Organic Chemistry Chapter 4: Reaction Mechanisms, Transition States, and Intermediates

What are reaction mechanisms?

Step by step accounts of how electrons move when bonds are broken and formed.

What does energetics refer to in chemical reactions?

An account of energy changes and/or requirements during a transformation.

What is synthetic strategy in organic chemistry?

Understanding the specific chemistry of functional groups to design molecules with specific structural features.

What is the first process that occurs during a chemical transformation?

The breaking of a bond, which requires energy input.

What happens during bond formation?

It always releases energy.

What are transition states?

Structures that lie at the point of highest energy in a reaction profile; they are not stable and cannot be directly observed.

What do energy maxima and minima represent in a reaction profile?

Energy maxima represent transition states, while energy minima represent reaction intermediates.

What are reaction intermediates?

Structures that form as a result of covalent bond cleavage, lying at an energy minimum between reactants and products.

What are the two types of covalent bond cleavage?

Homolytic cleavage and heterolytic cleavage.

What is homolytic cleavage?

The electrons in a covalent bond are equally distributed between the atoms.

What is heterolytic cleavage?

One atom gets both electrons from the bond, developing a negative charge, while the other atom develops a positive charge.

What is bond dissociation energy (BDE)?

The energy required to break a covalent bond homolytically; BDEs are always positive.

How are organic reaction mechanisms classified?

According to whether electrons are transferred singly or in pairs.

What is a carbocation?

A positively charged carbon atom, typically sp2 hybridized with a vacant p orbital.

How are carbocations classified?

As primary, secondary, or tertiary based on the type of carbon atom bearing the positive charge.

What stabilizes carbocations?

Alkyl groups and electron-donating groups, as well as resonance stabilization.

What is hyperconjugation?

The overlap of sigma bonding orbitals with an empty p orbital, stabilizing carbocations.

What is resonance stabilization in carbocations?

Delocalization of the positive charge over several atoms when adjacent to a C=C pi bond.

What are free radicals?

Species classified as primary, secondary, or tertiary based on the type of carbon atom bearing the single electron.

How are free radicals stabilized?

By alkyl substituents and resonance, similar to carbocations.

What is a carbanion?

A negatively charged carbon atom, similar to ammonia in structure.

What stabilizes carbanions?

Resonance; however, alkyl groups destabilize them due to increased negative charge.

What is the basicity of carbanions?

Carbanions are strong bases, stronger than amines, and can remove protons from ammonia.

What is the order of stability for carbocations?

Tertiary > Secondary > Primary > Methyl.

What is the order of stability for carbanions?

Usually the opposite of carbocations, with stability decreasing from tertiary to methyl.

What is the significance of curved arrow formalism?

It indicates the movement of electron pairs (full-headed arrows) and single electrons (half-headed arrows).

What happens during the conjugation of a carbocation?

The positive charge is delocalized over several atoms, providing stabilization.

What factors stabilize reaction intermediates?

The two major factors are resonance and hyperconjugation.

What is resonance in the context of reaction intermediates?

Resonance refers to the delocalization of electrons or charges over several atoms, providing a stabilizing force.

How does hyperconjugation stabilize carbocations?

Hyperconjugation involves overlap between sp3 orbitals of alkyl groups and the empty p-orbital of the carbocation, stabilizing it.

What is the stability order of carbanions?

3o carbanion < 2o carbanion < 1o carbanion < methyl carbanion.

How does resonance affect carbanion stability?

Resonance is the main factor stabilizing carbanions, allowing them to exist with lower energy.

What is the hybridization of the central carbon in a resonance-stabilized carbanion?

The hybridization is typically sp2.

What is the relationship between stability and reactivity of intermediates?

More stable intermediates form faster and tend to live longer, while less stable intermediates form more slowly and react quickly.

What is the inductive effect in relation to carbocations and carbanions?

Electron-withdrawing groups destabilize carbocations but stabilize carbanions.

What is the effect of alkyl groups on carbocations?

Alkyl groups are electron donors and stabilize carbocations through hyperconjugation.

How does BDE relate to radical stability?

Lower BDE indicates higher stability of the radical formed; it takes less energy to form a more stable radical.

What is the stability order of free radicals?

Tertiary free radicals > Secondary free radicals > Primary free radicals.

What is the significance of resonance structures?

The greater the number of resonance structures, the more stable the intermediate.

What is a benzyl carbocation?

A benzyl carbocation has a positive charge on a carbon directly attached to a benzene ring, stabilized by resonance.

What is the role of lone pairs in carbenes?

Carbenes have a lone pair in the sp2 orbital, allowing them to act as nucleophiles.

What is the geometry of carbon in carbenes?

The carbon in carbenes is sp2 hybridized with trigonal geometry.

What happens to the stability of carbanions with increasing alkyl groups?

Increasing alkyl groups destabilize carbanions due to the inductive effect.

What is the effect of halogens on carbocations?

Halogens destabilize carbocations due to their electron-withdrawing nature.

What type of charge do localized electrons or charges have?

Localized electrons or charges reside on a single atom and have the lowest stability.

What is the relationship between resonance and hyperconjugation in stability?

Resonance is a greater stabilizing force than hyperconjugation.

What is a vinyl carbocation?

A vinyl carbocation has a positive charge on a carbon that is part of a pi-bond, making it less stable.

How does the presence of electron-donating groups affect carbocations?

Electron-donating groups stabilize carbocations by diffusing the positive charge.

What is the significance of half-headed arrows in resonance structures?

Half-headed arrows represent the movement of single electrons.

What is the main factor stabilizing free radicals?

Resonance stabilization is the main factor for free radicals.

What is the effect of resonance on the stability of carbocations?

Resonance stabilization can make a primary allylic carbocation more stable than a tertiary carbocation.