4. A mixture of 1.6 g of methane and 1.5 g of ethane are chlorinated for a short time. The moles of methyl chloride produced is equal to the number of moles of ethyl chloride. What is the reactivity of the hydrogens in ethane relative to those in methane? Show your work.

Sample Problem

Solution:

Recall: The amount of product is proportional to the number of hydrogens that can produce it multiplied by their reactivity.

Number of hydrogens leading to methyl chloride =

1.6g * (1 mol/16 g) * (4 mol H/1 mol methane) = 0.40 mol H

Number of hydrogens leading to ethyl chloride =

1.5 g * (1 mol/30 g) * (6 mol H/ 1 mol ethane) = 0.30 mol H

0.40 mol H * Rmethane = 0.30 mol H * Rethane

Rethane/Rmethane = 0.4/0.3 = 1.3

How do we form the orbitals of the pi system…

First count up how many p orbitals contribute to the pi system. We will get the same number of pi molecular orbitals.

Three overlapping p orbitals. We will get three molecular orbitals.

If atomic orbitals overlap with each other they are bonding, nonbonding or antibonding

Anti-bonding, destabilizing.Higher Energy

pi type anti-bond sigma type anti-bonding

If atoms are directly attached to each other the interactions is strongly bonding or antibonding. Bonding, stabilizing the system. Lower energy.

But now a particular, simple case: distant atomic orbitals, on atoms not directly attached to each other. Their interaction is weak and does not affect the energy of the system. Non bonding

non-bonded

pi type bond sigma type bonding

or

or

or or

Molecular orbitals are combinations of atomic orbitals.

They may be bonding, antibonding or nonbonding molecular orbitals depending on how the atomic orbitals in them interact.

All bonding interactions.

Only one weak, antibonding (non-bonding) interaction.

Two antibonding interactions.

Example: Allylic radical

Allylic Radical: Molecular Orbital vs Resonance

Note that the odd electron is located

on the terminal carbons.

Molecular Orbital. We have three pi electrons (two in the pi bond and the unpaired electron). Put them into the molecular orbitals.

Resonance Result

Again the odd, unpaired electron is only on the terminal carbon atoms.

But how do we construct the molecular orbitals of the pi system? How do we know what the molecular orbitals look like?

Key Ideas:

For our linear pi systems different molecular orbitals are formed by introducing additional antibonding interactions. Lowest energy orbital has no antibonding, next higher has one, etc.

0 antibonding interactions

1 weak antibonding Interaction, “non-bonding”

2 antibonding interactions

Antibonding interactions are symmetrically placed.

This would be wrong.

Another example: hexa-1,3,5-triene

Three pi bonds, six pi electrons.Each atom is sp2 hybridized.

Have to form bonding and antibonding combinations of the atomic orbitals to get the pi molecular orbitals.

Expect six molecular orbitals.

# molecular orbitals = # atomic orbitals

Start with all the orbitals bonding and create additional orbitals. The number of antibonding interactions increases as we generate a new higher energy molecular orbital.

Nucleophilic Substitution and -elimination

Substitution Process

Nucleophiles have a pair of electrons which are used to form a bond to the electrophile.

A Leaving Group departs making room for the incoming nucleophile.

Nucleophiles can also frequently function as Lewis bases.

The electrophile can function as Lewis acid.

Note that the nucleophile converts a lone pair into a bond and becomes more positive by +1

Note that the bond from C to the Leaving Group is collapsed into a lone pair on the Leaving Group which becomes more negative by -1.

-Elimination

H

Lv

base

+ H-Lv

A pi bond is created.

Instead of substitution a base can remove both the leaving group and an adjacent hydrogen creating a pi bond. Recall dehydrohalogenation.

Competition between Nucleophilic Substitution and -elimination.

Br

+ Na+ C2H5O-

H

nucleophilicsubstitution

OEt

+ Br-

Note the change in charges on the nucleophile and the Leaving group

First the nucleophilic substitution. The ethoxide attacks the carbon bearing the bromine.

Now the -elimination.

Br

+ Na+ C2H5O-

H

Now the elimination. The ethoxide (base) attacks the hydrogen on a carbon adjacent to the carbon bearing the Br ().

elimination

+ C2H5OH + Br-

Since we are using Br as the leaving group this could also be called a dehydrohalogenation.

