Nomenclature of Organic Compounds - Definition, Different Classes, FAQs

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Nomenclature definition: It is used for assigning proper name to a particular organic compound on the basis of certain standard rules so that the study of these compounds may become systematic and easy to follow. In case of aliphatic compounds, two systems of naming system are generally used,

(A) Trivial system (B) IUPAC system

(A) Trivial system.

In previous days, the organic compounds were named after the source from which they were obtained, as in example, urea got its name because the compound was obtained from the urine of the mammals. Similarly, methyl alcohol was known as wood spirit since it could be obtained as one of the products during the destructive distillation of wood and formic acid derived its name from the Greek word formicus (red ants) since the acid could be obtained from red ants’ species. These names are without any systematic basis or rules and are known as common names or trivial matters. Such a type of a system of nomenclature is known as a trivial system.

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(A) Trivial system.
(B) What is the full form of IUPAC?
IUPAC System of Naming Organic Compounds:
Different Classes of Organic Compounds
General rules for naming organic compounds

(B) What is the full form of IUPAC?

IUPAC system of nomenclature of carbon compounds

Nomenclature meaning in Tamil is Peyargal

In an attempt to rationalise the system of naming an International Congress of Chemists was held in Geneva in 1892. They adopted certain uniform rules for naming or nomenclature of the organic compounds/ naming organic compound: The system of nomenclature was named as the Geneva system and since then the system of naming has been modified by the International Union of Pure and Applied Chemistry which is full form of IUPAC in chemistry.

The new system is called IUPAC system of naming the compound. Thus this system of nomenclature of organic compound was first introduced in 1947 and was changed from time to time. The most comprehensive rules for nomenclature of organic compounds and names were first published in 1979 and later revised, modified and updated in 1993.

The rules discussed are based on guide books published by IUPAC in 1979 (Nomenclature of Organic Chemistry by J Rigandy and S.P. Klesney). With the help of this above naming system, an organic compound having any number of carbon atoms can be easily named.

Many of the books discuss nomenclature based on the IUPAC recommendations of 1979, we include here the recommendations of 1993 also to provide LATEST UPDATED RULES.

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IUPAC System of Naming Organic Compounds:

In the IUPAC system, the naming of an organic compound consists

(i) Word root. The word root is the number of carbon atoms present in the chain and for chains containing up to four carbon atoms, special word roots (meth-C₁, eth-C₂. prop-C₃) have been used.

(ii) Suffix: In the nomenclature of organic compounds and naming the compound the word root is linked to the suffix which may be primary or secondary or both.

Primary suffix. In the naming, IUPAC nomenclature of organic chemistry it indicates the nature of linkages in the carbon atoms. For example, if the carbon atoms are linked by single covalent bonds (C-C), the primary suffix -ane is used.
In nomenclature of organic compounds double bond (C=C) between the carbon atoms, the suffix is -ene while for the triple bonded carbon atoms (C=C suffix -yne is used. Thus, the suffixes are:

ane -for single bonded compounds, C-C

ene- for double bonded compounds, C=C

yne - for triple bonded compounds, C=C

If the parent chain contains two, three or more double or triple bonds in the organic compound, by IUPAC name then the numerical prefixes such as di(for two),tri(for three),tetra(for four).etc are added to the primary suffix.

(b) Secondary suffix. The nomenclature of organic compounds by IUPAC name it indicates the presence of functional secondary suffixes used to represent functional groups are:

Class of organic compound	Functional	Secondary suffix by IUPAC name and its nomenclature
Alcohols	-HO	-ol
Aldehydes	-CHO	-al
Ketones	>C==O	-one
Carboxylic acids	-COOH	-oic acid
Esters	-COOR	alkyl oate
Acid chlorides	-COCI	-oyl chloride
Acid amides	-CONH₂	-amide
Nitriles	-C=N	-nitrile
Amines	-NH₂	-amine

It may be reported that while adding the secondary suffix to the primary suffix in the nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, the terminal 'e' of the primary suffix (i.e., ane, ene or yne) is omitted 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) and however, the terminal'-e' is retained if the secondary suffix begins with a consonant.

