Chapter 6 Substitution and Elimination Reactions: SN1, SN2, E1, E2

 

 

 

For each case, the counter ion (such as Na+) is not shown.

 

In general chemistry, what were these types of reactions called?

 

 substitution reactions

 

Acids and Bases

 

Bronsted acid: any compound that ______________ a H+ (proton) in a reaction

 

Bronsted base: any compound that ______________ a H+ in a reaction

 

Conjugate acids and bases are related through loss and gain of H+

 

H-Cl : ____________________________  of Cl-

 

 

Cl- : ____________________________  of H-Cl

 

conjugate acid                                   conjugate base

of B:-                                                   of H-B

 

Apart from gas phase, naked H+’s do not exist.

 

 

Hydronium ions often referred to as H+ .

 

 

Often compounds can be both acids and bases.

 

(counter ion not shown again)

 

Many organic reactions involve a “protonation step” in their mechanism.

 

protonation step : reaction in which a Bronsted acid-base reaction occurs

 

 

 

HA + H2O ®  H3O+ + A-

 

K = [H3O+][A-]/[HA][H2O]

 

Ka = [H3O+][A-]/[HA]

 

pKa = -log Ka

 

The lower the value of the pKa, the ______________ the acid

 

 

 

If pKa < 5 : strong acid         If pKa < 0 : very strong acid

 

Table 6.3 List of pKa values.  Do not memorize but be able to recognize which are strong and weak acids.

 

Lewis Acids and Bases

 

Lewis Base : any species with a reactive pair of electrons (donates a pair of electrons in reactions)

 

Lewis Acid : species that reacts with a Lewis base (accepts a pair of electrons in reactions)

 

            Lewis base         Lewis acid

 

In many cases in this chapter, lone pair electrons will be donated to an “empty orbital”.

           

 

 

 

In each reaction which is the Lewis base and which is the Lewis Acid?

 

In many cases in this chapter, lone pair electrons will be donated to any "empty' orbital.

 

Either electrons in a nonbonding MO will be donated to a p orbital

 

or electrons in a nonbonding orbital will be donated to a s* orbital.

 

 

            Lewis Base                                                                Lewis Acid

            filled orbital                                                                empty orbital

            nonbonding                                                                p or s*

            (n)

 

Since orbitals of similar energy interact more strongly, then for Lewis Acid-Base reactions, the interaction will be between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

 

 

 

 

 

HOMO: most often a filled nonbonding molecular orbital (n or NMBO)

 

LUMO: often either an empty p orbital or an empty antibonding orbital (s*)

 

Substitution reactions can be thought of as Lewis Acid-Base reactions.

 

Lewis base in                                                                        Lewis base in

forward reaction                                                                    reverse reaction

 

Lewis bases that donate electrons in substitution reactions are known as nucleophiles (Nu). 

 

A nucleophilic substitution reaction

 

ejected ion or atom(s) : leaving group (L)

molecule or ion that reacts with Nu : substrate  (R-L)

 

Case 1           Nu:-  +  R-L  ®  Nu-R + L:-

 

Case 2           Nu:  +  R-L  ®  Nu-R+ + L:

 

does not exhaust possibilities

 

 

 

e.g. Case 1

 

 

 

 

 

(azide anion)

 

Notice that in each reaction, the Nu structure is very different, and the products appear different.  Despite this, the reactions are the same.  They are all nucleophilic substitution reactions.

 

 

e.g. Case 2

                                                an oxonium ion

 

Does the reverse reaction involve a protonation step?

 

(lone pair electrons left off of bromide ion in last two examples)

 

 

 

 

There are 2 mechanisms for nucleophilic substitution reactions.

 

SN2 Mechanism

 

substitution nucleophilic bimolecular

 

 

 

 

backside attack

 

 

 

            2 explanations

 

                        - less hindrance to approach of nucleophile from behind C-L bond than from front (steric hindrance)

                                   

                        - MO theory

                       

 

Nu filled nonbonding             R-L empty antibonding

                        MO (n), HOMO                                  MO s*, LUMO

 

 

 

 

 

     

        

   HOMO (n)                           LUMO s* of R-L bond

 

                        - Is there good overlap?

                        - Is this a stabilizing interaction?

 

 

           

            filled n                         filled s of R-L bond

 

                        - Is there good overlap?

                        - Is this a stabilizing interaction?

 

Front side attack

 

            empty s* of R-L

 

            filled n of Nu

 

-         Is there good overlap?

-         Is this a stabilizing interaction?

-         What type of interaction is this called?

 

 

SN2 reaction will occur by Nu approaching from behind C-L bond.

 

 

 

partial bond,              charge is dispersed

 

 

            - 2 partial bonds are weaker than 1 full bond

            - nonbonded interactions are greater

 

Would you expect the intermediate structure to be higher or lower in energy than iodoethane and bromoethane?

 

 

 

 

 

 

 

 

 

Potential Energy Diagram of Reaction

 

 

 

 

 

 

 

Transition State: High point in energy between starting material and products. TS is an energy maximum and not an isolable compound.

 

 

 

 

The height of the TS above reactants is the amount of energy that is required for a molecule of starting material to be transformed into product.  What is this known as?