Organic Chemistry/Overview of Functional Groups



The number of known organic compounds is quite large. In fact, there are many times more organic compounds known than all the other (inorganic) compounds discovered so far, about 7 million organic compounds in total. Fortunately, organic chemicals consist of a relatively few similar parts, combined in different ways, that allow us to predict how a compound we have never seen before may react, by comparing how other molecules containing the same types of parts are known to react.

These parts of organic molecules are called functional groups. The identification of functional groups and the ability to predict reactivity based on functional group properties is one of the cornerstones of organic chemistry.

Functional groups are specific atoms, ions, or groups of atoms having consistent properties. A functional group makes up part of a larger molecule.

For example, -OH, the hydroxyl group that characterizes alcohols, is an oxygen with a hydrogen attached. It could be found on any number of different molecules.

Just as elements have distinctive properties, functional groups have characteristic chemistries. An -OH group on one molecule will tend to react similarly, although perhaps not identically, to an -OH on another molecule.

Organic reactions usually take place at the functional group, so learning about the reactivities of functional groups will prepare you to understand many other things about organic chemistry.

Memorizing Functional Groups


Don't assume that you can simply skim over the functional groups and move on. As you proceed through the text, the writing will be in terms of functional groups. It will be assumed that the student is familiar with most of the ones in the tables below. It's simply impossible to discuss chemistry without knowing the "lingo". It's like trying to learn French without first learning the meaning of some of the words.

One of the easiest ways to learn functional groups is by making flash cards. Get a pack of index cards and write the name of the functional group on one side, and draw its chemical representation on the other.

For now, a list of the most important ones you should know is provided here. Your initial set of cards should include, at the very least: Alkene, Alkyne, Alkyl halide (or Haloalkane), Alcohol, Aldehyde, Ketone, Carboxylic Acid, Acyl Chloride (or Acid Chloride), Ester, Ether, Amine, Sulfide, and Thiol. After you've learned all these, add a couple more cards and learn those. Then add a few more and learn those. Every functional group below is eventually discussed at one point or another in the book. But the above list will give you what you need to continue on.

And don't just look at the cards. Say and write the names and draw the structures. To test yourself, try going through your cards and looking at the names and then drawing their structure on a sheet of paper. Then try going through and looking at the structures and naming them. Writing is a good technique to help you memorize, because it is more active than simply reading. Once you have the minimal list above memorized backwards and forwards, you're ready to move on. But don't stop learning the groups. If you choose to move on without learning the "lingo", then you're not going to understand the language of the chapters to come. Again, using the French analogy, it's like trying to ignore learning the vocabulary and then picking up a novel in French and expecting to be able to read it.

Functional groups containing ...


In organic chemistry functional groups are submolecular structural motifs, characterized by specific elemental composition and connectivity, that confer reactivity upon the molecule that contains them.

Common functional groups include:

Sr No. Chemical class Group Formula Graphical Formula Prefix Suffix Example
1 Acyl halide Haloformyl RCOX   haloformyl- -oyl halide  
Acetyl chloride
(Ethanoyl chloride)
2 Alcohol Hydroxyl OH   hydroxy- -ol  
3 Aldehyde Aldehyde RCHO   oxo- -al  
4 Alkane Alkyl RHn   alkyl- -ane  
5 Alkene Alkenyl R2C=CR2   alkenyl- -ene  
6 Alkyne Alkynyl RC≡CR'   alkynyl- -yne  
7 Amide Carboxamide RCONR2   carboxamido- -amide  
8 Amines Primary amine RNH2   amino- -amine  
Secondary amine R2NH   amino- -amine  
Tertiary amine R3N   amino- -amine  
4° ammonium ion R4N+   ammonio- -ammonium  
9 Azo compound Azo
RN2R'   azo- -diazene  
Methyl orange
10 Toluene derivative Benzyl RCH2C6H5
  benzyl- 1-(substituent)toluene  
Benzyl bromide
11 Carbonate Carbonate ester ROCOOR   alkyl carbonate
12 Carboxylate Carboxylate RCOO  


