Practical Quiz - Chem 232 UMD

What lab is lab one?

Recrystallization of an impure solid

What is recrystallization of an impure solid?

a purification technique in which a crude solid is dissolved in minimum amount (volume) of hot solvent, and upon cooling, a purer solid precipitated from the solution

What is the most important consideration for good recrystallization?

Choosing a good solvent based on

• Polarity

• Differential solubility of solid in solvent

• Solvent Bp

• Chemical interaction between solid and solvent

• Solubility of impurity in solvent

insoluble, soluble

The solid should be infinitely _____ at low temperatures and infinitely _____ at high temperatures

What can you do if there are purities that result that do not dissolve?

Use hot gravity filtration to filter them out, or a decolorizing agent

What is the experimental set-up for experiment 1?

four test tubes in a warm/hot water bath, then cooled to room temp, then finally in an ice bath, best solvent should be observed (which one is solid at cold and room temp but no precipitate at hot)

• THERE MUST BE 4 TUBES, ONE FOR EACH SOLVENT

Then, for recrystallization, .40g of crude solid was placed in a 25 ml Erlenmeyer flask, best solvent was used, small magnetic stir bar was added, heated on hot plate until solid was completely dissolved

• Cool to room temp, any insoluble are filtered out via gravity filtration

• Recrystallized sample was collected via vacuum filtration using a Buchner funnel and filter flask

What is lab 3?

Extraction

What is extraction?

A technique defined as transferring solute from one solvent to another

• We match polarity as much as possible (like dissolves like)

• We want something to be completely solvated at room temp, not partially precipitated at room temp like for experiment 1

• Make sure the solvent has a high boiling point so we can remove it later

What is selectivity?

The solvent extracts only our compound of interest and not other components

• typically achieved by experience and trial and error

• In order to extract the max amount of solute into the solvent, we help the process along b raising the temperature, grinding the solid, and mixing well

What kind of extraction did we use?

Liquid-liquid extraction - use two immiscible solvents to separate a mixture of solids (must have different densities, refractive indices)

• MOST ORGANIC COMPOUNDS ARE NOT WATER SOLUABLE

• In order to separate the mixed solutes, we increase the solubility of one solid in water and “pull” it to the aqueous phase, thus separating it from the other organic solids in the mixture

• The process is repeated (take advantage of the acidity/basicity of the solution

What bases are used for what compounds?

Strong acids, like carboxylic acids, use weak bases (NaHCO3) to deprotonate it

Weak acids, like phenol, require strong bases like NaOH

Amines are protonated with strong acids like HCl

What reagents are used for experiment 1?

Benzoic acid, p-toluic acid, acetanilide, and benzyl

What solvents are used?

Hexanes, acetone, ethanol, and water

What reagents are used in experiment 3?

3-nitrobenzoic acid, 2-napthol, and 1,4-dimethyoxybenzene

What are the solvents used?

NaHCO3, NaOH, and HCl

What is experiment 4?

Thin layer chromatography of spinach

What is chromatography?

separation and purification technique

• thin layer chromatography is based on partitioning the sample between two phases, the mobile and stationary

• Planar version

• Stationary phase is the silica paper, and the mobile phase is capillary action

• Separates based on polarity (silica plate is polar, so the more polar a substance, the less distance it will travel.

What are the pigments that we plan to extract?

Chlorophyll-a, lutein (xanthophyll) and B-carotene.