Nomenclature & Isomerism of Organic Compounds

A. ORGANIC COMPOUNDS
1. Classification of organic compounds





2. Some common organic Compound :









3. IUPAC Rules for nomenculature of organic compounds :


(i) Longest chain rule Select longest possible carbon atom chain which may or may not be in one plane. It is called parent chain.

(ii) Numbering of carbon atoms on longest carbon chain is done is such a way that the carbon atom carrying the subsistent group gets the lowest possible number. The number which indicates the position of substituent on parent chain is called position number or locant. It is decided by two rules

(a) Lowest sum rule When two or more substituents are present the numbering of carbon atoms of parent chain is done is such a way that sum of locants is lowest. This is called lowest sum rule.

(b) Lowest set of locants rule When two or more substituents are present, the carbon atom of parent chain are numbered in such a way that the set of locants is lowest this is called lowest set of locants rule





Sum of locants = 3 + 4 correct
= 4' + 5' incorrect


4. Preference list of functional groups:

carboxylic acid> Sulphuric acid > anhydride > esters > acid halide > acid amide > aldehyde > nitrile > ketone > alcohol > amine

Functional Group	Prefix	Suffix
Sulphuric acid SO3H	Sulpho
Carboxylic acid COOH	Carboxy	- oic acid
Anhydride (RCOO)2O		- alkanoic anhydride
Ester COOR	carboxy	- alkyl oate
Acid halide COX	haloformyl	- oyl halide
Acid amide CONH2	carbamoyl	- amide
Nitrile C N	cyano	- nitrile
	formyl	- al
	Oxo	- one
Alcohol OH	hydroxyl	- ol
Amino NH2	Amino	amino

5. When using prefixes such as formyl, cyano, carboxy, carbamoly etc, the carbon of substituent is not counted in principal chain.

6. Some important names:






7. (i) When two like carbon containing functional groups are present on at two ends of chain, then carbon of both functional groups is counted in chain




(ii) when two different carbon containing functional groups are present at two ends of carbon chain, only the carbon atom of principal functional group is counted towards parent carbon.



8. If a hydrocarbon has both double and triple bond, it is name as alkenyne.



While numbering double bond is always preferred over triple bond if, both of these are at same footings, however, if the footings are different then the bond nearer to the end will get the preference irrespective of the bond type.

9. If two branches of different length are attached at same distance from the ends then the chain with lower no. of carbon atoms is preferred.

10. If two atoms or groups of same preference occupy identical potions from either end of the parent chain, the lower number must be given to atom/group whose prefix comes first in alphabetic order.


(B) Isomerism:
It is shown by different compounds which have same molecular formula but different properties,.



1. Chain isomerism :

• Different skeleton of carbon chains is present in different isomers e.g. C₅H₁₂ has three isomers n, iso and neo pentane.
  

2. Position isomerism :
Position of functional group or multiple bond is different is isomers. e.g.

(i) o, p and m isomers of same compound.



(ii) butanol-1 and butanol-2.


3. Functional isomerism :

• Different functional groups are possible by same molecular formula in different isomers.

e.g.,
(i) Alcohol and ether
(ii) Aldehydes and ketones
(iii) Carboxylic acids and esters
(iv) Cyanides and isocyanides
(v) Nitro alkanes and alkyl nitrites
(vi) 1°, 2° and 3° amines


4. Metamerism :

• Different alkyl groups are attached to same functional groups in different isomers.
• Ethers, esters, ketones, 2°-amines shows metamerism because they have two alkyl groups.

e.g. C₂H₅ O C₂H₅ and CH₃ O C₃H₇ diethyl ether methyl propyl ether
diethyl ethermethyl propyl ether


5. Tautomerism or demotropism or alleotropism or kryptomerism :

• It is dynamic isomerism
• Two isomers can never be separated.
• It is caused due to presence of labile H in compound.

Keto enol tatortomerism_

(i) In simple forms keto form is more stable due to greater strength of C- O bond as compared to C C p bond.

(ii) In some cases enolic form is stablised by intramolecular hydrogen bonding (Chelation).



(iii) When benzene ring is present in enolic form, double is in conjugation with the p cloud of benzene ring, the stablisation of enolic form further increases by conjugation.




(iv) Polar protic solvents (e.g. H₂O, CH₃OH) form hydrogen bonding with keto form and decrease the enol content. Aprotic solvents increase the enol content.

6. Ring-chain isomersm :

• Cyclic and open chain both structure are possible by the same formula
  e.g. Cycloalkane and alkenes.

7. Geometrical isomerism (cis-trans isomerism) or configurational isomerism :

(a) It is caused due to hindered ration across C = C bond (or C = N or N = N bond.).

(b) Conditions necessary for compound to exhibit geometrical isomersm.
(i) The compound must have at least one C = C.
(ii) The two group attached to same carbon atom must be different.





(c) Difference in properties of cis and trans isomer

(i) Boiling point- Higher for cis isomer.
(ii) Melting point- Higher or trans isomer (due to symmetry)
(iii) Density - Higher for trans isomer (due to close packing)
(iv) Dipole moment - Higher for cis isomer. trans Isomer has zero dipole moment (or less dipole moment than trans form.)
(v) Cis form forms cyclic compounds (due to nearness of group) but trans form does not form.




