Organic Chemistry : How to Identify Compounds

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #21 : Organic Chemistry

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How many signals would a  NMR spectrum of the molecule shown produce?

Possible Answers:

Correct answer:

Explanation:

 NMR spectrum produces one signal for each unique carbon and one signal for each group of similar carbons in a given molecule. There are 15 carbon atoms in this molecule. However, many of them can be grouped because they are similar to each other. Additionally, the molecule is perfectly symmetric, meaning that corresponding carbon atoms on each side of the line of symmetry are similar. The pictured molecule shows the carbon atoms that would show up as distinct signals on the  NMR spectrum:

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Example Question #21 : How To Identify Compounds

An unknown compound was processed through a mass spectrometer. The molecular formula of the compound reads . Based on this information, this compound can possibly be identified as __________.

Possible Answers:

Two of these

Hexene

Cyclohexane

Hexyne

Correct answer:

Two of these

Explanation:

The correct answer is "two of these."

The molecular formula was given. Therefore, we simply needed to use the following formula to find out how many DBE's (Double Bond Equivalents) that are present in the compound: 

A double bond equivalent is a ring or a pi bond (i.e. a double bond would count as one DBE while a triple bond would count as two DBEs). We know that the compound must have a ring or a double bond. However, without more information, we cannot tell if the compound is a cyclohexane or a hexene.

Example Question #22 : How To Identify Compounds

Name

What is the name of the given molecule?

Possible Answers:

(E)-5-methyl-5-heptene

(Z)-5-methyl-5-heptene

(Z)-3-methyl-2-heptene

(E)-3-methyl-2-heptene

Correct answer:

(E)-3-methyl-2-heptene

Explanation:

Count for the longest carbon chain starting at the end closest to the double bond. Seven carbons make up this chain, so use hept- and there is a double bond, so end with -ene. The methyl group is located at the 3rd carbon and the double bond starts at the 2nd carbon. The molecule has a trans configuration, so it should have E in its name.

Example Question #23 : How To Identify Compounds

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How many C-13 NMR signals would be seen from the given molecule?

Possible Answers:

8

16

9

7

14

Correct answer:

9

Explanation:

Single bonds undergo free rotation, and so this molecule had an internal plane of symmetry giving it only 9 signals. Total number of carbons were 16. If you cannot see the symmetry, rotate the bonds to match alkyl chains.

Example Question #24 : How To Identify Compounds

Which of the following procedures should be used to determine if a compound exhibits conjugation?

Possible Answers:

Scanning tunneling spectroscopy

Infrared spectroscopy

Scanning electron microscopy

Ultraviolet spectroscopy

Nuclear magnetic resonance spectrsocopy

Correct answer:

Ultraviolet spectroscopy

Explanation:

The best procedure to use when determining conjugation of a compound is ultraviolet spectroscopy. Conjugation is present in compounds of connected p-orbitals with delocalized electrons, this is often seen in alternating double bonds and single bonds. UV spectroscopy is used because these conjugated systems are able to absorb light in the ultraviolet and visible spectrum making them visible for observation.

Example Question #21 : Organic Chemistry

Which of the following is a component of the retention factor value in thin layer chromatography?

Possible Answers:

Temperature

Thickness of the adsorbent

Amount of material spotted

Distance traveled by the solvent front

Correct answer:

Distance traveled by the solvent front

Explanation:

Thin layer chromatography (TLC) is used to find the identity of a compound in a mixture. A TLC plate is a sheet of plastic or glass which is coated with a solid adsorbent, such as silica. A small portion of the mixture is spotted near the bottom of the TLC plate, which is placed in a liquid solvent so that the bottom of the plate is in the solvent. This liquid (solvent) slowly rises up the TLC plate by capillary action. As it moves past the spot that was applied to the plate, an equilibrium is formed for each portion of the mixture between the molecules of that portion which are adsorbed on the solid and the molecules which are still in solution. The components should differ in solubility and in the degree of their adsorption, and some components will travel further up the plate than others will. When the solvent reaches the top of the plate, the plate is removed from the liquid and the separated components of the mixture are seen, either with the naked eye or under UV light if components are not colored. Then, we can determine the identity of a component of the mixture using Rf value.

