Transcript Organic nomenclature

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Organic nomenclature
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What is it?
• The systematic naming of an organic compound
generally requires the identification and naming of a
parent structure.
• This name may then be modified by prefixes,
infixes, and, in the case of a parent hydride,
suffixes, which give the precise structural changes
required to generate the actual compound from the
parent structure.
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Naming straight-chain alkanes
• Naming of straight chain alkanes (alkanes that do not
branch) is a straightforward process.
• To give an alkane a name, a prefix indicating the number of
carbons in the molecule is added to the suffix ane,
identifying both the kind of molecule (an alkane) and how
many carbons the molecule has (the prefix).
• The name pentane, for example, tells you that the molecule
is an alkane (thus the ane ending) and that it has five
carbons (pent indicates five).
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• Prefixes for alkanes that have 1-4 carbons are
rooted historically.
• These are methane, ethane, propane, and butane,
respectively.
• On the other hand, for 5 carbons and up a prefix
derived from greek is given. (An easy way to
remember the first four names is the anagram Mary
eats peanut butter, standing for methane, ethane,
propane, butane).
• Learning the prefixes for up to twelve carbons is a
good idea.
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Number of Carbons
1
2
Prefix
Methane
Ethane
Structure
CH4
CH3CH3
3
4
5
6
Propane
Butane
Pentane
Hexane
CH3CH2CH3
CH3(CH2)2CH3
CH3(CH2)3CH3
CH3(CH2)4CH3
7
8
9
Heptane
Octane
Nonane
CH3(CH2)5CH3
CH3(CH2)6CH3
CH3(CH2)7CH3
10
11
12
Decane
Undecane
Dodecane
CH3(CH2)8CH3
CH3(CH2)9CH3
CH3(CH2)10CH3
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Naming branched alkanes
• The nomenclature becomes more complex if the
alkane branches. In such a case, there are several
rules that you must follow to give the alkane the
correct name.
• Find the longest chain of carbons in the molecule.
The number of carbons in the longest chain
becomes the parent name.
• After finding the parent chain, you number the
parent chain starting with the end nearest the first
substituent (a substituent is any fragment that juts
off the main chain).
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• Next, determine the names of all substituents.
Substituents are named as if the piece were a separate
molecule, except that the suffix of yl is used rather than
ane.
• Thus, a two-carbon substituent would be an ethyl
substituent (not an ethane substituent).
• Put the substituents in alphabetical order (ie. ethyl
before methyl) in front of the parent name.
• Next, identify the positions of all substituents in the
name by placing the carbon number where the
substituent attaches to the parent chain in front of it.
For example, 2-methylheptane indicates that a methyl
substituent is attached to the number 2 carbon.
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Try this one…
a)
• Which can be drawn as:
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Step 1
• Find the longest carbon chain in the molecule.
– First, begin by finding the parent chain in the molecule--that is,
the longest possible chain of connecting carbons.
– Note that the parent chain is not necessarily the chain that
simply follows from left to right.
– For example, if you were to count the number of carbons
directly from left to right in this molecule you would get 7
carbons. This is not the parent chain, however!
– If you start at the left and then count up where the molecule
branches, you find that there are 8 carbons by taking this route.
– This is is the longest chain and thus the parent chain is octane.
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Step 2
• Number the parent chain.
– The second step is to number the carbons in the parent chain
starting at the end closest to the first substituent.
– It is important to number the molecule from the correct end (in
other words, in this example do you number the alkane from
right to left or left to right).
– Following this rule, on this molecule you number from right to
left, as the 2-carbon substituent is closer to that end.
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8
7
6
2
1
4
3
5
1
2
3
7
8
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5
6
4
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Step 3
• Name all the substituents.
– You then identify the names of the substituents.
– In this case, the only substituent is a 2 carbon group at
the number 4 carbon.
– This is an ethyl group.
4
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Step 4
• Put the substituents in alphabetical order.
– The next step is to put the substituents in alphabetical
order (ie. ethyl before methyl) but since there is only one
substituent this is unnecessary.
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Step 5
• Locate the substituent on the parent change by
giving it a number.
– Thus, the proper nomenclature of this alkane is
4-ethyloctane.
– Note that a dash is used to separate the number from
the substituent.
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Your turn…
• What is this?
• And the structural formula is?
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Your turn…
• What is this?
• And the structural formula is?
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Your turn…
• What is this?
• And the structural formula is?
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• The prefixes di, tri, tetra etc., used to designate several groups
of the same kind, are not considered when putting the
substituents in alphabetical order.
Number of same groups
Prefix
1
mono
2
di
3
tri
4
tetra
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Your turn…
• What is this?
• And the structural formula is?
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Your turn…
• What is this?
• And the structural formula is?
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A little harder…
• Butanol… C4H10O
– What about these 4 structures?
– How many C, H, O?
– And what are they all called?
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A little harder…
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Homework…
•
Draw and name all the possible structures with
the formula:
1. C3H7Br (Br = bromo)
2. C5H12
•
Draw the following compounds:
–
–
–
3-ethyl-2,4-dimethylheptane
2,2,4-trimethylpentane
4-ethyl-3,5-dimethylheptane
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