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Topic: Hemiacetal

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  Hemiacetal - Wikipedia, the free encyclopedia
A hemiacetal is a functional group or compound containing the function group in the form of:
A hemiacetal can react with an alcohol under very acidic conditions to form an acetal, and can dissociate to form an aldehyde and an alcohol.
Conversion of hemiacetal to acetal is an SN1 reaction.
en.wikipedia.org /wiki/Hemiacetal   (242 words)

 Di- and Polysaccharides
There is another important difference between the hemiacetal and acetal linkages in sugars and saccharides, and that is their reaction with mild oxidizing agents.
The outcome is that in a reducing sugar the anomeric carbon is in an aldehyde or hemiacetal.
For this purpose, the hemiacetal includes the anomeric carbon of a monosaccharide and the alcohol role is played by a specific OH group of a second monosaccharide.
chemistry2.csudh.edu /rpendarvis/dipolysacch.html   (1325 words)

 Hemiacetal -- Facts, Info, and Encyclopedia article   (Site not responding. Last check: 2007-11-05)
A hemiacetal is a (Click link for more info and facts about functional group) functional group or compound containing the function group in the form of:
A hemiacetal can react with an (A liquor or brew containing alcohol as the active agent) alcohol under very acidic conditions
Coversion of hemiacetal to acetal is an SN1 reaction.
www.absoluteastronomy.com /encyclopedia/h/he/hemiacetal.htm   (283 words)

 additionto CO
One of the OH group is suitably positioned near the aldehyde group to form a hemiacetal.
A hemiacetal or a hemiketal can be further reacted with alcohols to convert them to acetals and ketals.
The net structural change that occurs during this reaction is theat the OH group in a hemiacetal or a hemiketal is now replaced with a OR group.
www.wiu.edu /users/mftkv/Chemistry102/addtocarbonyl.html   (535 words)

 Carbos   (Site not responding. Last check: 2007-11-05)
Since hemiacetals form and come apart readily even in neutral solution, there will be an equilibration leading to a mixture of the a and b isomers.
Note that lactose has a free hemiacetal at one end of the b (1® 4) disaccharide and is thus a reducing sugar.
If the hemiacetal is first oxidized to an acid, these problems will disappear, because acids or esters are less prone to base catalyzed reactions at the a-carbon than aldehydes.
instruct.uwo.ca /chemistry/223b-98/carbos.htm   (4143 words)

 Process for the preparation of glyceraldehyde and derivatives thereof - Patent 20010025124
The process of claim 3 wherein the hemiacetal of glyceraldehyde is subjected to a reductive treatment in the presence of ammonia or a primary or secondary amine in order to obtain a 3-aminopropane-1,2-diol derivative.
The process is characterized in that 3-butene-1,2-diol is dissolved in a lower alkanol and subjected to ozonolysis to obtain a 3-hydroperoxy-3-alkoxy-propane-1,2-diol, which is subjected to a reductive treatment to obtain a hemiacetal of glyceraldehyde, which optionally may be converted into glyceraldehyde, or an acetal or a hemiacetal thereof.
The reduction of the formed hydroperoxide and the reductive alkylation of the methoxy hemiacetal of glyceraldehyde were performed at ambient temperature without further cooling until 300 ml of the solution had been converted.
www.freepatentsonline.com /20010025124.html   (2313 words)

 Chemistry : Chapter 12 : Reactions of Sugars   (Site not responding. Last check: 2007-11-05)
The alcohol groups in the middle should remain on the same side of the carbon as in the linear version of the structure.
The alcohol part of the hemiacetal (in the ring form of the sugar) reacts with an alcohol in another sugar (also in the ring form).
Both sugars have a hemiacetal group and that group must participate in the polymerization.
www.wwnorton.com /chemistry/concepts/chapter12/ch12_5.htm   (307 words)

