Mostly every Thursday, to show the wonderful effects (no diagrams, sorry) of the many reagents from LDA to alkyl lithium, Reagent Thursday posts will be posted. Reagent Thursday, if forgotten, will be plugged by Reagent Friday or any other day of the week.
Let's start with the first reagent; the master of reduction, lithium aluminum hydride! Lithium aluminum hydride is ionic and I wish I could really show you how it looks like. The formula is LiAlH4. Can you imagine it?
No? It's a negatively charged previously electrophilic aluminum attached to four hydrogens. The lithium is positively charged and attracted to the aluminum. The aluminum has tetrahedral geometry (ionic bonds are not sharing).
Let's move on. LAH (short for lithium aluminum hydride) is a strong reducing agent used to reduce carboxylic acids and esters to primary alcohols. The mechanism is unimportant because after all this is Reagent Thursday and the focus is to learn the reagent's reactions without mechanisms; in fact, not many teach the mechanism of LAH.
Anyway, let's move on. Carboxylic acids and esters are too stable to be reduced by sodium borohydride (another ionic weaker reducing agent). So LAH is used. What sodium borohydride is used for, selectively reducing aldehydes and ketones are strong enough to be reduced by LAH.
We'll continue this discussion next Thursday with LAH. Bye!
~ReagentThursday
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