Organic Chemistry : Identifying Aromatic Compounds

Study concepts, example questions & explanations for Organic Chemistry

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Example Question #1 : Identifying Aromatic Compounds

Compound A. Benzene

Compound B.Cyclobutadiene

Compound C.Cyclotetraoctene

Compound D.1_3_5_hex-tri-ene

Which compound(s) shown above is(are) aromatic?

 

Possible Answers:

A and C

A, B, and C

A

D

Correct answer:

A

Explanation:

For a compound to be considered aromatic, it must be flat, cyclic, and conjugated and it must obey Huckel's rule. Huckel's rule states that an aromatic compound must have  pi electrons in the overlapping p orbitals in order to be aromatic (n in this formula represents any integer). Only compounds with 2, 6, 10, 14, . . .  pi electrons can be considered aromatic. Compound A has 6 pi electrons, compound B has 4, and compound C has 8. This eliminates answers B and C. Answer D is not cyclic, and therefore cannot be aromatic. The only aromatic compound is answer choice A, which you should recognize as benzene.

Example Question #1 : Organic Functional Groups And Molecules

Which of the compounds below is antiaromatic, assuming they are all planar?

Possible Answers:

(8) Annulene

(6) Annulene

(10) Annulene

(2) Annulene

(14) Annulene

Correct answer:

(8) Annulene

Explanation:

The correct answer is (8) Annulene. This is because all aromatic compounds must follow Huckel's Rule, which is 4n+2. Note that "n" in Huckel's Rule just refers to any whole number, and 4n+2 should result in the number of pi electrons an aromatic compound should have. For example, 4(0)+2 gives a two-pi-electron aromatic compound.

It is also important to note that Huckel's Rule is just one of three main rules in identifying an aromatic compound. An annulene is a system of conjugated monocyclic hydrocarbons. A compound is considered anti-aromatic if it follows the first two rules for aromaticity (1. Pi bonds are in a cyclic structure and 2. The structure must be planar), but does not follow the third rule, which is Huckel's Rule.

(8) Annulene follows the first two rules, but not Huckel's Rule, and is therefore antiaromatic; no value of a whole number for "n" will result in 8 with the formula 4n+2.

Example Question #1 : Organic Functional Groups

Img 0611

Which of the following best describes the given molecule?

Possible Answers:

Anti-aromatic

Aromatic

Non-aromatic

None of these

Correct answer:

Anti-aromatic

Explanation:

A molecule is anti-aromatic when it follows all of the criteria for an aromatic compound, except for the fact that it has  pi electrons rather than  pi electrons, as in this case.

Example Question #3 : Organic Functional Groups And Molecules

Img 0612

Which of the following best describes the given molecule?

Possible Answers:

Aromatic

None of these

Non-aromatic

Anti-aromatic

Correct answer:

Non-aromatic

Explanation:

A molecule is aromatic when it adheres to 4 main criteria:

1. The molecule must be planar

2. The molecule must be cyclic

3. Every atom in the aromatic ring must have a p orbital

4. The ring must contain  pi electrons

The carbon on the left side of this molecule is an sp3 carbon, and therefore lacks an unhybridized p orbital. The molecule is non-aromatic.

Example Question #164 : Organic Chemistry

Aromatic

How many pi electrons does the given molecule have?

Possible Answers:

Correct answer:

Explanation:

There are 14 pi electrons because oxygen must contribute 2 pi electrons to avoid antiaromaticity.  The other 12 pi electrons come from the 6 double bonds.

Nitrogen does not contribute any pi electrons, as it is  hybridized and it's lone pairs are stored in sp2 orbitals, incapable of pi delocalization.  Each nitrogen's p orbital is occupied by the double bond.

Example Question #165 : Organic Chemistry

Antiaromatic

How many pi electrons does the given compound have?

Possible Answers:

Correct answer:

Explanation:

If oxygen contributes any pi electrons, the molecule will have 12 pi electrons, or 4n pi electrons, and become antiarmoatic.  As it is now, the compound is antiaromatic.

Nitrogen cannot give any pi electrons because it's lone pair is in an sp2 orbital. Boron has no pi electrons to give, and only has an empty orbital.

Example Question #2 : Organic Functional Groups

Consider the molecular structure of anthracene, as shown below. 

Anthracene

Which of the following is true regarding anthracene?

Possible Answers:

Anthracene follows Huckel's rule

Anthracene is planar

There is an even number of pi electrons

All of these answer choices are true

Correct answer:

All of these answer choices are true

Explanation:

In this question, we're presented with the structure of anthracene, and we're asked to find which answer choices represent a true statement about anthracene. Let's go through each of the choices and analyze them, one by one.

