Naming Alkenes

Introduction

Alkenes are named with the same stems as alkanes — meth-, eth-, prop-, but- and so on — but with the ending -ene to show the double bond. For small alkenes that is all you need. From four carbons onwards, though, the double bond can sit in more than one place along the chain, giving different compounds. The name must then include a number saying where the double bond is. This page shows how to find and use that number.

Core explanation

Stem plus -ene. The stem counts the carbons in the chain that contains the double bond, and the suffix -ene shows that the compound is an alkene.

Carbons Name Formula --- --- --- 2 ethene CH₂=CH₂ 3 propene CH₂=CHCH₃ 4 butene (two forms) C₄H₈ 5 pentene (two straight-chain forms) C₅H₁₀

Why ethene and propene need no number. In ethene the double bond can only be between carbons 1 and 2. In propene, a double bond between carbons 2 and 3 is exactly the same molecule as one between carbons 1 and 2, just viewed from the other end. So there is only one propene.

Butene has two positions. With four carbons, the double bond can be at the end of the chain or in the middle:

- But-1-ene : CH₂=CHCH₂CH₃ — the double bond is between carbons 1 and 2. - But-2-ene : CH₃CH=CHCH₃ — the double bond is between carbons 2 and 3.

These are different compounds with the same molecular formula, C₄H₈. They have different boiling points (about −6 °C and about 1–4 °C respectively) and give different products in some reactions.

Numbering rules.

1. Find the longest carbon chain that contains the double bond. 2. Number the carbons starting from the end nearer the double bond . 3. The position number (locant) is the lower of the two carbon numbers joined by the double bond. 4. Write the number between the stem and -ene, separated by hyphens: pent-2-ene.

Check by numbering from the other end. CH₃CH₂CH=CH₂ looks different on paper from CH₂=CHCH₂CH₃, but numbering from the right-hand end puts the double bond at carbon 1. Both are but-1-ene. The rule "number from the end nearer the double bond" prevents names such as "but-3-ene", which is incorrect.

Older names. You may see "1-butene" in older books. The current IUPAC style puts the number next to the part of the name it describes: but-1-ene.

Step-by-step reasoning

To name CH₃CH₂CH₂CH=CH₂:

1. Count the carbons in the chain: five, so the stem is pent-. 2. Number from the end nearer the double bond, the right-hand end. 3. The double bond joins carbons 1 and 2, so the locant is 1. 4. The name is pent-1-ene.

Visual explanation

Draw a row of four carbon circles numbered 1 to 4 from left to right. Draw a double line between circles 1 and 2 and label it but-1-ene. Beneath, draw another row with the double line between 2 and 3 and label it but-2-ene. Flip the first row mentally: a double bond between 3 and 4 is simply but-1-ene seen from the other side.

Real-world analogy

Naming the double-bond position is like giving a seat number on a bus. A two-seat bench needs no number, but on a long bench you must say which pair of seats is taken — and you count from whichever end gives the smaller number.

Real-world example

Propene is produced on a huge scale for making poly(propene), used in food containers, ropes and carpets. Butenes from cracking are used to make other chemicals and fuel additives; but-1-ene is added in small amounts when making some grades of poly(ethene) to give tougher, more flexible film for packaging.

Why?

Why must the position be stated? But-1-ene and but-2-ene have the same formula but different structures and properties. Without the number, "butene" would not identify a single compound, and the whole point of systematic naming is one name for one structure.

Common misconception

"CH₃CH₂CH=CH₂ is but-3-ene." Chains must be numbered from the end nearer the double bond, so this molecule is but-1-ene. No straight-chain alkene is ever named with a locant bigger than necessary.

Worked example

Question: Name CH₃CH=CHCH₂CH₃.

Reasoning: Five carbons gives pent-. Numbering from the left, the double bond joins carbons 2 and 3; from the right, it joins carbons 3 and 4. The lower locant is 2.

Answer: Pent-2-ene.

Quick check

1. Why does propene not need a position number in its name? Answer: The double bond can only be at the end of the three-carbon chain, so there is only one possible propene.

Exam focus

Always number from the end nearer the double bond and use the lower number of the pair. Write names in the form stem-number-ene, such as but-2-ene, with hyphens. Examiners often ask you to draw both butenes and to explain that they are isomers.

Advanced insight

But-2-ene exists in two further forms, because there is no rotation about the double bond. In one, the two CH₃ groups are on the same side (cis or Z); in the other, they are on opposite sides (trans or E). These geometric isomers have slightly different physical properties and are studied in more advanced courses.

Summary

Alkenes are named with the carbon stem and the ending -ene. From butene onwards, a position number shows where the double bond starts, counting from the end of the chain nearer the double bond. But-1-ene and but-2-ene are different compounds with the same formula, C₄H₈. Ethene and propene need no number.

Practice questions

1. Draw the structural formula of but-2-ene. Answer: CH₃CH=CHCH₃. 2. Name CH₂=CHCH₂CH₂CH₂CH₃. Answer: Hex-1-ene. 3. A student names CH₃CH₂CH₂CH=CHCH₃ as hex-4-ene. Give the correct name and explain. Answer: Hex-2-ene; the chain must be numbered from the end nearer the double bond, which gives the lower locant 2. 4. Explain why but-1-ene and but-2-ene are different compounds even though both are C₄H₈. Answer: The double bond is in a different position in the chain, so the atoms are arranged differently and the compounds have different properties.