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Lucid Guideline For I.U.P.A.C. Nomenclature Of Organic Compounds: Part-4:

Updated on October 25, 2015

List of Topics Included (continued from part-3)

(12) I. U. P. A. C. nomenclature of aromatic hydrocarbons

(12/A/1) Nomenclature of benzenoids

(12/A/2) Nomenclature of substituted benzenoids

(12/B/1) Nomenclature of some cyclic anions and cations which are non-benzenoids

(12/B/2) Nomenclature of some neutral compounds which are non-benzenoids

(13) I. U. P. A. C. nomenclature of organic compounds containing one functional group

(13/A) Nomenclature of organic compounds which contain hydroxy (-OH) functional group

(13/A/1) Nomenclature of alcohols

(13/A/2) Nomenclature of phenols

(13/B) Nomenclature of organic compounds containing ether (-O-) functional group

@ References

(12) I. U. P. A. C. Nomenclature of Aromatic Hydrocarbons (also known as, "Arenes")

What is special about aromatic hydrocarbons?

Aromatic hydrocarbons are special type of hydrocarbon compounds because they show following characteristics. Other hydrocarbons do not show such exclusive properties.

(a) They are cyclic and planar compounds containing alternate double bonds.

(b) Even though they are unsaturated hydrocarbons, their characteristic reactions are "substitution reactions" instead of "addition reactions".

(c) The total number of π electrons present in their cyclic structure is given by formula:

Number of π electrons = (4 n + 2); where n is integer showing number of cycles present in their structures.

For example, if the given aromatic hydrocarbon contains one cycle (or one ring), then total number of π electrons in the ring must be: 4 (1) + 2 = 6. Likewise an arene containing two rings should contain a total of 10 π electrons.

(d) The π electrons present in their cyclic structure are continuously moving along the entire plane of molecule. This phenomenon is called, “delocalization of π electrons” or “resonance”.

(e) Due to the phenomenon of resonance, they are less reactive and more stable compared to other unsaturated hydrocarbons like alkenes and alkynes.

The above all characteristic properties of aromatic hydrocarbons are in agreement with Huckel’s rule.

Due to their special characteristics, their nomenclature is also special.

I. U. P. A. C. names of these compounds are not derived on the basis of number of carbon atoms. (This is valid only for benzenoids).

For example, a well known arene called, "benzene", contains six carbon atoms but its name does not give any hint about this. Thus, nomenclature of arenes requires some special considerations.

There are two types of aromatic hydrocarbons as explained below:

(a) Benzenoids.

Benzenoids are further divided into two sub-categories:

  • Benzenoids which contain one or more benzene rings fused with each other. They do not contain any hydrocarbon branch as substituent. They form a "homologous series of benzene".
  • Benzenoids which contain an alkyl or alkenyl group as substituent. They are known as, "substituted benzenoids".

(b) Non-benzenoids.

  • Non-benzenoids do not contain benzene ring at all. They obey Huckel's rule and show all aromatic characters.
  • Some cyclic anions and cations are included in this category. Moreover a special kind of some neutral compounds called, “annulenes” are also included in this category.

(12/A/1) Nomenclature of Benzenoids containing fused benzene rings, means homologous series of benzene

The general formula of benzenoids can be written as: CnH(2n-6m).

Here, “n” denotes number of carbon atoms while “m” denotes number of benzene rings present in their structures.

[Note: This general formula is valid only when two carbon atoms of any one benzene ring are involved in linkage. In compound like pyrene this formula does not work because more than two carbon atoms of the same benzene ring are involved in linkage.]

The initial few members of homologous series of aromatic hydrocarbons are listed in table below.

As their names have become very popular, I. U. P. A. C. has accepted them without any change.

Homologous Series of Benzenoids

I. U. P. A. C. name of compound
number of carbon atoms (n)
number of benzene rings (m)
molecualr formula: CnH(2n-6m)
 
 
 
Benzene
6
1
C6H(6)
 
 
 
Napthalene
10
2
C10H(8)
 
 
 
Anthracene
14
3
C14H(10)
 
 
 
Phenanthrene
14
3
C14H(10)
 
 
 
Please note that both anthracene and phenanthrene contain same number of benzene rings (means three); but the rings are joined in different way to give two different compounds.

Structures of some Benzenoids

Source
Source
Note carefully that both anthracene and phenanthrene are isomers containing three benzene rings. They differ only in that the joining of benzene rings in both of them is different.
Note carefully that both anthracene and phenanthrene are isomers containing three benzene rings. They differ only in that the joining of benzene rings in both of them is different. | Source
Source
Watch carefully how numbering is done in poly nuclear benzenoids
Watch carefully how numbering is done in poly nuclear benzenoids | Source

(12/A/2) Nomenclature of Substituted Benzenoids

Substituted benzenoids are derivatives of benzenoids containing one or more alkyl or alkenyl branch or branches attached to the benzene ring.