Summary.

Formal Charges and Nucleophilic Substitution

In the free nucleophile the pair of electrons is a lone pair belongs exclusively to the nucleophile.

In the product, it is a bond and shared.

The result is the nucleophile increases its charge by +1

Conversely the leaving group converts a shared pair of electrons (a bond) into unshared electrons (lone pair). The charge of the leaving group becomes more negative by -1.

NH

H

C

H

Br

HH

C

H

Br

HH

H2N

NH

H

C

H

Br

HH

C

H

Br

HH

H3N

H

Negative Nucleophile

Neutral NucleophileOther things being equal, the more basic species will be a better nucleophile. NH2

- is a better nucleophile than NH3

N: from -1 to 0 ; Br: from 0 to -1

N: from 0 to +1; Br: from 0 to -1

Negative Nucleophile, positive leaving group

Br C

H

OH2

HH

C

H

HH

Br OH2

Br: from -1 to 0; O: from +1 to 0

Two Nucleophilic Substitution Mechanisms: SN1 & SN2

SN2 mechanism: substitution, nucleophilic, 2nd order

Backside attack

Hydrogens flip to the other side. Inversion

of configutationExamine important points….

Look at energy profile next…

Energy Profile, SN2

SN1 reaction: substitution, nucleophilic, first order.

Step 1, Ionization,

Rate determining step.

Step 2, Nucleophile reacts with Electrophile.

Note stereochemistry: nucleophile can bond to either side of carbocation. Get both configurations.

Protonated ether.

Now the alternative mechanism: SN1

CH3OH + (CH3)3C-Br CH3OC(CH3)3 + H + + Br-

Step 3, lesser importance, deprotonation of the ether.

Next, energy profile….

Energy Profile of SN1, two steps.Slow step to form carbocation. Rate determining. Examine important points…..

Carbocation, sp2

Fast step to form product.

Kinetics: SN1 vs. SN2SN1, two steps.

SN2, one step.

Effect of Nucleophile on Rate:Structure of Nucleophile

SN1: Rate Determining Step does not involve nucleophile. Choice of Nucleophile: No Effect

SN2: Rate Determining Step involves nucleophile. Choice of nucleophile affects rate.

Note the solvent for this comparison: alcohol/water. Talk about it later…

Frequently, better nucleophiles are stronger bases.

Compare

Compare

But compare the halide ions!!

In aq. solution F – more basic than I -. (HI stronger acid.) But iodide is better nucleophile.

We need to discuss Solvents

Classifications

Polar vs non-polar solvents, quantified by dielectric constant. Polar solvents reduce interaction of positive and negative ions.

Water > EtOH > Acetic acid > hexane

Solvents. Another Classification

Protic vs aprotic solvents. Protic solvents have a (weakly) acidic hydrogen having a positive charge which stabilize anions.

ROH --- Br - --- HOR

Alcohols are protic solvents Aprotic solvents

CH3CN acetone tolueneIncreasing polarity

Role of Solvents

Some solvents can stabilize ions, reducing their reactivity.

Many nucleophiles are ions, anions.

Protic solvents can stabilize anions. Protic solvents have (weakly) acidic hydrogens bearing a positive charge. Anions may be stabilized

Methanol, protic solvent, stabilizing the fluoride ion, reducing its nucleophilicity.

Small, compact anions (like fluoride ion) are especially well stabilized and have reduced nucleophilicity. Iodide ion is large diffuse charge and less stabilization occurs.

Halide ion problem

Iodide ion Bromide ion Chloride ion Fluoride ion

basicity

Protic solvent solvation

nucleophilicity

The problem: basicity and nucleophilicity of the halide ions do not parallel each other in protic solvents.

nucleophilicity

The explanation. Fluoride most stabilized in protic solvents reducing its nucleophilicity.

Summary for Halide Ions

Iodide ion Bromide ion Chloride ion Fluoride ion

Nucleophilicity in aprotic solvents

Protic solvent solvation

Nucleophilicity in protic solvents

basicityBut in aprotic solvents.

Protic solvents.

basicity

Protic solvent solvation