Hemiacetal

Hemiacetal

Overview
Hemiacetals and hemiketals are compounds that are derived from aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s and ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

s respectively. The Greek word hèmi means half. These compounds are formed by formal addition of an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 to the carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 group.

When the alcohol group is replaced by a second alkoxy group
Alkoxy group
In chemistry, the alkoxy group is an alkyl group singular bonded to oxygen thus: R—O. The range of alkoxy groups is great, the simplest being methoxy . An ethoxy group is found in the organic compound phenetol, C6H5OCH2CH3 which is also known as ethoxy benzene...

, an acetal
Acetal
An acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...

 or a ketal, respectively, is formed.

The general formula of a hemiacetal is R1R2C(OH)OR, where R1 or R2 is often hydrogen and R (bonded to O) is not hydrogen.

While in the IUPAC definition of a hemiacetal R1 or R2 may or may not be a hydrogen, in a hemiketal none of the R-groups is an H.
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Encyclopedia
Hemiacetals and hemiketals are compounds that are derived from aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s and ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

s respectively. The Greek word hèmi means half. These compounds are formed by formal addition of an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 to the carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 group.

When the alcohol group is replaced by a second alkoxy group
Alkoxy group
In chemistry, the alkoxy group is an alkyl group singular bonded to oxygen thus: R—O. The range of alkoxy groups is great, the simplest being methoxy . An ethoxy group is found in the organic compound phenetol, C6H5OCH2CH3 which is also known as ethoxy benzene...

, an acetal
Acetal
An acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...

 or a ketal, respectively, is formed.

Formula and formation


The general formula of a hemiacetal is R1R2C(OH)OR, where R1 or R2 is often hydrogen and R (bonded to O) is not hydrogen.

While in the IUPAC definition of a hemiacetal R1 or R2 may or may not be a hydrogen, in a hemiketal none of the R-groups is an H. Hemiketals are regarded as hemiacetals that have no R-groups being H, thus a subclass of the hemiacetals.

Formation of hemiacetals

Formation of hemiketals

Cyclic hemiacetals and hemiketals

Left a lactol
Lactol
In organic chemistry, a lactol is the cyclic equivalent of a hemiacetal or a hemiketal.The compound is formed by the intramolecular nucleophilic addition of a hydroxyl group to the carbonyl group of an aldehyde or a ketone....

 of ribose, a cyclic hemiacetal.
Right a lactol of fructose, a cyclic hemiketal.


Hemiacetals and hemiketals are generally unstable compounds. In some cases however, stable cyclic hemiacetals and hemiketals, called lactol
Lactol
In organic chemistry, a lactol is the cyclic equivalent of a hemiacetal or a hemiketal.The compound is formed by the intramolecular nucleophilic addition of a hydroxyl group to the carbonyl group of an aldehyde or a ketone....

s, can be readily formed, especially when 5- and 6-membered rings are possible. In this case an intramolecular
Intramolecular
Intramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.- Examples :...

 OH group reacts with the carbonyl group. Glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 and many other aldoses exist as cyclic hemiacetals whereas fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...

 and similar ketoses exist as cyclic hemiketals.

Synthesis


In organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...

, hemiacetals can be prepared in a number of ways:
  • Nucleophilic addition
    Nucleophilic addition
    In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a π bond is removed by the creation of two new covalent bonds by the addition of a nucleophile....

     of an alcohol to a carbonyl group of an aldehyde
  • Nucleophilic addition
    Nucleophilic addition
    In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a π bond is removed by the creation of two new covalent bonds by the addition of a nucleophile....

     of an alcohol to a resonance stabilized hemiacetal cation
  • Partial hydrolysis
    Hydrolysis
    Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

     of an acetal
    Acetal
    An acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...


Reactions


Hemiacetals and hemiketals may be thought of intermediate between alcohols and aldehydes or ketones, and acetal
Acetal
An acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...

s and ketals:
-C=O + 2 ROH ⇌ -C(OH)(OR) + ROH ⇌ -C(OR)2 + H2O


A hemiacetal can react with an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 under acidic conditions
to form an acetal
Acetal
An acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...

, and can dissociate to form an aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

 and an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

.
  • hemiacetal + alcohol
    Alcohol
    In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

     (excess) + acid
    Acid
    An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

     (catalyst) ↔ acetal
    Acetal
    An acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...

     + water
    Water
    Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...



An aldehyde dissolved in water exists in equilibrium with low concentrations of its hydrate, R-CH(OH)2. Similarly, in excess alcohol, the aldehyde, its hemiacetal, and its acetal all exist in solution.

Hemiacetal results from addition of the alcohol's hydroxyl group to the carbon in the C=O bond. Acetals are products of substitution reactions catalyzed by acid. The presence of acid improves the leaving capacity of the hydroxyl group and enables its substitution with an alkoxyl group (-OR). The conversion of a hemiacetal to an acetal is an SN1 reaction
SN1 reaction
The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular...

.

Ketones give hemiketals and ketals. These do not form as readily as hemiacetals and acetals. To increase yields of ketals or acetals, water formed during the reaction can be removed.