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Chamras Chemistry 106 Lecture Notes

Examination 1 Materials

Chapter 14: Ethers, Epoxides, & Sulfides

Ethers General Formula: Types:

a) Symmetrical:

Examples:

b) Unsymmetrical:

Examples: Physical Properties:

a) ROR’ Bond Angle:

b) Polarity: (Comparison with alcohols)

H

O

H R

O

R'

R

O

R'

2

c) Reactivity: (Comparison with alcohols)

Advantage: d) Boiling Point:

i) Compared to Alcohols:

BP: 118oC 66oC 35oC Reason: Dipole Moments: 1.7 1.6 1.2 ***Note: Comparison is done with compounds of identical or similar molar masses, since molar mass is a factor affecting boiling point. What is the second important factor?

ii) Within Ethers:

OH

O

O

O

O

O

BP (oC)

91

35

–25

Ether

3

e) Ethers as Solvents: (Table 14-2, pg.625)

Relatively Unreactive

Range of BP’s

Range of Densities

Low MP’s

Superb Dissolution Ability of Cations and Organic Compounds

***Poor Solvation of Anions (In comparison to alcohols): Why?

f) Ether-Electrophile Complexes: Example: BH3.THF Example: BF3. Et2O Use:

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g) Crown Ether Complexes: Uses:

a) Dissolving cations in non-polar solvents b) Dissolving cations in aprotic media

O O

O

O

O

O

O

O

OO

O

Na+ K+

O O

O

O

O O

O

O

OO

O

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Nomenclature: Methods:

a) Common Naming Method:

Naming Template: “alkyl alkyl ether” (with the alkyl group names in the alphabetical order) Example:

b) IUPAC Naming Method:

Naming Template: “alkoxy alkane” (with the smaller alkyl group as the alkoxy side chain) Example:

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*Cyclic Ethers: Spectroscopy: IR: C–O Stretch 1000-1200 cm–1

OO

OO

O

O

O O

O O

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Synthesis of Ethers:

a) Williamson Ether Synthesis (Covered in Chp. 11): General Equation: Example: Mechanism: Details: Alkyl Halide (Step 2): Primary. Secondary and Tosylates also possible, but elimination competes, resulting in poor yields.

RO– + R'–X R–O–R' + X–

O

OEt

Na–HH

Br

OH

(1) NaH

(2) Et–Br

OEt

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b) Alkoxymercuration-demercuration (Covered in CHp.8):

General Equation:

c) Bimolecular Dehydration of Alcohols: Industrial method.

General Equation:

…Remember: Unimolecular Dehydration of Alcohols (Proceeds via Elimination) This process competes with substitution (resulting in the formation of ether). Example:

In order to have substitution dominate the mechanistic pathway, the following measures could be taken:

1. Use a relatively unhindered alcohol. 2. Use excess alcohol. 3. Keep the temperature low.

Example:

Hg(OAc)2

ROHORAcOHg

NaBH4

ORH

OH

H3O+, Heat

2 R–OH R–O–R + H2OH+

H+

OH O

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*Synthesis of Phenyl Ethers: A special case of Williamson Ether Synthesis. General Equation: General Formula for Phenyl Ethers: Example: Mechanism: Williamson Ether Synthesis *Why does the ring have a nitro-substitution?

OR

ArO– + R'–X Ar–O–R' + X–

Aromatic alcohol used instead of aliphatic one.

Phenyl Ether Product

OH

NO2

(1) NaOH

(2)Br

O

NO2

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Reactions of Ethers:

a) Ether Cleavage by HBr and HI: General Equation: Example:

Mechanism:

R–O–R' R–X R'–XExcess HX

X = Br, I+

O HBr (excess) Br

Br+

OH–Br

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Example: Predict the product for the following reaction: Exception: Phenyl Ethers

Example: Predict the product for the following reaction:

b) Autoxidation of Ethers: ***Note: This is a slower than usual reaction. ***Practical Hazard Associated with This Reaction:

O

HI (excess)+

O

HBr (excess)

R

O R' O2 (excess)R

O R'

O

R

O

O R'

+

HO

Hydroperoxide Dialkyl Peroxide

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Sulfides (Thioethers)

General Formula: Remember: The prefix “thio”… The function:

Example: Synthesis:

Williamson Ether (sulfide) Synthesis: General Equation: Mechanism: Very similar to that of the Williamson Ether synthesis from alcohols.

