Organic Reactions Chapter 6

This set covers organic reaction conventions, reaction types (substitution, elimination, addition), mechanisms, bond cleavage, thermodynamics, kinetics, energy diagrams, and catalysis based on Chapter 6 lecture notes.

In organic chemistry equations, what do the symbols $h\nu$ and $\Delta$ represent?

They are used for reactions that require light ($h\nu$) or heat ($\Delta$).

What is indicated when numbers (e.g., 1. and 2.) are placed above and below a reaction arrow?

They indicate two or more sequential reactions carried out without drawing an intermediate compound.

What occurs in a substitution reaction?

An atom or a group of atoms is replaced by another atom or group of atoms.

Describe the characteristic changes in an elimination reaction.

Elements of the starting material are lost and a $\pi$-bond is formed.

What is the defining feature of an addition reaction?

Elements are added to a starting material, typically involving the breaking of a $\pi$-bond.

What is a reaction mechanism?

A detailed description of how bonds are broken and formed as starting material is converted to product.

What is a concerted reaction?

A one-step reaction where bonds are made and broken in the same step.

What characterizes a stepwise reaction?

A reaction involving more than one step, where a starting material is first converted to an unstable reactive intermediate before forming the product.

Define homolysis (homolytic bond cleavage).

The breaking of a covalent bond where each atom leaves with one electron, generating uncharged reactive intermediates with unpaired electrons called free radicals.

Define heterolysis (heterolytic bond cleavage).

Bond breaking where one atom takes both electrons from the bond, generating charged intermediates (ions).

What are the three most important reactive intermediates generated by the homolysis and heterolysis of a C−Z bond?

Carbocations (heterolysis where Z is more electronegative), carbanions (heterolysis where C is more electronegative), and radicals (homolysis).

What is bond dissociation energy?

The energy needed to homolytically cleave a covalent bond.

How is the enthalpy change ($\Delta H^{\circ}$) related to endothermic and exothermic reactions?

When $\Delta H^{\circ}$ is positive (+), the reaction is endothermic (energy absorbed); when $\Delta H^{\circ}$ is negative (−), the reaction is exothermic (energy released).

What is the relationship between bond strength and bond dissociation energy?

The stronger the bond, the higher its bond dissociation energy.

How do you calculate the overall $\Delta H^{\circ}$ for a reaction using bond dissociation energies?

Sum the bond dissociation energies for bonds broken in starting materials (positive value) and add them to the sum of bond dissociation energies for bonds formed in products (negative value).

What does thermodynamics describe in a chemical reaction?

It describes energy and equilibrium, specifically the relative energies and amounts of reactants and products at equilibrium, symbolized by $\Delta G^{\circ}$, $\Delta H^{\circ}$, $\Delta S^{\circ}$, and $K_{eq}$.

What does kinetics describe in a chemical reaction?

It describes reaction rates, or how fast reactants are converted to products, symbolized by $E_a$, $\Delta G^{\ddagger}$, and $k$.

What is the equilibrium constant expression ($K_{eq}$) for the reaction $mA + nB \rightleftharpoons sC + tD$?

$$K_{eq} = \frac{[products]}{[reactants]} = \frac{[C]^s [D]^t}{[A]^m [B]^n}$$

What is the relationship between stability and the position of equilibrium?

The more stable the compound (lower in energy), the greater its concentration at equilibrium.

What is the mathematical relationship between $\Delta G^{\circ}$ and $K_{eq}$?

$$\Delta G^{\circ} = -2.303 RT \log(K_{eq})$$

Define entropy ($\Delta S^{\circ}$) and state when it is positive.

Entropy is the change in freedom of motion or degree of disorder; $\Delta S^{\circ}$ is positive (+) when the products are more disordered than the reactants.

Provide the Gibbs standard free energy equation.

$$\Delta G^{\circ} = \Delta H^{\circ} - T\Delta S^{\circ}$$

What is a transition state in an energy diagram?

An unstable energy maximum reached as reactants convert to products, where bonds are partially broken and partially formed.

Define energy of activation ($E_a$).

The minimum amount of energy needed to break bonds in the reactants; it represents the energy barrier for the reaction.

What is the rule of thumb for the relationship between temperature and reaction rate?

Increasing the temperature by $10\,^{\circ}C$ doubles the reaction rate.

What determines the order of a rate equation?

The sum of the exponents of the concentration terms in the rate equation.

What is the rate-determining step in a multistep reaction?

The slow step, which is the step with the transition state of highest free energy.

How does a catalyst increase the rate of a reaction?

By providing a lower energy pathway with a more stable transition state, thereby lowering the energy of activation ($E_a$).

Define 'active site' and 'substrate' in the context of enzymes.

The active site is the specific region of the enzyme that binds the organic reagent, which is called the substrate.