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Hemiacetal
 
 
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Hemiacetals and hemiketals are compounds of the general formula R1R'1C(OH)OR2, where R2 is not hydrogen. These types of compounds are formed from carbonyl compounds and alcohols, i.e. hemiacetals from aldehydes, hemiketals from ketones. Thus, for hemiacetals, either R1 or R'1 must be a hydrogen, while for hemiketals, neither will be.



a class="link1" onMouseover='showByLink("m341387",this)' onMouseout='hide("m341387")'href="http://www.absoluteastronomy.com/topics/Organic_synthesis">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_chemistry molecules can often contain a higher level of complexity compared to purely Inorganic_chemistry compounds, so the synthesis of organic compounds has developed into one of the most im...
, hemiacetals can be prepared in a number of ways:

acetals 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 oxygens attached to the same carbon atom.Traditional usages distinguish ketal from acetal . Current accepted terminology classifies ketals as a subset of acetals....
s and ketal
Ketal

A ketal is a functional group or molecule containing a carbon Chemical bond to two -OR groups, where O is oxygen and R represents any alkyl group....
s:

-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 any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
 under acidic conditions to form an acetal
Acetal

An acetal is a molecule with two single bonded oxygens attached to the same carbon atom.Traditional usages distinguish ketal from acetal . Current accepted terminology classifies ketals as a subset of acetals....
, and can dissociate to form an aldehyde
Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
 and an alcohol
Alcohol

In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
.

An aldehyde dissolved in water exists in equilibrium with low concentrations of its hydrate, R-CH(OH)2.






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Hemiacetals and hemiketals are compounds of the general formula R1R'1C(OH)OR2, where R2 is not hydrogen. These types of compounds are formed from carbonyl compounds and alcohols, i.e. hemiacetals from aldehydes, hemiketals from ketones. Thus, for hemiacetals, either R1 or R'1 must be a hydrogen, while for hemiketals, neither will be.

Formation of hemiacetals
Formation of 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_chemistry molecules can often contain a higher level of complexity compared to purely Inorganic_chemistry compounds, so the synthesis of organic compounds has developed into one of the most im...
, 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 p 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 p 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 one or more water are split into hydrogen and hydroxide ions which may go on to participate in further reactions....
     of an acetal
    Acetal

    An acetal is a molecule with two single bonded oxygens attached to the same carbon atom.Traditional usages distinguish ketal from acetal . Current accepted terminology classifies ketals as a subset of 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 oxygens attached to the same carbon atom.Traditional usages distinguish ketal from acetal . Current accepted terminology classifies ketals as a subset of acetals....
s and ketal
Ketal

A ketal is a functional group or molecule containing a carbon Chemical bond to two -OR groups, where O is oxygen and R represents any alkyl group....
s:

-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 any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
 under acidic conditions to form an acetal
Acetal

An acetal is a molecule with two single bonded oxygens attached to the same carbon atom.Traditional usages distinguish ketal from acetal . Current accepted terminology classifies ketals as a subset of acetals....
, and can dissociate to form an aldehyde
Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
 and an alcohol
Alcohol

In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
.
  • hemiacetal + alcohol
    Alcohol

    In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
     (excess) + acid
    Acid

    An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
     (catalyst) ? acetal
    Acetal

    An acetal is a molecule with two single bonded oxygens attached to the same carbon atom.Traditional usages distinguish ketal from acetal . Current accepted terminology classifies ketals as a subset of acetals....
     + water
    Water

    Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....


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 molecule....
.

Ketones give hemiketals and ketal
Ketal

A ketal is a functional group or molecule containing a carbon Chemical bond to two -OR groups, where O is oxygen and R represents any alkyl group....
s. 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.

Cyclic hemiacetals and cyclic hemiketals in nature

Hemiacetals and hemiketals are generally unstable compounds. In some cases however, stable cyclic hemiacetals and hemiketals can be readily formed, especially when 5- and 6-membered rings are possible. Glucose
Glucose

Glucose , a monosaccharide also known as grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology....
 and many other aldoses exist as cyclic hemiacetals whereas fructose
Fructose

Fructose is a simple Reducing sugar sugar found in many foods and is one of the three important dietary monosaccharides along with glucose and galactose....
 and similar ketoses exist as cyclic hemiketals.

See also

  • Acetal
    Acetal

    An acetal is a molecule with two single bonded oxygens attached to the same carbon atom.Traditional usages distinguish ketal from acetal . Current accepted terminology classifies ketals as a subset of acetals....
  • Ketal
    Ketal

    A ketal is a functional group or molecule containing a carbon Chemical bond to two -OR groups, where O is oxygen and R represents any alkyl group....