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More about alkanes

Naming them

linear or straight chain alkanes - each carbon bonded to a maximum of two other carbons

1-carbon

methane

CH4

2-carbons

ethane

CH3CH3

3-carbons

propane

CH3CH2CH3

4-carbons

butane

CH3CH2CH2CH3 = CH3(CH2)2CH3

5-carbons

pentane

CH3(CH2)3CH3

6-carbons

hexane

CH3(CH2)4CH3

7-carbons

heptane

CH3(CH2)5CH3

8-carbons

octane

CH3(CH2)6CH3

9-carbons

nonane

CH3(CH2)7CH3

10-carbons

decane

CH3(CH2)8CH3

Branched alkanes - certain carbons are bonded to three or four carbons

Branched alkanes can be viewed as straight chain alkanes in which hydrogens along the chain have been replaced by "substituents"

Above, the CH3 replaces a hydrogen and is termed a substituent.

Alkyl groups - When substituents are hydrocarbon groups the substituents are called alkyl groups

The names of alkyl groups follow the names of their parent alkanes  Replace the ane ending with yl

1-carbon

methyl

-CH3

2-carbons

ethyl

-CH2CH3

3-carbons

propyl

-CH3CH2CH3

4-carbons

butyl

-CH2(CH2)2CH3

5-carbons

pentyl

-CH2(CH2)3CH3

6-carbons

hexyl

-CH2(CH2)4CH3

7-carbons

heptyl

-CH2(CH2)5CH3

8-carbons

octyl

-CH2(CH2)6CH3

9-carbons

nonyl

-CH2(CH2)7CH3

10-carbons

decyl

-CH2(CH2)8CH3

Using substituent names and the names of linear alkanes, we can name branched alkanes:

Ex:  

1. Find the longest chain in the molecule.  This is considered the parent and for alkanes simply give the appropriate linear alkane name.:

longest chain 5-carbons long - pentane

2. Identify the substituent(s)

methyl substituent

3. Number the carbons in the main chain sequence, starting at the end that will give the smallest number at the alkyl substituent.

The methyl group is said to be at the "2" position

4. Construct the name from the above information as follows

General form:  #-substituentparent, where # = the substituent position

2-methylpentane

Ex:

2-ethylpentane

5. What if there is more than one substituent?

Use di, tri, tetra, penta, hexa, etc... and numbers of positions

2,3-dimethylpentane

6. What if the substituents are different, list each one separately with appropriate numbering.  Ordering of substituent list is alphabetical (the prefixes di,tri, etc... are ignored for purposes of alphabetization).

Parent name: octane

Substituents: methyl, propyl, ethyl

4-ethyl-2-metyl-3-propyloctane

Another example:

 

Parent name: decane

Substituents: ethyl, methyl (two of them)

6-ethyl-4,5-dimethylheptane

dimethyl follows ethyl because the di is ignored in alphabetization

 

Cycloalkanes - we have talked about linear and branched alkanes, but there is one more common type - cycloalkanes:

cyclopropane
cyclobutane
cyclopentane
cyclohexane

What do cycloalkanes look like?

cyclohexane - boat vs. chair comformation - also model in class

Other types of alkanes?

How bout this isomer of C20H20 - dodecahedrane

The properties of alkanes depend a great deal on the number of carbons and branching pattern. 

Increase number of carbons, both melting point and boiling point increases.

Branching generally lowers the boiling point

Isomers of C4H10  Boiling Point
-0.5 oC
-12 oC
Isomers of C5H12 Boiling point
36 oC
28oC

 

Effect of branching on melting point more difficult to predict.  For a given number of carbons, the more symmetric molecule will have the higher melting point

Isomers of C5H12 m.p.
-129.7oC
-159.9oC
-16.5oC