11th Organic Chemistry deals with some basic principles and techniques which help us to study the reactions and their mechanism of organic chemistry very easily. We shall learn in this chapter about the nomenclature and isomerism of organic compounds. We shall also know about reaction mechanism and various methods to understand the quantitative analysis of organic compounds. We shall learn the techniques of purification of organic compounds. We shall explain in the influence of electronic displacements on structure and reactivity of organic compounds. I hope that this chapter will be very helpful to read and understand the all over organic chemistry. Let’s start this chapter.
11th Organic Chemistry: Notes Some Basic Principles and Techniques
Organic chemistry. The branch of chemistry which deals with composition classification, preparation, properties and proper uses of organic compounds is called organic chemistry.
Organic compounds. All the hydrocarbons and their derivatives are known as organic compounds.
For example methane, ethane, benzene, chloroform, Phenol etc.
Hydrocarbons. Those compounds which are made of only carbon and hydrogen are called hydrocarbons.
For example CH4, C2H6, C2H4, C2H2, C6H6 etc.
Classification of Hydrocarbons
Based on the carbon skeleton, there are two types of hydrocarbons:
1. Acylic or open chain compounds
2. Cylic or closed chain compounds
Acylic or open chain compounds. An organic compound in which carbon atoms are are linked together in straight chain or branched chain is called Acylic or open chain compounds. For examples
Cylic or closed chain compounds.
Those compounds which contain one or more rings of carbon atoms in their molecules are called cylic or closed chain compounds. For examples
Types of Acylic or open chain compounds :
There are three types of open chain compounds.
1. Alkane. That hydrocarbon in which carbon atoms are linked together by single bonds only is called Alkane. Its general formula is CnH2n+2. For example methane, ethane, propane etc.
2. Alkene. That hydrocarbons in which at least one double bond must be present between two carbon atoms in open chain is called alkene. Its general formula is CnH2n. For example ethene, propene, butene etc.
3. Alkyne. That hydrocarbons in which at least one triple bond must be present between two carbon atoms in open chain is called alkyne. Its general formula is CnH2n-2. For example acetylene, propane, butyne etc.
Types of cylic or closed chain compounds:
There are mainly two types of cylic or closed chain compounds.
1. Alicyclic hydrocarbons
2. Aromatic hydrocarbons
Alicyclic hydrocarbons. Those cylic hydrocarbons which behave like open chain hydrocarbons are called alicyclic hydrocarbons. For example cycopropane, cyclobutene, oxirane.
Aromatic hydrocarbons. Those cylic hydrocarbons which behave the intermediate properties between alkane and alkene and have planar structures are called aromatic hydrocarbons. Ex:- benzene, toluene, furan, thiophene etc.
Types of Alicyclic hydrocarbons.
There are two types of alicyclic hydrocarbons .
1. Homocyclic. In this compound, only carbon atoms are present in the cylic structure. For example cyclopropane, cyclobutane, cycle-hexane etc.
2. Heterocyclic. In this compound, some other atoms like O, S, N are present in the cylic structure. For example oxetane, oxolane, pyrolidine etc.
Types of Aromatic hydrocarbons.
There are two types of aromatic hydrocarbons .
1. Benzenoid aromatic compounds. Organic compounds containing one or more fused or isolated benzene rings and their derivatives are called
benzenoid aromatic compounds. For example Benzene, toluene, nitrobenzene, aniline, naphthalene etc.
2. Non-benzenoid aromatic compounds. Aromatic compounds which do not contain a benzene ring but instead contain other highly unsaturated rings are called non-benzenoid aromatic compounds. For example tropons, azulene, tropolone etc.
Alkyl. The species obtained from an Alkane after removal of one H-atom is called alkyl. The general formula of an alkyl is CnH2n+1. For example methane (CH4) — methyl (CH3-)
Ethane (C2H6) — ethyl (C2H5-)
Propane (CH3CH2CH3) forms two type of alkyls as n- propyl and isopropyl.
