A hydride is a compound in which hydrogen is bonded to a more electropositive element, such as a metal or a metalloid. Hydrides play a crucial role in various organic chemistry reactions, including the hydration of alkenes, the oxidation of alcohols, the nucleophilic addition of hydrides and Grignard reagents, and the chemistry of esters.
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Hydrides can act as reducing agents, donating their hydrogen atoms to other molecules in organic reactions.
In the hydration of alkenes via hydroboration, a hydride is initially added to the alkene, followed by the addition of a hydroxyl group (OH) to form an alcohol.
During the oxidation of alcohols, hydrides are removed from the alcohol, converting it to an aldehyde or ketone.
Nucleophilic addition of hydrides and Grignard reagents to carbonyl compounds (such as aldehydes and ketones) results in the formation of alcohols.
Hydrides can also be involved in the chemistry of esters, where they can participate in reduction reactions to convert esters into alcohols.
Review Questions
Explain the role of hydrides in the hydration of alkenes via hydroboration.
In the hydration of alkenes by hydroboration, a borane (BH3) reagent first adds a hydride (H-) and a boron-containing group to the alkene, forming an organoborane intermediate. This is followed by the addition of a hydroxyl group (OH-) to the organoborane, ultimately converting the alkene into an alcohol. The hydride plays a crucial role in the initial addition step, providing the hydrogen atom that becomes part of the final alcohol product.
Describe how hydrides are involved in the oxidation of alcohols.
During the oxidation of alcohols, hydrides are removed from the alcohol, converting it to an aldehyde or ketone. This oxidation reaction typically involves the use of an oxidizing agent, such as chromic acid (H2CrO4) or pyridinium chlorochromate (PCC), which facilitates the removal of the hydride from the alcohol. The loss of the hydride results in the formation of a carbonyl group (C=O), either an aldehyde or a ketone, depending on the structure of the original alcohol.
Analyze the role of hydrides in the nucleophilic addition of Grignard reagents to carbonyl compounds.
In the nucleophilic addition of Grignard reagents to carbonyl compounds (aldehydes and ketones), the Grignard reagent, which contains a nucleophilic alkyl or aryl group, adds to the carbonyl carbon. Simultaneously, a hydride from the Grignard reagent is also added, resulting in the formation of an alkoxide intermediate. This alkoxide is then protonated, converting the intermediate into an alcohol product. The hydride plays a crucial role in this reaction by providing the necessary reducing power to convert the carbonyl group into an alcohol.
An organometallic compound with the general formula RMgX, where R is an alkyl or aryl group, and X is a halogen (usually Cl, Br, or I). Grignard reagents are highly reactive nucleophiles used in organic synthesis.