PTT 102 Organic Chemistry. Alcohol & Ether Reaction of Alcohol and Ethers MISS 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.
Alcohol & Ether
Reaction of Alcohol and Ethers
MISS NOORULNAJWA DIYANA YAACOB
CO2: Ability to EXPLAIN and DIFFERENTIATE the chemical, physical properties and reactions of alcohol, ether, aldehyde, ketone and carboxylic acids
1. Determine the longest hydrocarbon containing the functional group:
2. The functional group suffix should get the lowest number:
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:
5. The functional group substituent on a ring gets the number 1, but the functional group is not numbered in the name:
6. If there is more than one substituent, the substituents are cited in alphabetical order:
The 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.
The mechanism for acid-catalyzed dehydration depends on the structure of the alcohol.
B: = baseX = leaving group
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
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 the
carbocation is formed:
Alkyl groups decrease the concentration of positive
charge in the carbocation
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
Oxidation by chromic acid:
Secondary alcohols are oxidized to ketones
Primary alcohols are oxidized to aldehydes and eventually carboxylic acids:
The oxidation of primary alcohol will stop at aldehyde if pyridiniumchlorochromate (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:
An oxygen of chromic acid is protonated in the acidic solution
The alcohol molecule displaces a molecule of water in an SN2 reaction on chromium
A base present in the reaction mixture (H2O, ROH) removes a proton from the strongly acidic spesies
A base removes a proton from chromates ester in an E2 reaction, thereby forming the carbonyl compound
A tertiary alcohol cannot be oxidized and is converted to a stable chromate ester instead:
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 arelativelyatablecarbocation, an SN1 reaction occurs
Protonation converts the very basic RO- leaving group into the less basic ROH leaving group.
The leaving group departs
The halide ion combines with carbocation
Ether cleavage: an SN2 reaction:
Epoxides ,like ors, undergo substitution reaction withv hydrogen halides.
The acid protonates the oxygen atom of the epoxide
The protonatedepoxide undergoes back-side attack by the halide ion
Protonatedepoxides 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.
If different substituent are attached to the two carbons of the protonatedepoxide, 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 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
The alkoxide ion picks up a proton from the solvent
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
The ability of a host to bond only certain guests is an
example of molecular recognition