6.2: Classifying Organic Molecules
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There are a lot of possible organic compounds! Even just carbon and hydrogen can be combined in many ways depending on how many carbons are in the chain or ring, whether there are any branches coming off the chain, and whether there are any double or triple bonds (compared to only single bonds). Chemists classify organic compounds based on bonding patterns in their structures. These patterns are important because structure determines function. That is, these patterns in structure determine what properties a compound will have and the types of chemical reactions in which it can participate. Therefore, by grouping organic compounds based on their structures, we are also classifying them based on their physical and chemical properties.
Hydrocarbons
If an organic molecule contains only carbon and hydrogen, then it is a hydrocarbon. Hydrocarbons are further classified based on what types of bonds they contain as shown in the table below.
| Hydrocarbon Category | Type of bonds |
|---|---|
|
Alkane |
All single bonds |
| Alkene | At least one double bond |
| Alkyne | At least one triple bond |
| Aromatic | Contains alternating single and double bonds |
Notice that there is only one letter that is different in the first three names! You will find that suffixes are very important in organic chemistry. You should also remember that hydrogen can only form single bonds, so the double and triple bonds in alkenes and alkynes refer to C=C or C≡C bonds. Because this is only an introduction to organic chemistry, we will not deal with compounds that contain both double and triple bonds.
Alkanes are saturated hydrocarbons, meaning they have the maximum number of hydrogen atoms. Alkenes and alkynes are unsaturated hydrocarbons. Because they have at least one double or triple bond, they have fewer hydrogen atoms per carbon atom than alkanes do. The concept of saturated and unsaturated molecules will come up again in the Biochemistry chapter when we discuss saturated and unsaturated fatty acids.
Functional Groups
Functional groups are what we call the structural patterns that we use to classify organic compounds. Remember that the hydrocarbons discussed in the previous section are just one subgroup of organic chemicals. Organic compounds commonly contain other elements in addition to carbon and hydrogen such as oxygen and nitrogen. As you will see in the table below, functional groups are a pattern of certain elements bonded together with certain bonds (single, double, or triple). Because functional groups are only a portion of a molecule, we use a generic symbol to represent the rest of the molecule. Just like you commonly use x as a variable in algebra, we use R as a variable group in organic chemistry. So, H-O-R means that there is a hydrogen atom singly bonded to an oxygen atom and that oxygen atom is singly bonded to something else which has not been specified. R- will usually represent a chain of carbons and hydrogens. The example H-O-R could represent H-O-CH3 or H-O-CH2CH3 or H-O-CH2CH2CH3, etc. If the functional group connects to the rest of the structure in multiple places, then the symbol R can be modified with primes. Use R if there is one attachment, R’ for a second attachment, and R’’ for a third chain attached to the functional group.
The table below lists the functional groups you will be expected to recognize for this course. Each functional group has a name that represents a structure that is commonly found as a portion of some organic molecules. Remember that these functional groups give the molecule certain properties, e.g. the ability to dissolve in water rather than a non-polar solvent, and the ability to participate in certain chemical reactions, e.g. the addition of a chlorine atom. Finally, you will see a suffix listed for each functional group. You will see these suffixes again later in this chapter when you learn how to name organic molecules. The suffix of the name indicates the functional group found in the molecule. Thus, when you see the name of a new molecule you can quickly get an idea of some of its properties based on its suffix.
| Functional Group Name | Structure | Suffix(es) | Example |
|---|---|---|---|
| Alcohol |
R-OH R cannot be H |
-ol |
Ethanol CH3CH2OH |
| Ether |
R-O-R' R/R' cannot be H |
-oxy -ane (two parts) |
Methoxyethane CH3-O-CH2CH3 |
| Amine |
|
-amine |
Ethanamine CH3CH2NH2 |
| Aldehyde |
or abbreviated CHO |
-al |
Ethanal
|
| Ketone |
R/R' cannot be H |
-one |
Propanone
|
| Carboxylic Acid |
or abbreviated COOH |
-ic acid (two words) |
Propanoic acid
|
| Ester |
R' cannot be H |
-yl -oate (two words) |
Ethyl ethanoate
|
| Amide |  |
-amide |
Ethanamide
|