Functional groups. The atom or group of atoms linked with carbon chains and responsible for the particular properties of an organic compound is called functional group. For example. Alcohol (-OH), halogens (-X), aldehyde (-CHO), amine (- NH2) etc.
Nomenclature of Organic Compounds
Rules for IUPAC Nomenclature for Branched Chain Alkanes
The following rules are used for naming branched chain alkanes.
1. Longest chain rule. Select the longest continuous chain of carbon atoms. This chain is called parent chain while all other carbon atoms or chain of carbon atoms which are not included in the parent chain are called branch chains or side chains or substituents.
2. Rule for larger number of side chains. If two chains of equal lengths are possible, select the one which attached with more number of side chains.
3. Lowest number of rules. When two or more substituents are present, the lowest set of locants rule is applied. According the this rule, when two or more different sets of locants containing the same number of terms is possible, then that set of locants is the lowest which when compared term by term with other sets, each in order of increasing magnitude, has the lowest term at the first point of difference.
4. Name of the branched chain alkane. prefix the name of the substituents to the name of parent alkane and indicate its position by writing before it the number of the carbon atom carrying the substituent. The name of the substituents is separated from its locants by a hyphen ( – ) . The final name of the alkane is always written as one word.
5. Alphabetical order of the side chains. when two or more alkyl groups (side chains) are present on the parent chain, each alkyl group prefixed by its positional number is arranged in alphabetical order (irrespective of its positional number) before the name of the parent alkane.
Note:- It may be noted here that while deciding the alphabetical order of the various alkyl groups, prefixes iso and neo are considered to be the part of the fundamental name of the alkyl group while the prefixes sec and tert are not.
6. Numbering of different alkyl groups at equivalent positions. If two different alkyl groups are present at equivalent positions, the numbering of the parent chain is done in such a way that the alkyl group which comes first in the alphabetical order gets the lowest number.
7. Naming the same alkyl groups at different positions. When the same alkyl group occurs more than once on the parent chain at different positions. The positional number of each alkyl group is separated by commas and suitable prefixes such as di (for two), tri (for three), tetra (for four), etc. are attached to the name of the alkyl group. However, the prefixes di, tri, etc. are not considered while deciding the alphabetical order of the alkyl groups.
8. Numbering the complex substituent.
- In case the substituent on the parent chain is complex that means it has branched chain, it is named as a substituted alkyl group by numbering the carbon atom of this group attached to the parent chain as 1. The name of such a substituent is always enclosed in brackets to avoid confusion with the numbers of parent chain.
- If two complex substituents are of equal length, then the complex substituents with larger number of alkyl groups forms a part of the longest carbon chain while the other one is considered the real complex substituent.
- While deciding the alphabetical order of the various substituents, the name of the complex substituent is considered to begin with the first letter of the complete name.
- If the same complex substituent occurs more than once on the parent chain at different positions, prefix bis ( for two), tris (for three), tetrakis (for four) etc. are used before the name of the complex substituent.
Rules for IUPAC Nomenclature of Unsaturated Hydrocarbons (Alkenes and Alkynes)
While naming compounds containing multiple (double and triple) bonds, the following additional rules are followed:
1. The parent chain must contain the multiple bond regardless of the fact whether it also denotes the longest chain of carbon atoms or not.
2. If both double and triple bonds are present, the numbering of the parent chain should always be done from that end which is nearer to the double or the triple bond. That means lowest set of locants rule must be followed for the multiple bonds.
3. If, however, there is a choice in numbering the double bond is always given preference over the triple bond.
4. If the organic compound contains only one double bond or the triple bond, its locants or the positional number is always placed before its suffix according to the recommendation for IUPAC nomenclature of organic compounds. If both double and triple bonds are present, their locants are written before their respective suffixes, the ‘e’ from the suffix ‘ene’ is dropped while writing the complete name.