To find the Rf value, we divide the distance traveled by the unknown component by the distance traveled by the solvent front. This equation has nothing to do with the temperature, thickness of the adsorbent, or the amount of material spotted. 

Example Question #25 : How To Identify Compounds

How many proton signals would the given compound show when analyzed by 1H NMR?

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Possible Answers:

5

7

6

8

9

Correct answer:

8

Explanation:

Each proton (hydrogen) in a different environment gives a unique NMR signal. If we analyze this compound we see there are eight different protons, each noted by a different letter in the image.

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Example Question #26 : How To Identify Compounds

When the given compound is analyzed by 13C NMR, how many 13C signals would appear as a triplet?

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Possible Answers:

2

4

1

0

3

Correct answer:

2

Explanation:

To appear as a triplet, a carbon would have to have a signal that is split by two attached hydrogens. In the given compound there are two carbons that have two hydrogens attached to them.

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Example Question #21 : How To Identify Compounds

What functional group is indicated by an infrared absorption at ?

Possible Answers:

Hydroxyl group

Alkane

None of these groups

Carbonyl

Amine

Correct answer:

Carbonyl

Explanation:

One of the predominate diagnostic bands in IR spectroscopy is the strong absorption between  for a carbonyl group. The other groups have absorptions in the IR spectrum, but at differing frequencies:

 (stretch) at 

 at 

 at 

Example Question #21 : Organic Chemistry

What is the (desired) chemical method through which molecules are fragmented when analyzed through mass spectrometry?

Possible Answers:

Ionization through photon bombardment, depending on instability of the bond and stability of the fragments

Bombardment by free radical electrons which ionizes the molecule at different atoms, resulting in distinct fragments

Bombardment by free radical electrons in an ionization chamber which causes the formation of a radical cation, and subsequent spontaneous bond cleavage throughout the molecule, depending on instability of the bond and stability of the fragments

Spontaneous homolytic cleavage of unstable bonds with good leaving groups, unmediated by electron bombardment

Putting the molecule through a strong magnetic field, resulting in various electron spins of the hydrogens in the molecule

Correct answer:

Bombardment by free radical electrons in an ionization chamber which causes the formation of a radical cation, and subsequent spontaneous bond cleavage throughout the molecule, depending on instability of the bond and stability of the fragments

Explanation:

The correct answer is "Bombardment by free radical electrons which causes the formation of a radical cation and spontaneous bond cleavage throughout the molecule, depending on instability of the bond and stability of the fragments." First the parent molecule (the compound being identified) is bombarded by free radicals, resulting in a molecular radical cation, and then the molecular ion spontaneously fragments at the weak bonds and where the resulting fragments will be thermodynamically stable. These fragments then reach the detector plate at different places depending on the mass of each fragment.

Incorrect answers:

"Bombardment by free radical electrons which radicalizes the molecule at different atoms, resulting in distinct fragments" is incorrect because it indicates that the desired fragmentation of the molecule is through direct abstraction of a radical by the free radical, rather than a spontaneous self-cleaving process. If this occurs, it is not a desirable result, because the molecular ion will no longer be structurally in tact for the next chamber of the mass spectrometry in which it is meant to fragment and be separated based on mass/charge ratio. It is also somewhat sterically unlikely for a free radical electron to directly cause the splitting of carbon-carbon bonds in a sterically bulky organic molecule -- the free radical electron is more likely to abstract a proton and leave a positive radical charge on the molecule.

"Spontaneous homolytic cleavage of unstable bonds with good leaving groups, unmediated by electron bombardment" is incorrect because mass spectrometry methods use electrons to induce radical cation formation.

"Ionization through proton bombardment, depending on instability of the bond and stability of the fragments" is false because electrons, rather than protons, are used to ionize the molecule. 

"Putting the molecule through a strong magnetic field, resulting in various electron spins of the hydrogens in the molecule" is incorrect because it does not reflect the spectra produced by mass spectrometry, nor the method by which molecular fragments are produced.

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