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Fundamentals of Inorganic Chemistry For Competitive Exams: Ananya Ganguly
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I am indeed very delighted to present the book Fundamentals of Organic Chemistry for the JEE (Main and Advanced) Volume I to the readers with elaborated concepts along with more solved and unsolved problems.
In all competitive examinations, there are several vital areas that a candidate needs to master—organic chemistry being one of them. There are various books in the market on this topic having different approaches; however this book provides simple, shortcut methods and time-saving tactics which are helpful during the examination to the students. It will also help you to gain confidence through the right approach to a particular questions rather than attempting number of questions. This book would be extremely useful for the students who enroll for examinations like JEE (Main and Advanced) and for other engineering entrance examinations.
In order to bridge the gap between theory and practical, each concept is explained in detail, in an easy-to-understand manner supported with numerous worked-out examples and practical. These will stimulate thought and facilitate advanced learning.
One of the important factors that contribute to the success of a book is the way it has been developed. This book reflects my experience and understanding of the requirements of the students. The methods and approaches discussed in this book are tried and tested modes of instruction in the class.
Fundamentals of Organic Chemistry, Volume I cover topics like, IUPAC Nomenclature, General Organic Chemistry, Hydrocarbon, Electrophilic Aromatic Substitution, and Analysis of Organic Compounds. Volume II covers, Alkyl Halides and Aryl Halides, Alcohol, Phenol and Ether, Aldehyde and Ketone, Carboxylic Acids and Derivatives, Nitrogen Containing Compounds, Carbohydrates, Amino Acids and Polymers.
I am sure that readers will appreciate this book and will find this book very useful to prepare for various examinations. Your comments and suggestions would be very useful in improving the subsequent editions of this book.
Although we have taken utmost care to prepare the manuscript and checking subsequent proofs, there may be a possibility of some errors creeping inside the book. We will welcome suggestions for further improvement of the book. Please mail us your suggestions on: chemistrycoach1@gmail.com.
Ananya Ganguly
Acknowledgments
Fundamentals of Organic Chemistry, Volume I is the result of encouragement that I received from my students, who insisted that, my knowledge and experience should benefit a wider audience.
I would like to thank my friends who have, over the years, been my support and strength. Writing this book has been a long but fulfilling journey and I am fortunate to be assisted by a talented team of editors.
I am indebted to my family for keeping me motivated during all stages of the project. I always feel a divine power supporting my efforts when my family is around. They actually made me work harder on the project.
I extend my sincere thanks to Pearson editorial team for their constant encouragement and support during the publication of this book.
IUPAC Nomenclature
1
Introduction
Carbon is the fourth most abundant chemical element in the universe by mass after hydrogen, helium, and oxygen. Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation (by synthesis or by other means) of carbon-based compounds, hydrocarbons, and their derivatives. These compounds may contain any number of other elements, including hydrogen, nitrogen, oxygen, the halogens as well as phosphorus, silicon and sulphur.
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simpleoxides of carbon, and cyanides, as well as the allotropes of carbon such as diamond and graphite, are considered inorganic. The distinction between “organic” and “inorganic” carbon compounds, while ‛useful in organizing the vast subject of chemistry... is somewhat arbitrary’.
Organic chemistry is the science concerned with all aspects of organic compounds. Organic synthesis is the methodology of their preparation.
Vitalism
The word ‛organic’ is historical, dating back to the 1st century. For many centuries, Western alchemists believed in vitalism. This is the theory that certain compounds could be synthesized only from their classical elements — earth, water, air, and fire — by action of a ‛life-force’ (vis vitalis) possessed only by organisms. Vitalism taught that these ‛organic’ compounds were fundamentally different from the ‛inorganic’ compounds that could be obtained from the elements by chemical manipulation.
