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CH221 CLASS 5
CHAPTER 3: ORGANIC COMPOUNDS: ALKANES AND CYCLOALKANES
Synopsis. Class 5 begins with a brief review of functional groups, followed by a
preliminary discussion of the alkanes, including isomerism, alkyl groups, primary,
secondary carbons (etc) and finally, nomenclature.
Functional Groups
There are presently (2005) almost 20 million known organic compounds in the
CAS registry – each with its own physical and chemical properties. Luckily, like
the elements of the periodic table, all organic compounds can be arranged into a
relatively small number of families whose chemistry is reasonably predictable.
The compounds of each of these families are characterized by the presence of
particular groupings of atoms, known as functional groups. It is the functional
group that determines the chemical behavior of a family of compounds and the
chemistry of every organic molecule, regardless of size or complexity, is related
to the functional group it contains. Compare the reaction between the simple
alkene, ethylene, and bromine with the reaction of the more complex cycloalkene,
menthene, with the same reagent:
H
H
C
Br2
C
H
Br
H
CH3
H
C
C Br
H
H
CH3
Br2
CH(CH3)2
H
Br
Br
CH(CH3)2
Both molecules possess an alkene C=C
functional group: both undergo an addition
reaction
A summary of the common functional groups, according to type, is given below
and on the next page.
Family name
Functional group Simple example
Alkane
C-C
and
bonds only
Alkene
Arene
Alkyne
C=C
Aromatic C=C
CH2=CH2
C6H6
Ethene (ethylene)
Benzene
CHCH
CH3CH2Br
Ethyne (acetylene)
Halide
CC
R-Halogen
Alcohol
R-OH (hydroxyl)
CH3CH2OH
Ethanol
alcohol)
Ether
R-O-R
CH3OCH3
Amine
>N-
CH3NH2
Nitrile (cyanide)
R-CN
CH3CN
Dimethyl
ether
(methoxymethane)
Methylamine
(aminomethane)
Ethanenitrile
(acetonitrile, methyl
cyanide)
Nitroso
Nitro
Thiol (mercaptan)
R-N=O
R-NO2
R-SH
C6H5NO
CH3NO2
CH3CH2SH
Nitrosobenzene
Nitromethane
Ethanethiol (ethyl
mercaptan)
CH3-S-CH3
Dimethyl sulfide
Sulfide (thioether) R-S-R
C-H CH3CH3
Continued on
Next page!
Name
Ethane
Bromoethane
(ethyl bromide)
(ethyl
Family name
Functional group Simple example
Sulfoxide
Sulfone
Aldehyde
R2S=O
R2S(=O)2
R-CH=O
(CH3)2SO
(CH3)2SO2
CH3CHO
Ketone
R2C=O
(CH3)2CO
Carboxylic acid
R-C=O
OH
CH3COOH
Ethanoic
(acetic acid)
Ester
R-C=O
OR
CH3COOC2H5
Ethyl ethanoate
(ethyl acetate)
CH3CONH2
Ethanamide
(acetamide)
CH3COCl
Ethanoyl chloride
(acetyl chloride)
(CH3CO)2O
Ethanoic
anhydride (acetic
anhydride)
Dimethyl
Amide
Carboxylic
chloride
Carboxylic
anhydride
R-C=O
NR2
acid R-C=O
Hal
acid (R-C=O)2O
Carbonate ester
(R-O)2C=O
(CH3O)2CO
Name
Dimethyl sulfoxide
Dimethyl sulfone
Ethanal
(acetaldehyde)
Propanone
(acetone)
acid
carbonate
The Alkanes
The alkanes contain carbon and hydrogen only, bonded via single bonds only.
They are thus called saturated hydrocarbons because of their elemental
content and because their molecules possess the maximum number of hydrogen
atoms per carbon atom. The general formula is CnH2n+2. Another general name
for alkanes, or groups derived from alkanes (alkyl groups – see later), is
aliphatic, from the Greek aleiphos, meaning “fat”. This arises from the fact that
many long chain alkyl groups have been found in animal fats.