(iii) Prefixes. The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, there are many groups which are not regarded as functional groups in the IUPAC name of the compound and these are regarded as substituents or side chains. These are represented as prefixes and are placed before the word root while naming a compound. These may be

(a) Alkyl groups. In IUPAC naming rules, these groups in organic compounds contain one hydrogen atom less than the alkane which are named by substituting the suffix ane of the name of the corresponding alkane by yl.

(b) The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry functional groups not regarded as principal functional groups and if a compound contains more than one functional group, then one of the functional groups is regarded as principal functional group and is treated as a secondary suffix. The other functional groups are regarded as substituents and are indicated by prefixes according to the IUPAC naming system.

These are the iupac nomenclature rules to be followed.

Different Classes of Organic Compounds

These are the organic compounds containing carbon and hydrogen atoms only. They may be further classified into two classes: saturated and unsaturated hydrocarbons. (a) Saturated Hydrocarbons-Alkanes

1. Hydrocarbons: The nomenclature of organic compounds which contain only carbon-carbon single bonds where these were earlier named as paraffins (Latin: meaning little affinity) due to their least chemical reactivity and according to IUPAC these are named as alkanes (ane is suffix with word root). Various members of the series are given below as named by IUPAC system.

General formula=C, H₂_n+2 Suffix: ane

IUPAC name list/ IUPAC name chart

Formula	IUPAC name	Common name
CH₄	Methane	Methane
CH₃CH₃	Ethane	Ethane
CH₃CH₂CH₃	Propane	Propane
CH₃CH₂CH₂CH₃	Butane	n-Butane
CH₃(CH₂)₃CH₃	Pentane	n-Pentane
CH₃(CH₂)₄CH₃	Hexane	n-Hexane

Classification of carbon atoms in alkanes. The carbon atoms in an alkane molecule may be classified into four types as primary (1°), secondary (2), tertiary (3°) and quaternary (4°).

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The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, the nomenclature can be done by

(1) A carbon atom connected to one other (or no other) carbon is called primary carbon

(written as 1° carbon).

(2) A carbon atom connected to two other carbon atoms is called secondary carbon

(written as 2º carbon).

(3) A carbon atom connected to three other carbon atoms is called tertiary carbon

(written as 3º carbon).

(4) A carbon atom connected to four other carbon atoms is called quaternary carbon

(written as 4º carbon).

Hydrogen atoms attached to 1°, 2° and 3° carbon atoms are referred to as primary, secondary and tertiary H atoms respectively. The following example clearly differentiates the various carbon atoms and it is useful in naming the compounds.

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General rules for naming organic compounds

We have discussed the simple rules for naming organic compounds for different families of compounds. The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry. But many organic compounds have typical branching or substituent and due to branching at different positions in a molecule, it is not possible to name them directly.

Thus, the nomenclature includes simple branched chain compounds that can be named according to common names. In the common system, all the isomeric alkanes (having the same molecular formula) have the same parent’s name and the names of various isomers are distinguished by prefixes where the prefix indicates the type of branching in the molecule. Example,

(i) In nomenclature, prefix n-(normal) is used for those alkanes in which all the carbon atoms form a continuous chain where there is no branching.

CH₃CH₂CH₂CH₃

n-Butane

(ii)In nomenclature prefix iso- is employed for those alkanes during which one methyl group is connected to the next-to-end carbon atom (second last) of the continuous chain.

(iii) )In nomenclature prefix neo- is employed for those alkanes which have two methyl groups connected to the second last carbon atom of the continuous chain.

