Nomenclature of Aliphatic Hydrocarbons

Scope and Goal of This Chapter

In this chapter you learn how to name aliphatic hydrocarbons systematically according to simple IUPAC rules. You will focus on:

• Straight-chain (unbranched) alkanes, alkenes, and alkynes
• Branched alkanes (introduction to substituents)
• How to choose the main chain and number it
• How to write names correctly (order, prefixes, punctuation)

You do not yet need detailed mechanisms or properties of these compounds; the emphasis here is purely on naming and reading names.

Basic Idea of Systematic Names

The systematic name of an aliphatic hydrocarbon is built from three parts:

1. Root (or parent) name – indicates the length of the longest carbon chain
2. Infix/suffix – indicates the type and number of multiple bonds (alkane, alkene, alkyne)
3. Prefixes – indicate substituents (side chains) and how many of each

General pattern (for a simple example):

$$\text{[locants]-[prefixes][root][bond-type suffix]}$$

Example:
3-methylhexane

• hex → 6 carbon atoms in the main chain
• ane → only single bonds (alkane)
• 3-methyl → a methyl group on carbon 3 of the main chain

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Root Names for Chain Length

The root name is based on the number of carbon atoms in the longest continuous chain chosen as the parent:

For longer chains: undec- (11), dodec- (12), etc., but for beginners the table above is usually sufficient.

Names of Unbranched Alkanes, Alkenes, and Alkynes

Unbranched Alkanes

For straight-chain alkanes (no branches, only single bonds):

• Name = root + ane

Examples (condensed formulas for clarity):

• CH₄ → methane
• CH₃–CH₃ → ethane
• CH₃–CH₂–CH₃ → propane
• CH₃–CH₂–CH₂–CH₃ → butane

No locant (position number) is needed if there is no ambiguity and no substituents.

Unbranched Alkenes

Alkenes contain one or more C=C double bonds. For a single double bond:

• Name = root + -ene
• The main chain must include the double bond
• The position of the double bond is given by a number (locant)

Numbering rule:
Number the chain so that the double bond gets the smallest possible locant.

Two common ways of writing:

• but-1-ene or 1-butene → $CH_2=CH–CH_2–CH_3$
• but-2-ene or 2-butene → $CH_3–CH=CH–CH_3$

Here:

• Root but → 4 carbons
• ene → alkene
• 1 or 2 → first carbon of the double bond

For more than one double bond, use -diene, -triene, etc. (e.g., buta-1,3-diene), but the detailed system for multiple bonds is mainly developed in more advanced or later sections.

Unbranched Alkynes

Alkynes contain one or more C≡C triple bonds. For a single triple bond:

• Name = root + -yne
• The main chain must include the triple bond
• Number the chain so that the triple bond gets the smallest possible locant

Examples:

• propyne or prop-1-yne → $CH_3–C≡CH$
• but-1-yne → $CH_3–CH_2–C≡CH$
• but-2-yne → $CH_3–C≡C–CH_3$

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Choosing the Parent Chain

For branched hydrocarbons, you must first decide which chain is the parent chain. Basic beginner rules:

1. Maximum number of carbon atoms
• Choose the longest continuous carbon chain as the parent.
2. Include multiple bonds
• If there are double or triple bonds, the parent chain must contain all of them if possible, even if a slightly shorter chain would be longer in carbon count.
3. Lowest locants for multiple bonds
• When more than one chain of the same length includes the multiple bond(s), choose the one that gives the lowest possible locant(s) for the double and triple bonds.
4. As many substituents as possible (tie-breaker)
• If there is still more than one possible parent chain, choose the one that carries the most substituents.

Within this chapter we mostly apply these rules to simple small examples.

Alkyl Substituents and Their Names

Side chains (branches) of only carbon and hydrogen attached to a main chain are called alkyl groups.

