recrystallization

Question 1

what are the six steps for a recrystallization? give a brief description/tip for each

Answer 1

1. solvent selection (what polarity would be best)
2. dissolving compound (need to dissolve to recrystallize)
3. hot filtration (did you have insoluble impurities?)
4. cool liquid (cool on ice)
5. cold filtration (stir your crystals before you add them to the funnel)
6. dry crystals (takes 5-15 minutes, so spread them out on a watchglass)

Question 2

what are the 4 parameters for an ideal recrystallization solvent?

Answer 2

-one to two steps away in polarity
-nonreactive
-high boiling point
-volatile

Question 3

when is hot filtration necessary?

Answer 3

only when impurities do not dissolve

Question 4

what are the three solubility rules for recrystallization?

Answer 4

-a compound wth the same polarity as the solvent will be soluble in both hot and cold solvent
-a compound that is three steps away will never be soluble in hot or cold solvent
-a compound that is one or two steps away in polarity will give solubility hot and insolubility cold

Question 5

why is diethyl ether a common extracting solvent, but a horrible recrystalizaing solvent? what is a better choie for recrystallization and why?

Answer 5

-its boiling point is 35° C which is too low to dissolve anything that it will also precipitate out when cold
-dibutyl ether because it has a higher boiling point (142° C), so it has a much greater ability to dissolve compounds of different polarity

Question 6

why should you not chose compounds with a melting point below 0°C for recrystallization?

Answer 6

the solvent will generally freeze before the compound can precipitate out

Question 7

why is choosing a more volatil solvent better?

Answer 7

it will allow the crystals to dry faster

Question 8

what is the purpose of solvent selection?

Answer 8

to select a solvent (or mixture of solvents) that will fully dissolve the compound at the boiling point of the solvent but will not dissolve the compound at ice bath (or below) temperatures

Question 9

what is the purpose of dissolving the compound?

Answer 9

to fully dissolve the compound in boiling solvent to release/ separate the impurities from the compound

Question 10

what is the purpose of hot filtration?

Answer 10

to remove any insoluble impurities from the sample while the solvent is hot

Question 11

what is the purpose of cooling the liquid?

Answer 11

cooling the solvent reduces the compound solubility and induces precipitation

Question 12

what is the purpose of cold filtration?

Answer 12

to remove soluble impurities from the compound

Question 13

what is the purpose of drying the crystals?

Answer 13

to allow residual solvent molecules time to evaporate

Question 14

what is recrystallization? briefly describe the process

Answer 14

-a purification technique that manipulates a compound’s solubility to remove impurities
-an impure sample is dissolved in a boiling solvent, allowed to cool, and purified crystals are collected by vacuum filtration. Impurities are either removed via vacuum filtration or are left dissolved in solution

Question 15

what happens if you choose a solvent that is outside of 1-2 steps in polarity?

Answer 15

the compound will not dissolve

Question 16

how would you adjust the polarity of MP- EtOAc?

Answer 16

-adding a little bit of P water to make it slightly more polar without pushing it to be fully MP+

Question 17

in step 2, how do you know if you have no insoluble impurities? what does this allow you to do?

Answer 17

-everything will dissolve in the boiling solvent
-skip hot filtration

Question 18

why should you never recrystallize all of your available compound? about how much should you save?

Answer 18

-if your recrystallization fails, you won’t have any left over to characterize or use as a seed crystal
-10%

Question 19

if you have an insoluble impurity and perform hot filtration, what will be on the filter paper? what will be in the solution?

Answer 19

-the insoluble impurity will be on the filter paper
-the solution will hold any dissolved compounds

Question 20

what happens if your solvent begins to cool during hot filtration?

Answer 20

the other compounds may precipitate out

Question 21

what are some things you can do if no crystals are forming during step 4?

Answer 21

-remove half od the recrystallization solvent on a hot plate which will concentrate the solution and help induce precipitation
-modify the polarity of the solvent by moving it farther from the polarity of the compound
-add a seed crystal which gives crystals a starting point to grow from

Question 22

during cold filtration, what will be on your filter paper? what will be in the solution?

Answer 22

-crystals of compound are on the filter paper
-soluble impurities remain in the solution

Question 23

what solvent was ORG1 recrystallized in? what is its polarity?

Answer 23

-ethanol
-MP+

Question 24

why did we modify the polarity of ethanol?

Answer 24

-the naphthalene was not precipitating, so a P solvent (water) was added to make the solvent’s polarity less like that of naphthalene, encouraging it to precipitate

Question 25

wat can happen if you modify your solvent too much?

Answer 25

-you could change the solvent polarity too much and precipitate the yellow, MP+ m-nitroaniline out of the solution which would make the recrystallization fail and give you a MP that is too low and broad

Question 26

at what points should you measure melting point?

Answer 26

-the temperature at which the sample begins to melt -the final temperature when it is completely liquid

Question 27

what solvent was used to recrystallize ORG2? what is its polarity?

Answer 27

-cyclohexane -NP

Question 28

what was in ORG1? ORG2?

Answer 28

-naphthalene -m-nitroaniline

Question 29

what was added to cyclohexane to modify it? what is its polarity?

Answer 29

-ethyl acetate -MP-

Question 30

what would happen when you put ORG2 with cyclohexane in the ice bath without modifying it with ethyl acetate?

Answer 30

-the cyclohexane would freeze because it freezes at 6° C

Question 31

how will impurities affect your melting point?

Answer 31

it will make it lower and broader

Question 32

how do you know if your MP is too low? too broad?

Answer 32

- 5-10 °C -range > 5 °C