (I).

 

• Symmetric, Asymmetric and Dissymmetric molecules

• Symmetric molecules : If any symmetry is present in the molecule then molecule will be symmetric
• Dissymmetric molecules : Molecule will be a dissymmetric molecule if it has no plane of symmetry, no centre of symmetry and no alternating axis of
• Asymmetric molecules : Dissymmetric molecule having at least one asymmetric carbon is known as assymmetric molecule. All asymmetric molecules are also dissymmetric molecules but the reverse is not necessarily

 

COOH

|

H — C^* — OH

|

H — C^* — OH

|

CHO

|

H — C^* — OH

|

CH3

 

C6 H5

No plane of symmetry

ß

 

No plane of symmetry

ß

Dissymmetric molecule

ß

Asymmetric molecule

Dissymmetric molecule

ß

Asymmetric molecule

 

• Chiral or asymmetric carbon atom : A carbon bonded to four different groups is called a chiral carbon or a chirality The chirality centre is indicated by asterisk. e.g.,

 

a

|

d — C^* — b

|

c

CH 3

|

HO — C^* — H

|

COOH

Lactic acid

 

Note : ®Carbons that can be chirality centres are

sp ³ -hybridised carbons;

sp ²

and sp -hybridised carbons

 

cannot be chiral carbons because they cannot have four group attached to them.

• Isotopes of an atom behave as different group in

 

 

 

 

D

|

H — C^* — T

|

Br

H 1

|

Cl ³⁵ — C^* — Cl ³⁷

|

H 2

Lactic acid

 

• Carbon of the following groups will not be a chiral carbon

O

||

• CH3 , – CH2OH,  – CHX2 , – CHO,  – C– Z

 

• Maleic acid

(HOOC – CH = CH – COOH)

show  geometrical  isomerism  while   malic   acid

 

(HOOC – CH ₂ – CHOH – COOH) show optical isomerism.

(5)  Calculation of number of optical isomers

• If molecule is not divisible into two identical halves and molecule has n asymmetric carbon atoms then Number of optically active forms = 2ⁿ = a

 

Number of enantiomeric pair

Number of racemic mixture Number of meso form

*

= a / 2

= a / 2

= 0

*                             *

 

Examples : C6 H5 – C HOH – C HOH – C HOH – CH3

This molecule cannot be divided into two identical halves and it has three asymmetric carbons. Hence number of optical active isomers = a = 2ⁿ = 2³ = 8 .

*                             *                                *                             *

2

CH OH – C HOH – C HOH– – C HOH – C HOH – CHO

n=4

Number of optically active forms = a = 2⁴ = 16

 

*                                 *

3                                                      3

CH  – C HOH– – C HCl – CH

n=2

Number of optically active forms = 2² = 4

• If molecule is divisible into two identical halves, then the number of configurational isomers depends on the number of asymmetric carbon

Case I : When compound has even number of carbon atoms, i.e., n = 2, 4, 8,10,12,….. :

• Number of optically by active forms = a = 2^n-1
• Number of enantiomeric pairs = a / 2
• Number of racemic mixture = a / 2
• Number of meso forms = m = 2^{(n / 2)-1}
• Total number of configurational isomers = a + m

Example :

 

 

 

*                                 *

COOH – C HOH– – C HOH – COOH

• (II)

Two idenitcal halves (I) and (II) having n = 2 . Thus number of optical isomers = a = 2^2-1 = 2

Number of meso form = m = 2^{(n / 2)-1} = 2^{(2 / 2)-1} = 2⁰ = 1

Total number of configurational isomers = 2 + 1 = 3

 

*                         *                            *                        *

 

C6 H5 – C HCl – C HCl– – C HCl – C HCl – C6 H5

n=4, even

 

a = 2^4-1 = 2³ = 8

m = 2(n / 2)-1 = 21 = 2

Total number of configurational isomers = 8 + 2 = 10

Case II : When compound has odd number of carbon atoms, i.e.,

 

• Number of optically active forms = a = 2^n-1 – 2^{(n-1)/ 2}
• Number of enantiomeric pairs = a / 2
• Number of racemic mixutre = a / 2
• Number of meso forms = m = 2^{(n-1)/ 2}
• Total number of configurational isomers = a + m

Example :

 

 

 

 

 

n = 3, 5,7, 9,11,…… :

 

 

*                           *                             *

CH 2 OH – C HOH– C HOH – C HOH – CH 2 OH

• (II)

Compound has two identical halves and has three asymmetric carbons.

