Unit 2 - Chemistry Notes
, HYDROCARBONS
Certain organic compounds contain only two elements - hydrogen and carbon. These are known as
hydrocarbons. Hydrocarbons are divided into two main classes - aliphatics and aromatics. Aliphatic
hydrocarbons are further divided into four families: alkanes, alkenes, alkynes, and their cyclic analogs
(cycloalkanes, etc.).
Hydrocarbons
Aliphatic Aromatic
Alkanes Alkenes Alkynes Cyclic
A series of compounds in which each member differs from the next member by a constant amount is
called a homologous series and the members of the series are known as homologs.
ALKANES
The family of alkanes forms a homologous series. Each member differs from the next by CH2. Also,
in each alkane, the number of hydrogen atoms equals two more than twice the number of carbon
atoms. Therefore, the general formula for an alkane is CnH2n+2. A list of the first 10 alkanes is given
below.
CH4 methane
C2H6 ethane
C3H8 propane
C4H10 butane
C5H12 pentane
C6H14 hexane
C7H16 heptane
C8H18 octane
C9H20 nonane
C10H22 decane
Note in each case the Greek or Latin prefixes used to indicate the number of carbons in the particular
alkane. Also note that every one ends in -ane. Learn the above list very well as these root names are the
basis of all hydrocarbons.
,Expanded Structural Diagrams
The normal structures (alkanes) of a few alkanes are shown below.
methane propane hexane
Different compounds that have the same molecular formula are called isomers. The three different
isomers of pentane (C5H12) are shown below. Since they are unique compounds, they have unique
names and properties.
pentane 2-methylbutane 2,2-dimethylpropane
(bp 36°C) (bp 28°C) (bp 9oC)
Structures can also be shown using condensed or line diagrams.
Condensed Structural Diagrams
These diagrams all carbon atoms and the number of hydrogens bonded to each.
pentane 2-methylbutane 2,2-dimethylpropane
Line Diagrams
These diagrams show only the carbon-carbon bonds themselves.
pentane 2-methylbutane 2,2-dimethylpropane
In line diagrams, it is assumed that each carbon has four bonds and that hydrogens are attached at the
appropriate locations. The number of hydrogens present must be determined by completing the
bonding capacity of carbon (4).
, Hydrogens can be replaced or substituted by other elements or groups. For example a hydrogen of
methane (CH4) may be replaced by a chlorine atom to form chloromethane (CH3Cl). Branched
alkanes contain substitutions derived from smaller alkanes.
These substituted groups are named by dropping the -ane from the name of the corresponding alkane
and replacing it by -yl. They are known collectively as alkyl groups. The general formula for an alkyl
group is CnH2n+1 since it contains one less hydrogen than the parent alkane with the formula CnH2n+2.
To devise a system of nomenclature that could be used for even the most complicated compounds, the
International Union of Pure and Applied Chemists (IUPAC) developed a system that is used
throughout the world today. Since this system follows much the same pattern for all families of organic
cormpounds, we shall consider it in some detail as applied to alkanes.
ALKANES AND ALKYL RADICALS
Alkane Formula Melting Phase at Room Alkyl Group Formula
Point (oC) Temperature
Methane CH4 -183 gas methyl CH3
Ethane C2H6 -172 gas ethyl C2H5
Propane C3H8 -187 gas propyl C3H7
Butane C4H10 -135 gas butyl C4H9
Pentane C5H12 -130 liquid pentyl(amyl) C5H11
Hexane C6H14 -94 liquid hexyl C6H13
Heptane C7H16 -91 liquid heptyl C7H15
Octane C8H18 -57 liquid octyl C8H17
Nonane C9H20 -54 liquid nonyl C9H19
Decane C10H22 -30 liquid decyl C10H21
Naming Branched Alkanes
Branched alkanes can be named by following a simple sequence of steps:
1. Find the longest continuous chain of carbon atoms (the main chain). It is not necessary that the
longest chain be written in a straight line.
The longest continuous chain contains 5 carbon atoms.
2. Name this chain by adding "-ane" to the stem name (e.g. pentane).
3. Pick out the alkyl groups attached to the main chain.
This is an alkyl group.
