Alkynes: Electrophilic Addition, Redox Reactions, and Properties (CHEM-201 Lecture 14)

Chapter 10: Alkynes – Electrophilic Addition and Redox Reactions

Introduction to Alkynes

Alkynes are hydrocarbons containing a carbon-carbon triple bond. Their unique orbital structure imparts distinct chemical and physical properties, making them important in organic synthesis and biological activity.

• Alkyne-containing compounds are found in various anti-cancer agents, with biological activity derived from their orbital shape.

• Click chemistry is a fast, versatile method to cyclize alkynes using an azide and copper catalyst, forming 1,2,3-triazoles.

• General reaction: $R-N_3 + R'-C C-H \xrightarrow{Cu(I)} R-N=N=N-C(R')=CH$

Boiling Points of Alkynes

The linear geometry of alkynes leads to strong intermolecular interactions, resulting in higher boiling points compared to corresponding alkanes and alkenes.

C5 compound	Boiling point (°C)
Pentane	36
trans-pent-2-ene	37
pent-2-yne	54

• Alkynes have higher boiling points due to their linear shape and stronger interactions.

Classification and Terminology of Alkynes

Alkynes are classified based on the position and symmetry of the triple bond.

Type	General form	Example
Terminal	R–C≡C–H	1-butyne
Internal symmetrical	R–C≡C–R	2-butyne
Internal unsymmetrical	R–C≡C–R'	3-hexyne

• The reactivity of alkynes depends on their classification.

Index of Hydrogen Deficiency (IHD)

IHD is a measure of unsaturation in a molecule, indicating the number of rings and multiple bonds.

• Each alkyne contributes two indices of hydrogen deficiency, as two equivalents of H2 can be added to fully saturate the triple bond.

Formula:

$\text{IHD} = \frac{\text{number of carbons} + 1 - \text{number of hydrogens}/2 - \text{number of halogens}/2 + \text{number of nitrogens}/2}{1}$

Naming Alkynes (IUPAC Rules)

Alkynes are named according to IUPAC conventions, prioritizing the longest chain containing the triple bond and the lowest possible locant for the triple bond.

Rule	Example
Longest chain with triple bond	hex-2-yne
Number chain to give triple bond lowest number	2-methylbut-1-yne
Indicate position of triple bond	3-octyne
Multiple triple bonds: use -diyne, -triyne	hexa-1,5-diyne

• Alkynes do not have (E)/(Z) designations due to their linear geometry.

Enynes: Molecules with Both Alkene and Alkyne

An enyne contains both alkene and alkyne functional groups. When naming, the chain is numbered to give the lowest possible locants to both groups, with en (alkene) coming before yne (alkyne) in the name.

• Alkene and alkyne are treated equally in numbering.

• Example: (E)-5-methylhex-5-en-1-yne

Physical and Chemical Properties of Alkynes

Stability and Hydrogenation

Hydrogenation reactions are used to measure the stability of alkenes and alkynes.

• Terminal (mono-substituted) alkene: releases 30 kcal/mol

• Internal (di-substituted) alkene: releases 27 kcal/mol

• More substituted alkenes are more stable.

• Alkynes are less stable than alkenes; the first reduction of an alkyne releases 36 kcal/mol, the second 28 kcal/mol.

Takeaway: More highly substituted alkenes are more stable; alkynes are less stable than alkenes.

Cis vs. Trans Alkenes

Trans-alkenes are more stable than cis-alkenes due to reduced steric hindrance.

• Alkynes do not have cis/trans or E/Z designations because of their linear geometry.

Bond Length and Hybridization

Alkynes have sp-hybridized carbons (50% s character), resulting in shorter bond lengths compared to alkenes and alkanes.

• Bond length decreases as s character increases: ethane (sp3) > ethene (sp2) > ethyne (sp)

• Alkyne C–C bond length: 1.20 Å (shortest)

Acidity of Alkynes

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

Compound	Conjugate base	Hybridization	% s character	pKa
Alkane (CH4)	CH3-	sp3	25%	~50
Alkene (CH2=CH2)	CH2CH-	sp2	33%	~44
Alkyne (HC≡CH)	HC≡C-	sp	50%	~25

• Alkynes are more acidic; their protons are removed more easily.

Molecular Orbitals of Alkynes

Alkynes possess one sigma bond and two orthogonal pi bonds, resulting in a linear geometry (180° bond angle).

• Pi bonds are perpendicular to each other, pushing hydrogens outward.

• This differs from alkenes, which have one set of pi bonds above and below the plane.

Cylinder of Electron Density

The orthogonal pi bonds in alkynes create a cylinder of electron density, making them good nucleophiles.

• Deprotonated alkynes (acetylide anions) can attack electrophiles and form new C–C bonds.

Reactivity and Synthesis Involving Alkynes

Formation of New C–C Bonds

Alkynes can be deprotonated to form acetylide ions, which act as nucleophiles in substitution reactions to form new carbon-carbon bonds.

• General reaction: $HC\equiv CH + NaNH_2 \rightarrow HC\equiv C^- Na^+ + NH_3$

• Acetylide ion attacks alkyl halide: $HC\equiv C^- + R-X \rightarrow HC\equiv C-R + X^-$

• Alkyl chlorides are good electrophiles due to chloride being a good leaving group.

Nucleophilic Additions

Nucleophilic additions to alkynes are facilitated by their electron density and low steric hindrance.

• New C–C bonds can be formed by:

  • Pushing electrons up to oxygen by breaking a pi bond

  • Pushing electrons up to carbon by breaking a pi bond

  • Opening an epoxide

Double Alkylations of Alkynes

Alkynes can undergo sequential alkylation to form symmetrical or unsymmetrical internal alkynes.

• Use two equivalents of base and electrophile for symmetrical alkynes.

• Use two different alkyl halides for unsymmetrical alkynes.

Reduction of Alkynes – Hydrogenation

Alkynes can be reduced to alkenes or alkanes using hydrogen gas and catalysts.

• Full reduction with H2 and Pd/C yields alkanes.

• Partial reduction to cis-alkenes is achieved with Lindlar catalyst (Pd/CaCO3, Pb(OAc)2).

• Lindlar catalyst is a poisoned catalyst, allowing only syn addition of hydrogens.

• Trans-alkenes are formed via anti-addition using Na/NH3 (Birch reduction).

Summary Table: Alkyne Reduction Methods

Method	Reagents	Product	Mechanism
Full hydrogenation	H2, Pd/C	Alkane	Syn addition
Partial hydrogenation (cis)	H2, Lindlar catalyst	Cis-alkene	Syn addition
Partial hydrogenation (trans)	Na, NH3	Trans-alkene	Anti addition (radical)

Common Mistakes in Alkyne Chemistry

• Always draw a new line to represent the new C–C bond formed.

• Count your carbons at the end to check your work.

• Be careful with numbering and naming, especially with enynes and multiple functional groups.

Practice Problems and Applications

• Designate alkynes as terminal, internal symmetrical, or internal unsymmetrical.

• Calculate IHD for molecules containing alkynes.

• Name alkynes and enynes using IUPAC rules.

• Predict products of alkylation and reduction reactions.

• Supply appropriate reagents for synthesis involving alkynes.

Additional info:

• Deuterium (D2) behaves like hydrogen in reduction reactions; this is covered in more detail in Organic Chemistry II.

• Alkynes are important intermediates in organic synthesis, pharmaceuticals, and materials science.