Question

A cyclic major organic product is obtained when cyclopropanone and methyl magnesium bromide is mixed \(_{\text {in }}\) ratio of respectively: (A) \(1: 1\) (B) \(2: 1\) (C) \(1: 2\) (D) \(1: 3\)

Short answer

The correct ratio of cyclopropanone and methyl magnesium bromide for forming the cyclic major organic product is \(1: 1\). This is because one molecule of cyclopropanone reacts with one molecule of methyl magnesium bromide to form the four-membered ring, cyclobutan-1-ol, as the major product.

Step-by-step solution

Identify the Reaction Mechanism

The given reaction is a Grignard reaction. Grignard reagents such as methyl magnesium bromide are known to be strong nucleophiles and can react with carbonyl compounds like cyclopropanone.

Write down the reactants and the Grignard reaction mechanism

The reactants are given as follows: - Cyclopropanone: \(C_3H_4O\) - Methyl magnesium bromide: \(CH_3MgBr\) Now, let's examine the Grignard reaction mechanism: 1. Nucleophilic attack: The Grignard reagent attacks the carbonyl group in cyclopropanone, breaking the carbonyl double bond and forming a new carbon-carbon bond. 2. Protonation: After the nucleophilic attack, the negatively charged oxygen atom gets protonated by a water molecule or any other suitable proton source, leading to the formation of an alcohol.

Perform the reaction and determine the major product

When the Grignard reagent methyl magnesium bromide (\(CH_3MgBr\)) reacts with cyclopropanone, it results in the rupture of the carbonyl double bond and forms an additional carbon bond with methylene group (\(CH_3\)): Cyclopropanone + \(CH_3MgBr\) \(\rightarrow\) Cyclic major organic product (Cyclobutan-1-ol) In this case, the cyclic major organic product is the four-membered ring, cyclobutan-1-ol.

Determine the reactants' ratio based on the major product structure

Since the Grignard reagent \(CH_3MgBr\) reacts once with one molecule of cyclopropanone to form the cyclic major organic product, the ratio of cyclopropanone to methyl magnesium bromide should be 1:1. Hence, the correct answer is (A) \(1: 1\).

Key concepts

Cyclopropanone

Cyclopropanone is a fascinating molecule in organic chemistry known for its high reactivity due to its ring strain and the presence of a carbonyl group. Structurally, it consists of a three-membered ring of carbon atoms, also known as a cyclopropane, with an additional oxygen double-bonded to one of these carbons, forming a ketone.

Due to its strained ring system, cyclopropanone is particularly reactive in comparison to other less strained ketones. This reactivity makes it a prime candidate for nucleophilic attacks, where an electron-rich nucleophile, such as a Grignard reagent, can add to the carbonyl carbon, leading to a variety of organic transformations.

The inherent strain of cyclopropanone's three-membered ring plays a critical role in its reactions since the ring strain can be relieved through chemical reactions, thereby driving the reaction forward.

Methyl Magnesium Bromide

Methyl magnesium bromide, often written as \(CH_3MgBr\), is an example of a Grignard reagent, which is a cornerstone in the realm of organic synthesis. These reagents are formed by the reaction of an alkyl halide with magnesium metal in dry ether and are characterized by their polar carbon-magnesium bond.

Methyl magnesium bromide, in particular, contains a carbon atom bonded to three hydrogen atoms and a magnesium bromide group. The carbon is nucleophilic, which means it is electron-rich and tends to form new bonds by attacking electron-deficient areas, such as the carbon in a carbonyl group.

Since Grignard reagents like methyl magnesium bromide are highly reactive, they must be handled with care. They are sensitive to moisture and air and thus are typically prepared and used under an inert atmosphere in a solvent free from water and other protic substances that could potentially react with them.

Nucleophilic Attack

A nucleophilic attack refers to the process where a nucleophile, an electron-rich species, donates a pair of electrons to form a new chemical bond with an electrophile, an electron-deficient atom or region of a molecule. In the context of the Grignard reaction involving cyclopropanone and methyl magnesium bromide, the nucleophile is the carbon end of the methyl magnesium bromide molecule.

During the nucleophilic attack, the electrons from the nucleophile flow towards the electrophilic carbonyl carbon in cyclopropanone. This breaks the double bond of the oxygen, leading to the formation of a tetrahedral intermediate. The process is favored by the relief of ring strain in cyclopropanone and the stability of the carbocation formed during the reaction.

The strength of nucleophilicity can be influenced by several factors, including the presence of a negatively charged species, electronegative atoms in close proximity that can donate electron density, and the solvent used in the reaction which can stabilize or destabilize the nucleophile.

Cyclic Organic Product

A cyclic organic product results from reactions where the starting materials form ring structures. In the case of our Grignard reaction, the nucleophilic attack by methyl magnesium bromide on cyclopropanone leads to the formation of a new carbon-carbon bond. This consequently results in a larger ring system, specifically a four-membered ring, identified as cyclobutan-1-ol.

Cyclic products are of immense interest in chemistry due to their presence in many natural products and pharmaceuticals. The synthesis of such products often involves complex reactions that form new bonds and sometimes increase the size of the ring system. The formation of cyclic compounds can be favored over open-chain forms due to thermodynamic stability or kinetic factors during the reaction.

The creation of cyclic products, like cyclobutan-1-ol, is significant in the synthesis of more complex organic molecules, showcasing the versatility of Grignard reactions in forming various carbon skeletons and introducing functional groups to the organic framework.