You Gotta Know These Functional Groups
A functional group is a fragment of a molecule with a particular arrangement of atoms that allow it to undergo similar reactions to other molecules with the same functional group. Knowing the basic functional groups and how they react is fundamental to understanding organic chemistry. This article will give text-based descriptions of the functional groups similar to those given in quizbowl questions, but it’s a good idea to look at the to get a better idea of what the groups look like. Both in this article and in most skeletal formulas, the letter R refers to an arbitrary hydrocarbon chain. Sometimes, “prime” symbols are added for molecules with multiple R groups to illustrate that the two R groups need not be the same group (for example, ethers are denoted R–O–R′, pronounced “R O R prime”). Also, to better understand some of the reactions discussed in this article, it’s a good idea to learn .
- The simplest functional groups are alkanes, alkenes, and alkynes, which are all hydrocarbons with no atoms other than carbon and hydrogen. Alkanes have only single bonds, alkenes have a carbon-carbon double bond, and alkynes have a carbon-carbon triple bond. Alkynes can be reduced through a hydrogenation reaction to create alkenes, which can be reduced again to alkanes. Many “greasy” substances like oil and wax are purely alkanes. Fats that contain no double bonds (i.e. have alkane groups) are called saturated, while fats with an alkene double bond are called unsaturated. Alkenes can be classified as cis or trans based on whether the non-hydrogen groups are on the . Alkynes are classified as internal or terminal based on whether the triple bond is . The simplest alkene and alkyne are ethylene (responsible for fruit ripening) and acetylene (used as a fuel in welding), respectively.
- Alcohols (R–O H) are defined by an –O H (oxygen and hydrogen) bonded to a carbon in an alkyl group. The –O H group by itself is called a hydroxyl group, so another equivalent way of defining an alcohol is a carbon bonded to a hydroxyl group. Common examples of alcohols include the one-carbon alcohol methanol (also called “wood alcohol,” and which can cause blindness if ingested), the two-carbon ethanol found in alcoholic drinks, and the three-carbon isopropanol found in some rubbing alcohol products. Alcohols are the simplest oxygen-containing functional group and can be oxidized to carbonyls and carboxylic acids. Having two alcohol groups on a molecule results in a diol such as ethylene glycol, the primary ingredient in most antifreeze. There are three alcohol groups in glycerol, a molecule that serves as a building block for fats.
- Carbonyls (R–C(O)–R′) are characterized by a carbon-oxygen double bond. The two simplest functional groups that contain a carbonyl are the aldehyde, which has a carbonyl at the end of a hydrocarbon chain, and the ketone, which has a carbonyl in the middle of a hydrocarbon chain. Some examples of aldehydes and ketones include formaldehyde (used in tissue preservation) and acetone (a solvent used in nail polish remover). Other functional groups that contain a carbonyl include the carboxylic acids, esters, amides, and ureas. Due to resonance, the carbon atom in a carbonyl has a , while the oxygen has a partial negative charge. This causes the carbonyl carbon to act as an electrophile in reactions, meaning it is an “electron-loving” atom that will accept electrons from a nucleophile. Carbonyls can be produced through many methods, such as oxidation reactions of alcohols or cleavage of an alkene by ozone in ozonolysis.
- Ethers (R–O–R′) are defined by an oxygen atom bonded to two alkyl groups. They are similar to alcohols except the alcohol hydrogen is replaced by another carbon-containing fragment. Common ethers include diethyl ether (the first-developed anesthetic, where it was commonly known as just “ether”) and the laboratory solvent THF (tetrahydrofuran). Ethers can be produced through the Williamson ether synthesis reaction, in which a deprotonated alcohol an alkyl halide. In many organic reactions that employ explosive metal-containing reagents, such as a magnesium-based Grignard reagent, an etherated solvent is used to stabilize that reactive reagent.
- Carboxylic acids (R–C O O H) are one of the most oxidized oxygen-containing functional groups and are characterized by a carbon atom singly bonded to one oxygen atom and doubly bonded to another oxygen atom. Carboxylic acids are frequently sour-smelling, such as the two-carbon acetic acid (found in vinegar). Other common carboxylic acids include the one-carbon formic acid (found in ants), and the three-carbon lactic acid that causes muscle soreness after strenuous exercise. As indicated by their name, carboxylic acids are relatively acidic and can thus form salts when they interact with a base, leading to a negatively charged carboxylate (such as formate, acetate, lactate, etc.). Carboxylic acids and amines are the two functional groups common to every amino acid; the to form a peptide bond, which contains an amide group. The end of a protein that contains a free carboxylic acid is called the C-terminus.
