Basic Principles of Organic ChemistryClassification of Organic Compoundsa) Acyclic or open chain compounds: These contain alkanes, alkenes, alkynes and their derivatives. These are also called aliphatic compounds.b) Cyclic or closed chain compounds: Cyclic compounds whose rings are made up of only one kind of atoms, i.e. carbon atoms are called homocyclic or cabocyclic compounds. Aliphatic cyclic compounds are called alicyclic compounds eg cyclopropane, cyclobutane etcOrganic compounds containing one or more fused or isolated benzene rings and their functinalized derivatives are called benzenoids or aromatic compounds, eg benzene, toluene, naphthalene, anthracene etc.Cyclic compounds containing one or more heteroatoms (usually O,N, S etc) are called hetrocyclic compounds eg ethylene oxide, tetrahydrofuran (THF), furan, pyrole etcIUPAC Nomenclature of Organic Compounds:a) Nomenclature of saturated hydrocarbons:Select the longest continuous chain of carbon atoms in the molecule. The compound is named as a derivative of this alkaneNumber the carbon atoms in the parent chain starting from the end which gives lowest possible sum for the numbers of the carbon atoms carrying the substituentsThat set of locants is preferred, which when compared term by term with other set of locants, each in order of increasing magnitude, has the lowest term at the first point of difference. For example the set of locants (2,7,8) is preferred over the set of locants (3,4,9) since 2 comes before 3 even though the sum of locants in the former case is 17 while in the latter case, it is 16The correct name is: 2,7,8 - Trimethyldecane and not 3,4,9 - Trimethyldecaneb) Nomenclature of compounds containing functional group or multiple bonds:Select the longest continuous chain containing the carbon atoms having the functional group or those involved in the multiple bondsThe numbering of atoms in the parent chain is done in such a way that the carbon atom bearing the functional group or those carrying the multiple bond gets the lowest possible numberWhile writing the name of alkene (double bond) or alkyne (triple bond), the primary suffix 'ane' of the corresponding alkane is replaced by 'ene' and 'yne' respectively. However, if the multiple bond occurs twice or thrice in the parent chain, the prefix di- or tri- is attached to the primary suffix ene or yneIn naming the organic compounds containing one functional group a suffix known as secondary suffix is added to the primary suffix (giving number of carbon atoms in the chain) to indicate the nature of the functional group. A few important secondary suffixes are:Functional groupSecondary suffixFunctional groupSecondary suffixAlcohols (-OH)-olAldehydes (-CHO)-alKetones (>C=O)-oneCarboxylic acids (-COOH)-oic acidAmines (-NH2 )-amineAcid amides (-CONH2 )-amideAcid chlorides (-COCL)-oyl chlorideEsters (-COOR)-oateNitrites (-C=N)-nitriteThioalcohols (-SH)-thiolNomenclature of compounds having polyfunctional groups:When an organic compound contains two or more functional groups, one group is called the principal functional group while the others are called the secondary functional groups and are treated as substituents: The order of preference for principal group is: Carboxylic acid > acid anhydrides > esters > acid halides > amides > nitrites > aldehydes > ketone > alcohols > amines > double bond > triple bondWhen the functional groups act as substituents, they ar named as:Functional groupPrefixFunctional groupPrefix- COOHCarboxy-CHOFormyl-COORAlkoxy cabonyl or Carbalkoxy>COOxo or Kelo-COCLChloroformyl-OHHydroxy-CONH2 Carbamoyl-SHMecaplo-CNCyano-NH2 Amino-ORAkoxy=NHImino-XHalo-NO2 NitroNomenclature of simple aromatic compounds:a) Nuclear substituted: In these the functional group is directly attached to the benzene ring. Most of these compounds are better known by their common and historical names. In the IUPAC system, they are named as derivatives of benzene.b) Side chain substituted: In these the functional group is present in the side chain of the benzene ring. Both in the common and IUPAC systems, these are usually named as phenyl derivatives of the corresponding aliphatic compounds. Sturctural IsomerismComponds having the same molecular formula but different structures:Compounds having the same molecular formula but different arrangement of carbon chains are called chain or nuclear isomers. For example (i) pentane, isopentane and neopentane (ii) hexane, 2-methulpentane, 3-methylpentane, 