Review Orgo

8/13/2019 Review Orgo

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Review Questions

A sample for thin layer chromatography containing two compounds (say, two dye

molecules) was contaminated by a student with two other compounds. The next

student came and run the T! of the contaminated sample."ould the second student have still gotten the correct Rf values for the original two

compounds##

They would $ust hve extra rf values

%sing a polar solvent, a student will run a T! for two molecules, one polar andone nonpolar, on the same T! plate, which molecule will have a smaller Rf value#

One thing to note with your example capn, is that the adsorbent is normally apolar polymer. This means that even with a polar solvent, a nonpolar

molecule might travel farther up the TLC plate than a polar molecule, due toattraction to the adsorbent.

%sing a nonpolar solvent, a student will run a T! for two molecules, one polar

and one nonpolar, on the same T! plate, which molecule will have a smaller Rf

value#

The polar molecule will have the the smaller rf value because of attraction to the

adsorbent

!onsidering temperature and solubility, what would be the criteria for a good

solvent for recrystalli&ation#

An effective solvent must be mediocre, good enough to dissolve the sample at hightemperature (so that the impurities are freed), but wea' enough so that at least some

of your sample crystalli&es out after cooling (so that you get at least some yield

harvested.)

The compound should be very soluble at the boiling point of the solvent and onlysparingly soluble in the solvent at room temperature. This difference in solubility at hotversus cold temperatures is essential for the recrystallization process

The T! of a compound is run twice, once in a polar solvent, the other in a non

polar solvent. n which eluent solvent will the spot have a high Rf value#

t will probably have a higher rf value in a non polar solvent"hat 'ind of a*ueous solution should be added to the organic solution containing

the following two organic molecules in order to separate them by extraction#

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2/5

A basic a*uesous solution containing +a!- or other bases should be used so thatthe phcooh to phcoo +a/ and goes into a*ueous phase while the neutral molecules

may stay in the organic solvent

0ollowing the previous *uestion, how does a student recover the 1h!--2 fromthe a*ueous solution#

T23 student would use an acidic solution to elute the remaninig phcooh

"hat 'ind of a*ueous solution should be added to the organic solution containingthe following two organic molecules in order to separate them#

An acidic solution bcontaining 2!l should be used to convert R+24 to R+2 /!l

0ollowing the previous *uestion, how does a student recover the R+24from the

a*ueous solution#

Add basic a*ueous solution

2ow would you separate the following two molecules by extraction#

They cannot be easily separated by extraction because they are both neutral

5o what method(s) would you propose to separate the following two molecules#

6y columnchromatography or recrystallli&ation

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3/5

"hen a mixture of four compounds is eluded through a column, !ompound 7comes out the first, then !ompound 8, 9, and ". "hat is ran'ing of the four

compounds from high to low#

a. 7,8,9,"

b. ", 9, 8, 7

c. ",7,8,9

d. !annot be determined based on the information.

"hen a mixture of two compounds are eluded through a column to be separated,

but the compounds never come out after a long time. "hat may li'ely be the

solution to this problem#

a. :ust 'eep running the column with more eluent solvent.

b. Run the column all over again using new sample, and with more polar eluent

solvent.

c. Run the column all over again using new sample, and with less polar eluent

solvent.


"hen a mixture of two compounds is eluded through a column to be separated,both compounds come out rather *uic'ly as a mixture without any separation. "hat

may li'ely be the solution to this problem#

a. ;o not apply pressure, and let the solvent come out very slowly.

b. Rerun the column with more polar eluent solvent.

c. Rerun the column with less polar eluent solvent.


"hat would be $ustified as a good solvent for recrystalli&ation of a sample#

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4/5

!ompare a crystal that is pure of one component and a crystal that is impure, how

would their melting points be different# (there are two parts to the answer for this*uestion).

The melting point of the pure crystal would be higher and more li'e the temp

recorded in literature, range will be larger for the

Apart from the intermolecular interactions, what properties of molecules also

influence how easy it is to form crystals#

molecular packing controlled by shape symmetry

Apart from the intermolecular interactions, what properties of molecules also

influence how easy it is to form crystals#

If the bp of two or more compounds are separated

f the boiling points of two compounds are separated by more than


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