Organic Chemistry

Alkanes

• Alkanes are single-bonded organic compounds made of only carbon and hydrogen atoms.
• General formula: C_nH_2n+2

Alkanes are subdivided into three groups, as shown below:

1) Linear straight-chain alkanes

For linear straight-chain alkanes, carbon atoms are connected in a continuous linear chain, unbranched.

How to name straight-chain alkanes:

• Count the number of carbon atoms
• Choose the proper prefix from the table given
• Once the prefix is chosen, add the suffix “ane”

Example 1

Name the compound shown in the figure below:

In the above example:

• The straight-chain contains four carbons.
• The prefix chosen based on the number of carbons is "but-"
• Adding the suffix “ane” gives us the name "butane."

Table 1. Prefixes for unbranched straight-chain alkanes

Branched Alkanes

In branched alkanes, one of the hydrogen atoms from methylene group is replaced an alkyl group. Branched alkanes have the same general formula as linear alkanes, but with a different structure and name. 

Table 2: Common Alkyl Groups

Naming Branched Alkanes

• Count the longest continuous carbon chain (which becomes your parent chain) and name it. Look for all end-to-end options.
• Number the carbon atoms in your parent chain. Start from the end that is closest to a branch (i.e., the one that will give you the smallest number when counting the distance to a branch).
• Identify and name the group attached to the parent chain.
• To write the final name of the whole compound, write the alkyl group's names in alphabetical order with the numbers in the front showing to which carbon each group is attached. 
• Use prefixes di-, tri-, etc. if more than one of the same kind of group is found in the molecule.

Example 2

Name the following molecule:

• step 1

The longest continuous chain has 8 carbons, as shown above. Therefore, the parent chain is an octane.

• Step 2

The molecule has a methyl group at carbon number three and an ethyl group at carbon number five. 

• Step 3

Based on steps 1 and 2, the name becomes 5-ethyl-3-methyl octane.

More examples can be found here.

Cycloalkanes

Cycloalkanes are molecules that have carbon atoms arranged in a form of a ring. Polycyclic alkanes are those that have two or more cycloalkanes joined together to form multiple rings. Several examples are shown in the figure below:

Photo credit : libretext

Notice that the two end carbons are connected together forming a ring structure. Hence, cycloalkanes have two fewer hydrogens than straight chain alkanes. As a result, their general formula will be CnH2n. This also is the general formula for alkenes, discussed below. 

Alkene

Alkenes are unsaturated molecules with at least one carbon-to-carbon double bond.

Numbering is not required for cycloalkenes with only one double bond. Numbering is needed when there are multiple double bonds or other branches present.

Naming Alkenes

• First, identify and number the longest carbon chain giving the lowest number possible to the carbon-carbon double bond. 
• If the molecule contains substituents, add their names and positions as prefixes.
  • Give them the smallest number possible when numbering.
  • If you have more than one substituent, arrange them in alphabetical order.
  • Identify if there are any stereoisomers, and use the appropriate prefixes if needed (cis or trans)
  • If there are more than 2 (that is, 3 or 4) nonhydrogen atoms found in the alkene, then use the Entgegen (E), Zusammen (Z) system:
    • Entgegen is used when the higher priority groups are found on the opposite side of the double bond.
    • Zusammen is utilized when the higher priority groups are found on the same side as the double bond.
• If there is a hydroxyl group attached to the alkene, then they get the priority over double bond and the name suffix becomes -enol instead of -ene.

Alkyne

Alkynes are unsaturated molecules with at least one carbon-to-carbon triple bond.