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Organic Chemistry 1 · Alkene and Alkyne Chemistry

Structure and Nomenclature of Alkynes

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

An is a hydrocarbon containing a carbon–carbon . Each alkyne carbon is sp-hybridized, so the unit R–C≡C–R′ is linear (180°), and the triple bond is one sigma (σ) bond plus two pi (π) bonds. Terminal alkynes (R–C≡C–H) sit at the chain end and are weakly acidic; internal alkynes (R–C≡C–R′) sit between two carbon groups. IUPAC naming uses the -yne suffix, numbering the parent chain so the triple bond gets the lowest . Every triple bond adds two degrees of unsaturation.

Why this matters

burns in oxyacetylene torches (over 3000 °C) for welding and cutting — a reminder of the energy stored in the triple bond. In drug discovery, alkynes serve as rigid, linear linkers and as handles for click chemistry, where a reacts with an azide to join two molecular fragments.

The college version

1. The Triple Bond: sp Hybridization and Linear Geometry

Each alkyne carbon makes only two sigma-bonding directions (to the other alkyne carbon and to its substituent or H), so it mixes one 2s and one 2p orbital into two sp hybrid orbitals pointing 180° apart; the two remaining p orbitals stay unhybridized and perpendicular. This gives one σ bond (head-on sp overlap) and two π bonds (side-on p overlap), plus linear geometry (180°). The triple bond is shorter and stronger than a double or single bond (C≡C ≈ 1.20 Å, vs ≈1.34 Å for C=C and ≈1.54 Å for C–C).

2. Terminal vs Internal Alkynes and IUPAC Naming

A terminal alkyne ends in C≡C–H and is weakly acidic; an has carbon groups on both sides. IUPAC rules:

  1. Find the longest chain containing the triple bond (the parent) and replace -ane with -yne (ethane → ethyne).
  2. Number from the end nearest the triple bond; the locant is the lower alkyne-carbon number.
  3. Name substituents with locants, then assemble locants-substituents-parent, placing the triple-bond locant before "yne" (e.g., pent-2-yne).

3. Degree of Unsaturation and Physical Properties

Each triple bond counts as two degrees of unsaturation (two missing H₂ pairs). For CₙHₓ:

\[ \text{DoU} = \frac{2n + 2 - x}{2} \]

Ethyne (C₂H₂) gives (2·2 + 2 − 2)/2 = 2. Alkanes = 0, alkenes/cycloalkanes = 1, alkynes = 2. In a physical-property comparison with alkenes and alkanes, alkynes are likewise nonpolar and water-insoluble, but their linear, polarizable π system makes them slightly denser and higher-boiling than comparable alkenes; terminal alkynes are slightly more polar because of the acidic C–H.

How it works

  1. Find the longest chain that includes the C≡C.
  2. Number from the end giving the triple bond the lowest locant.
  3. Name substituents alphabetically and insert the triple-bond locant before "-yne."
  4. To draw from a name, lay out the parent chain, place the triple bond at its locant, and attach substituents.

Common confusions

Do not confuseWithDifference
Terminal alkyneInternal alkyneTerminal has C≡C–H (acidic); internal has R–C≡C–R′
Alkyne (C≡C)Alkene (C=C)Triple bond = 2 DoU, linear; double = 1 DoU, planar
sp hybridizationsp² hybridizationsp = linear (180°); sp² = trigonal planar (120°)
One triple bondThree single bondsOne σ + two π, not three equivalent σ bonds

Memory aids

"SP = Straight Pipe." Two sp carbons make a 180° line, and "yne" rhymes with "line." For naming, "lowest number wins" — put the triple bond at the smallest locant.

Quick review

Topic Recap

Alkynes contain a C≡C triple bond (one σ + two π) between two sp-hybridized carbons, giving linear 180° geometry. Terminal alkynes end in C≡C–H and are weakly acidic; internal alkynes have carbon groups on both sides. IUPAC names use the -yne suffix with the lowest locant, and each triple bond contributes two degrees of unsaturation.