In such compounds, either one or more hydrogen atoms of benzene are substituted with some alkyl or alkenyl groups.

(1) If they contain only one substituent group, the general I. U. P. A. C. name of compound is given as, “alkyl benzene”.

For example:

(a) Methylbenzene (which has a common name, "Toluene")

(b) Ethylbenzene

(c) Ethenylbenzene (which has a common name, "Styrene")

(d) Allylbenzene

(e) Ethynylbenzene (which has a common name, "Phenyl acetylene")

(f) (1-Methylethyl)benzene (which has a common name, "Cumene" or "Isopropylbenzene") etc.

See picture number: 12/A/2/1 given below.

(2) If they contain two or three similar substituent groups, then number of such groups is shown by prefixes: di (means two), tri (means three) etc.

For example:

(a) 1,2-Dimethylbenzene (which has a common name, "ortho-Xylene")

(b) 1,3-Dimethylbenzene (which has a common name, "meta-Xylene")

(c) 1,4-Dimethylbenzene (which has a common name, "para-Xylene")

(d) 1,3,5-Trimethylbenzene (which has a common name, "mesitylene" or "sym-trimethyl benzene").

See picture number: 12/A/2/2 given below.

(3) If they contain two dissimilar groups, then relative position of each group is denoted by suitable number based on alphabetical order of their names. However, while writing their names, alphabetically prior group should be written first irrespective of its number. This means prefixes “di”, “tri” etc. are not considered for alphabetical priority.

For example:

(a) 1-Ethyl-2-methylbenzene

(b) 1-Methyl-2-propylbenzene

(c) 1-Methyl-3-propylbenzene

(d) 3-Ethyl-1,1-dimethylbenzene

(e) 1-Ethyl-2-methyl-3-propylbenzene etc

See picture number: 12/A/2/3 given below.

(12/A/2/1) Structures of some substituted benzenoids

Source

Nomenclature of substituted poly nuclear benzenoids

Please note carefully how numbering is done in poly nuclear substituted benzenoids
Please note carefully how numbering is done in poly nuclear substituted benzenoids | Source

(12/B/1) Nomenclature of some cyclic anions and cations which are non-benzenoids

Some cyclic anions and cations having alternate double bonds show aromatic characters, even though they do not contain benzene ring.

For example:

(a) Cyclopropenyl cation

(b) 1,3-Cyclopentadienyl anion

(c) 1,3,5-Cycloheptatrienyl cation (also known as, "Tropylium cation")

(d) 1,3,5,7-Cyclooctatetraenyl dianion etc.

See the picture given below.

Structures of some cyclic anions and cations which are non-benzenoids

Source

(12/B/2) Nomenclature of some neutral compounds which are non-benzenoids

However, there are some neutral monocyclic compounds which contain extended double bond system. Such compounds called, "annulenes" also show aromatic characters. Such compounds are uncommon.

For example, “Cyclooctadecanonaene", which is commonly known as, “[18]Annulene”.

[Note: Here, the number included in square bracket, denotes total number of carbon atoms present in the compound.]

See the picture given below.

Structure of, "[18]Annulene"

Source

(13) I. U. P. A. C. nomenclature of organic compound which contain only one functional group

When a functional group is attached to the organic compound, naming is done such that name itself gives hint about functional group present. This is done by adding suitable suffix or prefix of that particular functional group.

As, each and every functional group has its characteristic prefix or suffix, it is easy to assign I. U. P. A. C. name to any compound which contains a functional group. Here the important information required is the knowledge regarding prefix and suffix used for various functional groups.

Following guidelines will explain how to assign correct I. U. P. A. C. names to compounds containing various functional groups.

(13/A) Nomenclature of organic compounds which contain hydroxy (-OH) functional group

The compounds which contain a hydroxy group are further classified into two categories:

(a) Alcohols and

(b) Phenols.

(13/A/1) Nomenclature of alcohols

When one or more hydrogen atoms of some aliphatic hydrocarbon are displaced by hydroxy groups, the resultant compounds are known as alcohols.

To assign correct name to such compounds, following points are taken into consideration:

(1) First of all a longest and continuous carbon chain is selected which contains hydroxy group. Based on number of carbon atoms contained by this chain, the root name of compound is determined.

(2) The suffix used for alcohols is “ol”. Hence “ol” is added after the root name.

(3) Numbering of the chain is done as per rules discussed previously.

Following examples will make this more clear. (common name of alcohol is written in the bracket).