Example:

R

S

H

(1) NaOH

(2) R'–BrR

S

R'

OH SH

O S

SH

(1) NaOH

(2) CH3Br

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Epoxides General Formula: Nomenclature: Covered in cyclic ethers. Synthesis: a) Peroxyacid Epoxidation of Alkenes: General Equation: Example: CH2Cl2

2

31

4R O

O

OH+

O

24

13 +

R OH

O

O

OH

OMCPBA

+

Cl

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Mechanism: Concerted & Stereospecific Commonly Used Peroxyacids:

MCPBA: MMPP:

b) Base-Promoted Cyclization of Vicinal Halohydrins: General Equation:

C

X

C

OH

X = Cl, Br, I

OH–

C C

O

+ X–

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Example: Mechanism:

KOH

OH

Cl

O

Cl

H

O H

–

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Reactions: Ring-Opening Reaction of Epoxides.

a) Acid-Catalyzed: Functional Group Transformation: Epoxide Acid Open-chain carbocation… Example: Mechanism: SN1-like, stereospecific. *The Fate of the Resulting Carbocation: Depends on what nucleophile adds to the carbocation. Example: Regiochemistry of Addition:

O

OH+

OH

+

OH

+HX H2O

OH OHX HO

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Example: Predict the product and write the mechanism for the following reaction:

b) Base-Catalyzed: Functional Group Transformation: Epoxide Base Open-chain substituted alkoxide… Example: Alkoxide ion of a vicinal diol

OH3O

+

OOH–

OH

–O

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Mechanism: SN2-like. Regiochemistry of Addition: Example: Predict the product and write the mechanism for the following reaction: Suggested Problems: 32, 33, 38, 39, 47,

ONaOCH3

CH3OH

ONaOH

H2O

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Chapter 15: Conjugated Systems

Introduction Compounds with two or more double bonds:

a) Isolated:

b) Cumulated:

c) Conjugated: Relative Stabilities of dienes: (As seen in Chp. 7) Experimental Conclusion:

C

E(H

eat of H

ydro

gen

ation)

292kJ

252kJ

225kJ

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…Also remember the relative stabilities of alkenes. Example: Rank the following dienes in the increasing order of heat of hydrogenation:

Structure & Properties of Conjugated Systems Observation: Length: Strength? Explanation: In a conjugated system, the π-electrons are delocalized and dispersed over the conjugated carbon skeleton of the molecule. As a result, the C–C “single” bonds have some characteristics of C=C double bonds and vice versa. Conclusion: A more realistic drawing of a conjugated system is as follows:

A CB D E

< < < <

1.30 angstroms 1.55 angstroms

An Isolated C=C An Isolated C–C

1.34 angstroms 1.48 angstroms

A Conjugated System

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s-cis (cisoid) Vs. s-trans (transoid)

Constructing the MO’s for the π-System of 1,3-Butadiene **{4 p-atomic orbitals combined 4 π-molecular orbitals}**

π-electrons

Related Terminology: Node, bonding, non-bonding, anti-bonding, HOMO, LUMO.

H

H

H

H

H

H

H

H

E

0.0

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1. Node: A region of MO with zero electron density 2. Bonding MO: MO’s (used for bonding) that are lower in energy than the isolated atomic orbitals. 3. Non-bonding MO: MO’s that are the same in energy as the isolated atomic orbitals. 4. Anti-bonding MO: MO’s (weaken the bonding) that are higher in energy than the isolated atomic orbitals. 5. HOMO: Highest Occupied Molecular Orbital. 6. LUMO: Lowest Unoccupied Molecular Orbital. ___________________________________________________________________________ Exercise: Construct the MO diagram of the π-system for 1,3,5-hexatriene molecule.

E

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Allylic Cations …Remember: (Chp. 7) Allyl Group: CH2

–––––CH––––CH2–––– Allylic Position: C = Allylic carbon, H’s = Allylic hydrogens Allyl Cation: Stability of Allyl Cation: A More Realistic Drawing of Allyl Cation: Stability of Allylic Cations:

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

2

1

2

1

CH3+ 1o 2o Allyl 3o Substituted

Allylic< < < < <

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Ionic Additions of HX to Conjugated Dienes Types:

a) 1,2-Addition Conjugated System with the Corresponding b) 1,4-Addition Addition Sites Numbered

Structural Outcome of 1,2-Addition: Structural Outcome of 1,4-Addition: Mechanistic Examples of 1,2 and 1,4-Additions:

1

2

3

4

3

4

H

X

2

3

H

X

H Cl

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Energetics of the 1,2 and 1,4 Additions:

Kinetic Condition Vs. Thermodynamic Condition

1. Kinetic condition favors the formation of kinetically preferred product (AKA: Kinetic product). The kinetic product is formed through the kinetic process.