 Hemiacetal and hemiketal forms (from carbohydrate) --  Britannica Student Encyclopedia   (Site not responding. Last check: 2007-11-05)
Although optical rotation has been one of the most frequently determined characteristics of carbohydrates since its recognition in the late 19th century, the rotational behaviour of freshly prepared solutions of many sugars differs from that of solutions that have been allowed to stand.
More results on "Hemiacetal and hemiketal forms (from carbohydrate)" when you join.
More from Britannica on "Hemiacetal and hemiketal forms (from carbohydrate)"...
www.britannica.com /ebi/article-72609   (827 words)

 Carbonyl Reactivity
Most hydrates and hemiacetals (Y = OH & OR), for example, are known to decompose spontaneously to the corresponding carbonyl compounds.
Following acid-catalyzed hydrolysis of the acetal, the resulting 4-hydroxyaldehyde is in equilibrium with its cyclic hemiacetal.
The ylides are another class of nucleophilic organic reagents that add rapidly to the carbonyl function of aldehydes and ketones.
www.cem.msu.edu /%7Ereusch/VirtualText/aldket1.htm   (4359 words)

 ARS | Publication request: The Aspergillus Parasiticus Esta-Encoded Esterase Converts Versiconal Hemiacetal Acetate to ...   (Site not responding. Last check: 2007-11-05)
We show that this enzyme is necessary for the conversion of the aflatoxin precursors versiconal hemiacetal acetate to versiconal and versiconal acetate to versiconol, respectively.
Further work may identify cultural practices or resistant corn or cottonseed that could reduce aflatoxin contamination of crops in the field by interfering with the action of the esterase.
Technical Abstract: The route for the conversion of 1-hydroxyversicolorone to versiconal hemiacetal acetate (VHA) to versiconal (VHOH) in the aflatoxin biosynthesis involves two metabolic grids.
ars.usda.gov /research/publications/publications.htm?SEQ_NO_115=150434   (467 words)

 [No title]   (Site not responding. Last check: 2007-11-05)
4.9 Reaction of Aldehydes and Ketones with Alcohols Hemiacetal Formation Aldehyde or ketone + alcohol (hemiacetal Hemiacetal: an organic compound in which a C atom is single-bonded to a —OH group and an —OR group.
Chemical reaction equation in a general form: Examples: Cyclic hemiacetal can be formed from a molecule with both an aldehyde group (-CHO) and an alcohol group (-OH).
Acetal Formation A hemiacetal may react with a second alcohol molecule to form an acetal.
www2.hawaii.edu /~yingbin/chem152/chapter4c.doc   (270 words)

 Carbohydrate - Wikipedia, the free encyclopedia
The straight-chain structure described here is only one of the forms a monosaccharide may take.
The aldehyde or ketone group may react with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, in which case there is an oxygen bridge between the two carbon atoms, forming a heterocyclic ring.
Rings with five and six atoms are called furanose and pyranose forms and exist in equilibrium with the straight-chain form.
www.wikipedia.org /wiki/Carbohydrate   (844 words)

 [No title]   (Site not responding. Last check: 2007-11-05)
It is assumed that C-2 "used to be" a D-hydroxyl before becoming a KETO group.
SUGAR ALCOHOLS: Delete "-OSE" and add "ITOL" Example: D-Ribitol, found in Riboflavin, is a reduced form of D-Ribose HEMIACETAL has hydroxyl and ether on the same carbon: Acetal has two "ethers." Hemiacetals are easy to hydrolyze, acetal is harder.
a hemiacetal or hemiketal, is a "reducing" sugar.
www.rci.rutgers.edu /~molbio/stryer/sugaruls.txt   (280 words)

 Hemiacetal - Encyclopedia Glossary Meaning Explanation Hemiacetal   (Site not responding. Last check: 2007-11-05)
Here you will find more informations about Hemiacetal.
If you find this encyclopedia or its sister projects useful,
A hemiacetal can react with an alcohol under very acidic conditions
www.encyclopedia-glossary.com /en/Hemiacetal.html   (256 words)

