PTT 102Organic Chemistry

Alcohol & EtherReaction of Alcohol and

EthersMISS NOORULNAJWA DIYANA YAACOB

Course Outcome

CO2: Ability to EXPLAIN and DIFFERENTIATE the chemical, physical properties and reactions of alcohol, ether, aldehyde, ketone and carboxylic acids

HOW ARE ALCOHOLS CLASSIFIED AND

NAMED?

Alcohols– An alcohol is an organic compound with an — OH group. – The —OH functional group in alcohols is called a hydroxyl group

or hydroxy function.

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Alcohols23.2

Alcohols23.2

Alcohols23.2

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1. Determine the longest hydrocarbon containing the functional group:

2. The functional group suffix should get the lowest number:

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3. When there is both a functional group suffix and a substituent, the functional group suffix gets the lowest number:

4. The chain is numbered in the direction that gives a substituent the lowest number:

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6. If there is more than one substituent, the substituents are cited in alphabetical order:

5. The functional group substituent on a ring gets the number 1, but the functional group is not numbered in the name:

CH3

OH3-methylcyclohexanol

not3-methylcyclohexan-1-ol

1-methylcyclohexanolnot

1-methylcyclohexan-1-ol

CH3HO OH

CH3

CH3

2, 2-dimethylcyclopentanolnot

2, 2-dimethylcyclopentan-1-ol

Chemical and Physical Properties

• Alcohols have an odor that is often described as “biting” and as “hanging” in the nasal passages.

• In general, the hydroxyl group makes the alcohol molecule polar.

• Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable hydrocarbons and ethers. The boiling point of the alcohol ethanol is 78.29 °C.

• Alcohols can also undergo oxidation to give aldehydes, ketones, or carboxylic acids, or they can be dehydrated to alkenes. They can react to form ester compounds, and they can (if activated first) undergo nucleophilic substitution reactions

Ethers

What is the general structure of an ether and how are the alkyl groups of an ether named?

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Ethers

– The general structure of an ether is R—O—R. The alkyl groups attached to the ether linkage are named in alphabetical order and are followed by the word ether.

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Ethers

• An ether is a compound in which oxygen is bonded to two carbon groups.

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Nomenclature of Ethers

As substituents:

Physical and Chemical Properties

• Ether molecules cannot form hydrogen bonds with each other, resulting in a relatively low boiling points.

• Ethers are slightly polar.• Ethers are more polar than alkenes but not as

polar as alcohols, esters, or amides of comparable structure.

• Ethers in general are of low chemical reactivity, but they are more reactive than alkanes

Reaction of Alcohol and Ethers

• 1. Alcohol Elimination Reaction• 2. Oxidation of Alcohol• 3. Ether substitution reaction• 4. Epoxide Reaction• 5.Crown Ethers Synthesis

1. Alcohol Elimination Reaction

• Alcohol can undergo elimination reaction by losing an OH from one carbon and an H from adjacent carbon.

• Overall, this amounts to the elimination of WATER.

• Lost of water from a molecule is called DEHYDRATION.

• The product of the reaction is an ALKENE.

• Dehydration of an alcohol requires an acid catalyst and heat.

• Sulfuric acid (H2SO4) and Phosphoric acid (H3PO4) are commonly used acid catalysts.

Mechanism of Alcohol DehydrationThe general form of alcohol dehydrations is as follows:

The first step involves the protonation of the alcohol by an acid, followed by loss of water to give a carbocation.

Elimination occurs when the acid conjugate base plucks off a hydrogen. Alcohol dehydrations generally go by the E1 mechanism.

Example

• The mechanism for acid-catalyzed dehydration depends on the structure of the alcohol.

• Dehydration of secondary and tertiary alcohol are E1 reactions.

What is E1 Reaction??

• In the E1 mechanism, the the first step is the loss of the leaving group, which leaves in a very slow step, resulting in the formation of a carbocation.

• The base then attacks a neighboring hydrogen, forcing the electrons from the hydrogen-carbon bond to make the double bond.

B: = baseX = leaving group

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Dehydration of Secondary and Tertiary Alcohols by an E1 Pathway

The acids protonates the most basic atom in the reactant. Protonation converts the very polar leaving group (OH) into a good leaving (H2O).Water departs, leaving behind a carbocation.A base in the reaction mixture removes a proton from a β carbon, forming an alkene

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Because the rate-determining step in the dehydration reaction of 2° or 3° alcohol is a formation Of a carbocation intermediate, The rate of dehydration reflects the ease with which thecarbocation is formed:

Carbocation Rearrangement• Dehydration of 2° and 3° alcohols involves the formation of carbocation

intermediate, so be sure to check the structure of the carbocation .• Crbocation will rearrange if rearrangement produces a more stable carbocation.