Vitalism survived for a while even after the rise of modern atomic theory and the replacement of the Aristotelian elements by those we know today. It first came under question in 1824, when Friedrich Wöhler synthesized oxalic acid, a compound known to occur only in living organisms, from cyanogen. A more decisive experiment was Wöhler’s 1828 synthesis of urea from the inorganic salts potassium cyanate and ammonium sulfate. Urea had long been considered to be an ‛organic’ compound, as it was known to occur only in the urine of living organisms. Wöhler’s experiments were followed by many others, where increasingly complex ‛organic’ substances were produced from ‛inorganic’ ones without the involvement of any living organism.
Modern Classification
Even after vitalism had been disproved, the distinction between ‛organic’ and ‛inorganic’ compounds has been retained through the present. The modern meaning of ‛organic compound’ is any one of them that contains a significant amount of carbon - even though many of the ‛organic compounds’ known today have no connection whatsoever with any substance found in living organisms.
1.2 IUPAC Nomenclature
There is no ‛official’ definition of an organic compound. Some textbooks define an organic compound as one containing one or more C-H bonds; others include C-C bonds in the definition. Others state that if a molecule contains carbon―it is organic.
Even the broader definition of ‛carbon-containing molecules’ requires the exclusion of carbon-containing alloys (including steel), a relatively small number of carbon-containing compounds such as metal carbonates and carbonyls, simple oxides of carbon and cyanides, as well as the allotropes of carbon and simple carbon halides and sulfides, which are usually considered to be inorganic
The ‛C-H’ definition excludes compounds that are historically and practically considered to be organic. Neither urea nor oxalic acid is organic by this definition, yet they were two key compounds in the vitalism debate. The IUPAC Blue Book on organic nomenclature specifically mentions urea and oxalic acid. Other compounds lacking C-H bonds that are also traditionally considered to be organic include benzenehexol, mesoxalic acid, and carbon tetrachloride. Mellitic acid, which contains no C-H bonds, is considered to be a possible organic substance in Martian soil. All do, however, contain C-C bonds.
The ‛C-H bond only’ rule also leads to somewhat arbitrary divisions in sets of carbon-fluorine compounds, as, for example, Teflon is considered by this rule ‛inorganic’ but Tefzel organic. Likewise, many Halons are considered inorganic, whereas the rest are considered organic. For these and other reasons, most sources consider C-H compounds to be only a subset of ‛organic’ compounds.
In summary, most carbon-containing compounds are organic, and most compounds with a C-H bond are organic. Not all organic compounds necessarily contain C-H bonds (e.g., urea).
DRAWING ORGANIC MOLECULES
This page explains the various ways that organic molecules can be represented on paper or on screen - including molecular formulae and various forms of structural formulae.
Molecular Formulae
A molecular formula simply counts the numbers of each sort of atom present in the molecule, but tells you nothing about the way they are joined together.
For example, the molecular formula of butane is C4H10, and the molecular formula of ethanol is C2H6O.
Molecular formulae are very rarely used in organic chemistry, because they don't give any useful information about the bonding in the molecule. About the only place where you might come across them is in equations for the combustion of simple hydrocarbons, for example:
C5H12 + 8O2 5CO2 + 6H2O
In cases like this, the bonding in the organic molecule isn't important.
Structural Formulae
A structural formula shows how the various atoms are bonded. There are various ways of drawing this and you will need to be familiar with all of them.
Displayed Formulae
A displayed formula shows all the bonds in a molecule as individual lines. You need to remember that each line represents a pair of shared electrons.
For example, this is a model of methane together with its displayed formula:
Notice that the way methane is drawn bears no resemblance to the actual shape of the molecule. Methane is not flat with 90° bond angles. This mismatch between what you draw and what the molecule actually looks like can lead to problems if you aren't careful.
For example, consider a simple molecule with the molecular formula CH2Cl2. You might think that there were two different ways of arranging these atoms if you drew a displayed formula.
The chlorines could be opposite each other or at right angles to each other. But these two structures are actually exactly the same. Look at how they appear as models.
catch hold of the top hydrogen and rotate the molecule a bit
One structure is, in reality, a simple rotation of the other one.