 Carbohydrates - Glucose   (Site not responding. Last check: 2007-11-05)
Due to the tetrahedral geometry of carbons that ultimately make a 6 membered stable ring, the -OH on carbon #5 is converted into the ether linkage to close the ring with carbon #1.
The chair structures are always written with the orientation depicted on the left to avoid confusion.
A carbon that has both an ether oxygen and an alcohol group is a hemiacetal.
www.elmhurst.edu /~chm/vchembook/543glucose.html   (575 words)

 Hemiacetals, Acetals and the Aldol Reaction
Alcohols react with the carbonyl groups of aldehydes and ketones to produce compounds called hemiacetals.
Let's look first at the first part of the overall reaction, the formation of the hemiacetal.
"How do we get from the hemiacetal to the acetal?" We have to lose an OH Since we know how to put an alcohol on a carbocation (making the hemiacetal), perhaps the way to lose a water is to just reverse the reaction that puts an ROH on a carbocation.
dwb.unl.edu /Teacher/NSF/C10/C10Links/www.chem.wsu.edu/chem102/102-AcetalAldol.html   (741 words)

 Glycosidic bond - Chicago Voyager : Travel & Tourism Guides : Information Portal   (Site not responding. Last check: 2007-11-05)
A glycosidic bond is formed between a hemiacetal group of a sugar molecule (such as glucose) and an alcohol functional group to form an acetal.
A N-glycosidic bond is formed between the hemiacetal group of a sugar molecule (such as ribose) and a N-H group on an aromatic ring.
An example of this is the bond between a nitrogenous base and the deoxyribose molecule in DNA.
chicagovoyager.com /info/gl/Glycosidic_bond.html   (122 words)

 Acetal Mechanism
The one we predicted is called a hemiacetal.) Since the hemiacetal seems to be about halfway to the acetal, we'll explore converting the hemiacetal into the acetal.
There are a couple of general ideas we can extract from what we did in working out this mechanism.
Intermediates: The aldehyde, the methanol, the hemiacetal, and the acetal are all stable molecules.
chemistry2.csudh.edu /rpendarvis/hemiacetal.html   (1610 words)

 dec2.html   (Site not responding. Last check: 2007-11-05)
One of the reactions we left out the other day was the one between an hemiacetal or hemiketal with an alcohol.
This compound will be an acetal if the alcohol reacted with an hemiacetal, or a ketal if the aclohol reacted with an hemiketal.
Unless we used the hydroxyl from the hemiacetal or hemiketal of the other sugar (that is, the OH in C1') to form the glycosidic bond, the other sugar still has the possibility of opening into its linear form (hydrolysis of the hemiacetal/hemiketal):
tonga.usip.edu /gmoyna/biochem341/lecture34.html   (2729 words)

 Preamble, 2-Carb-0 & 2-Carb-1
Cyclic hemiacetals or hemiketals of sugars with a five-membered (tetrahydrofuran) ring are called furanoses, those with a six-membered (tetrahydropyran) ring pyranoses.
The polyhydric alcohols arising formally from the replacement of a carbonyl group in a monosaccharide with a CHOH group are termed alditols (see 2-Carb-19).
Glycosides are mixed acetals formally arising by elimination of water between the hemiacetal or hemiketal hydroxy group of a sugar and a hydroxy group of a second compound.
www.chem.qmul.ac.uk /iupac/2carb/00n01.html   (2074 words)

 Orgo 3D Web : Chapter 21 : Tetrahydropyran-2-ol, a cyclic hemiacetal   (Site not responding. Last check: 2007-11-05)
Here is the product of intramolecular hemiacetal formation of 5-hydroxypentanal.
Note that the hemiacetal form is the more stable one.
The OH leans slightly toward the ring oxygen.
www.wwnorton.com /orgo3dweb/ch21/2_tetrahydropyranol.htm   (39 words)

