The hydrocarbons having one or more carbon to carbon double bond(s) are referred to as alkenes. They are also known as unsaturated hydrocarbons which reflect that addition reaction is possible. The general formula for alkenes is CnH2n. Like other organic compounds, alkenes are named according to IUPAC rules today.
Organic compounds were named based on their sources or properties in past e.g. methane gas was named marsh gas due to its presence in marshy places. However, organic compounds being in millions could not be handled this way. Therefore, an organization for nomenclature was necessary. The concept originated in 1889 when the International Congress of Chemistry was opened in Paris by Marcellin Berthelot who explained the concepts of Congress as follows:
“Theories are not to be considered, but only practical questions, such as relate to analytical methods and nomenclature. The last urgently needs revision and improvement. The system hitherto followed has become insufficient. So many new compounds have been discovered that they are bursting through the frames formerly intended and thought wide enough to contain them. We cannot keep on adding syllables and forming endless names for new combinations. A new and clear system is absolutely necessary, with lines broad enough to last for some time at least.”
Later in 1892, a report was accepted in Geneva (Switzerland) by some of the foremost organic chemists of that time. This report was the novel relationship between chemical substances, diagrams, and names that shaped the practice of chemical nomenclature ever since.
In 1930, the International Union of Chemistry (ICU) was given a modified report known as Liege Rules. In 1947, a further modified report was given by the International Union of Pure and Applied Chemistry.
Finally, in 1979 the most recent report was published by the International Union of Pure and Applied Chemistry (IUPAC).
Systems of nomenclature
The two systems used to name organic compounds are:
- Trivial system
- IUPAC system
In the trivial system of nomenclature, alkenes are named by ending with “-ylene”.
The use of trivial names becomes difficult for alkenes having four or greater numbers of carbon atoms. This is because a large number of isomers are possible for higher alkenes. So, the trivial system can be applied for small chain alkenes only.
Some of the simple alkenes are named as the derivative of ethylene.
IUPAC system is an internationally adapted system of nomenclature. When naming alkenes the IUPAC system of nomenclature suggests following some basic writing strategies in order to reduce the hard work and difficulties that one might face.
- Use a comma “,” to separate numbers.
- Use a hyphen “-” to separate numbers and letters.
How to name alkenes – IUPAC system
1. Nomenclature of Simple alkenes
1. Examine the structure of the compound and select the longest continuous carbon chain. This chain must contain the double bond i.e. (C = C).
2. The chain will be named by the ending “-ene”.
3. If alkyl substituents are present in the parent chain, the name of substituent alkyl will be according to the number of carbons. The suffix for the alkyl substituents is “-yl”.
For different alkyl substituents, different names are used.
The name methyl is used for 1 carbon chain length, ethyl for 2 carbon, propyl for 3 carbon, and so on.
4. The carbon atoms of the parent chain are numbered in such a way that it includes the double bond (C = C). The numbering starts from the end nearer to the double bond. It does not depend on the numbers that are assigned to the alkyl substituents.
5. The number assigned to the double bond is according to the number of the first carbon attached to it. This number is placed before the name of the alkene.
6. The alkyl substituents are named according to the number of the carbon atoms to which they are attached.
7. If the parent chain contains more than one different alkyl substituent, they are named in alphabetical order.
8. If multiple substituents of the same type are present on the parent chain. They are named by prefixes such as “di” for 2, “tri” for 3, “tetra” for 4, and “penta” for 5, etc.
2. Alkenes with more than one double bonds
9. If alkenes contain more than one double bond, they are indicated by suffixes. We use “diene” for two double bonds, “triene” for three double bonds, and “tetraene” for four double bonds, etc.
10. The total sum of numbers assigned to the double bonds must be kept lowest.
In other words, the direction having a maximum number of branching molecules on the nearest points is adopted to name the alkenes.