Consider a slightly more complicated molecule, C2H5Cl. The displayed formula could be written as either of these:
But, again these are exactly the same. Look at the models.
The commonest way to draw structural formulae
For anything other than the most simple molecules, drawing a fully displayed formula is a bit of a bother - especially all the carbon-hydrogen bonds. You can simplify the formula by writing, for example, CH3 or CH2 instead of showing all these bonds.
So for example, ethanoic acid would be shown in a fully displayed form and a simplified form as:
1.4 IUPAC Nomenclature
You could even condense it further to CH3COOH, and would probably do this if you had to write a simple chemical equation involving ethanoic acid. You do, however, lose something by condensing the acid group in this way because you can not immediately see how the bonding works.
You still have to be careful in drawing structures in this way. Remember from above that both these structures represent the same molecule:
All of the next three structures represent butane.
CH3 — CH2 — CH2 — CH3
All of these are just versions of four carbon atoms joined up in a line. The only difference is that there has been some rotation about some of the carbon-carbon bonds. You can see this in a couple of models. molecule has been twisted about this C-C bond
Not one of the structural formulae accurately represents the shape of butane. The convention is that we draw it with all the carbon atoms in a straight line–as in the first of the above structures. This is even more important when you start to have branched chains of carbon atoms. The following structures again represent the same molecule–2-methylbutane.
The two structures on the left are fairly obviously the same–all we have done is flip the molecule over. The other one is not so obvious until you look at the structure in detail. There are four carbons joined up in a row, with a CH3 group attached to the next-to-end one. That is exactly the same as the other two structures. If you had a model, the only difference between these three diagrams would be that you would have rotated some of the bonds and turned the model around a bit. To overcome this possible confusion, the convention is that you always look for the longest possible chain of carbon atoms, and then draw it horizontally. Anything else is simply hung off that chain.
It does not matter in the least whether you draw any side groups pointing up or down. All of the following represent exactly the same molecule.
If you made a model of one of them, you could turn it into any other one simply by rotating one or more of the carboncarbon bonds.
How to draw structural formulae in 3-dimensions
There are occasions when it is important to be able to show the precise 3-D arrangement in parts of some molecules. To do this, the bonds are shown using conventional symbols:
bond in plane of paper
bond going back into paper away from you
bond coming out of paper towards you
For example, you might want to show the 3-D arrangement of the groups around the carbon, which has the -OH group in butan-2-ol.
Butan-2-ol has the structural formula:
Using the conventional bond notation, you could draw it as, for example:
The only difference between these is a slight rotation of the bond between the two carbon atoms in the centre. This is shown in the two models below. Look carefully at them–particularly at what has happened to the lone hydrogen atom. In the left-hand model, it is tucked behind the carbon atom. In the right-hand model, it is in the same plane. The change is very slight.
1.6 IUPAC Nomenclature
It doesn't matter in the least which of the two arrangements you draw. You could easily invent other ones as well. Choose one of them and get into the habit of drawing 3-dimensional structures that way.
Notice that no attempt was made to show the whole molecule in 3-dimensions in the structural formula diagrams. The CH2CH3 group was left in a simple form. Keep diagrams simple–trying to show too much detail makes the whole thing amazingly difficult to understand.
Skeletal Formulae
In a skeletal formula, all the hydrogen atoms are removed from carbon chains, leaving just a carbon skeleton with functional groups attached to it.
For example, we have just been talking about butan-2-ol. The normal structural formula and the skeletal formula look like this:
In a skeletal diagram of this sort,
there is a carbon atom at each junction between bonds in a chain and at the end of each bond (unless there is something else there already, like the -OH group in the example);
there are enough hydrogen atoms attached to each carbon to make the total number of bonds on that carbon up to 4.
Diagrams of this sort take practice to interpret correctly, and may well not be acceptable to your examiners (see below).
There are, however, some very common cases where they are frequently used. These cases involve rings of carbon atoms, which are surprisingly awkward to draw tidily in a normal structural formula.
