Chapter 2: Nomenclature

Contents

2.0 Introduction
2.1 Hydrocarbons
2.2 Alkanes
2.3 Branched Alkanes
2.4 Cyclic Alkanes
2.5 Bicyclic Compounds
2.6 Alkenes
     2.6.1 Classification of Alkenes
     2.6.2 Stereoisomers of Alkenes
2.7 Alkynes
2.8 Naming Hydrocarbons with Several Double or Triple Bonds
2.9 Functional Groups
2.10 Naming Complex Organic Compounds
2.11 Naming Alkyl Halides, Ethers and Alcohols
     2.11.1 Naming Alkyl Halides
     2.11.2 Naming Ethers
     2.11.3 Naming Alcohols
2.12 Naming Carbonyl Compounds
     2.12.1 Naming Aldehydes
     2.12.2 Naming Ketones
2.13 Naming Acyl Compounds
     2.13.1 Naming Carboxylic Acids
     2.13.2 Naming Esters
     2.13.3 Naming Amides
     2.13.4 Naming Acid Anhydrides
     2.13.5 Naming Acyl Halides
2.14 Naming Amines
2.15 Naming Nitriles
2.16 Acetyl Functional Group
2.17 Naming Organic Compounds with Multiple Functional Groups
2.18 Index of Hydrogen Deficiency (Degrees of Unsaturation)
     2.18.1 Saturated and Unsaturated organic compounds and the Index of Hydrogen Deficiency (IHD) 
     2.18.2 Heteroatoms and determining the index of hydrogen deficiency

Learning Objectives

•  Understand the structural difference between hydrocarbons (alkanes, alkenes and alkynes).
•  Understand the differences between parent chains and substituents.
•  Understand and evaluate which functional group present in a molecule is the principal group.
•  Understand the IUPAC rules required to correctly and unambiguously identify each and every molecule with a unique name that describes its structure.
•  Understand the rules needed to correctly determine the stereochemistry of alkenes using the Cahn-Ingold-Prelog system.
•  Apply the IUPAC rules or naming hydrocarbons and other organic structures with more complex functional groups (aldehydes, ketones, carboxylic acids, nitriles, halogens, esters and alcohols).

2.0 Introduction

This chapter will introduce the nomenclature for organic compounds. The primary goal of nomenclature is to identify each and every molecule with a unique name that describes its structure.

The International Union of Pure and Applied Chemistry (IUPAC) has established a detailed set of standard rules that are used to name organic compounds. Names derived from using the IUPAC rules are called systematic names.

2.1 Hydrocarbons

Hydrocarbons are organic compounds composed only of carbon and hydrogen atoms. These compounds can be divided into two groups: aliphatic hydrocarbons and the aromatic hydrocarbons.

There are three subgroups of aliphatic hydrocarbons. They differ from each other by the number of bonds between carbon atoms.

• Alkanes contain only single bonds connecting all carbon atoms
• Alkenes contain at least one double bond connecting two adjacent carbon atoms
• Alkynes contain at least one triple bond connecting two adjacent carbon atoms.

Aromatic hydrocarbons have different properties and reactivity when compared to the aliphatic group. These types of compounds will be covered in a later chapter.

2.2 Alkanes

In alkanes, all carbon atoms are connected by σ bonds and therefore lack π bonds. These types of aliphatic hydrocarbons have the general molecular formula C_nH_2n+2 and the names of these compounds have an ane suffix.

Linear Alkanes (Parent Chain)

Alkanes are named based on the number of carbon atoms present in the parent chain. The parent chain in an alkane is defined as the longest continuous uninterrupted chain of carbon atoms. When naming the parent chain of a compound, the names in Table 2.1 are used.

The first step in naming an alkane is to identify the parent chain. In the example below, the parent chain is made of eight carbon atoms. Based on the longest continuous uninterrupted chain, the parent name for this compound is oct. The ending ane is used because it is an alkane, with all carbon atoms connected by single bonds. Therefore the name of the alkane is octane. The compound below is made of eight carbons and has the general molecular formula C₈H₁₈ (C_nH_2n+2).

2.3 Branched Alkanes

Branched alkanes are organic compounds that consist of a parent chain and substituents. These substituents can be thought of as branches connected to the parent chain, as in Figure 2.3.

Alkyl groups

Alkyl groups are substituents that are derived from alkanes but differ from alkanes in the lack of an H atom.

Alkyl groups can be either branched (branched carbon skeleton) or unbranched (linear or straight chain carbon skeleton).

Alkyl substituents are named based on the number of carbon atoms in the longest continuous chain of the substituent.