3. Nomenclature of cycloalkenes
11. If only one double bond is present in the ring of a cycloalkene, it is unnecessary to specify the position.
12. The numbering of cycloalkenes is begun from one of the carbon atoms of the double bond. The resultant name shall have minimum possible numbers.
13. If the ring contains any substituents, they are given the lowest sum of numbers.
4. Nomenclature of alkenyl groups
14. When one hydrogen atom is removed from an alkene, the ending “-ene” is replaced by “-enyl”. The numbering of alkenyl groups is started from the carbon atom from which hydrogen is removed.
15. There are some alkenyl groups for which common names are used preferably as compared to IUPAC names.
5. Alkenes with alkynes
16. If an organic compound contains double as well as triple bonds, the compound will be named as:
- Select the longest chain which includes double and triple bonds.
- The double bond takes precedence over the triple bond.
6. Alkenes with halogens
17. If any halogen substituent is present on alkenes, the compound will be named as:
- The alkenes are named by adding a prefix that indicates halogen substituent. The prefixes for the halogens are fluoro, chloro, bromo, and iodo.
- The numbering is started from the double bond.
7. Alkenes with alcohols
18. If an organic compound contains alkene (double bond) as well as alcoholic functional groups, the compound will be named as:
- Select the longest chain which contains both functional groups.
- The alcohol takes precedence over alkene, the numbering is started from the alcohol.
8. Cis-trans nomenclature of alkenes
The “cis-trans” isomerism arises due to the restriction in rotation about double bonds. This type of isomerization is also known as geometrical isomerism.
A molecule of 2-butene exists in two isomeric forms:
These two isomers of 2-butene can be differentiated by adding the prefixes ‘cis’ or ‘trans’.
The isomer in which two like groups are present on the same side of the double bond is known as the cis isomer. Whereas, the isomer in which two like groups are present on the opposite side of the double bond is known as the trans isomer.
The two isomers cannot be interconverted without breaking the π-bond of alkenes.
The cis-trans isomers of an alkene are diastereoisomers. They have different physical and chemical properties like melting points, boiling points, NMR spectra, etc.
The cis-trans isomerism is possible when each carbon atom of the double bond exhibits two different groups. If one of the carbon atoms of the double bond has two similar groups, the two isomers are superimposable.
Although when an alkene contains a chiral carbon, it exhibits geometrical as well as optical isomerism.
9. E/Z nomenclature of alkenes
The cis/trans nomenclature does not apply to complex systems so a new nomenclature system is required. The Cahn, Ingold, and Prelog system (CIP System) was developed in 1951 by Robert S. Cahn, Christopher K. Ingold, and Vladimir Prelog.
In this system, the two groups on each carbon of the double bond are categorized based on priority.
- If both higher priority groups are on the same side of the double bond, the geometrical isomer is designated as Z (derived from the German word ‘Zusammen’ meaning ‘together’).
- If both higher priority groups are on the opposite side of the double bond, the geometrical isomer is designated as E ( derived from the German word entgegen meaning ‘opposite’).
Cahn, Ingold, and Prelog rules
These are the CIP rules:
- Priority is given to the atom of the highest atomic number.
- If the first atoms are the same as the second atom in a molecule, the priority will be given to ethyl over the methyl group.
- A lone pair has the lowest priority, thus the priority will be given to +NH3 over NH2.
- When considering isotopes, priority will be given to the heavier isotope. For example, 13C takes precedence over 12C.
On the left side of this example, carbon takes priority over hydrogen. According to rule 1, on the right side, Br takes precedence over C. So the two higher priority groups are present on the opposite side, it is named as (E)-3-bromopent-2-ene.
In this example, on the left-hand side, two atoms are the same. According to rule 2, propyl takes precedence over ethyl. On the right-hand side, ethyl takes priority over methyl. The two higher priority groups are present on the same side, it is named Z.
10. E/Z nomenclature for dienes
The E/Z nomenclature can also be used for compounds with more than one double bond.