Knowledge Check

  1. What hybridization and bond angle does each carbon of a triple bond have?
  2. Draw the line-angle structure of 5-methylhex-2-yne.
  3. Is but-1-yne a terminal or internal alkyne? What about but-2-yne?
  4. Calculate the of C₆H₁₀.
  5. Why is ethyne linear while ethene is planar?

Answers and Rationales

  1. sp, 180°. Two sigma-bonding directions require two sp orbitals pointing straight apart.
  2. Six-carbon parent, C≡C between C2 and C3, CH₃ on C5. Number from the end nearest the triple bond (locant 2).
  3. But-1-yne is terminal (C≡C–H at the end); but-2-yne is internal (CH₃–C≡C–CH₃).
  4. (2·6 + 2 − 10)/2 = 2, consistent with one triple bond.
  5. Ethyne carbons are sp (180°); ethene carbons are sp² (120° trigonal planar).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine two friends gripping hands in three places at once. Connected at three spots, they lock into a perfectly straight line and cannot twist around one another the way two friends holding a single pair of hands could. That is the carbon–carbon triple bond: three shared connections holding two carbons in a stiff, straight, rod-like arrangement.

Compare the three carbon–carbon bonds: a single bond is one handshake (free spinning); a double bond is two handshakes (flat, but still bent at angles); a triple bond is three handshakes (shortest, strongest, and completely straight).

Where this stops being exact: "three pairs of hands" suggests three identical connections. In reality one is a strong head-on σ bond and the other two are weaker side-by-side π bonds whose electrons smear into a cylindrical cloud around the C–C axis — not three separate lines, and the π bonds break more easily than the σ bond.

Simple Example

  • Ethyne (acetylene), HC≡CH — the simplest alkyne, fully linear.
  • Propyne, CH₃–C≡C–H — a terminal alkyne.
  • But-2-yne, CH₃–C≡C–CH₃ — an internal alkyne.

Worked example

Work through naming + DoU, checking structure before conclusions.

  1. Parent chain. For CH₃–CH₂–CH₂–C≡C–CH(CH₃)–CH₃, the longest chain containing C≡C has seven carbons (hept-).
  2. Numbering. From the left the triple bond starts at 4; from the right it starts at 3, so number from the right — the triple bond must get the lowest locant (3); a methyl then falls on C4.
  3. Assemble. 4-methylhept-3-yne.
  4. Electron accounting. Each alkyne carbon shares 6 electrons with its partner (σ + 2π) plus 2 with its substituent/H → full octet, neutral, no lone pairs.
  5. DoU check. C₈H₁₄ → (2·8 + 2 − 14)/2 = 2, consistent with one triple bond.

Key takeaways

  • High yield: An alkyne carbon is sp-hybridized and linear (180°).
  • High yield: The triple bond is one σ + two π bonds; shorter and stronger than C=C.
  • High yield: Number so the triple bond gets the lowest number.
  • High yield: Each triple bond contributes 2 degrees of unsaturation.
  • Terminal alkynes are weakly acidic; internal alkynes are not.
  • Alkynes are nonpolar, water-insoluble, and slightly denser/higher-boiling than comparable alkenes.

Keep learning

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Practice Organic Chemistry 1

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Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Describe the carbon–carbon triple bond in terms of sp hybridization, sigma and pi bonds, and the resulting linear (180°) geometry.
  • Distinguish terminal alkynes from internal alkynes and explain why this difference matters.
  • Apply IUPAC rules to name alkynes and draw their line-angle structures from names.
  • Calculate degrees of unsaturation for alkynes and compare their physical properties with alkanes and alkenes.

Key vocabulary

Alkyne
Hydrocarbon with a C≡C triple bond
Triple bond
One σ + two π bonds
sp hybridization
One s + one p orbital → two linear orbitals
Terminal alkyne
Triple bond at chain end (R–C≡C–H)
Internal alkyne
Triple bond between carbon groups
Locant
Number showing where the triple bond starts
Degree of unsaturation
Count of missing H₂ pairs
Acetylene
Common name for ethyne (HC≡CH)

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