1. why should i use THF as solvent? is it creat any stability to the LDA?
2. why should i stirr for such long time? is it because the low temperature? why do i need the temperature of -78C?
3.what is the role of the ammonium chloride?
4. do you know good internet site that can give me good learning of flash chromotography and other ways of methods of chromotography?
1. You require a solvent that:
a) Won't freeze at -78C
b) Will dissolve your reagents
c) Is chemically inert to your reagents. You are useing a very strong base, so you require an aprotic solvent
THF just fits the bill
2. I wouldn't call 1 h a long time. -78C is used because your reagents are extremely reactive and so may cause unwanted side reactions/decomposition at higher temeratures.
3. Quenching your reaction essentially means destroying/neutralising excess reagent. Ammonium chloride is a weak acid... think about what you are generating in your reaction, why do you need a proton source at the end?
4. See any practical organic chemisrty textbook, and some general organic texts cover it (eg. Clayden, Greeves, Warren & Wothers).