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Why is tertiary alcohol more acidic in a gas phase?
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Ludeman Eng
Why is tertiary alcohol more acidic in a gas phase?
It comes down to the stability of the conjugate base, [math]mathrm{RO^-}[/math], which is known as an alkoxide ion. The more stable the conjugate base of an acid, the more acidic that acid will be.
In the solution phase, a tertiary alkoxide ion is less stable than a primary or secondary alkoxide ion because the bulkier tertiary alkyl group makes the anionic oxygen atom harder to solvate. Solvation = stabilization, and allows the negative charge to be partly spread to the nearby solvent molecules.
In the gas phase, there is no solvent. The only possible stabilization comes from charge delocalization, and in a saturated alkoxide this must be via inductive “pushing” of charge onto neighboring atoms. A tertiary alkoxide ion has more nearby atoms over which the charge can be spread by induction, and so in the gas phase a tertiary alkoxide ion is more stable than a primary or secondary alkoxide ion.
Note that inductive stabilization of negative charge is much weaker than resonance: while saturated alcohols have [math]pK_a[/math] values in the 16–18 range, phenols and enols (resonance delocalization of charge onto carbon) have [math]pK_a[/math] values around 10, and carboxylic acids (resonance delocalization of charge onto oxygen) have [math]pK_a[/math] values around 5.
It comes down to the stability of the conjugate base, [math]mathrm{RO^-}[/math], which is known as an alkoxide ion. The more stable the conjugate base of an acid, the more acidic that acid will be.
In the solution phase, a tertiary alkoxide ion is less stable than a primary or secondary alkoxide ion because the bulkier tertiary alkyl group makes the anionic oxygen atom harder to solvate. Solvation = stabilization, and allows the negative charge to be partly spread to the nearby solvent molecules.
In the gas phase, there is no solvent. The only possible stabilization comes from charge delocalization, and in a saturated alkoxide this must be via inductive “pushing” of charge onto neighboring atoms. A tertiary alkoxide ion has more nearby atoms over which the charge can be spread by induction, and so in the gas phase a tertiary alkoxide ion is more stable than a primary or secondary alkoxide ion.
Note that inductive stabilization of negative charge is much weaker than resonance: while saturated alcohols have [math]pK_a[/math] values in the 16–18 range, phenols and enols (resonance delocalization of charge onto carbon) have [math]pK_a[/math] values around 10, and carboxylic acids (resonance delocalization of charge onto oxygen) have [math]pK_a[/math] values around 5.
It comes down to the stability of the conjugate base, [math]mathrm{RO^-}[/math], which is known as an alkoxide ion. The more stable the conjugate base of an acid, the more acidic that acid will be.
In the solution phase, a tertiary alkoxide ion is less stable than a primary or secondary alkoxide ion because the bulkier tertiary alkyl group makes the anionic oxygen atom harder to solvate. Solvation = stabilization, and allows the negative charge to be partly spread to the nearby solvent molecules.
In the gas phase, there is no solvent. The only possible stabilization comes from charge delocalization, and in a saturated alkoxide this must be via inductive “pushing” of charge onto neighboring atoms. A tertiary alkoxide ion has more nearby atoms over which the charge can be spread by induction, and so in the gas phase a tertiary alkoxide ion is more stable than a primary or secondary alkoxide ion.
Note that inductive stabilization of negative charge is much weaker than resonance: while saturated alcohols have [math]pK_a[/math] values in the 16–18 range, phenols and enols (resonance delocalization of charge onto carbon) have [math]pK_a[/math] values around 10, and carboxylic acids (resonance delocalization of charge onto oxygen) have [math]pK_a[/math] values around 5.
It comes down to the stability of the conjugate base, [math]mathrm{RO^-}[/math], which is known as an alkoxide ion. The more stable the conjugate base of an acid, the more acidic that acid will be.
In the solution phase, a tertiary alkoxide ion is less stable than a primary or secondary alkoxide ion because the bulkier tertiary alkyl group makes the anionic oxygen atom harder to solvate. Solvation = stabilization, and allows the negative charge to be partly spread to the nearby solvent molecules.
In the gas phase, there is no solvent. The only possible stabilization comes from charge delocalization, and in a saturated alkoxide this must be via inductive “pushing” of charge onto neighboring atoms. A tertiary alkoxide ion has more nearby atoms over which the charge can be spread by induction, and so in the gas phase a tertiary alkoxide ion is more stable than a primary or secondary alkoxide ion.
Note that inductive stabilization of negative charge is much weaker than resonance: while saturated alcohols have [math]pK_a[/math] values in the 16–18 range, phenols and enols (resonance delocalization of charge onto carbon) have [math]pK_a[/math] values around 10, and carboxylic acids (resonance delocalization of charge onto oxygen) have [math]pK_a[/math] values around 5.
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