(a) CH3-CH2-CH2-OH → Propanol or Propan-1-ol (Propyl alcohol),

(b) CH3-CH(OH)-CH3 → Propan-2-ol (Isopropyl alcohol),

(c) C(CH3)3-OH → 2-Methylpropan-2-ol (tertiary-Butyl alcohol),

(d) C6H11-OH → Cyclohexanol (Cyclohexyl alcohol),

(e) CH3-CH(OH)-CH=CH2 → But-3-en-2-ol etc.

(13/A/2) Nomenclature of phenols

When hydroxy group has displaced one or more hydrogen atoms of benzene ring, the resultant compound is known as phenol.

I. U. P. A. C. names of various phenol compounds are as follows:

(A) When only one hydroxy group is attached to benzene ring:

C6H5-OH → Phenol or hydroxybenzene

(B) When two hydroxy groups are attached to benzene ring:

The general formula of such compounds is: C6H4-(OH)2.

In common system of nomenclature such compounds are given special names like:

(1) Catechol (here two hydroxy groups are situated on alternate carbon atoms of benzene),

(2) Resorciol (here both the hydroxy groups are two carbon away) and

(3) Quinol (here both the hydroxy groups are three carbon away).

But in I. U. P. A. C. system of nomenclature, general names of such compounds are dihydroxy benzene. However their individual names are derived by placing suitable numerical prefix to show relative position of both the hydroxy groups before the name, "dihydroxy benzene". This will be clear from following examples.

(1) Catechol is called, "1,2-Dihdroxybenzene

(2) Resorciol is called, "1,3-Dihdroxybenzene and

(3) Quinol is called, "1,4-Dihdroxybenzene.

Refer the picture given below.

(C) When one hydroxy group and one methyl group are attached to benzene ring:

General names of such compounds are cresols.

(1) If there is only one carbon between methyl group and hydroxy group, the compound is named as, "2-Methylphenol" (or ortho-cresol)

(2) If there are two carbon atoms between methyl group and hydroxy group, the compound is named as, "3-Methylphenol" (or meta-cresol) and

(3) If there are three carbon atoms between methyl group and hydroxy group, the compound is named as, "4-Methylphenol" (or para-cresol).

See the picture given below.

(D) When hydroxy group is attached to first carbon atom of napthalene compound is named as, "1-Napthol" or "alpha-napthol". However, when hydroxy group is attached to second carbon atom of napthalene, compound is named as, "2-Napthol" or "beta-napthol".

See the picture given below.

[Note: Due to their characteristic properties, phenols containing more than one functional groups are also discussed here. This will help to avoid confusion between nomenclature of phenol and phenolic compounds. It should also be noted that phenol is only one while phenolic compounds are large in number.]

Structures and I. U. P. A. C. names of some phenols

Source

Structures of Alpha-napthol and Beta-napthol

Please note that in old system of nomenclature, the carbon atom number one is called "alpha" carbon while carbon atom number two is called "beta" carbon.
Please note that in old system of nomenclature, the carbon atom number one is called "alpha" carbon while carbon atom number two is called "beta" carbon. | Source

(13/B) Nomenclature of ether compounds

Such compounds contain ether (means, "-O-") functional group.

these compounds may have one of the following general formulas:

(a) R-O-R; for example: CH3-O-CH3 known as methoxymethane

(b) R-O-R’; for example: CH3-O-CH2-CH3 known as methoxyethane

(c) R-O-Ar; for example: CH3-O-C6H5 known as methoxybenzene

(d) Ar-O-Ar; for example: C6H5-O-C6H5 known as phenoxybenzene

[Note:

(a) Here: R, R’ Ar and Ar’ are symbols used for different alkyl or aryl groups.

(b) If both the groups attached to oxygen atom are similar, the ether compound is known as, "symmetric ether" or "simple ether". Their general formula is: R-O-R or Ar-O-Ar.

(c) If both the groups attached to oxygen atom are dissimilar, the ether compound is known as, "asymmetric ether" or "mixed ether". Their general formula is: R-O-R’ or Ar-O-Ar’.]

In I. U. P. A. C. system of naming, ether compounds are regarded as alkoxy derivative of alkanes. Hence their general name is, “alkoxyalkane”.

What is meant by, “alkoxy”?

When a hydrogen atom of hydroxy group is removed from any alcohol, the remaining group left is called an alkoxy group.

For example:

(a) When hydrogen of hydroxy group is removed from methanol, remaining group is called, “methoxy group”.

(CH3-OH) - (H) → CH3O-.
(Methanol) - (hydrogen) → methoxy

(b) When hydrogen atom of hydroxy group is removed from ethanol, remaining group is called, “ethoxy group”.

