Hemiacetals and
hemiketals are compounds that are derived from
aldehydeAn 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
ketoneIn 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
alcoholIn 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
carbonylIn 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 groupIn 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
acetalAn 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 R
1R
2C(OH)OR, where R
1 or R
2 is often hydrogen and R (bonded to O) is not hydrogen.
While in the IUPAC definition of a hemiacetal R
1 or R
2 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.
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| Formation of hemiacetals |
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| Formation of hemiketals |
Cyclic hemiacetals and hemiketals
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| Left a 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
lactolIn 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
intramolecularIntramolecular 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.
GlucoseGlucose 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
fructoseFructose, 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 synthesisOrganic 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
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
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 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 acetalAn 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
acetalAn 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
alcoholIn 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
acetalAn 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
aldehydeAn 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
alcoholIn 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
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) + acidAn 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) ↔ acetalAn acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...
+ waterWater 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 reactionThe 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.