Ch. 2 - General Chemistry Translated: Finding the Electrons

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 2 - General Chemistry Translated: Finding the Electrons

Problem 51

Chapter 1, Problem 51

To which atom of formaldehyde would you expect H⁺ to add?

Verified step by step guidance

Step 1: Begin by identifying the structure of formaldehyde. Formaldehyde has the molecular formula CH₂O and consists of a carbon atom double-bonded to an oxygen atom and single-bonded to two hydrogen atoms.

Step 2: Analyze the electronic structure of formaldehyde. The carbon atom in the carbonyl group (C=O) is partially positive due to the electronegativity difference between carbon and oxygen. Oxygen is more electronegative and pulls electron density toward itself, creating a dipole moment.

Step 3: Consider the nature of H⁺ (a proton). H⁺ is an electrophile, meaning it seeks regions of high electron density to form a bond. It will preferentially add to the atom in formaldehyde that has the highest electron density or is most nucleophilic.

Step 4: Evaluate the atoms in formaldehyde. The oxygen atom in the carbonyl group has lone pairs of electrons and is highly nucleophilic, making it the most likely site for H⁺ addition. The carbon atom is less nucleophilic due to its partial positive charge.

Step 5: Conclude that H⁺ would add to the oxygen atom of formaldehyde, forming a protonated intermediate where the oxygen atom gains a positive charge.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electrophilic Addition

Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile reacts with a nucleophile. In the case of formaldehyde, the carbon atom is electrophilic due to its partial positive charge, making it susceptible to attack by nucleophiles such as H⁺. Understanding this concept is crucial for predicting the site of protonation in carbonyl compounds.

Carbonyl Group Reactivity

The carbonyl group (C=O) is a key functional group in organic chemistry, characterized by a carbon atom double-bonded to an oxygen atom. The carbon atom in the carbonyl is electrophilic, while the oxygen atom is more electronegative, creating a dipole. This polarity influences the reactivity of carbonyl compounds, including their interaction with acids like H⁺.

Formaldehyde Structure

Formaldehyde (H₂C=O) is the simplest aldehyde, consisting of a carbon atom bonded to a hydrogen atom and a hydroxymethyl group. Its structure allows for unique reactivity patterns, particularly in electrophilic addition reactions. Recognizing the arrangement of atoms in formaldehyde helps in understanding how H⁺ would preferentially add to the carbon atom, leading to the formation of a more stable intermediate.

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Drawing Resonance Structures

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Which atoms in the enol ether would you expect to react with Br⁺ ?

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Textbook Question

i) Which of the following resonance structures represents the 'actual' structure of the molecule shown? (ii) Which contributes more to the resonance hybrid? (iii) Why?

(a)

(b)

(c)

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Assign the hybridization of the nitrogen in each resonance structure of acetamide.

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How many valence electrons does each of the following contribute to a Lewis structure?

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Draw the resonance structure that would result from the indicated movement of electrons.

(a)