Question

Nitriles (RC\equivN) react with Grignard reagents (R'MgBr). The reaction product from 2 -methylpropanenitrile with methylmagnesium bromide has the following spectroscopic properties. Propose a structure. $$ \mathrm{CH}_{3} \mathrm{CHC} \equiv \mathrm{N} \quad \frac{1 . \mathrm{CH}_{3} \mathrm{MgBr}}{2 \mathrm{H}_{3} \mathrm{O}^{+}} \quad ? $$ 2-Methylpropanenitrile \(\mathrm{MS}: \mathrm{M}^{+}=86\) \(\mathrm{IR}: 1715 \mathrm{~cm}^{-1}\) \({ }^{1} \mathrm{H}\) NMR: \(1.05 \delta(6 \mathrm{H},\) doublet, \(J=7 \mathrm{~Hz}) ; 2.12 \delta(3 \mathrm{H},\) singlet \() ; 2.67 \delta\) \((1 \mathrm{H},\) septet \(, J=7 \mathrm{~Hz})\) \({ }^{13} \mathrm{C} \mathrm{NMR}: 18.2,27.2,41.6 .211 .2 \delta\)

Short answer

2,2,4-trimethyl-3-pentanone is the structure of the reaction product.

Step-by-step solution

Identify the Reaction Type

This is a nitrile hydrolysis reaction followed by nucleophilic addition. A Grignard reagent (methylmagnesium bromide, CH₃MgBr) will react with a nitrile group (RC≡N) to form a ketone after hydrolysis.

Predict the Intermediate Product

The initial step involves the formation of an imine-like intermediate where the Grignard reagent attacks the carbon of the nitrile group, leading to the formation of an imine complex R-CH₂-C(OH)=N. This intermediate is unstable and will hydrolyze to form the ketone.

Determine the Final Structure

With the intermediate hydrolyzed, the reaction yields a ketone. The Grignard reagent adds a methyl group to the nitrile carbon, so the final product structure will be 2,2,4-trimethyl-3-pentanone (a ketone with molecular formula C₇H₁₄O).

Analyze Spectroscopic Data to Confirm

1. **Mass Spectrometry (MS):** The base peak is at M⁺ = 86, likely for the ketone part. 2. **Infrared (IR):** A peak at 1715 cm⁻¹ indicates the presence of a C=O bond suggesting a ketone. 3. **¹H NMR:** Shows methyl groups with chemical shifts and splitting patterns consistent with a ketone. Doublet at 1.05 δ for two equivalent methyl groups, singlet at 2.12 δ for the ketone methyl, and a septet at 2.67 δ for a single hydrogen next to the central carbon. 4. **¹³C NMR:** Peak at 211.2 δ is typical for the carbon in a ketone.

Key concepts

Grignard Reagent Mechanism

Grignard reagents, like methylmagnesium bromide (CH₃MgBr), are powerful tools in organic chemistry known for their ability to form carbon-to-carbon bonds. These compounds are organomagnesium halides, characterized by the presence of a carbon-magnesium covalent bond. This bond makes the carbon atom nucleophilic, which means it can donate electrons to form new bonds.
In the reaction with nitriles, a Grignard reagent first performs a nucleophilic attack. It targets the carbon atom in the nitrile group (RC≡N), breaking the triple bond and forming an imine-like intermediate. This is the essence of its mechanism: transforming a nitrile into an imine through the addition of an alkyl group.

• The initial attack by CH₃⁻ (from CH₃MgBr) forms an intermediate R-CH₂-C(OH)=N.
• The presence of a charge on the nitrogen makes the imine intermediate unstable.
• The intermediate is then stabilized by hydrolysis, transforming it into a more stable ketone structure.

Understanding this mechanism helps underscore the versatility and utility of Grignard reagents in organic synthesis, especially in building complex molecules from simpler ones.

Spectroscopy Analysis in Organic Chemistry

Spectroscopy is a vital technique for determining the structure of organic compounds. It involves analyzing how molecules interact with electromagnetic radiation. Each type of spectroscopy gives unique information about the molecular structure.
The spectroscopic analysis of the product formed from nitrile reactions with Grignard reagents involves different methods:

• **Mass Spectrometry (MS):** This technique measures the mass-to-charge ratio of ions. For the product in question, a peak at M⁺ = 86 indicates the presence of a specific structure, likely a ketone.
• **Infrared Spectroscopy (IR):** IR spectroscopy identifies functional groups by measuring bond vibrations. A notable peak at 1715 cm⁻¹ suggests a carbonyl group (C=O), pointing to a ketone.
• **¹H Nuclear Magnetic Resonance (NMR):** By analyzing hydrogen environments, ¹H NMR provides insight into the hydrogen framework of the molecule. Here, a doublet at 1.05 δ (6 H) denotes two equivalent methyl groups, while the singlet at 2.12 δ (3 H) indicates the ketone's methyl group, and a septet at 2.67 δ (1 H) describes a single hydrogen near the central carbon.
• **¹³C NMR:** This technique focuses on the carbon skeleton. The peak at 211.2 δ is typical for a carbon atom in a ketone, ensuring correct identification of the product structure.

Combining these techniques provides comprehensive insights and confirms the molecular structure.

Nucleophilic Addition

Nucleophilic addition is a fundamental reaction mechanism in organic chemistry, especially crucial in transformations involving nitriles and Grignard reagents. This reaction occurs when a nucleophile, a species rich in electrons, "adds" to an electrophile, a species deficient in electrons.
Here’s how nucleophilic addition is showcased in the Grignard and nitrile interaction:

• The carbon in the nitrile group is electrophilic due to its triple bond with nitrogen. This bond is polar, making carbon susceptible to attack by a nucleophile.
• The nucleophile in this reaction is derived from the Grignard reagent, specifically the carbon atom bonded to magnesium.
• Once the Grignard reagent attacks the carbon in the nitrile, it forms a new alkyl bond with it, leading to the creation of the imine intermediate.

The successful completion of nucleophilic addition during this process is critical for transforming nitriles into ketones. This addition paves the way for further reactions that result in the final stable product after hydrolysis.