CHAPTER 2The Alkanes: Structure and Nomenclature of Simple Hydrocarbons

2.1 Hydrocarbons: An Introduction

• Hydrocarbons- organic compounds that only contain carbon and hydrogen.

• 2 Classes of hydrocarbons• 1. Saturated- all carbon to carbon bonds are single bonds (alkanes).

• 2. Unsaturated- molecules with at least one carbon to carbon double or triple bond (alkenes, alkynes, or aromatics).

2.1 Hydrocarbons: An Introduction

In this chapter we will be studying alkanes.

Formula: CnH2n+2

Used to calculate the number of hydrogen's if you are given the number of carbons in the chain.

Alkanes are the simplest hydrocarbons and only contain single bonds between the carbon atoms.

Ex: methane (CH4) and ethane (C2H6)

2.2 Molecular and Structural Formulas: Isomerism• There are an infinite number of organic compounds.

• They are usually represented by molecular or structural formulas.

Molecular formula- describe the exact number of each kind of atom in a compound.• Example: CH4, C2H6, C3H8

Structural formula- shows both the number of atoms and the bonding arrangement of the atoms.

Examples: Look on board for drawn structures of CH4, C2H6, and C3H8

2.2 Molecular and Structural Formulas: Isomerism

• The more atoms in a molecular formula the more compounds that can be made.

• There are 5 types of isomers that you will learn.• Skeletal, positional, and functional are all types of

structural isomers.• Geometric and conformational are types of

stereoisomerism.• Structural isomerism, also called constitutional isomers, is

where different atoms are attached to one another.• Stereoisomerism is where the same atoms are bonded to

one another, but their orientation in space is different.

2.3 Skeletal Isomerism in Alkanes

• Isomers are compounds with the same molecular formula but different structural formulas.

• For alkanes the arrangement of the carbons can be different because they all are single bonds and they demonstrate skeletal isomersism.

• This is isomers that differ in the arrangement of the carbon chain.

• There are two types of structural formulas for C4H10.

• Branched or a continuous chain (see board)

2.3 Skeletal Isomerism in Alkanes

Cycloalkanes• The general formula is CnH2n.• Instead of CnH2n+2 for alkanes.• A cycloalkane contains 2 hydrogen’s less than a regular

alkane.• To form a cycloalkane the two end carbons have to each

lose one hydrogen to form a bond and create the ring structure.

• For every acyclic molecule there is a corresponding cyclic or ring molecule.

2.4 Representations of Structural Formulas

• There are different ways that you can draw structural isomers.

• Figure 2.1• Space-Filling Model• Ball-and-Stick• Skeleton• Condensed formula• Line or stick

2.5 Positional Isomerism

• Remember skeletal isomers differ in the arrangement of the carbon atoms (that is why it is called the carbon skeleton).

• Positional isomers differ in the location of the noncarbon group or a double or triple bond. Basically, the carbon skeleton stays the same.

• A lot of time skeletal and positional isomers occur at the same time.

• See board for example.

2.6 IUPAC Nomenclature of Alkanes

• A. Naming of continuous-chain, unbranched alkanes.

• Greek name for the number of carbons followed by the suffix- ane.

• Ex: C5H12 5=pent called-pentane

• The first four prefixes have trivial names that were incorporated into the IUPAC because of extensive use.

• Memorize table 2.1 on page 49.

2.6 IUPAC Nomenclature of Alkanes

• Ring or cyclic compounds follow the same naming rules except the prefix “cyclo” is added.

• B. Naming branched-chain alkanes• 1. branched chains have shorter carbon chains (called alkyl groups) attached to the longer carbon skeletons (called-parent or base chain).

• Alkyl groups (also known as substituents) are derived by changing the ending of the hydrocarbon name from “ane” to “yl”.

2.6 IUPAC Nomenclature of Alkanes

• Example: CH4 = methane -CH3 = methyl

• Look at Table 2.2 page 50.

• Steps to naming:• 1. Count the longest carbon chain- name it with the Greek

number for carbon atoms followed by the suffix – ane (remember this is the parent chain). Please remember that the longest chain does not have to go in a straight line.

• -for ring compounds the ring is usually the base (parent) chain.

• 2. Name the attached shorter chains (alkyl groups).• 3. Number the longest carbon chain placing the first

substituent on the lowest carbon possible in that chain.

2.6 IUPAC Nomenclature of Alkanes

• Remember you can number left to right or right to left, which ever places the substituent on the lowest carbon possible in the parent chain.

• Ex: 3-methylpentane• 4. If there is more than one of a particular alkyl group you have to use a Greek prefix indicating the number of identical groups. Ex: dimethyl would be used if there are two methyl groups coming off of the parent chain.

2.6 IUPAC Nomenclature of Alkanes

• You must number each alkyl group so that the reader knows which carbon in the parent chain that it is attached to.

• Ex: 2,4-dimethylpentane• 5. Always name alphabetically. Do not take prefixes into consideration when alphabetizing.