- Esters (R–C O O–R′) are similar to carboxylic acids, but they have an alkyl group in place of the acid’s hydrogen atom. Esters are characterized by their fruity smells; for example, isoamyl acetate has a strong banana-like odor and is found in many fruits. Ester linkages are found in the class of fat molecules called triglycerides, where they connect the glycerol backbone to three long fatty acid tails. Polyesters are a class of polymer that contains ester linkages between monomers; examples include PET, which is used in clothing fibers and some water bottles. The most common reaction for creating esters is the Fischer esterification, which combines a carboxylic acid and an alcohol to create an ester, .
- Alkyl halides (R–X) are characterized by a halogen atom bonded to a hydrocarbon alkyl group (the letter X is used in organic chemistry to denote an arbitrary halogen). Many alkyl halides pose significant environmental risks, such as the CFCs (chlorofluorocarbons) that were once used as refrigerants and significantly contributed to the depletion of the ozone layer. PFAs (per- and polyfluoroalkyl substances) such as Teflon, PFOA, and PFOS are incredibly durable and are thus deemed “forever chemicals” due to their persistence as pollutants. Despite these hazards, alkyl halides represent a useful building block in organic synthesis; the halogen atom serves as an excellent leaving group, allowing the alkyl group to be connected to a different molecule in a substitution reaction.
- Thiols (also called mercaptans; R–SH) are defined by an –SH (sulfur and hydrogen) bonded to a carbon. They are effectively sulfur analogues of alcohols, which is sensible given that sulfur sits right below oxygen in the same column of the periodic table. Thiols are notably pungent, giving a characteristic stinky “rotten eggs” smell. A thiol group is found in the side chain of the amino acid cysteine; the to form disulfide bridges in protein folding. The ability of thiols to scavenge dangerous free radicals allows the molecule glutathione to serve as an antioxidant to protect cells from oxidative stress.
- Aromatic compounds contain a ring system that satisfies the criteria for aromaticity. These most often feature six-membered benzene rings in phenyl groups but can include many different ring sizes and also non-carbon atoms. Common aromatic compounds include benzene, toluene, and phenol. Aromatic compounds are non-polar, meaning they feel “greasy” or oily. Ring systems are described as aromatic if and only if they are planar, essentially meaning the molecule is flat; and if they satisfy Hückel’s rule, which means the ring has 4n + 2 pi electrons for some integer n. For example, benzene has six pi electrons, which is satisfied by n = 1 and thus 4·1 + 2 = 6, so benzene is aromatic, while cyclobutadiene has four pi electrons, which does not correspond to any integer value of n, so cyclobutadiene is not aromatic. Aromatic compounds are one of the easiest to identify by nuclear magnetic resonance spectroscopy as they feature a unique downfield peak around 7 ppm due to a diamagnetic ring current.
- Amines (R–N R′ R″) are characterized by a nitrogen atom that is only singly bonded to carbons and hydrogens. Another way to think about them is that they are analogues of ammonia (N H3) in which at least one nitrogen is replaced by an alkyl group. Amines can also be thought of as nitrogen analogues to alcohols, and they can sometimes react similarly; for example, the reaction between an alcohol and a carboxylic acid to form an ester is analogous to the reaction between an amine and a carboxylic acid to form an amide. Also like alcohols, amines can be categorized as primary, secondary, or tertiary depending on the number of alkyl groups bonded to the nitrogen. Amines are characterized by a “fishy” smell, such as in putrescine and cadaverine found in rotting fish. Amines can react with an acid (or a carbon electrophile) to generate ammonium salts, some of which are used in disinfectants. Because amines adopt a square pyramidal geometry, they can “flip” via quantum tunneling in the , which is often analogized to an umbrella turning inside-out due to a gust of wind
This article was contributed by ÎÞÓǶÌÊÓƵ writer Justin Zhang.