In naming highly branched alkanes, the system becomes complicated. However, they can be easily named for IUPAC Nomenclature of Branched Chain Saturated Hydrocarbons (Alkanes) according to IUPAC system as discussed below:

Rules for IUPAC Nomenclature of Branched Chain Saturated Hydrocarbons (Alkanes)

The following rules are used for naming/ nomenclature of the branched chain alkanes :

Longest chain rule-The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, the nomenclature can be done by

choose the longest continuous chain of carbon atoms within the molecule. the chosen chain, containing the utmost variety of carbon atoms, is considered the parent or root chain and it provides the name of the parent organic compound. Carbon atoms that don't seem to be enclosed within the parent chain are known as substituents or branched chains.

Position of the substituent-The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, the nomenclature can be done by

Number the carbon atoms of the parent chain as 1, 2, 3... etc. start from the end which gives lower number to the carbon atoms having the substituents in the compound.
Lowest set of locants- When two or more substituents are present, then the end of the parent chain which gives the lowest set of the locants is preferred for numbering.
This rule is called lowest set of locants which means that when two or more different sets of locants are present then that set of locants when compared term by term with other sets. This rule is used irrespective of the nature of the substitute.

Presence of more than one same substituent-In IUPAC nomenclature of the same substituent or side chain occurs more than once, the prefixes di (for 2), tri (for 3), tetra (for 4) etc.
Naming different substituent -In IUPAC nomenclature of two or more different substituents or side chains are present in the m, they are summed in the alphabetical order along with their appropriate positions.
Naming different substituents at equivalent positions- In IUPAC nomenclature of two different substituents are in equivalent positions from the two ends of the chain (two sets of locants are equally possible), then the numbering of the chain done in such a way that the group which comes first in the alphabetical order (written first in the name) ats lower number.

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NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. Full form of IUPAC

IUPAC full form is the International Union of Pure and Applied Chemistry.

2. What is Nomenclature's meaning?

Nomenclature Is a system used for assigning proper names to a particular organic compound on the basis of certain standard rules so that the study of these compounds may become systematic and easy to follow.

3. What is a trivial name?

Names are without any systematic basis or rules and are known as common names or trivial names.

4. What is the lowest locant rule?

When two or more substituents are present, then the end of the parent chain which gives the lowest set of the locants is preferred for numbering.

5. What is the word root?

The word root is the number of carbon atoms present in the chain and for chains containing up to four carbon atoms, special word roots (meth-C₁, eth-C₂. prop-C₃) have been used.

6. What is organic chemistry nomenclature?

In the nomenclature of organic chemistry, the word root is linked to the suffix which may be primary or secondary or both.

7. What is the significance of the "ortho-", "meta-", and "para-" prefixes in aromatic compound naming?

These prefixes are used in common names for disubstituted benzene rings. "Ortho-" indicates substituents on adjacent carbons (1,2-), "meta-" indicates substituents separated by one carbon (1,3-), and "para-" indicates substituents opposite each other (1,4-). In IUPAC nomenclature, numbers are used instead.

8. What is the significance of prefixes like "iso-" and "neo-" in organic nomenclature?

Prefixes like "iso-" and "neo-" are used in common names but not in IUPAC nomenclature. "Iso-" typically indicates a methyl branch on the second-to-last carbon, while "neo-" indicates a tert-butyl group. In IUPAC names, these structures are named systematically using alkyl substituents.

9. What is the difference between primary, secondary, and tertiary alcohols in naming?

The terms primary, secondary, and tertiary refer to the number of carbon atoms attached to the carbon bearing the -OH group. While these terms aren't used in IUPAC names, they affect the numbering of the parent chain to give the -OH group the lowest possible number.

10. How are aromatic compounds named in IUPAC nomenclature?

Benzene derivatives are often named using the prefix "phenyl-" or as substituted benzenes. For example, C6H5-CH3 can be named methylbenzene or toluene (a common name). Substituents on the benzene ring are numbered to give the lowest possible combination of numbers.

11. What are positional isomers and how are they named?

Positional isomers have the same molecular formula but differ in the position of a functional group or substituent. They are named by changing the number indicating the position of the group. For example, 1-propanol and 2-propanol are positional isomers.