 Isolation and Characterization of the Versicolorin B Synthase Gene from Aspergillus parasiticus. EXPANSION OF THE ...
O label (*) from 6, it was shown that heavy isotope was retained in 7 without loss in the critical cyclization to versicolorin B (8) (27, 28).
The cyclization of versiconal hemiacetal (7) can be carried out nonenzymatically in the presence of acid to yield versicolorin B (8) as its racemate, which is designated historically as versicolorin C (32, 33).
In this connection it is noteworthy that homogeneous VBS does not contain a bound flavin chromophore and preliminary kinetic evidence suggests that FAD, FMN, and glucose have little or no inhibitory effect on the cyclization of versiconal (7) to versicolorin B (8) (35, 80).
www.jbc.org /cgi/content/full/271/23/13600   (7211 words)

 ARS | Publication request: Conversion of Versiconal Hemiacetal Acetate to Versiconal and Versiconol Acetate to ...   (Site not responding. Last check: 2007-11-05)
Publication request: Conversion of Versiconal Hemiacetal Acetate to Versiconal and Versiconol Acetate to Versiconol by Aspergillus Parasiticus Esta-Encoded Esterase During Aflatoxin Biosynthesis
Technical Abstract: In aflatoxin biosynthesis, the pathway for the conversion of 1-hydroxyversicolorone to versiconal hemiacetal acetate (VHA) to versiconal (VHOH) involves two metabolic grids.
In the grids, the steps from VHA to VHOH and versiconol acetate (VOAc) to versiconol (VOH) had been proposed to be catalyzed by the same esterase.
ars.usda.gov /research/publications/publications.htm?SEQ_NO_115=158019   (356 words)

 [No title]
They are joined by single oxygen atoms (acetal linkages) between the C-1 of one pyranose ring and the C-4 of the next ring.
Since a molecule of water is lost when an alcohol and a hemiacetal react to form an acetal, the glucose units in the cellulose polymer are referred to as anhydroglucose units.
Cellulose hydroxyls can also be made to add across activated double bonds, as in the formation of cyanoethyl cellulose from reaction with acrylonitrile.
www.fibersource.com /f-tutor/cellulose.htm   (1994 words)

 Correlation of Low-Barrier Hydrogen Bonding and Oxyanion Binding in Transition State Analogue Complexes of ...
The structure is similar to the AcLF-CF adduct, except for the presence of two epimeric adducts in the R- and S-configurations at the hemiacetal carbons.
The protonation states of the hemiacetal and His 57 are explained by the high basicity of the hemiacetal oxygen (pK
CHO with chymotrypsin is consistent with a neutral hemiacetal between pH 7 and 13.
pubs.acs.org /cgi-bin/jtext?bichaw/40/i08/abs/bi002535a   (427 words)

 Organic Reactions
Hemiacetals can be recognized by looking for a carbon atom that has both an
Hemiacetals and acetals play an important role in the chemistry of carbohydrates.
In this case, a hemiacetal is formed that contains a five-membered furanose ring.
chemed.chem.purdue.edu /genchem/topicreview/bp/3organic/organic.html   (3490 words)

 112999 - Quiz - Organic Chemistry 354 - November 29, 1999 - Dr. Sundin - UWP   (Site not responding. Last check: 2007-11-05)
Monosaccharides exist in the solid state as cyclic hemiacetals and in water solution as an equilibrium mixture of cyclic hemiacetals and the open chain free aldehyde or ketone.
Which hemiacetal, A or B, is the hemiacetal of a ketose?
At first glance, one is surprised to find that ketoses can be reducing sugars just like aldoses.
www.uwplatt.edu /~sundin/354-9/5411299q.htm   (185 words)

 Di and polysaccharides   (Site not responding. Last check: 2007-11-05)
The left ring is a locked hemiacetal, but the other ring can open.
Galactose is part of nerve and brain biochemicals, so milk is essential to infants.
One of its OH groups from the formation of a hemiacetal is still intact, so it can exist with that sugar in an open chain or as a ring.
www.rpi.edu /dept/chem-eng/Biotech-Environ/FUNDAMNT/sugpoly.htm   (155 words)

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