Chapter 9: Alkynes – Structure, Properties, and Reactions

Alkynes

Introduction to Alkynes

Alkynes are hydrocarbons characterized by the presence of a carbon-carbon triple bond. They are an important class of unsaturated organic compounds and play a significant role in both natural products and synthetic organic chemistry.

• General Formula: $C_nH_{2n-2}$

• Hybridization: The carbons in the triple bond are sp hybridized.

• Common Name: Acetylene

• IUPAC Name: Ethyne

Natural Alkynes

Alkynes are found in nature and can have significant biological activity.

• Dynemicin A: An anticancer drug containing an alkyne functional group.

• Histrionicotoxin: A toxin found in poison dart frogs, also featuring an alkyne moiety.

• Applications: Many natural alkynes are used in pharmaceuticals or as toxins.

Preparation and Synthesis of Alkynes

Industrial Preparation of Acetylene

Acetylene is commonly produced by the reaction of calcium carbide with water.

• Reaction:

$\text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{HC} \equiv \text{CH}$

• Calcium carbide is produced by heating calcium oxide and carbon at high temperatures (1800–2100°C).

• Acetylene is a key starting material for many organic syntheses.

Nomenclature of Alkynes

IUPAC Naming Rules

Alkynes are named using the same basic rules as alkanes, with modifications to indicate the triple bond.

• Longest Chain: Identify the longest carbon chain containing the triple bond.

• Numbering: Number the chain to give the triple bond the lowest possible locant.

• Precedence: When both an alkene and an alkyne are present, number to give the first multiple bond the lowest number. If a tie, the alkene takes priority over the alkyne.

• Functional Group Priority: Alcohols > Alkenes, Alkynes > Alkanes > Alkyl halides (R-X)

• Substituent Name: The ethynyl group ($-C \equiv C-H$) is used as a substituent.

Example: Naming compounds with both alkene and alkyne groups requires careful numbering to follow IUPAC rules.

Structural Features of Alkynes

Bonding and Geometry

Alkynes have unique structural features due to their triple bond.

• Bond Angles: The carbons in the triple bond are linear (180° bond angle).

• Bond Lengths: Triple bonds are shorter than double or single bonds.

• Electron Density: The electron density is concentrated along the axis of the triple bond.

Comparison Table: Ethane, Ethylene, and Acetylene

The following table compares key structural features of ethane (alkane), ethylene (alkene), and acetylene (alkyne):

Property	Ethane	Ethylene	Acetylene
Formula	C2H6	C2H4	C2H2
Bond Distance (pm)	153	134	120
Bond Angles	111.0°	121.4°	180.0°
Hybridization	sp3	sp2	sp
% s-character	25%	33%	50%
pKa	62	45	26

Acidity of Alkynes

Relative Acidity

Terminal alkynes are significantly more acidic than alkanes and alkenes due to the high s-character of the sp-hybridized carbon.

• Order of Acidity: $\text{CH}_3\text{CH}_3$ (alkane, pKa ≈ 62) < $\text{H}_2\text{C}=\text{CH}_2$ (alkene, pKa ≈ 45) < $\text{HC} \equiv \text{CH}$ (alkyne, pKa ≈ 26)

• Reason: sp-hybridized carbons are more electronegative, stabilizing the negative charge on the conjugate base (acetylide ion).

Acid-Base Reactions of Alkynes

Terminal alkynes can be deprotonated by strong bases to form acetylide ions, which are useful nucleophiles in organic synthesis.

• With Hydroxide:

$\text{HC} \equiv \text{CH} + \text{OH}^- \rightarrow \text{HC} \equiv \text{C}^- + \text{H}_2\text{O}$

• With Amide Ion:

$\text{HC} \equiv \text{CH} + \text{NH}_2^- \rightarrow \text{HC} \equiv \text{C}^- + \text{NH}_3$

• Acetylide Ion: The resulting ion ($\text{HC} \equiv \text{C}^-$) is a strong nucleophile.

Additional info:

• Alkynes are more reactive than alkanes and alkenes due to the electron-rich triple bond.

• Terminal alkynes (with a hydrogen attached to the triple bond) are especially important in synthesis due to their acidity and ability to form acetylide ions.