More Groups

Here are the names and structures of some more functional groups. There's a lot of compounds mentioned on this page, but it's not crucial to memorize them all. They are there to give an idea of how common each group is and what each group is like in terms of its physical and chemical properties.

Esters

Esters are unions of an alcohol and a carboxylic acid. Many esters are commonly occurring, especially in plants. Most fruit flavors are based on esters. Some examples include:

• ethyl acetate , a component of wine that forms when the alcohol starts to oxidize to acetic acid (vinegar), then combines with more ethyl alcohol;
• isoamyl acetate , a banana flavoring;
• isoamyl isovalerate , an apple flavoring;
• n-butyl butyrate , resembling overripe fruit;
• ethyl levulinate , a pleasant fruit aroma with a peony-like quality;
• methyl salicylate , the main component of wintergreen oil;
• methyl cinnamate , a strawberry flavoring;
• methyl anthranilate , a grape flavoring;
• allyl phenoxyacetate , a floral, honey-like aroma;
• fructone , a fruit aroma and flavor with a minty quality to it;
• maltyl isobutyrate , a sugary, berry, cotton-candy-like flavor;
• p-cresyl pivalate , a leathery, horse-stable scent reminiscent of narcissus flowers;
• 1-octen-3-yl acetate , an ester of "mushroom alcohol" having a potato-skin aroma;

...and so on.

It's worth noting that words like "acetate", "salicylate", "pivalate", etc., can refer to either esters, where the acid is covalently bonded to some other group, or salts such as sodium acetate CH₃C(=O)O^- Na⁺, where the acid is in its ionized state.

Ethers

Ethers are unions of two alcohol molecules. Diethyl ether is the most familiar ether. Diphenyl ether has an aroma like geranium or new vinyl.

Aldehydes

Aldehydes are compounds whose molecules have the functional group -CHO (i.e. -CH=O). Their names end in -aldehyde or -al, for example hexanal derived from hexane. Some examples include:

• acetaldehyde , the cause of hangovers;
• isobutyraldehyde , the malty smell of cereal;
• benzaldehyde , the aroma and flavor of bitter almond and cherries;
• cinnamaldehyde , the aroma and flavor of cinnamon candy,
• and aldehyde MNA, or methyl nonyl acetaldehyde , the secret ingredient in Chanel No. 5.

Aldehyde means "alcohol dehydrogenated" and an aldehyde group can be formed when a primary alcohol group is oxidized, removing two of its hydrogens. Aldehydes readily oxidize further to form carboxylic acids.

Ketones

Ketones are similar to aldehydes in that they have a C=O group. But instead of having -CH=O, ketones have C-C(=O)-C. Some examples include:

• acetone , a solvent commonly available as nail polish remover;
• butanone or methyl ethyl ketone , another solvent;
• diacetyl or butanedione , formerly used as an artificial butter flavoring;
• acetophenone , an artificial flavoring used to construct cherry and other fruit flavors;
• ionone , a floral aroma present in small amounts in roses.

Ketones are more stable in air than aldehydes. They can be formed by oxidizing a secondary alcohol, removing two of its hydrogens. However they cannot oxidize further to form carboxylic acids without breaking the carbon chain, which takes a lot of energy.

Note that the main aroma compound of roses is not ionone but phenethyl alcohol .

Lactones

Lactones have a ring containing a -C(=O)O- group. They are similar to cyclic ketones. A lactone can be thought of as a hydroxy carboxylic acid esterified to itself. The carbon atoms are counted with Greek letters; the atom joined to the carboxylic acid is called alpha, the next one over is beta, the next one is gamma, etc. Whichever carbon the hydroxy would be attached to is the one that gives its Greek letter to the lactone.
Gamma lactones have a five-membered ring; a good example is gamma-nonalactone , a common scent for sunscreen.
Delta lactones have a six-membered ring, etc. Lactones that have all their carbon atoms in one huge ring are often called omega-lactones; one example is the artificial musk omega-pentadecalactone, with 15 carbons:

Lactones can also be unsaturated, such as coumarin , present in several plants including cinnamon.

Enols

Enols are isomers of ketones or aldehydes, except the two can freely interconvert. Isomers that can do this are called tautomers. Two simple examples would be the interconversion between acetaldehyde and vinyl alcohol, and between acetone and propen-2-ol:
⇌ ⇌
Enols are somewhat acidic. Maybe because of their resemblance to carboxylic acids? I'm not sure why. In most cases, ketones are more stable than enols and the molecule spends much more time as a ketone than as an enol. One exception is 2,4-pentanedione, where a hydrogen bond between the -OH and the =O stabilizes the enol form:

Amides

Amides have a C(=O)NH₂ group. The amide group is conjugated, planar, and triangular. It has a negative partial charge on the oxygen atom and a positive partial charge on the hydrogens of the nitrogen atom, making it very hydrophilic and capable of strong hydrogen bonds. It is possible to substitute one or both of the hydrogen atoms. Monosubstituted amides have cis- and trans- forms.

One useful amide is acrylamide , which polymerizes into a plastic that can be used to make hydrogels. These are sold as tiny beads that expand to many times their volume when placed in water.
Another common amide is nylon. Different types of nylon exist, but all have repeating units of either -(-NH(CH₂)_nC(=O)-)- or -(-NH(CH₂)_nNHC(=O)(CH₂)_n'C(=O)-)-.

Imines

Imines have a C=N bond, for example .

Oximes

Oximes are imines where the group attached to the nitrogen atom is a hydroxy, for example .

Nitriles

Nitriles are organic cyanides, for example decyl nitrile , a floral-citrus aroma compound. They do not have the extreme toxicity that inorganic cyanides often do.
The nitrile group is called cyano- when it is a substituent, for example methyl cyanoacrylate , known commonly as super glue.

Azo Groups

An azo compound has an N=N double bond. Often these compounds have a bright red or yellow color. The azo group has cis- and trans- configurations just like an alkene, however an azo group can easily interconvert between the two forms. Azobenzene is red in its cis- configuration and yellow in its trans- form. The reason is cis- bonds always absorb light at longer wavelengths than an otherwise identical bond in the trans- configuration. In the absence of light, azobenzene slowly relaxes to the more stable trans-azobenzene. When exposed to ultraviolet light, it changes to cis-azobenzene, and when exposed to blue light, it changes back to trans-azobenzene.

Schiff Bases

A Schiff base is a union of a primary amine and an aldehyde. The amine is in its protonated form, and the aldehyde gives up its oxygen atom and takes two of the amine's hydrogens, releasing water. A good example of a Schiff base occurs in the light sensing proteins of the eyes, where a molecule of 11-cis-retinaldehyde unites with a molecule of lysine to form:

When light strikes this complex, it reverts the cis- bond to the more stable trans- configuration, the protein changes shape, and a cascade of other proteins ultimately cause the cell to change its electrical firing rate into its attached neuron.

Chelates

A chelate ("KILL-ate") is a complex made up of an organic compound and a metal ion that fits together with it. The elements oxygen, nitrogen, and sulfur are particularly good at chelating metals. Metals that occur in chelated forms in nature include (but are not limited to) magnesium, iron, copper, and zinc. Some metal ions, such as copper and iron, have their own geometry that the chelating agent must match in order to fit them. Others, such as sodium and potassium, are more like a ball of uniform charge and the molecule most only offer the right size aperture. Here are some examples of chelating agents and chelated compounds:

Copper-salicylate complex

EDTA (binds manganese, calcium, magnesium)

Crown ether (binds lithium)

Crown ether (binds sodium)

Crown ether (binds potassium)

Heme

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