Organic Chemistry : Organic Concepts

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #1 : Identifying Aromatic Compounds

Consider the structure of cyclobutadiene, shown below:

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

Possible Answers:

Non-aromatic

Aromatic

It depends on the environment

Antiaromatic

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.

Example Question #11 : Identifying Aromatic Compounds

Octagon

Assuming that this molecule is planar, determine whether this molecule is aromatic, and name the number of  electrons ( molecular orbitals).

Possible Answers:

Aromatic, 6  electrons

Not aromatic, 8  electrons

Aromatic, 8  electrons

Non aromatic, 6  electrons

Correct answer:

Not aromatic, 8  electrons

Explanation:

There is no p orbital surrounding the Boron atom, so the ring does not have a fully conjugated pi system. In addition, there are 8  electrons, which does not follow Huckel's rule (an aromatic system contains 4n+2 electrons). The pi electrons include the lone pair (not shown, but implicit) on the nitrogen atom, which is why the answers with "6 electrons" are not correct.

Example Question #11 : Organic Functional Groups And Molecules

1

Assuming that this molecule is planar, determine whether this molecule is aromatic, and name the number of  electrons (not  molecular orbitals). 

Possible Answers:

Not aromatic, 6  electrons. 

Aromatic, 6  electrons. 

Not aromatic, 8  electrons.

Aromatic, 8  electrons.

Correct answer:

Aromatic, 6  electrons. 

Explanation:

The lone pairs on the two nitrogen atoms reside in sp2 orbitals, meaning they do not participate in resonance. Within the ring, there are three  bonds, meaning there are six  electrons. Thus, the molecule is aromatic because it is planar and follows Huckel's rule. 

Example Question #11 : Organic Functional Groups And Molecules

Identify the given organic functional group.

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Possible Answers:

Aldehyde

Acid anhydride

Ketone

Ester

Carboxylic acid

Correct answer:

Aldehyde

Explanation:

Aldehydes are carbonyls with one R-group and one hydrogen attached to the carbonyl carbon.

Example Question #1 : Identifying Carbonyl Compounds

Identify the given organic functional group.

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Possible Answers:

Acetal

Aldehyde

Ester

Carboxylic acid

Ketone

Correct answer:

Ketone

Explanation:

Ketones are carbonyls with two R-groups attached the the carbonyl carbon.

Example Question #1 : Identifying Carbonyl Compounds

Identify the given organic functional group.

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Possible Answers:

Amide

Ester

Acid halide

Carboxylic acid

Acid anhydride

Correct answer:

Acid halide

Explanation:

This is an acid halide, were X is any halogen (group VII). They are the most reactive of all the carboxylic acid derivatives.

Example Question #1 : Identifying Carbonyl Compounds

Identify the given organic functional group.

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Possible Answers:

Carboxylic acid

Aldehyde

Ester

Ketone

Acid anhydride

Correct answer:

Carboxylic acid

Explanation:

Carboxylic acids are composed of one R goup and a hydroxy group bound to the carbonyl carbon. The hydroxyl group gives the molecule acidic properties.

Example Question #2 : Identifying Carbonyl Compounds

Identify the given organic functional group.

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Possible Answers:

Aldehyde

Ester

Acid anhydride

Hemiketal

Acetal

Correct answer:

Hemiketal

Explanation:

This is a hemiketal. Hemiketals are formed from the reaction of a ketone with an alcohol. The alcohol attacks the carbonyl carbon, reducing the double bond and adding a hydroxyl group.

Example Question #3 : Identifying Carbonyl Compounds

Identify the given organic functional group.

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Possible Answers:

Ketal

Hemiacetal

Acetal

Acid anhydride

Hemiketal

Correct answer:

Ketal

Explanation:

Ketals have two -OR groups bound to a central carbon. The R groups attached to the oxygens are not necessarily the same. Ketals result from the reaction of a ketone with two equivalents of alcohol.

Example Question #1 : Identifying Carbonyl Compounds

Identify the given organic functional group.

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Possible Answers:

Ester

Hemiacetal

Acetal

Carboxylic acid

Acid anhydride

Correct answer:

Acid anhydride

Explanation:

Acid anhydrides are relatively reactive, since they have a fairly good leaving group (the conjugate base of a carboxylic acid) [COO-]. They are formed by reacting two carboxylic acids via dehydration synthesis.

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