Cyclohexane, C6H12, is a ring of carbon atoms, each with two hydrogens attached. This is what it looks like in both a structural formula and a skeletal formula.
And this is cyclohexene, which is similar but contains a double bond:
But the commonest of all is the benzene ring, C6H6, which has a special symbol of its own.
benzene ring
Bond-Line Formulae
We will use a very simplified formula called a bond-line formula to represent structural formulae. The bond-line representation is the quickest of all to write because it shows only the carbon skeleton. The number of hydrogen atoms necessary to fulfill the carbon atoms’ valences are assumed to be present, but we do not write them in. Other atoms (e.g., O, Cl, N) are written in and hydrogens attached to those atoms are also written. Each intersection of two or more lines and the end of a line represent a carbon atom unless some other atom is written in.
Bond-line formulae are often used for cyclic compounds:
Muliple bonds are also indicated in bond-line formulae. For example:
Problem
1. Outline the carbon skeleton of the following condensed structural formulae and then write each as a bond-line formula.
(a) (CH3)2CHCH2CH3
(b) (CH3)2CHCH2CH2OH
(e) CH3CH2CH(OH)CH2CH3
(f) CH2=C(CH2CH3)2O
(c) (CH3)2C=CHCH2CH3 (g)
(d) CH3CH2CH2CH2CH3
(h) CH3CHC1CH2CH(CH3)2
CLASSIFICATION OF CARBON AND HYDROGEN OF ORGANIC COMPOUND
(a) Classification ‘A’–Degree of Carbon Atom and Hydrogen Atom
The degree is assigned on the basis of connectivity of carbon atoms with other carbon atoms in the chain. According to this classification, we have four types of carbon atoms.
These are:
1. Primary carbon: (1°) A carbon atom linked to one carbon atom
2. Secondary carbon: (2°) A carbon atom linked with two carbon atoms
3. Tertiary carbon: (3°) A carbon attached to three carbon atoms
4. Quaternary carbon: (4°) A carbon attached to four carbon atoms
The hydrogens in a molecule are also referred to as primary, secondary, and tertiary. Primary hydrogens are attached to primary carbons, secondary hydrogens are attached to secondary carbons, and tertiary hydrogens are attached to tertiary carbons. There is no Quaternary Hydrogen atom.
Illustration
1. (a) Provide a structural formula for CH3CH2C(CH3)2CH2CH(CH3)2, and define and identify all the primary (1°), secondary (2°), tertiary (3°), and quaternary (4°) C’s. (b) Identify all the 1°, 2°, and 3° H’s. (e) Give number of H atoms bonded to a 1°, 2°, 3°, and 4° carbon atom in an alkane. (d) Give the number of C atoms bonded to a 1°, 2°, 3°, and 4° carbon atom in an alkane.
Ans. (a) In the condensed formula all atoms or groups written after a C are bonded to it.
1 °C is bonded to only one other C, a 2 °C to two other C’s, a 3 °C to three other C’s, and a 4° to four other C’s. (The C of CH4 is super F.)
(b) 1 °H’s are those attached to 1 °C’s, 2 °H’s to 2 °C’s, and 3 °H’s to 3 °C’s. Other atoms or groups bonded to C, like halogen, are similarly identified. (A 4 °H cannot exist because all four bonds of a 4 °C are bonded to other C’s.)
(c) 3, 2, 1, and 0.
(d) 1, 2, 3, and 4.
(b) Classification ‘B’–With Respect to the Functional Group
In this method, the relative positions of different carbon atoms in a molecule are designated by small letters of Greek alphabets with respect to the functional group it contains. The carbon atom directly attached to the functional group is designated as a-C, the carbon atom next to it is indicated as b-C atom and the next to b-C is g-C atom and so on. Each of the hydrogen’s is given the smaller a, b, g, d designation as the carbon atom to which it is attached.
