Nomenclature of Cycloalkenes, Alkenols, and Alkynes

Review of Cycloalkane Nomenclature

• Priority Groups in Cyclic Structures

  • When naming substituted cycloalkanes, the lowest possible numbers are given to the substituents.

  • Example 1: Cyclohexane with an ethyl and a methyl group:

    • Parent chain: Cyclohexane.

    • Substituents: Ethyl and Methyl.

    • Numbering: Start at the ethyl group (alphabetical priority between ethyl and methyl dictates it gets position 1 only if equal numbering possible), then number to give methyl the lowest possible number. In this case, 1-ethyl-2-methylcyclohexane.

  • Example 2: Cyclohexane with hydroxyl (-OH) and methyl groups (Cyclohexanol):

    • If a hydroxyl group is present on a cyclohexane ring, the compound is named as a cyclohexanol, not a hydroxy-substituted cyclohexane. This means the -OH group takes priority.

    • The carbon bearing the hydroxyl group is designated as position 1.

    • Numbering proceeds in the direction that gives the other substituents (e.g., methyl groups) the lowest possible numbers.

    • Correct Naming: For a cyclohexanol with two methyl groups, if numbering clockwise gives positions 2 and 3 for the methyls, and counter-clockwise gives positions 3 and 5, the correct numbering is 2,3.

    • Significance: The -OH group's designation as part of the parent name (e.g., cyclohexanol) means it implicitly occupies position 1. Therefore, it is incorrect to state "1-hydroxy" in the name.

Cycloalkene Nomenclature

• Definition: Cyclic molecules containing at least one carbon-carbon double bond.

• Basic Naming (No Substituents):

  • Identify the number of carbons in the ring.

  • Determine the parent cycloalkane name.

  • Replace the ane ending with ene.

  • Prefix with cyclo-.

  • Example: A five-membered ring with one double bond is cyclopentene.

  • Numbering Rule: If there are no substituents, the double bond is assumed to be between carbons 1 and 2 (the specific numbers don't matter as all positions are equivalent if no substituents are present).

• With Substituents:

  • Priority: The carbon-carbon double bond receives priority in numbering. The carbons of the double bond are designated as positions 1 and 2.

  • Direction of Numbering: Numbering proceeds around the ring in the direction that assigns the lowest possible numbers to the substituents.

  • Example 1: A cyclopentene ring with a methyl group:

    • If the methyl group is attached to one of the double bond carbons, numbering starts there, giving the double bond carbons positions 1 and 2. If the methyl is at position 1, it's 1-methylcyclopentene.

    • If the methyl group is not directly on the double bond, you choose the 1 and 2 positions for the double bond that lead to the lowest possible number for the methyl group.

    • For example, if the methyl is one carbon away from the double bond, it might be 3-methylcyclopentene.

  • Example 2: A cyclohexene with two methyl groups:

    • Parent: Cyclohexene (six-membered ring with a double bond).

    • Double bond carbons are 1 and 2.

    • Numbering direction: Compare potential substituent numbers (e.g., 3,5 vs. 4,6).

    • Choose the direction that gives the lowest sum of locants for the substituents.

    • Example: 3,5-dimethylcyclohexene (meaning methyl groups at positions 3 and 5 relative to the double bond at 1-2).

Alkenols (Alkenes with Hydroxyl Groups)

• Definition: Molecules containing both a carbon-carbon double bond and a hydroxyl (-OH) group.

• Priority: The hydroxyl group takes priority over the double bond functionality when naming and numbering.

• Naming Convention: The main compound name ends with enol (a combination of ene for the double bond and ol for the alcohol).

• Numbering Rules:

  • Begin numbering the main chain from the carbon closest to the hydroxyl group, assigning the -OH carbon the lowest possible number.

  • The position of the hydroxyl group is denoted just before the ol ending.

  • The position of the double bond is denoted just before the ene part of the name.

• Example 1: A linear 5-carbon chain with a double bond and a hydroxyl group.

  • Chain length: Pentane base.

  • Prioritize -OH: Number from the end closest to -OH.

  • If -OH is at carbon 2 and double bond at carbon 3, and a methyl group at carbon 4. Name becomes 4-methylpent-3-en-2-ol.

  • Alternate Naming Style: 4-methyl-3-penten-2-ol (both are acceptable).

• Example 2: Cyclohexene with a hydroxyl group.

  • Parent: Cyclohexenol (since -OH takes priority over the double bond).

  • The carbon bearing the -OH is position 1.

  • Numbering proceeds in the direction that gives the double bond and any other substituents the lowest possible numbers.

  • If -OH is at 1, and the double bond between 2 and 3, and a methyl group at 2, the name is 2-methylcyclohex-2-en-1-ol.

