Exam 2 ch 3+4

Free energy

- measure of a reactions tendency to go in the direction written. A negative value is favorable for a reaction

Entropy

- measure of disorder or freedom of motion. Increase in entropy is favorable for reaction

Enthalpy

- measure of the heat energy in a system. In a reaction, the heat absorbed or evolved is called the heat of reaction. Decrease is favored for reaction

Fused rings

- molecule in which two or more rings share two adjacent carbon atoms

Intermediate

- molecule or fragment of a molecule that is formed in a reaction and exists for a finite length of time before it reacts in the next step. An intermediate corresponds to a relative minimum in the reaction energy diagram

Chain reaction

- multistep reaction where a reactive intermediate formed in one step brings about a second step that generates the intermediate required for the following step

Axial bonds

- one of six bonds on the chair conformation of the cyclohexane ring that are parallel to the axis of the ring.

Equatorial bonds

- one of six bonds on the cyclohexane ring that are directed out towards the equator of the ring

Order

- power of a concentration term in the rate equation.

Initiation step

- preliminary step in a chain reaction, where the reactive intermediate is first formed

Rate constant

- proportionality constant Kr in the rate equation

Equilibrium constant

- quantity calculated from the relative amounts of the products and reactants present at equilibrium

Endothermic

- reaction consuming heat

Exothermic

- reaction giving off heat

Hammond's postulate

- related species that are closer in energy are also closer in structure. In an exothermic reaction, the transition state is closer to the reactants in energy and structure. In endothermic, transition state is closer to products in energy and structure

Rate equation

- relationship between the concentrations of the reagents and the observed reaction rate

Torsional strain

- resistance to twisting about a bond

Reactive intermediates

- short lived species that is never present in high concentrations because it reacts as quickly as it is formed

Equilibrium

- state of a system such that no more net change is taking place; the rare of the forward reaction equals rate of reverse reaction.

Propagation steps

- steps in chain reaction that are repeated over and over to form the product. Sum of propagation steps should give the net reactant.

1,3-diaxial interaction

- strong steric strain between two axial groups on cyclohexane carbons with one carbon in between them

Kinetics

- study of reaction rates

Catalyst

- substance that increases the rate of a reaction (by lowering Ea) without being consumed in the reaction

Boat conformation

- the less stable puckered conformation of cyclohexane, with both parts puckered upwards. The most stable boat is the twist boat conformation. Twisting minimizes torsional strain and steric strain

Flagpole hydrogens

- two hydrogens in the boat conformation of cyclohexane point upward like flagpoles. Twist boat reduces steric repulsion of the flagpole hydrogens

Half-chair conformation

- unstable conformation halfway between the chair conformation and boat conformation. Part of ring is flat in this conformation

Inductive effect

- a donation or withdrawal of electron density through sigma bonds

Free radicals

- a highly reactive species in which one of the atoms has a odd number of electrons. Most commonly, a radical contains a carbon atom with 3 bonds and an unpaired electron

Carbenes

- a highly reactive species with only two bonds to an uncharged carbon atom with a nonbonding pair of electrons. Simplest carbene is methylene

Carbanion

- a strongly nucleophilic species with a negatively charged carbon atom having only 3 bonds. Carbon atom has a nonbonding pair of electrons.

Chair-chair interconversion

- also known as ring flip. Process of one chair conformation of cyclohexane flipping into another one, with all the axial and equatorial positions reversed. Boat conformation is intermediate.

Bond-dissociation enthalpy (BDE or bond-dissociation energy)

- amount of enthalpy required to break a bond homolytically, to give radicals

Termination steps

- any steps where a reactive intermediate is consumed without another one being generated

Spirocyclic compounds

- bicyclic compounds in which the two rings share only one carbon atom

Thermodynamics

- branch of chemistry dealing with the energy changes accompanying chemical and physical transformations

Homolytic cleavage

- breaking of a bond in such a way that each atom retains one of the bonds two electrons. Produces two radicals

Heterolytic cleavage

- breaking of a bond in such a way that one of the atoms retains both of the bonds electrons. Forms two ions

Bridgehead carbons

- carbon atoms shared by two or more rings. Three chains of carbon atoms connect the bridgeheads

Bridged rings

- compound containing two rings joined at non adjacent carbon atoms

Activation energy

- energy difference between the reactants and the transition state; the minimum energy the reactant must have in order for the reaction to occur.

Standard Gibb's free energy change

- free-energy change corresponding to reactants and products in their standard states at 25 degrees celsius and 1 atm

Reaction-energy diagram

- graph with vertical axis = energy and horizontal = reaction coordinate

Change in enthalpy

- heat of reaction, the amount of heat evolved or consumed throughout the course of a reaction

Transition state

- highest energy state in a molecular collision that leads to reaction

Reaction coordinate

- horizontal axis, symbolizes progress of reaction

Steric strain

- interference between two bulky groups that are so close together that their electron clouds experience a repulsion

Carbocation

a strongly electrophilic species with a positively charged carbon atom having only 3 bonds

Chair conformation

most stable conformation of cyclohexane, with one part puckered upwards and another part puckered downward