Alkynes: Structure, Acidity of Terminal Alkynes, and Reactions

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Topic 4 — Key Facts for Kenyatta University (Kenya) Core concept: Alkynes contain a carbon-carbon triple bond (C≡C); the unique property of terminal alkynes is their Brønsted-Lowry acidity (pKa ~25), allowing formation of acetylide ions and subsequent substitution reactions High-yield point: Terminal alkynes react with NaNH₂ (strong base) or AgNO₃/NH₃ to form silver or copper acetylides; internal alkynes do not exhibit this acidity ⚡ Exam tip: The substitution of terminal alkynes to form alkynyl anions is a favourite Kenyatta University exam question — know the AgNO₃/NH₃ test and the NaNH₂ reaction mechanism

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Alkynes: Unsaturated Hydrocarbons with Triple Bonds

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. Their general formula is CₙH₂ₙ₋₂ for acyclic alkynes. Each triple bond reduces the hydrogen count by four compared to the corresponding alkane.

Nomenclature of Alkynes

IUPAC rules for naming alkynes:

1. Identify the longest chain containing the C≡C bond
2. Number from the end that gives the C≡C bond the lowest possible number
3. Use the suffix -yne (e.g., ethyne, propyne, but-1-yne)
4. If both double and triple bonds are present, the chain is numbered to give the lowest set of locants; the suffix is -en-yne

First few alkyne names:

• Ethyne (C₂H₂): HC≡CH (acetylene — the most important industrial alkyne)
• Propyne (C₃H₄): CH₃–C≡CH
• But-1-yne (C₄H₆): CH≡C–CH₂–CH₃
• But-2-yne (C₄H₆): CH₃–C≡C–CH₃

Structure and Bonding in Alkynes

sp Hybridisation:

• Each carbon in the triple bond is sp hybridised
• Two sp orbitals are formed from one s and one p orbital
• The sp orbitals are 180° apart (linear geometry)
• Two unhybridised p orbitals remain on each carbon
• The triple bond consists of one σ bond (sp–sp overlap) and two π bonds (p–p overlap)

Orbital Picture of the Triple Bond:

• σ bond: Strong, formed by head-on overlap of sp–sp orbitals along the bond axis
• π bonds: Two weaker bonds formed by lateral overlap of the two perpendicular pairs of p orbitals
• Total bond dissociation energy of C≡C is approximately 839 kJ/mol (vs. 614 kJ/mol for C=C and 347 kJ/mol for C–C)

Geometry:

• Linear geometry around each sp carbon: 180° bond angle
• The molecule is linear overall (no geometry around the triple bond axis)

Physical Properties of Alkynes

Property	Trend
Boiling point	Increases with molecular size; comparable to alkenes with same carbon count
Density	Less than 1.0 g/cm³
Solubility	Insoluble in water; soluble in organic solvents
Polarity	Slightly polar (ethyne is non-polar; propyne has small dipole moment)

Boiling Points:

• Ethyne (C₂H₂): −75°C (gas) — stored in acetone-filled steel cylinders under pressure
• Propyne (C₃H₄): −23°C (gas)
• But-2-yne (C₄H₆): 27°C (liquid)

⚡ Storage Note: Acetylene (ethyne) is stored dissolved in acetone in gas cylinders filled with a porous mass. Pure acetylene is shock-sensitive and can explode under pressure. Never store acetylene under pressure alone.

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Reactions of Alkynes

1. Addition of Hydrogen Halides (HX)

Alkynes undergo addition of HX following Markovnikov’s rule. The addition occurs in two steps:

Step 1: HX adds to the alkyne to form a vinyl halide:

HC≡CH + HCl → H₂C=CHCl   (vinyl chloride / chloroethene)

Step 2: A second molecule of HCl adds to the vinyl halide to give a geminal dihalide:

H₂C=CHCl + HCl → H₃C–CHCl₂   (1,1-dichloroethane)

⚡ Exam Tip: With excess HX, the second halogen always adds to the same carbon as the first (geminal addition), because the intermediate vinyl cation is stabilised by the halogen’s lone pair electrons.

2. Addition of Halogens (X₂)

Step 1: Addition of one molecule of X₂ gives a trans-vicinal dihalide:

HC≡CH + Br₂ → CHBr=CHBr   (1,2-dibromoethene)

Step 2: Second addition gives a tetrahalide:

CHBr=CHBr + Br₂ → CHBr₂–CHBr₂   (1,1,2,2-tetrabromoethane)

Test for Unsaturation: Like alkenes, alkynes decolorise bromine solution (orange to colourless). This test cannot distinguish between alkenes and alkynes.

3. Addition of Water (Hydration)

Alkynes can be hydrated to give ketones (for internal alkynes) or aldehydes (for terminal alkynes). The reaction requires:

Reagents: Dilute H₂SO₄ and HgSO₄ (mercuric sulfate catalyst) at 60°C

Mechanism: Tautomerisation via enol intermediates

Reaction:

CH₃–C≡C–CH₃ + H₂O → CH₃–CO–CH₂–CH₃   (butan-2-one)

For terminal alkynes:

HC≡CH + H₂O → CH₃CHO   (acetaldehyde)
CH₃–C≡CH + H₂O → CH₃–CO–CH₃   (acetone)

⚡ Exam Tip: The product of alkyne hydration is always a ketone, not an alcohol. This is because the enol intermediate tautomerises to the keto form. This is called the Kucherov Rule.