12. How are carboxylic acids named in IUPAC nomenclature?

Carboxylic acids are named by replacing the "-e" in the parent alkane name with "-oic acid". The carboxyl group is always at the end of the chain, so numbering starts from the carboxyl carbon. For example, CH3-COOH is ethanoic acid.

13. How are esters named in IUPAC nomenclature?

Esters are named by first naming the alkyl group attached to the oxygen, followed by the name of the carboxylate (derived from the carboxylic acid). For example, CH3-COO-CH2-CH3 is ethyl ethanoate.

14. How are amines named in IUPAC nomenclature?

Primary amines are named by adding the suffix "-amine" to the parent alkane name. For secondary and tertiary amines, N-substituted names are used. For example, CH3-CH2-NH2 is ethanamine, while (CH3)2NH is N-methylmethanamine.

15. How are organometallic compounds named in IUPAC nomenclature?

Organometallic compounds are named by specifying the organic groups attached to the metal, followed by the name of the metal. For example, (CH3)2Zn is dimethylzinc. More complex organometallics may use prefixes to indicate the number of organic groups.

16. How are ethers named in IUPAC nomenclature?

Ethers can be named in two ways: 1) as alkoxy derivatives of alkanes (e.g., methoxyethane), or 2) by naming the alkyl groups alphabetically followed by "ether" (e.g., ethyl methyl ether).

17. What are common functional groups in organic chemistry?

Common functional groups include alcohols (-OH), aldehydes (-CHO), ketones (C=O), carboxylic acids (-COOH), esters (-COO-), amines (-NH2), alkenes (C=C), and alkynes (C≡C). Each group has specific chemical properties and naming conventions.

18. How are cyclic compounds named in IUPAC nomenclature?

Cyclic compounds are named by adding the prefix "cyclo-" to the alkane name corresponding to the number of carbons in the ring. For example, a six-carbon ring is named cyclohexane. Substituents are numbered to give the lowest possible numbers.

19. How are alkenes and alkynes named in IUPAC nomenclature?

Alkenes use the suffix "-ene" and alkynes use "-yne". The parent chain must include the double or triple bond, and its position is indicated by the lowest possible number. For example, CH3-CH=CH-CH3 is named 2-butene.

20. What is the priority order for naming functional groups in organic compounds?

The priority order (from highest to lowest) is: acids > acid derivatives > aldehydes > ketones > alcohols > amines > alkenes > alkynes > alkanes. The highest priority group is named as a suffix, while others are named as prefixes.

21. How are branched alkanes named?

Branched alkanes are named by identifying the longest carbon chain as the parent alkane, then naming and numbering the branches (alkyl groups) as substituents. For example, (CH3)2CH-CH2-CH3 is 2-methylbutane.

22. What is organic compound nomenclature?

Organic compound nomenclature is a systematic way of naming organic compounds based on their structure and functional groups. It provides a standardized method for chemists to communicate about molecules, ensuring clarity and consistency in scientific discussions and literature.

23. What are the basic steps in naming an organic compound using IUPAC rules?

The basic steps are: 1) Identify the longest carbon chain (parent chain), 2) Determine the parent alkane name, 3) Identify and name substituents, 4) Number the parent chain to give substituents the lowest possible numbers, 5) List substituents alphabetically with their position numbers, and 6) Add prefixes and suffixes as needed for functional groups.

24. How do you determine the parent chain in an organic compound?

The parent chain is typically the longest continuous carbon chain in the molecule. If there are two equally long chains, choose the one with more substituents. For cyclic compounds, the ring is usually considered the parent chain.

25. What is the difference between a prefix and a suffix in organic nomenclature?

Prefixes are used to name substituents and are added before the parent chain name (e.g., methyl-, chloro-). Suffixes indicate the main functional group and are added to the end of the parent chain name (e.g., -ol for alcohols, -oic acid for carboxylic acids).

