Lecture (2)
1-Alkyl group:
If a hydrogen atom is removed from an alkane, the partial structure that remain is
called alkyl group.
n-alkyl groups are generated by removing hydrogen from an end carbon, while
branched alkyl groups are generated by removing a hydrogen atom from an internal
carbon.
C3
C4
CH3CH2CH2─
propyl
CH3CH2CH3
Propane
CH3 CH CH3
iso-propyl
CH3CH2CH2CH2─
butyl
CH3CH2CHCH3
sec-butyl
CH3
CH3
CH3
CH3CHCH3
CH3 CHCH2─
CH3CH2CH2CH3
Butane
C4
CH3─ C─
CH3
Iso-butane
iso-butyl
tert-butyl
It should be noted that primary carbon (1˚)is bonded to another carbon, secondary
carbon (2˚) is bonded to two other carbons, tertiary carbon(3˚) and quaternary carbon
(4˚) are bonded to 3 and 4 other carbons respectively.
H
H
R
R
R─ C─H
R─ C ─H
R─ C ─H
R─ C ─R
H
R
R
R
Primary Carbon (1˚)
Bonded to one other
Carbon
Secondary Carbon(2˚)
Bonded to two other
Carbons
Tertiary Carbon(3˚) Quaternary Carbon (4˚)
Bonded to three other Bonded to four other
Carbons
Carbons
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Practice problem:
Propose structures for two isomers with the formulaC2H6O
Strategy: we know that carbon forms four bonds, oxygen forms two, and hydrogen
forms one
Put the piece together by trial and error, along with intuition.
Solution: there are two possibilities:
H
H
H─C─C─ O─H
H H
H
H
H─C─O─C─ H
H
H
2-Naming branched-chain alkanes:
A chemical name has the three parts in the IUPAC system of nomenclature: prefix,
parent and suffix. The parent name selects a main part of the molecule and tells how
many carbon atoms are in the part, the suffix identifies the functional-group family
that the molecules belongs to, and the prefix specifies the location(s) of various
substituents on the main parts:
PREFIX ── PARENT ── SUFFIX
Where are the substituents?
How many carbons?
What family
As we cover new functional groups in later chapters, the applicable IUPAC rules of
nomenclature will be given. Let’s see how to name branched- chain alkanes. all but
the most complex branched-chain alkanes can be named by the following steps:
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STEP (1): find the parent hydrocarbon
(a)-find the longest continuous carbon chain in the molecule and use the name of that
chain as the parent name. the longest chain may not always be obvious; you may have
to “turn corners”
CH2CH3
CH3CH2CH2 CH─CH3
named as substituted hexane
(b)-if two chains of equal length are present, choose the one with the larger number of
branched points as the parent:
CH3
CH3
CH3 CH CHCH2CH2CH3
CH3 CH─CHCH2CH2CH3
CH2CH3
Named as hexane
With two substituents
CH2CH3
NOT
hexane with one substituents
STEP (2) number the atoms in the main chain
Beginning at the end nearer the first branched points, number each carbon atom in the
parent chain:
1
CH3
7
CH3
2
CH2
6
CH2
CH3─ 3CH4CH─CH2CH3
5
CH3─ 5CH4CH─CH2CH3
NOT
CH26CH27CH3
3
CH22CH21CH3
The first branched occurs at C3 in the proper system of numbering but at C4 in the
improper systems.
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STEP (3) identify and number the substituents
Assign a number to each substituents according to its points of attachment on the
parent chain. If there are two substituents on the same number. There must always be
as many numbers in the name as there are substituents.
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CH29CH3
CH3
CH2CH3
substituents
On C3,CH2CH3(3-ethyl)
7
6
5
4
3
2
1
CH3─ CH CH2 CH2 CH─ CH CH2 CH3
On C4,CH3(4-methyl)
On C7,CH3(7-methyl)
CH3
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substituents
CH35CH2─4C─ 3CH22 CH1CH3
CH2
On C2,CH3(2-methyl)
On C4,CH3(4-methyl)
On C4,CH2CH3(4-ethyl)
CH3
CH3
STEP(4) write the name as a single word
Use hyphens to separate the various prefixes and commas to separate numbers. If two
or more different side chains are present, cite them in the alphabetical order. if two or
more identical side chains are present, use one of the prefixes di-, tri-, and so forth.
Don’t use these prefixes for the alphabetizing, through.
2
6
CH21CH3
CH3
CH35CH24CH2 3CH─CH3
1
CH32CH3CH4CH52CH26CH3
CH2CH3
3-methyl hexane
3-ethyl-2-methyl hexane
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1
2
CH3
CH2
8
CH3─3CH 4CH─CH2CH3
5
CH39CH2
CH3
CH2CH3
CH3─7CH6CH25CH2 4CH─ 3CH2CH21CH3
CH26CH27CH3
4-ethyl-3-methyl heptane
3-ethyl-4,7-dimethyl nonane
Practice problems:
(1)-What is the IUPAC name of the following alkane?
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CH28CH3
CH3
CH36CH5CH24CH23CH22CH1CH3
Strategy: the molecule has a chain of eight carbons (octane) with two methyl
substituents. Numbering from the end of nearer the first methyl substituents indicates
that the methyls are at C2 and C6
Solution: the alkane is named 2,6-dimethyl octane
(2)-Draw the structure of 3-isopropyl-2-methyl hexane
Stategy:
First: look at the parent name (hexane) and draw its carbon structure:
C─C─C─C─C─C
Next: find the substitutions (3-isopropyl and 2-methyl), and place them on the proper
carbons
CH3CHCH3
Isopropyl group at C3
1C─2C─3C─4C─5C─6C
CH3
methyl group at C2
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Finally: add hydrogen to complete the structure
Solution:
CH3CHCH3
CH3─CH─CH─CH2─CH2─CH3
CH3
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3-isopropyl-2-methyl hexane
PROBLEM (1):
Give IUPAC names for the following alkanes:
a)The three isomers of C5H12
CH3
(b) CH3CH2CH CHCH3
CH2CH3
CH3
CH3
c) CH3CHCH2CHCH3
CH3
d) CH3
C
CH2CH3
CH2CH2 CHCH3
CH3
PROBLEM (2):
Draw structures corresponding to the following IUPAC names :
(a) 3-Ethyl-4,4-dimethylheptane
(b) 3,4-Dimethylnonane
(c) 2,2-Dimethyl-4-propyloctane
(d) 2,2,4-Trimethylpentane
PROBLEM (3):
Name the following alkane:
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