Lena Savoie
The acidity of alcohols and ketones can be compared by looking at the stabilisation of their conjugate bases. In the case of alcohols, the conjugate base is more stable because the negative charge is carried only by the oxygen atom, which is more electronegative than carbon. In contrast, when a ketone is deprotonated, an ionic carbon atom is formed, which is extremely unstable. This is because the carbon atom is smaller and has a higher Zeff. Additionally, the presence of electron-donating alkyl groups in alcohols further stabilises the conjugate base by destabilising the negative charge on the oxygen atom. Therefore, alcohols are more acidic than ketones.
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What You'll Learn
- The negative charge in an alcohol is carried by the oxygen atom, with no resonance structures
- Ketones have a more stable conjugate base than alcohols, making them less acidic
- The carbonyl group in ketones is electron-withdrawing, stabilising the negative charge
- Alcohol's negative charge lies on oxygen, which is more electronegative than carbon
- Ketones are not acidic because they don't form a carbocation
The negative charge in an alcohol is carried by the oxygen atom, with no resonance structures
The acidity of alcohols and ketones can be compared by examining the stability of their conjugate bases. A more stable conjugate base indicates a weaker acid. In the case of alcohols and ketones, the relative stability of their conjugate bases is influenced by the distribution of negative charges in their structures.
In the conjugate base of an alcohol, the negative charge is carried solely by the oxygen atom, and there are no resonance structures. Oxygen is highly electronegative and can accommodate the excess electron, resulting in a relatively stable conjugate base. Conversely, in the case of ketones, the negative charge is distributed between the oxygen atom and the carbon atom in the carbonyl group. While resonance structures can contribute to stabilising the negative charge in ketones, the presence of an ionic carbon atom introduces instability.
The carbonyl group in ketones is electron-withdrawing, which should, in theory, stabilise the negative charge and make ketones more acidic than alcohols. However, this is not the case due to the presence of the ionic carbon atom. When a ketone loses a proton, it forms an ionic carbon atom, which is highly unstable. In contrast, when an alcohol loses a proton, it forms an ionic oxygen atom, which is relatively more stable due to oxygen's higher electronegativity compared to carbon.
Furthermore, the presence of electron-donating alkyl groups in alcohols can further stabilise the negative charge on the oxygen atom, making the conjugate base stronger and the acid weaker. In contrast, in ketones, the presence of additional alkyl groups can destabilise the anion, making the conjugate base weaker and the acid stronger.
In summary, the absence of resonance structures and the localisation of the negative charge on the highly electronegative oxygen atom contribute to the relatively greater stability of the conjugate base of an alcohol compared to that of a ketone. This leads to the conclusion that alcohols are more acidic than ketones.
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Ketones have a more stable conjugate base than alcohols, making them less acidic
The relative acidity of molecules can be determined by examining the stability of their conjugate bases. A stable conjugate base corresponds to a weak base and a less acidic parent molecule.
In the case of ketones and alcohols, the conjugate base of a ketone is more stable than that of an alcohol. This is due to the presence of a carbonyl group in ketones, which has electron-withdrawing properties. When the ketone is deprotonated, the negative charge is stabilised by resonance and electron delocalisation. This results in the negative charge being distributed across multiple electronegative atoms, such as oxygen.
In contrast, when an alcohol is deprotonated, the resulting conjugate base carries the negative charge on the oxygen atom only, with no resonance structures. This lack of resonance makes the conjugate base less stable, and therefore, the parent alcohol molecule more acidic.
Furthermore, the presence of electron-donating alkyl groups in alcohol molecules can further destabilise the conjugate base. The alkyl groups push electrons towards the already negative oxygen atom, making the conjugate base less stable and the parent alcohol molecule more acidic.
Thus, the higher stability of the ketone conjugate base results in a weaker base and a less acidic molecule compared to alcohols. This relationship between conjugate base stability and molecular acidity helps explain why ketones are less acidic than alcohols.
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The carbonyl group in ketones is electron-withdrawing, stabilising the negative charge
The carbonyl group in ketones is electron-withdrawing, which stabilises the negative charge. This is due to the proximity of the adjacent π system, which allows for resonance stabilisation through delocalisation of the negative charge to the more electronegative oxygen atom. This makes the conjugate base of the ketone more stable, which should, in theory, make ketones more acidic than alcohols. However, this is not the case.
The reason for this discrepancy lies in the fact that when you deprotonate a ketone, you are left with an ionic carbon atom, which is extremely unstable. On the other hand, when you deprotonate an alcohol, you are left with an ionic oxygen atom, which is much more stable due to its higher electronegativity and larger size.
