Question

1.) What is the purpose of making derivatives of unknowns?

2.) Give three reasons why acetone might interfere with your results.

3.) An unknown sample does not react with 2,4-dinitrophenylhydrazine reagent, but a color change is observed on reaction with ceric nitrate reagent. Draw the structure of a compound that would give this result.

4.) An unknown sample produces a precipitate upon reaction with 2,4-dinitrophenylhydrazine reagent. Draw the structure of a compound that would give this result.

5.) The unknown in question 4 causes Schiff's reagent to turn a magenta color. Draw the structure of a compound that would give this result.

6.) An unknown sample produces a reddish precipitate upon reaction with 2,4-dinitrophenylhydrazine reagent, no color change with Schiff's reagent, and a yellow precipitate when mixed with iodine and base. Draw the structure of a compound that would give this result.

7.) Using chemical tests how would you distinguish among 1-pentanol, 2-pentanone, 3-pentanone, and pentanal?

8.) Draw the 1H NMR spectrum of 2-pentanone. Include chemical shifts, splittings, and areas of signals

Answer 1

Answers:

1.) What is the purpose of making derivatives of unknowns??

A derivative of an unknown provides the final evidence in the identification and confirmation for the functional group in the unknown compound. The melting point of the derivative of unknown is an additional physical constant for comparison with known standard value.

Usually, crystalline derivatives such as a semicarbazone or 2,4-dinitrophenylhydrazone (derived from aldehydes or ketones) are prepared as a simple way of verifying the identity of the original compound.

2.) Give three reasons why acetone might interfere with your results.?

Acetone is a simple methyl ketone. It may interfere with 2,4-dinitrophenylhydrazine reagent used for carbonyl group detection.

It may also interfere in iodoform test of methyl ketones.

It may also mislead in the IR and 1H NMR spectral analysis of unknown compounds.

3.) An unknown sample does not react with 2,4-dinitrophenylhydrazine reagent, but a color change is observed on reaction with ceric nitrate reagent. Draw the structure of a compound that would give this result.

?Negative result for 2,4-dinitrophenylhydrazine reagent indicates the unknown does not have a carbonyl group in it. The color change upon reaction with ceric nitrate reagent indicates that it is an alcohol (usually below C10 alcohols). The molecular formula is usually indicated as shown here: ROH where is an alkyl group of C1 to C10 carbons), Example: n-pentanol (CH₃CH₂CH₂CH₂CH₂OH)

4.) An unknown sample produces a precipitate upon reaction with 2,4-dinitrophenylhydrazine reagent. Draw the structure of a compound that would give this result.

Formation of a bright yellow to dark red precipitate upon reaction of an unknown compound with 2,4-dinitrophenylhydrazine reagent is a confirmatory test for a carbonyl compound i.e an aldehyde or a ketone.

Example: RCHO + 2,4-dinitrophenylhydrazine ...............> RCH=NNH(2,4-dinitrophenyl)

5.) The unknown in question 4 causes Schiff's reagent to turn a magenta color. Draw the structure of a compound that would give this result.

Formation of a magenta color with Schiff's reagent is a confirmatory evidence for an aldehyde compound. Example: n-pentanal, CH₃CH₂CH₂CH₂CHO.

6.) An unknown sample produces a reddish precipitate upon reaction with 2,4-dinitrophenylhydrazine reagent, no color change with Schiff's reagent, and a yellow precipitate when mixed with iodine and base. Draw the structure of a compound that would give this result.

The above sequence of tests is a confirmatory test for methyl ketone, such as 2-pentanone: CH₃CH₂CH₂COCH₃.

7.) Using chemical tests how would you distinguish among 1-pentanol, 2-pentanone, 3-pentanone, and pentanal??

These compounds may be grouped into alcohol and carbonyl compounds.

Subjecting the above compounds for 2,4-dinitrophenylhydrazine reagent and a negative result is a confirmatory test for 1-pentanol.

Among the remaining carbonyl compounds (2-pentanone, 3-pentanone, and pentanal) pentanal is an aldehyde.

Positive results with 2,4-dinitrophenylhydrazine reagent and Schiff's reagent is indicative of pentanal.

Positive results with 2,4-dinitrophenylhydrazine reagent and iodoform test is indicative of 2-pentanone.

The remaining compound is 3-pentanone.

8.) Draw the 1H NMR spectrum of 2-pentanone. Include chemical shifts, splittings, and areas of signals

Following are the chemical shifts, splittings, and areas of signals of protons in 2-pentanone.

CH3................1 Carbon.............2.1 ppm, singlet, 3H

CH2.................3 Carbon...........2.5 ppm, triplet, 2H

CH2.................4 Carbon...........1.5 ppm, sextet, 2H

CH3.................5 Carbon............0.9 ppm, triplet, 3H