The aldehydes which will not form Grignard product with one equivalent Grignard reagents are
(A) Ben

Q: Why doesn't the Grignard reagent attack the aldehyde group first?

Acid-base reactions involve the transfer of a tiny, highly mobile proton ($H^+$), which has a very low activation energy barrier. Nucleophilic addition requires the bulky Grignard carbon to approach the carbonyl carbon, which is sterically and electronically more demanding. Thus, acid-base reactions are kinetically much faster.

Q: What exactly happens to the Grignard reagent after reacting with the active hydrogen?

The alkyl or aryl group ($R^-$) from the Grignard reagent ($RMgX$) abstracts the proton ($H^+$) to form an alkane ($RH$). For example, if you use methylmagnesium bromide ($CH_3MgBr$), it will form methane gas ($CH_4$).

Q: How many equivalents of Grignard reagent are needed to react with the aldehyde group in compounds B and D?

You would need at least two equivalents. The first equivalent is sacrificed to deprotonate the active hydrogen (forming a carboxylate or alkoxide salt). The second equivalent can then act as a nucleophile and attack the aldehyde group.

Q: Does the methoxy group in compound C react with Grignard reagents?

No, ethers (like the methoxy group) do not contain acidic hydrogens and are generally inert to Grignard reagents. In fact, ethers like diethyl ether or THF are commonly used as solvents for Grignard reactions precisely because they don't react with them.

Q: What is the final product if excess Grignard reagent is used for compound B?

If excess $RMgX$ is used, the carboxylic acid group forms a carboxylate salt ($-COOMgX$), and the aldehyde group undergoes nucleophilic addition to form a secondary alkoxide salt ($-CH(R)OMgX$). Upon acidic workup, you would get a compound with both a carboxylic acid group and a secondary alcohol group.

Question

The aldehydes which will not form Grignard product with one equivalent Grignard reagents are

Options:

(A) (C), (D)

(B) (B), (D)

(C) (B), (C), (D)

(D) (B), (C)

Sigma Percentile · Accepted Answer

### The Dual Nature of Grignard Reagents

When we think of a Grignard reagent ($RMgX$), our minds immediately jump to its role as a fantastic nucleophile, ready to attack the electrophilic carbon of a carbonyl group. However, it is crucial to remember that a Grignard reagent is also an exceptionally strong base.

In the grand arena of chemical kinetics, there is a golden rule: **acid-base reactions are almost always faster than nucleophilic addition reactions**. If a molecule contains an acidic proton (often called an active hydrogen), the Grignard reagent will preferentially act as a base, snatching that proton to form an alkane ($RH$) and leaving the carbonyl group completely untouched.

### Analyzing the Given Compounds

Let's inspect the four aromatic aldehydes provided in the question to see if they harbor any hidden active hydrogens.

**Compound (A): Benzaldehyde**
This molecule consists solely of a benzene ring and an aldehyde group. There are no acidic hydrogens present. Therefore, $1$ equivalent of $RMgX$ will directly attack the $-CHO$ group, successfully forming the Grignard addition product (a secondary alcohol).

**Compound (B): p-Formylbenzoic acid**
Here, we have a carboxylic acid group ($-COOH$) attached to the ring. The hydrogen in the $-COOH$ group is highly acidic. When $1$ equivalent of $RMgX$ is added, it is instantly consumed in an acid-base reaction:

$HO_2C-C_6H_4-CHO + RMgX \rightarrow XMgO_2C-C_6H_4-CHO + RH$

Because the single equivalent of the Grignard reagent is entirely used up neutralizing the acid, there is none left to attack the aldehyde group. To get the Grignard product from the aldehyde, we would need **more than one equivalent**.

**Compound (C): p-Methoxybenzaldehyde**
This compound features a methoxy group ($-OCH_3$). Ethers do not contain active hydrogens and are generally inert to Grignard reagents. Thus, just like in compound (A), the Grignard reagent will act as a nucleophile and attack the $-CHO$ group, forming the desired product.

**Compound (D): p-Formylbenzyl alcohol**
This molecule contains a primary alcohol group ($-CH_2OH$). The hydroxyl proton is an active hydrogen. Similar to compound (B), $1$ equivalent of $RMgX$ will be consumed in an acid-base reaction to form an alkoxide salt and an alkane. The aldehyde group remains unreacted.

### The Final Verdict

Compounds (B) and (D) contain active hydrogens that prematurely consume the Grignard reagent via an acid-base reaction. Therefore, they will **not** form the Grignard addition product when treated with only one equivalent of the reagent.

The correct option is **(b) (B), (D)**.

Animated Solution for Chemistry - Organic Chemistry: The aldehydes which will not form Grignard product with one equivalent Grignard reagents are

Select Answer:

Visualized Solution

$Analyzing the Problem Statement$

$R M g X as Base vs Nucleophile$

$Identifying Active Hydrogens$

$Consumption of R M g X$

$Nucleophilic Addition$

$Final Answer$

The Sigma Insight: Carbonyl Compounds

The Dual Nature of Grignard Reagents

Analyzing the Given Compounds

The Final Verdict

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