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:


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

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


INDUSTRIAL SOURCE

Principle source is petroleum and natural gas.

Two common processes to convert alkanes into other alkanes.


MECHANISM OF HALOGENATION

 


REACTIONS OF ALKANES

 

Halogenation


Combustion


Pyrolysis (cracking)