Organic Chemistry : Using Other Organic Chemistry Lab Techniques

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #11 : Using Other Organic Chemistry Lab Techniques

A mass spectrum produces the following data:

M: m/z = 102; Relative abundance = 100%

M+1: m/z = 103; Relative abundance = 6.9%

Considering the data, which of the following answer choices can possibly represent the number of carbon and nitrogen atoms present in the compound?

Possible Answers:

Carbon atoms: 5

Nitrogen atoms: 2

Carbon atoms: 6

Nitrogen atoms: 1

Carbon atoms: 8

Nitrogen atoms: 0

Carbon atoms: 6

Nitrogen atoms: 2

Correct answer:

Carbon atoms: 6

Nitrogen atoms: 2

Explanation:

A mass spectrum is very useful in the fact that it can help find an unknown compound's molecular formula.

To find the number of carbons from the data given, we divide the relative abundance of M+1 by 1.1:

 so we know that the compound must have six or seven carbon atoms (probably 6).

We cannot find exactly how many nitrogen atoms are present based on the data given. However, we can use the data and the nitrogen rule to determine that there must be an even number of nitrogens (0, 2, 4, ...) present in the compound. (The nitgrogen rule states that if the M peak has an m/z that is even, the compound must contain an even number of nitrogens).

The only answer that fits into both of these requirements is: 6 carbon atoms and 2 nitrogen atoms

Example Question #11 : Using Other Organic Chemistry Lab Techniques

Nmr

What is the NMR description of the given molecule?

Possible Answers:

1 doublet, 1 triplet ,1 quadruple (multiplet)

2 singlet, 1 doublet

1 singlet, 1 doublet, 1 quadruple (multiplet)

1 singlet,  2 quadruple (multiplet)

1 singlet, 1 triplet, 1 quadruple (multiplet)

Correct answer:

1 singlet, 1 triplet, 1 quadruple (multiplet)

Explanation:

The molecule has one  and one  next to each other. This results in the  reading with a quadruple, since it is next to 3 similar hydrogen. The   reading will have a triplet. The   group that is alone will have only a singlet peak.

Example Question #63 : Organic Chemistry

Rank the following IR bond resonances in order of increasing frequency:

 sp3,  sp2, ,

Possible Answers:

, ,  sp3,  sp2

, ,  sp2,  sp3

, ,  sp3,  sp2

, ,  sp3,  sp2

, ,  sp3,  sp2

Correct answer:

, ,  sp3,  sp2

Explanation:

Frequency is directly proportional to wavenumber, as both quantities are inversely proportional to wavelength. Thus one can order the resonances in order of increasing wavenumber:

 has the lowest wavenumber 

 

 sp3 

 sp2 

 

Example Question #64 : Organic Chemistry

Which analytical instrument is used primarily to determine the functional groups present in a compound?

 

Possible Answers:

NMR Spectroscopy

UV-vis Spectroscopy

IR Spectroscopy 

Mass Spectrometry

Correct answer:

IR Spectroscopy 

Explanation:

Infrared Spectroscopy is used to determine the functional groups present in a molecule. It makes use of the IR region of the electromagnetic spectrum. When an analyte is exposed to IR radiation, different functional groups absorb radiation at their characteristic wavelengths causing vibrational and rotational excitation to bonds within the molecule. The frequencies absorbed correspond to the amount of energy required to increase the molecular vibration of the functional groups in a molecule and can be plotted on a graph for analysis. 

Example Question #65 : Organic Chemistry

Which of the following spectroscopic techniques provides the most information about an organic molecule's framework/structure? 

Possible Answers:

mass spectrometry

NMR

UV-Visible

IR

Correct answer:

NMR

Explanation:

It is most useful for determining the type of nuclei (most commonly studied nuclei are  and ) present and their relative locations within a molecule. H-NMR is most commonly used because it is practically present in all organic compounds. This technique is useful for a complete determination of the structure of organic compounds. 

Example Question #66 : Organic Chemistry

Thin layer chromatography (TLC) is being used to separate a crude mixture of the following compounds using a 3:1 hexanes:acetone. What would be the order of the compounds on the TLC plate once run, highest Rf to lowest Rf?

1q1

Possible Answers:

I, III, II, IV

II, I, IV, III

IV, III, I, II

IV, I, II, III

I, IV, III, II

Correct answer:

IV, I, II, III

Explanation:

The most polar compounds will stick to the TLC plate and not follow the solvent very much, if at all. The more non-polar compounds will tend to follow the solvent front and therefore have the highest Rf. Rf is the distance the compound travels divided by the distance the solvent travels. The most non-polar compound is iso-propylbenzene, IV, followed by acetophenone, I. Benzyl alcohol, II, would be followed by benzoic acid, III, which would not move up the plate much at all. Alcohols are polar compounds, as are carboxylic acids, which are the most polar of the four listed. 

Example Question #67 : Organic Chemistry

1q3

A compound was analyzed using the TLC plate above in diethyl ether. The TLC plate is 60 mm long and the compound was spotted 5 mm from the bottom of the plate. The solvent was allowed to travel up the plate until it was 5 mm from the top. If the compound traveled a total of 15 mm, what is the Rf of this compound?

Possible Answers:

Correct answer:

Explanation:

To calculate the Rf you must determine the length the solvent traveled starting at the position of the compound. Then determine how far the compound traveled. 

To determine the distances you must analyze the given information. The compound distance traveled is given (15 mm). The solvent distance is started from where the compound was spotted to where the solvent was stopped (the dashed line). If the TLC plate is 60 mm, then subtract 5 mm since the compound was spotted 5 mm up the plate from the bottom. Then subtract another 5mm because the solvent was stopped 5 mm from the top:

Example Question #61 : Organic Chemistry

What is the correct way to handle broken glass in the laboratory?

Possible Answers:

Throw it away in the trash.

Leave it alone.

Place broken glass in the "broken glass" receptacle.

Leave it for the janitor to clean.

Place in the recycle bin.

Correct answer:

Place broken glass in the "broken glass" receptacle.

Explanation:

The only place to dispose of broken glass is the "broken glass" receptacle. This is provided in every lab so no one gets hurt when transferring waste from the lab. If glass is disposed of in the trash, it can cut someone through the bag when handling the refuse. Always clean broken glass as soon as possible. Broken glass in a lab can have chemical residue and is considered a health hazard. Never walk away from broken glass thinking someone else will find it.

Example Question #61 : Organic Chemistry

The theoretical yield for a reaction is . If  of product is isolated, what is the percent yield for the reaction?

Possible Answers:

44.2%

22.9%

29.6%

70.4%

77.1%

Correct answer:

77.1%

Explanation:

The percent yield of a reaction is calculated by dividing the actual yield by the theoretical yield, then multiplying that number by 100%:

Example Question #70 : Organic Chemistry

The starting compound used in a reaction, which proceeds by a SN1 mechanism, has an optical rotation of . What is the expected optical rotation of the product? (Assume the substitution occurs at the chiral center.)

Possible Answers:

Correct answer:

Explanation:

A SN1 reaction proceeds with loss of stereochemistry. A pure chiral compound rotates plane polarized light. This is measured as an optical rotation in degrees. Two enantiomers have equal and opposite optical rotations. If the starting material has an optical rotation of , then it's enantiomer has the same rotation, but negative . In this case the reaction will become racemic (equal amounts of both isomers) and have an optical rotation of .

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