, HYDROCARBONS
Certain organic compounds contain only two elements - hydrogen and carbon. These are known as
hydrocarbons. Hydrocarbons are divided into two main classes - aliphatics and aromatics. Aliphatic
hydrocarbons are further divided into four families: alkanes, alkenes, alkynes, and their cyclic analogs
(cycloalkanes, etc.).
Hydrocarbons
Aliphatic Aromatic
Alkanes Alkenes Alkynes Cyclic
A series of compounds in which each member differs from the next member by a constant amount is
called a homologous series and the members of the series are known as homologs.
ALKANES
The family of alkanes forms a homologous series. Each member differs from the next by CH2. Also,
in each alkane, the number of hydrogen atoms equals two more than twice the number of carbon
atoms. Therefore, the general formula for an alkane is CnH2n+2. A list of the first 10 alkanes is given
below.
CH4 methane
C2H6 ethane
C3H8 propane
C4H10 butane
C5H12 pentane
C6H14 hexane
C7H16 heptane
C8H18 octane
C9H20 nonane
C10H22 decane
Note in each case the Greek or Latin prefixes used to indicate the number of carbons in the particular
alkane. Also note that every one ends in -ane. Learn the above list very well as these root names are the
basis of all hydrocarbons.
,Expanded Structural Diagrams
The normal structures (alkanes) of a few alkanes are shown below.
methane propane hexane
Different compounds that have the same molecular formula are called isomers. The three different
isomers of pentane (C5H12) are shown below. Since they are unique compounds, they have unique
names and properties.
pentane 2-methylbutane 2,2-dimethylpropane
(bp 36°C) (bp 28°C) (bp 9oC)
Structures can also be shown using condensed or line diagrams.
Condensed Structural Diagrams
These diagrams all carbon atoms and the number of hydrogens bonded to each.
pentane 2-methylbutane 2,2-dimethylpropane
Line Diagrams
These diagrams show only the carbon-carbon bonds themselves.
pentane 2-methylbutane 2,2-dimethylpropane
In line diagrams, it is assumed that each carbon has four bonds and that hydrogens are attached at the
appropriate locations. The number of hydrogens present must be determined by completing the
bonding capacity of carbon (4).
, Hydrogens can be replaced or substituted by other elements or groups. For example a hydrogen of
methane (CH4) may be replaced by a chlorine atom to form chloromethane (CH3Cl). Branched
alkanes contain substitutions derived from smaller alkanes.
These substituted groups are named by dropping the -ane from the name of the corresponding alkane
and replacing it by -yl. They are known collectively as alkyl groups. The general formula for an alkyl
group is CnH2n+1 since it contains one less hydrogen than the parent alkane with the formula CnH2n+2.
To devise a system of nomenclature that could be used for even the most complicated compounds, the
International Union of Pure and Applied Chemists (IUPAC) developed a system that is used
throughout the world today. Since this system follows much the same pattern for all families of organic
cormpounds, we shall consider it in some detail as applied to alkanes.
ALKANES AND ALKYL RADICALS
Alkane Formula Melting Phase at Room Alkyl Group Formula
Point (oC) Temperature
Methane CH4 -183 gas methyl CH3
Ethane C2H6 -172 gas ethyl C2H5
Propane C3H8 -187 gas propyl C3H7
Butane C4H10 -135 gas butyl C4H9
Pentane C5H12 -130 liquid pentyl(amyl) C5H11
Hexane C6H14 -94 liquid hexyl C6H13
Heptane C7H16 -91 liquid heptyl C7H15
Octane C8H18 -57 liquid octyl C8H17
Nonane C9H20 -54 liquid nonyl C9H19
Decane C10H22 -30 liquid decyl C10H21
Naming Branched Alkanes
Branched alkanes can be named by following a simple sequence of steps:
1. Find the longest continuous chain of carbon atoms (the main chain). It is not necessary that the
longest chain be written in a straight line.
The longest continuous chain contains 5 carbon atoms.
2. Name this chain by adding "-ane" to the stem name (e.g. pentane).
3. Pick out the alkyl groups attached to the main chain.
This is an alkyl group.