For a straight-chain alkyl group of $n$ carbons:

• Name = root (for $n$ carbons) + yl

Starting from alkanes:

• methane → methyl
• ethane → ethyl
• propane → propyl
• butane → butyl

They are written as prefixes in the entire compound name:

• methyl = $–CH_3$ (as a substituent)
• ethyl = $–CH_2–CH_3$
• propyl = $–CH_2–CH_2–CH_3$

Example (simple view):

• Parent: propane ($CH_3–CH_2–CH_3$)
• If a hydrogen on carbon 2 is replaced with $–CH_3$ (methyl), we get 2-methylpropane.

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Numbering the Parent Chain: Locants

Once the parent chain is identified, you:

1. Number its carbon atoms consecutively
2. Assign each substituent (or double/triple bond) to its carbon number (locant)
3. Choose the direction of numbering that gives the lowest possible set of locants

For simple branched alkanes (no multiple bonds), use the lowest set of locants rule for substituents:

• Look at all substituent positions derived from numbering left-to-right
• Compare with the positions from numbering right-to-left
• Choose the direction that yields the smaller list when compared from left to right

Example:

Structure (in words): A 5-carbon chain with a methyl group on the middle carbon.

Chain: $CH_3–CH_2–CH(CH_3)–CH_2–CH_3$

If you number from the left:

• C1, C2, C3(methyl), C4, C5 → locant for methyl = 3

If you number from the right:

• C1, C2, C3(methyl), C4, C5 → locant also = 3

Here both directions give the same locant; there is no conflict. Name: 3-methylpentane.

More instructive example:

Chain: $CH_3–CH(CH_3)–CH_2–CH_3$

Numbering left-to-right:

• C1 $CH_3$
• C2 $CH$ (with methyl)
• C3 $CH_2$
• C4 $CH_3$

Locant for methyl substituent = 2 → 2-methylbutane.

If you number right-to-left:

• C1 $CH_3$
• C2 $CH_2$
• C3 $CH$ (with methyl)
• C4 $CH_3$

Locant = 3 → 3-methylbutane.

The correct choice is always the smaller locant, so the IUPAC name is 2-methylbutane.

For chains with multiple substituents, the full set of locants is compared, not just the first one.

One Substituent: Basic Naming Pattern

For a branched alkane with one substituent:

1. Choose and number the parent chain.
2. Determine the substituent name (e.g., methyl, ethyl).
3. Determine the carbon number where it is attached.
4. Assemble the name:
• locant-substituentparent

Example:

$CH_3–CH_2–CH(CH_3)–CH_2–CH_3$

• Parent chain: 5 carbons → pentane
• Substituent: methyl
• Position: carbon 3

Name: 3-methylpentane.

Another example:

$CH_3–CH(CH_2CH_3)–CH_3$

Visual: main 3-carbon chain (propane) with an ethyl group on carbon 2.

• Parent chain: 3 carbons → propane
• Substituent: ethyl
• Position: 2

Name: 2-ethylpropane.

Several Identical Substituents

If the same substituent occurs more than once:

• Use multiplicative prefixes:
• di- (2), tri- (3), tetra- (4), etc.
• List all locants, separated with commas
• Place the multiplicative prefix immediately in front of the substituent name

General pattern:

$$\text{locant₁,locant₂- (di/tri/…)substituentparent}$$

Example:

Structure: A hexane chain with two methyl groups at carbons 2 and 3:

$CH_3–CH(CH_3)–CH(CH_3)–CH_2–CH_2–CH_3$

• Parent chain: 6 carbons → hexane
• Substituents: two methyl groups
• Locants: 2 and 3

Name: 2,3-dimethylhexane.

Example with equivalent choices:

$CH_3–CH(CH_3)–CH_2–CH(CH_3)–CH_3$

If numbered from the left:

• Methyls at C2 and C4 → 2,4-dimethylpentane

From the right:

• Methyls at C2 and C4 again. Tie. So no conflict; either way yields the same name.