Unbranched or linear alkyl substituents are named by replacing the suffix ane of the derived hydrocarbon with yl.

An alkyl group with one carbon atom is derived from methane (CH₄) and is called a methyl group (-CH₃). An alkyl group with two carbon atoms is derived from ethane (CH₃CH₃) is called an ethyl group (-CH₂CH₃).

Naming branched alkyl groups is more complex than naming un-branched alkyl substituents.

When naming branched alkyl groups, each carbon atom in the substituent is numbered sequentially. The carbon directly attached to the parent chain is assigned the number 1 and longest chain of the substituent is numbered sequentially (Figure 2.6).

In the above example, the longest chain of the branched substituent is composed of four carbon atoms and is considered a butyl group. This butyl group also contains a methyl group attached to the second carbon. This branched substituent is called (2- methylbutyl). When naming branched alkyl groups, the name is placed inside parenthesis to avoid confusion with the rest of the compound’s name.

An sp³ hybrid carbon atom in a structure can be classified as primary (1°), secondary (2°), tertiary (3°), or quaternary (4°) depending on the number of carbon atoms directly bonded to it. A primary carbon is attached to one more carbon atom; a secondary carbon is attached to two more carbon atoms; a tertiary carbon will be bonded to three carbon atoms; and a quaternary carbon will be bonded to four carbon atoms (Figure 2.7).

Some branched alkyl groups contain common names. It is important to be able to recognize both the systematic and the common names for such substituents (Figure 2.8). To help you learn the common names, look for the similarities between the structures.

To name branched alkanes, these five steps must be followed:

• Identify the parent chain: this is the longest continuous uninterrupted chain of carbons in the molecule. Note that the chain may be bent or not.
• Identify the substituents: these are the groups that are directly connected to the parent chain.
• Assign the location (locant) to each substituent: the parent chain is numbered in the direction that gives the lowest locant to the substituent at the first point of branching.
• Name the compound: the parent chain name is the last part of the name and is preceded by the substituents and their numerical location. Substituents are listed in alphabetical order.

Note that when assembling the name of the compound, a hyphen is always used to separate numbers from letters.

The video below will show you how to name a branched alkane by using the steps outlined above.

As seen in the previous video, when multiple substituents are present in a molecule, the parent chain is numbered in the direction that gives the lowest number to the first substituent encountered.

When a substituent appears more than once in a compound, a prefix is used to identify the number of the times the substituent appears in the compound: di is two, tri is three, tetra is four, penta is five, etc. In the example above, there are three methyl groups present in the molecule, so the prefix tri should be used.

Once all substituents are identified and their proper locant established, they are alphabetized. Prefixes di, tri, tetra, penta, and hexa are not included as part of the alphabetized scheme. Prefixes that are hyphenated, such as sec- and tert-, are also ignored in the alphabetization; however, iso and neo are not ignored. A hyphen is used to separate numbers from letters and a comma to separate numbers from numbers.

The branched alkane in Figure 2.10 contains seven carbon atoms in the parent chain. Its parent name is heptane. There are three methyl groups – trimethyl – directly attached to the parent chain. The parent chain is numbered from left to right as this gives the lowest location to the first substituent encountered. The name of this compound is 2,5,5-trimethylheptane.

If when numbering the parent chain, there is a tie in the first locant, then the second locant should be considered. If there is still a tie in the second locant, the third locant is evaluated and so forth (Figure 2.11).

If there is a competition between two parent chains of equal length, choose the one that has the greatest number of substituents. In Figure 2.12, the longest uninterrupted continuous carbon chain is made of nine carbon atoms, but the structure to the right has more alkyl substituents. The systematic name for this compound is 3,6-diethyl-2,7-dimethylnonane.

2.4 Cyclic alkanes

Cyclic alkanes are hydrocarbons that contain a ring and in which all carbon atoms including those in the ring are connected through a single bond. Cyclic alkanes have a general molecular formula C_nH_2n, as there are two less hydrogen atoms when going from a linear to a cyclic structure.

Cyclic alkanes are named using the IUPAC rules used for naming alkanes. However, the term cyclo is used when naming cyclic alkanes to indicate the presence of the ring in the structure. The name of the parent chain follows based on the number of carbon atoms in the ring.

When an alkyl group is attached to a ring, the ring is treated as the parent if the ring has more carbons than the alkyl group. If not, the ring is treated as the substituent (Figure 2.13).

A cyclic alkane must be numbered in the direction that will give the lowest locant to the first substituent encountered. If there is a tie, then the second locant should be as low as possible.