Hydrocarbons are nonpolar and fairly unreactive. Swap one hydrogen for an oxygen- or nitrogen-containing group and everything changes: the molecule can become polar, dissolve in water, smell of vinegar or fruit, and react in new ways. A functional group is the atom or group of atoms that gives a family of organic compounds its characteristic properties and reactions. Chemists write the rest of the molecule as R (for "rest"), so every alcohol is R–OH no matter how long its carbon chain is. Learn to spot the functional group first; it tells you the name ending, the likely boiling point and solubility, and which reactions the compound can undergo.
| Class | Group | Name ending or prefix | Example |
|---|---|---|---|
| Alcohol | R–OH (hydroxyl) | -ol | ethanol, |
| Ether | R–O–R′ | alkoxy- + alkane | methoxyethane, |
| Aldehyde | R–CHO (C=O at the end of the chain) | -al | ethanal, |
| Ketone | R–CO–R′ (C=O inside the chain) | -one | propanone, |
| Carboxylic acid | R–COOH (carboxyl) | -oic acid | ethanoic acid, |
| Ester | R–COO–R′ | alkyl …-oate | methyl ethanoate, |
| Amine | R–NH₂ (or N bonded to 2 or 3 carbons) | -amine | methanamine, |
| Amide | R–CONH₂ (C=O bonded to N) | -amide | ethanamide, |
Naming Each Class
- Alcohols: the parent chain includes the carbon holding –OH. Replace the -e of the alkane with -ol and give the –OH carbon the lowest number: is propan-2-ol. Two –OH groups make a diol: ethane-1,2-diol.
- Ethers: name the smaller side as an alkoxy group (methoxy, ethoxy) attached to the larger alkane: is ethoxyethane.
- Aldehydes: the C=O carbon is always carbon 1, so no number is needed: is butanal.
- Ketones: the C=O is inside the chain, so give its carbon the lowest number: pentan-2-one and pentan-3-one are different compounds. Propanone and butanone need no number because only one position is possible.
- Carboxylic acids: the –COOH carbon is carbon 1 and counts in the chain: is propanoic acid. Vinegar is a dilute solution of ethanoic (acetic) acid.
- Esters: split the molecule at the single-bonded O. The alkyl group on that O is named first as a separate word; the part containing the C=O is named like the acid with -oic acid changed to -oate: is ethyl ethanoate.
- Amines: name the alkyl group plus -amine: is ethanamine; is propan-1-amine. Extra alkyl groups on the N get an N- prefix: is N-methylmethanamine.
- Amides: the C=O carbon is carbon 1; change -oic acid to -amide: is propanamide. Alkyl groups on the N again use N-, as in N-methylethanamide.
- Haloalkanes (alkyl halides), which appear in reactions, simply use halogen prefixes: is 2-bromopropane.
Telling Look-alike Groups Apart
- Is there a C=O? If no: an O between two carbons is an ether, an –OH on a carbon is an alcohol, and an N bonded to carbon is an amine.
- If yes, look at what else is on the C=O carbon: H means aldehyde, two carbons means ketone, –OH means carboxylic acid, –O–C means ester, N means amide.
- In condensed formulas, –CHO is an aldehyde, –CO– between carbons is a ketone, –COOH is an acid, –COO– between carbons is an ester, and –CONH₂ is an amide.
Polarity, Hydrogen Bonding and Boiling Point
Boiling point measures how hard it is to pull molecules apart, so it depends on intermolecular forces. C–O, C=O, O–H and N–H bonds are polar because O and N are much more electronegative than C and H. Hydrogen bonding needs a hydrogen bonded directly to O or N (or F), because only then is that H exposed enough to be attracted strongly to a lone pair on a neighbouring molecule. Alcohols, carboxylic acids, and amines and amides with N–H bonds can hydrogen bond with each other. Aldehydes, ketones, ethers and esters are polar but have no O–H or N–H, so their molecules attract each other only by dipole-dipole and London forces. To compare fairly, pick molecules of similar molar mass so the London forces are about equal and the functional group makes the difference.
| Compound | Class | Molar mass (g/mol) | Strongest force between molecules | Boiling point (about) |
|---|---|---|---|---|
| butane | alkane | 58.14 | London dispersion | −1 °C |
| methoxyethane | ether | 60.11 | weak dipole-dipole | 7 °C |
| methyl methanoate | ester | 60.06 | dipole-dipole | 32 °C |
| propanal | aldehyde | 58.09 | dipole-dipole | 48 °C |
| propanone | ketone | 58.09 | dipole-dipole | 56 °C |
| propan-1-ol | alcohol | 60.11 | hydrogen bonding (one O–H) | 97 °C |
| ethanoic acid | carboxylic acid | 60.06 | hydrogen bonding (O–H and C=O; pairs of molecules) | 118 °C |
| ethanamide | amide | 59.08 | hydrogen bonding (two N–H and C=O) | 221 °C |
- Alcohols (O–H)
- Carboxylic acids (O–H plus C=O; they pair up, so boiling points are very high)
- Amines with N–H (N–H bonds are less polar than O–H, so weaker than alcohols)
- Amides with N–H (highest boiling points of the eight)
- Ethers (C–O–C)
- Aldehydes (C=O)
- Ketones (C=O)
- Esters (C=O and C–O)
- Each can still accept hydrogen bonds from water through its O lone pairs