Module 3: Nomenclature

This module, Organic Nomenclature, aligns with the AAMC MCAT Content Category 4D: Structure, Function, and Reactivity of Organic Compounds, which includes mastery of systematic naming as a foundational skill. While the MCAT does not expect memorization of obscure IUPAC rules, it does require fluency in interpreting, constructing, and reasoning through the names of organic compounds—particularly those related to functional groups, substituents, and parent chains. A strong grasp of nomenclature allows students to visualize molecular structures from names, compare isomers, and follow complex reaction pathways. This module focuses on the most MCAT-relevant naming conventions, including alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, carboxylic acids, esters, amides, and amines, with attention to naming priorities, stereochemical descriptors (R/S and E/Z), and common naming pitfalls. Every concept presented is tied directly to AAMC-style passage reasoning and question formats.

Why Nomenclature Matters on the MCAT

Knowing how to name organic molecules — and more importantly, how to decode names into structures — is a fundamental MCAT skill. While you’re rarely asked to give full IUPAC names from scratch, you must be able to:

• Interpret IUPAC names into structures
• Identify functional groups from naming conventions
• Recognize parent chains, substituents, and stereochemistry from naming cues
• Spot naming traps or inconsistencies in answer choices

IUPAC vs. Common Names

• IUPAC: The International Union of Pure and Applied Chemistry provides a standardized naming system used universally in textbooks and on the MCAT.
• Common names: Still appear on the MCAT (e.g., formic acid, acetone, toluene, isopropyl alcohol), so you must recognize both.

Key Features of an IUPAC Name

An IUPAC name typically has 5 components, ordered as follows:

[Prefix(es)] – [Parent chain] – [Saturation & functional group suffix] – [Highest-priority functional group ending]

Component	Function	Example
Prefix	Substituents or branches	methyl-, bromo-
Parent Chain	Main carbon backbone (longest continuous chain)	but-, pent-
Unsaturation	Indicates double/triple bonds	-ene, -yne
Functional Group	Highest priority group with suffix (e.g., -ol, -al)	-ol, -one
Locants	Numbers to show position of groups	2-pentanol

Steps for IUPAC Naming: Core Algorithm

Here is a foolproof 6-step system used to name any organic molecule:

Step 1: Identify the Longest Carbon Chain

• This becomes your parent chain
• Must include the highest-priority functional group
• Choose the chain with most substituents if tied

Step 2: Number the Chain

• Start from the end closest to the highest-priority group
• If no high-priority group, start from end closest to first substituent or multiple bond
• Goal: Give the lowest possible numbers to important features

Step 3: Identify and Name Substituents

• Common ones: methyl-, ethyl-, bromo-, chloro-, nitro-
• Each gets a locant (number indicating carbon it’s on)

Step 4: Assign Locants to Double/Triple Bonds

• Double bonds: use –ene
• Triple bonds: use –yne
• If both are present: use –en–yne, with numbering indicating both locations

Step 5: Add Functional Group Suffix

• Find the highest-priority functional group (see table below)
• Assign its suffix (e.g., –ol, –al, –one, –oic acid)
• Drop final “e” from hydrocarbon if suffix begins with a vowel

Step 6: Put It All Together

• List substituents in alphabetical order
• Combine all parts with correct punctuation:
  • Use hyphens (-) between numbers and letters
  • Use commas (,) between numbers
  • Do not leave spaces

Functional Group Priority Table

Priority	Functional Group	Suffix	Prefix (if not highest)
1	Carboxylic acid	–oic acid	carboxy–
2	Ester	–oate	alkoxycarbonyl–
3	Acid chloride	–oyl chloride	chloroformyl–
4	Amide	–amide	carbamoyl–
5	Nitrile (cyano)	–nitrile	cyano–
6	Aldehyde	–al	formyl–
7	Ketone	–one	oxo–
8	Alcohol	–ol	hydroxy–
9	Amine	–amine	amino–
10	Alkene	–ene	–
11	Alkyne	–yne	–
12	Alkane	–ane	–
13	Ether, Halide, Nitro	No suffix	methoxy–, bromo–, nitro–

MCAT Organic Nomenclature Tip: When naming, only the highest-priority group gets the suffix. Others become prefixes.