 

Thus,

a = 2^n-1 – 2^{(n-1)/ 2} = 2² – 2¹ = 4 – 2 = 2

m = 2(n-1) / 2 = 21 = 2

 

Hence total number of configurational isomers = 2 + 2 = 4

 

*                         *                           *                         *                             *

COOH – C HCl – C HOH– C HBr – C HOH – C HCl – COOH

 

(I)

n=5

 

(6)  Optical activity of compounds containing one asymmetric carbon

 

 

Examples :

*

CH₃ – CHOH– COOH ;

*

CH₃ – CHOH– CHO

 

 

 

*

CH₂OH – C HOH– CHO  ;

*

C6 H5 – CHCl– CH3

 

 

Any molecule having one asymmetric carbon atom exists in two configurational isomers which are nonsuperimposible mirror images.

 

COOH

|

H — C — OH

|

CH3 (I)

COOH

|

HO — C — H

|

CH3 (II)

 

• and (II) have the same molecular formula, the same structure but different configurations, hence (I) and (II) are known as configurational (I) and (II) are nonsuperimposable mirror images, hence (I) and (II) are optical isomers. Configurational isomers which are nonsuperimposable mirror images are known as enantiomers. Thus (I) and (II) are enantiomers. Pair of (I) and (II) is known as enantiomeric pair.
  • Properties of Enantiomers : All chemical and physical properties of enantiomers are same except two physical
• Mode of rotation : One enantiomer rotates light to the right and the other by an equal magnitude to the left For example

Enantiomer	[a]	bp	d
(+) 2-methyl-1-	+ 5.78	128.9	1.41
butanol
(–2) -methyl-1-	– 5.78	128.9	1.41
butanol

• Rate of chemical reaction with an optically active compound : Both the enantiomers of 2-methyl-

 

1-butanol are converted to 2-methyl butene when treated with conc.

CH2OH

H 2 SO4 . The rate of the reactions is the same.

 

|

CH₃ — C— H

|

C2 H5

(+)

CH2OH

|

¾¾con¾c. H¾2SO¾4  ® CH3

D

 

 

 

 

 

conc. H SO

• CH2

• C = CH2 K1

|

CH3

 

H — C— CH₃ ¾¾¾¾2   ¾4  ® CH₃ – CH₂ –  C =  CH₂            K 2

 

|

C2 H5

(-)

D

 

 

K1  = K2

|

CH3

 

When both these compounds are treated with lactic acid, the rate of the reaction is different. (+) –2–methyl-1-butanol

K₃ (–) lactic acid

Ester

(–) –2-methyl-1-butanol

K₄ (–) lactic acid Ester

K 3  ¹  K 4

 

 

 

Thus rate of reactions of enantiomers with optically active compound is different.

• Racemic Mixture : An equimolar mixture of two enantiomers is called a racemic mixture (or racemate,

± form, (dl) form or racemic modification). Such a mixture is optically inactive because the two enantiomers rotate the plane polarised light equally in opposite directions and cancel each other’s rotation. This phenomenon is called external compensation.

Þ Racemic mixture can be separated into (+) and (–) forms. The separation is known as resolution.

Þ The conversion of (+) or (–) form of the compound into a racemic mixture is called racemisation. It can becaused by heat, light or by chemical reagents.

Þ Racemic mixture is designated as being ( ± ) or (dl).

• Enantiomeric Excess : A sample of an optically active substance that consists of a single enantiomer is said to be enantiomerically pure or to have an enantiomeric excess of 100%. An enantiomeric pure sample of (+)-2-butanol shows a specific rotation of 52^o . On the other hand, a sample of (+)-2-butanol that contains less

 

than an equimolar amount of (–)-2-butanol will show a specific rotation that is less than

0^o .

+ 13.52^o

but greater than

 

Such a sample is said to have an enantiomeric excess less than 100%. The enantiomeric excess (ee) is defined as follows :

% Enantiomeric excess = (moles of one enantiomer – moles of other enantiomer) ´100 Total number of moles of both enantiomers

The enantiomeric excess can be calcualted from optical rotation :

 

 

% Enantiomeric excess =

Observed specific rotation Specific rotation of pure enantiomer

´ 100

 

Enantiomeric excess is also known as optical purity.

(7)  Optical activity of compounds containing two asymmetric carbon

Case I : When molecule is not divisible into two identical halves.

The number of optical isomers possible in this case is four (a = 2² = 4). Further there will be two pairs of

enantiomers and two racemic modifications. In practice also it is found to be so. For example dibromocinnamic acid exists in the following four optically active forms.

