Acetal: Difference between revisions
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{{Short description|Organic compound with the structure >C(O–)2}} |
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{{For|the engineering plastic|polyoxymethylene}} |
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{{for multi|the engineering plastic|Polyoxymethylene|the flavor compound|1,1-Diethoxyethane{{!}}1,1-diethoxyethane}} |
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{{distinguish|acetyl}} |
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[[File:Acetal general structure.svg|thumb|Generic structure of acetals]] |
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{{Technical|date=September 2019}} |
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[[File:Acetal_allgemein2.svg|upright|thumb|Structure of a generic acetal]] |
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An '''acetal''' is a [[functional group]] with the connectivity R<sup>1</sup>R<sup>2</sup>C(OR')(OR{{'}}{{'}}). Here, R<sup>1</sup> and R<sup>2</sup> are organic fragments or hydrogen, while R' and R{{}}{{'}} can only be organic fragments. (If one of R' or R{{'}}{{'}} is H, the functional group is known as a ''[[hemiacetal]]'', while if both are H, the functional group is a ''ketone or aldehyde [[hydrate]]''.) The central carbon atom has four bonds to it, and is therefore [[saturation (chemistry)|saturated]] and has [[tetrahedral geometry]]. The R' and R{{'}}{{'}} groups can be equivalent to each other (a "symmetric acetal") or not (a "mixed acetal"). Acetals are formed from and convertible to [[aldehyde]]s or [[ketone]]s. The term '''ketal''' is sometimes used to identify structures associated with ketones (R<sup>1</sup>, R<sup>2</sup> ≠ H) rather than aldehydes and, historically, the term '''acetal''' was used specifically for the aldehyde cases (R<sup>1</sup> or R<sup>2</sup> = H; the term '''formal''' was used for derivatives of formaldehyde, R<sup>1</sup> = R<sup>2</sup> = H).<ref>{{GoldBookRef|title=ketals|file=K03376}}</ref> The IUPAC originally deprecated the usage of the word ketal but has since reversed its decision. However, in contrast to historical usage, ketals are now a ''subset'' of acetals. |
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In [[organic chemistry]], an '''acetal''' is a [[functional group]] with the connectivity {{chem2|R2C(OR')2}}. Here, the R groups can be organic fragments (a [[carbon]] atom, with arbitrary other atoms attached to that) or [[hydrogen]], while the R' groups must be organic fragments not hydrogen. The two R' groups can be equivalent to each other (a "symmetric acetal") or not (a "mixed acetal"). Acetals are formed from and convertible to [[aldehyde]]s or [[ketone]]s and have the same [[oxidation state]] at the central carbon, but have substantially different [[chemical stability]] and [[Reactivity (chemistry)|reactivity]] as compared to the analogous [[carbonyl]] compounds. The central carbon atom has four bonds to it, and is therefore [[Saturated and unsaturated compounds|saturated]] and has [[tetrahedral geometry]]. |
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The term '''ketal''' is sometimes used to identify structures associated with [[ketone]]s (both R groups organic fragments rather than hydrogen) rather than [[aldehyde]]s and, historically, the term '''acetal''' was used specifically for the aldehyde-related cases (having at least one hydrogen in place of an R on the central carbon).<ref>{{GoldBookRef|title=ketals|file=K03376}}</ref> The IUPAC originally deprecated the usage of the word ketal altogether, but has since reversed its decision. However, in contrast to historical usage, ketals are now a subset of acetals, a term that now encompasses both aldehyde- and ketone-derived structures. |
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If one of the R groups has an oxygen as the first atom (that is, there are more than two oxygens single-bonded to the central carbon), the functional group is instead an [[orthoester]]. In contrast to variations of R, both R' groups are organic fragments. If one R' is a hydrogen, the functional group is instead a [[hemiacetal]], while if both are H, the functional group is a ketone [[hydrate]] or aldehyde hydrate. |
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| ⚫ | Formation of an acetal occurs when the [[hydroxyl]] group of a [[hemiacetal]] becomes [[protonation|protonated]] and is lost as water. The [[carbocation]] that is produced is then rapidly attacked by a molecule of [[Alcohol (chemistry)|alcohol]]. Loss of the proton from the attached alcohol gives the acetal. |
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Acetals are stable compared to hemiacetals but their formation is a reversible [[chemical equilibrium|equilibrium]] as with [[ester]]s. As a reaction to create an acetal proceeds, water must be removed from the reaction mixture, for example, with a [[ |