MCAT Organic Nomenclature Key Priority Rules:

• Only one group gets to be the suffix — the rest become prefixes.
• The higher the priority, the more likely it is to end the IUPAC name.
• Functional group suffixes replace the terminal -e in the hydrocarbon name (but not if they begin with a consonant — e.g., ketone = pentanone, not pentaneone).

Alkanes and Substituent Naming Rules

What Are Alkanes?

Alkanes are the simplest class of hydrocarbons — they contain only single bonds between carbon atoms and follow the general molecular formula:

$$
C_nH_{2n+2}
$$

They are:

• Saturated hydrocarbons (max hydrogen per carbon)
• Nonpolar
• Undergo combustion and substitution reactions

Examples of Straight-Chain Alkanes:

Number of Carbons	Name	Molecular Formula
1	Methane	CH₄
2	Ethane	C₂H₆
3	Propane	C₃H₈
4	Butane	C₄H₁₀
5	Pentane	C₅H₁₂
6	Hexane	C₆H₁₄
7	Heptane	C₇H₁₆
8	Octane	C₈H₁₈
9	Nonane	C₉H₂₀
10	Decane	C₁₀H₂₂

MCAT Organic Nomenclature Tip: Memorize the first 10 alkane names. They’re used constantly in both IUPAC names and MCAT passages.

Branched Alkanes and Alkyl Groups

When alkanes form branches, the side chains are named as alkyl groups — that is, they are named like alkanes but with a –yl suffix instead of –ane.

Alkyl Group	Formula	Derived from	Example Name
Methyl	–CH₃	Methane	2-methylbutane
Ethyl	–CH₂CH₃	Ethane	3-ethylhexane
Propyl	–CH₂CH₂CH₃	Propane	2-propylpentane
Isopropyl	–CH(CH₃)₂	Propane	3-isopropylhexane
Butyl	–CH₂CH₂CH₂CH₃	Butane	2-butylpentane
Tert-butyl	–C(CH₃)₃	Butane	2-tert-butylhexane

MCAT Organic Nomenclature Tip: Recognize common alkyl groups like isopropyl, isobutyl, sec-butyl, and tert-butyl — these often appear as distractors or in naming tasks.

Naming Simple Branched Alkanes: Step-by-Step

Let’s break this into a process using a worked-out example:

Example Molecule:

A six-carbon chain (hexane) with:

• A methyl group at carbon 2
• An ethyl group at carbon 3

Naming Steps:

1. Longest chain = 6 carbons → hexane
2. Substituents:
   • 2-methyl
   • 3-ethyl
3. Alphabetize substituents:
   • Ethyl comes before methyl
4. Combine into final name:
   • 3-ethyl-2-methylhexane

Rules to Remember:

• Use lowest possible numbers when assigning substituents
• Alphabetize substituent names (not including prefixes like di-, tri-)
• If multiple identical substituents exist, use prefixes (di-, tri-, tetra-), but these don’t affect alphabetical order

Numbering Tie-Breakers

When more than one numbering scheme gives the same lowest number for the first substituent, compare the next one:

• Example: 2,3-dimethylbutane vs. 3,4-dimethylbutane → Choose 2,3
• MCAT may present you with two correct chains — only one will follow all rules

Summary: Alkane Naming Flow

1. Identify the longest continuous carbon chain.
2. Number the chain to give substituents the lowest numbers.
3. Name and number all substituents.
4. Alphabetize substituents (ignore di-/tri- when alphabetizing).
5. Combine everything into a single IUPAC name.

Naming Alkenes and Alkynes (Unsaturation)

Guide to hydrocarbons with double and triple bonds for MCAT Organic Nomenclature.

What Are Alkenes and Alkynes?

Alkenes and alkynes are unsaturated hydrocarbons — they contain double or triple bonds between carbon atoms, respectively.

