Question

From the nature of the carbon-metal bonds in organometallic compounds, predict the products of the following reactions. Give your reasoning. a. \(\mathrm{CH}_{3} \mathrm{MgCl}+\mathrm{ICl}\) b. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Li}+\mathrm{CH}_{3} \mathrm{OH}\) c. \(\mathrm{CH}_{3} \mathrm{Li}+\mathrm{HC} \equiv \mathrm{CH}\) d. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Li}+\mathrm{CuI}\)

Short answer

a. \( \mathrm{CH}_3\mathrm{I} + \mathrm{MgClI} \); b. \( \mathrm{C}_6\mathrm{H}_6 + \mathrm{LiOCH}_3 \); c. \( \mathrm{CH}_4 + \mathrm{LiC} \equiv \mathrm{CH} \); d. \( \mathrm{(C}_6\mathrm{H}_5)_2\mathrm{CuLi} \).

Step-by-step solution

Analysis of Reaction a

The reaction is between an organomagnesium compound, \( \mathrm{CH}_3\mathrm{MgCl} \), and iodine chloride, \( \mathrm{ICl} \). The carbon-metal bond in \( \mathrm{CH}_3\mathrm{MgCl} \) is ionic in nature with carbon being more electronegative compared to magnesium, hence, the \( \mathrm{CH}_3^- \) carbanion attacks the electrophilic \( \mathrm{I^+} \) of \( \mathrm{ICl} \), leading to the formation of \( \mathrm{CH}_3\mathrm{I} \) and \( \mathrm{MgClI} \).

Analysis of Reaction b

Here, \( \mathrm{C}_6\mathrm{H}_5\mathrm{Li} \) is reacting with \( \mathrm{CH}_3\mathrm{OH} \). \( \mathrm{C}_6\mathrm{H}_5^- \) is a strong base and abstracts a proton from \( \mathrm{CH}_3\mathrm{OH} \), producing benzene, \( \mathrm{C}_6\mathrm{H}_6 \), and lithium methoxide, \( \mathrm{LiOCH}_3 \).

Analysis of Reaction c

The reaction involves \( \mathrm{CH}_3\mathrm{Li} \) with acetylene, \( \mathrm{HC} \equiv \mathrm{CH} \). \( \mathrm{CH}_3^- \) will deprotonate acetylene, resulting in methane, \( \mathrm{CH}_4 \), and lithium acetylide, \( \mathrm{LiC} \equiv \mathrm{CH} \). This occurs because acetylene has acidic hydrogens capable of being abstracted by strong bases.

Analysis of Reaction d

Here, \( \mathrm{C}_6 \mathrm{H}_5 \mathrm{Li} \) reacts with copper(I) iodide, \( \mathrm{CuI} \). \( \mathrm{C}_6 \mathrm{H}_5^- \) coordinates with \( \mathrm{Cu}^+ \) from \( \mathrm{CuI} \) to form a Gilman reagent, \( \mathrm{(C}_6\mathrm{H}_5)_2\mathrm{CuLi} \). Gilman reagents are organocopper compounds used in many organic synthesis reactions.

Key concepts

Carbon-Metal Bonds

In organometallic chemistry, carbon-metal bonds are vital as they define the nature and reactivity of the compound. These bonds typically exist between carbon and a metal, such as lithium or magnesium, creating unique chemical behavior.
The bond between carbon and a metal can be polarized, where carbon often carries a partial negative charge while the metal carries a partial positive charge. This happens because carbon is generally more electronegative than metals like lithium or magnesium.
This polarization makes the carbon in organometallic compounds act as a carbanion, or a negatively charged carbon species, allowing it to react with both electrophiles (positively charged or electron-deficient species) and as a nucleophile in various reactions. Understanding these properties is crucial to predict and rationalize the behavior of organometallic compounds in reactions.

Nucleophilic Attack

One of the key reactions involving organometallic compounds is nucleophilic attack. This involves the carbon in a carbon-metal bond acting as a nucleophile, meaning it donates an electron pair to an electrophile.
In the instance of a nucleophilic attack, such as with Grignard reagents or organolithium compounds, the carbon can effectively attack electrophilic centers, usually an atom with a positive charge or a partial positive charge, resulting in the formation of new bonds.
For example, when a Grignard reagent like \( \mathrm{CH}_3 \mathrm{MgCl} \) reacts with \( \mathrm{ICl} \), the carbanion methyl \( \mathrm{CH}_3^- \) attacks the iodine, resulting in methyl iodide \( \mathrm{CH}_3 \mathrm{I} \) as a product. This reaction showcases the nucleophilic nature of the carbon-metal bond, leading to significant transformations in organic synthesis.

Organolithium Compounds

Organolithium compounds are organometallic compounds in which the lithium atom is bonded to an organic group. These compounds are highly reactive due to the polar nature of the carbon-lithium bond.
The carbon in organolithium compounds acts as a strong nucleophile and base. For instance, in reactions where organolithium compounds are involved, the organic group can deprotonate acids, like in the reaction between \( \mathrm{CH}_3 \mathrm{Li} \) and acetylene \( \mathrm{HC} \equiv \mathrm{CH} \), producing methane and lithium acetylide.
Additionally, organolithium compounds, like \( \mathrm{C}_6 \mathrm{H}_5 \mathrm{Li} \) in reaction scenarios, can also coordinate with metal ions, forming useful synthetic intermediates like Gilman reagents. Their strong nucleophilic character makes them invaluable in forming carbon-carbon bonds and expanding molecular frameworks.

Grignard Reagents

Grignard reagents, typically represented as \( \mathrm{RMgX} \) where \( \mathrm{R} \) is an organic group and \( \mathrm{X} \) is a halogen, are widely utilized in organic synthesis for their versatility and reactivity.
The carbon in a Grignard reagent acts similarly to that in organolithium compounds, being both nucleophilic and basic. This characteristic makes Grignard reagents exceptionally useful for carbon-carbon bond formations, which is a key step in building complex organic molecules.
For example, when \( \mathrm{CH}_3 \mathrm{MgCl} \) reacts with \( \mathrm{ICl} \), the nucleophilic \( \mathrm{CH}_3^- \) attacks the electrophilic iodide, creating \( \mathrm{CH}_3 \mathrm{I} \). This kind of transformation showcases the practical utility of Grignard reagents in organic chemistry, allowing chemists to synthesize a wide variety of compounds efficiently.