Acetals are stable compared to hemiacetals but their formation is a reversible [[chemical equilibrium|equilibrium]] as with [[ester]]s. As a reaction to create an acetal proceeds, water must be removed from the reaction mixture, for example, with a [[Dean–Stark apparatus]], lest it [[hydrolyse]] the product back to the hemiacetal. The formation of acetals reduces the total number of molecules present (carbonyl + 2 alcohol → acetal + water) and therefore is generally not favourable with regards to [[entropy]]. One situation where it is not entropically unfavourable is when a single [[diol]] molecule is used rather than two separate alcohol molecules (carbonyl + diol → acetal + water). |
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==Acetalisation and ketalization== |
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Acetals are used as [[protecting group]]s for [[carbonyl]] groups in organic synthesis because they are stable with respect to hydrolysis by [[base (chemistry)|bases]] and with respect to many oxidizing and reducing agents. They can either protect the carbonyl in a molecule (by temporarily reacting it with an alcohol) or a diol (by temporarily reacting it with a carbonyl). That is, either the carbonyl, or the alcohols, or both could be part of the molecule whose reactivity is to be controlled. |
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{{anchor|Acetalisation}} |
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Acetalisation and ketalization are the [[organic reaction]]s that involve the formation of an acetal (or ketals) from aldehydes and ketones, respectively. These conversions are [[acid]] [[catalysis|catalysed]]. They eliminate water. Since each step is often a rapid equilibrium, the reaction must be driven by removal of water. Methods for removing water include [[azeotropic distillation]] and trapping water with desiccants like [[aluminium oxide]] and [[molecular sieve]]s. Steps assumed to be involved: protonation of the carbonyl oxygen, addition of the alcohol to the protonated carbonyl, protonolysis of the resulting [[hemiacetal]] or hemiketal, and addition of the second alcohol. These steps are illustrated with an aldehyde RCH=O and the alcohol R'OH: |
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:{{chem2| RCH\dO + H+ <-> RCH\dOH+ }} |
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:{{chem2|RCH\dOH+ + R'OH <-> RCH(OH)(OR') + H+}} |
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:{{chem2| RCH(OH)(OR') + H+ <-> RC+H(OR') + H2O}} |
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:{{chem2|RC+H(OR') + R'OH <-> RCH(OR')2 + H+}} |
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Another way to avoid the entropic cost is to perform the synthesis by acetal exchange, using a pre-existing acetal-type reagent as the OR'-group donor rather than simple addition of alcohols themselves. One type of reagent used for this method is an orthoester. In this case, water produced along with the acetal product is destroyed when it hydrolyses residual orthoester molecules, and this [[side reaction]] also produces more alcohol to be used in the main reaction. |
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Various specific carbonyl compounds have special names for their acetal forms. For example, an acetal formed from [[formaldehyde]] is sometimes called a "formal"<ref>Morrison, Robert T. and Boyd, Robert N., "Organic Chemistry (6th ed)". p683. Prentice-Hall Inc (1992).</ref> or the [[methylenedioxy]] group. The acetal formed from [[acetone]] is sometimes called an [[acetonide]]. |
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==Examples== |
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===Sugars=== |
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Acetalisation is the [[organic reaction]] that involves the formation of an acetal (or ketals). One way of acetal formation is the [[nucleophilic addition]] of an alcohol to a ketone or an aldehyde. Acetalisation is often used in organic synthesis to create a protecting group because it is a reversible reaction. |
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| ⚫ | Since many sugars are polyhydroxy aldehydes and ketones, sugars are a rich source of acetals and ketals. Most [[glycosidic bond]]s in [[carbohydrate]]s and other [[polysaccharide]]s are acetal linkages.<ref>{{GoldBookRef|title=glycosides|file=G02661}}</ref> [[Cellulose]] is a ubiquitous example of a polyacetal. |
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[[Benzylidene acetal]] and [[acetonide]] as protecting groups used in research of modified sugars. |
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Acetalisation is [[acid]] [[catalysis|catalysed]] with elimination of water; acetals do ''not'' form under [[base (chemistry)|basic]] conditions. The reaction can be driven to the acetal when water is removed from the reaction system either by [[azeotropic distillation]] or trapping water with [[molecular sieve]]s or [[aluminium oxide]]. |