Type	Bond Type	General Formula
Alkene	C=C double	CnH2nC_nH_{2n}
Alkyne	C≡C triple	CnH2n−2C_nH_{2n-2}

• Alkenes exhibit cis/trans (or E/Z) isomerism if there are distinct substituents on the double bond.
• Both alkenes and alkynes are considered functional groups when naming organic molecules.

Basic Naming Rules

Step 1: Identify the Parent Chain

• Must include the double or triple bond
• Choose the longest chain that includes the unsaturation

Step 2: Number the Chain

• Number so that the multiple bond gets the lowest possible number
• If there’s a tie, use substituent priorities next

Step 3: Assign Suffix and Locant

• Use the suffix:
  • –ene for alkenes
  • –yne for alkynes
• Use a number to indicate where the double or triple bond starts
• For molecules with both double and triple bonds, use the combined suffix –en–yne

MCAT Style: “2-butyne” means a triple bond starting at carbon 2

MCAT Organic Nomenclature Examples:

Example 1: But-2-ene

• Four carbon chain (butane base)
• Double bond between C-2 and C-3
• Final name: but-2-ene

Example 2: Hex-1-yne

• Six carbon chain
• Triple bond starting at carbon 1
• Final name: hex-1-yne

Example 3: 4-methylpent-2-yne

• Parent chain = pentane (5 carbons)
• Triple bond starts at C-2 → pent-2-yne
• Methyl group at C-4
• Final name: 4-methylpent-2-yne

Example 4: 2-methylpent-1-en-4-yne

• Parent = pentane (5 carbon chain)
• Double bond at C-1, triple bond at C-4
• Methyl group at C-2
• Final name: 2-methylpent-1-en-4-yne

MCAT Organic Nomenclature Tip: For -en-ynes, the double bond gets priority in numbering.

Unsaturation Priority Recap:

Functional Group	Naming Suffix	Priority Order (in numbering)
Alkene (C=C)	–ene	Higher than alkyne
Alkyne (C≡C)	–yne	Lower than alkene
Alkane (C–C)	–ane	Lowest

Naming Functionalized Hydrocarbons

Alcohols, Ketones, Aldehydes, Carboxylic Acids, and More

What Are Functionalized Hydrocarbons?

A functionalized hydrocarbon contains one or more functional groups that alter its reactivity and naming priority. These groups replace or modify the standard hydrocarbon name with special suffixes and rules.

On the MCAT:

• You will not be expected to name highly complex polyfunctional molecules.
• But you must recognize how functional groups change naming and interpret IUPAC names back into structures.

General Naming Strategy for Functionalized Compounds

When a functional group is present:

1. Find the longest carbon chain that includes the functional group.
2. Number the chain so that the functional group gets the lowest possible number.
3. Replace the terminal “-e” of the alkane name with the appropriate suffix.
4. Any lower-priority groups (e.g., halides, alkyls, alcohols if not highest) become prefixes.

Most Common Functional Groups and Their Naming Rules

Functional Group	IUPAC Suffix	Prefix (if not highest)	Example Name
Alcohol (–OH)	-ol	hydroxy-	Propan-2-ol
Aldehyde (–CHO)	-al	formyl-	Butanal
Ketone (C=O)	-one	oxo-	Pentan-2-one
Carboxylic Acid (–COOH)	-oic acid	carboxy-	Ethanoic acid (acetic)
Amine (–NH₂)	-amine	amino-	Propan-1-amine
Amide (–CONH₂)	-amide	carbamoyl-	Butanamide
Nitrile (–C≡N)	-nitrile	cyano-	Propanenitrile

Example 1: Butan-2-ol

• 4 carbon chain = butane
• OH group at carbon 2 = butan-2-ol

Example 2: 3-hydroxybutanal

• 4 carbon chain with an aldehyde on C-1 (gets suffix –al)
• OH group on C-3 = hydroxy-
• Final name: 3-hydroxybutanal

Example 3: 2-aminopropanoic acid

• 3 carbon chain with carboxylic acid = propanoic acid
• NH₂ group at C-2 = 2-amino
• Final name: 2-aminopropanoic acid (aka alanine!)

