Start with two molecules. Ethanol has an O–H on a two-carbon chain. Cholesterol has an O–H on a steroid of twenty-seven carbons. Both O–H groups do the same kinds of reaction with the same reagents.
That is what a functional group is: a small, specific arrangement of atoms that behaves the same way whichever molecule carries it. Organic chemistry is organized around these groups rather than around molecules. There are well over a hundred million 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: it is there, but it takes no part in the reaction. The previous section explains why. A C–C bond joins two atoms of identical electronegativity (2.55 against 2.55). A C–H bond is close enough (2.55 against 2.20) that the difference does not matter. Neither bond has a δ+ end worth attacking, and neither carries a lone pair to attack with.
A functional group is the opposite. Each one has a polar bond, a lone pair, or a π bond whose electrons sit exposed above and below the plane of the atoms. The group is where the electrons are uneven or easy to reach; the skeleton is where they are not.
First: reading the notation
This section writes molecules as condensed formulas, the compact notation introduced in the Lewis Structures section, and draws each one out in full beside it. Skeletal drawings, which most books use for this material, arrive at the start of Drawing Molecules & Moving Electrons. Four conventions do the work here, and the third and fourth are new.
- Hydrogens follow their own atom, and heavy atoms join left to right. CH3CH2OH is C–C–O, with three H on the first carbon, two on the second and one on the oxygen. Parentheses mark branches: (CH3)3C–OH is a carbon carrying three methyl groups and an OH, with no hydrogen of its own.
- R means "any carbon chain, and it does not matter which". R–OH is the alcohol group with the rest of the molecule abbreviated away. R′ (said "R prime") is a second chain that need not match the first. So R–O–R′ covers both CH3OCH3 and CH3OCH2CH3. This is the whole idea of the section in one symbol: the R is the part that does not matter.
- A C followed by O or OO, with only two other neighbors written, carries a C=O. In CH3COOH the second carbon has only the CH3 and the two oxygens as neighbors. Three neighbors give it only three bonds, so it needs a double bond to reach four, and the first O takes it. So CH3COOH is not a chain C–O–O–H. Its carbon has a double bond to the first oxygen and a single bond to the second, and the second carries the H. Written out, it is CH3–C(=O)–OH. In the same way, CH3COOCH3 is CH3–C(=O)–O–CH3, and CH3CHO is CH3–CH=O, with no O–H at all. This convention is the commonest reason a condensed formula gets misread, so wherever a formula could be read two ways these notes write the explicit C(=O) form beside it.
The fourth convention is a ring. Benzene is C6H6: six carbons joined in a flat ring, one hydrogen on each carbon, and three double bonds that alternate with single bonds around the ring.
Take one hydrogen off and attach the ring to something else. Five hydrogens are left, so the ring is written C6H5–, a substituent hanging off one side. Toluene, C6H5CH3, is a benzene ring with a CH3 where one hydrogen was.
Take two hydrogens off and the ring sits between two groups. Four hydrogens are left, so it is written –C6H4–, a connector with a group at each end. So HOOC–C6H4–O–CO–CH3 is a benzene ring with a COOH on one carbon and an O–CO–CH3 on another. Count the ring hydrogens and you know how many groups are attached.
The formula –C6H4– does not say which two carbons carry the groups. There are three choices. The groups can sit on neighboring carbons, called ortho or 1,2. They can have one carbon between them, called meta or 1,3. Or they can sit straight across the ring, called para or 1,4. The drawing has to show which.
The groups this course uses
Eighteen functional groups cover essentially everything in a first-year course, and the alkane, which has no group, is the baseline they are compared against. They fall into four families, drawn below one family at a time, with the full list in a table at the end of this section. Every drawing writes in each atom; R stands for any carbon chain.
Carbon and hydrogen only, plus the nitrile. Propane has only C–C and C–H bonds, so it has no functional group: it is an alkane. Put a double bond between two carbons and you have an alkene; a triple bond gives an alkyne. A carbon triple-bonded to a nitrogen, C≡N, is a nitrile.
Carbon single-bonded to a halogen, oxygen, sulfur or nitrogen. A halogen on a carbon chain makes an alkyl halide, R–X, where X stands for F, Cl, Br or I. An O–H on a chain is an alcohol. An oxygen with a carbon on each side is an ether. Swap the alcohol's oxygen for sulfur and you have a thiol, R–SH.
An amine is a nitrogen bonded to carbons and hydrogens, with one lone pair. The nitrogen can carry one, two or three carbons and still be an amine, which is why the table lists three structures for it: R–NH2, R2NH and R3N. That choice matters in the section on degree below.
The carbonyl family. Seven groups contain a carbon double-bonded to oxygen. They get their own section next, with their own drawing.
Rings, and the nitro group. A benzene ring is itself a group, the aromatic ring. An OH bonded directly to a carbon of that ring is a phenol. It has the same atoms as an alcohol's OH, but its chemistry is different. The three C=C inside the ring are not counted as three alkenes: they do not react like alkenes, and the whole ring is one group. Why is the business of Aromatic Chemistry.
The nitro group, R–NO2, needs formal charges. Its nitrogen has four bonds and no lone pair, so it carries a formal charge of +1. The singly bonded oxygen has one bond and three lone pairs, so it carries −1. That is the formal-charge arithmetic from earlier in this chapter.
