Transcript Details of Nomenclature Rules.

Drawing Chemical Structures
•
Condensed structures: C-H and C-C and single bonds aren't shown but
understood
– If C has 3 H’s bonded to it, write CH3
– If C has 2 H’s bonded to it, write CH2; and so on. Sometimes bonds between
carbons aren't shown in condensed structures—here the CH3, CH2, and CH units
are simply drawn next to one another, but some bonds are added for clarity The
compound called 2-methylbutane, for example, is written as follows:
Constitutional Isomers
• Isomers that differ in how their atoms are arranged in chains are called
constitutional isomers
• Compounds other than alkanes can be constitutional isomers of one
another
• They must have the same molecular formula to be isomers
Alkanes and Alkane Isomers
• Alkanes: Compounds with C-C single bonds and C-H bonds
only (no functional groups)
• Connecting carbons can lead to large or small molecules
• The formula for an alkane with no rings in it must be CnH2n+2
where the number of C’s is n
• Alkanes are saturated with hydrogen (no more can be added
• They are also called aliphatic compounds
Alkane Isomers
• CH4 = methane, C2H6 = ethane, C3H8= propane
• The molecular formula of an alkane with more than three carbons can
give more than one structure
– C4 (butane) = butane and isobutane
– C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane
• Alkanes with C’s connected to no more than 2 other C’s are straightchain or normal alkanes
• Alkanes with one or more C’s connected to 3 or 4 C’s are branchedchain alkanes
Names of Small Hydrocarbons
Formula Name
(CnH2n+2)
1
2
3
4
Methane
Ethane
Propane
Butane
CH4
C2H6
C3H8
C4H10
5
6
7
Pentane
Hexane
Heptane
C5H12
C6H14
C7H16
8
9
10
Octane
Nonane
Decane
C8H18
C9H20
C10H22
No. of
Carbons
Alkane Structures
Planar
condensed-line
zig-zag
condensed
A perfect 3-D zig-zag for
n-butane
Isomers: butane (C4H10)
Isomers: pentane (C5H12)
Which are isomeric?
To tell the isomers apart you can (should):
• Count total # of carbons and classify them
as 1°, 2°, 3°, or 4°. (sometimes
isomers give same count! but this is quick
and dirty.)
• Build models (must practice!)
• Name (foolproof!).
General Nomenclature Rules.
1. Find a parent chain (longest continuous).
2. Number the main chain atoms so as to start
nearer to the first branch.
3. Name substituents and assign numbers
appropriate to their positions along the main
chain.
4. Write a name as a single word.
5. Complex (branched) substituents are named as if
they were a compound.
Details of Nomenclature Rules.
1a. Find a parent chain (longest continuous). Always try
all possible permutations!
2
2
1
3
4
1
5
3
4
6
5
branched substituent
smaller chain
2
1
3
6
4
5
correct
Details of Nomenclature Rules.
1b. Find a parent chain (longest continuous). Maximize
the number of branch points, i.e. avoid substituents
that are branched (if possible).
correct
branched substituent
Details of Nomenclature Rules.
2a. Number the main chain atoms so as to start nearer to
the first branch.
1
6
4
3
7
5
8
8
3
2
2
1
correct
5
4
6
7
Details of Nomenclature Rules.
2b. Number the main chain atoms so as to start nearer to the first
branch. If the substituents are at the same distances from chain
ends give lower number to the substituent with the alphabetical
priority of the first letter.
5
6
3
4
2
1
2
1
5
3
6
4
Note that the ethyl group will get position number 3, not the methyl.
Details of Nomenclature Rules.
2c. Number the main chain atoms so as to start nearer to the first
branch. If there are more than two substituents, begin numbering
at the end nearer to the two closest branch points.
8
1
7
2
2
4
6
3
5
7
5
3
6
1
8
4
Details of Nomenclature Rules.
4a. Write a name as a single word. Pay attention to
hyphens and commas.
7
2
1
6
5
3
6
4
3
5
2
1
4
3-ethyl-4-methylhexane
2,3-dimethyl-5-ethylheptane
Details of Nomenclature Rules.
4b. Write a name as a single word. Branched substituents are named
as a separate molecule. The numbering within the name always
starts nearest to the attachment point of the complex
substituent.
2
5
7
1
6
4
1
3
2
4-(1-methylethyl)heptane
4
1
2
6
3
7
5
10
8
9
2,3-dimethyl-6-(2-methylpropyl)decane
5
6
4
3
10
7
9
2
8
1
6-(1,2-dimethylpropyl)-3-ethyldecane