Chapter 3 - Alkenes

Naming Alkenes

Change the suffix from ane to ene

CH3 CH3 CH2 CH2

ethane ethene

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

12

345

76

3-propyl-2-heptene

Naming Alkenes

Cl

1234

56

789

8-chloro-4-nonene

Double bond gets precedence over substituents

Naming Alkenes

For more than one double bond

use diene, triene etc

Br

1234

56

78

6-bromo-2,4-octadiene

Naming Alkenes

Double bond in a cyclic always get

numbers 1 and 2

BrBr

3,4-dibromocyclohexene1

2

3

45

6

Naming Subtleties

ClBrCH3 Cl

Cl12345

67

8

2-chloro-4-bromo-7-methyl-4-octane

1,6-dichlorocyclohexene

Terms• Vinylic carbons – sp2 carbons

• Allylic carbons – ones next to the vinylic carbons

AlkenesDouble 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

Z (zusammen) E (entgegen)

C

higherC

higher

lowerlower

C

lower higherC

lowerhigher

E/Z Rules

Assign priority based on the higher atomic number

(53)(35)(17)(16)(8)(7)(6)(1)

Increasing priority

-H -CH 3 -NH2 -OH -SH -Cl -Br -I .

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.

CH2CH2Br

CH2CH2F

Cl

H

E/Z Rules

When a double or tripple bond shows up, count it as though there are two

or three of the same atom

C CH2R R C CH2

C

R C H

O

R CH

OO

E/Z nomenclature

Thermodynamics and KineticsHow Far and How Fast

• Thermodynamics–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

• 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

Go = Ho - TSo

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

Go , Gibbs free energy change…

– Predicts whether a reaction will happen “spontaneously”

– Exergonic reactions = - Go (spontaneous)

– Endergonic reactions = + Go

Ho = energy bonds broken - energy bonds formed

Ho = energy bonds broken - energy bonds formed

Calculate ΔH

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.

H

H

H

HH Cl

HH

H

H

HBr

Where are Calculations Valid

• Just in the Gas Phase

• The solvent makes a difference!– Solvents can have a large effect on H– Water can solvate cations +++ or anions - - -

Rate of Reaction

• Rate depends on:

1.The number of collisions/time

2.Fraction of collisions with sufficient energy

3.Fraction of collisions with proper orientation

Kinetics

• Energy of Activation, Ea (or G**)

• 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 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

A B

A BB+A A B+