2. Thermodynamic condition favors the formation of thermodynamically preferred product

(AKA: Thermodynamic product). The thremodynamic product is formed through the thermodynamic process.

Kinetic Process = = = Thermodynamic Process = = =

Allylic CationE

Reaction Coordinate

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Detailed Energetics of the 1,2 and 1,4 Additions: Example:

Allylic CationE

Reaction Coordinate

H Br

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Allylic Radicals Stability: Follows the same trend as for the allylic cations. Allylic Bromination (A review): Mechanistic Example: How was Br-radical generated? a) b)

Br

H

H

H

H

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MO’s of Allyl Cation, Allyl Radical, & Allyl Anion **Made of 3 combined atomic orbitals**

E

0.0

Cation Radical Anion

!-electrons !-electrons!-electrons

H

H

H

H

H

H

H

H

H

HH

H

H

H

H

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The Diels –Alder Reaction

• Discovered in 1928: By Atto Diels & Kurt Alder Reaction Specifications:

o Concerted Mechanism

o Usually Thermally Initiated

o An addition Reaction (A Cycloaddition, more specifically)

o A [4 + 2] Cycloaddition

o Involves Movement of π-electrons Classification of Diels–Alder Reaction: *Pericyclic Reactions: Involve a Cyclic Transition State Structure 1. Electrocyclic Reactions 2. Chelatropic Reactions 3. Sigmatropic Reactions 4. Cycloaddition Reactions: Result in the Formation of A Cyclic Product (An Adduct) 1. [4+2] (AKA: Diels-Alder Reactions) 2. [6+4] 3… General Equation for Diels–Alder Reaction:

Diene + Dienophile Diels Alder Adduct

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The Simplest Example for Diels–Alder Reaction: 4π-e– + 2π-e–

Mechanism: Suggested Transition State Structure:

+

+

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MO Consideration to Account for the Bonding of the Diels–Alder Reaction: HOMO of the Diene interacts with the LUMO of the Dienophile (Full MO) (Vacant MO) (electron-rich) (electron-poor) Why Such Interaction between the Two Mentioned MO’s? How to Enhance the Reactivities of Dienes and Dienophiles? A Good Diene: Has EDG Examples: A Good Dienophile: Has EWG Examples:

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More Details on Diels–Alder Reaction: 1. The diene assumes an s-cis conformation: s–cis s-trans 2. Addition in syn (from one face of the double bond) with respect to the dienophile. Example: 3. The geometrical isomerism of the dienophile is maintained and present in the product: 4. When the diene is 1,4-substituted, it assumes the s-cis conformation, which points the substituents away from the s-cis cavity of the diene: Example:

H

Br

Br

H

H

Br

Br

H

CH3

H

CH3

H

H

CH3

H

CH3

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5. Alder Endo Rule: With dienophiles equipped with π-bonds in their EWG’s, due to orbital overlap of the π-system of the diene with the π-bond of the EW substituent on the dienophile, the transition state energy is stabilized. Therefore, this approach (AKA: Endo approach) is energetically favored over the opposite (AKA: Exo approach). The above-mentioned stabilizing effect of the orbitals is called Secondary Orbital Effect. Transition States:

O

H

H

HHO

H

H

H

H

H

H

H

H

O

endo product exo product(favored)

O

O

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How to Tell the Endo Vs. the Exo Product? Example: Predict the major product for the Diels-Alder reaction below:

endo

exo

exo

endo

exo

endo

O

O

O

+

35

6. When both of the reactants are unsymmetrically substituted, the major product for the Diels–Alder reaction is as follows: Examples: Predict the product for the Diels–Alder reaction between the reactants below: A) Analysis:

EDG

EDG

+

+

EWG

EWG

EDG

EWG

EDG EWG

EDG

EWG

EDG

EWG

A)

B)

H2N

+

O

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B) Analysis: UV-Vis Spectroscopy:

+

O

NH2

UV & Visible EMRSAMPLE

Absorption at a specificwavelength of the EMRby the !-bonds of the sample

!-max

Absorbance

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1. Strength of π-bonds Vs. Energy of EMR absorbed Vs. Wavelength of EMR absorbed: (Wavelength of λ-Max) 2. Strength of π-bonds Vs. Extent of Delocalization: 3. Extent of π-Conjugation Vs. Wavelength of λ-Max: Examples:

H2C CH2

Structural Example !-Max

171 nm

217 nm

290 nm

600 nm(orange)