Organic Chemistry Lab Theory Quiz 2

Health Hazards

irritant, carcinogen, acute toxicity

physical hazards

flammable, corrosive, explosive, gasses under pressure, oxidizer

environmental hazard

hazard classes

health, physical, environmental

1

most severe hazard

4

least severe hazard

chromatography

a technique used to separate mixtures based on differences in adsorption of analytes to an absorbent

TLC (Thin-layer chromatography)

used to monitor reaction progress, determine endpoint, confirm identity and purity

endpoint

completion

stationary phase

The phase that does not move

TLC stationary phase

silica, SiO4, polar

Mobile phase

the phase that moves

TLC mobile phase

moves up by capillary action, the developer

capillary action

spontaneous flow of a liquid through a narrow space, often in opposition to gravity

What happens if you increase the polarity of a solvent?

all compounds will move faster

What decreases as polarity increases?

Rf value

list functional groups in increasing polartiy

alkane, alkene, aromatic hydrocarbons, ethers and halides, aldehydes ketones and esters, amines, alcohols, carboxylic acids

amide

can be more or less polar depending on protenation by silica

cis vs trans alkenes

cis alkenes more polar

Retention factor

the ratio that describes the distance traveled by the analyte relative to the solvent

Rf equation

distance traveled by analyte / distance traveled by solvent

determining endpoint

one row will have the pure starting material, and as the reaction goes on it will separate and eventually there will be no more starting material and only product

In identical development conditions with two pure samples with different retention factors, they are?

different compounds

in identical development conditions with two pure samples with the same retention factor, they are?

maybe the same compound but we need additional information

What are the requirements to get purity from a TLC

The Rf must be reproducible, compared against a known standard, know the number of compounds in a sample

TLC solvent

must be volatile

TLC optimal Rf range

0.2 - 0.8

< 0.2 Rf

poor separation

> 0.8 Rf

coelution with impurities

Limitations of TLC

• UV only detects active compounds and non active compounds go undetected

• doesn’t detect all organic compounds, like iodine

• stains are compound-specific so impurities can go undetected

active compounds

conjugated systems and aromatic compounds

inactive compounds

alkanes, amines, sugars

other staining reagents

ninhydrin, KmNO4, vanillin, DNP, dragendorff

What did we do for the isomerization experiment?

We reacted a cis alkene, dimethyl maleate with Bromine as a catalyst and refluxed it to get both the cis and trans version, dimethyl fumerate. Then we added hexane to precipitate the products, wash the crystals, dissolve them, and spot them on a TLC plate.

What happens when we add hexane to the dimethyl maleate and fumarate mixture?

only dimethyl fumerate solidifies because it has a higher melting point

why does the cis alkene have a lower melting point than the trans alkene?

steric hindrance makes it unable to be closely packed or solidify

What does a TLC chamber contain?

a beaker, a solvent below the origin, half a filter paper to saturate the air, and a watchglass

Column chromatography

used to purify compounds

column chromatography stationary

silica

column chromatography mobile phase

eluent

silica

polar and porous

column

a glass tube packed with stationary phase

eluate

what comes out of the column

column volume

the volume of solvent required to pass through the entire column, once

CV equation

1/Rf

if the Rf is 0.2, how many column volumes are needed?

5

polar vs non polar analytes

Nonpolar moves quicker

quinone synthesis experiment

reacting tetrachloro-1,4-benzoquinone (yellow) with NH3 (aq) to create 2,5-diamino-3,6-dichloro-1,4-benzoquinone (purple) and purify it using column chromatography

On the TLC plate, the product section is spotted after purification. Can you assert that the reaction went to completion?