For example,
(c) Alkyl Groups
An alkyl substituent (or an alkyl group) is an alkane from which a single hydrogen has been removed. Alkyl substituents are named by replacing the ‛ane’ ending of the alkane with ‛yl.’ The letter ‛R’ is used to indicate any alkyl group.
Iso alkyl group: An alkyl group in which the second last carbon in the chain is branched to one –CH3 group.
For Example,
Neoalkyl Group
The alkyl group in which second last carbon of the chain is branched to two CH3 groups.
For Example
(d) Unsaturated Alkyl Group
CH2 = CH – Vinyl or ethenyl
CH2 = CH – CH2 – Alkyl or prop-2-enyl
CH3 – CH = CH – x prop-1-enyl
CH3 – CH = CH – CH2 – Crotyl
C6H5 – CH = CH – CH2 – Cinnamyl
C6H5 – Phenyl
C6H5CH2– Benzyl Tolyl
NOMENCLATURE
OF ORGANIC COMPOUNDS
(A) Common or Trivial Nomenclature
In the early days, an organic compound was named as per its origin or its characteristic properties. These names are called common names or trivial names. Take an example of formic acid (HCOOH). It originates from ants and the Latin name of ant is formicium and also it behaves like an acid. Thus the name formic acid is given. More examples,
Wood alcohol from wood
Cocaine front Peruvian cocoa leaves
Butyric acid from rancid butter
Buckminsterfullerene is a common name given to the newly discovered C60 cluster (a form of carbon) noting its structural similarity to the geodesic domes popularized by the famous architect R. Buckminster Fuller
Common names are useful, and in many cases indispensable, particularly when the alternative systematic names are lengthy and complicated. Common names of some organic compounds are given in Table 1.1.
Table 1.1 Some Common Names of Compounds
CH4
H3CCH2CH2CH3
(H3C)2CHCH3
Methane
n-Butane
Isobutane (H3C)4C
Neopentane
H3CCH2CH2OH
n-Propyl alcohol HCHO
(H3C)2CO
CHCl3
CH3COOH
C6H6
C6H5OH
C6H5NH2
C6H5COCH3
CH3OCH3
Formaldehyde
Acetone
Chloroform
Acetic acid
Benzene
Phenol
Aniline
Acetophenone
Dimethyl ether
(B) IUPAC (International Union of Pure and Applied Chemistry) Nomenclature
The IUPAC nomenclature of organic chemistry is a systematic method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). Ideally, every possible organic compound should have a name from which an unambiguous structural formula can be drawn. There is also an IUPAC nomenclature of inorganic chemistry.
For ordinary communication, to spare a tedious description, the official IUPAC naming recommendations are not always followed in practice except when it is necessary to give a concise definition to a compound, or when the IUPAC name is simpler (e.g. ethanol against ethyl alcohol). Otherwise the common or trivial name may be used, often derived from the source of the compound. We will observe while naming the organic compounds that in some cases, trivial names or common names have been retained by IUPAC.
Nomenclature of Organic Compounds
The term nomenclature means the system of naming. The systematic nomenclature of organic compounds is known as IUPAC (International Union of Pure and Applied Chemistry) system, which is briefly discussed below.
According to IUPAC system, the name of an organic compound, in general, consists of the following parts:
(i) Word root
(ii) Primary suffix
(iii) Secondary suffix
(iv) Prefix(es)
1.12 IUPAC Nomenclature
(i) Word root: The word root represents the number of carbon atoms in the parent chain. The general word root for different aliphatic compounds is ALK. The word root for different lengths of carbon chains are given below in Table 1.2.
Table 1.2 Some Prefixes
(ii) Primary Suffix: Primary suffix is used to represent saturation or unsaturation in the carbon chain. While writing the name, primary suffix is added to the word root. Some of the primary suffixes are given below in Table 1.3.
