A functional group is a small, specific arrangement of atoms that behaves the same way whichever molecule it is bolted onto. An O–H on a two-carbon chain and an O–H on a thirty-carbon steroid do the same chemistry, at roughly the same rate, with the same reagents. That is why organic chemistry is organized around groups rather than around molecules: there are tens of millions of known compounds and only a few dozen groups, and the group is what a molecule does.
The rest of the molecule — the carbon chain or ring the group sits on — is a spectator. The previous section explains why. A C–C bond joins two atoms of identical electronegativity (2.55 against 2.55), and a C–H bond is close enough (2.55 against 2.20) that the difference does not matter. Neither has a δ+ end worth attacking, and neither carries a lone pair to attack with. Every bond in a functional group, by contrast, is either polar, or a lone pair, or a π bond sitting exposed above and below the plane of the atoms. The group is where the electrons are uneven or accessible; the skeleton is where they are not.
The groups this course uses
Nineteen groups cover essentially everything in a first-year course. The structures below are drawn in condensed and Lewis form — R stands for any carbon chain, and R′ for a second one that need not match. The naming column is a preview: how a group shows up in a compound's name is the business of IUPAC Nomenclature, and you do not need it yet. It is here because the suffix is often the fastest way to recognize the group in a name you meet on a label.
| Group | Structure | Name preview | An example you have met |
|---|---|---|---|
| Alkane | C–C and C–H bonds only, CH3CH2CH3 | suffix -ane | Propane, the gas in a camping stove |
| Alkene | C=C, CH2=CH2 | suffix -ene | Ethylene, the hormone that ripens fruit |
| Alkyne | C≡C, HC≡CH | suffix -yne | Acetylene, the fuel in a welding torch |
| Alkyl halide | R–X (X = F, Cl, Br, I), CH2Cl2 | prefix fluoro-, chloro-, bromo-, iodo- | Dichloromethane, the solvent in paint stripper |
| Alcohol | R–OH, CH3CH2OH | suffix -ol | Ethanol |
| Ether | R–O–R′, CH3CH2OCH2CH3 | prefix alkoxy- (methoxy-, ethoxy-) | Diethyl ether, the first surgical anesthetic |
| Amine | R–NH2, R2NH or R3N; (CH3)3N | suffix -amine, prefix amino- | Trimethylamine, the smell of fish |
| Thiol | R–SH, CH3CH2SH | suffix -thiol | Ethanethiol, added to natural gas so a leak can be smelled |
| Aldehyde | R–CH=O, written RCHO; CH3CHO | suffix -al | Acetaldehyde, what ethanol becomes in the liver |
| Ketone | R–C(=O)–R′, written RCOR′; CH3COCH3 | suffix -one | Acetone, nail-polish remover |
| Carboxylic acid | R–C(=O)–OH, written RCOOH; CH3COOH | suffix -oic acid | Acetic acid, vinegar |
| Ester | R–C(=O)–OR′, written RCOOR′; CH3COOCH2CH3 | suffix -oate, two words: ethyl acetate | Ethyl acetate, the pear-drop smell of nail polish |
| Amide | R–C(=O)–NH2, NHR′ or NR′2; CH3CONH2 | suffix -amide | Acetaminophen; the peptide bond in every protein |
| Acid chloride | R–C(=O)–Cl, written RCOCl; CH3COCl | suffix -oyl chloride | Acetyl chloride — a lab reagent, too reactive toward water to be sold in a shop |
| Anhydride | R–C(=O)–O–C(=O)–R, (CH3CO)2O | suffix -oic anhydride | Acetic anhydride, the reagent that turns salicylic acid into aspirin |
| Nitrile | R–C≡N, CH2=CHCN | suffix -nitrile, prefix cyano- | Acrylonitrile, spun into acrylic sweaters |
| Aromatic ring | C6H6 benzene, or C6H5– as a substituent | parent benzene, prefix phenyl- | Toluene, C6H5CH3, in paint thinner |
| Phenol | an OH directly on an aromatic ring, C6H5OH | parent phenol | The active part of many antiseptics |
| Nitro group | R–NO2, drawn R–N+(=O)–O− | prefix nitro- (it is never the suffix) | TNT, trinitrotoluene, CH3C6H2(NO2)3 |
Two rows deserve a second look with the previous sections in hand. The nitro group's nitrogen has four bonds, so it carries a formal charge of +1, and the singly bonded oxygen carries −1: that is the formal-charge arithmetic from earlier in this chapter, and a nitro group drawn with a neutral four-bonded nitrogen is the five-bonded-carbon error in disguise. And the amine row lists three structures rather than one, because the nitrogen can carry one, two or three carbons and still be an amine — which matters below.
