Ethers. Chapter 14: Ethers, Epoxides, & Sulfides. General Formula: Types: a) Symmetrical: Examples: b) Unsymmetrical: Examples: Physical Properties:

Transcription

1 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) RR Bond Angle: R R' b) Polarity: (Comparison with alcohols) R R' 1

2 c) Reactivity: (Comparison with alcohols) Advantage: d) Boiling Point: i) Compared to Alcohols: Reason: BP: 118 o C 66 o C 35 o C Dipole Moments: ***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: Ether BP ( o C)

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 rganic Compounds ***Poor Solvation of Anions (In comparison to alcohols): Why? f) Ether-Electrophile Complexes: Example: B 3.TF Example: BF 3. Et 2 Use: 3

4 g) Crown Ether Complexes: Na + K+ Uses: a) Dissolving cations in non-polar solvents b) Dissolving cations in aprotic media 4

5 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: 5

6 *Cyclic Ethers: Spectroscopy: IR: C Stretch cm 1 6

7 Synthesis of Ethers: a) Williamson Ether Synthesis (Covered in Chp. 11): General Equation: R + R' X R R' + X Example: (1) Na (2) Et Br Et Mechanism: Na Br Et Details: Alkyl alide (Step 2): Primary. Secondary and Tosylates also possible, but elimination competes, resulting in poor yields. 7

8 b) Alkoxymercuration-demercuration (Covered in Cp.8): General Equation: g(ac) 2 NaB 4 R Acg R R c) Bimolecular Dehydration of Alcohols: Industrial method. General Equation: 2 R + R R + 2 Remember: Unimolecular Dehydration of Alcohols (Proceeds via Elimination) This process competes with substitution (resulting in the formation of ether). Example: 3 +, eat In order to have substitution dominate the mechanistic pathway, the following measures could be taken: Example: 1. Use a relatively unhindered alcohol. 2. Use excess alcohol. 3. Keep the temperature low. + 8

9 *Synthesis of Phenyl Ethers: A special case of Williamson Ether Synthesis. General Equation: Ar + R' X Ar R' + X Aromatic alcohol used instead of aliphatic one. Phenyl Ether Product General Formula for Phenyl Ethers: R Example: N 2 (1) Na N 2 (2) Br Mechanism: Williamson Ether Synthesis *Why does the ring have a nitro-substitution? 9

10 Reactions of Ethers: a) Ether Cleavage by Br and I: General Equation: Excess X R R' R X + R' X X = Br, I Example: Br (excess) Br + Br Mechanism: Br 10

11 Example: Predict the product for the following reaction: + I (excess) Exception: Phenyl Ethers Example: Predict the product for the following reaction: Br (excess) b) Autoxidation of Ethers: R R' 2 (excess) R R' + R R' ydroperoxide Dialkyl Peroxide ***Note: This is a slower than usual reaction. ***Practical azard Associated with This Reaction: 11

12 Sulfides (Thioethers) General Formula: Remember: The prefix thio The function: Example: S S Synthesis: Williamson Ether (sulfide) Synthesis: General Equation: R S (1) Na (2) R' Br R S R' Mechanism: Very similar to that of the Williamson Ether synthesis from alcohols. Example: S (1) Na (2) C 3 Br 12

13 Epoxides General Formula: Nomenclature: Covered in cyclic ethers. Synthesis: a) Peroxyacid Epoxidation of Alkenes: General Equation: R R Example: MCPBA + Cl C 2 Cl 2 13

14 Mechanism: Concerted & Stereospecific Commonly Used Peroxyacids: MCPBA: MMPP: b) Base-Promoted Cyclization of Vicinal alohydrins: General Equation: C C X X = Cl, Br, I C C 14 + X

15 Example: K Cl Mechanism: Cl 15

16 Reactions: Ring-pening Reaction of Epoxides. a) Acid-Catalyzed: Functional Group Transformation: Epoxide Acid pen-chain carbocation Example: + + Mechanism: S N 1-like, stereospecific. *The Fate of the Resulting Carbocation: Depends on what nucleophile adds to the carbocation. Example: X X + 2 Regiochemistry of Addition: 16

17 Example: Predict the product and write the mechanism for the following reaction: 3 + b) Base-Catalyzed: Functional Group Transformation: Epoxide Base pen-chain substituted alkoxide Example: Alkoxide ion of a vicinal diol 17