Table 1.3 Some Primary Suffixes
Nature of Carbon Chain Primary Suffix
Saturated Carbon Chain ane
Unsaturated Carbon Chains
One C = C bond ene
Two C = C bonds adiene
Three C = C bonds atriene
One C = C bond yne
Two C = C bonds adiyne
It may be noted that an extra ‘a’ is added to the word root if the primary suffix to be added begins with a constant (other than a, e, i, o, u.)
For example, for two double bonds, suffix is diene and if it is to be added to word root ‘but’ (for 4C atoms, it becomes butadiene)
(iii) Secondary Suffix: Secondary suffix is used to indicate the functional group in the organic compounds. The terminal ‘e’ of the primary suffix is dropped if it is followed by a suffix beginning with ‘a’, ‘i’, ‘o’, ‘u’ or ‘y’. But it is retained if secondary suffix begins with a consonant.
Secondary suffixes for various functional groups are given in Table 1.4.
Table 1.4 Some Organic families and Secondary Suffixes
It may be noted that while adding the secondary suffix to the primary suffix, the terminal ‘e’ of the primary suffix (i.e. ane, ene or yne) is dropped if the complete secondary suffix (the suffix plus multiplying affix, if any, such as di tri tetra) begins with a vowel (a, e, i, o, u). However, the terminal ‘–e’ is retained if the secondary suffix begins with a consonant.
For example,
–CH3 – CH2 – CH2 – CH2 – OH Word root but
Primary suffix ane
Sec. suffix –ol
So, its name is But + ane + ol = Butanol or Butan – 1– ol.
[Here, ‘e’ of ane is dropped because sec. suffix –‘ol’ begins with a vowel.]
(iv) Prefix: Prefix is a part of the name which appears before the word root.
Primary prefix: Indicates that whether the compound is cyclic or acyclic etc., primary prefix cyclo is used immediately before the word root.
Primary prefix
Word root
Primary Suffix
Secondary Suffix
Cyclo hex ane –
Secondary Prefix
IUPAC name
Cyclohexane
Prefixes that are used to represent the names of alkyl groups (branched chains) or some functional groups, which are regarded as substituents, are secondary prefix.
(a) Alkyl groups are formed by the removal of H atom from the alkanes. These are represented by the general formula C nH2n+1 or R-. Some alkyl groups along with their prefixes are given in Table 1.5.
Table 1.5 Some Alkyl Groups along with their Prefixes
Alkane
CH4
C2H6
C3H8
Alkyl Group Abbreviation Prefix
CH3– Me- Methyl
CH3CH2 Et- Ethyl
CH3CH2CH2– n-Pr- n-Propyl
C3H8 CH3– CH–| CH3 Iso-Pro Isopropyl
Some unsaturated groups are given below :
(b) Some functional groups are always indicated by the prefixes instead of secondary suffixes. These functional groups along with their prefixes are listed in Table 1.6.
Table 1.6 Functional groups which are always represented by prefixes
Functional Group
— NO2 Nitro
— OR' alkoxy
— Cl Chloro
IUPAC Name
R— NO2 Nitroalkane
R— OR' Alkoxyalkane
R— Cl Chloroalkane — Br Bromo R— Br Bromoalkane — I lodo R— I Iodoalkane — F Fluoro
R— F Fluoroalkane — NO Nitroso
R— NO Nitrosoalkane
Table 1.7 Some Characteristic Groups with their Prefixes
Characteristic group Prefix
— CIO
Chlorosyl— CIO2
Chloryl— CIO3
— IO
Perchloryl-
Iodosyl-
— IO2 Iodyl-(replaces iodoxy-)
— I(OH)2
— IX2
= N2
Dihydroxy-l3- iodanyl-(replaces dihydroxyiodo-)
Dihydroxy-l3- iodanyl-(replaces dihaloiodo-)
Diazo-
— N3 Azido-
— SR (R)-sulfanyl- (and similarly, (R)-selanyl- and (R)-tellanyl-)
— SH3 l4-Sulf Anyl-
(c) In polyfunctional compounds, i.e., compounds with more than one functional group, one of the functional groups is treated as the principal functional group and is indicated by the secondary suffix, whereas the other functional groups are treated as substituents and are indicated by prefixes. The prefixes for various functional groups are given in Table 1.8.
