The treatment of CH3MgX with CH3C≡C-H produces
(a) CH3-CH=CH2
(b) CH3C≡C-CH3
(c) CH3-CH=CH-CH3
(d) C

Q: Why doesn't the Grignard reagent attack the triple bond?

Triple bonds are highly electron-rich, which naturally repels incoming nucleophiles. Furthermore, acid-base reactions are kinetically much faster than nucleophilic additions, so the proton transfer happens before any attack can occur.

Q: What makes the terminal hydrogen so acidic?

The carbon atom it is attached to is $sp$-hybridized. This means it has 50% s-character, holding its electrons very close to the nucleus. This high electronegativity pulls electron density away from the hydrogen, making it easy to remove as $H^+$.

Q: Would this reaction happen with an alkene like propene?

No. The carbons in an alkene are $sp^2$-hybridized (33% s-character). They are not electronegative enough to make the attached $C-H$ bonds sufficiently acidic to react with a Grignard reagent.

Q: What is the other product formed alongside methane?

The remaining alkynide ion pairs with the magnesium halide to form a new Grignard reagent: propynylmagnesium halide ($CH_3-C \equiv C-MgX$).

Q: Can we use this reaction practically in synthesis?

Absolutely! The new Grignard reagent formed ($CH_3-C \equiv C-MgX$) is an excellent nucleophile. It can be reacted with alkyl halides or carbonyl compounds to synthesize larger, more complex alkynes.

Question

The treatment of $CH_3MgX$ with $CH_3C \equiv C-H$ produces

Options:

(A) $CH_3-CH=CH_2$

(B) $CH_3C \equiv C-CH_3$

(C) $CH_3-CH=CH-CH_3$

(D) $CH_4$

Sigma Percentile · Accepted Answer

### The Dual Nature of Grignard Reagents
Grignard reagents, like $CH_3MgX$, are often introduced as powerful nucleophiles. However, it is crucial to remember their alter ego: they are exceptionally strong bases.

When a Grignard reagent encounters a molecule, it faces a choice. It can either attack an electrophilic center (nucleophilic addition) or snatch an acidic proton (acid-base reaction).

**Acid-base reactions are kinetically much faster** than nucleophilic attacks. Therefore, if an acidic hydrogen is present anywhere in the reaction mixture, the Grignard reagent will act as a base first.

### The Acidity of Terminal Alkynes
Now, let's look at our second reactant, propyne ($CH_3-C \equiv C-H$). This is a terminal alkyne, meaning the triple bond is at the end of the carbon chain.

The carbon atom participating in the terminal triple bond is $sp$-hybridized. An $sp$-hybridized orbital has **50% s-character**, which means the electrons in this orbital are held very close to the positively charged nucleus.

This high s-character makes the $sp$ carbon highly electronegative. It pulls the electron density of the $C-H$ bond towards itself, leaving the terminal hydrogen atom electron-deficient and, consequently, **acidic**.

### The Proton Transfer Mechanism
When we mix $CH_3MgX$ and $CH_3-C \equiv C-H$, the stage is set for a rapid acid-base reaction.

The carbon-magnesium bond in the Grignard reagent is highly polarized. The methyl group ($CH_3$) carries a partial negative charge ($\delta^-$), acting as a strong carbanion base.

This methyl carbanion quickly abstracts the acidic terminal proton ($H^+$) from propyne.

$$CH_3^- + H^+ \rightarrow CH_4 \uparrow$$

The methyl group grabs the hydrogen to form **methane gas** ($CH_4$), which bubbles out of the solution.

### The Final Outcome
The remaining part of the alkyne becomes a propynide ion ($CH_3-C \equiv C^-$). This anion pairs up with the magnesium halide cation ($MgX^+$) to form a brand new Grignard reagent: propynylmagnesium halide.

$$CH_3-C \equiv C-H + CH_3MgX \rightarrow CH_4 + CH_3-C \equiv C-MgX$$

Therefore, the major organic product of this reaction is **methane** ($CH_4$).

This specific reaction is actually a classic analytical test known as the **Zerewitinoff determination**, used to estimate the number of active (acidic) hydrogens in an unknown chemical substance.

### What About Internal Alkynes?
It is important to contrast this behavior with internal alkynes, such as 2-butyne ($CH_3-C \equiv C-CH_3$).

Internal alkynes do not have a hydrogen atom attached to an $sp$-hybridized carbon. They lack an acidic proton.

If we were to treat 2-butyne with methylmagnesium halide, **no reaction would occur**. The Grignard reagent would have no acidic proton to abstract, and the electron-rich triple bond would repel any nucleophilic attack.

Animated Solution for Chemistry - Organic Chemistry: The treatment of $C H_{3} M g X$ with $C H_{3} C \equiv C - H$ produces

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Visualized Solution

$R e a c t an t s$

$A c i d - B a se v s N u c l eo p hi l i c A dd i t i o n$

$T h e P r o t o n T r an s f er S e t u p$

$E x ec u t in g t h e R e a c t i o n$

$C o n c l u s i o n$

$T h e W a y F or w a r d$

The Sigma Insight: Hydrocarbons

The Dual Nature of Grignard Reagents Grignard reagents, like $C H_{3} M g X$ , are often introduced as powerful nucleophiles

The Acidity of Terminal Alkynes Now, let's look at our second reactant, propyne ( $C H_{3} - C \equiv C - H$ )

The Proton Transfer Mechanism

The Final Outcome The remaining part of the alkyne becomes a propynide ion ( $C H_{3} - C \equiv C^{-}$ )

What About Internal Alkynes?

Similar Questions

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The major product [B] in the following reactions is $CH_{3} - CH_{2} - CH_{3} ∣ CH - CH_{2} - OCH_{2} CH_{3} HI Heat [A] alcohol H_{2} SO_{4} Δ [B]$

Comprehension Passage

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Animated Solution for Chemistry - Organic Chemistry: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The treatment of CH3​MgX with CH3​C≡C−H produces

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Visualized Solution

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Reactants

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Acid−Base vs Nucleophilic Addition

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The Proton Transfer Setup

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Executing the Reaction

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Conclusion

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The Way Forward