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Chapter 6: Problem 2

By taking into account electronegativity differences, draw the products formed by heterolysis of the carbon-heteroatom bond in each molecule. Classify the organic reactive intermediate as a carbocation or a carbanion. a. CC(C)(C)O b. BrC1CCCCC1 c. \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{Li}\)

Short Answer

Expert verified
Products: (a) Carbocation. (b) Carbocation. (c) Carbanion.

Step by step solution

01

Understanding Heterolysis

Heterolysis is the cleavage of a chemical bond where one atom retains both electrons previously shared in the bond. This often occurs between atoms with significant differences in electronegativity. In each example, identify the carbon-heteroatom bond in question.
02

Electronegativity Consideration

Determine which atom in each carbon-heteroatom bond is more electronegative. Electronegativity differences will determine which atom receives the electron pair after bond cleavage. A more electronegative atom will take the electron pair, leading to ions as products.
03

Step 3a: Analyzing Molecule a (CC(C)(C)O)

In molecule (a), the C-O bond is the focus. Oxygen is more electronegative than carbon, so upon heterolysis, oxygen will take both electrons, forming a negative oxygen ion (alkoxide) and a positive carbocation (a tert-butyl cation, \(\mathrm{C}^{+}(\mathrm{CH}_3)_3\)).
04

Step 3b: Analyzing Molecule b (BrC1CCCCC1)

In molecule (b), the C-Br bond is targeted. Bromine is more electronegative than carbon, so the electron pair will remain with bromine post-cleavage, producing a positive carbocation (cyclohexyl cation) and a negative bromide ion (\(\mathrm{Br}^-\)).
05

Step 3c: Analyzing Molecule c (CH3CH2-Li)

In molecule (c), the C-Li bond is considered. Lithium, being a metal, is less electronegative than carbon, so upon bond cleavage, lithium becomes a cation (\(\mathrm{Li}^+\)), and the carbon forms a carbanion (\(\mathrm{CH}_3\mathrm{CH}_2^-\)).
06

Classification of Reactive Intermediates

Based on each heterolytic bond cleavage:- (a) forms a carbocation (\(\mathrm{C}^{+}(\mathrm{CH}_3)_3\))- (b) forms a carbocation (cyclohexyl cation)- (c) forms a carbanion (\(\mathrm{CH}_3\mathrm{CH}_2^-\))

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Key Concepts

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

Electronegativity
Electronegativity is a measure of how strongly an atom can attract or hold onto electrons in a chemical bond. It is a key factor in predicting the behavior of atoms when a bond breaks through heterolysis.
In a heterolysis process, the more electronegative atom in a bond will tend to keep the pair of electrons, leaving the less electronegative atom with a positive charge.
Consider some essential points about electronegativity:
  • Electronegativity usually increases across the periodic table from left to right.
  • It decreases down a group in the periodic table.
  • Fluorine is the most electronegative element, followed by oxygen and nitrogen.
Understanding electronegativity trends helps predict which atom will develop a negative or positive charge during heterolytic bond cleavage. In organic molecules, oxygen, bromine, and similar atoms are generally more electronegative than carbon.
Carbocation
A carbocation is a positively charged carbon atom. It forms when a carbon loses a pair of electrons during heterolysis. This ion often participates as an intermediate in chemical reactions and has some distinctive features.
Carbocations are:
  • Typically unstable, as they lack a full set of electrons.
  • Highly reactive, seeking to stabilize themselves by gaining an electron pair.
  • More stable when they are more substituted, meaning that tertiary carbocations (attached to three other carbons) are more stable than secondary and primary carbocations.
In the context of the exercise, when the bond heterolyses, such as in the C-O bond in the first molecule, a carbocation is formed at the carbon. This tert-butyl carbocation is relatively stable for a carbocation due to the stabilizing effects of the surrounding methyl groups.
Carbanion
A carbanion is a negatively charged carbon atom. It forms when carbon becomes more electronegative than the accompanying atom in a heterolytic bond cleavage, such as a metal in an organometallic compound.
Key characteristics of carbanions include:
  • Having a lone pair of electrons, which grants them a full octet.
  • Being nucleophilic, meaning they tend to donate their lone pair to an electron-poor center.
  • Usually more stable with electron-withdrawing groups adjacent, as these groups can delocalize the negative charge.
In the exercise, the C-Li bond is a classic example where the carbanion forms. Since lithium is less electronegative, carbon retains the electron pair, resulting in a carbanion. This carbanion can engage in various chemical reactions, acting as a nucleophile and forming new bonds.

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Most popular questions from this chapter

a. Which \(K_{\text {eq }}\) corresponds to a negative value of \(\Delta G^{\circ}, K_{\mathrm{eq}}=1000\) or \(K_{\mathrm{eq}}=.001 ?\) b. Which \(K_{\text {oq }}\) corresponds to a lower value of \(\Delta G^{\circ}, K_{\text {eq }}=10^{-2}\) or \(K_{\text {eq }}=10^{-5} ?\)

a. Which value corresponds to a negative value of \(\Delta G^{\circ}: K_{\text {oq }}=10^{-2}\) or \(K_{\text {oq }}=10^{2}\) ? b. In a unimolecular reaction with five times as much starting material as product at equilibrium, what is the value of \(K_{\text {eq }} ?\) Is \(\Delta G^{\circ}\) positive or negative? c. Which value corresponds to a larger \(K_{\text {eq }}: \Delta G^{\circ}=-2\) kcal/mol or \(\Delta G^{\circ}=5 \mathrm{kcal} /\) mol?

For which of the following reactions is \(\Delta S^{\circ}\) a positive value? a. A / M \(\longrightarrow\) CCCCC b. \(\mathrm{CH}_{3} \cdot+\mathrm{CH}_{3}{ } \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{3}\) c. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH})_{2} \longrightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=0 \quad+\mathrm{H}_{2} \mathrm{O}\) d. \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{CH}_{3} \mathrm{COOH}+\mathrm{CH}_{3} \mathrm{OH}\)

\(\Delta H^{\circ}\) for the \(\mathrm{O}-\mathrm{O}\) bond in \(\mathrm{H}_{2} \mathrm{O}_{2}\) is lower than for many other bonds involving second-row elements \(\left(\Delta H^{\circ}=51 \mathrm{kcal} / \mathrm{mol}\right) .\) For example, \(\Delta H^{\circ}\) for the \(\mathrm{C}-\mathrm{C}\) bond in \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) is \(88 \mathrm{kcal} / \mathrm{mol}\) and \(\Delta H^{\circ}\) for the \(\mathrm{C}-\mathrm{O}\) bond in \(\mathrm{CH}_{3} \mathrm{OH}\) is \(93 \mathrm{kcal} / \mathrm{mol}\). Offer an explanation.

Draw an energy diagram for a two-step reaction, \(A \rightarrow B \rightarrow C\), where the relative energy of these compounds is \(\mathrm{C}<\mathrm{A}<\mathrm{B}\), and the conversion of \(\mathrm{B} \rightarrow \mathrm{C}\) is rate-determining.
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