Organic Chemistry I: Structures and Nomenclature (IUPAC Naming of Organic Compounds)

Organic Chemistry I – Structures (Ch. 20 & 3)

Overview

This section covers the foundational principles of organic compound structures and their systematic naming using the IUPAC system. Topics include hydrocarbons, functional groups, isomerism, and detailed strategies for naming organic molecules.

3.5 Naming Organic Compounds

Introduction to Nomenclature

• Nomenclature is a set of rules for systematically naming organic molecules.

• Chemists use the IUPAC (International Union of Pure and Applied Chemistry) naming system for consistency and clarity.

• The IUPAC name allows chemists to draw the molecular structure directly from the name.

• Example: Vitamin B3 (IUPAC: pyridine-3-carboxamide), Vitamin A (IUPAC: (2E,4E,6E,8E)-3,7-dimethyl-9-(2,6,6-trimethylcyclohex-1-enyl)nona-2,4,6,8-tetraen-1-ol)

Strategy for Naming Molecules

• 1. Identify the parent structure (longest carbon chain or ring).

• 2. Identify the suffix (highest priority functional group).

• 3. Assign the locants (carbon numbers).

• 4. Name the substituents (other groups attached to the parent).

• 5. Assemble the name following IUPAC conventions.

Key Components of IUPAC Names

• Parent: The main chain or ring, described by the number of carbons using Greek prefixes.

• Suffix: Indicates the highest priority functional group and its locant (position).

• Substituents: Other functional groups attached to the parent, with their locants.

Functional Group Priority

Each functional group is assigned a priority, which determines the suffix and numbering of the parent chain.

Functional group	Structural formula	Condensed formula	Suffix	Prefix
Carboxylic acid	RCOOH	RCOOH	-oic acid	N/A
Ester	RCOOR'	RCOOR'	-oate	N/A
Amide	RCONH2	RCONH2	-amide	N/A
Aldehyde	RCHO	RCHO	-al	oxo
Ketone	RCOR'	RCOR'	-one	oxo
Alcohol	R–OH	ROH	-ol	hydroxy
Amine	RNH2	RNH2	-amine	amino
Alkene	RCH=CHR'	RCH=CHR'	-ene	alkenyl
Alkyne	RC≡CR'	RC≡CR'	-yne	alkynyl
Aromatic	C6H5R	C6H5R	-benzene	phenyl
Ether	ROR'	ROR'	N/A	alkoxy
Alkyl halide	RX	RX	N/A	halo
Alkyl	RH	RH	N/A	alkyl

Flowchart for Naming Organic Molecules

• Determine if the molecule contains an aromatic group.

• If aromatic is the highest priority, use it as the parent; otherwise, treat it as a substituent.

• Identify the highest priority group and base.

• Assign locants and determine cis/trans isomerism if present.

• Assemble the IUPAC name.

Parent Chain Selection

• The parent is the longest continuous carbon chain or the ring containing the highest priority functional group.

• Chains can be saturated (alkanes) or unsaturated (alkenes, alkynes).

• Add the "cyclo" prefix for rings.

# of C	Greek prefix	# of C	Greek prefix
1	meth	6	hex
2	eth	7	hept
3	prop	8	oct
4	but	9	non
5	pent	10	dec

Locants: Numbering the Parent Chain

• Locant is the position of a substituent or functional group on the parent structure.

• Number the carbons so that the highest priority functional group has the lowest possible locant.

• If multiple numbering schemes are possible:

  1. List all locants for substituents (lowest to highest).

  2. Choose the scheme with the lowest locant at the first point of difference.

  3. If still ambiguous, assign the lowest locant to the substituent that comes first alphabetically.

Substituents: Naming and Prefixes

• Substituents are named using specific prefixes, often based on the functional group.

Functional group	Structural formula	Prefix
Alcohol	R–OH	hydroxy
Amine	RNH2	amino
Alkene	RCH=CHR'	alkenyl
Alkyne	RC≡CR'	alkynyl
Aromatic	C6H5R	phenyl
Ether	ROR'	alkoxy
Alkyl halide	RX	halo
Alkyl	RH	alkyl

Alkyl Substituents: Simple and Branched

• Simple alkane substituents use the Greek prefix and the -yl ending (e.g., methyl, ethyl).

• Branched alkane substituents use common names (e.g., isopropyl, sec-butyl, isobutyl, tert-butyl).

# of C	Greek prefix	# of C	Greek prefix
1	meth	6	hex
2	eth	7	hept
3	prop	8	oct
4	but	9	non
5	pent	10	dec

Suffixes: Determined by Highest Priority Functional Group

• The suffix is taken from the highest priority functional group present in the molecule.

Functional group	Structural formula	Suffix
Carboxylic acid	RCOOH	-oic acid
Ester	RCOOR'	-oate
Amide	RCONH2	-amide
Aldehyde	RCHO	-al
Ketone	RCOR'	-one
Alcohol	R–OH	-ol
Amine	RNH2	-amine
Alkene	RCH=CHR'	-ene
Alkyne	RC≡CR'	-yne
Aromatic	C6H5R	-benzene

Putting It All Together: Steps for IUPAC Naming

• Identify the parent (longest carbon chain).

• Identify the highest priority group to determine the suffix.

• Number the parent chain according to the rules.

• Assign each substituent its locant and prefix.

• Group like substituents and add the multiplier prefix (di, tri, tetra, penta, etc.) to indicate quantity.

• Organize substituents (and locants) in alphabetical order, ignoring multiplier prefixes.

• Assemble the name and punctuate:

  • Numbers separated by commas

  • Numbers and letters separated by hyphens

  • Letters joined together

Naming Alkenes and Alkynes

• Alkynes: Molecules with C≡C bonds; use suffix -yne.

• Alkenes: Molecules with C=C bonds; use suffix -ene.

• For naming, alkenes and alkynes are divided into:

  1. With functional groups higher than itself.

  2. With functional groups lower than itself.

• If the functional group is lower priority, apply the IUPAC algorithm and use ene/yne suffix with locant and multiplier prefixes.

• If higher priority, add en/yn before the family suffix, with locant and multiplier prefixes.

Naming Aromatic Molecules

• Identify the parent aromatic structure as the highest priority group.

• Assign locants using alkane protocols.

• Name each substituent and group common substituents.

• Assemble the name.

• Geometric isomers (e.g., ortho, meta, para) are distinguished by locants for substituents.

Examples and Applications

• Example: Naming a molecule with a chlorine and alcohol group: Number the chain so the alcohol gets the lowest locant, then name as 2-chloroethanol.

• Example: Drawing hexanoic acid: Six-carbon chain with a terminal carboxylic acid group.

• Example: Naming an aromatic compound with two methyl groups: Use locants to specify positions, e.g., 1,3-dimethylbenzene.

Key Equations and Notation

• General formula for alkanes: $C_nH_{2n+2}$

• General formula for alkenes: $C_nH_{2n}$

• General formula for alkynes: $C_nH_{2n-2}$

Summary Table: Steps for IUPAC Naming

Step	Description
1	Identify parent chain/ring
2	Identify highest priority group (suffix)
3	Number chain for lowest locant
4	Name substituents (prefixes)
5	Assemble name (alphabetical order, punctuation)