acetal
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ac·e·tal
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acetal
A geminal diether (R1 = H). Ketals, considered a subclass of acetals, are also geminal diethers (R1 = C, aliphatic or aromatic). Acetals are (1) independent structural units or a part of certain biological and commercial polymers, (2) blocking or protecting groups for complex molecules undergoing selective synthetic transformations, and (3) entry compounds for independent organic chemical reactions. See also Polyacetal.
Acetals are easily prepared by the reaction of aldehydes with excess alcohol, under acid-catalyzed conditions. This is usually a two-step process (see reaction below) in which an aldehyde is
treated with an alcohol to yield a less stable hemiacetal, which then reacts with additional alcohol to give the acetal. Protonic or Lewis acids are effective catalysts for acetal formation; dehydrating agents, such as calcium chloride and molecular sieves, can also be used for molecules, such as sugars, where acids may cause problems. Less common acetal preparations are Grignard reagent condensation with orthoformates and mercuric-catalyzed additions of alcohols to acetylenes. See also Aldehyde; Ketone; Organic synthesis.
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any member of a class of organic compounds having the general formula R1HC(OR3)OR4 or R1R2C(OR3)OR4 — in a thioacetal the corresponding formulae are R1HC(SR3)SR4 or R1R2C(SR3)SR4 — where R3 and R4 are alkyl groups (or R4 is H in a hemiacetal or hemithioacetal). An acetal molecule is formed by the acid-catalysed combination of the carbonyl group of an aldehyde or ketone molecule with either one or two alcohol(or mercaptan) molecules (which may be the same or different), or with a diol (or dithiol), by a reaction of the following general type, where X is O (or S in a mercaptan or dithiol):
R1R2C=O + R3XH = R1R2C(XR3)OH.
The hemiacetal (or hemithioacetal) so formed may then undergo a further reaction:
R1R2C(XR3)OH + R3XH = R1R2C(XR3)2 + H2O
or
R1R2C(XR3)OH + R4XH = R1R2C(XR3)XR4 + H2O.
In carbohydrates such compounds are formed at the carbonyl group of the acyclic form of a saccharide or saccharide derivative. The terms 'ketal' (or 'thioketal') and 'hemiketal' (or 'thiohemiketal'), may be applied respectively to any acetal of general formula R1R2C(XR3)XR4 or R1R2C(OH)XR3, i.e. to those derived from ketones. These terms, at one time abandoned, have recently been reintroduced as the respective names of subclasses of acetals and hemiacetals and as functional class names.
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An organic compound formed by a combination of an aldehyde with an alcohol.
Wikipedia on Answers.com:
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Acetal
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It has been suggested that Acetalisation be merged into this article or section. (Discuss) Proposed since February 2011. |
For the plastic, see Polyoxymethylene plastic.
Not to be confused with Acetyl.
An acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.
Traditional usages distinguish ketals from acetals (whereas a ketal has two carbon-bonded R groups and is formally derived from a ketone, an acetal has one or both carbon-bonded R groups as a hydrogen and is formally derived from an aldehyde). Current IUPAC terminology classifies ketals as a subset of acetals.[1]
Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water. The carbocation ion 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 will hydrolyse the product back to the hemiacetal. The formation of acetals reduces the total number of molecules present and therefore is not favourable with regards to entropy. A way to improve this is to use an orthoester as a source of alcohol. Aldehydes and ketones undergo a process called acetal exchange with orthoesters to give acetals. Water produced along with the acetal product is used up in hydrolysing the orthoester and producing more alcohol to be used in the reaction.
Most glycosidic bonds in carbohydrates and other polysaccharides are acetal linkages.[2] Acetaldehyde diethyl acetal is an important flavouring compound in distilled beverages.[3]
The plastic known as acetal is a polyacetal of formaldehyde.
Acetals are used as protecting groups for carbonyl groups in organic synthesis as they are stable with respect to hydrolysis by bases and with respect to many oxidizing and reducing agents.
Examples
- Dimethoxymethane, a solvent
- Dioxolane
- Methylenedioxy group
- Metaldehyde
- Paraldehyde
- 1,3,5-Trioxane
References
- ^ Nic, M.; Jirat, J.; Kosata, B., eds. (2006–). "ketals". IUPAC Compendium of Chemical Terminology (Online ed.). doi:10.1351/goldbook.K03376. ISBN 0-9678550-9-8. http://goldbook.iupac.org/K03376.html.
- ^ Nic, M.; Jirat, J.; Kosata, B., eds. (2006–). "glycosides". IUPAC Compendium of Chemical Terminology (Online ed.). doi:10.1351/goldbook.G02661. ISBN 0-9678550-9-8. http://goldbook.iupac.org/G02661.html.
- ^ Volatile Compounds in Foods and Beverages, ISBN 0-8247-8390-5, http://books.google.com/books?id=_OvXjhLUz-oC, p.554
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Translations:
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Acetal
Nederlands (Dutch)
acetaal (chemische verbinding)
Deutsch (German)
n. - (chem.) Acetal
Ελληνική (Greek)
n. - (χημ.) ακετάλη
Português (Portuguese)
n. - acetal (m) (Quím.)
中文（简体）(Chinese (Simplified))
乙缩醛, 乙缩醛二乙醇
中文（繁體）(Chinese (Traditional))
n. - 乙縮醛, 乙縮醛二乙醇
日本語 (Japanese)
n. - アセタール, アセタール樹脂
العربيه (Arabic)
(الاسم) أسيتال : سائل طيار عديم اللون
עברית (Hebrew)
n. - תרכובת אורגנית שנוצרה מדחיסה של שתי פרודות אלכוהול עם פרודת אלדהיד, אצטאל (תרכובת)
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The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
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McGraw-Hill Science & Technology Encyclopedia
McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.
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Oxford Dictionary of Biochemistry
Oxford University Press. Oxford Dictionary of Biochemistry and Molecular Biology © 1997, 2000, 2006 All rights reserved.
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Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.
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This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article Acetal.
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Mentioned in
acetal resins
(organic chemistry)
dioxolane
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