assil assil
2013-11-13, 20:25
asalam 3alaikom
habit tfahmoni fi wahd l'anomalie ta3 la structure lineaires des oses fi biochimie
un aldehyde +alcool=hémiacéta l
l fahmoooooooooni please li raho fahem:mad::mad::mad:
chromato
2013-11-14, 11:30
Les aldéhydes s'additionnent sur les monoalcools pour donner un hémiacétal
instable qui peut ensuite se condenser avec une autre molécule d'alcool pour donner un
acétal
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http://www.google.dz/imgres?hl=ar&tbm=isch&tbnid=qrczJKh_hki2pM:&imgrefurl=http://unt-ori2.crihan.fr/unspf/2010_Nancy_Perkadis_Chimie_Organique/co/077_comp_acetals.html&docid=ySGYigBqMKhgaM&imgurl=http://unt-ori2.crihan.fr/unspf/2010_Nancy_Perkadis_Chimie_Organique/res/res16_1.png&w=493&h=256&ei=ZqeEUpLRLsPetAb76ICAAQ&zoom=1&ved=1t:3588,r:0,s:0,i:77&iact=rc&page=1&tbnh=162&tbnw=312&start=0&ndsp=11&tx=70&ty=93
Les cétones ne donnent pas les composés correspondants aux hémiacétals (hémicétals)
et aux acétals (cétals), Par contre,
la création de cétals cycliques est possible, la structure étant stabilisée par la
présence du cycle (comme la forme cyclique des sucres)
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le mécanisme
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L'intérêt de la formation de ces acétals et cétals, c'est leur stabilité en milieu basique
(base alcaline, ou base plus forte, tel qu'un organométallique). On pourra ainsi protéger
un groupement carbonyle, si on doit faire réagir un autre groupement de la molécule en
milieu basique (généralement, les bases provoquent L'aldolisation du dérivé carbonylé,
ce que L'on veut éviter. Exemple :
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