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Addition to Alkynes
and Partial Reductions

Master the chemistry of triple bonds: learn how alkynes react with reagents, and how Lindlar's catalyst and dissolving metal reductions give you stereochemical control.

AP Chemistry

What Are Alkynes?

Alkynes are hydrocarbons containing a carbon-carbon triple bond \( \text{C} \equiv \text{C} \). The triple bond consists of one \(\sigma\) bond and two \(\pi\) bonds, making alkynes electron-rich and highly reactive toward electrophilic addition.

C
C
R
R'
 Triple Bond sp hybridized carbons 180° bond angle, linear geometry

Why Alkyne Additions Matter

Two-Stage Reactivity
A triple bond can add reagents once (to form an alkene) or twice (to form an alkane or disubstituted product).
Stereochemical Control
Partial reductions let you choose between cis and trans alkenes with precision.
Synthetic Utility
Alkynes are versatile building blocks: halogenation, hydration, and hydrogenation give diverse products.

Types of Addition Reactions

Alkynes undergo many of the same addition reactions as alkenes, but they can react once or twice. The key categories are:

1
Halogenation: Addition of \(\text{Br}_2\) or \(\text{Cl}_2\) across the triple bond (can add 2 or 4 halogen atoms).
2
Hydrohalogenation: Addition of \(\text{HBr}\) or \(\text{HCl}\) following Markovnikov's rule (or anti-Markovnikov with peroxides).
3
Hydration: Addition of water via acid-catalyzed (Markovnikov) or hydroboration (anti-Markovnikov) pathways.
4
Hydrogenation: Addition of \(\text{H}_2\) to reduce the triple bond: full reduction to alkane, or partial reduction to alkene.

Hydrogenation: Full vs. Partial

Alkyne
\(\text{R-C}{\equiv}\text{C-R'}\)
 Lindlar's cat. cis-Alkene syn addition
\(\text{Na/NH}_3\text{(l)}\)
 trans-Alkene anti addition
excess \(\text{H}_2\)
 Pt or Pd/C Alkane Partial Reduction (stops at alkene) Full Reduction (goes to alkane) Dissolving Metal (trans selectivity)

Lindlar's Catalyst: Syn Hydrogenation

Lindlar's catalyst is a "poisoned" palladium catalyst: Pd deposited on \(\text{CaCO}_3\) and deactivated with lead acetate and quinoline. This poisoning makes it selective: it reduces alkynes to alkenes but cannot reduce the alkene further.

R
R'
\(\text{H}_2\), Lindlar's cat.
R
H
R'
H
 cis (Z) alkene Both H atoms add to same face syn addition → cis product

Dissolving Metal Reduction

Treating an alkyne with sodium (or lithium) in liquid ammonia \((\text{Na/NH}_3\text{(l)})\) produces a trans (E) alkene. This is called a dissolving metal reduction or Birch-type reduction.

R
R'
\(\text{Na, NH}_3\text{(l)}\)
R
H
R'
H
 trans (E) alkene H atoms on opposite faces anti addition → trans product

Cis vs. Trans: Side by Side

Lindlar's Catalyst \(\text{Na/NH}_3\text{(l)}\)
Reagents \(\text{H}_2\), \(\text{Pd/CaCO}_3\), quinoline Na (or Li), liquid \(\text{NH}_3\)
Addition Type Syn Anti
Product Geometry cis (Z) trans (E)
Mechanism Surface catalysis Radical anion intermediate
Mnemonic "Lindlar → Z-alkene (cis)" "Na → traNs-alkene (E)"

Halogenation of Alkynes

Alkynes react with \(\text{Br}_2\) in two stages. One equivalent of \(\text{Br}_2\) gives a trans-dibromoalkene (anti addition via a bromonium ion). Two equivalents give a tetrabromoalkane.

\(\text{R-C}{\equiv}\text{C-R'}\)
\(1\text{ eq Br}_2\)
 trans-Dibromoalkene (anti addition)
\(1\text{ eq Br}_2\)
 Tetrabromoalkane (fully saturated) Control with stoichiometry: 1 eq stops at alkene, 2 eq goes to alkane

Hydration: Markovnikov vs. Anti-Markovnikov

Acid-Catalyzed Hydration
Reagents: \(\text{H}_2\text{O}\), \(\text{H}_2\text{SO}_4\), \(\text{HgSO}_4\)

Markovnikov addition: \(\text{OH}\) goes to the more substituted carbon.

