Chapter 5:  Organic Reactions

Millions of Compounds

Dozens of Functional Groups

Infinite number of reactions

BUT….There are only a few underlying principles for all organic reactions.

 

 

Goals for this chapter

 

Identify reaction types

Use homolytic, heterolytic bond breaking and homogenic, and heterogenic bond making in:

Radical reactions and mechanisms

Polar reactions and mechanisms

Addition reactions and mechanisms

Use concepts of nucleophilic and electrophilic

Discuss energy changes in reactions from

Thermodynamic perspective and

Bond perspective

Describe intermediate and energy for reactions

 

Radical Reactionsl

 

Radicals can be formed by peroxides or photolytic processes

These reactions occur in a series of steps

Initiation

 

 

Propagation

 

 

 

 

 

 

Termination

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Polar Reactions: Lewis Acid-Base Reactions

 

Polar reactions occur because in polar molecules nucleophiles are attracted to electrophiles.

Nucleophiles may be negative or neutral and are electron rich.

Electrophiles may be positive or neutral and are electron poor.

Electrons flow from Nucleophile to electrophile.

Octet rule must be followed.

Be sure you are able to identify the positive and negative side of each functional group.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Be sure you know how electrons transfer during reactions.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Addition Reactions


A+ B --> C

 

 


Electrons in p bonds are

uAccessible

uNegative (electron rich)

uNucleophilic

 


Electrons in highest occupied molecular orbital and electrostatic potential

Energy profile for reaction of ethene and HBr.

 


Equilibrium Rates and energy

 

Reactions reach equilibrium when the forward rate of reaction (formations of products) equals the reverse rate of reaction (formation of reactants.

Reactants <--> Products

 

Keq=[Products]/[reactants]

 

DG° Key to understanding equilibrium

 

When DG° is negative

uSpontaneous

uExergonic

uKeq > 1

When DG° is positive

uNonspontaneous

uEndergonic

uKeq < 1

DG° = -RT ln Keq

 

 

Keq = e-DG°/RT

 

 

 

What is DG° for the reaction between HBr and CH2=CH2 if Keq is 7.5x107?

 

 

 

 

 

 

 

 

Enthalpy

 

nMeasures difference in strength of bonds formed and bonds broken

nHBr + CH2=CH2 à CH3-CH2Br

uBroken - Endothermic

«One H-Br bond             366 kJ/mol

«One C=C double bond              611 kJ.mol

uFormed - Exothermic

«One CH3CH2-H bond                 -420 kJ/mol

«One CH3CH2-Br bond                -285 kJ/mol

«One CH3-CH3 single bond          -376 kJ/mol

                                                                        -104 kJ/mol

Why isn’t this equal to –84.1 kJ/mole as in the book? What is the actual strength of the C-C bond in bromoethane?

 

 

 

 

 

Entropy: A measure of randomness           DG° = DH ° - T DS °

uLarge values = disorder

«Greater number of particles

«Gas phase

«Weaker bonds

«Less structure

uSmall values = order

«Fewer particles

«Liquid phase

«Stronger bonds


«More structurte