Even if other parts of the molecule are quite different, certain functional groups tend to react in certain ways.  As we progress in our study of organic chemistry, it will become extremely important to be able to quickly recognize the most common functional groups, because they are the key structural elements that define how organic molecules react.  For now, we will only worry about drawing and recognizing each functional group, as depicted by Lewis and line structures.  Much of the remainder of your study of organic chemistry will be taken up with learning about how the different functional groups behave in organic reactions.

The ‘default’ in organic chemistry (essentially, the lack of any functional groups) is given the term alkane, characterized by single bonds between carbon and carbon, or between carbon and hydrogen.  Methane, CH₄, is the natural gas you may burn in your furnace.  Octane, C₈H₁₈, is a component of gasoline.

Alkenes (sometimes called olefins) have carbon-carbon double bonds, and alkynes have carbon-carbon triple bonds.  Ethene, the simplest alkene example, is a gas that serves as a cellular signal in fruits to stimulate ripening.  (If you want bananas to ripen quickly, put them in a paper bag along with an apple – the apple emits ethene gas, setting off the ripening process in the bananas). Ethyne, commonly called acetylene, is used as a fuel in welding blow torches.

We will have more to say about the subject of cis and trans alkenes in chapter 3, and we will learn much more about the reactivity of alkenes in chapter 10.

Alkanes, alkenes, and alkynes are all classified as hydrocarbons, because they are composed solely of carbon and hydrogen atoms. Alkanes are said to be saturated hydrocarbons, because the carbons are bonded to the maximum possible number of hydrogens  – in other words, they are saturated with hydrogen atoms.  The double and triple-bonded carbons in alkenes and alkynes have fewer hydrogen atoms bonded to them – they are thus referred to as unsaturated hydrocarbons.

The aromatic group is exemplified by benzene (which used to be a commonly used solvent in the organic lab, but which was shown to be carcinogenic), and naphthalene, a compound with a distinctive ‘mothball’ smell. Aromatic groups are planar (flat) ring structures, and are widespread in nature.  We will learn more about the structure and reactions of aromatic groups next semester.

The distinction is important, because as we will see later, there is a significant difference in the reactivity of alcohols and phenols.

We are already familiar with several common alcohols. For example, ethanol $$\left( \ce{CH_3CH_2OH} \right)$$ is the alcohol present in alcoholic beverages. It is also widely used in the industrial manufacture of other chemicals. Methanol $$\left( \ce{CH_3OH} \right)$$ is used as a gasoline additive or alternative. Additionally, methanol can be used to manufacture formaldehyde, which is employed in the production of plastics, paints, and other useful substances. Isopropanol is commonly known as rubbing alcohol. In addition to its industrial uses, isopropanol is used to clean various surfaces, including computer monitors, whiteboards, and even skin (e.g., before getting blood drawn).

The deprotonated forms of alcohols, phenols, and thiols are called alkoxides, phenolates, and thiolates, respectively. A protonated alcohol is an oxonium ion.

In an ether functional group, a central oxygen is bonded to two carbons. Below is the structure of diethyl ether, a common laboratory solvent and also one of the first compounds to be used as an anesthetic during operations. Ethers are good solvents for other organic compounds because of their low reactivity. They readily dissolve nonpolar molecules. The sulfur analog of an ether is called a thioether or sulfide.

One of the most important properties of amines is that they are basic, and are readily protonated to form ammonium cations. In the case where a nitrogen has four bonds to carbon (which is somewhat unusual in biomolecules), it is called a quaternary ammonium ion. 

Many smaller amines have very strong and offensive odors. For example, the aptly-named compounds cadaverine and putrescine are foul-smelling amines, formed as a part of the decay process after death.

Amines serve a wide variety of uses. Diphenylamine acts as a stabilizer for certain types of explosives. Amines are found as components in some lubricating materials, in developers, and are a part of waterproofing textiles. Some amines, such as novocaine, are used as anesthetics. Many pharmaceutical compounds contain amines, including 8 of the 10 most prescribed medications in 2012.

Phosphate and its derivative functional groups are ubiquitous in biomolecules.  Phosphate linked to a single organic group is called a phosphate ester; when it has two links to organic groups it is called a phosphate diester.  A linkage between two phosphates creates a phosphate anhydride. 

There are a number of functional groups that contain a carbon-oxygen double bond, which is commonly referred to as a carbonyl.   Ketones and aldehydes are two closely related carbonyl-based functional groups that react in very similar ways.  In a ketone, the carbon atom of a carbonyl is bonded to two other carbons.  In an aldehyde, the carbonyl carbon is bonded on one side to a hydrogen, and on the other side to a carbon.  The exception to this definition is formaldehyde, in which the carbonyl carbon has bonds to two hydrogens.

The simplest aldehyde has a carbonyl group bonded to two hydrogen atoms. This molecule, called formaldehyde, can be used as a tissue preservative or as a very harsh disinfectant. It is also used as a precursor to various materials, including plastics, resins, and other polymers.

Ketones undergo most of the same reactions as aldehydes, but they tend to be slightly less reactive. The simplest ketone is acetone, in which the carbonyl carbon is bonded to two $$\ce{CH_3}$$ groups. This ketone is commonly used to remove fingernail polish and serves as an industrial solvent. Methyl ethyl ketone is used as a paint stripper and a solvent. Ketones are also used in the production of various polymers, either as a building block or as a solvent.

A group with a carbon-nitrogen double bond is called an imine, or sometimes a Schiff base (in this book we will use the term ‘imine’).

The hormone testosterone, the amino acid phenylalanine, and the glycolysis metabolite dihydroxyacetone phosphate all contain multiple functional groups, as labeled below.

While not in any way a complete list, this section has covered most of the important functional groups that we will encounter in synthetic and biological organic chemistry.   A summary table is included in your workbook, and is reproduced here for reference; the “priority” is used when determining how to name a compound with multiple functional groups.  We will formally learn how to name functional groups later.

Priority order for functional groups in IUPAC nomenclature

Functional Group Family	Functional Group Structure	Name if main group	Name if substituent
Carboxylic acid		alkanoic acid	carboxy
Ester		alkyl alkanoate	alkoxycarbonyl
Amide		alkanamide	amido
Nitrile		alkanenitrile	cyano
Aldehyde		alkanal	formyl
Ketone		alkanone	oxo
Alcohol		alkanol	hydroxy
Amine		alkanamine	amino
Alkene		alkene	alkenyl
Alkyne		alkyne	alkynyl
Alkane		alkane	alkyl
Ether			alkoxy
Halide			halo
Nitro compound			nitro

Khan Academy video tutorial on functional groups: