How to name alkanes and write their formulas

Note: There’s a summary table at the end of this page.

Definitions

Organic: Contains carbon

Hydrocarbons: Organic compounds made from hydrogen and carbon only

Alkanes: Organic compounds that come from crude oil

Homologous series: A series of compounds with the same functional group and similar chemical properties

Saturated: Compounds that don’t have a carbon carbon (c=c) double bond (like alkanes)

General formula

General formula for alkanes: C_nH_2n+n

Each homologous series has its own general formula.

To use the general formula, replace n in the general formula with the number of C (carbon) atoms.
eg. An alkane compound with 3 carbon atoms:

C_nH_2n+2
= C₍₃₎H₂₍₃₎₊₂
= C₃H₈

How to name alkanes

Every alkane ends with “-ane”, but starts differently depending on the number of C (carbon) atoms.

1 – meth
2 – eth
3 – prop
4 – but
5 – pent
6 – hex

eg. An alkane compound with 4 carbon atoms: butane

Here’s a trick I use to remember how each compounds starts:
(1) Monkeys (2) Eat (3) Peanut (4) Butter
5 & 6 are easy enough to remember because they are the same as shapes (pentagon, hexagon)

Displayed / structural formula

Important:

H (Hydrogen) has 1 bond
C (Carbon) has 4 bonds
(This will help you draw the structure)

Steps:

1. Draw a chain with the correct number of C’s (carbons).
2. Add H’s (hydrogens) around the C’s (carbons) to make each carbon have 4 bonds.
3. Ensure that the lines are drawn in and that each letter has the correct number of bonds.

---

---

Condensed formula

This is similar to the structural formula but it is written.

1. Look at the structural formula and write each individual carbon alone.
2. Write how many hydrogen atoms are bonded to the individual carbons.

1. There are 2 C’s
   C C
2. There are 3 H’s bonded to each C
   Therefore: CH₃CH₃

---

When the “chain of C’s” is longer, there are repeated parts of the formula which you can group together.

1. There are 5 C’s
   C C C C C
2. There are 3 H’s bonded to the first and last C’s
   CH₃ C C C CH₃
3. There are 2 H’s bonded to the other 3 C’s
   CH₃CH₂CH₂CH₂CH₃
4. We can group the repeated parts together
   Therefore: CH₃(CH₂)₃CH₃

Summary

Have any questions about this topic? Leave a comment below and I’ll get back to you 🙂