Editing Acetal

{{Short description|Organic compound with the structure >C(O–)2}}
{{for multi|the engineering plastic|Polyoxymethylene|the flavor compound|1,1-Diethoxyethane{{!}}1,1-diethoxyethane}}
{{distinguish|acetyl|acetaldehyde}}
[[File:Acetal general structure.svg|thumb|Generic structure of acetals]]

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]].

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.

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 [[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.
{{multiple image|caption_align=center
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 | image1            = Acetal formation 2.png
 | caption1          = Aldehyde to acetal conversion
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 | caption2          = Ketone to ketal conversion
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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 [[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==
{{anchor|Acetalisation}}
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:
:{{chem2| RCH\dO  +  H+  <->  RCH\dOH+ }}
:{{chem2|RCH\dOH+  +  R'OH  <->  RCH(OH)(OR')  +  H+}}
:{{chem2| RCH(OH)(OR')  +  H+  <->  RC+H(OR')  +  H2O}}
:{{chem2|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==
===Sugars=== 
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.  

[[Benzylidene acetal]] and [[acetonide]] as protecting groups used in research of modified sugars.

===Chiral derivatives===
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.&nbsp;164–167.</ref>
[[File:Lardolure_is.svg|center|500x500px]]

===Formaldehyde and acetaldehyde===
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]].

===Unusual acetals===
[[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>

===Flavors and fragrances===
[[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}}), a propylene glycol ketal, are commercial fragrances.

==Related compounds==
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.

''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.

==See also==
*[[Hemiaminal]]
*[[Orthoformate]]

==References==
<references/>

{{Functional groups}}
{{Authority control}}

[[Category:Acetals| ]]
[[Category:Functional groups]]
[[Category:Protecting groups]]