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Carbocation Stabilities

Alkyl groups decrease the concentration of positive charge in the carbocation

Primary Alcohols Undergo Dehydration by an E2 Pathway

• Why??• Because primary carbocation are too unstable to be formed.• E2 reaction :one-step process of elimination with a single

transition state.• Any base (B: ) in the reaction mixture (ROH,ROR, H2O.HSO4

-) can remove the proton in the elimination reaction

• An ether is also obtained: it is the product of competing SN2 reaction since 1° alcohol are one most likely to form substitution products in SN2/E2 reaction

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Primary Alcohols Undergo Dehydration by an E2 Pathway

• We can summarized what we have learned about the mechanisms by which alcohol undergo substitution and elimination reaction:

2° & 3° Alcohol: Undergo SN1 and E1 reaction

1° Alcohol: Undergo SN2 and E2 reaction

2. Oxidation of Alcohol

• Primary alcohols can be oxidized to aldehydes or further to carboxylic acids

• Secondary alcohols can be oxidised to ketones but no further

• Tertiary alcohols cannot be oxidized

Oxidation by Chromium (VI)

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Oxidation of AlcoholsOxidation by chromic acid:

Secondary alcohols are oxidized to ketones

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Primary alcohols are oxidized to aldehydes and eventually carboxylic acids:

The oxidation of primary alcohol will stop at aldehyde if pyridinium chlorochromate (PCC) is used as the oxidizing agent in a solvent such as dichloromethane (CH2Cl2).In the absence of water, the oxidation stops at the aldehyde:

No water present

Mechanism:

An oxygen of chromic acid is protonated in the acidic solutionThe alcohol molecule displaces a molecule of water in an SN2 reaction on chromiumA base present in the reaction mixture (H2O, ROH) removes a proton from the strongly acidic spesiesA base removes a proton from chromates ester in an E2 reaction, thereby forming the carbonyl compound

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A tertiary alcohol cannot be oxidized and is converted to a stable chromate ester instead:

Di-tert-Butyl ChromateNo hydrogen on this carbon

Cr

O

O

OO

3. Ether substitution reaction

• The OR group of an ether and the OH group of an alcohol have nearly the same basicity.

• Both groups are strong bases, so both are very poor leaving group.

• Consequently, ethers, like alcohols, needs to be activated before they can undergo a nucleophilic substitution reaction

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Nucleophilic Substitution Reactions of Ethers

Ethers, like alcohols, can be activated by protonation:

What happenster the ether is protonated depend on the structure of ether. If departure of ROH creates arelatively atable carbocation, an SN1 reaction occurs

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Ether cleavage: an SN1 reaction:Protonation converts the very basic RO- leaving group into the less basic ROH leaving group.The leaving group departsThe halide ion combines with carbocation

Ether cleavage: an SN2 reaction:

4. Epoxide Reaction

• Alkene can be converted into epoxide by a peroxyacid

• Or by the addition of ClOH (by using Cl2 and H2O) followed by HO-

• Epoxides ,like ors, undergo substitution reaction withv hydrogen halides.

• The mechanisms of the reaction depends on whether it is carried out under acidic or neutral/basic conditions.

• Nucleophilics Substitution: Acid Conditions

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Acid-Catalyzed Epoxide Ring OpeningHBr:

Protonated epoxides are so reactive that they can be opened by poor nucleophiles, such as water and alcohols, where HB+ is any acid in the solution and :B is any base.

The acid protonates the oxygen atom of the epoxideThe protonated epoxide undergoes back-side attack by the halide ion

• If different substituent are attached to the two carbons of the protonated epoxide, and the nucleophile is something other than H2O, the product obtained from nucleophilic attack on the 2- position of the oxirane will be different from that obtained from nucleophilic attack on the 3-position .

• The major product is the one resulting from nucleophilic attack on the more substituted carbon

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Reaction of an epoxide in the presence of methanol and acid

Mechanism:

Regioselectivity:

Nucleophilics Substitution: Basic/Neutral Conditions

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When a nucleophile attacks an unprotonated epoxide,the reaction is a pure SN2 reaction:The C-O bond does not begin to break until the carbon is attacked by the nucleophile.The nucleophile is more likely to attack the less substituted carbon because it is less sterically hinderedThe alkoxide ion picks up a proton from the solvent

Therefore:

5.Crown Ethers Synthesis

• Crown ethers are cyclic compounds containing several ether linkage around a central cavity

• A crown ether specifically binds certain metal ions or organic molecules.

• The crown ether is called the “host” and the species it binds is called the “guest”

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Crown Ethers

The ability of a host to bond only certain guests is an example of molecular recognition

Because the ether linkages are chemically inert, the crown ether can bind the guest without reacting with it.The crown-guest complex is called INCLUSION COMPOUND