 

COOH

|

H — C— Br

|

H — C— Br

|

C6 H5 (I)

COOH

|

Br — C— H

|

Br — C— H

|

C6 H5 (II)

COOH

|

H — C— Br

|

Br — C— H

|

C6 H5 (III)

COOH

|

Br — C— H

|

H — C— Br

|

C6 H5 (IV)

 

First pair of enantiomers                                                                             Second pair of enantiomers

Thus there are two pairs (I), (II) and (III), (IV) of enantiomers. Further, more equimolar mixutre of (I) and (II) will give one racemic mixture. Similarly, equimolar mixture of (III) and (IV) will give other racemic mixutre.

Now let us examine the relationship between the structures (I) and (III), (I) and (IV), (II) and (III) and (II) and (IV). These are configurational isomers but these are not mirror images. Configurational isomers which are not mirror images are known as diastereomers.

 

 

 

Thus (I) and (III) are diastereomers

• and (IV) are diastereomers
• and (III) are diastereomers

(II) and (IV) are diastereomers

Properties of Diastereomes : Diastereomers have different physical properties, e.g., melting and boiling points, refractive indices, solubilities in different solvents, crystalline structures and specific rotations. Because of differences in solubility they often can be separated from each other by fraction crystallisation; because of slight differences in molecular shape and polarity, they often can be separated by chromatography.

Diastereomers have different chemical properties toward both chiral and achiral reagents. Neither any two diastereomers nor their transition states are mirror images of each other and so will not neccessarily have the same energies. However, since the diastereomers have the same functional groups, their chemical properties are not too dissimilar.

Case II : When molecule is divisible into two identical halves.

Number of optical isomers = a = 2^2-1 = 2

Number of meso forms = m = 2⁰ = 1

Total number of configurational isomers = 3

For example, tartaric acid exists in the following three forms :

COOH |	COOH |	COOH |
H — C — OH	HO — C — H	H — C — OH
|	|	|
HO — C — H	H — C — OH	H — C — OH
|	|	|
COOH	COOH	COOH
(I)	(II)	(III) No non-super imposible mirror image because it has a plane of

symmetry

 

(I) and (II) are enantiomers

• and (III) diastereomers
• and (III) are diastereomers
• is optically inactive due to symmetry of the It is known as meso form.
• Optical activity in compounds containing no assymmetric carbon : Although the largest number of known optically active compounds are optically active due to the presence of chiral carbon atom, some compounds are also known which do not possess any chiral carbon atom, but on the whole their molecules are chiral (such molecules were earlierly called dissymmetric); hence they are optically active. Various types of compounds belonging to this group are allenes, alkylidene cycloalkanes, spiro compounds (spirans) and properly substituted
• Allenes : Allenes are the organic compounds of the following general

C = C = C

 

 

 

Allenes exhibit optical isomerism provided the two groups attached to each terminal carbon atom are different, i.e.,

 

• a

C = C = C            or

• b

a                                     x

C = C = C

b                                     y

 

For example, 2 3-pentadiene shows enantiomerism (optical isomerism)

 

H

 

H3 C

C = C = C

H

 

CH3

H

 

H3 C

C = C = C

H

 

CH3

 

Non-superimposable mirror images of 2, 3-pentadiene

• Alkylidene cycloalkanes and spiro compounds : When one or both of the double bonds in allenes are replaced by one and two rings, the resulting systems are respectively known as alkylidene cycloalkanes are

 

H3 C

 

H

H

= C

COOH

• CH2

C

• CH2

CH2                   a

C                        C

CH2                   b

 

1-Methylcyclohexylidene-4- acetic acid (Alkylidene cycloalkane)

Spirans

 

 

• Biphenyls : Suitably substituted diphenyl compounds are also devoid of individual chiral carbon atom, but the molecules are chiral due to restricted rotation around the single bond between the two benzene nuclei and hence they must exist in two non-superimposable mirror images of each other. Such types of stereoisomerism which is due to restricted rotation about single bond, is known as atropisomerism and the stereoisomers are known and

 

atropisomers. Examples

F  HOOC

 

 

 

 

COOH F

The above discussion leads to the conclusion that the essential condition for optical isomerism is the molecular disymmetry or molecular chirality and not the mere presence of a chiral centre. However, it may be noted that the molecules having only one chiral centre are always chiral and exhibit optical isomerism.

• Fischer projection formulae : The arrangement of the atoms or groups in space that characterizes a stereoisomer is called its

Emil Fischer (1891) provided an easy method to represent the three dimensional formulae of various organic molecules on paper. Fischer projection is, thus, a planar representation of the three dimesional structure.

