1. slaked lime is not caustic soda, but I don't think it makes a difference here.

2. The base is need to neutralise the HCl or the quinoline HCl salt forms.

3. On this sort of scale, it is probably advisable to try it on small scale first.

If you are not sure of how to do the reaction, it is better to buy the compound or to find a simpler method.

See attachment for a similar procedure

Skraup synthesis perse involves anilines and glycerol.

Check Org. React. 7, 59, 1953 for variants of your kind

Dear Ayman Mohamed Ibrahim , the given procedure is simple, clear cut and well stated. In the procedure there is no use of sodium hydroxide at all. First they have allowed the reaction of aniline hydrochloride (formed by reaction of aniline and aqueous HCl) with acetaldehyde solution and it has been followed by reaction with ZnCl2. In this process an intermolecular aldol reaction of acetaldehyde takes place first to produce crotonaldehyde (CH3-CH=CH-CHO, it is a Michael acceptor) ant then a Michael addition takes place. Finally it undergo ZnCl2 catalysed intramolecular cyclization, dehydration and aromatization by aerial oxidation to furnish the 2-methylquinoline which remain as a hydrochloride salt and/or as ZnCl2 complex of it . After basification with slaked lime (it is nothing but calcium hydroxide, a base and much cheaper than NaOH ; hence it is used preferentially over NaOH) the free 2-methylquinoline is liberated and it is steam distilled. 2-methylquinoline being steam volite comes out with water as distillate. Hence in the distillate you are getting two layers...one organic layer , it is mainly 2-methylquinoline and the other aqueous aqueous part. This aqueous part may contain little dissolved 2-methylquinoline. To isolate it, the aqueous part(after separation of organic part) of the distillate is extracted with chloroform. Ether can also be used instead of chloroform. Possibly density of 2-methylquinoline is more than 1.0 hence chloroform with higher density (>1.0) is used so that the organic layer comes in the bottom part. Otherwise large excess of ether may be necessary to bring the product in the upper organic layer (in case ether is used).

Now about the ppt formation.: if you use NaOH to basify along with slaked lime ppt of Zn(OH)2 (as ZnCl2 was used in the reaction) will form along with Ca(OH)2. Zn(OH)2 become soluble in excess NaoH as it is converted to water soluble sodium zincate but Ca(OH)2 will not. Hence you are getting the ppt. So please avoid use of NaOH along with slaked lime. No need at all to use NaOH, slaked lime itself is enough to make free 2-methylquinoline from 2-methylquinoline hydrochloride or it's zinc complex.

Thanks Prof. Gandhi K. Kar for your simplification. i greatly appreciate it. i need to know when does the reaction end ? procedure says when vigorous reaction occurs. what does mean ' vigorous' word mean? In the image above a gummy substance is formed after refluxing for 7 h, is that means ending of reaction?

Also, can i use cotonaldehyde instead of acetaldeyde as acetaldehyde can polymerize in presence of acid to paraldeyde which decrease yield ?

thanks for help prof. Gandhi K. Kar

Sincerely

Dear Ayman Mohamed Ibrahim ; This is in response to your query. The reaction will be almost complete unless it is a reversible one. You can check the completion by TLC (thin layer chromatography) or GC or HPLC of the crude products. The "vigorous reaction" means it can warm up or can become too hot to reflux due to exothermic reaction. It can be controlled by slow addition of reagents or by cooling externally (say with cold water) when the reaction becomes too hot. Too hot reaction condition may lead to some polymeric product on the other hand lower temperature makes the reaction very slow. So you have to find out an optimum condition. I think it's a standardised procedure any it needs 7 hours as mentioned for completion. Gummy reaction mixture or emulsion can form but during steam distillation only the 2-methylquinoline will come out and not the gum.

I hope use of crotonaldehyde should also lead to the product. But it is costly. You can go through the Collective Volumes of Organic synthesis (Vol-1 to 10), I hope a procedure may itself be available with use of crotonaldehyde and aniline derivative. I am now very busy with my administrative works for UG and PG admission. Once I get time I will also try to check it. If available I will let you know.

Thanks