Question

Write structural formulas for substances (one for each part) that fit the following descriptions: (a) a phenol that would be a stronger acid than phenol itself (b) that isomer of dichlorophenol that is the strongest acid (c) the Claisen rearrangement product from a \(\alpha\) -methylally1 2,6 -dimethylpheny 1 ether (d) a quinone which does not undergo Diels-Alder addition (e) a quinone that would be a better charge-transfer agent than quinone itself.

Short answer

(a) The structural formula of the stronger acid is: \( \begin{array}{c} \ce{O-Ph-NO2}\\ \ce{NO2}\\ \end{array} \) (b) The structural formula of the strongest acid among the dichlorophenol isomers is: \( \ce{O-Ph(Cl2)}\\ \ce{2,4-(Cl2)}\\ \) (c) The structural formula of the Claisen rearrangement product is: \( \ce{OC6H3(CH3)2CH=CHCH2CH3} \) (d) The structural formula of the quinone that does not undergo Diels-Alder addition is: \( \ce{O=C\C=C\CO} \) (e) The structural formula of the quinone that acts as a better charge-transfer agent is: \( \ce{O=C\C=C(\ce{OCH3})\CO} \)

Step-by-step solution

(a) Stronger Acidic Phenol than Phenol Itself

To make a phenol stronger than phenol itself, we should add electron-withdrawing groups to the phenol ring, which will stabilize the conjugate base after deprotonation. Here, we can add a nitro group (-NO2) to the phenol ring as it is an electron withdrawing group that will increase the acidity of the phenol. The structural formula of the stronger acid is as follows: \( \begin{array}{c} \ce{O-Ph-NO2}\\ \ce{NO2}\\ \end{array} \) Here, "\(-\)Ph" represents the phenol group and "\(-\)NO2" is the nitro group connected to the phenol ring. This form of phenol, with the nitro group added, is a stronger acid than phenol itself.

(b) Strongest Acidic Isomer of Dichlorophenol

The strongest acidic isomer of dichlorophenol will have chlorine atoms as substituents in positions that maximally enhance the acidity of the phenolic group by stabilizing its conjugate base. This can be achieved by placing the chlorine atoms at positions 2 and 4 (ortho and para to the phenolic group). These positions will have the strongest inductive and resonance effects on the compound. The structural formula of this isomer is: \( \ce{O-Ph(Cl2)}\\ \ce{2,4-(Cl2)}\\ \) Here, the "\(-\)Ph(Cl2)" represents the phenol group with chlorine atoms at positions 2 and 4, and this is the strongest acid among the dichlorophenol isomers.

(c) Claisen rearrangement product from α-methylally1 2,6-dimethylphenyl 1 ether

The Claisen rearrangement involves a [3,3] sigmatropic rearrangement of allyl vinyl ethers. In this case, the starting compound is α-methylallyl 2,6-dimethylphenyl 1 ether, and we need to find the rearrangement product. The structural formula of the starting compound is as follows: \( \ce{O(C6H3(CH3)2)C=C(CH3)CH2} \) The rearrangement will involve the migration of the aryl group to the terminal carbon of the allyl group and the formation of a new carbon-carbon bond. The structural formula of the Claisen rearrangement product is: \( \ce{OC6H3(CH3)2CH=CHCH2CH3} \)

(d) A Quinone that does not undergo Diels-Alder addition

Diels-Alder reactions involve the cycloaddition of a diene with a dienophile. To make a quinone that cannot undergo Diels-Alder reactions, we need to remove the diene or dienophile characteristics of the molecule. We can achieve this by putting a double bond between the carbonyl groups in the quinone ring. This will prevent the structure from acting as a dienophile. The structural formula of this quinone is: \( \ce{O=C\C=C\CO} \)

(e) A Quinone that acts as a better charge-transfer agent than quinone itself

To enhance the charge-transfer ability of a quinone, we can introduce electron-donating groups that will stabilize the radical cation formed upon charge-transfer. We can achieve this by attaching a methoxy group (-OCH3) to one of the ring carbons. The structural formula of this quinone is: \( \ce{O=C\C=C(\ce{OCH3})\CO} \)

Key concepts

Phenol Acidity Enhancement

Understanding phenol acidity is crucial in organic chemistry, as it revolves around the ability of the phenolic compound to donate a proton. This ability can be enhanced by the introduction of electron-withdrawing groups. Such groups stabilize the conjugate base by delocalizing the negative charge after deprotonation.

By adding a nitro group (-NO2), which is a powerful electron-withdrawing group, to the phenol ring, the acidity is substantially increased. The delocalization of the negative charge over the electron-withdrawing group helps stabilize the conjugate base of phenol, making it easier for the compound to lose a proton.

For example, introducing a nitro group to the phenolic ring yields "4-nitrophenol", a substance significantly more acidic than phenol itself due to the strong resonance effect. This is an important concept when designing phenolic compounds with enhanced acid strength.

Claisen Rearrangement

The Claisen rearrangement is a pivotal reaction in organic synthesis, known for its role in forming carbon-carbon bonds via a [3,3] sigmatropic rearrangement. It generally involves the transformation of allyl vinyl ethers into gamma, delta-unsaturated carbonyl compounds.

In a typical Claisen rearrangement, the allyl group migrates to the ortho position relative to the phenol ether, generating a new bond and altering the backbone of the molecule without breaking any aromatic or key structural features. This reaction is thermally driven and proceeds via a concerted mechanism.

Using the example of α-methylallyl 2,6-dimethylphenyl ether, the rearrangement leads to the migration of the aryl component resulting in the formation of a new alkene bond in the product, an essential step in synthetic organic transformations.

Quinone Properties

Quinones are fascinating compounds in organic chemistry, marked by their distinctive cyclic structure with conjugated double bonds and carbonyl groups. Two key properties distinguish quinones — their reactions in Diels-Alder processes and charge-transfer capabilities.

In Diels-Alder reactions, quinones typically act as dienophiles due to their electron-deficient nature. However, modifying their structure, such as inserting a double bond between carbonyls, can negate their dienophile properties, preventing them from participating in this type of cycloaddition.

Moreover, the charge-transfer capability of quinones can be enhanced by introducing electron-donating groups. These groups facilitate the formation of radical cations, improving the electron mobility within the quinone structure. Thus, adding a methoxy group, an electron-donating entity, can transform quinones into better charge-transfer agents.

Structural Formula Design

Designing structural formulas in organic chemistry involves understanding reactivity, properties, and desired chemical behavior. This process demands careful consideration of substituents and their impact on the molecules.

For enhanced acidity, position electron-withdrawing groups strategically to maximize their effect, such as placing nitro groups on a phenol. Similarly, for determining diene or dienophile behavior in quinones, structural changes can prevent or facilitate specific reactions, such as Diels-Alder reactions.

When attempting to enhance electron-donating capability, consider adding alkoxy groups as seen with methoxy's influence on quinones. Each structural change has a clear objective: to modulate the chemical behavior of the molecule, fitting it into desired chemical pathways and functions.