11. Naturally occurring alkenes
Alkenes are present in nature as a group of natural products named terpenes. Some of the examples of naturally occurring alkenes are:
- Limonene (1-Methyl-4-(1-methylethenyl)-cyclohexene)
Limonene is an alkene used as a flavoring and fragrance additive in foods, beverages, cosmetics, etc.
The 𝛃-carotene is a conjugated molecule and is found in carrots etc. It is a precursor of vitamin A and possesses many different beneficial effects. It boosts the immune system and is much useful for healthy skin and muscles as well.
How do you name alkenes with more than one double bond?
If alkenes contain more than one double bond, they are indicated by the suffixes.
- The term “diene” is used for two double bonds.
- The term “triene” is used for three double bonds.
- The term “tetraene” is used for four double bonds.
If the two double bonds are separated by a single bond, alkenes are known as conjugated dienes.
If the double bonds are separated by two or more single bonds, those alkenes are known as isolated alkenes.
What are the first four alkenes?
- Ethene (H2C = CH2)
- Propene (CH2= CHCH3)
- Butene (CH3CH2CH = CH2) (CH3CH = CHCH3)
- Pentene (CH3CH2CH2CH = CH2) (CH3CH2CH = CHCH3)
How to name alkenes with halogens?
If any halogen substituent is present on alkenes, alkenes are named by adding a prefix that indicates a halogen substituent. The prefixes for the halogens are fluoro, chloro, bromo, and iodo.
In the case of halogens, the numbering starts from the double bond e.g. 5-chloropentene, etc.
How is the general formula for alkanes and alkenes determined?
The general formula for alkenes and alkanes is given below:
- For alkanes→ CnH2n+2
- For alkenes→ CnH2n
“n” is the number of carbon atoms present in the chain. For example, for the ethyl group “n” shall be 2.
How do I name organic compounds having double as well as triple bonds?
If an organic compound contains double as well as triple bonds, the rules for naming these compounds are:
- Select the longest chain which includes double and triple bonds.
- The priority shall be given to the double bond.
How to name alkenes with alcohol?
If an organic compound contains alkene as well as alcohol functional group, the rules for naming these compounds are:
- Select the longest chain which contains alkene and alcohol, functional groups.
- The priority shall be given to the alcoholic functional group.
What are the smallest alkenes?
The smallest alkenes are ethene, propene, butene, etc.
What are symmetrical alkenes and unsymmetrical alkenes?
An alkene in which both carbon atoms of the double bond contain different substituents is known as an unsymmetrical alkene. Whereas, an alkene in which both carbon atoms of the double bond contain different substituents is known as an unsymmetrical alkene” or “asymmetrical alkene”.
What are some common uses of alkenes?
- Alkenes are used in the formation of polymers like polythene.
- In daily life, alkenes are used in the manufacturing of plastics.
- They are used in the artificial ripening of fruits.
- Alkenes produce vegetable ghee by hydrogenation reaction.
- They are used in the manufacturing of many functional groups like alcohols, alkyl halides, etc.
Is alkene a functional group?
A functional group is a group of atoms that are responsible for the characteristics reaction of a particular compound. In alkenes, the functional group is a carbon-carbon double bond that undergoes many types of reactions.
Which will be more stable conjugated or unconjugated dienes?
The conjugated dienes are more stable than the nonconjugated dienes, due to the presence of delocalization of charge through resonance.
For example, Butadiene is stable due to the delocalization of electrons.
What is the IUPAC name of limonene?
The IUPAC name of limonene is (1-Methyl-4-(1-methylethenyl)-cyclohexene). It is a naturally occurring alkene and belongs to the class of terpenes.
What are the conditions for alkenes to show geometrical isomerism?
The geometrical isomerism is possible when each carbon atom of the double bond exhibit two different groups. If one of the carbon atoms of the double bond has two like groups, the two isomers are superimposable and geometrical isomerism will not be possible.