CH2
O
CO
(CH2)16CH3
CH
O
CO
(CH2)16CH3
CH2
O
CO
(CH2)16CH3
Glycerol
Stearic acid
This is a typical component of animal fat:
it is a triglyceride ester called
glyceryl tristearate. The long chain part
is derived from stearic acid, a "fatty acid"
Alkane Isomers
The alkanes can be built from the simplest member, methane (CH 4) by formally
inserting successive CH2 (methylene) units in C-H bonds. For the first two
alkanes after methane - ethane (C2) and propane (C3) – there is only one way
that this can be done:
H
H
C
H
Methane
H
H
H
C
H
Ethane
H
CH2
H
H
C
CH2
CH2
H
H
Propane
However, beyond propane, there are at least two ways in which to extend the
carbon chain: either in a linear fashion or in a branched fashion, as shown for
C4H10 and C5H12 overleaf.
CH3CHCH2
CH3
extend
by CH2
H
CH3CH2CH2CH3
H
and
Butane
CH3CHCH3
2-Methylpropane
(isobutane)
Propane
C4H10
C 3H 8
extend
by CH2
CH3CH2CH2CH3
CH3
CH3CH2CH2CH2CH3 and CH3CH2CHCH3
Pentane
CH3
CH3CHCH3
Butane and
isobutane
C4H10
extend
by CH2
2-Methylbutane
CH3
CH3CCH3
CH3
2,2-Dimethylpropane
C5H12
Butane and pentane, with no branches, are called straight chain or normal (n)
alkanes, whereas all the others are called branched-chain alkanes.
Compounds that have the same molecular formula are called isomers. For
example, butane is the straight chain isomer and 2-methylpropane is the
branched-chain isomer with the molecular formula C4H10. Isomers like these,
whose atoms are connected in different ways are called constitutional isomers
or structural isomers. You will see later that there is another, more subtle, type
of isomerism, called stereoisomerism. There are three main types of
constitutional isomerism, as summarized in the table on the next page.
Alkyl Groups
A general name for a hydrocarbon group in organic molecules is alkyl group.
Saturated alkyl groups are named by replacing the ending –ane of the parent
hydrocarbon with the ending –yl.
E.g.
CH4
methane
CH3methyl
As in
CH3-I
methyl iodide or
iodomethane
CH3CH2CH2CH3
butane
CH3CH2CH2CH2butyl or n-butyl
As in
CH3CH2CH2CH2-Li
n-butyllithium
Primary, Secondary, Tertiary and Quaternary Atoms
By inspection of some of the foregoing examples, it will be noticed that in the
alkyl groups of organic molecules there are different types of carbon atoms,
depending on the number of C-C bonds to those carbon atoms. This gives rise to
the nomenclature primary (1o), secondary (2o), tertiary (3o) and quaternary (4o),
as illustrated below (R designates unspecified groups) for hydrocarbon groups.
H
R
C
H
H
R
C
H
R
R
H
R
R
C
H
R
C
R
R
R
1 C-C bond
2 C-C bonds
3 C-C bonds
4 C-C bonds
primary (1o)
secondary (2o)
tertiary (3o)
quaternary (4o)
methyl
methylene
methine
These terms are routinely applied to all organic compounds, not just the
hydrocarbon groups above:
OH
CH3CH2
1o
OH
CH3
CH
2o
CH3
CH3
CH3
C
Cl
3o
CH3
CH3
CH3
+
N
4o
CH3
CH3
The terms are important because, although within a family, functional groups at
1o, 2o and 3o carbon atoms will have a similar chemistry, it will not be exactly the
same for each. For example, 1-propanol (1o) may be more or less reactive in
certain reactions than 2-propanol (2o).
Naming Alkanes
Trivial and Systematic Nomenclature
The original names of organic compounds are now known as trivial names.
