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Organic Chemistry Lab > Module 5: RECRYSTALLIZATION AND SUBLIMATION > Flashcards

Module 5: RECRYSTALLIZATION AND SUBLIMATION Flashcards

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1
Q
  • also known as fractional crystallization
A

RECRYSTALLIZATION

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2
Q
  • involves the dissolution of the solid in an appropriate solvent at an elevated temperature and the subsequent “re-formation of the crystals upon cooling”, so that any impurities remain in solution
A

RECRYSTALLIZATION

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3
Q
  • desired compound should be ____________________________
A

+ soluble in the hot solvent
+ insoluble or nearly insoluble in the cold solvent

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3
Q

RECRYSTALLIZATION PROCESS

A

= Selection of an appropriate solvent
= Dissolution of the solid to be purified in the solvent
= Decoloration with activated charcoal
= Filtration of the hot solution to remove insoluble impurities
= Formation of the crystals as it cools
= Isolation of the purified crystals by filtration
= Drying the crystals

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3
Q

_____________ should either be insoluble in the solvent at all temperatures or must remain at least moderately soluble in the cold solvent

A

impurities

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3
Q
  • “most critical step” in the process: selected to form a product of high purity and in good recovery or yield
A

selection of an appropriate solvent

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4
Q
  • the best solvent will dissolve the solute in hot solution but not in the cold solution); vice versa, it is called a poor solvent
A

good solvent

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5
Q

the boiling point of the solvent should be low enough so that it can readily be removed from the crystals

(True or False)

A

True

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5
Q
  • the solvent should not react chemically with the substance being purified

(True or False)

A

True

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6
Q
  • most ______________ dissolve in hot solvents than in cold ones
A

organic compounds

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6
Q

high degree of microporosity; has large surface area

A

activated charcoal

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6
Q

▪ using excessive amounts may decrease recovery; use “dropwise method”

A

too much solvent is used

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6
Q

too less solvent will not yield crystals

A

supersaturated solution

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7
Q

“removes high molecular weight impurities”, which are often
colored and relatively less soluble

A

decolorization with activated charcoal

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8
Q
  • a ______________ should be used to minimize crystallization in the funnel
A

short-stemmed or stemless glass funnel

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8
Q
  • sometimes the sample is ______________ with small quantity of insoluble impurity that will never dissolve
A

contaminated

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8
Q
  • using ______________ will minimize crystallization on the filter
A

fluted filter paper

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8
Q
  • it is important to filter the solution while hot to prevent _____________ during filtration
A

premature crystallization

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9
Q

reduce the temperature to induce crystal formation; generally the solution should not be disturbed as it cools, since this also leads to production of small crystals

A

Cooling

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10
Q

crystal of the original solid is added to the solution to induce crystallization, which may then be quite rapid

A

Seeding

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10
Q

▪ _____________ the solution will induce bumping of crystals which causes to reformation of more crystals

A

Stirring and Agitation

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10
Q

a glass rod is to rub the inside surface of the crystallization vessel at or just above the air/solution interface; microscopic scratches provide sharp edges where crystals may grow

A

Scratching

11
Q
  • during cooling and crystallization, the solution should be protected from _________________ by covering the opening with a piece of filter paper, an inverted beaker, or by loosely plugging it with a clean cork
A

airborne contaminants

12
Q

solution should not be ___________ as it cools

A

disturbed

12
Q

crystals normally are washed with a small amount of

A

pure, cold solvent

13
Q

crystals easily form at a ___________

A

low temperature

14
Q

be careful not to use too low temperature as it may form ____________

A

small crystals

15
Q
  • the solid product is isolated by ____________ using a Büchner or Hirsch funnel and a clean, dry filter flask or by _____________
A
  1. vacuum filtration
  2. gravitational filtration
16
Q
  • removing the last traces of solvent from the crystalline product may be accomplished by _______________
A

air or oven drying

16
Q

percentage recovery formula

A

% 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑦 =
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑟𝑦𝑠𝑡𝑎𝑙𝑠 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑
——————————————- × 100
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑠𝑎𝑚𝑝𝑙𝑒

17
Q
  • the purity of the crystals is usually assessed using ______________, as the presence of the impurities decreases the melting point of the product
A

melting point determination

17
Q

___________ is employed to detect any sodium chloride impurity left on the purified crystals (white precipitate or turbid solution indicates presence of NaCl)