Rules for IUPAC Nomenclature of Compounds Containing one Functional Group, Multiple Bonds and Substituents
1. Selection of the parent chain. Select the longest possible chain of carbon atoms containing the functional group and the maximum number of multiple bonds as the parent chain without caring whether it represents the longest possible carbon chain or not.
2. Lowest locants rule for the functional group. Number the parent chain in such a way that the functional group gets the lowest possible number followed by the double and triple bonds even if it violates the lowest set of locants rule.
3. Numbering the chain terminating functional groups. When a chain terminating functional group such as – CHO, – COOH, – COOR, – CONH2, – COCl, – C=- N, etc. is present, it is always given number 1 and number 1 is usually omitted from the final name of the compound when there is no ambiguity.
Rules for IUPAC Nomenclature of Polyfunctional Compounds
Organic compounds which contain two or more functional groups are called polyfunctional compounds. Their IUPAC names are written as follows:
1. Selection of principal functional group. When an organic compound contains two or more different functional groups, one of the functional groups is selected as the principal functional group while all other groups are treated as substituents and also called secondary functional groups.
The choice of the principal functional group is made on the basis of the following order of preference.
Amine salt > carboxylic acid > sulphonic acid > anhydrides > esters > acid chlorides > acid amides > nitriles > aldehydes > ketones > alcohols > phenols > thiols > amines > ethers > alkenes > alkynes.
Note- All the remaining functional groups such as halo (fluoro, chloro, bromo, iodo), nitroso (- NO), nitro (- NO2), and alkoxy (- OR), alkyl (-R) are treated as substituents.
2. Selection of the principal chain. while selecting the principal chain present in a polyfunctional compound are, we must care that the principal chain must contain the principal functional group and the maximum number of secondary functional groups and multiple bonds, if any.
3. Numbering of the principal chain. The principal chain present in a polyfunctional compound must be numbered in such a way that the principal functional group gets the lowest possible number followed by double bond, triple bond and the substituents.
4. Alphabetical order. The prefixes for the secondary functional groups and other substituents should be placed in alphabetical order before the word root.
Rules for Naming Alicyclic Compounds
The following rules are generally used.
1. The names of the alicyclic compounds are written by adding the prefix, ‘cyclo’ to the word root for this compound.
2. If two or more alkyl groups or or other substituent groups are present in the ring, their positions are indicated by arabic numerals, i.e. 1, 2. 3. 4 ….. etc. While numbering the carbon atoms of the ring, the substituents which comes first in the alphabetical order is given the lowest number provided it does not violate the lowest sum rule.
3. (a) If the ring contains more or equal number of carbon atoms than the alkyl group attached to it, it is named as a derivative of cyclo alkane and the alkyl group is treated as a substituent group. otherwise it is named as a derivative of alkane and the cycloalkyl group is considered as a substituent group.
(b) If the side chain contains a multiple bond or a functional group, the alicyclic ring is treated as the substituent irrespective of the size of the ring.
(c) If more than one alicyclic ring is attached to a single chain, the compound is named as a derivative of alkane irrespective of the number of carbon atoms in the ring or chain.
4. If a multiple (double or triple) bond and some other substituents are present in the ring, the numbering is done in such a way that the multiple bond gets the lowest number.
5. If the ring contains a multiple bond and the side chain contains a functional group, then the ring is treated as the substituent and the compound is named as a derivative of the side chain.
6. If the ring as well as the side chain contain functional group, the compound is named as a derivative of the side chain or the alicyclic ring according as the side chain or the ring contains the principal functional group.
Note- If the alicyclic ring and the side chain contain the same functional group, the compound is named as a derivative of the side chain of the ring according as the side chain or the ring contains higher number of carbon atoms.
7. If a compound contains an alicyclic ring directly linked to the benzene ring, it is named as a derivative of benzene.
8. If some functional group along with other substituent groups are present in the ring, it is indicated by some appropriate prefix or suffix and its position is indicated by numbering the carbon atoms of the ring in such a way that the functional group gets the lowest number.