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===Chiral derivatives=== |
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The carbonyl group in '''1''' takes a proton from [[hydronium]]. The protonated carbonyl group 2 is activated for nucleophilic addition of the alcohol. The structures '''2a''' and '''2b''' are [[mesomer]]s. After [[deprotonation]] of '''3''' by water the [[hemiacetal]] or hemiketal '''4''' is formed. The hydroxyl group in '''4''' is protonated leading to the [[oxonium ion]] '''6''' which accepts a second alcohol group to '''7''' with a final deprotonation to the acetal '''8'''. The reverse reaction takes place by adding water in the same acidic medium. Acetals are stable towards basic media. In a transacetalisation or crossacetalisation a diol reacts with an acetal or two different acetals react with each other. Again this is possible because all the reaction steps are equilibria. |
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Acetals also find application as [[Chirality (chem)|chiral]] auxiliaries. Indeed acetals of chiral glycols like, e.g. derivatives of tartaric acid can be asymmetrically opened with high selectivity. This enables the construction of new chiral centers.<ref>P.J. Kocieński: ''Protecting Groups'', S. 164–167.</ref> |
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[[File:Acetalisation.png|thumb|center|500px|Acetalisation mechanism]] |
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[[File:Lardolure_is.svg|center|500x500px]] |
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===Formaldehyde and acetaldehyde=== |
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==Examples== |
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Formaldehyde forms a rich collection of acetals. This tendency reflects the fact that low molecular weight aldehydes are prone to self-condensation such that the C=O bond is replaced by an acetal. The acetal formed from [[formaldehyde]] (two hydrogens attached to the central carbon) is sometimes called a ''formal''<ref>Morrison, Robert T. and Boyd, Robert N., "Organic Chemistry (6th ed)". p683. Prentice-Hall Inc (1992).</ref> or the [[methylenedioxy]] group. The acetal formed from [[acetone]] is sometimes called an [[acetonide]]. Formaldehyde forms [[Paraldehyde]] and [[1,3,5-Trioxane]]. [[Polyoxymethylene]] (POM) plastic, also known as "acetal" or "polyacetal", is a polyacetal (and a polyether), and a polymer of [[formaldehyde]]. [[Acetaldehyde]] converts to [[Metaldehyde]]. |
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*[[benzylidene acetal]], a protecting group |
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*[[Dimethoxymethane]], a solvent, a.k.a. methylal, a.k.a. formal [ambiguous] |
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===Unusual acetals=== |
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*[[Dioxolane]] |
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| ⚫ | [[Phenylsulfonylethylidene]] (PSE) acetal is an example of arylsulfonyl acetal possessing atypical properties, like resistance to acid hydrolysis which leads to selective introduction and removal of the protective group.<ref name=":0">{{Cite journal|last=Chéry|first=Florence|last2=Rollin|first2=Patrick|last3=De Lucchi|first3=Ottorino|last4=Cossu|first4=Sergio|date=2000|title=Phenylsulfonylethylidene (PSE) acetals as atypical carbohydrate-protective groups|journal=Tetrahedron Letters|volume=41|issue=14|pages=2357–2360|doi=10.1016/s0040-4039(00)00199-4|issn=0040-4039}}</ref> |
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*[[Metaldehyde]] |
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*[[Paraldehyde]] |
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===Flavors and fragrances=== |
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*[[1,3,5-Trioxane]] |
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[[1,1-Diethoxyethane]] (acetaldehyde diethyl acetal), sometimes called simply "acetal", is an important flavouring compound in [[distilled beverage]]s.<ref>{{Cite book |last=Maarse |first=Henk |url=https://books.google.com/books?id=_OvXjhLUz-oC |title=Volatile Compounds in Foods and Beverages |date=1991-03-29 |publisher=CRC Press |isbn=978-0-8247-8390-7 |language=en}}</ref> Two ketals of ethyl acetoacetate are used in commercial fragrances.<ref>{{cite book |doi=10.1002/14356007.t11_t02 |chapter=Flavors and Fragrances, 3. Aromatic and Heterocyclic Compounds |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2016 |last1=Panten |first1=Johannes |last2=Surburg |first2=Horst |pages=1–45 |isbn=978-3-527-30673-2 }}</ref> [[Fructone]] ({{chem2|CH3C(O2C2H4)CH2CO2C2H5}}), an ethylene glycol ketal, and fraistone ({{chem2|CH3C(O2C2H3CH3)CH2CO2C2H5}}), an propylene glycol ketal, a commercial fragrances. |
