Alkene Naming & Isomerism - Quick Notes

IUPAC Naming Rules for Alkenes

  • Alkene: hydrocarbon with C=CC=C bond.

  • Parent chain: longest continuous chain containing the double bond; suffix -ene.

  • Steps for naming alkenes:
    1) Identify the longest chain containing the carbon–carbon double bond; name the parent chain with the suffix -ene.
    2) Number the chain from the end closest to the double bond.
    3) Indicate the position of the double bond by the lower of the two numbers assigned to the double-bonded carbons (e.g., a double bond starting at carbon 2 yields -2-ene).
    4) Designate the location of any substituent groups with appropriate numbers and names.
    5) If the three‑dimensional relationship is known, add cis or trans before the name.

  • Examples:

    • 2-butene: double bond starts at C-2; name as 2−ene2-ene for the parent butene with the double bond location.
    • trans-2-hexene and cis-2-hexene: same base, different stereochemistry; trans means opposite sides, cis means same side.

  • Cis–trans isomerism:

    • Occurs when each double-bond carbon is bonded to two different substituent groups.
    • If one double-bond carbon has two identical groups, cis–trans is not possible.

  • Cycloalkenes:

    • Double-bond carbons numbered 1 and 2 to give the first substituent the lower number.

  • Common vs IUPAC naming:

    • Ethene is the preferred name; ethylene is also accepted.
    • Propene is usually called propylene in common usage.
    • Ethyne is more often called acetylene.

  • Alkynes:

    • Named with the suffix -yne (e.g., 2-butyne).
    • Alkynes do not exhibit cis–trans isomerism because the triple bond is linear and each sp orbital holds only one substituent.

  • Trans fats and Omega‑3 fatty acids (context):

    • Hydrogenation can convert cis double bonds to trans; trans fats are linked to health risks.
    • Omega‑3 fatty acids (e.g., EPA, linolenic acid) are notable for health benefits.

  • Functional groups (brief recall):

    • Carbon–carbon double bond (alkene).
    • Alcohol, ether, thiol, amine, phenol.
    • Aldehyde, ketone, hemiacetal, acetal.
    • Carboxylic acid, ester, amide, thioester.
    • Phosphate ester.

  • Memorization tip:

    • Practice drawing and recognizing these groups; note properties like pH, solubility, and hydrogen bonding.