5.12 Acetals, Ketals, and Schiff Bases

An aldehyde can reversibly combine with an alcohol to form a molecule with two oxygen atoms on the same carbon atom. Taking the example of hexanal CH3(CH2)4CHO and ethanol CH3CH2OH, they can spontaneously react to form CH3(CH2)4C(OH)OCH2CH3, called a hemiacetal. Further reaction with another ethanol molecule can form a full acetal CH3(CH2)4C(OCH2CH3)2, with release of a water molecule.

Analogous reversible reactions are also possible between ketones and alcohols, forming hemiketals and ketals such as (H3C)2C(OCH2CH3)2 the ketal of acetone and ethanol.

If the alcohol is a polyol, such as a glycol, the resulting acetal or ketal will form a ring. To form a (hemi) acetal or (hemi) ketal, the alcohol should be a primary alcohol, since the bulky hydrocarbon chains of secondary and tertiary alcohols make the reaction inefficient.

An amine, in its positively charged protonated state, can react with an aldehyde, the pair giving up two of the amine hydrogens and the aldehyde oxygen to release a molecule water, and double-bonding the aldehyde carbon to the amine nitrogen. The result is called a Schiff base, and it has the structure R-NH+=CH-R', where R and R' are unspecified groups.

The reaction is reversible; a Schiff base can easily hydrolyze back to the original amine and aldehyde. A hydrogen bond acceptor near the Schiff base stabilizes it, helping it stay bound together.

A Schiff base can also be made analogously from a ketone and an amine. Schiff bases are examples of secondary imines.

The light-sensing proteins in our eyes, called opsins, incorporate a Schiff base. A molecule of 11-cis retinaldehyde is joined to an amino group from a unit of lysine (yep, that lysine 🦕) in the protein. When light hits this moiety, the 11-cis-retinal reverts back to the all-trans isomer, causing changes in the shape of the protein molecule that ultimately lead to neural signals conveying the perception of light. Parts of the protein that are near this retinal base can fine-tune the range of wavelengths that it responds to, and having more than one type of opsin with sensitivities to different wavelengths makes it possible to distinguish colors.


Schiff base of lysine and 11-cis retinal.		 hν
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Lysine and all-trans retinal.

The hν or "aitch-nu" means light causes the reaction to take place.

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