1. Review of Organic Chemistry I

8/3/2019 1. Review of Organic Chemistry I

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Your C342 Team: Part I

This course will be team taughtwe will manage theentire course as a team, including lectures, quizzes and

exams1

Prof. Martha G. Oakley Prof. Laura C. Brown


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C342 Organic Chemistry II

Time

Understanding

A Challenging Course

Many of life's failures are people who did not realize how close

they were to success when they gave up. ~ Thomas Edison


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Discussions: Attend Any You Like

You may attend more than one discussion per week, if space

You may earn up to 10 bonus points over the semester for

attending office hours (max 1 per week) 4


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OFFICE HOURS

We will hold 12 office hours weekly beginning next week Professor Oakley will hold office hours this week from

3:00-4:00 on Tuesday and from 4:00 to 5:05 onThursday, Simon Hall (across from Jordan), Room 320B.

Professor Brown will hold office hours this week onMonday 9-10 am and Wednesday from 12:20-1:20 in

Chemistry A112

Regular office hours begin next week.

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Grading

Quizzes (see syllabus for dates) best 3 out of 4 (100 pts) Early quizzes available only for university sanctioned

events, etc.

Exams 3 x 2 hr exams; Tues evenings (150 pts each) Final Exam (300 pts) Drop lowest exam or of final grade, whichever is lowest YOU ARE RESPONSIBLE FOR READING THE

SYLLABUS

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Grading Cutoffs

The instructors do not grade on a curve (A) understands the material well andcan apply it

creatively to new situations. Cutoff for an A- no higher than 90%. (B) Good, solid understanding of the material Cutoff for a B- no higher than 75% (C) Major gaps in understanding Cutoff for a C- no higher than 60%. (D or F) Still larger gaps

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Active Learning

You should be thinkingduring class We will do lots of problem-solving in class We will call on students to help us work problems You should be trying to work the problem at the same

time as the student who has been called on

You get a bonus point every time you participate right orwrong

There are no other ways to earn bonus points, except fordiscussion attendance

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Courtesy in Class

This is a large room and there are a lot of people in it. Whispering in class makes it difficult for others to hear

the lectures

This class costs a lot of money! Lecture attendance is optional in this course Should you choose to attend lecture, we expect you to

be courteous to us and to your classmates

If we are slowed or cannot continue because of noise,you are responsible forallthe material we would havecovered without the noise.

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Review of Organic Chemistry I

Work practice problems on Oncourse (Resources,Practice Problems for Quiz 1)

Read Chapter 4.2-4.6 (Review)Work problems 4.3, 4.16, 4.17, 4.18, 4.19, 4.22,

4.29, 4.30, 4.31

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Lewis Structures and Formal Charge

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B. Write a Lewis structure for diazomethane

(CH2N2) and assign each atom a formal charge

(connectivity as shown)

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II. Curved Arrows

B. Move from electron source to electron sink

source: ________ charge or _____________

sink: __________ charge or partial _____________

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III. Resonance

A. Provide appropriate curved arrows to show how togenerate additional resonance structures (if possible)

O

O

O

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B. How many resonance structures can you draw for the

benzyllic cation (5 possible)? Show the electron flow

between each one.

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IV. Curved Arrows in Reaction Mechanisms

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V. Following Electron Flow in Reactions You

Haven't Seen Before

A. Thioester Claisen Condensation

S CH2

O

S CH3

O

+

Note: I will give you either the arrows or the products18


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continued

S

O -O CH3

S

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A.

B.

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Provide Arrows

HO NH O

-O

H HHO

H N

Provide mechanistic arrows, indicating the direction of electron flow, for the following one-step

transformation. Hint:number your carbons. Where are bonds broken or formed?

O-

H

OO O-

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NH

NH

CH3

O

H

CO2-

+HN

HN

H

NH+

NH

CH3

O

H

CO2-

NH

NH

CH3

O

H

CO2-

+HN

HN

H

NH+

NH

CH3

O

H

CO2-

Which of these groups of mechanistic arrows

is correct? Why? Electron source? Sink?


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VI. Nomenclature

Examples:1. isopropanol (2 propanol) 2. cis 3-hexene (Z-hexene)

3. tert-butanol (2,2-dimethylethanol) 4. trans-2-butene (E-butene)

5. n-butylamine (1-butyl amine)

You should be able to write structures from IUPAC or common names

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VII. Thermodynamics and Equilibrium

(1) Gravitational (2) Electrical (3) Chemical

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G = H TH

Free Energy is a Combination of Enthalpy and Entropy

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IfG < 0, reaction is product-favored

IfG >0, reaction is reactant-favored

IfG < 0, reaction is thermodynamically favorable

(higher to lower energy)

IfG >0, reaction is is thermodynamically unfavorable

(lower to higher energy)

In other words:


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Example of an Exothermic Reaction. Where does the energy

come from?

Consider the reaction H2(g) + Cl2 (g) --> 2 HCl

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Estimating Reaction Enthalpy from Bond

EnthalpiesOOH

These molecules are a special kind of isomer of one

another called tautomers (molecules that differ by the

placement of a double bond and a proton).

Enol tautomer Keto tautomer

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C=O C-O:

C=C C-C:

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Bonds formed:

Enthalpy of reaction:

OOH

Bonds broken:

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Practice: Predicting Entropy Changes

1. CO2 (s) ----> CO2 (g)2. 2CO + O2 (g) ----> 2 CO2 (g)3. Ag+(aq) + Cl-(aq) ----> AgCl (s)4. CO2 (g) ---> CO2 (aq)5.

+ H2


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Chemical Equilibrium

Concentrations of products and reactants at equilibrium are

CONSTANT.