The carbonyl family: what is attached to the C=O
Seven of the groups in the table contain a carbonyl, a carbon double-bonded to oxygen, and they are the seven most confused. The distinction is mechanical once you know where to look: find the C=O, then read the other two things attached to its carbon. One of them is usually carbon or hydrogen; the other one names the group.
| Attached to the C=O carbon | Group | Condensed |
|---|---|---|
| H (and a carbon, or a second H) | Aldehyde | RCHO |
| Carbon on both sides | Ketone | RCOR′ |
| OH | Carboxylic acid | RCOOH |
| O–carbon (OR′) | Ester | RCOOR′ |
| N | Amide | RCONH2, RCONHR′, RCONR′2 |
| Cl | Acid chloride | RCOCl |
| O attached to a second C=O | Anhydride | RCO–O–COR |
The single most tested recognition skill in the course is that table, read in reverse: given a structure, say which row it is. Practice it on the condensed formulas until RCOOR′ and RCOR′ look as different to you as they do to a chemist.
CH3COCH3. The C=O carbon is bonded to CH3 and CH3: carbon on both sides, so a ketone (acetone).
CH3COOCH3. Same start, but now there is an oxygen between the carbonyl and the second methyl. One side carbon, the other side O–C, so an ester (methyl acetate). The extra O is the whole difference, and it is the atom people skip when reading quickly.
CH3OCH3. No C=O at all: an oxygen with a carbon on each side is an ether. An ester is an ether-like O plus a carbonyl on one side; an ether has the O alone.
Primary, secondary, tertiary
Several groups are further classified by degree of substitution, and the rule has two versions.
For a carbon, count the other carbons bonded to it: one is primary (1°), two is secondary (2°), three is tertiary (3°), four is quaternary (4°). Alcohols and alkyl halides take the degree of the carbon that carries the OH or the halogen. CH3CH2OH is a primary alcohol; (CH3)2CHOH is secondary; (CH3)3COH is tertiary. Same for CH3CH2Br, (CH3)2CHBr and (CH3)3CBr.
For an amine, count the carbons bonded to the nitrogen, not to any carbon. CH3NH2 is primary, (CH3)2NH is secondary, (CH3)3N is tertiary. The reason for the different rule is that the nitrogen is the reactive atom, and what matters about it is how crowded it is.
Why polarity tells you where a group will react
The previous section said a polar bond has a δ+ end and a δ− end, and the section before it gave the numbers. Put them against the table above and a pattern falls out.
- C–O and C=O (2.55 against 3.44, a difference of 0.89): carbon is clearly δ+, oxygen clearly δ−, and oxygen carries two lone pairs besides. A carbonyl doubles the effect, since both bonds of the C=O pull the same way, which makes the carbonyl carbon the most electron-poor carbon in the whole table.
- C–N (difference 0.49): weaker than C–O, and nitrogen's single lone pair is the more important feature. Amines react through that lone pair far more than through the δ+ carbon.
- C–Cl and C–Br (0.61 and 0.41): the carbon is δ+, and the halogen brings three lone pairs, but those lone pairs are held tightly by an electronegative atom and rarely do much. The action in an alkyl halide is at the δ+ carbon.
- C–S and S–H (differences of 0.03 and 0.38): barely polar at all. A thiol reacts through sulfur's lone pairs and its easily removed S–H proton, not through any charge separation.
