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the reaction of one functional group in the presence of another same or different functional group
Colby Leffler
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Why is sodium borohydride a better reducing agent then lithium aluminum hydride?
Can't say it is a better reducung agent : actually it is a less powerfull reducing agent, il will reduce less functions then LiAlH4. However, its reactivity will allow better chemoselectivity : you can reduce an aldehyde on a compound with also an ester : that's why its mighty usefull. (it is also far more practical : you can use it in water or alcohol, no special handle is necessary : less accident gain of time, cheapier simpler)
What features can enzymes change in chemical reactions?
Enzymes display three major types of selective advantages: Chemoselectivity: Since the purpose of an enzyme is to act on a single type of functional group, other sensitive functionalities, which would normally react to a certain extent under chemical catalysis, survive. As a result, biocatalytic reactions tend to be "cleaner" and laborious purification of product(s) from impurities emerging through side-reactions can largely be omitted. Regioselectivity and Diastereoselectivity: Due to their complex three-dimensional structure, enzymes may distinguish between functional groups which are chemically situated in different regions of the substrate molecule. Enantioselectivity: Since almost all enzymes are made from L-amino acids, enzymes are chiral catalysts. As a consequence, any type of chirality present in the substrate molecule is "recognized" upon the formation of the enzyme-substrate complex. Thus a prochiral substrate may be transformed into an optically active product and both enantiomers of a racemic substrate may react at different rates. These reasons, and especially the latter, are the major reasons why synthetic chemists have become interested in biocatalysis. This interest in turn is mainly due to the need to synthesise enantiopure compounds as chiral building blocks for drugs and agrochemicals. Another important advantage of enzymes are that they are environmentally acceptable, being completely degraded in the environment. Furthermore the enzymes act under mild conditions, which minimizes problems of undesired side-reactions such as decomposition, isomerization, racemization and rearrangement, which often plague traditional methodology.