9. If an alicyclic ring is directly attached to a carbon containing functional group, the carbon atom of the functional group is no included in the parent name of the alicyclic system. Therefore, for such system, the following prefixes and suffixes for the functional groups are commonly used.
| Functional Group | Prefix | Suffix |
| − CHO | Formyl | Carbaldehyde |
| − COOH | Carboxy | Carboxylic acid |
| − COX (X = F, Cl, Br, I) | Halocarbonyl | Carbonyl halide |
| − COOR | Alkoxycarbonyl or Carbalkoxy | Alkyl carboxylate |
| − CONH2 | Carbamoyl | Carboxamide |
| − CN | Cyano | Carbonitrile |
Nomenclature of Aromatic Compounds
An aromatic compound consists of two parts:
1. Nucleus 2. Side chain
1. Nucleus. The most ideal aromatic compound is Benzene. It is represented by a regular hexagon of six carbon atoms with three alternate single and double bonds. This is called the nucleus.
2. Side chain. The alkyl group or any other aliphatic group containing at least one carbon atom which is attached to the benzene ring is called side chain. For example methyl benzene, ethyl benzene etc.
As we know that aromatic compounds consists of two types, we write their names in the following ways:
1. Nuclear substituted – These are the compounds in which the functional group is directly attached to the benzene ring. Most of these compounds are better known by their common names. In the IUPAC system, many of their common names have also been adopted by the IUPAC system. The positions of the substituents in disubstituted benzene are indicated by prefixes or by arabic numerals such as o (ortho) for 1, 2 ; m (meta) for 1, 3 and p (para) for 1, 4.
2. Side chain substituted – These are the compounds in which 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 excepts arenes which are named as derivatives of benzene. The positions of the substituents on the side chain including the benzene ring are indicated by Greek letters i.e., α, Β, γ …. in the common system, and by arabic numerals, i.e. 1, 2, 3 …. in the IUPAC system.
3. When an aromatic compound contains two or more functional groups, it is named as a derivatives of the compound with the principal functional group at position 1.
4. If all the functional groups are present in the benzene ring are such which are normally treated as substituent group, the various groups are arranged in alphabetical order with the group named first in the alphabetical order getting the lowest locants provided it does not violate the lowest locant rule for all the substituents.
5. When a substituent is such which when taken together with the benzene ring gives a special name to the molecule, then it is named as a derivative of that molecule with the substituent at position 1.
6. When a benzene ring is attached to an aliphatic chain having a functional group, it is named as phenyl derivative of that aliphatic compound.
Isomerism
The phenomenon due to which two or more compounds have the same molecular formula but different chemical and physical properties is called isomerism and these compounds are known as isomers.
Types of isomerism: There are two types of isomerism.
1. Structural Isomerism 2. Stereoisomerism
Structural isomerism
Compounds having the same molecular formula but different arrangement of atoms within molecules are called structural isomers and the phenomenon is called structural isomerism.
Types of structural isomerism: There are six types of structural isomerism
1. Chain or Nuclear isomerism. Compounds having the same molecular formula but different arrangement of carbon chain within molecule are called chain isomers and this phenomenon is called chain isomerism. For example 2- methyl propane and butane.
2. Position isomerism. Compounds which have the same structure of the carbon chain but differ only in the position of the multiple bond or the functional group are called position isomers and this phenomenon is called position isomerism. For example but-1-ene and but-2-ene.
3. Functional isomerism. Compounds having the same molecular formula but different functional groups are called functional isomers and this isomerism is called functional isomerism. For example, The molecular formula C2H6O represents two functional isomers as Ethanol and Methoxymethane.
4. Metamerism. Compounds having the same molecular formula but different number of carbon atoms on either side of the functional group are called metamers and this phenomenon is known as metamerism. This isomerism occurs among the members of the same homologous family. For example Ethoxyethane is a metamer of 1-methoxypropane.
