chapter 3 alcohol & ether1

8/13/2019 Chapter 3 Alcohol & Ether1

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Chapter 3: Alcohol & Ether


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Alcoholscontain a hydroxy group (OH) bonded to an sp3hybridized carbon.

IntroductionStructure and Bonding


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Alcohols and ethers exhibit dipole-dipole interactions becausethey have a bent structure with two polar bonds.

Alcohols are capable of in termolecu lar hydrogen bond ing. Thus,alcohols are more polarthan ethers.

Physical Properties

Steric factors affect hydrogen bonding.


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Interesting Alcohols


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Interesting Alcohols


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Preparation of Alcohols and Ethers

Alcohols and ethers are both common products ofnucleophilic substitution.

The preparation of ethers by this method is called theWilliamson ether synthesis.


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Unsymmetrical ethers can be synthesized in twodifferent ways, but often one path is preferred.


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An alkoxide salt is needed to make an ether.

Alkoxides can be prepared from alcohols by a Brnsted-Lowry acidbase reaction. For example, sodiumethoxide (NaOCH2CH3) is prepared by treating ethanolwith NaH.

NaH is an especially good base for forming alkoxidebecause the by-productof the reaction, H2, is a gas that

just bubbles out of the reaction mixture.


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Reactions of Alcohols

Recall that, unlike alkyl halides in which the halogen atomserves as a good leaving group, the OH group in alcohols is avery poor leaving group.

For an alcohol to undergo nuc leoph i l ic sub st i tu t ion, OH mustbe converted into a better leaving group. By using acid,OH

can be converted into H2O, a good leaving group.


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Reactions of AlcoholsDehydration

Dehydration, like dehydrohalogenation, is a el iminat ionreact ionin which the elements of OH and H are removed fromthe and carbon atoms respectively.

Dehydration is typically carried out using H2SO4and otherstrong acids, or phosphorus oxychlor ide (POCl3) in thepresence of an am ine base.


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Typical acids used for alcohol dehydration are H2SO4or p-tolu enesul fon ic acid (TsOH).

More substituted alcohols dehydrate more easily, giving riseto the following order of reactivity.


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Whenan alcohol has two or three carbons, dehydration isregioselect iveand follows the Zaitsev ru le.

The more subs t i tu ted alkene is the major product when amixture of constitutional isomers is possible.


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Secondary and 30alcoholsreact by an E1 mechanism ,whereas 10alcoholsreact by an E2 mechanism.


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The E1 dehyd rat ion o f 20and 30alcoholswith acid givesclean elimination products without any by-productsformed from an SN1 reaction.

Clean elimination takes place because the reactionmixture contains no good nucleophi le to react with theintermediate carbocation, so no competing SN1 react ionoccurs.

This makes the E1 dehydrat ion o f alcohols much moresynthetically useful than the E1 dehydro halogenat ion ofalkyl halides.


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Since 10carbocat ions are h igh ly unstab le, their dehydrationcannot occur by an E1 mechanism involving a carbocationintermediate. Therefore, 10 alcohols undergo dehydrat ionfo l lowing an E2 mechanism.


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Carbocation Rearrangements

Often, when carbocations are intermediates, a less stablecarbocation can rearrange to a more stable carbocation by ashift of a hydrogen or an alkyl group. This is called arearrangement.

Because the migrating group in a 1,2-shift moves with two

bonding electrons, the carbon it leaves behind now has onlythree bonds (six electrons), giving it a net positive (+) charge.


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Carbocation Rearrangements

A 1,2-shift converts a less s table carbocation to a morestable carbo cation .

Rearrangements are not unique to dehydrationreactions. Rearrangements occur whenever acarbocation is formed as a reactive intermediate.

Consider the example below. 20

Carbocation A rearrangesto the more stable 30 carbocation by a 1,2-hydride shift,whereas carbocation B does not rearrange because it is 30to begin with.


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Reaction of Ethers with Strong Acid

In order for ethers to undergo substitution or eliminationreactions, their poor leaving group must first be converted into agood leaving group by reaction with strong acids such as HBrand HI. HBr and HI are strong acids that are also sources of goodnucleophiles(Brand Irespectively).

When ethers react with HBr or HI, bothCO bonds are cleaved

and two alkyl halides are formed as products.


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The mechanism of ether cleavage is SN1 or SN2, depending onthe identity of R.

With 20 or 30 alkyl groups are bonded to the ether oxygen, theCO bond is cleaved by an SN1 mechanism involving acarbocation. With methyl or 10 R groups, the CO bond iscleaved by an SN2 mechanism.

Example: in the reaction of (CH3)3COCH3 with HI, the 30 alkylgroup undergoes nucleophilic substitution by an SN1mechanism, resulting in the cleavage of one CO bond. Themethyl group undergoes nucleophilic substitution by an SN2mechanism, resulting in the cleavage of the second CO

bond.


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chapter 3 alcohol & ether1

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