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==Related compounds== |
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**[[Cellulose]] is a ubiquitous example of a polyacetal. |
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Used in a more general sense, the term '''X''','''Y'''-''acetal'' also refers to any functional group that consists of a carbon bearing two heteroatoms '''X''' and '''Y'''. For example, ''N'',''O''-acetal refers to compounds of type R<sup>1</sup>R<sup>2</sup>C(OR)(NR'<sub>2</sub>) (R,R' ≠ H) also known as a ''hemiaminal ether'' or [[Aminal]], a.k.a. aminoacetal. |
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Although many compounds contain an acetal functional group, at least two acetal compounds are called "acetal" for short: |
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*[[Polyoxymethylene]] (POM) plastic, also known as "acetal" or "polyacetal", is a polyacetal (and a polyether), and a polymer of [[formaldehyde]]. |
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''S'',''S''-acetal refers to compounds of type R<sup>1</sup>R<sup>2</sup>C(SR)(SR') (R,R' ≠ H, also known as [[thioacetal]] and [[thioketal]]s. |
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*[[1,1-Diethoxyethane]] (acetaldehyde diethyl acetal), sometimes called simply "acetal", is an important flavouring compound in [[distilled beverage]]s.<ref>Volatile Compounds in Foods and Beverages, {{ISBN|0-8247-8390-5}}, https://books.google.com/books?id=_OvXjhLUz-oC, p.554</ref> |
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==See also== |
==See also== |
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*[[Aminal]], a.k.a. aminoacetal |
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*[[Hemiaminal]] |
*[[Hemiaminal]] |
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*[[Orthoformate]] |
*[[Orthoformate]] |
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*[[Thioacetal]] |
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*[[Thioketal]] |
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==References== |
==References== |
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<references/> |
<references/> |
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{{Functional groups}} |
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{{Authority control}} |
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[[Category:Acetals| ]] |
[[Category:Acetals| ]] |
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Latest revision as of 09:22, 18 August 2024
In organic chemistry, an acetal is a functional group with the connectivity R2C(OR')2. Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragments not hydrogen. The two R' groups can be equivalent to each other (a "symmetric acetal") or not (a "mixed acetal"). Acetals are formed from and convertible to aldehydes oder ketones and have the same oxidation state at the central carbon, but have substantially different chemical stability and reactivity as compared to the analogous carbonyl compounds. The central carbon atom has four bonds to it, and is therefore saturated and has tetrahedral geometry.
The term ketal is sometimes used to identify structures associated with ketones (both R groups organic fragments rather than hydrogen) rather than aldehydes and, historically, the term acetal was used specifically for the aldehyde-related cases (having at least one hydrogen in place of an R on the central carbon).[1] The IUPAC originally deprecated the usage of the word ketal altogether, but has since reversed its decision. However, in contrast to historical usage, ketals are now a subset of acetals, a term that now encompasses both aldehyde- and ketone-derived structures.
If one of the R groups has an oxygen as the first atom (that is, there are more than two oxygens single-bonded to the central carbon), the functional group is instead an orthoester. In contrast to variations of R, both R' groups are organic fragments. If one R' is a hydrogen, the functional group is instead a hemiacetal, while if both are H, the functional group is a ketone hydrate or aldehyde hydrate.
Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water. The carbocation that is produced is then rapidly attacked by a molecule of alcohol. Loss of the proton from the attached alcohol gives the acetal.
Acetals are stable compared to hemiacetals but their formation is a reversible equilibrium as with esters. As a reaction to create an acetal proceeds, water must be removed from the reaction mixture, for example, with a Dean–Stark apparatus, lest it hydrolyse the product back to the hemiacetal. The formation of acetals reduces the total number of molecules present (carbonyl + 2 alcohol → acetal + water) and therefore is generally not favourable with regards to entropy. One situation where it is not entropically unfavourable is when a single diol molecule is used rather than two separate alcohol molecules (carbonyl + diol → acetal + water).
