Nomenclature || Lec 02 || Alkenes and Alkynes || Dr. Rizwana

Naming alkenes and alkynes in organic chemistry hinges on one core rule: replace the alkane ending with the correct alkene/alkyne suffix, then assign the double or triple bond the lowest possible number by counting along the parent carbon chain. For alkenes, the ending changes from “-ane” to “-ene” (for example, an ethene-type structure is commonly called “ethene” and also appears as “vinyl” in common naming contexts). For alkynes, the ending changes to “-yne,” and the bond position must be explicitly included in the IUPAC-style name.

The transcript emphasizes that the double or triple bond does not have to sit at the end of the chain. When a C=C or C≡C appears anywhere in the carbon skeleton, numbering must reflect its location. That means the parent chain is chosen first—typically the longest chain available, with the “best” numbering determined by the lowest set of locants for the multiple bond(s). If the chain can be numbered in more than one direction, the numbering that gives the multiple bond the smaller number wins.

Concrete examples show how substituents and bond positions combine in the final name. For instance, a four-carbon straight chain with a methyl substituent and a double bond at a specific carbon is named by listing substituents with their positions (e.g., “2-methyl…”) and then inserting the double-bond locant (e.g., “...2-ene” style). When substituents are present on the chain, their positions are numbered relative to the parent chain, and multiple substituents are ordered alphabetically in the name (with “di-” used for repeated groups like dimethyl). The transcript also notes that the double bond position is written as part of the main chain name, not treated as an afterthought.

Cycloalkenes and cycloalkynes follow the same logic but with ring numbering. The parent ring is treated as the base structure, and substituents plus the C=C or C≡C positions are assigned so the multiple bond gets the lowest possible locant. The examples include substituted cyclohexene derivatives where methyl or ethyl groups attach at specific ring carbons, and the double bond location is included in the systematic name.

The most nuanced part comes when both double and triple bonds appear in the same molecule. In that case, the transcript stresses a preference rule: give priority to the double bond during numbering and naming. The numbering direction should be chosen so the double bond receives the lowest locant set; then the triple bond locant is appended as well. The final naming format keeps the complete parent-chain name with the double-bond suffix, while the triple-bond position is included in the appropriate “-yne” locant form. Overall, the message is that consistent parent-chain selection plus lowest-locant numbering for multiple bonds—especially the double-bond priority rule—makes even complex alkene/alkyne nomenclature manageable.

By the end, the lecture frames these examples as practice tools: once the rules for suffix changes, parent chain selection, and lowest-number locants are internalized, naming increasingly complicated alkenes, alkynes, and mixed unsaturation systems becomes a repeatable process rather than a memorization exercise.