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Organic Chemistry. Chapter 10. Functional Groups. The Key To Substitution Reactions. The Leaving Group Goes. Substitution Rx of R-OH. Alcohols have polar groups which make substitution probable Alcohols have a strongly basic leaving group (OH - ) which make it not probable

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Organic chemistry

Organic Chemistry

Chapter 10



The key to substitution reactions
The Key To Substitution Reactions

  • The Leaving Group Goes


Substitution rx of r oh
Substitution Rx of R-OH

  • Alcohols have polar groups which make substitution probable

  • Alcohols have a strongly basic leaving group (OH-) which make it not probable

  • Protonationconverts an alcohol to a good leaving group


Mechanisms
Mechanisms

  • Wow… we get to draw one……..

From yahoo images


Substitution rx of r oh1
Substitution Rx of R-OH

  • HBr and HI work well for SN2 reactions.

  • HCl does not work well because Cl is a poorer nucleophile.

  • Rate can be increased using ZnCl2

    • Zn2+ is a Lewis Acid

    • Complexes with the O:

      • (this weakens the C-O bond)



Rearramgement
Rearramgement

  • OH ya, don’t forget -



Conversion of alcohols
Conversion of Alcohols

  • Use phosphorous trihalides or thionyl chloride

    • Better yields and no rearrangements

    • PCl3, PBr3, PI3, or SOCl2

Do the Mechanism……………………..

From yahoo images


Conversion of alcohols1
Conversion of Alcohols

Pyridine is used as the solvent because it prevents formation of HCl or HBr and is a poor nucleophile.


Conversion of alcohols2

D

D

D

pyridine

pyridine

pyridine

Conversion of Alcohols

  • Commonly Used Methods for Converting Alcohols into Alkyl Halides

    • ROH + HBr RBr

    • ROH + HI RI

    • ROH + HCl RCl

    • ROH + PBr3 RBr

    • ROH + PCl3 RCl

    • ROH + SOCl2 RCl


Sulfonate esters leaving groups
Sulfonate esters leaving groups

  • Conversion of alcohols to sulfonyl chlorides

    • p-toluenesulfonyl chloride (tosyl chloride, TsCl)

    • methylsulfonyl chloride (mesyl chloride, MsCl)

    • trifluoromethanesulfonyl chloride (trif)

  • They are up to 100 x better than Cl-as leaving groups


Sulfonic acid has a pka of 6 5 wow
Sulfonic acid has apKa of – 6.5 wow!!

That ought to be on stable base now don’t ya think?



Dehydration of alcohols
Dehydration of Alcohols

  • Zaitsev’s Rule – more substituted formed


Dehydration of alcohols1
Dehydration of Alcohols

  • Dehydration is the reverse of Hydration

    • vary conditions to control equilibrium

    • Remove the alkene by distillation


Dehydration of alcohols2
Dehydration of Alcohols

  • Reaction may involve rearrangement


Dehydration of alcohols3
Dehydration of Alcohols

  • rearrangements with ring opening


Dehydration of alcohols4
Dehydration of Alcohols

E and Z produced, major product will have most bulky groups on opposite sides


Dehydration with pocl 3
Dehydration with POCl3

  • Uses a better leaving group

  • Conditions are not as extreme

  • Phosphorous oxychloride and pyridine

  • No rearrangements

  • Mildly basic conditions favor E2


Oxidation of alcohols
Oxidation of Alcohols

Look at reaction and conditions

For Primary and Secondary

Alcohols

From yahoo images


Substitution rx of ethers
Substitution Rx of Ethers

  • Ethers can be activated by acid

  • High concentration of HI, HBr will form the alkyl halide


Ethers as solvents
Ethers as Solvents

  • Ethers are relatively unreactive so they are frequently used as solvents

    • diethyl ether (ether)

    • tetrahydrofuran (THF)

    • 1,4-dioxane

    • 1,2-dimethoxyethane (DME)

    • methyl t-butyl ether (MTBE)


Addition of peroxyacids
Addition of Peroxyacids

...from chapter 4...

  • Alkenes can be oxidized to an epoxide by a peroxyacid.


Reactions of epoxides
Reactions of Epoxides

  • Because of 3 membered ring, epoxides are much more reactive than normal ethers

  • Undergo ring opening reactions at room temp


Reactions of epoxides oxiranes
Reactions of Epoxides(oxiranes)

Formation of glycols (addition of H2O)


Reactions of epoxides oxiranes1
Reactions of Epoxides(oxiranes))

Unsymmetrical additions yield the product resulting from Nu: attack on the more substituted carbon


Reactions of epoxides oxiranes2
Reactions of Epoxides(oxiranes)

Under basic conditions, Nu: attack is at the less hindered C

The epoxide is reactive enough that you don’t need to protinate to get the reaction to go



Crown ethers1
Crown Ethers

  • Cyclic compounds with ether linkages

  • Bind cations as “host” and “guest”


Crown ethers2
Crown Ethers

  • Naming – [x]-crown-Y

  • X = total number of atoms in the ring

  • Y = total number of oxygens



Thiols and sulfides1
Thiols and Sulfides

  • Thiols are sulfur analogs of alcohols

  • Also called mercaptans (mercury capturing)


Thiols and sulfides2
Thiols and Sulfides

  • Thiols are named by adding suffix “thiol”

  • Remember to keep the e

  • Common names are alkyl mercaptans


Physical properties thiols
Physical Properties-Thiols

  • The difference in electronegativity between S (2.5) and H (2.1) is 0.4.

  • This creates a bond with low polarity

  • show little association by hydrogen bonding

  • have lower boiling points and are less soluble in water than alcohols of comparable MW


Thiols act as nucleophiles
Thiols act as Nucleophiles

  • Thiolate anions are weak bases (weaker than alkoxides) and in protic solvents are better nucleophiles (better than alkoxides since they don’t H bond)


Sulfides or thioethers
Sulfides or thioethers

  • Sulfur analogs are called sulfides or thioethers

  • Most sulfides react readily to form sulfonium salts


Sulfides or thioethers1
Sulfides or thioethers

  • The sulfonium salt easily reacts in a substitution reaction:


Organometallic compounds
Organometallic Compounds

So far, we have seen reaction in which carbon is bonded to a more electronegative atom. What happens when it is connected to a less electronegative atom?


Organometallic compounds1
Organometallic Compounds

A compound that contains a carbon-metal bond

Organolithium

Organomagnesium (Grignard Reagent)



Organometallic compounds3
Organometallic Compounds

Organometalics act as nucleophiles

Reactions must be carried out in very dry solvents and

nothing acidic Can be in the reaction mixture


Organometallic compounds4
Organometallic Compounds

The greater the polarity difference, the greater the reactivity of an organometalic reagent


Coupling reactions
Coupling Reactions

  • Gilman Reaction

  • Heck Reaction

  • Stille Reaction

  • Suzuki Reaction


Gilman reagents

Henry Gilman

1893-1986

Gilman Reagents

Prepared from an organolithium reagent and copper(I) iodide


Gilman reagents1

Henry Gilman

1893-1986

Gilman Reagents

  • Gilman reagents can be used to form new carbon-carbon bonds by cross-coupling with alkyl oraryl or vinylic halides

(Note: cannot use SN2 with aryl or vinylic halides)





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