Acetalisation and ketalization
[edit]Acetalisation and ketalization are the organic reactions that involve the formation of an acetal (or ketals) from aldehydes and ketones, respectively. These conversions are acid catalysed. They eliminate water. Since each step is often a rapid equilibrium, the reaction must be driven by removal of water. Methods for removing water include azeotropic distillation and trapping water with desiccants like aluminium oxide and molecular sieves. Steps assumed to be involved: protonation of the carbonyl oxygen, addition of the alcohol to the protonated carbonyl, protonolysis of the resulting hemiacetal or hemiketal, and addition of the second alcohol. These steps are illustrated with an aldehyde RCH=O and the alcohol R'OH:
- RCH=O + H+ ⇌ RCH=OH+
- RCH=OH+ + R'OH ⇌ RCH(OH)(OR') + H+
- RCH(OH)(OR') + H+ ⇌ RC+H(OR') + H2O
- RC+H(OR') + R'OH ⇌ RCH(OR')2 + H+
Another way to avoid the entropic cost is to perform the synthesis by acetal exchange, using a pre-existing acetal-type reagent as the OR'-group donor rather than simple addition of alcohols themselves. One type of reagent used for this method is an orthoester. In this case, water produced along with the acetal product is destroyed when it hydrolyses residual orthoester molecules, and this side reaction also produces more alcohol to be used in the main reaction.
Examples
[edit]Sugars
[edit]Since many sugars are polyhydroxy aldehydes and ketones, sugars are a rich source of acetals and ketals. Most glycosidic bonds in carbohydrates and other polysaccharides are acetal linkages.[2] Cellulose is a ubiquitous example of a polyacetal.
Benzylidene acetal and acetonide as protecting groups used in research of modified sugars.
Chiral derivatives
[edit]Acetals also find application as chiral auxiliaries. Indeed acetals of chiral glycols like, e.g. derivatives of tartaric acid can be asymmetrically opened with high selectivity. This enables the construction of new chiral centers.[3]
Formaldehyde and acetaldehyde
[edit]Formaldehyde forms a rich collection of acetals. This tendency reflects the fact that low molecular weight aldehydes are prone to self-condensation such that the C=O bond is replaced by an acetal. The acetal formed from formaldehyde (two hydrogens attached to the central carbon) is sometimes called a formal[4] or the methylenedioxy group. The acetal formed from acetone is sometimes called an acetonide. Formaldehyde forms Paraldehyde and 1,3,5-Trioxane. Polyoxymethylene (POM) plastic, also known as "acetal" or "polyacetal", is a polyacetal (and a polyether), and a polymer of formaldehyde. Acetaldehyde converts to Metaldehyde.
Unusual acetals
[edit]Phenylsulfonylethylidene (PSE) acetal is an example of arylsulfonyl acetal possessing atypical properties, like resistance to acid hydrolysis which leads to selective introduction and removal of the protective group.[5]
Flavors and fragrances
[edit]1,1-Diethoxyethane (acetaldehyde diethyl acetal), sometimes called simply "acetal", is an important flavouring compound in distilled beverages.[6] Two ketals of ethyl acetoacetate are used in commercial fragrances.[7] Fructone (CH3C(O2C2H4)CH2CO2C2H5), an ethylene glycol ketal, and fraistone (CH3C(O2C2H3CH3)CH2CO2C2H5), an propylene glycol ketal, a commercial fragrances.
Related compounds
[edit]Used in a more general sense, the term X,Y-acetal also refers to any functional group that consists of a carbon bearing two heteroatoms X and Y. For example, N,O-acetal refers to compounds of type R1R2C(OR)(NR'2) (R,R' ≠ H) also known as a hemiaminal ether or Aminal, a.k.a. aminoacetal.
S,S-acetal refers to compounds of type R1R2C(SR)(SR') (R,R' ≠ H, also known as thioacetal and thioketals.
See also
[edit]References
[edit]- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "ketals". doi:10.1351/goldbook.K03376
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "glycosides". doi:10.1351/goldbook.G02661
- ^ P.J. Kocieński: Protecting Groups, S. 164–167.
- ^ Morrison, Robert T. and Boyd, Robert N., "Organic Chemistry (6th ed)". p683. Prentice-Hall Inc (1992).
- ^ Chéry, Florence; Rollin, Patrick; De Lucchi, Ottorino; Cossu, Sergio (2000). "Phenylsulfonylethylidene (PSE) acetals as atypical carbohydrate-protective groups". Tetrahedron Letters. 41 (14): 2357–2360. doi:10.1016/s0040-4039(00)00199-4. ISSN 0040-4039.
- ^ Maarse, Henk (1991-03-29). Volatile Compounds in Foods and Beverages. CRC Press. ISBN 978-0-8247-8390-7.
- ^ Panten, Johannes; Surburg, Horst (2016). "Flavors and Fragrances, 3. Aromatic and Heterocyclic Compounds". Ullmann's Encyclopedia of Industrial Chemistry. pp. 1–45. doi:10.1002/14356007.t11_t02. ISBN 978-3-527-30673-2.
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