2,3-dimethylbutane and 2,2-dimethulbutane etcCompounds having the same structure of the carbon chain but differing in position of multiple bonds, functional groups or substituents are called position isomers. For example, (i) but-1-ene and but-2-ene (ii) prpan-1-ol and propan-2-ol (iii) o-,m- and p- xylenes etc.Compounds having the same molecular formula but different functional groups are called functional isomers. For example, (i) alcohols and ethers (ii) aldehydes and ketones (iii) alkynes and dienes (iv) various amines (1o , 2o , 3o ) (v) cyanides and isocyanides etc.Compounds having the same molecular formula but different number of carbon atoms on either side of the functional group (O,S or NH) are caled metamers. For example, (i) diethyl ether and methyl n-propyl ether (or isopropyl methyl ether). 2-pentanone and 3-pentanone are position isomers as well as metamers.Isomers obtained by 1,3-migration of a proton from one polyvalent atom to the other within the same molecule are called tautomers. If one tautomer contains a keto group and the other the enol group, such a tautomerism is called keto-enol tautomerism. For example, acetaldehyde and vinyl alcohol.The percentage of enol form is negligible in simple aldehydes and ketones, ie, acetone, acetaldehyde etc. The percentage of enol form, however, increases, if the enol form is stabilized by intramolecular H-bonding. For example, the percentage of enol form in acetoacetic ester is 7% while that in acetylacetone is 76%.Only those aldehydes and ketones which contain one or more - hydrogens show keto-enol tautomerism.Primary and secondary nitroalkanes also show tautomerism but tertiary nitroalkanes and nitroarenes do not.Compounds having the same molecular formula but possessing open chain and cyclic structures are called ring chain isomers. For example, propene and cyclopropane. StereoisomerismIsomers which have the same structures but differ in the relative arrangements of atoms or groups in space are called stereoisomers. It is of three types, i.e. conformational isomerism, optical isomerism and geometrical isomerism.Geometrical Isomers: Stereoisomers which have the same structural formula but differ in the spatial arrangement of atoms around the double bond are called geometrical isomers. The isomer in which the two similar atoms/groups lie on the same side of the double bond is called the cis-isomer while that isomer in which the two similar atoms/groups lie on the opposite side of the double bond is called the trans-isomer.The necessary and sufficient condition for a molecule to exhibit geometrical isomerism is that each ot the two carbon atoms of the double bond must have different atoms/groups which may be same of different. Gor example , alkenes of the types abC=Cab and abC=Cde show geometrical isomerismCompounds containing C=N and N=N also show geometrical isomerism1,2-1,3 and 1,4-disubstituted cycloalkanes also show geometrical isomerismThe dipole moment of a cis-isomer is higher than that of a trans-isomerE and Z configuration of geometrical isomersGive priority to the four groups attached to the double bonded C-atoms. Higher the atomic number of the atom of the group attached to the double bonded carbon atom higher will be its priority. If priority cannot be decided the atoms directly attached to double bonded C-atoms then next atoms of the group are comparedIf two higher priority groups are on the same side of the double bond then it is called a Z-isomer otherwise it is called an E-isomerConformational Isomerism: The infinite number of momentary arrangement of the atoms in space which result through rotation about a single bond are called conformations or rotational isomersEthane has infinite number of conformations, of which, only two, i.e. staggered and eclipsed are important. The staggered conformation of ethane is more stable than the eclipsed conformation by about 3.0 kcal or 12.55 kJ mol-1 Important conformations of n-butane are staggered (or anti), gauche (or skew), partially eclipsed and fully eclipsed. Their relative stability is in the order: staggered>gauche>partially eclipsed>fully eclipsedCyclohexane exists in two non-planer conformations, i.e., chair and the boat form. Both these conformations are free from angle strain (i.e., have tetrahedral angles)The boat form of cyclohexane is less stable than the chair form by about 44 kJ mol-1 The two conformations of cyclohexane cannot be separated but ar readily interconvertible