AgNO3
(l) + NaCl (aq) → AgCl (s) + NaNO3 (l)

A

silver nitrate test

18
Q
  • defined as the “direct conversion” of a substance from its “solid phase to its gas phase” without passing through the liquid phase
A

SUBLIMATION

18
Q
  • this process entirely “relies on the equilibrium” between the solid and gas phases rather than solid and liquid phases
A

SUBLIMATION

19
Q

SUBLIMATION SET-UP

  • an “outer or lower vessel” is where the ____________ is placed while an “inner or upper vessel” is where the ____________ is formed again
A
  1. impure sample
  2. pure solid
19
Q
A
20
Q

enough “energy builds up” in the
molecules during heating and then escape into gas phase

A

SUBLIMATION

21
Q
  • the crystals may appear in different forms or habit:
A
  • light and feathery crystals
  • round and scaly crystals
  • needle-like crystals
  • blocks of crystal
21
Q

SUBLIMATION SET-UP

  • the upper vessel must be cold for the pure solids to form on it
A

note!

22
Q
  • the crystals, deposit, or material obtained when a substance is heated
A

SUBLIMATE

23
Q
  • has high vapor pressure, low melting point, and more volatile
A

SUBLIMATE

24
Q

__________, it also depends on volatility

A

PURIFICATION

24
Q
  • has low vapor pressure, high melting point, and less or non-volatile
A

RESIDUE

24
Q
  • the solid particle that remained on the dish
A

RESIDUE

25
Q
  • “less volatile or non-volatile impurities” remain as a __________, while the sublimate is being formed
A

residue

26
Q
  • the process of sublimation is very convenient to apply; however, its application is limited since it :cannot be used: for separating compounds that have “similar vapor pressures”
A

LIMITATION

27
Q

__________ was employed to differentiate the organic substance from the inorganic residue

A

barium chloride test

27
Q
  • BaCl2 will react with ___________ producing __________, thereby identifying the residue from the sublimate

BaCl2 (l) + Na2SO4 (aq) → BaSO4 (s) + 2 NaCl (aq)

A
  1. Na2SO4
  2. white precipitate
28
Q

RECRYSTALLIZATION
= BENZOIC ACID
* Appearance:?
* Chemical formula:?
* Molecular Weight:?
* Melting Point:?
* Boiling Point:?
* Use:?
* Synonym:?
* Density:?
* Toxicity:?

A
  • Appearance: colorless crystalline solid
  • Chemical formula: C7H6O2 or
    C6H5COOH
  • Molecular Weight: 122.12 g/mol
  • Melting Point: 122-123°C
  • Boiling Point: 249°C
  • Use: antifungal and preservative
  • Synonym: Carboxybenzene, Benzene carboxylic acid
  • Density: 1.32 g/cm3
  • Toxicity: toxic to the lungs & nervous system (prolonged exposure may lead to target organ damage)
28
Q

sodium sulfate
amorphous powder
polar
inorganic
3.4 × 10-5 mmHg at
25°C

A

RESIDUE

29
Q

NaCl was identified if separated through ____________

! the crystals, upon the test must not yield white precipitate to be able to conclude a complete separation

A

silver nitrate test

29
Q

methylene blue was separated upon filtration after the addition of ____________

! the crystals will remain blue if methylene blue is incompletely separated

A

activated charcoal

30
Q

SUBLIMATION
= SALICYLIC ACID
* Appearance:?
* Chemical formula:?
* Molecular Weight:?
* Melting Point:?
* Boiling Point:?
* Use:?
* Synonym:?
* Density:?
* Toxicity:?

A
  • Appearance: colorless needle-like crystals
  • Chemical formula: C7H6O3
  • Molecular Weight: 138.12 g/mol
  • Melting Point: 158°C
  • Boiling Point: 211°C
  • Use: keratolytic, used to treat acne, seborrhea, corn, calluses and warts
  • Synonym: 2-hydroxybenzoic acid
  • Density: 1.44 g/cm3
  • Vapor pressure: 8.2 × 10-5
    mmHg at 25°C
  • Toxicity: salicylism (through
    ingestion)
30
Q

salicylic acid
needle-like crystal
non-polar
organic
8.2 × 10-5 mmHg at
25°C

A

SUBLIMATE

Organic Chemistry Lab (6 decks)

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