18 Mechanism: S N 2-like. Regiochemistry of Addition: Example: Predict the product and write the mechanism for the following reaction: NaC 3 C 3 Na 2 Suggested Problems: 32, 33, 38, 39, 47, 18

19 E 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) C (eat of ydrogenation) 292kJ 252kJ 225kJ Experimental Conclusion: 19

20 Also remember the relative stabilities of alkenes. Example: Rank the following dienes in the increasing order of heat of hydrogenation: A B C D E < < < < Structure & Properties of Conjugated Systems bservation: An Isolated C=C An Isolated C C A Conjugated System Length: 1.30 angstroms 1.55 angstroms 1.34 angstroms 1.48 angstroms 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: 20

21 s-cis (cisoid) Vs. s-trans (transoid) Constructing the M s for the π-system of 1,3-Butadiene **{4 p-atomic orbitals combined 4 π-molecular orbitals}** E 0.0 π-electrons Related Terminology: Node, bonding, non-bonding, anti-bonding, M, LUM. 21

22 1. Node: A region of M with zero electron density 2. Bonding M: M s (used for bonding) that are lower in energy than the isolated atomic orbitals. 3. Non-bonding M: M s that are the same in energy as the isolated atomic orbitals. 4. Anti-bonding M: M s (weaken the bonding) that are higher in energy than the isolated atomic orbitals. 5. M: ighest ccupied Molecular rbital. 6. LUM: Lowest Unoccupied Molecular rbital. Exercise: Construct the M diagram of the π-system for 1,3,5-hexatriene molecule. E 22

23 Allylic Cations Remember: (Chp. 7) Allyl Group: C 2 C C 2 Allylic Position: C = Allylic carbon, s = Allylic hydrogens Allyl Cation: Stability of Allyl Cation: A More Realistic Drawing of Allyl Cation: Stability of Allylic Cations: C + 3 < 1 o < 2 o < Allyl < 3 o < Substituted Allylic 23

24 Ionic Additions of X to Conjugated Dienes Types: a) 1,2-Addition Conjugated System with the Corresponding b) 1,4-Addition Addition Sites Numbered Structural utcome of 1,2-Addition: 3 4 Structural utcome of 1,4-Addition: X 3 Mechanistic Examples of 1,2 and 1,4-Additions: 2 X Cl 24

25 Energetics of the 1,2 and 1,4 Additions: E Allylic Cation Reaction Coordinate 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 = = = 25

26 Detailed Energetics of the 1,2 and 1,4 Additions: Example: Br E Allylic Cation Reaction Coordinate 26

27 Allylic Radicals Stability: Follows the same trend as for the allylic cations. Allylic Bromination (A review): Mechanistic Example: Br ow was Br-radical generated? a) b) 27

28 M s of Allyl Cation, Allyl Radical, & Allyl Anion **Made of 3 combined atomic orbitals** E Cation Radical Anion 0.0!-electrons!-electrons!-electrons 28

29 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 29

30 The Simplest Example for Diels Alder Reaction: + Mechanism: 4π-e + 2π-e + Suggested Transition State Structure: 30

31 M Consideration to Account for the Bonding of the Diels Alder Reaction: M of the Diene interacts with the LUM of the Dienophile (Full M) (electron-rich) (Vacant M) (electron-poor) Why Such Interaction between the Two Mentioned M s? ow to Enhance the Reactivities of Dienes and Dienophiles? A Good Diene: as EDG Examples: A Good Dienophile: as EWG Examples: 31

32 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: Br Br 3. The geometrical isomerism of the dienophile is maintained and present in the product: Br Br 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: C 3 C 3 C 3 C 3 32

33 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 rbital Effect. endo product (favored) exo product Transition States: 33

34 ow to Tell the Endo Vs. the Exo Product? exo endo exo endo endo exo Example: Predict the major product for the Diels-Alder reaction below: + 34

35 6. When both of the reactants are unsymmetrically substituted, the major product for the Diels Alder reaction is as follows: EDG EDG EDG EWG A) + EWG EWG EDG EDG EDG EWG EWG B) + EWG Examples: Predict the product for the Diels Alder reaction between the reactants below: A) 2 N + Analysis: 35

36 B) + N 2 Analysis: UV-Vis Spectroscopy: UV & Visible EMR S A M P L E Absorption at a specific wavelength of the EMR by the!-bonds of the sample A b s o r b a n c e!-max 36

37 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: Structural Example!-Max 2 C C nm 217 nm 290 nm 600 nm (orange) 37