By convention, the following points are followed in writing the Fischer formula.

• The carbon chain of the compound is arranged vertically, with the most oxidised carbon at the
• The asymmetric carbon atom is in the paper plane and is represented at the interaction of crossed lines.

 

 

 

Asymmetric Carbon atom

 

C

 

• Vertical lines are used to represent bonds going away from the observer, e., groups attached to the vertical lines are understood to be present behind the plane of the paper.
• Horizontal lines represent bonds coming towards the observer, e., groups attached to the horizontal lines are understood to be present above the plane of the paper.

Some Fischer projections are given below :

 

 

COOH

COOH

CH 3

 

 

HO                         H

 

 

CH3

Fischer projection of one of the enantiomers of Lactic acid

COOH

CH3

 

Fischer projection of one of the enantiomers of Tartaric acid

Fischer projection of one of the enantiomers of 2,3-dibromobutane

 

• Name of optical isomers : Following three nomenclatures are used for optically active compounds,
• D,L. System of nomenclature : This nomenclature is mainly used in sugar chemistry or optically active polyhydroxy carbonyl compounds. This nomenclature was given by Emil Fischer to designate the configurations of various sugars relative to the enantiomeric (+) and (–) glucose as

All sugars whose Fischer projection formula shows the OH group on the chiral carbon atom adjacent to the

 

terminal

CH2 OH

group on the right hand side belong to the D -series. Similarly if OH is on the left hand side,

 

then the sugars belong to the L -series.

|

H — C — OH

|

CH2OH

D-series

|

HO — C — H

|

CH2OH

L-series

 

Examples :

CHO

|

H — C— OH

|

CH2OH

D(d)glyceraldehyde

or

D(+)glyceraldehyde

CHO

|

HO — C— H

|

CH2OH

L(l)glyceraldehyde

or

L(-)glyceraldehyde

 

Þ It must be noted that there is no relation between the sign of rotation (+, – or d, l) and the configuration (D and L) of an enantiomer.

 

 

 

Þ Any compound that can be prepared from, or converted into D(+) glyceraldehyde will belong to D-series and similarly any compound that can be prepared from, or converted into L(–) glyceraldehyde will belong to the L– series.

Þ This nomenclature is also used in a -amino acids.

• Erythro and Threo System of Nomenclature : This nomenclature is used only in those compounds which have
• Only two chiral carbons and
• The following structure, R¢- Cab – Cbc – R¢

i.e., out of six substituents on two asymmetric carbons, at least two should be same.

When two like groups (in the given example, group is b ) in Fischer projection formula are drawn on the same side of the vertical line, the isomer is called erythro form; if these are placed on the opposite sides, the isomer is said to be threo form.

R¢ |	CH3 |	CH3 |
a — C — b	H — C — Cl	H — C — Cl
|	|	|
c — C — b	H — C — Br	Br — C — H
|	|	|

 

R¢

Erythro form

C6 H5

Erythro form

C6 H5

Threo form

 

• R,S Nomenclature (Absolute configuration)

The order of arrangement of four groups around a chiral carbon (stereocentre) atom is called the absolute configuration around that atom. System which indicates the absolute configuration was given by three chemists

R.S. Cahn, C.K. Ingold and V. Prelog. This system is known as (R) and (S) system or the Cahn-Ingold Prelog system. The letter (R) comes from the latin rectus (means right) while (S) comes from the latin sinister (means left). Any chiral carbon atom has either an (R) configuration or an (S) configuration. Therefore, one enantiomer is

(R) and other is (S). A racemic mixture may be designated (R) (S), meaning a mixture of the two. (R) (S) nomenclature is assigned as follows :

Step I : By a set of sequence rules given below the atoms or groups connected to the chiral carbon are assigned a priority sequence.

Sequence Rules for Order of Priority

Rule 1 : If all four atoms directly attached to the chiral carbon are different, priority depends on their atomic number. The atom having highest atomic number gets the highest priority, i.e., (1). The atom with the lowest atomic number is given the lowest priority, i.e., (4), the group with next higher atomic number is given the next higher priority (3) and so on. Thus,

 

Cl

|

F — C — I º

|

Br

3

|

— C — 1

| 2

 

¾¾¾¾F¾¾C¾l¾B¾r¾¾I ¾¾ ¾®

Increasing atomic number

 

 

 

 

Increasing priority

COOH

|

H₂ N — C — Br

|

OH

 

4

|

º 3 — C — 1

| 2

 

¾¾¾¾C¾¾N¾¾O¾¾B¾r ¾¾ ¾®

Increasing priority