Chapter 9

What is an alkyne in organic chemistry?

An alkyne is a molecule that contains a carbon-carbon triple bond (C≡C).

Why are alkynes considered nucleophilic?

They have two π bonds with high electron density, allowing them to donate electrons.

Why does converting π bonds to σ bonds increase stability?

σ bonds are stronger and lower in energy than π bonds.

What is the simplest alkyne?

Acetylene (C≡C with hydrogens).

What is a common use of acetylene?

Used in blowtorches and as a starting material for synthesis.

What is the key rule for naming alkynes?

The parent chain must include the triple bond and be numbered to give it the lowest possible number.

How is the triple bond location indicated in naming?

By placing a number before the -yne suffix (e.g., 2-yne).

What is the difference between terminal and internal alkynes?

Terminal alkynes have the triple bond at the end (C≡C–H), internal alkynes have it in the middle.

Why are terminal alkynes more acidic than alkenes or alkanes?

The sp orbital holds electrons closer to the nucleus, stabilizing the conjugate base.

What is the approximate pKa of a terminal alkyne?

Around 25.

Why is a strong base required to deprotonate a terminal alkyne?

Because it is still weaker than water, so only strong bases can remove the proton.

What must be true about a base to deprotonate a terminal alkyne?

Its conjugate acid must have a pKa greater than 25.

What ion forms when a terminal alkyne is deprotonated?

An acetylide (alkynide) ion.

Why is the acetylide ion useful in synthesis?

It is a strong nucleophile that can form new carbon-carbon bonds.

How are alkynes commonly prepared from alkyl halides?

By double elimination (E2) using a strong base.

What types of dihalides can form alkynes?

Geminal (same carbon) and vicinal (adjacent carbons).

What mechanism is used to form alkynes from dihalides?

E2 elimination.

Why is excess NaNH₂ often used in alkyne formation?

To drive elimination to completion and deprotonate terminal alkynes.

Why is a proton source needed after forming an acetylide ion?

To regenerate the neutral alkyne.

What happens if no proton source is added after NaNH₂?

The alkyne remains deprotonated as an anion.

What happens when alkynes undergo full hydrogenation?

They are converted to alkanes using 2 equivalents of H₂.

What catalyst is used in full hydrogenation?

Pt, Pd, or Ni.

What type of addition is hydrogenation of alkynes?

Syn addition.

What intermediate forms during hydrogenation of alkynes?

A cis alkene.

Why is the intermediate cis in hydrogenation?

Because hydrogen adds to the same side of the metal surface.

What does a poisoned catalyst (Lindlar’s) do?

It stops hydrogenation at the alkene stage.

What product does Lindlar’s catalyst produce?

A cis alkene.

What type of addition occurs with Lindlar’s catalyst?

Syn addition.

What conditions produce a trans alkene from an alkyne?

Dissolving metal reduction (Na/NH₃).

What type of addition occurs in dissolving metal reduction?

Anti addition.

Why must dissolving metal reactions be cold?

NH₃ boils at -33°C.

Why can’t water be used in dissolving metal reduction?

It would react with the strong base/electrons.

What intermediate forms in dissolving metal reduction?

A radical anion.

Why does the radical anion prefer trans geometry?

To minimize electron repulsion.

What happens in step 1 of dissolving metal reduction?

A single electron is transferred to the alkyne.

What is hydrohalogenation of alkynes?

Addition of HX across the triple bond.

Does hydrohalogenation of alkynes follow Markovnikov’s rule?

Yes.

Why are vinylic carbocations unstable?

The positive charge is on an sp carbon, which is very unstable.

What is unusual about the rate law for alkyne hydrohalogenation?

Rate = k[alkyne][HX]² (third order).

What does the unusual rate law suggest?

A termolecular collision mechanism.

What happens when peroxides are added to hydrohalogenation?

Anti-Markovnikov addition occurs.

What mechanism occurs with peroxides?

A radical mechanism.

What is acid-catalyzed hydration of alkynes?

Addition of water to form a ketone.

What catalyst is used in hydration of alkynes?

HgSO₄ with acid.

What intermediate forms in hydration of alkynes?

An enol.

Why is the enol not usually observed?

It rapidly converts to a ketone.

What is tautomerization?

Rapid interconversion between two constitutional isomers (enol ↔ ketone).

What is the final product of Markovnikov hydration of alkynes?

A ketone.

What is the final product of anti-Markovnikov hydration of alkynes?

An aldehyde.

What reagents are used for anti-Markovnikov hydration?

1. BH₃ (or bulky borane) 2. H₂O₂, NaOH.

Why are bulky boranes used in hydroboration?

To prevent multiple additions.

Why is a C=O bond more stable than C=C?

Oxygen stabilizes the bond through electronegativity and resonance.

What happens in halogenation of alkynes?

Halogens (X₂) add across the triple bond.

What happens if 2 equivalents of halogen are added?

A tetrahalide is formed.

What stereochemistry is observed with one equivalent of halogen?

Mixture of syn and anti addition.

Why is alkyne halogenation different from alkene halogenation?

The mechanism is not fully understood and is less stereospecific.

What happens in ozonolysis of alkynes?

The triple bond is cleaved completely.

What are the products of alkyne ozonolysis?

Carboxylic acids (or CO₂ if terminal).

What happens when a terminal alkyne undergoes ozonolysis?

It forms a carboxylic acid and CO₂.

How can terminal alkynes be used in synthesis?

They can be deprotonated and used as nucleophiles.

What types of alkyl halides react best with acetylide ions?

Methyl and 1° alkyl halides.

Why do 2° and 3° alkyl halides fail in alkylation?

They undergo elimination instead of substitution.

What is double alkylation of acetylene?

Adding two alkyl groups sequentially using strong base and alkyl halides.

Why must reagents be added stepwise in alkylation?

To control deprotonation before substitution occurs.

How can you convert an alkene into an alkyne?

Halogenation followed by double elimination with strong base.

Why is water added after strong base in alkyne synthesis?

To protonate the final alkyne.