(d) E and Z system of nomenclature

(i) The group on each carbon atom are assigned priority no. 1 and 2 as per sequence rules. (Sequnce rules states that group or atom with higher atomic mass has higher priority, double and triple bonds are treated as if they have duplicate or triplicates single bonds)

(ii) If two groups of higher priority are on same side of double bond, it is assigned Z configuration and if on opposite side it is assigned E configuration.





(e) Geometrical isomerism in compound having C = N

Syn -if H (or alkyl group) are on same side of C = N.
anti -if H (or alkyl group) are on opposite side C = N.





8. Optical isomerism:

(a) Plane polarised light has vibrations only in one plane.

(b) Optically active compounds rotate plane polarised light.


(c) Angle of rotation () is the angle through which plane polarised light is rotated when passed through solution of optically active substance. It is measured by polarimeter.
(d) Specific rotation is angle of rotation produced by solution of length 10 cm (or 1 dm) and having unit concentration (1 g/cm³) for given wavelength () of light at given temperature (t).





(e) Optical isomers have identical physical and chemical properties but have different behaviour towards plane polarised light.
(f) Conditions for compound to show optical isomerism

(i) The compound must have at least one asymmetric or chiral carbon atom (all four groups attached to chiral carbon atom are different).
(ii) The compound must not have plane of symmetry.

(g) Enantiomers are pair of optical isomers which are related as non-superimposable mirror images of each other. They have identical properties but differ in their behaviour towards plane polarised light.

(h) Diastereomers are pair of optical isomers which cannot be related as non-superimposable mirror images of each other. They have different physical properties.

(i) Meso compounds do not show optical activity inspite of presence of chiral carbon atom due to presence of molecular symmetry. It is called internal compensation.




I and II are enantiomers.
I and III are diasteromers.
Ill - meso compound.

(j) Substituted allenes and substituted biphenyls do not have asymmetric carbon atom but they show optical isomerism due to chirality (dissymmetry) in molecule.
(k) Asymmetric synthesis is synthesis of optically active compound from an optically inactive molecule.

(I) Walden inversion is conversion of d-form of an optically active compound into 1-form or vice versa.

(m) Resolution is separation of d and I forms from racemic mixture.

(n) Racemisation is process of formation of recemic mixture by adding 50% d and 50% 1 form of same compound.


TIPS TO SOLVE PROBLEMS

1. While counting structural isorners of any compound tautomers are neglected.
2. To find total no. of optical isomers compound use following formula :
a) No. of d and l isomers (a) = 2ⁿNo. of meso forms (m) = 0
Total no. of optical isomers = a + m = 2ⁿ
(where n = no. of chiral C atoms)

(b) When molecule is symmetrical and has even number of chiral carbon atoms.

No. of d and l isomers (a) = 2^{(n - 1)}

No. of meso forms (m) = 2^{(n/2 - 1)}

Total no. of isomers = a + m

(c) When molecule is symmetrical and has odd number of chiral carbon atoms.

No. of d and l isomers (a) = 2^{(n - 1)} - 2^{(n/2 - 1/2)}

No. of meso forms (m) = 2^(n/2-l/2)

Total no. of isomers = a + m


3. 1° carbon atom - Only one valency of carbon atom is satisfied by other carbon atoms.

2° carbon atom - Two valencies of carbon are satisfied by other carbon atoms.

3° carbon atom - Three valencies of carbon atom are satisfied by other carbon atoms.

4° carbon atom - Four valencies are satisfied by other carbon atoms.


4. Topological Representation or Bond Line Notations It is a simple, brief and convenient method of representing organic molecules. In these notations, the bonds between the carbon atoms are represented by lines. A single line (-) represents a single bond, two parallel lines (=) represent a double bond and three parallel lines () represent a triple bond, the intersection of lines represents carbon atoms carrying appropriate number of H atoms

• Comparison of sp, sp² and sp³ hybrid orbital of carbon

The percent of s (or p) orbital in a hybrid orbital is given as

Percent of s(or p) contribution =

sp orbital : 50% contribution from each of s and p - orbitals.

sp² orbital : 33.33% contribution from s orbital and 66.67 from p-orbitals.
sp³ orbital : 25% contribution from s orbital and 75% from p-orbitals.

1. Size of Orbitals : Size of a hybrid orbital increases with decrease (or increase) of s
(or p) contribution in it. Hence, the relative sizes of hybrid orbitals follows the order

sp < sp² < sp³

2. Electronegativity : The electronegativity of an orbital increases with increase (or decrease) of s(or p) contribution in it. Hence, the electronegativity of hybrid orbitals follows the order.

sp >sp²>sp³

Groups and their short - hand notation :

Group	Short-hand notation
Methyl -CH3	Me
Ethyl -C2H5	Et
n-propyl CH3CH2CH2 -	n-Pr, Pra or Pr
Iso-propyl (CH3)2-CH-	iso Pr, Prb or Pri
N- butyl CH3CH2CH2 CH2 -	n-Bu, Bua or Bu
Iso-butyl (CH3)2CHCH2 -	iso Bu or Bui
T - butyl (CH3)3(C) -	t-Bu, But
s-butyl CH3CH2CH-CH3	s-Bu, Bub or Bus