Figure 2.1 - Four Classes of Hydrocarbons

General formula: CnH2n+2 CnH2n CnH2n-2 none

Alkanes

Alkanes: the simplest class of organic compounds

• Because there are so many different organic compounds, we must group them into classes based on their structure. The simplest class of organic compounds is the alkanes:

An alkane is a compound that contains only C and H atoms, and contains only single bonds.

• What are some examples of alkanes? Where do alkanes come from? What are the uses of alkanes?

• What is the general molecular formula of an alkane?

• How many constitutional isomers have the molecular formula C5H12?

Skeletal Structures• One of the most useful skills in organic chemistry is the ability to write and

understand skeletal structures. Let’s take a look at some skeletal structures for several alkanes:

Heptane 2,2-dimethylpentane

Cyclooctane tert-butylcyclohexane

• For each of the following skeletal structures, count the number of carbon and hydrogen atoms in the structure, and provide a systematic name for the alkane:

No constitutional isomers with only three carbons in the longest chain are possible for C6H14

Constitutional Isomers and Line-Angle Formulas

Constitutional isomers: Compounds with the same molecular formula but a different connectivity of their atoms.

Nomenclature of Alkanes

• How can we assign names to alkanes? • Straight-Chain Alkane

• Branched Alkanes

Table 2.1 - Names, Molecular Formulas, and Condensed Structural Formulas for the First 20 Alkanes with Unbranched Chains

Nomenclature of Alkanes - The IUPAC System

• IUPAC - International Union of Pure and Applied Chemistry• IUPAC name of an alkane with an unbranched carbon chain consists

of:• Prefix that indicates the number of carbon atoms in the chain • Suffix -ane to show that the compound is a saturated hydrocarbon

• IUPAC name of an alkane with a branched chain consists of: • A parent name that indicates the longest chain of carbon atoms in the

compound• Substituent names that indicate the groups bonded to the parent chain

Table 2.2 - Prefixes Used in the IUPAC System to Show the Presence of 1 to 20 Carbon Atoms in an Unbranched Chain

1. Name for an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms in the chain and the ending -ane

2. The longest chain of carbon atoms should be selected as the parent chain for branched-chain alkanes• Name of the parent chain becomes the root name

3. Each substituent is given a name and a number• Number shows the carbon atom of the parent chain to which the substituent is

bonded• Hyphen should be used to connect the number to the name

4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number

5. In case of two or more identical substituents:• Number the parent chain from the end that gives the lower number to the

substituent encountered first• Indicate the number of times the substituent appears by the prefix di-, tri-, tetra-,

and so on• Use commas to separate position numbers

Rules of the IUPAC System for Naming Alkanes1. Name for an alkane with an unbranched chain of carbon atoms consists of a prefix

showing the number of carbon atoms in the chain and the ending -ane2. The longest chain of carbon atoms should be selected as the parent chain for

branched-chain alkanes• Name of the parent chain becomes the root name

3. Each substituent is given a name and a number• Number shows the carbon atom of the parent chain to which the substituent is

bonded• Hyphen should be used to connect the number to the name

4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number

5. In case of two or more identical substituents:• Number the parent chain from the end that gives the lower number to the

substituent encountered first• Indicate the number of times the substituent appears by the prefix di-, tri-, tetra-,

and so on• Use commas to separate position numbers

6. In case of two or more different substituents:• List the substituents in alphabetical order• Number the chain from the end that gives the substituent encountered first the lower

number• If there are different substituents in equivalent positions on opposite ends of the

parent chain, give the substituent of lower alphabetical order the lower number7. Prefixes di-, tri-, tetra-, and so on are not included in alphabetizing

• Alphabetize the names of substituents first and then insert theprefixes

8. In case of two or more parent chains of identical length, choose the parent chain with the greater number of substituents

The IUPAC System - A General System of Nomenclature• Name given to any compound with a chain

of carbon atoms consists of a prefix, an infix, and a suffix

• Prefix indicates the number of carbon atoms in the parent chain

• Infix - Modifying element inserted into a word

• Indicates the nature of the carbon-carbon bonds in the parent chain

• Suffix indicates the class of compound to which the substance belongs

Draw the Compounds’ Structures

prop-en-e = propene

eth-an-ol = ethanol

but-an-one = butanone

but-an-al = butanal

pent-an-oic acid = pentanoic acid

cyclohex-an-ol = cyclohexanol

eth-yn-e = ethyne

eth-an-amine = ethanamine

Classification of Carbon and Hydrogen Atoms• Classification of carbon atoms

• Primary (1°) - Carbon bonded to one carbon atom• Secondary (2°) - Carbon bonded to two other carbon atoms• Tertiary (3°) - Carbon bonded to three other carbon atoms• Quaternary (4°) - Carbon bonded to four other carbon atoms

• Hydrogen atoms are classified as primary, secondary, or tertiary depending on the type of carbon to which each is bonded

