Alkenes and Alkynes

Alkanes have only single bonds, which makes them saturated - every carbon holds the maximum possible number of hydrogens. But many organic molecules contain double bonds (alkenes) or triple bonds (alkynes), and these “unsaturated” compounds need a way to indicate where the multiple bond is and what kind it is. That is where the suffixes “-ene” and “-yne” come in.

The Dating Analogy

Naming Alkenes

Alkenes contain at least one carbon-carbon double bond (C=C). Naming follows the same IUPAC rules as alkanes, with two modifications:

Modification 1: Replace the “-ane” suffix with “-ene.” Ethane becomes ethene. Propane becomes propene. Hexane becomes hexene.

Modification 2: The double bond gets a locant number. Number the parent chain so the double bond gets the lowest possible position. The locant indicates the first carbon of the double bond.

Example: A five-carbon chain with a double bond between carbons 1 and 2 is pent-1-ene (or 1-pentene in older notation). A double bond between carbons 2 and 3 is pent-2-ene.

General Formula for Alkenes

Acyclic alkenes with one double bond follow the formula CnH2n. Compare this to CnH2n+2 for alkanes - you lose two hydrogens for every double bond you add. This makes sense because forming a double bond requires each carbon to give up one hydrogen.

Naming Alkynes

Alkynes contain at least one carbon-carbon triple bond (C≡C). The suffix is “-yne,” and the locant rules are identical to alkenes.

Example: A four-carbon chain with a triple bond between carbons 1 and 2 is but-1-yne. Between carbons 2 and 3 is but-2-yne.

Acyclic alkynes with one triple bond follow the formula CnH2n-2. You lose four hydrogens compared to the corresponding alkane because a triple bond consists of one sigma bond and two pi bonds, replacing four C-H bonds.

Parent Chain Must Include the Multiple Bond

When a molecule has a double or triple bond, the parent chain must pass through that bond, even if a longer all-single-bond chain exists elsewhere. The multiple bond is more important than maximum chain length for determining the parent chain.

After ensuring the parent chain includes the multiple bond, number the chain so the double or triple bond gets the lowest locant.

Cis/Trans Designation for Alkenes

Because double bonds cannot rotate freely (the pi bond locks the geometry), groups attached to the double-bond carbons can be on the same side or opposite sides. This creates geometric isomers:

• Cis = same side. Both substituents of interest are on the same side of the double bond.
• Trans = opposite sides. The substituents are on opposite sides.

The cis/trans system works well when each double-bond carbon has one hydrogen and one non-hydrogen group. It becomes ambiguous when both carbons have two non-hydrogen substituents, which is why chemists developed the E/Z system.

E/Z Designation (Replaces Cis/Trans)

The E/Z system uses the Cahn-Ingold-Prelog (CIP) priority rules to assign priorities to the two groups on each carbon of the double bond. The higher-priority group on each carbon is identified, and then:

• Z (zusammen, German for “together”): The two higher-priority groups are on the same side of the double bond.
• E (entgegen, German for “opposite”): The two higher-priority groups are on opposite sides.

CIP Priority Rules (Brief Overview)

The Cahn-Ingold-Prelog priority system ranks substituents by atomic number:

1. Higher atomic number = higher priority. Br (35) > Cl (17) > O (8) > N (7) > C (6) > H (1).
2. If the atoms directly attached are the same, move outward and compare the next set of atoms along each branch until you find a difference.
3. Double and triple bonds are treated as if each bond were two or three single bonds to duplicate atoms. A C=O double bond treats the carbon as if it were bonded to two separate oxygen atoms.

You will study these rules in much greater depth in the Isomers chapter. For now, the key point is that E/Z assignments always use CIP priorities, not just “which group is bigger.”

Degree of Unsaturation (Index of Hydrogen Deficiency)

The degree of unsaturation (also called the index of hydrogen deficiency, IHD) tells you how many double bonds, triple bonds, or rings a molecule has compared to a fully saturated acyclic molecule with the same formula.

Example: C6H6 (benzene). DoU = (2(6) + 2 - 6) / 2 = (12 + 2 - 6) / 2 = $\frac{8}{2}$ = 4. Benzene has three double bonds and one ring = 4 DoU. Checks out.

Example: C3H6. DoU = (2(3) + 2 - 6) / 2 = (6 + 2 - 6) / 2 = $\frac{2}{2}$ = 1. This molecule has one degree of unsaturation - it could be propene (one double bond) or cyclopropane (one ring).

Naming Molecules with Multiple Double or Triple Bonds

When more than one double bond is present, use the multiplying prefix before “-ene”:

• Two double bonds: “-diene” (example: buta-1,3-diene)
• Three double bonds: “-triene” (example: hexa-1,3,5-triene)

For multiple triple bonds, use “-diyne,” “-triyne,” etc.

Each multiple bond gets its own locant. Commas separate multiple locants: buta-1,3-diene tells you there are double bonds starting at carbon 1 and carbon 3.

Terminal vs. Internal Alkynes

An important distinction for alkynes:

• Terminal alkyne: The triple bond is at the end of the chain (C1). The terminal carbon bears a hydrogen (R-C≡C-H). This hydrogen is slightly acidic (pKa ~ 25) because the sp-hybridized carbon holds the electrons tightly.
• Internal alkyne: The triple bond is in the interior of the chain (e.g., between C2 and C3). Both carbons of the triple bond are bonded to other carbons (R-C≡C-R’).

This distinction matters for reactivity. Terminal alkynes can be deprotonated by strong bases (like NaNH2) to form acetylide anions, which are excellent nucleophiles. Internal alkynes cannot.