2-hexyne gives trans-2-hexene on treatment with
(a) Pt / H2
(b) Li / NH3
(c) Pd / BaSO4
(d) LiAlH4

Q: Why doesn't LiAlH4 reduce the alkyne?

LiAlH4 is a source of nucleophilic hydride ions (H-). Alkynes are already very electron-rich (due to the pi bonds) and will repel the incoming nucleophile. LiAlH4 only reduces alkynes if they are activated by a nearby polar group, like a propargylic alcohol.

Q: What makes Lindlar's catalyst stop at the alkene?

Lindlar's catalyst is 'poisoned' (usually with lead acetate and quinoline). This poisoning reduces the catalytic activity of the palladium just enough so that it can reduce the highly reactive alkyne, but it is too weak to reduce the resulting alkene.

Q: Why does Lindlar's catalyst give cis-alkenes?

The reaction happens on the solid surface of the metal catalyst. The alkyne sits flat on the metal, and the two hydrogen atoms are delivered from the metal surface to the same face of the alkyne simultaneously. This forced 'syn-addition' results in a cis geometry.

Q: Why does Birch reduction specifically give the trans isomer?

The Birch reduction proceeds via a radical anion intermediate. To minimize both the electrostatic repulsion between the lone pair and the radical, and the steric repulsion between the two bulky alkyl groups, the intermediate twists into a trans conformation before it gets protonated.

Q: Can I use Sodium (Na) instead of Lithium (Li) for the Birch reduction?

Yes! Sodium in liquid ammonia (Na/NH3) works via the exact same solvated electron mechanism and will also perfectly reduce an internal alkyne to a trans-alkene.

Question

2-hexyne gives trans-2-hexene on treatment with

Options:

(A) Pt / H$_2$

(B) Li / NH$_3$

(C) Pd / BaSO$_4$

(D) LiAlH$_4$

Sigma Percentile · Accepted Answer

### The Art of Alkyne Reduction: Steering the Stereochemistry

When you look at an alkyne like **2-hexyne**, you are looking at a molecule loaded with potential. The carbon-carbon triple bond is a dense region of electron density, ripe for reduction. But in organic chemistry, it's rarely just about adding hydrogen; it's about *how* you add it.

Our goal in this problem is highly specific: we need to convert 2-hexyne into **trans-2-hexene**. This means we must not only stop the reduction halfway (at the alkene stage) but also ensure that the two hydrogen atoms add to opposite sides of the molecule (anti-addition). Let's evaluate our chemical toolkit to see which reagent is up to the task.

### The Sledgehammer: Catalytic Hydrogenation

Our first option is $Pt/H_2$. Platinum, palladium, and nickel are fantastic catalysts for hydrogenation. However, they are like sledgehammers. When you expose an alkyne to $H_2$ gas over a standard platinum catalyst, the reaction doesn't stop at the alkene.

The intermediate alkene is actually more reactive towards hydrogenation than the starting alkyne! As a result, the molecule is rapidly reduced all the way down to an alkane.

$$CH_3CH_2CH_2-C\equiv C-CH_3 \xrightarrow{Pt/H_2} CH_3CH_2CH_2CH_2CH_2CH_3$$

This gives us **n-hexane**, completely missing our target.

### The Poisoned Path: Lindlar's Catalyst

Next, we consider $Pd/BaSO_4$, famously known as **Lindlar's catalyst**. To prevent the "sledgehammer" effect, chemists intentionally "poison" the palladium catalyst with lead or quinoline. This drastically reduces its reactivity, allowing the reaction to pause at the alkene stage.

However, there is a stereochemical catch. Because the reaction occurs on the solid surface of the metal catalyst, both hydrogen atoms are delivered to the alkyne from the exact same face. This is called **syn-addition**.

$$CH_3CH_2CH_2-C\equiv C-CH_3 \xrightarrow{Pd/BaSO_4} 	ext{cis-2-hexene}$$

Syn-addition exclusively produces the **cis-alkene**. While elegant, it's the wrong geometric isomer for our needs.

### The Electron Donor: Birch Reduction

Now we turn to $Li/NH_3$ (Lithium metal dissolved in liquid ammonia). This is the classic **Birch reduction** conditions. Unlike catalytic hydrogenation, this reaction does not use $H_2$ gas. Instead, it relies on solvated electrons.

The alkali metal donates an electron into the antibonding orbital of the alkyne, creating a **radical anion**. This intermediate is highly unstable and immediately seeks to minimize repulsion. The bulky alkyl groups and the negative charges push as far away from each other as possible, naturally adopting a **trans geometry**.

$$CH_3CH_2CH_2-C\equiv C-CH_3 \xrightarrow{Li/NH_3} 	ext{trans-2-hexene}$$

Subsequent protonation by the ammonia solvent locks this geometry in place. The result is a beautifully selective **anti-addition**, yielding exactly the **trans-2-hexene** we desire.

### The Verdict

For completeness, we must mention $LiAlH_4$. While a powerhouse for reducing polar bonds like carbonyls, it is generally inert towards isolated, non-polar alkynes.

By understanding the distinct mechanisms—complete reduction, syn-addition, and anti-addition—we can confidently select **$Li/NH_3$** as the master key to unlock the trans-alkene.

Animated Solution for Chemistry - Organic Chemistry: 2-hexyne gives trans-2-hexene on treatment with

Select Answer:

Visualized Solution

\text{The Reactant: 2-hexyne}

\text{Reduction of Alkynes}

\text{Catalytic Hydrogenation}

\text{Lindlar's Catalyst}

\text{Lithium Aluminum Hydride}

\text{Birch Reduction}

\text{Conclusion}

The Sigma Insight: Hydrocarbons

The Art of Alkyne Reduction

The Sledgehammer

The Poisoned Path

The Electron Donor

The Verdict

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