Lab Midterm Flashcards
Temperature at which the solid phase is in equilibrium with the liquid phase
Melting point
The melting point range of a pure compound is…
Sharp, meaning it has a narrow range
Why is the melting point of an impure compound lower than that of a pure compound?
The impurity disrupts the crystalline lattice of the sample so less energy is required to convert the solid phase into liquid phase.
What are three possible causes for an inaccurate melting point determination?
Poor heat transfer to the sample, sagging or shrinking of the crystals being mistaken for melting, presence of a solvent causing the crystals to “sweat” before melting occurs
What is the melting point range for pure benzoic acid?
122-123 C
What is the melting point range for (+-)mandelic acid?
120-122 C
How tall should the sample be in a capillary tube for melting point?
1-2mm, about the same height as the width of the capillary tube
This type of spectroscopy give us information about what functional groups are present in a molecule
IR spectroscopy
This type of spectroscopy gives us information about the structure and bonding in a molecule
NMR spectroscopy
Refers to the number of hydrogens on a specific carbon (in H-NMR)
Integration
Refers to the number of hydrogens on adjacent carbons (H-NMR)
Splitting
What are the two different ways to synthesize acetophenetidin?
Williamson-Ether Synthesis and Amide Synthesis (we only did the Amide route in lab)
What are the reactants in the amide pathway of synthesis of acetophenetidin?
p-phenetidin and acetic anhydride
What is are the “solvents?” (goes above the reaction arrow in equation?
NaOAc and HCl
What are the products of synthesis of acetophenetidin?
Acetophenetidin and HOAc
What is recrystallization used for?
To purify organic solids
What are the three main criteria for choosing an organic solvent for recrystallization?
It should dissolve both the compound and impurities at the boiling point, the desired compound should be insoluble at cold temperatures, and the impurities must be soluble and remain in solution at cold temperatures.
How does recrystallization work?
An impure sample is dissolved in a minimum amount of solvent at the boiling point, insoluble impurities are removed by hot gravity filtration, the hot and saturated solution is allowed to cool slowly to room temperature and then placed in an ice bath to maximize crystal formation. The crystals are collected by vacuum filtration.
What are safety concerns with acetic anhydride and HCl?
Can cause burns, should avoid breathing vapors and skin contact
What are safety concerns with p-phenetidin?
Irritant
What are the safety concerns with sodium methoxide?
Flammable and corrosive
What is the safety concern with ligroin and ethanol?
Flammable
What color is pure p-phenetitin?
Colorless
What is the purpose of the sodium acetate solution in the amide synthesis of acetophenetidin?
It acts as a buffer to control acidity and maximize yield.
Why is HCl used instead of plain d.i. Water in acetophenetidin synthesis?
HCl is more polar and drives the reaction to the right, protonating the amine group and increasing solubility.
Why is the amide route a greener pathway?
It produces a larger crude mass.
A technique used to remove selected components from a mixture, usually based on solubility differences or chemical reactivity
Extraction
What solvent is commonly used for solid-liquid extractions?
CO2
What makes CO2 a “greener” alternative to traditional organic solvents?
It is readily available, nontoxic, nonflammable, and safe for the environment
To be insoluble in one another
Immiscible
What is the melting point of benzoic acid?
123 C
What is the melting point of p-toluic acid?
182 C
What is the melting point of B-naphthol?
122 C
What is the melting point of 4-t-butylphenol?
101 C
Weak bases extract…
Strong acids
Strong bases extract…
Weak acids
Why is sodium bicarbonate used in the extraction of a carboxylic acid from a mixture of carboxylic acid and phenol?
Sodium bicarbonate is a weak base, therefore it can deprotonate COOH but not ph-OH. Ph-OH must be deprotonated by a stronger base such as NaOH.
Is the aqueous phase on the top or bottom in the separatory funnel?
Bottom
Why do we wash the organic layer with brine and then add anhydrous magnesium salt?
We start with brine to remove the majority of the water and then remove the rest with the anhydrous MgSO4.
What is the difference between extracting and washing?
Extraction is pulling out a specific compound from a solution and washing is cleaning the extracted compound to remove impurities from solution.
Explain the purpose of hot gravity filtration. Why is it good to use the stemless funnel for this technique?
Hot gravity filtration separates insoluble impurities out of a hot solution. It is better to use a stemless funnel to prevent recrystallization from occurring inside the stem prematurely as it cools.
What is ligroin? Is it polar or non polar?
It is a colorless, flammable solvent. It is non polar.
Can IR spectroscopy be used to distinguish 2-pentanone from 2-hexanone? Why or why not?
No because IR spectroscopy is used to identify the presence of functional groups. H-NMR would work in this case.
What is the purpose of seed crystals?
They serve as nucleation sites where more molecules of the desired compound deposit themselves in layers
How do we remove the reddish brown impurities from p-phenetidin?
Decolorizing carbon
What is the best recrystallization solvent for acetophenetidin? Why?
Water because the solute is completely soluble at boiling point, but crystals reappear when placed in an ice bath.
What is the purpose of the wooden applicator stick?
Helps with gentle boiling
Hexane vs. water? Immiscible or miscible? What is on the top?
Immiscible; Hexane
Methylene chloride vs. water? Immiscible or miscible? What is on top?
Immiscible; water
Ethanol vs. water? Immiscible or miscible? What is on top?
Miscible
Ether and water? Immiscible or miscible? What is on top?
Immiscible; ether
Could you separate 2,4-dinitrophenol (pH 3.96) from benzoic acid using extraction procedures in this experiment?
No, because they are very close in acidity; both would be neutralized.
What criteria must be met in order for a liquid-liquid extraction to be successful?
Two solvents chosen must be immiscible and the compound to be extracted must have different affinities for the two solvents.
What is present in the aqueous layer of the extraction experiment?
Carboxylate ions
How do we regenerate the carboxylic acid from the carboxylate ions in the aqueous solution?
Add HCl in an ice bath to induce crystallization, then perform vacuum filtration to isolate the crystals.
What is present in the organic layer in the extraction experiment?
Phenol and possibly water
How do you isolate the phenol from the water in the organic layer?
Dry the solution with brine first, remove the brine, add anhydrous magnesium sulfate until it flows freely. Use rotovap to evaporate solution under pressure to leave pure phenol in the flask.
Which is more polar: chlorophyll a or chlorophyll b?
Chlorophyll b because it has a CHO group where chlorophyll a has a CH3 group
Which is more polar: xanthophyll or B-carotene?
Xanthophyll because it has an OH group at the end of the molecules. B-carotene is NONPOLAR
Techniques for separating, analyzing, and identifying compounds
Chromatography
Which pigment is the least polar? The most polar?
B-carotene is the least polar, chlorophyll b is the most polar
