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Chapter 21: Problem 6

Find the most acidic hydrogen in each of the following and write a chemical equation for the proton-transfer process that occurs on reaction with hydroxide ion. Use curved arrows to show electron flow and label the acid, base, conjugate acid, and conjugate base. (a) tert-Butyl methyl ketone (c) Methyl propanoate (b) 3-Methylbutanal

Short Answer

Expert verified
Identify acidic hydrogens on alpha carbons adjacent to carbonyls; hydroxide abstracts them forming water and organic anions.

Step by step solution

01

Identify the Most Acidic Hydrogen

In organic compounds, the acidic hydrogen is typically connected to electronegative atoms, like oxygen or situated on carbons adjacent to carbonyl groups (alpha carbons). For (a) tert-butyl methyl ketone, the alpha hydrogen is on the carbon adjacent to the C=O bond. For (c) methyl propanoate, the alpha hydrogen is also adjacent to the ester carbonyl group. In (b) 3-methylbutanal, again, the alpha hydrogen adjacent to the aldehyde group is the most acidic.
02

Write the Proton-Transfer Equation

The general reaction involves hydroxide ion ( ext{OH}^- ext{)} abstracting the most acidic hydrogen to form water and the deprotonated organic anion. For (a), this is \\[\text{(CH}_3\text{)}_3\text{CCH}_2\text{C}\text{(O)-CH}_3 + \text{OH}^- \rightarrow \text{(CH}_3\text{)}_3\text{CCH}_2\text{C=O-} + \text{H}_2\text{O} \]Similar reactions are written for (b) and (c), with the corresponding alpha hydrogens being abstracted and forming their anions.
03

Indicate Electron Flow with Curved Arrows

Using curved arrows, show the electron pair from the hydroxide ion ( ext{OH}^- ext{)} attacking the acidic hydrogen, and the electrons from the C-H bond moving onto the carbon. This notation is essential for illustrating the movement of electrons.
04

Label Acid, Base, Conjugate Acid, and Conjugate Base

In the reaction, the original organic compound acts as the acid because it donates a proton, and ext{OH}^- acts as the base because it accepts a proton. After proton donation, the deprotonated organic ion is the conjugate base, and ext{H}_2 ext{O} is the conjugate acid. Label these in your chemical equation for clarity.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Alpha Hydrogen
An alpha hydrogen in organic chemistry is a hydrogen atom that is bonded to a carbon atom adjacent to a carbonyl group, such as an aldehyde, ketone, or ester. These alpha hydrogens are particularly important because their acidity is enhanced by the electron-withdrawing capability of the carbonyl group. The presence of the carbonyl oxygen increases the acidity by stabilizing the negative charge that forms when the hydrogen is removed. For instance:
  • In 3-methylbutanal, the most acidic hydrogen is the alpha hydrogen adjacent to the aldehyde functional group.
  • In tert-butyl methyl ketone, the alpha hydrogen next to the ketone group is highly acidic for similar reasons.
Understanding and identifying the alpha hydrogen is crucial for predicting reactivity in organic reactions, particularly in those involving enol and enolate formation.
Proton-Transfer Mechanisms
Proton-transfer mechanisms are processes in which a proton (hydrogen ion) is moved from one molecule to another. This is a fundamental concept in acid-base chemistry. The transfer occurs because of differences in acidity and basicity, where the proton moves from a more acidic molecule to a more basic one.
The steps are straightforward:
  • Identify the most acidic proton in the molecule, often an alpha hydrogen near a carbonyl group.
  • A base, such as the hydroxide ion (\( \text{OH}^- \)), will abstract this proton to form water.
  • The loss of a proton transforms the original molecule into a conjugate base.
These mechanisms are vital for chemical transformations and synthesis, as they often lay the groundwork for further reactions in organic chemistry.
Curved Arrow Notation
Curved arrow notation is a visual tool used to demonstrate the flow of electrons during chemical reactions. It's particularly handy in illustrating the step-by-step sequence of electron movements that occur during proton-transfer reactions. To effectively use this notation:
  • Start the arrow at the electron pair that is doing the "attacking" - in proton-transfer, typically the lone pair on the base (e.g., \( \text{OH}^- \)).
  • Point the arrow to where the electrons are moving - in this case, towards the acidic hydrogen (the alpha hydrogen).
  • When the hydrogen is released, show the electron pair from the C-H bond moving onto the carbon atom to generate a negative charge.
This method simplifies understanding by providing a clear, step-by-step depiction of electron movements, ensuring that reactions and mechanisms are both predictable and interpretable.
Conjugate Acid-Base Pairs
Conjugate acid-base pairs are formations that result from the donation and acceptance of a proton. In any acid-base reaction, when an acid donates a proton, it transforms into its conjugate base, while the base that accepts the proton becomes its conjugate acid. This concept is crucial in understanding chemical equilibria and reaction dynamics in organic chemistry.
Consider the reaction of 3-methylbutanal with hydroxide ion:
  • The original compound is the acid, donating an alpha hydrogen.
  • The hydroxide ion (\( \text{OH}^- \)) acts as the base, accepting this hydrogen to form water, the conjugate acid.
  • As the original acid is deprotonated, it becomes a conjugate base.
Recognizing these relationships is essential, not only for understanding individual reactions but also for predicting the behavior of compounds in a larger chemical context.

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Most popular questions from this chapter

Show how each of the following compounds could be prepared from 3 -pentanone. In most cases more than one synthetic transformation will be necessary. (a) 2 -Bromo-3-pentanone (d) 3-Hexanone (b) 1-Penten-3-one (e) 2-Methyl-1-phenyl-1-penten-3-one (c) 1-Penten-3-ol

The use of epoxides as alkylating agents for diethyl malonate provides a useful route to \(\gamma\) -lactones. Write equations illustrating such a sequence for styrene oxide as the starting epoxide. Is the lactone formed by this reaction 3 -phenylbutanolide, or is it 4-phenylbutanolide?

The \(\mathrm{pK}_{\text {a's }}\) of ethyl acetoacetate and diethyl malonate are 11 and 13 , respectively. Write a structural formula for the most stable enolate of each using resonance to show the electron delocalization in each enolate.

Acrolein \(\left(\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}=\mathrm{O}\right)\) reacts with sodium azide \(\left(\mathrm{NaN}_{3}\right)\) in aqueous acetic acid to form a compound, \(\mathrm{C}_{3} \mathrm{H}_{5} \mathrm{~N}_{3} \mathrm{O}\), in \(71 \%\) yield. Propanal \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{O}\right)\), when subjected to the same reaction conditions, is recovered unchanged. Suggest a structure for the product formed from acrolein, and offer an explanation for the difference in reactivity between acrolein and propanal.

Chlorination of 2 -butanone yields two isomeric products, each having the molecular formula \(\mathrm{C}_{4} \mathrm{H}_{7} \mathrm{ClO}\) (a) What are these two compounds? (b) Write structural formulas for the enol intermediates that lead to each of these compounds. (c) Using curved arrows, show the flow of electrons in the reaction of each of the enols with \(\mathrm{Cl}_{2}\).
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