Sorry for the delay on Reagent Thursday. New Note: Reagent Thursday has been cancelled and replaced with Reagent Day, which will be held off until I complete the preparation of functional groups blog posts. But here's one Reagent Day article for us to see:
For the first installment of Reagent Day, chromic acid will be the featured reagent for us to talk about today. What makes chromic acid so confusing is the number of sources it can be made from. Usually, due to safety reasons, it is often made in situ (in the flask) rather than being dispensed in a bottle. Potassium chromate, potassium dichromate, chromium trioxide, sodium dichromate, or sodium chromate mixed with acid all form this very chromic acid.
Oxidation from chromic acid oxidizes primary alcohols to carboxylic acids, secondary alcohols to ketones, and aldehydes to carboxylic acids.
How? Primary alcohol oxidation proceeds via 1,2-addition where chromic acid is the electrophile. Then a base performs an "E2" reaction, only the bond goes to the carbonyl bond. Then H3CrO4 leaves the molecule and deprotonating the carbonyl turns it into an aldehyde, further oxidized to a carboxylic acid via hydration, 1,2-addition, E2, and deprotonation.
Secondary alcohol oxidation goes via ***not shown***
Sunday, January 13, 2013
Thursday, December 13, 2012
Reagent Thursday Introduction and LAH
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
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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