• Primary hydrogen - Hydrogen bonded to a 1° carbon• Secondary hydrogen - Hydrogen bonded to a 2° carbon• Tertiary hydrogen - Hydrogen bonded to a 3° carbon

Intramolecular Strain

• One is said to put a strain on the system if one disturbs the optimal structure • Strain: Measure of the energy stored in a compound due to a structural distortion

• Types - Torsional strain, steric strain, and angle strain• Torsional strain is the repulsion that arises between atoms or group of atoms when a

molecule is rotated around a sigma bond.• The steric strain is the repulsion between two atoms or groups of atoms when two atoms or

groups bumped into one another . This is also called steric hindrance.• Angle strain is expansion or compression of bond angles away from the optimal values.

Conformations of Alkanes• Conformation: Any three-dimensional arrangement of atoms in a molecule that

results from rotation about a single bond• Different conformations are often called conformational isomers or

conformers• Staggered conformation: Conformation about a C—C bond in which the atoms or

groups on one carbon are as far apart as possible from atoms or groups on an adjacent carbon

• Newman projection: A way to view a molecule by looking along a C—C bond to help evaluate the relative orientations of attached groups

Staggered Eclipsed

Dihedral angle

Conformations of Ethane

• Consider the molecule ethane, C2H6. This molecule can exist in different conformations. How can we represent these conformations?

• What are the relative energies of these conformations?

Figure 2.8 - Energy of Ethane as a Function of Dihedral Angle

+12.6 kJ/mol

Torsional strain 12.6 kJ (3.0 kcal)/mol

Each H/H eclipse ~1.0 kcal/mol

Conformation of Butane

• Consider rotations around the central C—C bond the molecule n-butane (the straight- chain C4H10 alkane). What are the conformations that result from rotations around this central bond?

• What are the relative energies of these conformations?

Figure 2.9 - Energy of Butane as a Function of the Dihedral Angle about the Bond between Carbons 2 and 3

Staggered confirmationsAnti conformation: Conformation at a dihedral angle of 180°Gauche conformation: Conformation at a dihedral angle of 60°

Cycloalkanes

Cyclic Compounds

Cyclohexane

Cyclohexanes: Chairs

Two types of H atoms

Conformations of Cycloalkanes

• Structures and energies are highly dependent on the size of the rings• Small ring strain: Associated with ring sizes below six that arises from

nonoptimal bond angles

Observed C—C—C bond angles are 60°Combined angle and torsional strain energy is about 116 kJ (27.7 kcal)/mol

Measured C—C—C bond angles are 88°. Combined angle and torsional Strain energy is about 110 kJ (26.3 kcal)/mol

Measured C—C—C bond angles are 105°. Combined angle and torsional Strain energy is about 27 kJ (6.5 kcal)/mol

Figure 2.25 - Strain Energy of Cycloalkanes as a Function of Ring Size

Conformations of Cycloalkanes - Cyclohexane

• Chair conformation: Most stable puckered conformation of a cyclohexane ring• All bond C—C—C bond angles are 110.9°• All hydrogens on adjacent carbons are staggered• No two atoms are close enough to each other for nonbonded interaction strain to exist

Axial bondsvs.Equatorial bonds

Figure 2.17 - Boat and Twist-Boat Conformations of Cyclohexane

Difference in energy between chair and boat conformations - Approximately 27 kJ (6.5 kcal)/molTwisted boat conformationApproximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformationApproximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation

Figure 2.19 - Energy Diagram for the Interconversion of Chair, Twist-Boat, and Boat Conformations of Cyclohexane

Calculation of the Ratio of the Two Conformations at Equilibrium• Uses the equation ΔG0 = −RT ln Keq

• ΔG0 = Change in Gibbs free energy• Keq = Equilibrium constant• T = Temperature in kelvins• R = Universal gas constant that has the value 8.314 J (1.987 cal)·K−1·mol−1

• For methylcyclohexane• At any given instant at room temperature, percentage of equatorial is (18.9/19.9)

×100% = about 95%

1

eq 1 1

eq

( 7280 J mol )ln 2.9398.314 J K mol 298 K

18.9 equatorial1 axial

K

K

-

- -

- -= =

´

= =

!

! !

Cyclohexanes: Two Chairs are in Rapid Equilibrium

Draw the Chair Conformation of Cyclohexane

Draw the flip of the chair conformation of cyclohexane

Drawing Alternative Chair Conformations of Cyclohexane

• Step 1 - Draw two sets of parallel lines, each set at a slight angle

• Step 2 - Complete each chair by drawing the ends connected to the parallel lines, in each case making one end tip up and the other end tip down

• Step 3 - Draw axial bonds as vertical lines drawn on the side of the larger angle (greater than 180°) at each ring atom

• Step 4 - Draw the equatorial bonds using the bonds of the ring as guides for the angles