Difference Between Hemiacetal and Hemiketal

Key Difference – Hemiacetal vs Hemiketal
 

The key difference between Hemiacetal and Hemiketal is that hemiacetal is formed via the reaction between an alcohol and an aldehyde whereas a hemiketal is formed via the reaction between an alcohol and a ketone.

Hemiacetal and hemiketal are organic compounds that can be observed as hybrid molecules containing two functional groups in the same molecule; hydroxyl group and an ether group.

CONTENTS

1. Overview and Key Difference
2. What is Hemiacetal
3. What is Hemiketal
4. Side by Side Comparison – Hemiacetal vs Hemiketal in Tabular Form
5. Summary

What is Hemiacetal?

Hemiacetal is an organic compound in which a central carbon atom is bonded with four different groups; -OR group, -OH group, -R group and –H group. A Hemiacetal is formed from an aldehyde. In contrast, if this compound is formed from a ketone, then it is called a Hemiketal. The chemical formula of Hemiacetal is RHC(OH)OR’. The “R” group is an alkyl group.

A Hemiacetal is formed when an alcohol is added to an aldehyde. In hemiacetal compounds, there are two types of bonds which represent the two starting molecules lead to the formation of the hemiacetal.

1. The –C-O-H represents the alcohol.
2. The –C-O-R bond represents the ether group formed from the carbonyl group of aldehyde.

A hemiacetal is formed when the –OR group of an alcohol attack the carbon atom of an aldehyde. (-OR group is formed when the hydrogen atom of ROH alcohol is released). The carbon atom of the aldehyde has a partial positive charge because of the difference between electronegativity values of the carbon atom and the oxygen atom in the carbonyl group. The oxygen –OR group of alcohol can act as a nucleophile since it is rich with electrons. This nucleophile attacks the carbon atom of the carbonyl group of aldehyde, leading to a nucleophilic addition reaction. The result is hemiacetal. This reaction occurs in an acidic medium. Another way of formation of a hemiacetal is partial hydrolysis of an acetal.

What is Hemiketal?

Hemiketal is an organic compound in which a central carbon atom is bonded with four different groups; -OR group, -OH group, and two -R groups (either identical or different from each other). A hemiketal is formed from the reaction between an alcohol and a ketone. The general chemical formula of a hemiketal is R₁R₂C(OH)OR’. Unlike in hemiacetal, there is no hydrogen atom directly bonded to the central carbon atom of the hemiketal.

The carbon atom of the carbonyl group of a ketone has a partial positive charge because of the difference between electronegative values of the carbon atom and the oxygen atom. Therefore, this carbon atom undergoes nucleophilic attack by the –OR group derived from an alcohol. The –OR group of the alcohol acts as a nucleophile since it is rich with electrons.

What is the Difference Between Hemiacetal and Hemiketal?

Summary – Hemiacetal vs Hemiketal

Hemiacetal and hemiketal are two forms of organic compounds containing two functional groups in the same molecule. The difference between Hemiacetal and Hemiketal is that hemiacetal is formed via the reaction between an alcohol and an aldehyde whereas a Hemiketal is formed via the reaction between an alcohol and a ketone.

Reference:

1. Libretexts. “Addition of Alcohols to Form Hemiacetals and Acetals.” Chemistry LibreTexts, Libretexts, 22 Aug. 2016. Available here   
2. “Hemiacetal.” Wikipedia, Wikimedia Foundation, 23 Feb. 2018. Available here 

Image Courtesy:

1.’Hemiacetal formation’By akane700 – Own work, (CC BY-SA 3.0) via Commons Wikimedia 
2.’Hemiketal formation’By akane700 – Own work, (CC BY-SA 3.0) via Commons Wikimedia