Hydroxide Ion

Definition

The hydroxide ion (OH-) is a negatively charged species consisting of one oxygen atom and one hydrogen atom. It is an important chemical species that plays a crucial role in various organic chemistry reactions, particularly in the context of nucleophilic addition of water (hydration) and the chemistry of esters.

5 Must Know Facts For Your Next Test

1. The hydroxide ion (OH-) is a strong base, meaning it can readily accept a proton (H+) to form water (H2O).
2. In the context of nucleophilic addition of water (hydration), the hydroxide ion acts as a nucleophile, attacking an electrophilic carbon atom to form a new carbon-oxygen bond.
3. During the hydrolysis of esters, the hydroxide ion acts as a nucleophile, attacking the carbonyl carbon of the ester to break the carbon-oxygen bond and form a carboxylate ion and an alcohol.
4. The hydroxide ion is a key participant in the mechanism of many organic reactions, including saponification (the hydrolysis of esters to form carboxylate salts and alcohols).
5. The concentration of hydroxide ions in a solution can be used to determine the pH of the solution, with higher hydroxide ion concentrations corresponding to a more basic (higher pH) environment.

Review Questions

• Explain the role of the hydroxide ion (OH-) in the nucleophilic addition of water (hydration) reaction.
  • In the nucleophilic addition of water (hydration) reaction, the hydroxide ion (OH-) acts as a nucleophile, attacking an electrophilic carbon atom to form a new carbon-oxygen bond. This process typically occurs in the presence of an electrophilic carbonyl compound, where the hydroxide ion adds to the carbonyl carbon, resulting in the formation of a tetrahedral intermediate. The hydroxide ion's strong nucleophilic character and ability to donate a pair of electrons make it a key participant in this type of organic reaction.
• Describe the role of the hydroxide ion (OH-) in the hydrolysis of esters.
  • During the hydrolysis of esters, the hydroxide ion (OH-) acts as a nucleophile, attacking the carbonyl carbon of the ester compound. This nucleophilic attack breaks the carbon-oxygen bond of the ester, forming a carboxylate ion and an alcohol. The hydroxide ion's ability to accept a proton and its strong nucleophilic character are essential for this hydrolysis reaction, which is a key step in the saponification process (the conversion of esters into carboxylate salts and alcohols).
• Analyze the relationship between the concentration of hydroxide ions (OH-) and the pH of a solution.
  • The concentration of hydroxide ions (OH-) in a solution is directly related to the pH of that solution. Higher concentrations of hydroxide ions correspond to a more basic, or higher pH, environment. This is because the hydroxide ion is a strong base that can readily accept a proton (H+) to form water (H2O), thereby increasing the pH of the solution. Conversely, lower concentrations of hydroxide ions result in a more acidic, or lower pH, environment. Understanding the relationship between hydroxide ion concentration and pH is crucial in various organic chemistry reactions and applications, where the pH of the reaction medium can significantly influence the outcome of the process.

Related terms

Nucleophile: A nucleophile is a species that donates a pair of electrons to form a new covalent bond, often attacking an electrophilic carbon atom.

Electrophile: An electrophile is a species that is attracted to electrons and can form a new bond by accepting a pair of electrons, often from a nucleophile.

Hydrolysis: Hydrolysis is a chemical reaction in which a molecule is cleaved into two or more smaller molecules by the addition of water.