Alkenes are hydrocarbons that contain a double bond. The double bond is the functional group or center of reactivity of the alkene. The functional group suffix of an alkene is “ene.” The general molecular formula for a hydrocarbon is CnH2n+2 minus two hydrogens for every bond or ring in the molecule. The number of bonds and rings is called the degree of unsaturation. Because alkenes contain fewer than the maximum number of hydrogens, they are called unsaturated hydrocarbons.
Because of restricted rotation about the double bond, an alkene can exist as cis–trans isomers. The cis isomer has its hydrogens on the same side of the double bond; the trans isomer has its hydrogens on opposite sides of the double bond. The Z isomer has the high-priority groups on the same side of the double bond; the E isomer has the high-priority groups on opposite sides of the double bond.
The relative priorities depend on the atomic numbers of the atoms bonded directly to the sp2 carbons.
All compounds with a particular functional group react similarly. Due to the cloud of electrons above and below its π bond, an alkene is an electron-rich molecule, or nucleophile. Nucleophiles are attracted to electrondeficient atoms or molecules, called electrophiles. Alkenes undergo electrophilic addition reactions.
The description of the step-by-step process by which reactants are changed into products is called the mechanism of the reaction. Curved arrows show which bonds are formed and which are broken and the direction of the electron flow that accompany these changes.
Thermodynamics describes a reaction at equilibrium; kinetics describes how fast the reaction occurs. A reaction coordinate diagram shows the energy changes that take place in a reaction. The more stable the species, the lower is its energy. As reactants are converted into products, a reaction passes through a maximum energy transition state. An intermediate is the product of one step of a reaction and the reactant for the next step.
Transition states have partially formed bonds; intermediates have fully formed bonds. The rate-determining step has its transition state at the highest point on the reaction coordinate.
The relative concentrations of reactants and products at equilibrium are given by the equilibrium constant Keq. The more stable the compound, the greater is its concentration at equilibrium.
If products are more stable than reactants, Keq is >1, ΔG° is negative, and the reaction is exergonic; if reactants are more stable than products,Keq <1, ΔG° is positive, and the reaction is endergonic.
ΔG° = ΔH° - TΔS°
ΔG° is the Gibbs free-energy change. ΔH° is the change in enthalpy—the heat given off or consumed as a result of bond making and bond breaking. An exothermic reaction has a negative an endothermic reaction has a positive ΔH°.
ΔS° is the change in entropy—the change in the degree of disorder of the system. A reaction with a negative ΔG° has a favorable equilibrium constant: The formation of products with stronger bonds and greater freedom of motion causes ΔG° to be negative.
ΔG° and Keq are related by the formula ΔG° = -RT ln(Keq)
The interaction between a solvent and a species in solution is called solvation.
The free energy of activation, ΔG*, is the energy barrier of a reaction. It is the difference between the free energy of the reactants and the free energy of the transition state. The smaller the ΔG* the faster is the reaction. Anything that destabilizes the reactant or stabilizes the transition state makes the reaction go faster. Kinetic stability is given by ΔG*, thermodynamic stability by ΔG°.
The rate of a reaction depends on the concentration of the reactants, the temperature, and the rate constant. The rate constant, which is independent of concentration, indicates how easy it is to reach the transition state. A first-order reaction depends on the concentration of one reactant, a second-order reaction on the concentration of two reactants.
from - Organic chemistry - Bruice - Chapter 3 summary
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