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Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 3 - Alkanes and Cycloalkanes: Properties and Conformational AnalysisProblem 17b
Chapter 2, Problem 17b

When we begin studying reactive intermediates, the designation of substitution will be especially important. Label the following carbocations as 1° , 2°, 3°, or 4° or based on the carbon bearing the positive charge.
(b) Skeletal structure of a carbocation with a positive charge on a carbon atom, indicating its classification.

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1
Step 1: Understand the concept of carbocation substitution. The degree of substitution (1°, 2°, 3°, or 4°) refers to the number of carbon atoms directly attached to the carbon bearing the positive charge. A 1° carbocation has one carbon attached, a 2° carbocation has two carbons attached, and so on.
Step 2: Examine the structure of the carbocation provided in part (b). Identify the carbon atom that bears the positive charge.
Step 3: Count the number of carbon atoms directly bonded to the positively charged carbon. These are the substituents that determine the degree of substitution.
Step 4: Based on the count, assign the appropriate designation: 1° (primary), 2° (secondary), 3° (tertiary), or 4° (quaternary). For example, if the positively charged carbon is bonded to three other carbons, it is a 3° carbocation.
Step 5: Verify your answer by ensuring that you have correctly identified the substituents and their direct attachment to the positively charged carbon. Double-check the structure for accuracy.

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Key Concepts

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

Carbocation Stability

Carbocations are positively charged carbon species that are classified based on the number of alkyl groups attached to the carbon bearing the positive charge. The stability of carbocations increases with the degree of substitution: tertiary (3°) carbocations are more stable than secondary (2°), which are more stable than primary (1°). This stability is due to hyperconjugation and the inductive effect of alkyl groups, which help to disperse the positive charge.
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Degree of Substitution

The degree of substitution refers to the number of carbon atoms directly attached to the carbon atom that carries the positive charge in a carbocation. A carbon atom can be classified as primary (1°) if it is attached to one other carbon, secondary (2°) if attached to two, tertiary (3°) if attached to three, and quaternary (4°) if attached to four. This classification is crucial for predicting the reactivity and stability of carbocations in organic reactions.
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Reactivity of Carbocations

Carbocations are highly reactive intermediates in organic chemistry, often formed during reactions such as electrophilic addition or rearrangements. Their reactivity is influenced by their stability; more stable carbocations are less likely to undergo rearrangement and more likely to participate in subsequent reactions. Understanding the reactivity patterns of different carbocation types is essential for predicting the outcomes of organic reactions.
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Related Practice
Textbook Question

Name the following alkanes using the IUPAC system of nomenclature. [Each molecule exemplifies one of the nomenclature rules in Tables 3.7 and 3.8.]

(n) rule 7

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Textbook Question

Name the following alkanes using the IUPAC system of nomenclature. [Each molecule exemplifies one of the nomenclature rules in Tables 3.7 and 3.8.]

(k) rule 6

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Textbook Question

When we begin studying functional groups, the designation of substitution will be especially important. Label the following bromoalkanes as 1° , 2°, 3°, or 4° based on the carbon to which the bromine is attached.

(b)

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Textbook Question

When we begin studying reactive intermediates, the designation of substitution will be especially important. Label the following carbocations as 1° , 2°, 3°, or 4° or based on the carbon bearing the positive charge.

(c)

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Textbook Question

Name the following alkanes using the common names for branched substituents.

(c)

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Textbook Question

In Chapter 8, we study the chemistry of alkenes, like ethene. In contrast to ethane, there is no free rotation around the C = C double bond of ethene. Explain this in the context of the molecular orbital picture of ethene.

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