The reduction of aldehydes and ketones with a suitable hydride-containing reducing agent is a good way of synthesizing alcohols. This approach would be even more effective if, instead of a hydride, we could use a source of nucleophilic carbon. Attack by a carbon atom on a carbonyl group would give an alcohol and simultaneously form a carbon-to-carbon bond. How can we make a C atom in an alkane nucleophilic? This was achieved by Victor Grignard, who created the organometallic reagent \(\mathrm{R}-\mathrm{MgBr},\) with the following reaction in diethyl ether: $$\mathrm{R}-\mathrm{Br}+\mathrm{Mg} \longrightarrow \mathrm{R}-\mathrm{MgBr}$$ The Grignard reagent is rarely isolated. It is formed in solution and used immediately in the desired reaction. The alkylmetal bond is highly polar, with the partial negative charge on the \(\mathrm{C}\) atom, which makes the C atom highly nucleophilic. The Grignard reagent \((\mathrm{R}-\mathrm{MgBr})\) can attack a carbonyl group in an aldehyde or ketone as follows: Addition of dilute aqueous acid solution to the metal alkoxide furnishes the alcohol. The important synthetic consequence of this procedure is that we have prepared a product with more carbon atoms than present in the starting material. A simple starting material can be transformed into a more complex molecule. (a) What is the product of the reaction between methanal and the Grignard reagent formed from 1-bromobutane after the addition of dilute acid? (b) By using a Grignard reagent, devise a synthesis for 2-hexanol. (c) By using a Grignard reagent, devise a synthesis for 2 -methyl- 2 -hexanol. (d) Grignard reagents can also be formed with aryl halides, such as chlorobenzene. What would be the product of the reaction between the Grignard reagent of chlorobenzene and propanone? Can you think of an alternative synthesis of this product, again using a Grignard reagent? (e) The basicity of the \(C\) atom bound to the magnesium in the Grignard reagent can be used to make Grignard reagents of terminal alkynes. Write the equation of the reaction between ethylmagnesium bromide and 1-hexyne. [Hint: Ethane is evolved.] (f) By using a Grignard reagent, suggest a synthesis for 2 -heptyn-1-ol.

Step by step solution

01

Synthesis using Grignard Reagent

The Grignard reagent for the reaction with methanal is \( CH_3CH_2CH_2CH_2MgBr \), which is formed from 1-bromobutane. The Grignard reagent reacts with methanal (\( HCHO \)) to form a primary alcohol. The structure we would get after the reaction is \( CH_3CH_2CH_2CH_2CH_2OH \) after addition of dilute acid.

02

Synthesis of 2-Hexanol

We need to use a Grignard reagent and an aldehyde or ketone to get 2-hexanol. We will use ethylmagnesium bromide (\( CH_3CH_2MgBr \)) as the Grignard reagent and pentanal (\( CH_3(CH_2)_4CHO \)) as the aldehyde. The reaction will give 2-hexanol \( CH_3CH_2CH_2CH(CH_3)CH_2OH \).

03

Synthesis of 2-Methyl-2-Hexanol

We will use ethylmagnesium bromide as the Grignard reagent and 2-methylpentanal as the aldehyde. The reaction will give 2-methyl-2-hexanol \( CH_3CH_2C(CH_3)CH_2CH_2CH_2OH \).

04

Reaction of Grignard Reagent with Chlorobenzene and Propanone

The Grignard reagent formed from chlorobenzene is \(C_6H_5MgBr\). This reacts with propanone to give 1-phenylpropan-1-ol. Another synthesis method could also use a benzene Grignard reagent and 2-butanone.

05

Reaction of Ethylmagnesium Bromide with 1-Hexyne

The reaction of ethylmagnesium bromide (\( CH_3CH_2MgBr \)) with 1-Hexyne (\( CH_3CH_2C≡CCH_2CH_2CH_3 \)) forms a terminal alkyne Grignard reagent. The hint suggests that Ethane \( CH_3CH_3 \) is removed during the reaction to form the Grignard reagent.

06

Synthesis of 2-Heptyn-1-ol

We will use propylmagnesium bromide with 2-butyne to get the Grignard reagent of 2-heptyne. Further treating it with methanol gives 2-heptyn-1-ol.

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