  • Alternate Naming Style: 2-methyl-2-cyclohexen-1-ol.

  • Often, the 1 for the -OH position is omitted if it is understood to be at carbon 1.

Alkene Substituents (Alkenyl Groups)

• Concept: Alkene functionalities can also act as substituents (similar to alkyl groups).

• Naming Convention: They are called alkenyl groups.

• Key Alkenyl Groups to Know:

  • Vinyl Group: CH_2=CH-

    • Structure: A double bond where one end is directly attached to the main chain.

    • Common Name: Vinyl.

    • Example: If chlorine is attached, it's vinyl chloride.

    • IUPAC Name (as a substituent): Ethenyl (from ethene + yl ending for a substituent).

  • Allyl Group: CH2=CH-CH2-

    • Structure: A double bond with one intervening carbon between the double bond and the point of attachment to the main chain.

    • Common Name: Allyl.

    • Example: If chlorine is attached, it's allyl chloride.

    • IUPAC Name (as a substituent): Prop-2-en-1-yl.

      • Derivation: Three carbons (prop-), double bond at position 2, attached at position 1 (-1-yl).

• Using Alkenyl Substituents in Nomenclature:

  • Example: A cyclohexanol with an allyl group.

    • Parent: Cyclohexanol (as -OH has priority).

    • -OH at position 1.

    • Allyl group at position 3.

    • Name: 3-allylcyclohexanol.

    • Alternate IUPAC Name: 3-(prop-2-en-1-yl)cyclohexanol (using brackets for the complex substituent).

• Comparison to Alkyl Substituents: Similar to how --CH(CH_3)_2 (isopropyl) can also be named 1-methylethyl.

  • Isopropyl is a 1-methylethyl group (1 refers to the point of attachment, methyl is on carbon 1 of the ethyl fragment).

Stereoisomers: Cis/Trans for Alkenes

• Reason for Stereoisomerism: The carbon-carbon double bond prevents rotation, locking substituents into fixed positions relative to each other.

• Requirement for Cis/Trans: Each carbon of the double bond must be attached to two different groups.

• Cis Isomer: Substituent groups on the same side of the double bond.

• Trans Isomer: Substituent groups on opposite sides of the double bond.

• Nomenclature Inclusion: The prefix cis- or trans- is placed at the beginning of the name, usually in italics.

• Example: Dichloroethene (ClCH=CHCl)

  • Parent: Ethene.

  • Substituents: Two chloro groups at positions 1 and 2 (1,2-dichloroethene).

  • If both chlorines are on the same side: cis-1,2-dichloroethene.

  • If chlorines are on opposite sides: trans-1,2-dichloroethene.

• General Rule: Any stereoisomer (including future types like R/S) must have its stereochemistry indicated in the name.

Alkyne Nomenclature

• Definition: Organic compounds containing at least one carbon-carbon triple bond.

• Basic Naming: Similar to alkanes and alkenes.

  • Identify the longest continuous carbon chain that includes the triple bond.

  • Replace the ane ending of the corresponding alkane with yne.

  • Example: A 5-carbon chain with a triple bond is a pentyne.

• Numbering Rules:

  • Priority Ranking: Usually, the triple bond takes priority over alkyl groups and halogens.

  • Number the longest chain from the end closest to the triple bond to give it the lowest possible locant.

  • Example 1: A 5-carbon chain with a triple bond.

    • If numbering from the left gives the triple bond at position 3 (pent-3-yne), and from the right gives it at position 2 (pent-2-yne), then the correct name is pent-2-yne.

  • With Substituents:

    • The main chain is numbered according to the alkyne's priority.

    • Substituents are then located and named based on this numbering.

    • Example 2: A 6-carbon chain with a triple bond at position 1 and a methyl group at position 5.

      • Parent: Hexyne.

      • Triple bond at 1: hex-1-yne.

      • Methyl at 5: 5-methylhex-1-yne.

      • Alternate Naming Style: 5-methyl-1-hexyne.

• Priority with Halogens:

  • The alkyne group takes priority over halogens.

  • Example 3: A 4-carbon chain with a triple bond at position 1 and a chloro group at position 4. 4-chlorobut-1-yne.

  • Tie-breaking Rule: If numbering from either direction gives the triple bond the same low number (e.g., 2 from left or right in a symmetrical alkyne), then number from the end that gives the substituent the lowest possible number.

    • Example 4: A 4-carbon chain with a triple bond at position 2 (from either direction) and a chloro group at position 1. 1-chlorobut-2-yne (instead of 4-chlorobut-2-yne).

• Exceptions: Hydroxyl Groups:

  • If a hydroxyl group is present, it takes priority over the alkyne group (similar to alkenols).

  • In such cases, the compound would be named as an alkynol.