Priority Note:

When multiple functional groups are present, use only one suffix — the highest-priority group takes it, and all others become prefixes.

For example:

• A compound with a ketone and a carboxylic acid is named as a carboxylic acid, with the ketone named as a prefix (oxo-).

Special Case: Dropping the “-e”

When the suffix starts with a vowel (e.g., –ol, –al, –one), the terminal “-e” of the alkane is dropped:

• Butane + –ol → butanol
• Hexane + –one → hexanone

Naming Molecules with Multiple Functional Groups

Overview

When a molecule contains two or more different functional groups, we must:

1. Determine the highest-priority functional group
2. Use its suffix as the primary naming ending
3. Convert all other functional groups into prefixes
4. Ensure correct numbering: the chain is numbered to give the highest-priority group the lowest possible locant

This is essential MCAT material — you must recognize which group takes naming priority and how others are named in relation to it.

Functional Group Priority List (Simplified)

Priority Rank	Functional Group	Suffix (used if highest)	Prefix (if not highest)
1	Carboxylic acid (–COOH)	–oic acid	carboxy–
2	Ester (–COOR)	–oate	alkoxycarbonyl–
3	Acid chloride (–COCl)	–oyl chloride	chloroformyl–
4	Amide (–CONH₂)	–amide	carbamoyl–
5	Nitrile (–C≡N)	–nitrile	cyano–
6	Aldehyde (–CHO)	–al	formyl–
7	Ketone (C=O)	–one	oxo–
8	Alcohol (–OH)	–ol	hydroxy–
9	Amine (–NH₂)	–amine	amino–
10	Alkene (C=C)	–ene	–
11	Alkyne (C≡C)	–yne	–
12	Alkane (C–C)	–ane	–
13	Halides, nitro, ether	no suffix	fluoro–, nitro–, methoxy–

Example 1: 4-hydroxybutanoic acid

• Functional groups: –COOH (carboxylic acid) and –OH (alcohol)
• Carboxylic acid wins → name ends in –oic acid
• Alcohol becomes prefix: hydroxy–
• Chain: 4 carbon atoms → butanoic acid
• OH on carbon 4 → 4-hydroxy

Final Name: 4-hydroxybutanoic acid

Example 2: 3-oxo-2-pentenoic acid

• Functional groups: –COOH, C=C, C=O
• Carboxylic acid is highest → suffix = –oic acid
• Ketone becomes oxo– prefix
• Alkene becomes –ene in the name

Final Name: 3-oxo-2-pentenoic acid

Example 3: 2-amino-3-hydroxybutanoic acid

• Functional groups: –COOH, –OH, –NH₂
• Carboxylic acid wins → base = butanoic acid
• OH → 3-hydroxy–
• NH₂ → 2-amino–

Final Name: 2-amino-3-hydroxybutanoic acid

This is the IUPAC name for serine, an amino acid.

MCAT Organic Nomenclature Tips:

• Be ready to identify the highest-priority group quickly — questions often ask “What is the correct name?” with competing suffixes.
• If all groups are lower-priority, the one that comes first alphabetically is used as the prefix (not the suffix!).
• Stereochemistry and numbering rules still apply (covered in Module 2).

Naming Cyclic Compounds and Rings

Aromatic and non-aromatic ring structures in IUPAC nomenclature

What Are Cyclic Compounds?

Cyclic compounds are ring-shaped molecules — either aliphatic (non-aromatic) or aromatic (e.g., benzene). Ring structures are prevalent in biomolecules, pharmaceuticals, and MCAT-style questions.

Naming Non-Aromatic Rings (Cycloalkanes)

The base name for a ring is formed by adding “cyclo” before the alkane name.

