How to name alkenes 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

Alkenes: Organic compounds that come from crude oil that have at least 1 carbon carbon double bond (c=c)

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

Unsaturated: Compounds that have a carbon carbon (c=c) double bond (like alkenes)

Single covalent bond: eg. C-C

Double covalent bond: eg. C=C

General formula

General formula for alkenes: C_nH_2n

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 alkene compound with 3 carbon atoms:

C_nH_2n
= C₍₃₎H₂₍₃₎
= C₃H₆

How to name alkenes

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

Note: An alkene with only 1 carbon doesn’t exist because then it can’t have a C=C

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

eg. An alkene compound with 2 carbon atoms: ethene

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)

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However… From 4 carbons and up, we need to add something extra:

Eg. you get but-1-ene and but-2-ene.

• The number shows where the double bond is
  • You start counting from the side of the double bond
  • If it is exactly in the middle, you can count from any side

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. Make a double bond between 2 C’s (carbons).
3. Add H’s (hydrogens) around the C’s (carbons) to make each carbon have 4 bonds. BE AWARE OF THE DOUBLE BOND!
4. 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.
3. Draw in the double bond.

1. There are 5 C’s
   C C C C C
2. Make the double bond between the 2 C’s
   C=C C C C
3. There are 2 H’s bonded to the first C, 1 H to the second C, 2 H’s to the third and fourth C’s, and 3 H’s to the last CH₂C=CHCH₂CH₂CH₃
4. We can group the repeated parts together
   Therefore: H₂C=CH(CH₂)CH₃

Summary

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