These names describe some aspect of the compound, often its origin, and are
non-systematic (i.e. they cannot be logically derived from the molecular structure).
Many compounds are still preferentially given trivial names, examples including
formic acid, derived from ants (Latin: Formica) and salicylic acid, which was first
discovered as a component of the bark of willow trees (Latin: Salix). As more and
more complex organic compounds were discovered, the need for a more
systematic kind of naming system became urgent: a system that will allow the
organic chemist to draw the structure of the molecule from its name, and viceversa. Nowadays, the most widely used naming system is that of the
International Union of Pure and Applied Chemistry (IUPAC). IUPAC names are
generally composed of at least two, and usually three parts:
PREFIX
Gives type and
positions of
substituents
ROOT
SUFFIX
Gives number
of carbon
atoms
Indicates
family
The names of the first four straight chain alkanes take their roots from trivial
sources: methane, ethane, propane and butane. After butane the root is derived
from the Greek for the number of carbon atoms: pentane (C5), hexane (C6),
heptane (C7), etc.
Naming more complex (branched) alkanes requires more thought: carry out the
following steps.
1
Find the parent hydrocarbon: the longest continuous
chain of carbon atoms gives the ROOT of the name.
E.g.
2 1
CH2CH3 Name is based on
hexane
CH3CH2CH2CHCH3
6 5
4 3
If two different chains of equal length are present,
choose the one with the greater number of branch
points.
CH3
NOT
5
6
3
1
4
E.g. CH3CHCHCH2CH2CH3
CH3
2
CH3CHCHCH2CH2CH3
CH2CH3
CH2CH3
Name based on hexane
with two substituents
2
Identify and number the substituents. Number the
main chain so that substituents take the lowest
possible numbers: give EACH substituent a number
E.g.
Substituent: 3-methyl
CH3
6 5 4
2 1
CH3CH2CH2CHCH2CH3 Root: hexane
3
CH3
1
3 4 5 6
CH3CHCHCH2CH2CH3
2
CH2CH3
Substituent: 2-methyl
Root: hexane
Substituent: 3-ethyl
3
Write the name as one word, with hyphens (-) between
numbers and words, and with substituents in
alphabetical order.
E.g. For the above examples, 3-methylhexane
and 3-ethyl-2-methylhexane, respectively.
If there are two substituents of the same type, use the
prefix di (for three substituents, tri, etc), and a comma.
2 CH
33 4 5
E.g. CH3CCH2CH2CH3
1
CH3
2,2-dimethylpentane
For alkanes with a more complex (e.g. branched) substituent,
the substituent is named as if it were a compound itself. Its
name appears in brackets.
E.g.
1
CH3
3
6
10
CH3CHCHCH2CH2CHCH2CH2CH2CH3
2
CH3
CH2CHCH3
} 2-methylpropyl
CH3 2
2,3-dimethyl-6-(2-methylpropyl)decane
Class Question
1. Name the following alkanes according to the IUPAC system.
8
CH2CH3
2 1
(a) CH3CHCH2CH2CH2CHCH3
6
CH3
2,6-dimethyloctane
(b)
CH3
1 2CH3
CH3CCH2CH2CHCH2CH3
5
7
CH3
2,2,5-trimethylheptane
2. Draw structures corresponding to the following IUPAC names.
(a) 2,2-Dimethyl-4-propyloctane
(b) 3-Ethyl-4,4-dimethylheptane
(a)
2,2-dimethyl-4-propyloctane
8 carbon atoms in longest chain
two CH3 branches
at position 2
1
CH3
one propyl branch
at position 4
CH3
CH2CH2CH3
C 2 CH2
CH
4
CH2
CH2
CH2
8
CH3
CH3
(b)
3-ethyl-4,4-dimethylheptane
7 carbon atoms in longest chain
one ethyl branch
at position 3
CH3
1
CH3
CH2
two CH3 branches
at position 4
CH2
CH
2
CH3
3
C CH2
CH3
CH2
7
CH3