Hammett Constant And The Hydrophobic (Hansch) Substituent Constant

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- Medicinal Chemistry -
Question No. 3.

Define the Hammett Constant and the hydrophobic (Hansch) substituent constant.
Comment on how inductive and mesomeric polar effects are treated in substituted aromatic systems.

A quantitative structure-activity relationship (QSAR) is an equation which correlates measurable or calculable physical or molecular properties to some specific biological activity.
Once this relationship has been determined, it is possible to predict the biological activity of related drug candidates before they are put through expensive and time-consuming biological testing.
The electronic effects of a substituent have an effect on the ionisation or polarity of a drug. This in turn may affect how easily a drug can pass through
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The substituents should therefore be hydrophobic and electron donating for maximum activity.

• log 1/C is the concentration of drug that has a defined biological activity. The bigger the value of log 1/C , the smaller the value of c, the better the drug.

• P is a measure of overall hydrophobicity.

• π is the substituent hydrophobicity constant and measures the hydrophobicity of a specific region on the drug’s skeleton.
When π is negative, activity decreases with hydrophobicity. You want substituents therefore that are very hydrophilic
When π is positive, activity increases with hydrophobicity. You want substituents therefore that are very hydrophobic

• σ is the Hammett constant discussed above; a measure of the electronic effect of substituents
When σ is positive, electron-withdrawing substituents add to activity
When σ is negative, electron-donating substituents add to activity

References

[xxx] Medicinal Chemistry: An Introduction By Gareth

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