Under the basic conditions, why dehydration of 3-hydroxybutanal occurs but no dehydration occurs for penta-4-en-2-ol , is to be explained. Concept introduction: When treated with a strong base, a ketone or aldehyde can react via an aldol addition, resulting in a β-hydroxy carbonyl compound. Under these basic conditions, an enolate anion acts as a nucleophile. Heating an aldol reaction facilitates an aldol condensation, in which the β-hydroxy carbonyl product undergoes dehydration to yield an α,β -unsaturated carbonyl compound. Alpha hydrogen of β-hydroxy carbonyl compounds is sufficiently acidic and the pKa is around 20 , thus, it can be abstracted by a strong base such as sodium hydroxide. The pKa of the alpha hydrogen atom in an alkene is around 42 . The alpha hydrogen atom in an alkene is not sufficiently acidic, so the sodium hydroxide cannot abstract the acidic hydrogen in alkenes. The aldol condensation reaction of a β-hydroxy carbonyl compound follows an E1cb mechanism that stands for elimination, unimolecular, conjugate base. Because of the enhanced acidity at the α carbon, Step 1 is a proton transfer, generating a resonance-stabilized enolate anion. In Step 2, the HO − leaving group departs to yield the overall product.

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Definition Definition Organic compounds that have a carbonyl group, C=O, as their functional group. The carbonyl group in aldehydes is placed at the end of the molecular structure, which means the C=O is attached to one hydrogen atom and an alkyl group or a benzene ring. Just like all the other homologous series in organic chemistry, the naming of aldehydes uses the suffix “-al”. The general molecular formula is C n H 2n O.

Chapter 18, Problem 18.25P

Interpretation Introduction

Interpretation:

Under the basic conditions, why dehydration of 3-hydroxybutanal occurs but no dehydration occurs for penta-4-en-2-ol, is to be explained.

Concept introduction:

When treated with a strong base, a ketone or aldehyde can react via an aldol addition, resulting in a β-hydroxy carbonyl compound. Under these basic conditions, an enolate anion acts as a nucleophile. Heating an aldol reaction facilitates an aldol condensation, in which the β-hydroxy carbonyl product undergoes dehydration to yield an α,β-unsaturated carbonyl compound. Alpha hydrogen of β-hydroxy carbonyl compounds is sufficiently acidic and the pKa is around 20, thus, it can be abstracted by a strong base such as sodium hydroxide. The pKa of the alpha hydrogen atom in an alkene is around 42. The alpha hydrogen atom in an alkene is not sufficiently acidic, so the sodium hydroxide cannot abstract the acidic hydrogen in alkenes. The aldol condensation reaction of a β-hydroxy carbonyl compound follows an E1cb mechanism that stands for elimination, unimolecular, conjugate base. Because of the enhanced acidity at the α carbon, Step 1 is a proton transfer, generating a resonance-stabilized enolate anion. In Step 2, the HO− leaving group departs to yield the overall product.

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Answer 2

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Definition Definition Organic compounds that have a carbonyl group, C=O, as their functional group. The carbonyl group in aldehydes is placed at the end of the molecular structure, which means the C=O is attached to one hydrogen atom and an alkyl group or a benzene ring. Just like all the other homologous series in organic chemistry, the naming of aldehydes uses the suffix “-al”. The general molecular formula is C n H 2n O.

Chapter 18, Problem 18.25P

Interpretation Introduction

Interpretation:

Under the basic conditions, why dehydration of 3-hydroxybutanal occurs but no dehydration occurs for penta-4-en-2-ol, is to be explained.

Concept introduction:

When treated with a strong base, a ketone or aldehyde can react via an aldol addition, resulting in a β-hydroxy carbonyl compound. Under these basic conditions, an enolate anion acts as a nucleophile. Heating an aldol reaction facilitates an aldol condensation, in which the β-hydroxy carbonyl product undergoes dehydration to yield an α,β-unsaturated carbonyl compound. Alpha hydrogen of β-hydroxy carbonyl compounds is sufficiently acidic and the pKa is around 20, thus, it can be abstracted by a strong base such as sodium hydroxide. The pKa of the alpha hydrogen atom in an alkene is around 42. The alpha hydrogen atom in an alkene is not sufficiently acidic, so the sodium hydroxide cannot abstract the acidic hydrogen in alkenes. The aldol condensation reaction of a β-hydroxy carbonyl compound follows an E1cb mechanism that stands for elimination, unimolecular, conjugate base. Because of the enhanced acidity at the α carbon, Step 1 is a proton transfer, generating a resonance-stabilized enolate anion. In Step 2, the HO− leaving group departs to yield the overall product.

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Answer 3

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Definition Definition Organic compounds that have a carbonyl group, C=O, as their functional group. The carbonyl group in aldehydes is placed at the end of the molecular structure, which means the C=O is attached to one hydrogen atom and an alkyl group or a benzene ring. Just like all the other homologous series in organic chemistry, the naming of aldehydes uses the suffix “-al”. The general molecular formula is C n H 2n O.

Chapter 18, Problem 18.25P

Interpretation Introduction

Interpretation:

Under the basic conditions, why dehydration of 3-hydroxybutanal occurs but no dehydration occurs for penta-4-en-2-ol, is to be explained.

Concept introduction:

When treated with a strong base, a ketone or aldehyde can react via an aldol addition, resulting in a β-hydroxy carbonyl compound. Under these basic conditions, an enolate anion acts as a nucleophile. Heating an aldol reaction facilitates an aldol condensation, in which the β-hydroxy carbonyl product undergoes dehydration to yield an α,β-unsaturated carbonyl compound. Alpha hydrogen of β-hydroxy carbonyl compounds is sufficiently acidic and the pKa is around 20, thus, it can be abstracted by a strong base such as sodium hydroxide. The pKa of the alpha hydrogen atom in an alkene is around 42. The alpha hydrogen atom in an alkene is not sufficiently acidic, so the sodium hydroxide cannot abstract the acidic hydrogen in alkenes. The aldol condensation reaction of a β-hydroxy carbonyl compound follows an E1cb mechanism that stands for elimination, unimolecular, conjugate base. Because of the enhanced acidity at the α carbon, Step 1 is a proton transfer, generating a resonance-stabilized enolate anion. In Step 2, the HO− leaving group departs to yield the overall product.

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Answer 4

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Definition Definition Organic compounds that have a carbonyl group, C=O, as their functional group. The carbonyl group in aldehydes is placed at the end of the molecular structure, which means the C=O is attached to one hydrogen atom and an alkyl group or a benzene ring. Just like all the other homologous series in organic chemistry, the naming of aldehydes uses the suffix “-al”. The general molecular formula is C n H 2n O.

Chapter 18, Problem 18.25P

Interpretation Introduction