Product: ketone (via enol tautomerization)
Hydroboration-Oxidation
Reagents: \(\text{R}_2\text{BH}\), then \(\text{H}_2\text{O}_2\)/\(\text{NaOH}\)

Anti-Markovnikov: \(\text{OH}\) goes to the less substituted carbon.

Product: aldehyde (via enol tautomerization)

Step-by-Step: Choosing a Reagent

1
Identify your starting material: Is it a terminal alkyne \((\text{R-C} \equiv \text{C-H})\) or an internal alkyne \((\text{R-C} \equiv \text{C-R'})\)?
2
Identify your target product: Alkane, cis-alkene, trans-alkene, ketone, or aldehyde?
3
Match reagent to product: cis-alkene → Lindlar's; trans-alkene → \(\text{Na/NH}_3\); alkane → excess \(\text{H}_2\)/Pt; ketone → \(\text{HgSO}_4\)/\(\text{H}_2\text{O}\); aldehyde → hydroboration.
4
Check stereochemistry: Lindlar = syn (cis), \(\text{Na/NH}_3\) = anti (trans), \(\text{Br}_2\) = anti (trans for first eq).
5
Check regiochemistry: Markovnikov (acid-catalyzed) vs. anti-Markovnikov (hydroboration, HBr/peroxides).

Example 1: Partial Reduction

Problem: Convert 2-butyne to cis-2-butene. What reagents do you need?

\(\text{CH}_3\)
\(\text{CH}_3\)
 2-butyne
\(\text{H}_2\), Lindlar's cat.
\(\text{CH}_3\)
H
\(\text{CH}_3\)
H
 cis-2-butene (Z)

Example 2: Trans Product

Problem: Convert 3-hexyne to trans-3-hexene. What reagents do you need?

\(\text{CH}_3\text{CH}_2\)
\(\text{CH}_2\text{CH}_3\)
 3-hexyne
\(\text{Na, NH}_3\text{(l)}\)
Et
H
Et
H
 trans-3-hexene (E)

Example 3: Hydration Products

Problem: What product forms when 1-hexyne is treated with \(\text{H}_2\text{O}\), \(\text{H}_2\text{SO}_4\), and \(\text{HgSO}_4\)?

H
\(\text{C}_4\text{H}_9\)
 1-hexyne
\(\text{H}_2\text{O, H}_2\text{SO}_4\)
\(\text{HgSO}_4\)
 Enol (unstable) tautomerize 2-hexanone (methyl ketone) Markovnikov: OH adds to more substituted C (C-2) Enol tautomerizes → ketone at C-2

Common Mistakes

✕ Mixing Up Catalysts
Confusing Lindlar's catalyst (cis) with \(\text{Na/NH}_3\) (trans). Remember: Lindlar → cis (Z), Na → trans (E).
✕ Over-Reducing
Using Pd/C or Pt instead of Lindlar's will reduce the alkyne all the way to an alkane, skipping the alkene entirely.
✕ Wrong Regiochemistry
Forgetting that acid-catalyzed hydration gives a ketone (Markovnikov) while hydroboration gives an aldehyde (anti-Markovnikov).

Alkyne Reactions Summary

Reaction Reagents Product
Syn Hydrogenation \(\text{H}_2\), Lindlar's cat. cis-Alkene
Dissolving Metal Na, \(\text{NH}_3\text{(l)}\) trans-Alkene
Full Hydrogenation \(\text{H}_2\) (excess), Pt or Pd/C Alkane
Halogenation (1 eq) \(\text{Br}_2\) (1 eq) trans-Dihaloalkene
Halogenation (2 eq) \(\text{Br}_2\) (2 eq) Tetrahaloalkane
Markovnikov Hydration \(\text{HgSO}_4\), \(\text{H}_2\text{O}\) Ketone
Anti-Markov. Hydration Hydroboration-oxidation Aldehyde

Question 1 of 5

What reagent system converts an internal alkyne to a cis-alkene?

Question 2 of 5

Dissolving metal reduction of 2-butyne with \(\text{Na/NH}_3\text{(l)}\) produces:

Question 3 of 5

Acid-catalyzed hydration of 1-pentyne \((\text{HgSO}_4, \text{H}_2\text{SO}_4, \text{H}_2\text{O})\) produces:

Question 4 of 5

How many equivalents of \(\text{Br}_2\) are needed to convert an alkyne into a tetrabromoalkane?

Question 5 of 5

Which reagent sequence would convert 1-hexyne into hexanal (an aldehyde)?