High School Organic Chemistry
IN THIS LESSON
Become familiar with concepts and nomenclature from organic chemistry.
1/4/25: I consider this lesson complete right now, but I am willing to edit it as needed.
Molecules can be considered organic if they have carbon-to-carbon bonds or carbon-to-hydrogen bonds. Hydrocarbons are molecules that only contain hydrogen and carbon atoms. Saturated molecules are molecules that only contain single bonds between the carbon atoms. Unsaturated molecules are molecules that contain at least one double or triple bond between carbon atoms. The molecular formula of a molecule indicates the atoms in a molecule and the number of each. The structural formula of a molecule indicates how the atoms in a molecule are arranged. Isomers are molecules that share molecular formulas (contain the same types and numbers of atoms) but have different structural formulas (are arranged differently from one another).
For our purposes, we will be classifying hydrocarbons into four basic categories. They are the following:
Alkanes are hydrocarbons that only have single bonds between the carbon atoms. This means that alkanes are always saturated. The general molecular formula for alkanes is CₙH2n+2. The names of alkanes always end in “ane.”
Alkenes are hydrocarbons that contain at least one double bond between the carbon atoms. These molecules are always unsaturated. The general molecular formula for alkenes is CnH2n, however this only applies when all bonds are single bonds with the only exception being one double bond. For each double bond in an alkane, subtract two hydrogens from the molecular formula. The names of alkenes always end in “ene.”
Alkynes are hydrocarbons that contain at least one triple bond between the carbon atoms. These molecules are always unsaturated. The general molecular formula for alkynes is CₙH₂ₙ₋₂, however, like alkenes, this only applies when all bonds are single bonds except for one triple bond. For each triple bond, subtract four hydrogens from the molecular formula. The names of alkynes always end in “yne.”
Cycloalkanes are alkanes that form a ring structure instead of a long chain structure. The general molecular formula for cycloalkanes is CₙH₂ₙ. The names of cycloalkanes always start with “cyclo” and end in “ane.”
When naming organic molecules, the root words used to indicate the number of carbon atoms go as follows: meth- is used for one carbon, eth- is used for two, prop- for three, and but- for four. The rest until ten are pent, hex, hept, oct, non, and dec.
The factors considered in naming organic molecules are: (1) the number of carbons in the longest carbon-to-carbon chain, (2) which “substituents” (functional groups) are attached to the hydrocarbon, and (3) where the functional groups are attached to the hydrocarbon.
Functional groups are small groups of molecules that may be attached to a larger hydrocarbon chain. They determine the class of a molecule and effect how the molecule reacts with other chemicals.
Steps in naming organic molecules
Step 1: Number the carbon atoms in the longest carbon-to-carbon chain.
Identify the longest carbon-to-carbon chain and count the number of carbons in this chain. The number of carbons you find here will indicate which root word to use in the molecule name. Then, number the carbons from one end to another, making sure that, once numbered, the carbons where functional groups are attached have the lowest possible numbers. In other words, if there are more functional groups toward one end of a hydrocarbon chain, start numbering the carbons from that end. The lowest number possible should also be given to the carbon attached to the most important functional group.
Step 2: Identify which functional groups are attached to the hydrocarbon.
The next lesson should go into the names of functional groups. These names and the number of these functional groups will be used in the final molecule name.
Step 3: Find which carbons the functional groups are attached to.
Use the numbers you assigned to the carbons in the longest chain. These numbers are used in the final molecule name.
For example:
For this example, know that a CH₃ group is a methyl group.
We can see that the longest carbon-to-carbon chain (the horizontal chain in the middle) contains five carbons. The root word for five is “pent.” We can also see that this molecule is an alkane as there are only single bonds between the carbon atoms. This means the basic structure is called “pentane.” Seeing that there are functional groups attached closer to the right side, we start the numbering from right to left. The right-most carbon in the longest chain is 1 and the left-most one is 5. Next, we can see that there are two methyl groups hanging off of the primary chain. Since there are two of these groups, we will use the prefix “di-“ in front of the word “methyl.” Lastly, we can see that, using our numbering system, the functional groups are attached to carbons 2 and 3. The final name of this molecule is 2,3-dimethylpentane.