Chapter 3 - Alkenes

Change the suffix from ane to ene Naming Alkenes Change the suffix from ane to ene

Naming Alkenes The longest chain must include the double bond Chain is numbered to give the double bond carbons the lowest numbers Numbers indicating location of double bond are placed before chain length

Double bond gets precedence over substituents Naming Alkenes Double bond gets precedence over substituents

For more than one double bond Naming Alkenes For more than one double bond use diene, triene etc

Double bond in a cyclic always get Naming Alkenes Double bond in a cyclic always get numbers 1 and 2

Naming Subtleties

Terms Vinylic carbons – sp2 carbons Allylic carbons – ones next to the vinylic carbons

Energy for single bond 2.9 kcal/mol for double 62 kcal/mol Alkenes Double bond locks the molecule and there is no rotation about the double bond Energy for single bond 2.9 kcal/mol for double 62 kcal/mol

Lowest Energy The Second Bond Must Break (Promote Electrons to the Antibonding Orbital). Then it Can Rotate

Different Molecules The connections are the same, but that’s about all!!!

Cis/Trans – Just Not Good Enough

Configuration - E,Z

E/Z Rules Assign priority based on the higher atomic number Increasing priority

E/Z Rules If priority cannot be assigned on the basis of the atoms bonded to the CARBON, look to the next set of atoms. Priority is assigned at the first point of difference.

E/Z Rules When a double or tripple bond shows up, count it as though there are two or three of the same atom

E/Z nomenclature

Thermodynamics and Kinetics How Far and How Fast Description of a reaction at equilibrium (Keq) Kinetics Description of rates of a chemical reaction rate = k [A] [B]2 / [C]0

For The Following Energy Discussion, Consider this Reaction

Energy considerations Mechanisms describe bond breaking-bond making

Energy considerations Exergonic vs. Endergonic Reactions ( Note: The more stable the species, the lower its energy. ) Ok, OK, OK, - What is exergonic and endergonic? Isn’t that supposed To be exothermic and endothermic???? Next slide!!!!!!!!

Energy Terms DGo = DHo - TDSo Endergonic and Exergonic describe the free energy used or expelled from a reaction. Endothermic and exothermic describe the heat ΔH used or expelled from a reaction DGo = DHo - TDSo G and H are often close to the same unless you have a large temperature value For almost all normal processes the +/- value of G and H will be the same

Energy considerations DGo , Gibbs free energy change… Predicts whether a reaction will happen “spontaneously” Exergonic reactions = - DGo  (spontaneous) Endergonic reactions = + DGo 

DHo = energy bonds broken - energy bonds formed

Calculate ΔH DHo = energy bonds broken - energy bonds formed Notice the table doesn’t list the breaking of the pie bond without the breaking of the sigma bond. This value is 62 kcal/mol. DHo = energy bonds broken - energy bonds formed

Where are Calculations Valid Just in the Gas Phase The solvent makes a difference! Solvents can have a large effect on DH Water can solvate cations +++ or anions - - -

Rate of Reaction The number of collisions/time Rate depends on: The number of collisions/time Fraction of collisions with sufficient energy Fraction of collisions with proper orientation

Kinetics Energy of Activation, Ea (or DG**) Fast vs. Slow reactions:

Organic Chemistry Lingo Kinetic Product : formed most rapidly Thermodynamic Product: most stable product Many times the Kinetic Product is the Thermodynamic Product!

Thermodynamic vs. Kinetic Control If the kinetic product and thermodynamic product differ, the major product will depend on reaction conditions. If carried out at mild (low-temp) conditions, the reaction will be irreversible and the kinetic product will be favored.

Thermodynamic vs. Kinetic Control If carried out at sufficiently vigorous (hi-temp) conditions, the reaction will be reversible and the thermodynamic product will be favored.

Thermodynamic vs. Kinetic Control Kinetic product and thermodynamic product: Formed fastest Most stable

Reaction Coordinate Diagrams Intermediate = product of one step, becomes reactant of next step Transition state = intermediate that cannot be isolated (partially formed bonds, highest energy structures!) Rate determining step = step with highest energy Transition state

Reaction Coordinate Diagrams 1) How many intermediates are there? 2) Is the first formed intermediate more apt to revert to reactants or go on to form products? 3) Which step in the reaction is the rate determining? 4) Is the reaction endergonic or exergonic? What is the sign of Go? 5) Which step has the greatest free energy of activation? 6) Where are the transition states?

Reaction Order First Order Reaction – reaction is proportional to the concentration of one reactant Second Order Reaction – reaction that is proportional to the concentration of two reactants