Noun
- any of a class of compounds of general formula R2C(OH)OR' (where R' is not H)
Read full definition at wiktionary.org
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Synthesis
In organic synthesis, hemiacetals can be prepared in a number of ways:- Nucleophilic addition of an alcohol to a carbonyl group of an aldehyde
- Nucleophilic addition of an alcohol to a resonance stabilized hemiacetal cation
- Partial hydrolysis of an acetal Read full entry
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- 1.Hemiacetal - Wikipedia, the free encyclopedia
- Skeletal formula of a hemiacetal ... an alcohol to a resonance stabilized hemiacetal cation ... hemiacetal + alcohol (excess) + acid (catalyst) acetal + water ...
- http://en.wikipedia.org/wiki/H
emiacetal
- 2.Hemiacetal: Definition from Answers.com
- hemiacetal ( ¦hemē′asə′tal ) ( organic chemistry ) A class of compounds that have the grouping &uldlbnd;C(OH)(OR) and that result from the
- http://www.answers.com/topic/h
emiacetal
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What is the major difference a. the cyclic hemiacetal is an
alcoho, whereas the cyclic
acetal is an ether.
b. the cyclic hemiacetal is an
acid, and the cyclic acetal is
a base.
c. the cyclic hemiacetal
contains more carbons in the
ring than the cyclic acetal.
d. All of the responses are
correct.
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The term "hemiacetal" is not commonly used any more. Normally a cyclic acetal is formed from a carbonyl compound and a diol (such as ethylene glycol) with removal of water. But if it's a hemiacetal, you only form one ether bond and the other remains an OH group ... so how is it going to be cyclic unless the substrate is cyclic? Your professor needs to write better questions. |
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What is the structure of the |
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I don't know how much experience you have with detailed mechanisms, but try to imagine (acid-catalyzed hemiacetal formation): 1) protonation of the carbonyl group 2) nucleophilic attack of the methanol on the highly electropositive carbon, with the C=OH(+) bond breaking onto the OH to give oxygen back its lone pair; note the methanol OH is now +'ve 3) the H-O bond from the methanol OH(+) breaks onto the oxygen, regenerating your H+ catalyst, and giving you your hemiacetal. So, your structure is: CH3 - CH2 - C(OH)(O-CH3) - CH3 The carbon has an OH and OCH3 branching off of it, if you can't tell from the notation. The acid-catalyzed reaction can go on to form an acetal, whereas the base-catalyzed version does not, but either way you can get a hemiacetal (although acid-catalyzed would give it in low yield as you would mainly get conversion to the more stable acetal). For the base-catalyzed reaction you just have deprotonation of methanol instead of protonation of the carbonyl, but other than that it's very similar. |
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why hemiacetal linkage formed there are similar carbons at
c2-c4. still the linkage is
between c1 and c5 only. same
is the case with hemiketal. i
would like to know the
chemistry behind it.
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Here's the rule: "Hemiacetals formed are not stable only if they are cyclic. If the hemiacetal formed is cyclic, it should be a 5 or 6-membered ring. Less than that the strain in the molecule will increase causing it to be unstable to exist freely. Therefore, the hemiacetal linkage is formed between C-1 and C-5 which gives a 6-member pyran ring (as in Glucose). Hemiacetal formed between C-2 and C-4 gives a 5-member furan ring in case of fructose. Both furan and pyran rings are stable and their existence in nature is the proof. |
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