Give The Systematic Name Of The Alkene

Giving the Systematic Name of an Alkene: A Comprehensive Guide

Alkenes, also known as olefins, are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond. Naming alkenes systematically, using IUPAC nomenclature, is crucial for clear and unambiguous communication in organic chemistry. This comprehensive guide will walk you through the process, covering various complexities and providing numerous examples.

Understanding the IUPAC Nomenclature System for Alkenes

The International Union of Pure and Applied Chemistry (IUPAC) established a systematic naming system to avoid confusion and ensure universal understanding. This system follows a set of rules, applied sequentially, to derive the name of any alkene.

1. Identifying the Parent Chain

The parent chain is the longest continuous carbon chain containing the double bond. This is the foundation upon which the entire name is built.

Example 1: Consider the molecule CH₂=CHCH₂CH₃. The longest chain containing the double bond has four carbons, making it a butene.
Example 2: In CH₃CH=CHCH₂CH₃, the longest chain is again four carbons long, resulting in a butene. Note that the double bond can be located anywhere within the parent chain.
Example 3: A slightly more complex case: CH₃CH₂CH=CHCH₂CH₃. This molecule also has a parent chain of five carbons (pentene) even though other chains with equal carbon number exist; the longest one containing the double bond is selected.

2. Numbering the Carbon Chain

The carbon chain must be numbered to indicate the position of the double bond and any substituents. Numbering begins from the end of the chain closest to the double bond. The double bond is assigned the lowest possible number.

Example 4: CH₂=CHCH₂CH₃. Numbering from the left gives the double bond the lowest number (1). The name is therefore 1-butene.
Example 5: CH₃CH=CHCH₂CH₃. Numbering from either end gives the double bond position 2. The name is 2-butene.
Example 6: CH₃CH₂CH=CHCH₂CH₃. Numbering from the left results in 3-hexene. Numbering from the right would give 4-hexene – 3 is lower than 4, so the former is preferred.

3. Identifying and Naming Substituents

Any branches or functional groups attached to the parent chain are considered substituents. These are named alphabetically and their position is indicated by the number of the carbon atom to which they are attached.

Example 7: CH₃CH=CHCH₂CH₂Cl. The parent chain is a pentene. The chlorine atom (chloro) is attached to carbon 5. The name is 5-chloro-2-pentene.
Example 8: CH₃CH(CH₃)CH=CHCH₃. This molecule has a methyl group (CH₃) as a substituent at carbon 3. The name is 3-methyl-2-pentene. Note that the methyl group is alphabetized before the pentene.
Example 9: (CH₃)₂C=CHCH₂CH₃. This molecule has two methyl groups attached to the same carbon. The name is 2,2-dimethyl-1-pentene.

4. Incorporating Stereoisomerism (Cis/Trans or E/Z)

Alkenes exhibit isomerism due to restricted rotation around the double bond. This can be described using the cis/trans notation (older system) or the E/Z notation (CIP priority system).

Cis/Trans: Cis indicates substituents on the same side of the double bond, while trans indicates substituents on opposite sides.
Example 10: CH₃CH=CHCH₃ can exist as cis-2-butene (methyl groups on the same side) and trans-2-butene (methyl groups on opposite sides).
E/Z: The E/Z notation uses the Cahn-Ingold-Prelog (CIP) priority rules to assign priority to the substituents on each carbon of the double bond. 'E' stands for entgegen (opposite) and 'Z' stands for zusammen (together). Higher atomic number has higher priority.
Example 11: If we have CH₃CH=CHCl, the chlorine atom has higher priority than the methyl group. If chlorine and methyl are on opposite sides, it is (E)-1-chloropropene; if they are on the same side, it is (Z)-1-chloropropene.

5. Combining Everything: A Comprehensive Example

Let's consider a complex alkene: CH₃CH=C(CH₃)CH₂CH(CH₃)₂.

Parent chain: The longest chain containing the double bond has six carbons (hexene).
Numbering: Numbering from the left gives the double bond position 2.
Substituents: There is a methyl group at position 3 and an isopropyl group (CH(CH₃)₂) at position 5.
Stereoisomerism: The molecule exhibits stereoisomerism. Using the E/Z nomenclature we need to examine the two groups at each carbon of the double bond. On carbon 2 we have CH₃ and a CH(CH₃)₂ group; the latter has higher priority. On carbon 3 we have CH₃ and H; again, CH₃ has higher priority. Since these high-priority groups are on opposite sides, it’s an E isomer.
Final Name: The complete name is (E)-3-methyl-5-isopropyl-2-hexene.

Advanced Considerations and Common Pitfalls

While the basic rules are straightforward, certain aspects can be challenging.

Cyclic Alkenes: Alkenes can be part of rings. The double bond is implicitly given position 1, and numbering continues around the ring to give the substituents the lowest numbers possible.
Multiple Double Bonds (Polyenes): Compounds with multiple double bonds are named using prefixes like "diene" (two double bonds), "triene" (three double bonds), etc. The positions of all double bonds are specified.
Complex Substituents: Dealing with complex substituents may require applying the IUPAC rules for naming branched alkyl groups and functional groups.

Practical Application and Examples

Let's solidify our understanding with some examples:

1. Name the following alkene: CH₂=CHCH(CH₃)₂

Parent chain: Butene
Substituents: 3-methyl
Double bond position: 1
Name: 3-methyl-1-butene

2. Name the following alkene: (CH₃)₂C=CHCH₂CH₃

Parent chain: Pentene
Substituents: 2,2-dimethyl
Double bond position: 2
Name: 2,2-dimethyl-2-pentene

3. Name the following alkene: CH₃CH₂CH=CHCH₂CH₂CH₃

Parent chain: Heptene
Double bond position: 3
Name: 3-heptene

4. Name the following alkene and indicate its stereoisomerism: CH₃CH=CHCl

Parent chain: Propene
Substituents: 1-chloro
Double bond position: 1
Stereoisomerism: This molecule has two different groups attached to each carbon of the double bond. We need to apply the E/Z convention. Chlorine has a higher priority than methyl. If chlorine and methyl are on opposite sides, it's (E)-1-chloropropene; if they are on the same side, it's (Z)-1-chloropropene.

Conclusion

Mastering the systematic naming of alkenes is fundamental to success in organic chemistry. By following the IUPAC rules diligently and understanding the subtle nuances, you'll confidently name and communicate the structure of even the most complex alkenes. Remember to always check for the longest carbon chain containing the double bond, assign the lowest numbers to the double bond and substituents, and correctly apply the E/Z or cis/trans nomenclature when applicable. With practice, this will become second nature, allowing you to confidently navigate the world of organic molecules.