Two wrong drawings of the nitro group are common. One gives nitrogen four bonds but no charges; that is the missing-charge error from the Lewis Structures section. The other gives nitrogen a double bond to each oxygen. That is five bonds on nitrogen, the same impossibility as a five-bonded carbon.
The table below lists all eighteen groups, with the alkane first for comparison. The structure column uses condensed formulas, with the explicit bonding written out wherever the condensed form could be misread. The naming column is a preview. How a group shows up in a compound's name belongs to 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 widely used 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, a solvent in 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 | Phenol itself, the first surgical antiseptic; thymol, the phenol in mouthwash |
| Nitro group | R–NO2, drawn R–N+(=O)–O− | prefix nitro- (it is never the suffix) | TNT, trinitrotoluene, CH3C6H2(NO2)3 |
The carbonyl family: what is attached to the C=O
Compare acetone, CH3–C(=O)–CH3, with acetic acid, CH3–C(=O)–OH. Both have a carbonyl, a carbon double-bonded to oxygen. In acetone that carbon also carries two carbons, so it is a ketone. In acetic acid it carries a carbon and an OH, so it is a carboxylic acid.
Seven of the groups in the table contain a carbonyl, and they are the seven most often confused. Telling them apart 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 a carbon or a 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 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 makes it a ketone (acetone).
CH3COOCH3. The start is the same, but now an oxygen sits between the carbonyl and the second methyl. One side is carbon and the other is O–C, so it is an ester (methyl acetate). The extra O is the whole difference, and it is the atom people skip when they read quickly.
CH3OCH3. There is 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
Look at ethanol, CH3CH2OH. The carbon that carries the OH is bonded to one other carbon. Now look at (CH3)2CHOH: the carbon carrying the OH is bonded to two carbons. In (CH3)3COH it is bonded to three.
That count is the degree of substitution. For a carbon, count the other carbons bonded to it. One is primary (1°), two is secondary (2°), three is tertiary (3°), and four is quaternary (4°). An alcohol takes the degree of the carbon that carries its OH. So ethanol is a primary alcohol, (CH3)2CHOH is secondary, and (CH3)3COH is tertiary.
Alkyl halides follow the same rule, using the carbon that carries the halogen. CH3CH2Br is primary, (CH3)2CHBr is secondary and (CH3)3CBr is tertiary.
Amines use a different count. CH3NH2 has one carbon on its nitrogen, (CH3)2NH has two, and (CH3)3N has three. They are the primary, secondary and tertiary amines. For an amine, count the carbons bonded to the nitrogen, not to any carbon. 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 Electronegativity section gave the numbers. Put those numbers against the groups 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+ and oxygen clearly δ−, and oxygen carries two lone pairs besides. A carbonyl strengthens the effect, since both bonds of the C=O pull the same way, which makes the carbonyl carbon among the most electron-poor carbons in the 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. 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, since both atoms are carbon, yet still a functional group. The π bond's electrons sit above and below the plane, held only by the sideways overlap of two p orbitals. That makes them 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. They are exposed in the same way but unusually stable, which is why the ring 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 also has a hydrogen on the carbonyl carbon, which means its 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 (one with only single bonds) is an alcohol. An OH directly on an aromatic ring is a phenol. It is far more acidic: it gives up its O–H hydrogen as H+ roughly a million times more readily. (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. The degree of an amine is carbons on nitrogen. The 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. In aspirin the two sit on neighboring ring carbons (ortho).
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 and O–C on the other make it an ester. It is not an ether, because the O sits next to a carbonyl. 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, this time on opposite carbons (para).
HO– on the ring. The OH is directly on an aromatic carbon, so this is a phenol, not an alcohol.
–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.
Three groups: aromatic ring, phenol, amide. Despite the “acet” in its name, acetaminophen has no carboxylic acid; its acetyl group, CH3CO, is part of the 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: a branched four-carbon chain called isobutyl. It is alkane, a spectator. Notice how much of the molecule that is.
The ring: aromatic, with two substituents on opposite carbons (para).
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. It also carries four different groups (H, CH3, the ring and COOH), which will matter in Stereochemistry.
Two groups, aromatic ring and carboxylic acid, and a lot of spectator. Of the thirteen carbons, one is the acid carbon, six are the ring and six are spectator. Ibuprofen, aspirin and acetaminophen are all painkillers, and the only group the three share is the aromatic ring. What tells them apart chemically is the acid, the ester and the amide.
Compare aspirin and acetaminophen once more. In aspirin an oxygen on the ring leads to a carbonyl, which makes an ester. In acetaminophen a nitrogen on the ring leads to a carbonyl, which makes an amide. That one atom decides how each is broken down in the body: aspirin's ester is hydrolyzed (split apart by water) within minutes, and acetaminophen's amide is not.
What carries forward
All of it, in three directions. Drawing Molecules & Moving Electrons redraws these groups in skeletal notation, which hides the spectator carbons so 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 several later chapters are named after these rows: Alkenes & Alkynes, Alcohols, Ethers & Related Chemistry, Carbonyl Chemistry, Carboxylic Acids & Derivatives, Aromatic Chemistry and Amines.