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17 Reactions of Aromatic Compounds -- Part 7
Key Mechanism 18-5 shows the mechanism for this reaction.
The acid-catalyzed addition to the carbonyl group is the first step.
The acid catalyst and alcohol attack the carbonyl group.
The hemiacetal is halfway to becoming a full acetal after adding one molecule of alcohol.
Most hemiacetals are too unstable to be isolated and purified.
The hemiacetal is converted to the more stable acetal in the second half of the mechanism.
A resonance carbocation occurs after the loss of water.
The acetal is given by the attack on the carbocation and the loss of a protons.
The alcohol is added to the carbonyl group in the first half.
There is a substitution of the hemiacetal in the second half.
There is a mechanism for the acid-catalyzed reaction of benzaldehyde with methanol.
You might wonder why acetal formation is only catalyzed by acid.
The formation of the hemiacetal can be base-catalyzed, with attack by alkoxide ion followed by the formation of the hemiacetal.
A resonance-stabilized cation is created by OH group and loss of water.
The equilib rium constant is used to control the proportions of reactants and products.
The acid-catalyzed reaction of acetaldehyde with alcohol gives important mechanisms in the acetal.
The equilibrium constants favor the process consisting of the two carbonyl compounds rather than the acetals.
The alcohol simple mechanisms are often used as the solvent to ensure a large excess.
The equilibrium is driven by the large amount of water.
The mechanism reverses acetal formation.
When cyclohexanone of water is attacked by the dimethyl acetal, it is quantitatively hydrolyzed to cyclohexanone by a brief treatment with alcohol.
The acid-catalyzed hydrolysis of cyclohexanone dimethyl acetal can be proposed.
The acetal is formed using a diol and alcohol.
Sugars and other sugars are called hemiacetals.
The most stable type of sugar is six-carbon sugar.
Lactose has one acetal and one hemiacetal.
In Chapter 23, we discuss the structures of carbohydrates.
Show what alcohols and carbonyl compounds give the following derivatives.
The type of mechanism is acid-catalyzed.
We assume it involves strong base and base elements, but no strong nucleophiles or free radicals.
You have to decide what products are formed by the hydrolysis of the acetal.
The affected atoms should be shown in an equation.
Water must add and the ring must be cleaved according to the equation.
The reactant won't react with water until it is activated.
It can become ionized at either atom.
The ring oxygen will be chosen.
The compound is well suited for ring cleavage to form a cation.
The product of this bond formation can be drawn.
Attack by water on the cation causes a hemiacetal.
OCH3 group can be lost if it is protonated.
Combining the preceding equations gives the complete mechanism.
The mechanism to review the steps should be written down.
Problems 18-27 and 18-28 can be completed by completing the five steps listed in the section.
The ring oxygen atom was protonated first, the reaction was reverse, and then the methoxy group was lost.
The mechanism of the forward could be written to show the methoxy oxygen leaving first, followed by ring reaction, as long as they take place.
The mechanism for the acid-catalyzed reaction of cyclohexanone with ethylene should be proposed.
There is a mechanism for the acid-catalyzed hydrolysis of cyclohexanone.
You drew mechanisms in parts.
The acid-catalyzed hydrolysis of the acetal is given in Problem 18-26(f).
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