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Lucid Guideline For I.U.P.A.C. Nomenclature Of Organic Compounds: Part-2:

Updated on October 25, 2015

List of Topics Included (Continued from part-1)

(8) Nomenclature of unsaturated hydrocarbons containing double bonds (called alkenes)

(8/1) Simple alkenes

(8/2) Branched alkenes

(8/3) Alkadienes and

(8/4) Alkatrienes

(9) Nomenclature of unsaturated hydrocarbons containing triple bond (called alkynes)

(9/1) Simple alkynes

(9/2) Branched alkynes and

(9/3) Alkadiynes

(10) Nomenclature of unsaturated hydrocarbons containing both double as well as triple bonds

(10/1) Straight chain hydrocarbon containing double as well as triple bond but no branch

(10/2) Branched hydrocarbon containing double as well as triple bond

@ References

(8) Nomenclature of Unsaturated Hydrocarbons containing Double Bonds (alkenes)

These are the compounds which contain one or more double bonds between two carbon atoms. They are known as, "alkenes". Their general formula is, "CnH2n". Homologous series of alkenes is shown in the following table.

In double bond between two carbon atoms, one of the bonds is σ (known as sigma) bond while another is a π (known as pi) bond.

In a sigma bond, the sharing pair of electrons is held very tightly, hence it is strong bond. However, in a pi bond the sharing pair of electrons is loosely held hence it is a weak bond.

The weak pi bond of alkene can be broken easily by supplying only a small amount of energy. It is due to this reason that alkenes are unstable or reactive compounds.

Homologous Series of Alkenes

Name of alkene
Its molecular formula
Its structural formula
ethene
C2H4
CH2=CH2
propene
C3H6
CH3-CH=CH2
but-1-ene
C4H8
CH2=CH-CH2-CH3
but-2-ene
C4H8
CH3-CH=CH-CH3
pent-1-ene
C5H10
CH2=CH-CH2-CH2-CH3
pent-2-ene
C5H10
CH3-CH=CH-CH2-CH3
hex-1-ene
C6H12
CH2=CH-CH2-CH2-CH2-CH3
hex-2-ene
C6H12
CH3-CH=CH-CH2-CH2-CH3
hept-1-ene
C7H14
CH2=CH-CH2-CH2-CH2-CH2-CH3
hept-2-ene
C7H14
CH3-CH=CH-CH2-CH2-CH2-CH3
hept-3-ene
C7H14
CH3-CH2-CH=CH-CH2-CH2-CH3
oct-1-ene
C8H16
CH2=CH-CH2-CH2-CH2-CH2-CH2-CH3
oct-2-ene
C8H16
CH3-CH=CH-CH2-CH2-CH2-CH2-CH3
oct-3-ene
C8H16
CH3-CH2-CH=CH-CH2-CH2-CH2-CH3
In any alkene, the double bond is situated between two carbon atoms, but its location is shown by minimum number. For example, in but-2-ene, the double bond is between second and third carbon, but its location is shown by minimum number 2.

(8/1) Nomenclature of simple alkenes with no alkyl branch

Simple alkenes are hydrocarbon compounds which:

(a) Contain only one carbon-carbon double bond

(b) Do not contain any alkyl branch.

General formula of these compounds is: “CnH2n”.

As double bond requires at least two carbon atoms, the first member of homologous series of alkene is ethene.

The table given above shows names and formulas of initial eight alkenes

To derive their names, a suffix, “ene”, is added to the corresponding root name given previously in part-1. For example, Eth + ene = Ethene, Prop + ene= Propene etc.

However, in case the longest and continuous carbon chain of given alkene contains more than three carbon atoms, it becomes necessary to indicate the location of double bond. This is done by placing a numerical letter between “root” and suffix “ene”, separated by a hyphen. This is necessary to distinguish isomers of alkene having similar molecular formula but different structures. For example, an alkene with formula: C4H8 has three isomers (two linear isomers + one branched isomer) having names: But-1-ene, But-2-ene and 2-Methylprop-1-ene.

See the following figure.

Structures and I. U. P. A. C. names of three isomers of butene

See all have common molecular formula of C4H8, hence they are isomers. The first two are simple alkenes while third is a branched alkene.
See all have common molecular formula of C4H8, hence they are isomers. The first two are simple alkenes while third is a branched alkene. | Source

(8/2) Nomenclature of branched alkenes

(8/2) Branched alkenes

Such alkenes contain at least one branch of an alkyl group.

In order to assign a correct name to such compound, following points should be taken into consideration:

(a) A principal chain should be selected such that it contains double bond, no matter if it is not the longest chain.

(b) Numbering should be given such that, carbon bearing double bond gets lowest possible number. For example, the correct name of an alkene, “CH3-CH2-CH2-CH(CH=CH2)-CH2-CH2-CH2-CH3” is, “3-Propylhept-1-ene”. The other names used for this alkene like, “4-Vinyloctane” or “5-Vinyloctane” or “3-Butylhex-1-ene” are wrong.