- C=C and C≡C: no polarity, both atoms carbon, yet still a functional group. The π bond's electrons sit above and below the plane, held by nothing but the sideways overlap of two p orbitals, so they are accessible in a way the electrons of a C–C σ bond are not. Uneven is one way for electrons to be available; exposed is the other.
- An aromatic ring is six of those π electrons in a loop, exposed in the same way but unusually stable, which is why it gets a chapter of its own.
Common mistakes
- Ether against ester. An ester has a C=O next to its oxygen; an ether does not. RCOOR′ against ROR′.
- Aldehyde against ketone. Both are a C=O with carbon attached. The aldehyde has a hydrogen on the carbonyl carbon, which means the C=O is at the end of a chain; the ketone's C=O is inside one. A CHO written at the end of a condensed formula is the giveaway.
- Amide against amine. Both have nitrogen. An amide's nitrogen is attached to a C=O; an amine's is attached to carbons and hydrogens with no carbonyl in sight. RCONH2 against RNH2. They behave completely differently, which is why the distinction is drilled.
- Alcohol against phenol. An OH on a saturated carbon is an alcohol; an OH directly on an aromatic ring is a phenol, and it is roughly a million times more acidic. (An OH on a C=C carbon, an enol, is a third case you will meet much later.)
- Calling an ester's C=O a ketone. A ketone needs carbon on both sides of the carbonyl. If one side is oxygen, it is not a ketone, whatever it looks like at a glance.
- Counting the wrong atom for an amine. Degree of an amine is carbons on nitrogen. Degree of an alcohol is carbons on the carbon. Two rules, not one.
Worked examples: real molecules, every group named
Find the non-alkane pieces. A C6H4 unit, a COOH, and an O–CO–CH3.
C6H4 with two things attached is a benzene ring carrying two substituents: an aromatic ring.
HOOC– is a C=O with an OH on it: a carboxylic acid. Its other neighbor is a ring carbon.
–O–CO–CH3 is the one to read slowly. There is a C=O; one side of it is CH3, the other side is an oxygen that continues to a carbon (the ring). Carbon on one side, O–C on the other: an ester. It is not an ether, because the O sits next to a carbonyl, and the C=O is not a ketone, because one of its neighbors is oxygen.
Three groups: aromatic ring, carboxylic acid, ester. The two-carbon CH3CO fragment is the acetyl group, which is what the "a" in aspirin and the "acet" in acetylsalicylic acid refer to.
The ring is again an aromatic ring with two substituents.
HO– on the ring. The OH is directly on an aromatic carbon, so this is a phenol, not an alcohol. (Same atoms as an alcohol, different group, and a different chemistry.)
–NH–CO–CH3. The nitrogen is bonded to a C=O. That makes it an amide, not an amine. Read the carbonyl too: its neighbors are N and CH3, and the N is the one that names it. It is not a ketone, and there is no amine anywhere in the molecule.
Two groups plus a ring: phenol and amide. A student who writes "alcohol, amine, ketone" has found all the right atoms and named all three groups wrongly, which is why this molecule is a favorite on exams.
Left of the ring: (CH3)2CHCH2– is carbon and hydrogen only, an isobutyl chain. Alkane, spectator. Notice how much of the molecule that is.
The ring: aromatic, two substituents.
Right of the ring: –CH(CH3)–COOH. The COOH is a carboxylic acid. The CH that carries it is bonded to three carbons (the methyl, the ring, and the carboxyl carbon), so that carbon is tertiary, a detail that matters later for its stereochemistry.
One aromatic ring, one carboxylic acid, and a lot of spectator. Ibuprofen, aspirin and acetaminophen are all painkillers, and the only group the three share is the aromatic ring. What they do in the body has more to do with the acid, the ester and the amide respectively than with the ring, which is the point of the section.
What carries forward
All of it, in three directions. Organic Structure & Electron Movement redraws these groups in skeletal notation, which hides the spectator carbons so that the groups stand out even more, and then names the electron-rich and electron-poor sites you have just located. IUPAC Nomenclature turns the preview column of the table into a system, including what to do when one molecule has several groups. And from Acids & Bases onward, every chapter is titled after one of these rows.