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Ch. 8 - Alkenes 1: Properties and Electrophilic Additions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Non è quello che usi tu?Cambia libro di testo
Capitolo 7, Problema 79a

In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH2.
(a) Draw the Lewis structure of CH2.

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Step 1: Begin by identifying the molecular formula CH₂. This indicates that the molecule contains one carbon atom and two hydrogen atoms.
Step 2: Recall that carbon has four valence electrons and hydrogen has one valence electron. The total number of valence electrons in CH₂ is calculated as follows: Carbon contributes 4 electrons, and each hydrogen contributes 1 electron, giving a total of 4 + 1 + 1 = 6 valence electrons.
Step 3: Arrange the atoms with carbon as the central atom, since it is less electronegative than hydrogen. Connect the two hydrogen atoms to the carbon atom using single bonds. Each single bond represents two shared electrons.
Step 4: After forming the two single bonds, 4 electrons are used (2 electrons per bond). This leaves 2 valence electrons remaining. Place these remaining electrons as a lone pair on the carbon atom.
Step 5: Verify the structure by checking the formal charges and ensuring that the total number of valence electrons matches the calculated total. Carbon will have a lone pair and two single bonds, making it neutral, and each hydrogen atom will have a full valence shell with 2 electrons.

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Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They help visualize the arrangement of electrons and the connectivity of atoms, which is crucial for understanding molecular geometry and reactivity. In the case of CH₂, the Lewis structure will show how the carbon atom is bonded to two hydrogen atoms and the distribution of its valence electrons.
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Drawing the Lewis Structure for N2H4.

Carbenes

Carbenes are neutral reactive intermediates that contain a carbon atom with only six valence electrons, resulting in a divalent state. They can be classified as singlet or triplet carbenes based on the spin state of their unpaired electrons. Understanding carbenes is essential for grasping their role in organic reactions, as they can act as nucleophiles or electrophiles depending on their electronic configuration.
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Reaction with a simple carbene.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial for chemical bonding. In the context of CH₂, carbon has four valence electrons, while each hydrogen has one. The arrangement and sharing of these electrons determine the molecule's structure and stability, making it important to account for them when drawing Lewis structures and predicting molecular behavior.
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Valence Electrons of Transition Metals
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The acid-catalyzed hydration we learned here in Chapter 8 is reversible:

(d) How might you shift the equilibrium to the right?

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In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH2 .

(e) Carbenes have great synthetic utility, especially in the synthesis of cyclopropanes from alkenes. Based on your answers to (a)–(d), show a mechanism for the cyclopropanation of cyclohexene.

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The reaction of (E)-3-chlorohex-3-ene with HCl results in the selective formation of 3,3-dichlorohexane instead of 3,4-dichlorohexane. Explain this result.

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Domanda del libro di testo

In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH2.

(f) Cyclopropanation of (E)- and (Z)-3-methylhex-3-ene gives two different products. Rationalize this outcome.

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Domanda del libro di testo

In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH2.

(b) What are the hybridization and shape of the central carbon of CH2?

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Domanda del libro di testo

The acid-catalyzed hydration we learned here in Chapter 8 is reversible:

(e) How might you shift the equilibrium to the left?

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