(c) In case of having two alkyl branches on the same number of carbon atom counting from either side, numbering should be given in such a way that alphabetically prior group gets lower number. For example, an alkene, "CH3-CH2-C(C2H5)=C(CH3)-CH2-CH3" should be named as, "3-Ethyl-4-methylhex-3-ene" but not as, "3-Methyl-4-ethylhex-3-ene".

See the following figure.

(Here, it must be remembered that, in case both substituted carbon are at similar position from either end, then more substituted carbon should bear lower number).

(d) In case of having a complex alkyl branch, its entire name (included in parenthesis) is considered for alphabetical order. For example, an alkene, "CH3-C(CH3)2-C{CH(CH3)2}=C(CH3)-CH3" is named as, "2,4,4-Trimethyl-3-(1-methylethyl)pent-2-ene" but not as, "3-(1-methylethyl)-2,4,4-trimethylpent-2-ene".

(Note: It must be noted that the group, "Trimethyl" is written prior to the group, "(1-methylethyl)" because the word "methyl" is alphabetically prior to word "methylethyl". It is not due to the priority of letter "T" over letter "m", because prefixes like di, tri etc. are not considered in alphabetical order).

(8/3) Nomenclature of alkadiens

Such alkenes contain two double bonds. Their general name formula is: CnH(2n-2).

Their I. U. P. A. C. names are derived by removing terminal "e" from their corresponding alkene name and then by adding "a-X,Y-diene".

Here, X and Y are numerical letters showing location of double bonds. For example, alkadiene, "CH3-CH=CH-CH=CH2" is named as, Penta-1,3-diene".

Likewise, compound, "CH2=CH-CH=C(CH3)-CH3" and "CH2=CH-C(CH3)=CH2" are named as, "4-Methylpenta-1,3-diene" (it is also a good example of lowest set of locant rule) and "2-Methylbuta-1,3-diene" respectively.

Structure and I. U. P. A. C. name of a typical Alkadiene

In 4-methylpenta-1,3-diene, numbering is done such that double comes early. But in 2-methylbuta-1,3-diene, position of double bond is same. Hence, numbering is done such that substituent comes early.
In 4-methylpenta-1,3-diene, numbering is done such that double comes early. But in 2-methylbuta-1,3-diene, position of double bond is same. Hence, numbering is done such that substituent comes early. | Source

Illustration Showing That Alphabetically Prior Alkyl Group Should Bear Lower Number

Numbering started from left hand side gives correct name, but if it is started from opposite side, it gives wrong name. Ethyl is alphabetically prior to methyl.
Numbering started from left hand side gives correct name, but if it is started from opposite side, it gives wrong name. Ethyl is alphabetically prior to methyl. | Source

(8/4) Nomenclature of alkatriens

Such alkenes contain three double bonds.

They are of two types:

(a) If all the three double bonds are accommodated in the principal chain, compound is named as "Alka-X,Y,Z-triene", where X, Y, Z are numerical letters indicating locations of double bonds.

(b) If all the three double bonds can not be accommodated in the principal chain, but one of the double bond acts as a branch, then compound is considered as a derivative of respective alkadiene and is named accordingly.

[Note: Nomenclature of "cis-trans" and "E-Z" geometrical isomers of alkenes is not included here. This is because it is more appropriate to discuss them separately under the title, "Stereo isomerism in alkenes". This will also help remaining focused on the present discussion.]

Make organic chemistry easy, refer dictionary

Can you Assign Correct I. U. P. A. C. name to the following Alkenes?


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(9) Nomenclature of unsaturated hydrocarbons having carbon-carbon triple bond (Alkynes)

Such Hydrocarbons contain at least one triple bond. They are of three types:

(a) Simple alkynes

(b) Branched alkynes and

(c) alkadiynes.

These are the compounds which contain one or more triple bonds between two carbon atoms. They are known as, "alkynes". Their general formula is, "CnH2n-2". Homologous series of alkynes is shown in the following table.

In triple bond between two carbon atoms, one of the bonds is σ (known as sigma) bond while remaining two bonds are π (known as pi) bonds.

As explained above, sigma bond is a strong bond but two π bonds are weak bonds.

The two weak pi bonds of alkyne can be broken easily hence alkynes are unstable or reactive compounds.

[Note: As π bond is weak bond and as alkynes contains two π bonds, it is assumed that alkynes should be more reactive than alkenes. But this is not true. On the contrary alkenes are more reactive than alkynes.

This can be explained by the concept of, "percentage s character" as explained below.]

The concept of, "percentage s character"

Two carbon atoms of triple bond of alkynes are "sp" hybrid.

As sp hybridization involves mixing of 1 s orbital and 1 p orbital, the proportion of s orbital is 50 %. This means the % s character is 50 % as per calculation shown below.

% "s" character = (number of s orbitals / number of total orbitals) x 100

= (1 / 2) x 100 = 50 %

But in case of alkenes, carbon atoms of pi bond are "sp2" hybrid. This involves mixing of 1 s orbital and 2 p orbitals. Hence, the % s character is counted as 33 % as per calculation shown below.