Equilibrium is DYNAMIC; the rates of the forward andreverse reactions are equal.

Equilibrium concentrations are independent of the direction

of approach (same reaction, same temp).32


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Direction of Approach Doesnt Matter

Figure 14.1

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N2(g) + 3H2 (g) 2NH3(g)


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A brief look at kinetics

The rate of a reaction depends on the concentration ofthe reactant(s) and a constant that is characteristic for

the reaction.

For example, isomerization of butene:

Rateforward = kforward [cis-butene]

Ratereverse = kreverse [trans-butene]

What happens to the concentrations of cis and trans

butene as we approach equilibrium?35


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How do these rate constants relate to the

equilibrium constant Kc?

At equilibrium, Rateforward = Ratereverse

Thus: kforward [cis-butene]eq = kreverse [trans-butene] eq

Rearranging:

kforward [trans-butene] eq=

kreverse [cis-butene]eq

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Writing Equilibrium Constant Expressions

Only consider substances whose concentrations change asthe reaction occurs.

Should we consider:

A) Pure solidsB) Pure liquidsC) GasesD) Solutes in dilute solutions (the stuff thats dissolved)E) Solvents in dilute solutions

For the reaction

aA + bB cC + dD

Keq = --------------------

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Shifting a Chemical Equilibrium: Le

Chateliers Principle

If a system is at equilibrium and conditions are changed so

that the system is no longer at equilibrium, the system will

Example: isomerization of butene. Keq = 1.5 at 600 K

Remove 1/2 of cis butene at equilibrium. What happens?

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Figure 14.6

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Can drive a reaction to completion by removing one of the

products.

OH

OOH

O

O+ H2O

init. 1 mmol 1 mmol 0 mmol 0mmoleq. .33 mmol .33 mmol .67 mmol .67mmol

propyl acetateH2SO4


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The concentrations of reactants and products at equilibrium are

dictated by the difference in free energy between them:

At room temperature,

For example calculate Keq

for following reaction:

N2(g) + 3H2 (g) 2NH3(g)

G =-32.90 kJ

Or to estimate, Keq =


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A word about arrows

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Irreversible Reactions:

Reversible Reactions:

Note: your book uses equilibrium arrows only when the reaction is actually at

equilibrium. This is technically correct but not very useful pedagogically. We will usereversible arrows to indicate that both forward and reverse reactions are occurring.

OH

O Oheat O

+ CO2 (g)

OH

O

OHO

O + H2O

H2SO

4 Keq = 4

OH

OH

NH3OH

O NH4 Keq ~ 1016


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VIII. Acid-Base Reactions and pKas

Normally written in simplified form:

Ka =[HA]

[H3O+ ][A- ]

H-A + H2O H3O+ + A-

Or

Ka is simply the equilibrium constant for this reaction:

H+

+ A-H-A H+ + AH-A

+

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Arrow pushing for acid-base reactions:

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Why is this wrong?


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Directionality of Acid-Base Reactions

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In which direction does the equilibrium lie?

?


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Which is the stronger acid, HA or HB?

Stronger base?


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Acidity and pKa

Ka =[H-A]

[H+ ][A- ]

So, the higher the Ka, the stronger or weaker the acid?

So, the higher the pKa, the stronger or weaker the acid?

If the pKa values differ by 1 unit, by how much do Kas differ?

Define pKa:

If Ka = 10-7, what is pKa?

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Acid-Base Equilibria

In which direction doesthe equilibrium lie?

Is Keq < 1 or > 1?

Keq=

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Where K = Ka


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Acid Conjugate Base pKa Ka

H-Cl

OH

O

acetic acid

H O H

NH3+

NH4+

O

NH2

Review from Organic Chemistry I

Note:

Ka =[H-A]

[H+ ][A- ]

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C C HH H C N:

NH3

H2O HF

Structure and Acidity

Conj base?

pKa?

Conj base?

pKa?

38 16 3

25 9


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How about within a column of the periodic table?

H-I

H-Br

H-Cl

H-F

CH3OH

CH3SH


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O

OH OH

4.8 15.9

OH OH

v.

10 15


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What about this series?

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HH

H

H

H

H

H3C CH3


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OH

O

OH

O

IOH

O

BrOH

O

Cl

name?

pKa 4.8 3.2 2.9 2.6As halogenelectronegativity

increases,pKa_____________

OH

O

OH

O

ClOH

O

ClOH

O

Cl

pKa 4.8 2.9 1.5 0.7

Cl ClCl

Inductive Effects


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OH

O

_________

4.8

ClOH

O

______________

4.5

OH

O

_________

4.0

OH

O

______________

2.8

Cl

Cl

Inductive effect falls off rapidly with distance:

Example 16.2. Pick the stronger acid in each pair.

OH

O

OH

Ov.

name?


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OH OH

NO2

OH

NO2

10 8.4 7.2

Explain:


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OO

Which is the most acidic proton in thismolecule? Next? Least?

When the acid is neutral, consider the stability of the conjugate base.

The more stable (less reactive) the conjugate base, the more reactive the acid,

lower pKa.


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How to decide which atom is most basic?

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Draw resonance structures for each form:


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OCH3

O

OCH3

O H

acid conjugate base

NH2

O

NH2

OH

pKa

-6.5

-0.5

Why is the protonated estermore acidic than the

protonated amide?

Hint: think about resonance

structures.

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List the five structural factors important for

determining the pKa of an acid:

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1)

2)

3)

4)

5)


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IX. pKas and Substitution Reactions

A. Which molecule is more likely to react via an SN2

mechanism? SN1? Predict products?

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1. Review of Organic Chemistry I

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