carboxy- -oate  
Sodium acetate
(Sodium ethanoate)
13 Carboxylic acid Carboxyl RCOOH   carboxy- -oic acid  
Acetic acid
(Ethanoic acid)
14 Cyanates Cyanate ROCN   cyanato- alkyl cyanate
Thiocyanate RSCN   thiocyanato- alkyl thiocyanate
15 Ether Ether ROR'   alkoxy- alkyl alkyl ether  
Diethyl ether
16 Ester Ester RCOOR'   -oate  
Ethyl butyrate
(Ethyl butanoate)
17 Haloalkane Halo RX   halo- alkyl halide  
(Ethyl chloride)
18 Hydroperoxide (see organic peroxide) Hydroperoxy ROOH   hydroperoxy- alkyl hydroperoxide  
Methyl ethyl ketone peroxide
19 Imine Primary ketimine RC(=NH)R'   imino- -imine
Secondary ketimine RC(=NR'')R'   imino- -imine
Primary aldimine RC(=NH)H   imino- -imine
Secondary aldimine RC(=NR')H   imino- -imine
20 Isocyanide Isocyanide RNC isocyano- alkyl isocyanide
21 Isocyanates Isocyanate RNCO   isocyanato- alkyl isocyanate
Isothiocyanate RNCS   isothiocyanato- alkyl isothiocyanate  
Allyl isothiocyanate
22 Ketone Ketone RCOR'   keto-, oxo- -one  
Methyl ethyl ketone
23 Nitrile Nitrile RCN   cyano-

alkyl cyanide

(Phenyl cyanide)
24 Nitro compound Nitro RNO2   nitro-    
25 Nitroso compound Nitroso RNO   nitroso-    
26 Peroxide Peroxy ROOR   peroxy- alkyl peroxide  
Di-tert-butyl peroxide
27 Benzene derivative Phenyl RC6H5   phenyl- -benzene  
28 Phosphine Phosphino R3P   phosphino- -phosphane  
29 Phosphodiester Phosphate HOPO(OR)2   phosphoric acid di(substituent) ester di(substituent) hydrogenphosphate DNA
30 Phosphonic acid Phosphono RP(=O)(OH)2   phosphono- substituent phosphonic acid  
Benzylphosphonic acid
31 Phosphate Phosphate ROP(=O)(OH)2   phospho-  
Glyceraldehyde 3-phosphate
32 Pyridine derivative Pyridyl RC5H4N





33 Sulfide RSR'   di(substituent) sulfide  
Dimethyl sulfide
34 Sulfone Sulfonyl RSO2R'   sulfonyl- di(substituent) sulfone  
Dimethyl sulfone
35 Sulfonic acid Sulfo RSO3H   sulfo- substituent sulfonic acid  
Benzenesulfonic acid
36 Sulfoxide Sulfinyl RSOR'   sulfinyl- di(substituent) sulfoxide  
Diphenyl sulfoxide
37 Thiol Sulfhydryl RSH   mercapto-, sulfanyl- -thiol  
(Ethyl mercaptan)

Note: The table above is adapted from the Functional Groups table on Wikipedia.

Combining the names of functional groups with the names of the parent alkanes generates a powerful systematic nomenclature for naming organic compounds.

The non-hydrogen atoms of functional groups are always associated with each by covalent bonds, as well as with the rest of the molecule. When the group of atoms is associated with the rest of the molecule primarily by ionic forces, the group is referred to more properly as a polyatomic ion or complex ion. And all of these are called radicals, by a meaning of the term radical that predates the free radical.

The first carbon after the carbon that attaches to the functional group is called the alpha carbon.

Alcohol containing two hydroxyl groups are called glycols. They have both common names and IUPAC names.

Mnemonics for Functional Groups


These are possible mnemonics for the common functional groups.

Vowels: Remember the vowels "A", "E", and "Y" for Alkane, Alkene, and Alkyne. Alkanes have only single covalent bonds. Alkenes have at least one double bond. Alkynes have at least one triple bond. The letters "I", "O", and "U" are not used. Furthermore, "O" and "U" would result in awkward pronunciations.

Alcohol: Look for the "C-O-H" in "Alcohol."

Ether: Ethers were anesthetics used in the 1800s. Dr. Kellogg also lived at the same time. Corn Flakes are made by Kellogg's. A rooster or cock (C-O-C) is the cornflake mascot. Or, think there is a C on "either" side of the O.

Amine: Remember the "N" stands for nitrogen.

Aldehyde: This sounds like "Adelaide," the Australian city. Australia is at the end of the Asian islands, and aldehydes are at the end of the hydrocarbon chain. The "Y" indicates a C=O double bond.

Ketone: Imagine the diagonal strokes of "K" forming the C=O double bond.  

Carboxylic Acid: "Box" stands for boxed wine or C-O-H, alcohol. The "Y" indicates a C=O double bond.

Ester: This sounds like "Estelle" George Costanza's mother in the TV show Seinfeld. George's nickname was Koko or Coco. So think of O=C-O-C.

Amide: Amine with a "D". D for double.