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Acetal

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An acetal is a molecule with two single bonded oxygens attached to the same carbon atom. Aldehydes and ketones undergo a reversible reaction with alcohols in the presence of an acid catalyst to yield acetals. Acetal formation involves the acid catalysed nucleophile addition of an alcohol to an aldehyde or ketone in a process analogous to that of the acid catalysed addition of water.


Definition

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A hemiacetal is defined as a compound derived from an aldehyde with an OH and an OR group on the same carbon.
"An acetal is defined as a compound derived from an aldehyde, where one carbon is attached to two OR units."
When an aldehyde or ketone is treated with an excess of alcohol in the presence of an acid catalyst, two molecules of alcohol are added to the carbonyl compound to give an acetal or a ketal respectively.

Acetal Group

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An acetal group is identified by a C atom that is bonded to two alkoxy (R-O-) groups. The two alkoxy groups may be the same, such as two CH3O groups or they may be different, such as CH3O and (CH3)3CO. A hemiacetal is identified by a C atom that is bonded to both an OH and an OR group.

Examples of an acetal and a hemiacetal functional group, not in rings. The examples of an acetal functional group, hemiacetal functional group, molecule of glucose containing one hemiacetal functional group and dimer of glucose containing an acetal functional group are shown below.

Acetal Groups

Formation

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If a small amount of acid catalyst is added to a hemiacetal reaction mixture, the hemiacetal reacts with a second alcohol molecule, in a condensation reaction, to form an acetal. An acetal is not an ether even though both types of compounds have structures in which one or more alkoxy (OR) groups are present.

Acetal formation is similar to the hydration reaction. Like water alcohols are weak nucleophiles that add to aldehydes and ketones only slowly under neutral conditions. Under acidic conditions however the reactivity of the carbonyl group is increased by protonation, so the addition of an alcohol occurs rapidly.

Acetal Formation

Formation Mechanism

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The mechanism of acetal formation is quite complex by considering the reaction of methanol with acetaldehyde. The aldehyde is the electrophile and the electrophilic center is the carbonyl carbon. The first step of the mechanism involves the oxygen of the carbonyl group using a lone pair of electrons to form a bond to a proton. This results in a charged intermediate where the positive charge is shared between the carbon and oxygen of the carbonyl group.

The intermediate formed from this first nucleophilic addition is called a hemiacetal. If a ketone had been the starting material, The structure obtained would have been a hemiacetal. Once the hemiacetal is formed, it is protronated and water is eliminated by the same mechanism described in the formation of imines.

Formation of Hemiacetal

The difference is that oxygen donates a lone pair of electrons to force the removal of water rather than nitrogen. The resulting oxonium ion is extremely electrophilic and a second nucleophilic addition of alcohol takes place to give the acetal.

Formation Mechanism

Properties

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Some properties of acetal has given below
  • The properties of Acetal depend on several factors including type, molecular weight, crystallinity and additive type and concentration.
  • The copolymers have an excellent balance of properties and processing characteristics. Melt temperature can range from 182 to 232oC with little effect on part strength. UV-resistant grades, glass reinforced grades, low wear grades and impact modified grades are standard.
  • Acetal copolymers have excellent resistance to chemicals and solvents. For example, specimens immersed for 12 months at room temperature in various inorganic solutions were unaffected except by strong mineral acids - sulfuric, nitric and hydrochloric.

Acetal Reaction

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Acetal undergo most of the functional group reactions on their side chains as long as the reactions are carried out at neutral or basic conditions. For example, olefinic groups can be halogenated or oxidised. Halogen group, depending on their position, can be condensed with other compounds or can be eliminated to give an olefinic group. Very important reactions are the transacetalization and transketalization reactions, wherein an existing alkoxy group are exchanged for others, usually by reacting with higher boiling alcohols.

Condensation reactions of Halo acetals, Hydroxy acetals and Olefinic acetals
  • The side chain attached to the acetal functional group is capable of undergoing the ordinary condensation reactions. For example, hydroxy methyl groups may be reacted further to give esters or ethers.
Condensation of Halo Acetals
  • Haloalkyl groups may be condensed further as with malonates to give substituted malonic esters.
Malonic Esters
  • In addition, halogen groups on acetals react with Grignard reagents as shown below.
Acetal Group with Grignard Reagent

Hydrolysis

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Acetals unlike hemiacetals are easily isolated from reaction mixtures. They are stable in basic solution but undergo hydrolysis in acidic solution. A hydrolysis reaction is the reaction of a compound with H2O in which the compound splits into two or more fragments as the elements of water are added to the compound. The product of acetal hydrolysis are the aldehyde or ketone and alcohols that originally reacted to form the acetal.

Acetal Hydrolysis
For example,

Acetal Hydrolysis Example
The carbonyl hydrolysis product is an aldehyde if the acetal carbon atom has a hydrogen atom attached directly to it, and it is a ketone if no hydrogen attachment is present. In the preceding example, the carbonyl product is a ketone because the two additional acetal carbon atom attachments are methyl groups.

Copolymer

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Acetal polymers also known as polyoxymethylene (POM) or polyacetal are formaldehyse based thermoplastics that have been commercially available. The homopolymer is more crystalline than the copolymer. The homopolymer provides the better mechanical properties except for elongation. The copolymer is also more stable than the homopolymer in an alkaline environment.

The advantages of acetal copolymer is excellent rigidity, impact toughness, abrasion resistance, creep resistance and solvent resistance. It has a good appearence, hydrolytic stability, fatigue endurance and low coefficient of friction. The disadvantage is high mould shrinkage. Post moulding shrinkage of about 0.1% normally completes within 48hrs. Attacked by acids and bases, very rapid attack by nitric acid. Very poor resistance to UV radiation. Homopolymers have high tensile strength, flexural strength, fatigue resistance and hardness.

Applications

Due to low coefficient of friction, commonly used as bearings, gears and conveyor belt limits. Electric kettles and water jugs. Components with snap fits. Chemical pumps, bathroom scales. Telephone key pads, pulley wheels, housing for domestic appliances, shower heads, fuel expansion tanks and toys.