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Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Non è quello che usi tu?Cambia libro di testo
Capitolo 10, Problema 64b

In Section 10.12, we saw that S-adenosylmethionine (SAM) methylates the nitrogen atom of noradrenaline to form adrenaline, a more potent hormone. If SAM methylates an OH group attached to the benzene ring instead, it completely destroys noradrenaline's activity.
b. Which reaction is more apt to occur, methylation on nitrogen or methylation on oxygen?

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Understand the context: The problem involves the methylation of noradrenaline by S-adenosylmethionine (SAM). Noradrenaline has functional groups that can be methylated, specifically the nitrogen atom and the oxygen atom of the hydroxyl group attached to the benzene ring. The question asks which site is more likely to undergo methylation.
Analyze the reactivity of the nitrogen atom: The nitrogen atom in noradrenaline is part of an amine group. Amines are nucleophilic due to the lone pair of electrons on the nitrogen, making them reactive toward electrophiles like the methyl group from SAM. This nucleophilicity makes the nitrogen a favorable site for methylation.
Analyze the reactivity of the oxygen atom: The oxygen atom in the hydroxyl group attached to the benzene ring is also nucleophilic due to its lone pairs of electrons. However, the hydroxyl group is part of a phenol structure, where the lone pairs on oxygen are partially delocalized into the aromatic ring. This delocalization reduces the nucleophilicity of the oxygen compared to the nitrogen.
Consider steric and electronic factors: Methylation on the nitrogen atom is more likely because the nitrogen is more nucleophilic and less hindered compared to the oxygen atom. Additionally, the methylation of the nitrogen atom leads to the formation of adrenaline, which is biologically significant, suggesting that the enzyme catalyzing this reaction is optimized for nitrogen methylation.
Conclude the comparison: Based on the nucleophilicity and the biological context, methylation on the nitrogen atom is more apt to occur than methylation on the oxygen atom of the hydroxyl group attached to the benzene ring.

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Methylation Reactions

Methylation is a biochemical process where a methyl group (CH3) is added to a molecule, often affecting its function. In the context of organic chemistry, methylation can occur on various functional groups, such as nitrogen or oxygen. The reactivity and stability of the resulting compound depend on the nature of the atom being methylated and the surrounding chemical environment.
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Functional Group Reactivity

Different functional groups exhibit varying reactivities due to their electronic and steric properties. Nitrogen atoms, especially in amines, are generally more nucleophilic than oxygen atoms in alcohols or phenols, making them more likely to undergo methylation. Understanding the reactivity of these functional groups is crucial for predicting the outcome of methylation reactions.
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Biological Significance of Methylation

Methylation plays a vital role in biological systems, influencing the activity of hormones and neurotransmitters. For instance, the methylation of noradrenaline to form adrenaline enhances its potency as a hormone. Conversely, methylation on an oxygen atom can lead to loss of biological activity, highlighting the importance of the specific site of methylation in determining molecular function.
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