4. Oxidation Reactions

Strong Oxidants (KMnO₄ or CrO₃): Complete cleavage of the C≡C bond:

Terminal alkyne:

CH₃–C≡CH + [O] → 2CH₃COOH   (acetic acid)

Internal alkyne:

CH₃–C≡C–CH₃ + [O] → 2CH₃COOH   (acetic acid)

Both carbons become carboxylic acids (no CO₂ formation from internal alkynes).

⚡ Exam Tip: Unlike alkenes, alkynes do not produce CO₂ upon oxidation — they produce carboxylic acids (or further decompose). This is a key difference used in analytical organic chemistry.

Mild Oxidants (O₃ or cold dilute KMnO₄): No reaction with simple alkynes under mild conditions. Alkynes are less reactive than alkenes toward mild oxidation.

5. Acidity of Terminal Alkynes

This is the most distinctive chemical property of terminal alkynes (alkynes with H on the terminal carbon).

The Terminal Alkyne as an Acid: Terminal alkynes (R–C≡C–H) are considerably more acidic than alkenes and alkanes:

Compound	Conjugate Base	pKa of Acid
HCl	Cl⁻	−7
H₂O	OH⁻	15.7
Ethanol	EtO⁻	16
Ammonia	NH₂⁻	38
Terminal alkyne	R–C≡C⁻	~25
Ethene	CH₂=CH⁻	~44
Ethane	CH₃–CH₂⁻	~50

Why Terminal Alkynes are Acidic: The acidic hydrogen is attached to an sp-hybridised carbon, which has 50% s-character. The higher s-character means the bonding orbital is closer to the nucleus and holds electrons more tightly. This makes the C–H bond more polar (H is more δ⁺), facilitating deprotonisation by a base.

Formation of Acetylide Ions:

Using NaNH₂ (sodium amide, a very strong base):

R–C≡C–H + NaNH₂ → R–C≡C⁻Na⁺ + NH₃

Using AgNO₃ in NH₃ (Tollen’s test for terminal alkynes):

R–C≡C–H + AgNO₃ + NH₃ → R–C≡C⁻Ag⁺ + NH₄NO₃

Silver acetylide precipitates out as a cream/yellow solid.

⚡ Exam Tip: Only terminal alkynes give this AgNO₃/NH₃ reaction. Internal alkynes do not react because they have no acidic hydrogen. This is a qualitative test to distinguish terminal from internal alkynes.

6. Alkylation of Acetylide Ions

The acetylide ion (R–C≡C⁻) is a strong nucleophile and can undergo SN2-type substitution with primary alkyl halides:

R–C≡C⁻Na⁺ + R'–CH₂–Br → R–C≡C–CH₂–R' + NaBr

This is a key C–C bond-forming reaction that allows chain extension. It converts a terminal alkyne into a longer internal alkyne.

Example:

NaC≡CH + CH₃–CH₂–Br → CH₃–CH₂–C≡CH   (but-1-yne)

⚡ Exam Tip: This reaction only works with primary alkyl halides (SN2 mechanism). Tertiary and secondary alkyl halides undergo elimination instead (E2). Aryl halides (halobenzene) do not react at all.

Industrial Importance of Ethyne (Acetylene)

Use	Process
Oxyacetylene welding	Combustion of C₂H₂ in O₂: C₂H₂ + 2.5O₂ → 2CO₂ + H₂O; flame temperature ~3,500°C
PVC production	H₂C=CHCl (vinyl chloride) → polymerises to polyvinyl chloride
Synthetic organic chemistry	Precursor for numerous industrial chemicals
Metal cutting	High-temperature flame

⚡ Safety: Acetylene gas is highly flammable and forms explosive mixtures with air (2.5–80% v/v). It must be stored in acetone-filled cylinders with proper pressure regulation. Never use acetylene above 15 psi — above this pressure it becomes unstable and explosive.

Summary of Alkyne Reactions

Reagent	Product	Mechanism
H₂ (Pt, Pd, Ni)	Alkane	Syn addition
HX (1 equiv.)	Vinyl halide	Electrophilic addition
HX (2 equiv.)	Geminal dihalide	Electrophilic addition
X₂ (1 equiv.)	Vicinal dihalide (trans)	Electrophilic addition
X₂ (2 equiv.)	Tetrahalide	Electrophilic addition
H₂O/Hg²⁺	Ketone	Electrophilic addition + tautomerisation
NaNH₂	Acetylide anion	Acid-base
AgNO₃/NH₃	Ag acetylide precipitate	Acid-base
R–X (1°)	Extended alkyne	Nucleophilic substitution
[O] (strong)	Carboxylic acid	Oxidation

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