26. How are multiple substituents of the same type named?

When there are multiple identical substituents, use prefixes like di-, tri-, tetra-, etc., before the substituent name. For example, 2,4-dimethylpentane has two methyl groups attached to the 2nd and 4th carbons of a pentane chain.

27. What is the difference between common names and IUPAC names?

Common names are traditional names that may not follow systematic rules (e.g., acetone), while IUPAC names are systematic and based on the compound's structure (e.g., propanone for acetone). IUPAC names are preferred in scientific communication for clarity and consistency.

28. Why is IUPAC nomenclature important in organic chemistry?

IUPAC (International Union of Pure and Applied Chemistry) nomenclature is crucial because it provides a universal language for naming organic compounds. This standardization allows chemists worldwide to communicate effectively, avoid ambiguity, and understand molecular structures based on their names alone.

29. How are alcohols named in IUPAC nomenclature?

Alcohols are named by replacing the "-e" in the parent alkane name with "-ol". The position of the -OH group is indicated by a number. For example, CH3-CH2-CH2-OH is 1-propanol.

30. What are the rules for alphabetizing substituents in IUPAC names?

Substituents are listed in alphabetical order, ignoring any prefixes like di-, tri-, etc. For example, in "3-ethyl-2-methylpentane", ethyl comes before methyl alphabetically.

31. How are aldehydes and ketones named differently?

Aldehydes use the suffix "-al" and are always at the end of the carbon chain, so numbering starts from the aldehyde carbon. Ketones use the suffix "-one" and their position is indicated by a number. For example, CH3-CHO is ethanal (an aldehyde), while CH3-CO-CH3 is propanone (a ketone).

32. What is the significance of bracketed numbers in organic nomenclature?

Bracketed numbers are used in several contexts: 1) In spiro compounds to indicate ring sizes, 2) In bridged compounds to show the number of atoms in each bridge, and 3) To clarify complex substituent positions, especially in polycyclic systems.

33. What is a substituent in organic nomenclature?

A substituent is an atom or group of atoms that replaces a hydrogen atom on the parent chain of an organic compound. Common substituents include alkyl groups (e.g., methyl, ethyl) and halogens (e.g., chloro, bromo).

34. What is the importance of numbering in organic nomenclature?

Numbering is crucial for specifying the exact positions of substituents and functional groups on the parent chain. It ensures that the same compound always receives the same name, regardless of who names it.

35. What are the rules for naming compounds with multiple functional groups?

When a compound has multiple functional groups, the highest priority group is named as a suffix, and others are named as prefixes. The priority order is: acids > acid derivatives > aldehydes > ketones > alcohols > amines > alkenes > alkynes > alkanes.

36. How are stereoisomers distinguished in IUPAC nomenclature?

Stereoisomers are distinguished using prefixes like "cis-", "trans-", "E-", "Z-", "R-", and "S-". For example, cis-2-butene and trans-2-butene are geometric isomers, while (R)-2-butanol and (S)-2-butanol are enantiomers.

37. What is the difference between a homologous series and an isomer series?

A homologous series consists of compounds with the same functional group but differing by a CH2 unit (e.g., methanol, ethanol, propanol). An isomer series consists of compounds with the same molecular formula but different structures (e.g., butane and 2-methylpropane).

38. What is the significance of locants in organic nomenclature?

Locants are numbers or letters used to specify the position of substituents or functional groups in a molecule. They are crucial for distinguishing between isomers and providing a clear, unambiguous description of the molecular structure.

39. How are polyfunctional compounds named when functional groups have equal priority?

When functional groups have equal priority, the group that appears first in the name (alphabetically) is given suffix priority. The other group is named as a prefix. For example, 4-hydroxy-2-pentanone has both a ketone and an alcohol group.

40. What is the difference between saturated and unsaturated compounds in naming?

Saturated compounds (alkanes) use the suffix "-ane", while unsaturated compounds use "-ene" for alkenes and "-yne" for alkynes. The presence of double or triple bonds affects the numbering of the parent chain to give these bonds the lowest possible numbers.

