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WSU / Chemistry / CHM 1250 / What is the purpose of recrystallization?

What is the purpose of recrystallization?

What is the purpose of recrystallization?

Description

School: Wayne State University
Department: Chemistry
Course: Organic Chem 1 Lab
Professor: Matti
Term: Spring 2019
Tags: Organic Chem Lab
Cost: 50
Name: CHM 1250 Lab Midterm Study Guide
Description: These notes cover all the info needed for the midterm.
Uploaded: 06/14/2019
3 Pages 31 Views 1 Unlocks
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Recrystallization - impure organic solid (will look like a crystal)If you want to learn more check out What are the three levels of personality analysis?

        Purpose: to be recrystallizedWe also discuss several other topics like How is chemical property different from physical property?

                  : to purify a solid organic compound

                        * will have an unknown (10 of them)We also discuss several other topics like Who purchases manhattan for $26 worth of items from native americans for the dutch, and it is now new amsterdam?

        Impure crystal → dissolve in hot solvent  → filter, cool  → obtain pure crystals

  • Solvent selection can be complicated (TA will give us a solvent once they boil it in water)
  • Minimal amount of hot solvent
  • Use water as the solvent in this experiment but that is not always the case

We also discuss several other topics like What is the minimum distance that enables one to distinguish two objects as separate, distinct entities?

Steps: (recrystallization)

  1. Choose a solvent that dissolves the material when hot but not when cold (or no seeing crystals)
  2. Acid a minimal amount of hot solvent and bring to near boiling, swirl in heat

Not too much water = dilute everythingDon't forget about the age old question of What is the first war to be televised to the american public?
Don't forget about the age old question of If water is essential for life and diamonds are not, why do diamonds cost more than water?

                *Acid charcoal*

  1. Stop boiling, cool slightly, and acid charcoal, return to boiling (or it will be a volcano)
  2. Charcoal will remove colored impurities. The impurities absorb the charcoal.

Hot Gravity Filtration - to remove charcoal once impurities from the unknown

  • Filter by gravity while hot using fluted filter paper
  • Mane fluted filter paper from a flat, round piece of filter paper
  • Heep the filtrate

*too much solvent = no crystal*

  1. Cool the filtrate, in air, then ice water bath - fluted paper has charcoal and impurities
  2. Use suction filtration to obtain the crystal
  • Must brean vacuum before turning off the water

Solid stays in the funnel, and separate the filtrate

Filtration Hints

  • When you want the filtrate = use gravity filtration
  • When you want the crystalline solid = use suction filtration

Properties of a good recrystallization solvent

  1. Doesn’t dissolve the compound to be purified when cold
  2. Does dissolve the compound when hot (near boiling)
  3. Has a relatively low bp for easy evaporation
  4. Does not react with the compound
  5. Cold solvent heeps impurities dissolved
  6. Does not have a bp near the mp of the compound

After your crystal dry…

  • Weigh your product
  • Calculate % recovery (you will never get 100% recovery) = what you obtained after recrystallization divided by the amount of crude materials you started with x100
  • Take mp, determine what your unknow may be do mixed mp

* the higher the range, the purer the compound*

Identification by mp and mixed mp

  • Take mp of unknown
  • Take mp of a mixture of your unknown and possible unknow.
  • If they are identical, the mp will not change
  • If they are not identical, there will be a drastic depression in the melting point
  • Record your unknown number
  • Never add charcoal to a boiling mixture
  • The filtrate after suction filtration should go

In aqueous acidic waste

Save all of your recrystallized material

Hand yor sample in with your report. Do not take it home.

Distribution Coefficient

        Liquid - liquid extraction

Extraction:

  • Used as  a purification technique
  • Work-up a reaction, isolate product
  • Recovery of a substance from a mixture by bringing it into contact with a solvent that preferentially dissolve it

Ex. coffee, tea, vanilla

Liquid - liquid extraction (get 2 liquids to mix then separate)

  • Requires two immiscible liquid (don’t mix stay separate)
  • water/dichloromethane ↘
  • water/diethyl ether    → nonpolar organic solvent is immiscible to water

                *go from aq to organic  ← organic compound= benzoic acid → solvent

  • Remove an organic compound from a mixture (removing organic compound from one solvent and moving it to other solvent)
  • Equilibrium process controlled by solubility of the substance in each of the two solvents

Distribution (partition) coefficient

        Liquid-liquid extraction is based on the principle that if a substance is soluble to some extent in two immiscible liquids, most of it can be transferred from one liquid to the other by thorough mixing.

        

        Npart coeff. = [A] in S2/ [A] in S1

        S1 is the original solvent (H2O)

        S2 is the extracting solvent (CH2Cl2)

        Np

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