Nomenclature and Isomerism of Alkenes

You are in a drug store, looking for your prescription medication. All the names on bottles appear to be a mix of alphabets and numerical values. However, each name conveys a specific meaning that allows pharmacists and chemists to handle the compounds appropriately. There is a system in organic Chemistry regarding the nomenclature and isomerism for Alkenes, according to which compounds are named so as to distinguish one from another. Alkenes are distinguished by having a carbon-carbon double bond. Also, they can be applied to very wide uses in plastics and pharmaceuticals, even in the ripening of many fruits.

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This Story also Contains

1. How Much Do You Know About Alkenes?
2. Nomenclature of Alkenes
3. Types of Isomerism in Alkenes
4. Importance and Applications of Alkenes
5. Some Solved Examples
6. Summary

How Much Do You Know About Alkenes?

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. Their general formula is C_nH_2n Their names are uniform and clear due to the IUPAC system of nomenclature. The system for the nomenclature of alkenes starts by identifying the longest continuous chain of carbon atoms that contains the double bond. This chain forms the base name of the alkene, ending in the suffix "-ene." The numbering of the chain starts from the end nearest the double bond to give the double bond the lowest possible position number. For example, in but-1-ene the double bond starts at the first carbon atom in a four-carbon chain. The side groups, called substituents are then named and numbered such that the name shows the structure of the molecule.

Nomenclature of Alkenes

The general rules and principles of the IUPAC nomenclature are already discussed in the earlier chapter. Here we will discuss some common examples of naming them.

IUPAC name: Octa-1,3,5,7-tetraene

IUPAC name: 2-n-propylpent-1-ene

Isomerism

Alkenes show two kinds of isomerism i.e, stereoisomerism and geometrical isomerism

• Stereoisomerism: Ethene (C₂H₄) $({\mathrm{C}}_2{\mathrm{H}}_4)$ and propane (C₃H₆)$({\mathrm{C}}_3{\mathrm{H}}_6)$ can have only one structure but alkenes higher than propene have different structures. For example, But-1-ene can have three different structures as follows:
  $
  \mathrm{CH}_2=\mathrm{CH}-\mathrm{CH}_2-\mathrm{CH}_3
  $But-1-ene

  $\mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_3
  $
  $\begin{gathered}
  \text { But-2-ene } \\
  \mathrm{CH}_2=\mathrm{C}-\mathrm{CH}_3 \\
  \mathrm{CH}_3
  \end{gathered}
  $

  2-Methyprop-1-ene
• Geometrical isomerism: When the groups attached to doubly bonded carbon atoms are different, they can be represented with different geometries. This type of isomerism is known as geometrical isomerism. The two structures or isomers are known as cis and trans isomers.
  
  NOTE: The trans isomer has a strong packing than the cis isomer, thus the melting point of the trans isomer is higher than the cis isomer. Further, the molecules of the cis isomer are loosely held thus they are more soluble in the particular solvent than the trans isomer.

Types of Isomerism in Alkenes

There are several kinds of isomerism exhibited by alkenes; the most important ones include structural and geometrical. Structural isomerism occurs when different compounds have the same molecular formula but different structural formulas. For example, butene can exist as but-1-ene and but-2-ene which differ in the position of the double bond. Restricted rotation due to a double bond introduces geometrical isomerism, also called cis-trans isomerism. In the cis isomers, the substituents on the double-bonded carbons are on the same side; in the trans isomers, they are on opposite sides. For instance, the two methyl groups in 2-butene can be either on the same- cis-2-butene - or on opposite sides. These variations have a significant impact on the physical and chemical properties of such compounds; therefore, the study of isomerism in alkenes is of great significance.

Importance and Applications of Alkenes

The significance of alkenes stretches beyond theoretical chemistry into numerous relevant industrial and biological applications. Two quite important feedstocks in the production of plastics, solvents, and other chemicals in the petrochemical industry are ethylene and propylene. For instance, ethylene is polymerized into the plastic packaging material polyethylene. In agriculture, alkenes will hasten the fruit-ripening process so that it gets to its best when it reaches the customer. Alkenes are also the precursors to an enormous number of pharmaceuticals. For example, highly critical drugs against cancer or cardiovascular diseases. At an academic level, the subject that helps in alkenes and their isomerism at a very fundamental level is knowledge of organic chemistry, which subsequently bridges up to higher-level studies like biochemical, medicinal, and material sciences. Alkenes are thus of so far-flung practical applications that they literally can't be dispensed with in modern society, hence requiring mastery over the nomenclature and isomerism.

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Some Solved Examples

Example 1
Question: Write the IUPAC name for the following compound:

1)3,5,5 tri-methyl hex-3-ene

2) (correct)2,2,4 tri-methyl hex-3-ene

3)3,5,5 tri-methyl hex-4-ene

4)None

Solution

-Parent Prefix will appear as hex- with an alkene modifier

-3 methyls are present at C2,C2 and C4 respectively

-IUPAC name is 2,2,4 tri-methyl hex-3-ene

Therefore, option (2) is correct.

Example 2

Question:
Amongst the following compounds, which is the optically active alkane having the lowest molecular mass?

1)$\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}_3$ ${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$

2)

3) (correct)

4)${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{C}\equiv \mathrm{CH}$.$\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{C} \equiv \mathrm{CH}$.

Solution:
An optically active compound contains an asymmetrical carbon atom. The correct answer is the compound that has an asymmetrical carbon atom and the lowest molecular mass. Therefore, option (3) is correct.

Example 3
Question:
Which of the following alkenes exhibits geometrical isomerism?
1. Propene
2. 2-Methyl propene
3. 2-Butene
4. 2-Methyl-2-butene

Solution:
2-Butene exhibits geometrical isomerism. Therefore, option (3) is correct.

Summary

Or, put more simply, alkenes nomenclature and isomerism form the backbone of Organic Chemistry that otherwise turns into messed-up studies of those versatile hydrocarbons. In that respect, the rules of nomenclature for the alkenes according to IUPAC and the kinds of isomerism related to them did was project insight into their structural variability and functional significance. Industrial applications of alkenes in industry and agriculture underline their relevance to everyday life and academic research. Getting all this has made us realize that we have gained much understanding not just in Organic Chemistry but in the art of how to invent and solve any problem.