Ring Size	Name	Formula
3	Cyclopropane	C₃H₆
4	Cyclobutane	C₄H₈
5	Cyclopentane	C₅H₁₀
6	Cyclohexane	C₆H₁₂
7	Cycloheptane	C₇H₁₄

Example 1: 1-chlorocyclopentane

• 5-membered ring = cyclopentane
• Cl attached at position 1 → 1-chloro

Final name: 1-chlorocyclopentane

Numbering in Substituted Cycloalkanes

1. Begin numbering at the highest-priority functional group.
2. If only alkyl groups are present, begin at the group that gives the lowest total numbers.
3. Ring size is the parent if it’s longer than any side chain.

Disubstituted and Trisubstituted Rings

• Use numbers to indicate position of each substituent.
• Follow alphabetical order for naming.

Example: 1-ethyl-3-methylcyclohexane

Aromatic Rings (Benzene and Derivatives)

Aromatic rings, especially benzene, follow their own naming rules.

Simple Benzene Derivatives:

Substituent	IUPAC Name	Common Name (if applicable)
–CH₃	Methylbenzene	Toluene
–OH	Hydroxybenzene	Phenol
–NH₂	Aminobenzene	Aniline
–COOH	Benzenecarboxylic acid	Benzoic acid
–NO₂	Nitrobenzene	—

Numbering Substituents on Benzene:

Use 1,2–, 1,3–, 1,4– or their common names:

Position	IUPAC Form	Common Prefix
1,2–	ortho–	o–
1,3–	meta–	m–
1,4–	para–	p–

Example: 1,4-dichlorobenzene = para-dichlorobenzene

Example: 3-nitroaniline

• Benzene with an –NH₂ at position 1 and –NO₂ at position 3
• Final name: 3-nitroaniline

MCAT Organic Nomenclature Pitfalls to Avoid:

• Don’t assume the ring is the parent unless it is larger or functionally dominant.
• Aromatic rings with multiple groups may require alphabetization and lowest numbering rules.
• Don’t forget common names like phenol, toluene, aniline are allowed on the MCAT and often appear.

Naming Flowchart, Tips, and Common Pitfalls

Full Naming Algorithm (Step-by-Step)

1. Find the Longest Carbon Chain
   • Must contain the highest-priority functional group and any multiple bonds.
2. Number the Chain
   • Give lowest numbers to:
     • Highest-priority group
     • Multiple bonds (double/triple)
     • Substituents (if ties)
3. Identify and Name Substituents
   • Use correct prefixes: methyl-, ethyl-, bromo-, hydroxy-, etc.
   • Use di-, tri-, tetra- for multiples of the same group (not in alphabetical order)
4. Assign Suffix
   • Based on highest-priority functional group
   • Drop the “e” from alkane if suffix begins with a vowel (e.g., –ol, –al)
5. Assemble the Name
   • Prefixes (in alphabetical order)
   • Numbers for positions
   • Suffix as the ending

Common Naming Suffixes Recap

Group	Suffix	Prefix (if not highest)
Alkane	–ane	—
Alkene	–ene	—
Alkyne	–yne	—
Alcohol	–ol	hydroxy–
Ketone	–one	oxo–
Aldehyde	–al	formyl–
Carboxylic Acid	–oic acid	carboxy–
Amine	–amine	amino–
Halide	—	fluoro–, chloro–, etc.

Common Mistakes on the MCAT

Mistake	How to Avoid It
Choosing the wrong parent chain	Always include the highest-priority group and longest path
Incorrect numbering	Functional groups > double/triple bonds > substituents
Alphabetizing prefixes incorrectly	Ignore di-, tri-, etc., but not iso-, sec-, tert-
Using multiple suffixes in one name	Only the highest-priority group gets a suffix
Forgetting to drop the final “e” when needed	Drop “-e” before vowel-starting suffixes like –ol, –al

Final MCAT Tips

• You won’t be asked to name overly complex molecules, but you will be tested on:
  • Interpreting IUPAC names
  • Recognizing correct vs. incorrect naming
  • Identifying structures from names
  • Matching priority to suffix usage
• Learn to spot common functional group patterns and aromatic rings quickly

IUPAC Naming Strategy Flowchart: A Step-by-Step Guide to Organic Nomenclature