No

On the TLC plate, the product is spotted before purification and there is one spot, did it go to completion?

yes, in comparison with the standard

Factors affecting column chromatography

The mobile phase must dissolve the analyte but cannot dissolve the stationary phase, must load the right amount of silica, and the solvent must move at a good pace

NH2

Polar, due to hydrogen bonding

Column chromatography vs TLC plate

inverse relationship

what happens if we add too much silica to the column?

the product will become too dilute because it will require a lot of mobile phase

what happens if we add too little silica to the column?

the analyte wont separate

what happens if the solvent moves too fast down the column?

there will be no separation

what happens if the solvent moves too slow down the column?

there will be diffusion and less purity

advantage of column chromatography

very effective in purifying

disadvantage of column chromatography

it is expensive, so we use TLC to confirm conditions first

Column solvent

Rf between 0.2 and 0.4, so we use around 5 column volumes. The separation between Rf values must be greater than or equal to 0.2 Rf

What if the Rf is too high in column chromatography?

it will move too fast

what happens if the Rf is too small in column chromatography?

it will move too slow

if you already spotted a fraction, do you spot it again?

no

what happens if only hexane is added on top of the analyte?

only the nonpolar yellow starting material will move

what is used to wet the column?

hexane

what is the eluent in the experiment?

4:1 hexane: ethyl acetate

Esters

contribute to fruity smells

isoamyl acetate

isopentyl acetate, banana like odor

Naming esters

First, replace the -ol from the alkyl part of the ester with -yl, then replace the -ic from the acyl portion with -ate

Formula for making esters

Carboxylic acid + alcohol → ester + water

isoamyl acetate reaction

reversible reaction with acetic acid, isopentanol, and sulfuric acid as a catalyst, in reflux

what is the limiting reagent in the isoamyl acetate reaction? and why?

alcohol because carboxylic acid is easier to remove with acid/base extraction

How is the alcohol measured in ester synthesis?

by weight

what is the formula to figure out how much alcohol is needed?

mass in grams = moles x MW

How is acid measured in ester synthesis?

by volume

what is the formula for how much acid volume is needed for ester synthesis?

volume in mL = (moles x MW) / density

What happens after the completion of ester synthesis?

there is products, catalyst, and excess acid left so we have to do liquid-liquid extraction

liquid - liquid extraction

extraction when the solute partitions between two immiscible liquid layers depending on differences in density and solubility

what are some solvents immiscible in water

benzene rings, dichloromethane, diethyl ether, ethyl acetate, toluene

extraction

keep what moves

washing

remove impurities

How would you do liquid liquid extraction after your ether reflux is over?

add diethyl ether and water to create an organic and aqueous layer

what goes into the aqueous layer after ether synthesis?

water, the catalyst, some carboxylic acid

what goes into the organic layer after ether synthesis?

the ether and some carboxylic acid because it still has hydrocarbons

how do we remove carboxylic acid from the organic layer?

We deprotonate it using a dilute base, sodium carboxylate

acid base extraction

special liquid-liquid extraction that separates compounds based on their acid-base capabilities and solubility

chemically active extraction

changing solubilities by reaction

to remove an acid

add a dilute base to create a charged conjugate base that will move to the aqueous layer

to remove a base

add a dilute acid to create a charged conjugate acid that will move to aqueous layer

neutral compounds

don’t react with an acid or base, and remain in the organic respond

what happens after removing the carboxylic acid from the ether solution?

diethyl ether and the product remain in solution, and since it is volatile, it can be rotary evaporated out

why dont we use NaOH to do the acid base extraction?

it is too strong and can react with the ether

what happens before we can rotary evaporate the diethyl ether and why?

we have to remove all of the water by using the magnesium sulfate drying agent and then filter it out because we dont want water mixed in the organic layer

Lets say inststead of adding excess carboxylic acid we added the difficult to remove alcohol?

we would remove it by column chromatography due to the differences in polarity

What do we do with the ester after rotary evaporation?

we have to purify it using distillation

why do we distill the ester?

to purify out any side products from side reactions

why did we need to remove the water before simple distillation?

There is not a 50-degree difference in boiling points between the water and ester

Since we have little product, how will we know when we have pure product even if the temperature readings are inaccurate?

the temperature will remain constant

What can IR spectroscopy tell us?

what functional groups are present, like an ester, but not the identity unless there is enough product to compare the fingerprint region

what does gas chromatography tell us?

the exact purity of a sample