(CH3-CH2-OH) - (H) → CH3-CH2O-.
(Ethanol) - (hydrogen) → ethoxy

In the same way, removal of one hydrogen atom from propanol and butanol will give alkoxy groups known as, “propoxy group” and “butoxy group” respectively.

However, in case of removal of such hydrogen from phenol, remaining group is called, “phenoxy” group.

How to assign a correct I. U. P. A. C. name to the given ether compound?

The nomenclature of ether compounds is done on the basis of following guideline.

(a) First of all check the two alkyl groups on both the side of oxygen and determine which one has longer carbon chain. If both the groups are similar, consider any one group.

(b) Now, divide the given ether compound into two fragments such that oxygen atom remains with a smaller alkyl group. This smaller part is consiered as an alkoxy part of the given ether. In case of symmetrical ethers, any part can be considered as alkoxy group.

(c) Determine the corresponding alkane name of the remaining alkyl group.

(d) Assign name to the given ether as, “alkoxyalkane”.

This will be clear from following examples:

(1) CH3-O-CH3 (common name: dimethyl ether)
On dividing this compound, we get two fragments; one is CH3O and another is CH3. As both the groups contain same number of carbon atoms, CH3O can be considered as alkoxy group means, “methoxy group”; while CH3 as alkane means methane. Hence, I. U. P. A. C. name of this ether is, “Methoxymethane”.

(2) CH3-O-CH2-CH2-CH3 (common name: methyl propyl ether)
On dividing this compound, we get two fragments; one is CH3O and another is -CH2-CH2-CH3. Name of CH3O can be taken as, “methoxy group” because it is smaller, while that of -CH2-CH2-CH3 as “propane”. Hence, I. U. P. A. C. name of this ether is, “Methoxypropane”.

Details of some common ether compounds with their I. U. P. A. C. names are given in following table.

Details and I. U. P. A. C. names of some common ether compounds

Formula of ether
Its common name
Name of alkoxy part present in it
Name of alkane part present in it
Its I. U. P. A. C. name (Alkoxyalkane)
 
CH3-CH2-O-CH2-CH2-CH3
ethyl propyl ether
ethoxy
propane
Ethoxypropane
 
CH3-CH2--CH(O-CH3)-CH3
nil
methoxy
butane
2-Methoxybutane
 
CH3-O-C(CH3)2-CH3)
nil
methoxy
propane
2-Methyl-2-methoxypropane
 
CH3-O-C6H5
methyl phenyl ether, anisole
methoxy
benzene
Methoxybenzene
 
CH3-CH2-O-C6H5
ethyl phenyl ether, phenetole
ethoxy
benzene
Ethoxybenzene
 
Please note that in common method of naming, alphabetical prior group is written first.

Can you assign correct I. U. P. A. C. names to following ether compounds?


view quiz statistics

Structures and I. U. P. A. C. names of some complex ether compounds

Note: Numbering is started from most substituted end.
Note: Numbering is started from most substituted end. | Source

@ References

(1) Organic Chemistry by: Robert Thornton Morrison and Robert Neilson Boyd, Seventh Edition, Published by, "Dorling Kindersley(India) Pvt. Ltd., licensees of Pearson Education in South Asia

(2) Oxford Dictionary Of Chemistry, published by Oxford University Press Inc., New York

(3) I. I. T. Chemistry, by Dr. O.P. Agarwal, 135th edition, Jai Prakash Nath Publications, Meerut, India

(4) Pradeep's New Course Chemistry, Class XI, Vol. II, 27th edition, Pradeep Publication, Jalandhar, India

(5) Pradeep's New Course Chemistry, Class XII, Vol. II, 27th edition, Pradeep Publication, Jalandhar, India

(6) Fundamentals Of Chemistry, Class 11, by J. D. Lee, Solomons & Fryhle, Published by: Wiley India Pvt. Ltd., 4435-35/7, Ansari Road, Daryaganj, New Delhi-110002

(7) Modern's abc of Chemistry, For Class XI, Part-II, by Dr. S. P. Jauhar, Published by: Modern Publishers, MBD House, Railway Road, Jalandhar City, India

(8) Modern's abc Of Chemistry, For Class XII, Part-II, by Dr. S. P. Jauhar, Published by: Modern Publishers, MBD House, Railway Road, Jalandhar City, India

(9) Organic Chemistry, by Bhupinder Mehta & Manju Mehta, Published by: Prentice-Hall Of India Private Limited, M-97, Connaught Circus, New Delhi, -110001, India

(10) Nootan ISC Chemistry, Class XI & XII, by Dr. H. C. Srivastava, Published by: Nageen Prakashan (Pvt.) Ltd., 310, Western Kutchery Road, Meerut-250001, U.P., India

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