Organic Chemistry LAB Questions

[WorcesterPHOBoy]

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Hi, I have two questions I could not even find the answer on google I wonder if any of you know what is this?

1) What would be the effect of collecting larger fractions of elutes instead of collecting small fractions in the test tube when carrying out the experiments described (Column Chromatography)

2) Once the chromatographic column has been prepared, why is is important to allow the level of the liquid to the column to drop to the level of the alumina before applying the soution of the compound to be seperated?

 

[ersoco]

1. Collecting larger fractions means you’ll have less fractions that are pure in each of your elute. For example, even if your compounds separate, a bigger fraction makes it likely that the beginning drips of a fraction could be one compound but the final drips could be another compound… making the fraction a useless mixture. Whereas the drips could have gone into two separate fractions if they’re smaller fractions.
2. Dropping the liquid to the alumina means that your solution you add won’t be diluted by the mobile phase that’s above the alumina… leading to bad separation. Your solution won’t all reach the alumina at one time, so as some of your sample is passing through the beginning of the column, there’s still some that hasn’t reached the alumina yet = bad.

Hope this makes sense.

 

[WorcesterPHOBoy]

1. Collecting larger fractions means you’ll have less fractions that are pure in each of your elute. For example, even if your compounds separate, a bigger fraction makes it likely that the beginning drips of a fraction could be one compound but the final drips could be another compound… making the fraction a useless mixture. Whereas the drips could have gone into two separate fractions if they’re smaller fractions.
2. Dropping the liquid to the alumina means that your solution you add won’t be diluted by the mobile phase that’s above the alumina… leading to bad separation. Your solution won’t all reach the alumina at one time, so as some of your sample is passing through the beginning of the column, there’s still some that hasn’t reached the alumina yet = bad.

Hope this makes sense.

Thank you, but for # 2, are you saying it is BAD?? if allow the level of the liquid to the column to drop to the level of the alumina before applying the soution of the compound to be seperated??