First, let's determine if anthracene is planar, which is essentially asking if the molecule is flat. When looking at anthracene, we see that the molecule is conjugated, meaning there are alternating single and double bonds. If we look at each of the carbons in this molecule, we see that all of them are  hybridized. This means that each of the three other atoms connected to the carbon are organized at a  angle in a single plane. Since ALL of the carbons are this way, we can conclude that anthracene is a planar compound.

Now let's determine the total number of pi electrons in anthracene. Remember, pi electrons are those that contribute to double and triple bonds. So, we'll need to count the number of double bonds contained in this molecule, which turns out to be . But, don't forget that for every double bond there are two pi electrons! Therefore, the total number of pi electrons is twice the amount of the number of double bonds, which gives a value of  pi electrons. This is indeed an even number.

Lastly, let's see if anthracene satisfies Huckel's rule. This rule is one of the conditions that must be met for a molecule to be aromatic. It states that when the total number of pi electrons is equal to , we will be able to have  be an integer value. So let's see if this works.

Since we arrived at an integer value for , we can conclude that Huckel's rule has indeed been satisfied.

All of the answer choices are true statements with regards to anthracene.

Example Question #5 : Organic Functional Groups And Molecules

Consider the molecule furan, shown below:

Screen shot 2015 12 26 at 9.11.51 pm

Is this molecule aromatic, non-aromatic, or antiaromatic?

Possible Answers:

Aromatic

Antiaromatic

Non-aromatic

It depends on the environment

Correct answer:

Aromatic

Explanation:

When determining whether a molecule is aromatic, it is important to understand that aromatic molecules are the most stable, followed by molecules that are non-aromatic, followed by molecules that are antiaromatic (the least stable). Therefore, if it is possible that a molecule can achieve a greater stability through switching the hybridization of one of its substituent atoms, it will do this.

An aromatic must follow four basic criteria: it must be  a ring  planar,  have a continuous chain of unhybridized p orbitals (a series of sp2-hybridized atoms forming a conjugated  system), and  have an odd number of delocalized electron pairs in the  system. Furan is planar ring (fulfilling criteria  and , and its oxygen atom has a choice of being sp3-hybridized or sp2-hybridized. Depending on what hybridization the oxygen atom chooses will determine whether the molecule is aromatic or not.

If the oxygen is sp3-hybridized, the molecule will not have a continuous chain of unhybridized p orbitals, and will not be considered aromatic (it will be non-aromatic). If the oxygen is sp2-hybridized, it will fulfill criterion . Placing one of its lone pairs into the unhybridized p orbital will add two more electrons into the conjugated system, bringing the total number of  electrons to  (or, it will have  pairs of  electrons). Because it has an odd number of delocalized electrons it fulfills criterion , and therefore the molecule will be considered aromatic.

Because an aromatic molecule is more stable than a non-aromatic molecule, and by switching the hybridization of the oxygen atom the molecule can achieve aromaticity, a furan molecule will be considered an aromatic molecule.

Example Question #3 : Organic Functional Groups

Consider the following molecule. 

Screen shot 2015 12 26 at 9.12.09 pm

Is this molecule aromatic, non-aromatic, or antiaromatic?

Possible Answers:

Non-aromatic

Antiaromatic

Aromatic

It depends on the environment

Correct answer:

Non-aromatic

Explanation:

An aromatic must follow four basic criteria: it must be  a ring  planar,  have a continuous chain of unhybridized p orbitals (a series of sp2-hybridized atoms forming a conjugated  system), and  have an odd number of delocalized electron pairs in the  system. This molecule cannot be considered aromatic because this sp3 carbon cannot switch its hybridization (it has no lone pairs). Therefore, it fails to follow criterion  and is not considered an aromatic molecule. It is a non-aromatic molecule.

Example Question #4 : Organic Functional Groups

Consider the structure of cyclobutadiene, shown below:

Screen shot 2015 12 26 at 9.12.26 pm

Is this molecule aromatic, non-aromatic, or antiaromatic?

Possible Answers:

It depends on the environment

Antiaromatic

Aromatic

Non-aromatic

Correct answer:

Antiaromatic

Explanation:

An aromatic must follow four basic criteria: it must be  a ring  planar,  have a continuous chain of unhybridized p orbitals (a series of sp2-hybridized atoms forming a conjugated  system), and  have an odd number of delocalized electron pairs in the  system. If the molecule fails any of the first three criteria, it is considered non-aromatic, and if it fails the only the fourth criterion (it has an even number of delocalized electron pairs), the molecule is considered antiaromatic. In the case of cyclobutadiene, by virtue of its structure follows criteria  and . However, it violates criterion  by having two (an even number) of delocalized electron pairs. Although it's possible that a molecule can try to escape from being antiaromatic by contorting its 3D shape so it is not planar, cyclobutadiene is too small to do this effectively. Therefore, cyclobutadiene is considered antiaromatic.

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