In organic chemistry, a hemiacetal or a hemiketal has the general formula R1R2C(OH)OR, where R1, R2 is hydrogen or an organic substituent. They generally result from the addition of an alcohol (a compound with at least one −OH group) to an aldehyde (R−CH=O) or a ketone (R2C=O), although the latter are sometimes called hemiketals. Most sugars are hemiacetals.
According to the IUPAC definition, in R1R2C(OH)OR R1 and R2 may or may not be a hydrogen. In a hemiketal, neither R-group can be a hydrogen. Hemiketals are regarded as hemiacetals where none of the R-groups are H, and are therefore a subclass of the hemiacetals.[1] The Greek prefix hèmi means half, refers to the fact that a single alcohol has been added to the carbonyl group, in contrast to acetals or ketals, which are formed when a second alkoxy group has been added to the structure.[2]
Cyclic hemiacetals and hemiketals are sometimes called lactols.[3] They often form readily, especially when they are 5- and 6-membered rings. In this case an intramolecular OH group reacts with the carbonyl group. Glucose and many other aldoses exist as cyclic hemiacetals whereas fructose and similar ketoses exist as cyclic hemiketals.
Formationedit
Formation of hemiacetals
Formation of hemiketals
Solutions of simple aldehydes in alcohols mainly consist of the hemiacetal. The equilibrium is easily reversed and dynamic. The equilibrium is sensitive to steric effects.[4]
Acetalization of aldehydes and ketones
Carbonyl compound
alcohol solvent
%hemiacetal
acetaldehyde
methanol
97
acetaldehyde
ethanol
91
propionaldehyde
methanol
95
bromoacetone
methanol
47
Structures of some readily isolable hemiacetals and hemiketals. Chloral and ethyl glyoxalate illustrate the stabilizing influence of electron-withdrawing groups. The cyclopropanone case illustrates the effect of ring-strain.[5] The two cases on the right illustrate the effect of ring-closure.[4]
Hemiacetals in natureedit
Arguably, the most common hemiacetals are sugars, for example glucose. The favorability of the formation of a strain-free six-membered ring and the electrophilicity of an aldehyde combine to strongly favor the acetal form.
Left, glucose, a cyclic hemiacetal. Right a lactol of fructose, a cyclic hemiketal.
Reactionsedit
Hemiacetals and hemiketals may be thought of as intermediates in the reaction between alcohols and aldehydes or ketones, with the final product being an acetal or a ketal:
R2C=O + R'OH ⇌ R2C(OH)(OR')
R2C(OH)(OR') + R'OH ⇌ R2C(OR')2 + H2O
Usually, the second reaction is unfavorable. In the presence of a dehydrating agent, it proceeds.
^ abSchmitz, Ernst; Eichhorn, Inge (1967). "Acetals and Hemiacetals". The Ether Linkage (PATAI'S Chemistry of Functional Groups). pp. 309–351. doi:10.1002/9780470771075.ch7. ISBN9780470771075.
^Salaun, Jacques (1983). "Cyclopropanone Hemiacetals". Chemical Reviews. 83 (6): 619–632. doi:10.1021/cr00058a002.
February 15, 2024
hemiacetal, organic, chemistry, hemiacetal, hemiketal, general, formula, r1r2c, where, hydrogen, organic, substituent, they, generally, result, from, addition, alcohol, compound, with, least, group, aldehyde, ketone, although, latter, sometimes, called, hemike. In organic chemistry a hemiacetal or a hemiketal has the general formula R1R2C OH OR where R1 R2 is hydrogen or an organic substituent They generally result from the addition of an alcohol a compound with at least one OH group to an aldehyde R CH O or a ketone R2C O although the latter are sometimes called hemiketals Most sugars are hemiacetals Contents 1 Nomenclature 2 Formation 2 1 Hemiacetals in nature 3 Reactions 4 ReferencesNomenclature editAccording to the IUPAC definition in R1R2C OH OR R1 and R2 may or may not be a hydrogen In a hemiketal neither R group can be a hydrogen Hemiketals are regarded as hemiacetals where none of the R groups are H and are therefore a subclass of the hemiacetals 1 The Greek prefix hemi means half refers to the fact that a single alcohol has been added to the carbonyl group in contrast to acetals or ketals which are formed when a second alkoxy group has been added to the structure 2 Cyclic hemiacetals and hemiketals are sometimes called lactols 3 They often form readily especially when they are 5 and 6 membered rings In this case an intramolecular OH group reacts with the carbonyl group Glucose and many other aldoses exist as cyclic hemiacetals whereas fructose and similar ketoses exist as cyclic hemiketals Formation edit nbsp Formation of hemiacetals nbsp Formation of hemiketalsSolutions of simple aldehydes in alcohols mainly consist of the hemiacetal The equilibrium is easily reversed and dynamic The equilibrium is sensitive to steric effects 4 Acetalization of aldehydes and ketones Carbonyl compound alcohol solvent hemiacetalacetaldehyde methanol 97acetaldehyde ethanol 91propionaldehyde methanol 95bromoacetone methanol 47 nbsp Structures of some readily isolable hemiacetals and hemiketals Chloral and ethyl glyoxalate illustrate the stabilizing influence of electron withdrawing groups The cyclopropanone case illustrates the effect of ring strain 5 The two cases on the right illustrate the effect of ring closure 4 Hemiacetals in nature edit Arguably the most common hemiacetals are sugars for example glucose The favorability of the formation of a strain free six membered ring and the electrophilicity of an aldehyde combine to strongly favor the acetal form nbsp nbsp Left glucose a cyclic hemiacetal Right a lactol of fructose a cyclic hemiketal Reactions editHemiacetals and hemiketals may be thought of as intermediates in the reaction between alcohols and aldehydes or ketones with the final product being an acetal or a ketal R2C O R OH R2C OH OR R2C OH OR R OH R2C OR 2 H2OUsually the second reaction is unfavorable In the presence of a dehydrating agent it proceeds References edit IUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected version 2006 hemiketals doi 10 1351 goldbook H02776 Fox Marye Anne Whitesell James K 2004 Organic Chemistry Jones amp Bartlett Learning p 590 ISBN 9780763721978 IUPAC Gold Book lactols a b Schmitz Ernst Eichhorn Inge 1967 Acetals and Hemiacetals The Ether Linkage PATAI S Chemistry of Functional Groups pp 309 351 doi 10 1002 9780470771075 ch7 ISBN 9780470771075 Salaun Jacques 1983 Cyclopropanone Hemiacetals Chemical Reviews 83 6 619 632 doi 10 1021 cr00058a002 Retrieved from https en wikipedia org w index php title Hemiacetal amp oldid 1181885822, wikipedia, wiki, book, books, library,