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Different Substituents and Alphabetical Order

If several different kinds of substituents are present, their prefixes are ordered alphabetically according to the base substituent name (ignoring multiplicative prefixes like di-, tri-).

General pattern:

$$\text{[locants-substituent₁][locants-substituent₂]…parent}$$

with substituent₁, substituent₂, … sorted alphabetically.

Alphabetical order uses:

• ethyl under e
• methyl under m
• propyl under p
  and ignores di-, tri-, etc., when sorting.

Example:

Structure: $CH_3–CH(CH_3)–CH(CH_2CH_3)–CH_3$

Choose the main chain: 4 carbons → butane.

Substituents:

• methyl at C2
• ethyl at C3

Alphabetical order: ethyl (e) before methyl (m).

So:

• Locants: 3-ethyl, 2-methyl
• Name: 3-ethyl-2-methylbutane.

Note the punctuation:

• Commas between numbers (3,4)
• Hyphens between numbers and letters (3-ethyl)

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Position of Double and Triple Bonds in Branched Chains

For branched alkenes and alkynes, the same basic principles apply, but with one additional priority:

1. Choose a parent chain that contains the maximum number of multiple bonds.
2. Number the chain to give the lowest possible locants to the multiple bond(s).
3. Only then apply locant rules to substituents.

The double or triple bond’s locant is the number of the first carbon of the multiple bond.

Naming pattern (simple systems):

• hex-2-ene or 2-hexene (double bond starts at C2)
• hex-3-yne or 3-hexyne (triple bond starts at C3)

With substituents:

Example (described): a 5-carbon chain with a double bond between C2 and C3, and a methyl group at C4.

Steps:

• Parent chain: 5 carbons → pent
• Double bond: between C2 and C3 → -2-ene
• Substituent: methyl at C4 → 4-methyl

Name: 4-methylpent-2-ene.

The presence of -ene in the parent name does not change the basic alphabetization of substituents.

Basic Use of Common vs. Systematic Names

In practice, some short alkanes and simple branched alkanes have common (trivial) names (e.g., isobutane, isopentane). Systematic IUPAC names are preferred in a scientific context:

• isobutane → IUPAC: 2-methylpropane
• isopentane → IUPAC: 2-methylbutane

In introductory practice, you should:

• Recognize that trivial names exist
• Be able to convert small common names into systematic names (at least for the simplest cases)
• Use systematic names consistently when naming from structures

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Typical Beginner Mistakes and How to Avoid Them

1. Using too short a parent chain
• Always check if there is a longer continuous chain available.
2. Ignoring multiple bonds when choosing the parent chain
• The parent must contain the double or triple bond.
3. Numbering to give substituents low numbers rather than the multiple bond
• For alkenes/alkynes, the multiple bond takes priority in numbering.
4. Incorrect alphabetical order
• Alphabetize by the core substituent name, not by the multiplicative prefix (di-, tri-).
5. Missing or misplaced hyphens and commas
• Commas between numbers
• Hyphens between numbers and letters

Correct example: 3-ethyl-2-methylhexane

Incorrect: 3 ethyl, 2 methyl hexane.

Summary of the Stepwise Naming Procedure

To name a simple aliphatic hydrocarbon:

1. Identify the parent chain
• Longest continuous chain
• Contains all multiple bonds (if present)
2. Determine the bond type
• Only single bonds → -ane
• Double bond(s) → -ene (with locant)
• Triple bond(s) → -yne (with locant)
3. Number the parent chain
• For alkanes: lowest set of substituent locants
• For alkenes/alkynes: lowest possible locant for the multiple bond(s), then substituents
4. Identify and name substituents
• As alkyl groups: methyl, ethyl, propyl, etc.
• Determine their locants
5. Combine everything
• Sort different substituents alphabetically
• Use di-, tri-, etc. for multiple identical groups
• Write: [locants-prefixes][root][suffix]
• Use proper commas and hyphens

With practice, these rules allow you to decode and construct names for a wide variety of simple aliphatic hydrocarbons.