2 ‫ מתוך‬1 ‫עמוד‬
3,4 ‫ניסויים‬
Separation and purification of a mixture of Benzoic acid, isooctane and anisole.
Reference: Journal of Chemical Education Vol. 75 No. 2 p.219 February 1998
Duration: two 5-hour lab periods.
You will receive a mixture of benzoic acid, isooctane and anisole which contains
traces of picric acid. Your task is to separate these compounds, purify them and
report their amounts. The second part of the lab will consist in recrystallizing an
unknown solid.
Very important, mix the solution you are given with sodium carbonate solution in a
250ml Erlenmeyer and not directly in a separatory funnel, think why doing otherwise
could be dangerous. Only after gas evolution subsides introduce the mixture in a
separatory funnel. Use sodium sulfate instead of calcium chloride for drying, and
filter with fluted filter paper instead of glass wool. Do not perform the predistillation
procedure, distill the isooctane directly from the mixture after the extraction.
Remember to discard the fore-run fraction during the distillation to obtain a cleaner
product. Do not distill your compound to dryness! (You must always leave a small
amount of liquid in the distillation flask, this might lower the yields but heating the
glass directly may in some cases be hazardous.) Do a full NMR analysis for anisole
(two per group). Check the index of refraction for your liquid samples and melting
point for benzoic acid. Send one sample of isooctane and of anisole per group to be
analyzed by GC-MS.
Strongly work on these three techniques: distillation, recrystallization and extraction.
You will be tested on your knowledge of the separation and purification techniques in
general and also specifically on what you will do during this lab session. Pay
attention to how you build your systems. For the recrystallization, work on how much
solvent to add, if unsure consult with your lab assistant. Talk about theoretical plates
when distilling with your group and lab assistant. Analyze the anisole NMR spectrum
with your group (isolate the anisole in the first week). Make sure you understand the
splitting patterns (ortho, metha, talk about the J3 values). Talk about impurities and
solvent signals in the NMR, water, TMS (tetramethylsilane), CHCl3, etc. Use
Nudelman et al J. Org. Chem. 1997, 62, 7512 as a guide for deuterated solvent
impurity peaks.
Separatory funnel (‫)משפך מפריד‬, iron ring and Erlenmeyer, three things you must have
to carry out a successful extraction.
2 ‫ מתוך‬2 ‫עמוד‬
How to realize a successful and proper recrystallization (or everything you ever
wanted to know about recrystallizing a compound but were afraid to ask your lab
assistant).
Your solvent must dissolve your compound when boiled (or heated), and must
dissolve the least amount of compound at room temperature (or cooled). Add a small
amount of solvent to your compound and heat it to boiling. If all your compound is
dissolved you probably added too much solvent, try to reduce the amount of solvent
by blowing a gentle stream of air over the sample. If not all your compound was
dissolved add small amounts until your entire compound is dissolved. The hardest
part of a recrystallization is to decide whether all your compound was dissolved or if
there are impurities that are not soluble in the hot solvent. The best way to resolve
this is to try to see if the insoluble amount of compound changes when you add a
small amount of hot solvent. Once you are sure you have dissolved your entire
compound you must filter it hot to remove insoluble impurities. To make sure your
compound does not crystallize on the filter keep the filter and filter paper hot at all
times. It’s recommended to keep an Erlenmeyer with a small amount of hot solvent
over a hot plate and the filter above it, the vapor from the boiling solvent will keep the
filter hot and your compound from precipitating. Once you have hot filtered (by
gravity!) all your solution, let it rest undisturbed until it reaches room temperature.
Sometimes it is helpful to cool the solvent to obtain higher yields, but sometimes this
will cause impurities to precipitate too, you must judge every case by itself. Once you
think most of the compound precipitated you must filter it by using a Büchner funnel.
Wash the remaining crystals with a small amount of cold solvent (you may put a
small amount of solvent in a vial or Erlenmeyer and keep it in the refrigerator until
you need it) to remove soluble impurities. Let the crystals dry for a few minutes and
collect them. Depending on the solvent used drying might take longer times, make
sure your crystals are dry of solvent before measuring a melting point to determine
purity.
Summarizing, there are two main purification steps in recrystallization.
1) Hot filtration removes insoluble impurities.
2) Cold filtration removes soluble impurities.
To test your abilities as a recrystallizer you will be given a sample of an unknown
from the following list
Acetylsalicylic acid
Benzoic acid
Benzoin
Dibenzoyl ethylene
Succinimide
o-Toluic acid
138-140C
121-122C
135-136C
108-111C
122-124C
108-110C
You will have at your disposal the pure compounds if you need them (think why!).
3,4 ‫ניסויים‬