It is well known that distilled alcoholic beverages contain a group of aroma compounds called acetals. Acetals are formed by a reaction in which an aldehyde molecule adds a molecule alcohol to form a labile hemiacetal. This combines with a second molecule of the alcohol and yields a stable acetal.
The reaction is catalyzed by acids but it is reversible, and consequently acetals are hydrolyzed in dilute alcoholic solution when acids are present. Ketones react in a similar way to form ketales.
Acetal is defined as "A molecule containg two OR groups bonded to the same carbon".
The acetal group serves as a good "protecting group". That is since the acetal is inert to most substances, this "protects" the molecules in question from undergoing unwanted reactions.
Aldehydes and ketones undergo a reversible reaction with alcohols in the presence of an acid catalyst to yield acetals, R2
, compounds that have two ether like -OR groups bonded to the same carbon.
Acetal formation involves the acid catalyzed nucleophilic addition of an alcohol to an aldehyde or ketone in a process analogous to that of the acid catalyzed addition of water.
Before elaborating some of the specific methods for cyclic acetal formation we might first consider a few general principles which apply to acyclic as well as cyclic acetal formation.
- Acetals are more easily prepared from aldehydes than from ketones.
- Cyclic acetals are easier to form than acyclic acetals
- Conjugation deactivates the carbonyl group towards acetalization
- Sterically hindered carbonyls react more slowly
- In aromatic aldehydes and ketones electron donating substituents on the arene ring retard acetal formation whereas electron withdrawing substituents facilitate it
- Gem-dialkyl substitution as in 2, 2-dimethylpropan-1, 3-diol, promotes cyclic acetal formation
Acetal can be hydrolyzed to regenerate the aldehyde or ketone from which it was made. Protonation of one oxygen atom occurs as the first step, and then a molecule of alcohol is displaced to produce the resonance stabilized carbocation intermediate.
The carbocation is subsequently intercepted by a molecule of water and deprotonation yields the hemiacetal.
Acetal copolymer appears susceptible to both free chlorine and combined chlorine attack. Scanning electron microscopy demonstrated that acetal copolymer is subject to surface attack, producing a chalky, friable surface layer that spalls easily.
Acetyl copolymer is widely used and has a melting point of 163o
C whereas the related acetal homopolymers have a higher melting point of 175o
C with greater mechanical strength. However polyoxymethylene is a strong enough material to be used for the manufacture of plastic fasteners of all types. It is resistant to attack by oxidation and by solvents and fuels.
Mechanism of formation of an acetal from a hemiacetal is shown below. Protonation and loss of water give an intermediate cation, which undergoes a nucleophilic addition reaction with methanol.
- The OH of the hemiacetal is protonated by an acid H-A making it a good leaving group.
- An electron pair on the -OCH3 group moves toward carbon, expelling water and giving a C-OCH3 bond with a positively charged trivalent oxygen.
Nucleophilic addition of methanol to the C=O bond pushes the pi electrons towards oxygen and neutralizes the positive charge.
- Deprotonation by the base :A- gives the neutral acetal and regenerates the acid catalyst.