Table 1.8 Prefixes for functional groups in polyfunctional compounds
Functional Group Prefix
Functional Group Prefix
—OH Hydroxy — COOH
Carboxy — CN Cyano — COOR
Carbalkoxy — NC Isocyano — COCl
Chloroformyl — CHO Formyl or Aldo — CONH2
Carbamoyl — SH Mercapto — NH2
Amino — SR Alkylthio >CO
Keto or Oxo
Arrangement of Prefixes, Word root and Suffixes: The prefixes, word root and suffixes are arranged as follows while writing the name.
Prefix(es) + Word root + p. suffix + sec. suffix
RULES FOR NOMENCLATURE (ALKANES)
(A) Saturated Hydrocarbons (Alkanes)
Case-1. Unbranched alkane
Count the number of C-atoms and write the name depending on the number of C-atoms as mentioned in table 1.2, i.e. for 1 carbon–methane, for 2 carbons–ethane etc.
Case-2. Branched alkane
Rule 1. Longest chain rule
Select the longest continuous chain of carbon atoms in the molecule. The selected chain, containing the maximum number of carbon atoms, is regarded as the parent or root chain . It gives the name of the parent hydrocarbon. Carbon atoms which are not included in the parent chain are identified as substituents.
For example, in the following compound, the parent chain consists of nine carbon atoms (structure I) and not eight (structure II). So, the compound is named as derivative of nonane.
1.16 IUPAC Nomenclature
Rule 2. Lowest Number or Lowest Sum Rule
The selected parent chain is numbered using Arabic numerals and the position of the alkyl group is indicated by the number of the carbon atom to which the alkyl group is attached. The numbering is done in such a way that the substituted carbon atoms have the lowest possible numbers.
For example, if X represents a substituent, then the correct numbering of the carbon chain is as given in structure A. The numbering of the carbon chain as given in structure B is wrong because it gives higher number to the carbon atom carrying the substituent.
The number that indicates the position of the substituent or side chain is called locant When series of the locants containing the same number of terms are compared term by term, that series is lowest which contains the lowest number on the occasion of first difference. Some examples are given below to illustrate the above idea:
The names of alkyl groups attached as a branch are then prefixed to the name of the parent alkane, and the position of the substituents is indicated by the appropriate numbers. If different alkyl groups are present, they are listed in alphabetical order.
For example,
It must be remembered that numbers are separated from the groups by hyphens, and there is no break between substituent and alkane names, i.e., between methyl and pentane, or ethyl and hexane.
Rule 3. Use of Prefixes Di, Tri, etc.
If the compound contains more than one similar alkyl groups, their positions are indicated separately and an appropriate numerical prefix, di, tri, etc., is attached to the name of the substituents. The positions of the substitutents are separated by commas. For example,
If there are different alkyl substituents present in a compound, their names are written in the alphabetical order. However, the numerical prefixes such as di, tri, etc. are not considered for the alphabetical order. For example,
Rule 5. Naming different alkyl substituents at the equivalent positions
If two different alkyl groups are located at the equivalent positions, then numering of the chain is done in such a way that the alkyl group which comes first in alphabetic order gets the lower position. For example, if ethyl and methyl groups are present at equivalent positions, then carbon bearing ethyl group should get the lower number as illustrated in the following example.
1.18 IUPAC Nomenclature
Rule 6. Naming the Complex Alkyl Substituents
If the alkyl substituent is further branched, it is named as substituted alkyl group. In the complex substituent, the numbering of the carbon atoms is also done. The carbon atom attached to the parent chain is always numbered as 1. The prefix/ name of such a substituents is enclosed in brackets as illustrated in the following examples.
