ORGANIC CHEMISTRY I CHEM 2323

CHAPTER 3 - ALKANES

CLASSIFICATION BY STRUCTURE: THE FAMILY

Structural theory relies on the structures of the molecules. These structures are divided into families. Alkanes are also referred to as paraffins.

STRUCTURE OF ETHANE

FREE ROTATION ABOUT THE CARBON-CARBON SINGLE BOND

Conformations - different arrangement of atoms that can be converted into another about single bonds.

Torsional energy - energy required to rotate about the carbon-carbon bond.

Staggered has the least energy, therefore, is the most stable.

Eclipsed has the most energy, therefore, is the least stable.

Skewed conformations are between the eclipsed and staggered.

ALKANE SERIES

ALKANES		ALKYLS
methane	CH4	methyl	CH3
ethane	C2H6	ethyl	C2H5
propane	C3H8	propyl	C3H7
butane	C4H10	butyl	C4H9
pentane	C5H12	propyl	C5H11
hexane	C6H14	hexyl	C6H13
heptane	C7H16	heptyl	C7H15
octane	C8H18	octyl	C8H17
nonane	C9H20	nonyl	C10H19
decane	C10H22	decyl	C10H21

NOMENCLATURE

Two systems of naming organic compounds are normally used:

• common - derived from various origins

• IUPAC - International Union of Pure and Applied Chemistry

ALKYLS

alkyl groups are derived from the alkanes (one less hydrogen) - see table above

Drop the -ane and replace with -yl

example: methane (CH₄) becomes methyl (CH₃-)

CH₄ to CH₃-

ALKYL ISOMERS

Isomers are compounds with the same molecular formula but different structures.

Alkyls can also form isomers:

COMMON NAMES OF ALKANES

n - alkanes - compounds of carbon which form one continuous chain.

isoalkanes - compounds of six carbons or fewer in which all carbons except one form a continuous chain and that one carbon is attached to the next- to-end carbon.

IUPAC NAMES OF ALKANES

Steps to name (IUPAC) alkanes:

1. Trace the longest continuous chain of carbons (parent chain)

six carbons = hexane

2. Locate the side group and circle it.

one side group which is a methyl

3. Starting from each end of the parent chain, number from the end to give the first side group you come to the lowest possible number.

If you number from left to right, the methyl would be on the 5th carbon

If you number from right to left, the methyl would be on the 2nd carbon

Choose the direction that gives the methyl the lowest carbon number which would be right to left. Therefore, the methyl is on the second carbon.

The name of the compound above would be 2 - methyl hexane (note that a "-" goes between numbers and words)

4. If there are two side groups, number from the direction which gives the lowest number for the first side group encountered.

The methyl would be on the 2nd carbon numbering from right to left and the bromo would be on the 3rd carbon numbering from left to right. The lowest number would be the methyl on the 2nd carbon. Therefore, number from right to left.

To name the compound, list the side groups in alphabetical order.

4 - bromo - 2 - methyl hexane

5. If numbering from each end of the parent chain gives the same initial number (note below that the bromo and the methyl would be on the second carbon), default to the second group of side groups (in this case the chloro side group).

The chloro side group gives the next lowest set of numbers, so we would number from left to right.

We would then name the compound based on the parent name of hexane and order the side groups in alphabetical order.

2 - bromo - 3 - chloro - 5 - methyl hexane

If this doesn't give a second set of lowest numbers, try a third set of lowest numbers, etc.

6. If there is no difference in numbering to find the lowest numbered side group (both the bromo and methyl group below would be on the second carbon),

default to the alphabetical names of the side groups.

Since bromo is alphabetically before methyl, we would number from left to right,

2 - bromo - 5 - methyl hexane

7. If more than one side group is the same, use di (2) - tri (3) - tetra (4) - etc.

We would name this 2,5 - dimethyl hexane.

Notice that we separate numbers using a comma (,) and numbers and words using a dash (-).

HALOGENS

• Br - bromo

• Cl - chloro

• F - fluoro

• I - iodo

The symbol X in an organic compound indicates a halogen.

CLASSES OF HYDROGENS AND CARBONS

Classes of carbons are based on how many carbons they themselves are directly attached to. For example a carbon directly attached to only one other carbon would be called a primary carbon.

Classes of hydrogen are based on the class of carbon they are attached to. For example, if the hydrogen is attached to a primary carbon, that hydrogen would be called a primary hydrogen.

PHYSICAL PROPERTIES

• Alkanes are nonpolar (Van der Waals forces)

• As the molecular weight increases, boiling point increases.

• Except for smaller alkanes, the boiling or melting point increases approximately 20 - 30^oC per carbon increase.

  • C₁ - C₄ = gas

  • C₅ - C₁₇ = liquid

  • C₁₈ and above = solid

• As the branching on a molecule increases (given the same molecular weight), the boiling and melting points decrease.

• Alkanes are less dense than water and are relatively insoluble.

INDUSTRIAL SOURCE

Principle source is petroleum and natural gas.

Two common processes to convert alkanes into other alkanes.

• isomerization - straight chains into branched chains

• cracking - long chains into smaller chains

MECHANISM OF HALOGENATION

REACTIONS OF ALKANES

Halogenation

Combustion

Pyrolysis (cracking)