% "s" character = (number of s orbitals / number of total orbitals) x 100

= (1 / 3) x 100 = 33%

Due to high % of s character, bonded electrons of pi bond of alkyne are more attracted towards the nucleus of carbon. Hence electrons of pi bond of alkyne are less readily available for reaction as compared to that of alkene.

It is due to the above explanation, that alkenes are more reactive than alkynes.

(9/1) Nomenclature of simple alkynes

They contain a straight chain of carbon with one triple bond. Their general formula is: CnH2n-2. Their names are derived by adding the suffix "yne" to their root name. e.g. Ethyne, Propyne etc. In case of having more than three carbon atoms, the location of triple bond is denoted to avoid confusion among their isomers. For example: But-1-yne, Pent-2-yne, etc.

Homologous series of alkynes

Name of alkyne
Its molecular formula
 
ethyne
C2H2
 
propyne
C3H4
 
but-1-yne
C4H6
 
but-2-yne
C4H6
 
pent-1-yne
C5H8
 
hex-1-yne
C6H10
 
hex-2-yne
C6H10
 

(9/2) Nomenclature of branched alkynes

They contain a branch of alkyl group. Their names are derived by placing name suitable number of alkyl group. For example: 3-Methylbut-1-yne, 4,4-Dimethylhex-2-yne, 2,5-Dimethylhex-3-yne.

(9/3) Nomenclature of alkadiynes

They contain two triple bonds in their structure. Their names are derived by adding, "a-X,Y-diyne" after their root name, where X and Y are the numerical letters indicating locations of triple bonds.

For example:

(a) A hydrocarbon (C6H6), containing six carbon atoms in the continuous chain with two triple bonds, one between 1st and 2nd and another between 5th and 6th carbon atoms, is named as, "Hexa-1,5-diyne".

(b) Likewise, a hydrocarbon (C5H4), containing five carbon atoms in the continuous chain with two triple bonds, one between 1st and 2nd and another between 4th and 5th carbon atoms, is named as, "Penta-1,4-diyne".

I. U P. A. C. names and structures of some Alkynes

Source

I. U. P. A. C. names and structures of some Alkadiynes

Both hexa-1,5-diyne as well as penta-1,4-diyne contain two triple bonds in their structures. Hence they are known as, "alkadiyne".
Both hexa-1,5-diyne as well as penta-1,4-diyne contain two triple bonds in their structures. Hence they are known as, "alkadiyne". | Source

(10) Nomenclature of unsaturated hydrocarbons containing both double as well as triple bonds

Such hydrocarbons contain both double as well as triple bond in their structures.

Naming of such compounds requires a special knowledge regarding comparative chemical reactivity of alkenes and that of alkynes.

However, such compounds are of two types:

(1) Straight chain hydrocarbons and

(2) Branched hydrocarbons.

These are discussed below.

(10/1) Nomenclature of straight chain hydrocarbons containing both double as well as triple bonds but no branch

In such compound, both the bonds (double bond as well as triple bond) are accommodated in the principal chain. Here, numbering should be started from such end which is nearer to the double or triple bond.

For example: Pent-3-en-1-yne and Hept-5-en-1-yne.

Structures of both these compounds are given in the following picture.

Further, reactivity of alkene is higher than that of alkyne. Hence, while naming and numbering such compound, due care should be taken that, "the double bond is always given preference over the triple bond".

For example: Pent-1-en-4-yne and Hex-1-en-5-yne.

Structures of both these compounds are given in the following picture.

[Note: The terminal "e" of primary suffix "ene" is dropped if the next letter is either "vowel" or "y"].

(10/2) Nomenclature of branched hydrocarbons containing both double as well as triple bonds

In this type of compound, both the bonds (double bond as well as triple bond) exist along with one or more alkyl branches.

Naming and numbering of such compounds requires some special considerations as given below:

(1) The parent chain should be selected such that it contains maximum number of multiple bonds, ignoring the longest chain rule.

(2) The primary suffix "ene" should always come before "yne".

(3) Numbering should be in accordance with, "lowest set of locant rule".

(4) In case the side chain also contains a multiple bond like, "=CH2", "-CH=CH2 " etc. then appropriate prefix must be used as shown below:

(a) For "=CH2"→ prefix used is either methylidene or methylene

(b) For "-CH=CH2"→ prefix used is either ethenyl or vinyl

(c) For "-CH2-CH=CH2"→ prefix used is allyl

(d) For "-CH2-CH2-CH(CH3)-CH3" → prefix used is either 3-methylbutyl or isopentyl

(e) For "-CH2-CH(CH3)-CH2-CH3" → prefix used is 2-methylbutyl

(f) For "-CH2-C(CH3)2-CH3" → 2,2-dimethylpropyl(or neopentyl) etc.

Illustration Showing that Double Bond is Given Priority Over Triple Bond

Source

Structures and I. U. P. A. C. names of some complex hydrocarbons containing both double as well as triple bonds

@ 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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