The numerical prefixes ‘bis-’, ‘tris-’, ‘tetrakis-’, ‘pentakis-’ etc. are used to indicate a multiplicity of substituted substituents. The name of the substituted substituent is enclosed in parentheses. For example,
Some Important Notes
1. For 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. We know that in case of simple substituents the multiplying prefixes are not considered , however, they are regarded to be the part of the name for the complex substituent. For example,
2. It may be noted that dimethyl propyl (a complex substitution) is alphabetized under ‛d’ and not under ‛m’. Therefore, it is cited before ethyl (e).
3. When the names of two or more complex substituents are composed of identical words, priority is given to the sub substituent with lower locant at the first cited point of difference within the complex substituent. For example,
The substituent (1-methylbutyl) is written first because it has lower locant that the substituent (2-methylbutyl).
4. When there are more than one same complex substituents, it is indicated by the multiplying prefix bis (for two), tris (for three), tetra kis (for four), etc.
5. The prefixes iso- and neo- are considered to be part of alkyl group. The prefixes sec- and tert- are not considered to be part of fundamental name. The use of ‘iso’ and related common prefixes for naming alkyl group is also allowed by the IUPAC nomenclature as long as these are not further substituted.
RULES FOR NAMING COMPOUNDS CONTAINING DOUBLE AND TRIPLE BONDS
1. Select the longest continuous chain of carbon atoms as the parent chain. If some carbon-carbon multiple bond is present, the parent chain must contain the carbon atoms involved in it. The number of carbon atoms in the parent chain determines the word root. The carbon atoms which are not included in the parent chain are considered alkyl substituents and they determine the prefixes.
If two equally long chains are possible, the chain with maximum number of side chains is selected as parent chain.
Note: It may be noted that the selected chain containing double or triple bonded carbon atoms (must include both the carbon atoms of the double bond or triple bond) may or may not be the longest chain in the structure. For example, in the above structure, the longest chain containing double bonded carbon atoms is of five carbon atoms and not of six.
Suffixes Denoting Multiple Bonds
The presence of one or more double or triple bonds in an otherwise saturated parent hydride (except for parent hydrides with Hantzsch-Widman names) is denoted by changing the ‛-ane’ ending of the name of a saturated parent hydride to one of the following:
1. The presence of both double and triple bonds is similarly denoted by endings such as ‛-enyne’, ‛-adienyne’, ‛-enediyne’, etc. Numbers as low as possible are given to double and triple bonds as a set, even though this may at times give ‛-yne’ a lower number than ‛-ene’. If a choice remains, preference for low locants is given to the double bonds. Only the lower locant for a multiple bond is cited except when the numerical difference between the two locants is greater than one, in which case the higher-numbered locant is cited in parentheses.
2. While writing the name of the alkene or alkyne, the suffix ‘ane’ of the corresponding alkane is replaced by ‘ene’ or ‘yne’ respectively.
3. The numbering must begin with the one end of the parent chain, but this is done in such a way that carbon atom carrying the multiple bonds gets the lowest number. The position of the multiple bond is then indicated by using the number of the first C-atom of the multiple bond.
Table 1.10
Note: It may be noted that we give the lowest number to the carbon atom having double or triple bond and not to any side chain (as in alkanes).
4. All the rules for naming side chains or substituents are then followed (same as in alkanes).
5. If the multiple bond occurs two or more times in the chain, then it is named as diene or diyne, triene or triyne etc. The positions or multiple bonds are also prefixed with numbers. Some examples are given below:
5. If the parent chain contains both the double and triple bonds, the following points should be followed: If double bond and triple bond get the same locant from two sides of the parent carbon chain, double bond gets the priority in the numbering but the name ends with triple bond. Example is given below.
The terminal ‘e’ in the name of alkene is dropped if it is followed by the suffix beginning with a,e, i, o, u. For example, terminal ‘e’ of the ene is dropped in the following cases: en – yne adien – yne
However, terminal ‛e’ is not dropped in case of ‛ene – diyne’ because it comes before diyne.