41. How are halogen-substituted compounds named?

Halogens are named as prefixes (fluoro-, chloro-, bromo-, iodo-) with position numbers. For example, CH3-CHCl-CH3 is 2-chloropropane. When halogens are the only substituents, they're listed alphabetically like other substituents.

42. How are cyclic alkenes named?

Cyclic alkenes are named by adding "cyclo-" to the alkene name. The double bond is assumed to be at the 1,2 position unless specified otherwise. For example, cyclohexene is a six-membered ring with one double bond.

43. What is the difference between a prefix and an infix in organic nomenclature?

A prefix is added to the beginning of a name (e.g., "chloro-" in chloromethane), while an infix is inserted into the name (e.g., "-oxa-" in 2-oxapropane, which is another name for dimethyl ether). Infixes are less common and mainly used for heterocyclic compounds.

44. How are spiro compounds named in IUPAC nomenclature?

Spiro compounds, which have two rings joined at a single carbon atom, are named using the prefix "spiro-" followed by numbers in square brackets indicating the number of carbons in each ring. For example, spiro[4.5]decane has a 4-carbon ring and a 5-carbon ring joined at one carbon.

45. How are bridged compounds named in IUPAC nomenclature?

Bridged compounds are named using the prefix "bicyclo" (for two rings) or "tricyclo" (for three rings), followed by numbers in square brackets indicating the number of atoms in each bridge. For example, bicyclo[2.2.1]heptane is the systematic name for norbornane.

46. What is the difference between a substituent and a functional group in naming?

A substituent is any atom or group that replaces a hydrogen in a parent structure, while a functional group is a specific arrangement of atoms within a molecule that gives it characteristic chemical properties. Functional groups often determine the suffix in IUPAC names, while other substituents are usually named as prefixes.

47. How are polymers named in organic chemistry?

Polymers are named using the prefix "poly-" followed by the name of the monomer unit in parentheses. For example, polyethylene is poly(ethene), and polystyrene is poly(1-phenylethene). For more complex polymers, the structure of the repeating unit is specified.

48. What is the significance of the "R" group in organic chemistry nomenclature?

The "R" group is a general term used to represent any alkyl group or hydrocarbon side chain. It's often used in general formulas or when discussing a class of compounds without specifying the exact structure of the hydrocarbon portion.

49. How are fused ring systems named in IUPAC nomenclature?

Fused ring systems are named by combining the names of the individual rings with a letter indicating the side of fusion. For example, naphthalene is named as a fused benzene system: [1]benzopyran. More complex systems use prefixes like "benzo-", "naphtho-", etc.

50. What is the difference between a root name and a stem name in organic nomenclature?

A root name is the base name of a compound derived from its parent structure (e.g., "butan-" for a four-carbon chain), while a stem name is the part of the name to which prefixes and suffixes are added (e.g., "but-" in "butanol"). The stem is often formed by dropping the final "-e" from the root name.

51. What is the significance of the "iso-" prefix in common names, and how is it handled in IUPAC nomenclature?

The "iso-" prefix in common names typically indicates a methyl branch on the second-to-last carbon of a chain. In IUPAC nomenclature, this structure is named systematically. For example, the common name "isobutane" is 2-methylpropane in IUPAC nomenclature.

52. How are heterocyclic compounds named in IUPAC nomenclature?

Heterocyclic compounds are named using prefixes that indicate the heteroatom (e.g., "oxa-" for oxygen, "aza-" for nitrogen) and suffixes that indicate the size and saturation of the ring. For example, a six-membered ring with one nitrogen is called pyridine.

53. What is the difference between a trivial name and a semisystematic name?

A trivial name is a common name that doesn't follow any systematic rules (e.g., "acetone"), while a semisystematic name partially follows IUPAC rules but retains some non-systematic elements (e.g., "acetic acid" instead of "