02 Functional Groups and Nomenclature

Recognizing Functional Groups

A functional group is a recognizable structural pattern that helps determine the characteristic behavior and classification of an organic compound. In general patterns, $\mathrm{R}$ represents a carbon-containing group, $\mathrm{Ar}$ an aromatic group, and $\mathrm{X}$ a halogen such as fluorine, chlorine, bromine, or iodine.

Common patterns include:

• An alkene contains a carbon–carbon double bond, $\mathrm{C=C}$; an alkyne contains a carbon–carbon triple bond, $\mathrm{C\equiv C}$.

• A haloalkane has the pattern $\mathrm{R{-}X}$.

• An alcohol contains $\mathrm{R{-}OH}$, while an ether contains $\mathrm{R{-}O{-}R'}$.

• An amine may have the pattern $\mathrm{R{-}NH_2}$, $\mathrm{R_2NH}$, or $\mathrm{R_3N}$.

• An amide contains $\mathrm{R{-}C(=O){-}NH_2}$, or a substituted version of that pattern.

The carbonyl group, $\mathrm{C=O}$, appears in several families. A carbonyl bonded to hydrogen is characteristic of an aldehyde, $\mathrm{R{-}C(=O)H}$. A carbonyl bonded to two carbon groups is a ketone, $\mathrm{R{-}C(=O){-}R'}$. In a carboxylic acid, it is bonded to $\mathrm{OH}$, giving $\mathrm{R{-}C(=O)OH}$. An ester has a carbonyl bonded to an oxygen that is bonded to another carbon group, as in $\mathrm{R{-}C(=O){-}O{-}R'}$.

Takeaway: Identify the bonds and neighboring atoms around a group, not just the presence of a particular atom, to distinguish compound families.

Building Systematic Names

Systematic naming describes a structure through a parent name and information about its functional groups, substituents, and unsaturation. A practical approach is:

1. Choose a suitable longest carbon chain that includes the principal functional group and, when possible, the multiple bonds.

2. Number the parent from the end that gives the principal functional group the lowest appropriate number. Then account for multiple bonds and substituents under the applicable naming rules.

3. Name and locate substituents with position numbers. Examples of prefixes include methyl-, bromo-, and hydroxy-. Different prefixes are generally listed alphabetically.

4. Show unsaturation with -ene for a carbon–carbon double bond and -yne for a carbon–carbon triple bond, adding a locant when needed.

5. Use the principal functional group as a suffix when the naming system allows. Other functional groups are generally named as prefixes.

Common suffixes and prefixes include:

• Carboxylic acid: -oic acid as a suffix; carboxy- as a prefix.

• Aldehyde: -al as a suffix; formyl- or, in suitable cases, oxo- as a prefix.

• Ketone: -one as a suffix; oxo- as a prefix.

• Alcohol: -ol as a suffix; hydroxy- as a prefix.

• Amine: -amine as a suffix; amino- as a prefix.

• Halogen: fluoro-, chloro-, bromo-, or iodo- as a prefix.

• Ether: often named using an alkoxy- substituent, such as methoxy- or ethoxy-.

For example, $\mathrm{CH_3CH_2OH}$ is ethanol: its parent has two carbons, and the alcohol suffix is -ol. The compound $\mathrm{CH_3COCH_2CH_3}$ is butan-2-one: its parent has four carbons, and its ketone carbonyl begins at carbon 2. In $\mathrm{CH_3CH(OH)COOH}$, the carboxylic acid supplies the suffix, while the alcohol group is named with the prefix hydroxy-; the name is 2-hydroxypropanoic acid.

Takeaway: Select the parent and principal group first, then use numbering, prefixes, and unsaturation markers to describe the rest of the structure.

Common and Systematic Names

Common names are traditional names that remain widely used, although they do not follow the same naming pattern as systematic names. For example:

• Methanoic acid is also called formic acid.

• Ethanoic acid is also called acetic acid.

• Propanone is also called acetone.

• Methanal is also called formaldehyde.

• Ethanal is also called acetaldehyde.

A common name can be convenient, while a systematic name more directly communicates structural information. Some traditional names are established and accepted in chemical usage; preferred names and other acceptable naming forms are distinguished in IUPAC nomenclature.

Takeaway: Recognize familiar common names, but use systematic naming to see how a name represents molecular structure.

Constitutional Isomerism

Isomers have the same molecular formula but differ in structure. Constitutional isomers differ in how their atoms are connected. This distinguishes them from stereoisomers, which have the same connections but differ in spatial arrangement.

Three useful patterns of constitutional isomerism are:

• Skeletal isomers have different carbon frameworks. Butane and 2-methylpropane both have the formula $\mathrm{C_4H_{10}}$, but one has a straight carbon chain and the other a branched framework.

• Positional isomers have the same framework and functional group, but the group is attached at a different position. Propan-1-ol and propan-2-ol both have the formula $\mathrm{C_3H_8O}$, but their $\mathrm{OH}$ groups are attached to different carbons.

• Functional-group isomers share a molecular formula but belong to different functional-group families. Ethanol and methoxymethane (dimethyl ether) both have the formula $\mathrm{C_2H_6O}$; one is an alcohol and the other an ether.

To check whether two structures are constitutional isomers, first compare their molecular formulas. If the formulas match, compare atom-to-atom connections. Different connections indicate constitutional isomers; identical connections mean the pair is not a constitutional-isomer pair.

Takeaway: Matching formulas are necessary, but different connectivity is what makes a pair constitutional isomers.