Additionally, alcohols have an electron-donating alkyl group in the alkoxide ion, which increases their basicity. In contrast, ketone anions also exhibit electron donation by the alkyl groups, but to a greater extent than in alkoxide ions. This should, in theory, make ketones a weaker base than alkoxide.
However, the pi bond and resonance stabilisation in ketones are not sufficient to overcome the instability of the ionic carbon atom. As a result, a substantial portion of the negative charge still resides on the carbon atom, making it less stable than the corresponding alcohol where the negative charge is predominantly on the oxygen atom.
Furthermore, the presence of additional alkyl groups in ketones can further destabilise the anion due to their weak electron-donating ability. This effect is more pronounced in ketones with multiple alkyl groups, leading to decreased stability of the conjugate base and, consequently, reduced acidity.
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Alcohol's negative charge lies on oxygen, which is more electronegative than carbon
The acidity of alcohols and ketones can be compared by observing the stability of their conjugate bases. The more stable the conjugate base, the stronger the acid. In the conjugate base produced from an alcohol, the negative charge is carried only by the oxygen atom, which is more electronegative than carbon. Oxygen's higher electronegativity makes it more stable than the ionic carbon atom produced from a ketone.
Ketones are not considered acidic. This is because forming a carbocation on the carbon associated with the double bond to the oxygen never occurs due to geometry. Carbocations like to be flat and not on an sp2 hybridized carbon.
In the case of alcohols, the negative charge lies on the more electronegative oxygen atom, which is left after the deprotonation of the alcohol. Oxygen is more electronegative than carbon and is therefore more stable. This is because oxygen is more willing to take on excess electrons.
The carbonyl group on ketones is electron-withdrawing, which stabilizes the negative charge. However, this does not make their conjugate bases more stable, and thus ketones are less acidic than alcohols.
The presence of electron-donating alkyl groups in ketone anions further complicates the comparison. While these groups increase electron donation, the pi bond and resonance stabilization may not be sufficient to make the ketone anion a weaker base than the alkoxide ion found in alcohols.
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Ketones are not acidic because they don't form a carbocation
Ketones are less acidic than alcohols, and this can be explained by the stability of the conjugate base formed. When alcohols are deprotonated, an ionic oxygen atom is left, which is relatively stable. On the other hand, when a proton is removed from a ketone, an ionic carbon atom is formed, which is unstable. This is because oxygen is more electronegative than carbon and is more willing to take on electrons.
Ketones are not acidic because they do not form a carbocation. A carbocation is a positively charged carbon atom that results from the loss of a proton (a positively charged hydrogen ion, H+) from a carbon atom. Carbocations are formed in the SN1/E1 reaction mechanism, where a good leaving group, usually a weak base, is lost, and a carbocation is formed. This often occurs in polar solvents such as water or acetic acid.
The stability of carbocations depends on the donation of electron density from neighboring electron-rich atoms. The more stable the carbocation, the faster the reaction. The stability of carbocations increases as C-H bonds are replaced with C-C bonds. This is because the electron density from multiple C-H dipoles can accumulate, creating a small partial negative charge on the carbon atom, which can then be donated to the adjacent carbocation, making it less electron-deficient.
Ketones do not form carbocations because there is no proton attached to the carbonyl carbon to be removed. The carbonyl carbon in ketones is already partially positive due to the electron-withdrawing effect of the carbonyl group. Therefore, it cannot form a carbocation by losing a proton, as there is no proton to lose.
Additionally, the carbonyl group in ketones does not have any neighboring electron-rich atoms that can donate electron density to stabilize a potential carbocation. Instead, the carbonyl group is electron-withdrawing, which means it stabilizes the negative charge of the conjugate base. This further supports the idea that ketones are less acidic than alcohols due to the stability of their conjugate bases.
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Frequently asked questions
Ketones are not considered acidic, but their alpha hydrogen is relatively more acidic than most hydrogens bonded to sp3 carbons. When you de-protonate an alcohol, you are left with an ionic oxygen atom, which is more stable than the ionic carbon atom you are left with when de-protonating a ketone.
A stable base is a weak base, and a weaker conjugate base is a stronger acid. The carbonyl group on ketones is electron-withdrawing, which stabilizes the negative charge well, but the negative charge still rests on the carbon atom, which is less stable.
Alkyl groups are weakly electron-donating, and they tend to destabilize anions. In the case of alcohols, the presence of alkyl groups further destabilizes the negative charge on the oxygen atom, making it a stronger conjugate base.