5. Tautomerism. It is a special kind of functional isomerism in which the isomers exists in dynamic equilibrium with each other. It arises due to migration of a hydrogen atom from one polyvalent atom to the other within the same molecule with necessary rearrangement of linkages. The isomers thus obtained are called tautomers and this phenomenon is called tautomerism. For example Phenol and Cyclohexa-2,4-dien-1-one.
6. Ring-chain isomerism. Compounds having the same molecular formula but possessing open chain and cyclic structures are called ring-chain isomers and this isomerism is called ring-chain isomerism. For example Propene and cyclopropane.
Stereoisomerism
Isomers which have the same structural formula but have different relative arrangement of atoms or groups in space are called stereoisomers and this phenomenon is called stereoisomerism.
There are two types of stereoisomerism
1. Configurational isomerism
2. Conformational isomerism
1. Configurational isomerism. That isomerism in which one isomer can not be converted into another isomers without breaking the bonds present between two atoms is called configurational isomerism. This isomerism is also of two types as Geometrical isomerism and Optical isomerism.
a. Geometrical isomerism. That isomerism in which atoms or groups are arranged differently along double bonded atoms is called geometrical isomerism.
⇒ When similar atoms or group of atoms are present in the same direction along the double bonded atoms, this type of isomers are called cis isomers.
⇒ When similar atoms or group of atoms are present in the opposite direction along the double bonded atoms, this type of isomers are called trans isomers.
b. Optical isomerism. That isomerism in isomers rotate the plane polarised light in opposite direction, such like isomers are called optical isomers and this phenomenon is called optical isomerism. There are two types of isomers in this isomerism.
⇒ When an isomer rotates the plane polarised light in right direction, it is called Dextrorotatory form (d-form).
⇒ When an isomer rotates the plane polarised light in left direction, it is called Laevorotatory form (l-form).
2. Conformational isomerism. That isomerism in which one isomer can be converted into another isomers without breaking the bonds present between two atoms is called conformational isomerism.
Electronic Displacement in a covalent bonds
There are four types of electronic displacement in covalent bonds
1. Inductive effect. The displacement of @ electron along a saturated carbon chain when polar bond is present at terminal, it is called inductive effect. There are two types of inductive effects.
- – I-effect. If the substituent attached to the end of the carbon chain is electron withdrawing, the effect is called -I effect. For example -NO2 > -CN > -COOH > -F > -Cl > -Br.
- + I-effect. If the substituent attached to the end of the carbon chain is electron donating, the effect is called +I effect. For example (CH3)3C- > (CH3)2CH- > CH3CH2– > CH3– > D > H.
2. Electromeric effect. The complete transfer of π electrons of a multiple bonds in the presence of attacking reagent, it is called electromeric effect. There are two types of electromeric effect.
- + E -effect. If the electrons of the π bond are transferred to that atom of the double bond to which the reagent gets finally attached, the effect is called + E -effect. For example, addition of acids to alkenes.
- – E -effect. If the electrons of the π bond are transferred to an atom of the double bond other than the one to which the reagent gets finally attached, the effect is called – E -effect. For example, the addition of cyanide ion to the carbonyl group.
3. Resonance effect or mesomeric effect. In case of conjugated system (having alternate σ- and π- bonds), the electrons can flow from one part of the system to the other due to resonance. This movement creates centres of low and high electron density and this activity is called resonance effect. It is of two types:
(1) Groups which donate electrons to the double bond or to a conjugated system are said to have +R or +M-effect. For example, – OH, – OR, -NH2, – NR2, -Cl, -Br etc.
(2) Groups which withdraw electrons from the double bond or from a conjugate system towards themselves due to resonance are said to have -R or -M-effect. For example, >C=O, -CHO, -COOR, -CN, -NO2 etc.
4. Hyperconjugation effect. When an alkyl group is attached to an unsaturated system such as double bond or a benzene ring, the order of inductive effect is actually reversed. This effect is called hyperconjugation or Baker- Nathan effect. The order of hyperconjugation effect decreases in the order: CH3– > CH3CH2– > (CH3)2CH- > (CH3)3C-