Chemistry 20
Chapters 2
Alkanes
Hydrocarbons: a large family of organic compounds and they contain only carbon and
hydrogen.
Hydrocarbons are divided into two groups:
1. Saturated hydrocarbon: a hydrocarbon that contains only carbon-carbon single bonds
(alkanes, also called Aliphatic hydrocarbons). Saturated in this context means that each
carbon in the hydrocarbon has the maximum number of hydrogen atoms bonded to it.
2. Unsaturated hydrocarbon: a hydrocarbon that contains one or more carbon-carbon
double bonds, triple bonds, or benzene rings.
Alkanes: They are saturated hydrocarbons (they have only carbon-carbon single bonds). The
molecular formula of this group is CnH2n+2 (n is the number of carbon atoms).
Naming the unbranched alkanes: Chemists have adopted a set of rules established by the
International Union of Pure and Applied Chemistry (IUPAC). The IUPAC name for an alkane
consists of two parts: 1. A prefix that shows the number of carbon atoms (meth-, eth-, prop-,
but-, pent-, hex-, hept-, oct-, non- and dec-). 2. The suffix “-ane”.
CH4 Methane
C2H6 Ethane
C3H8 Propane
C4H10 Butane
Note: we can represent the formula of an organic compound by the molecular formula or by
the structural formula. Structural formulas can be represented by three ways: Expanded
(Complete) structural formula, Condense structural formula, and Line-angle formula:
Expanded (complete) structural formula: to represent this model, the carbon atom is shown
attached to the hydrogen atoms (we show all connections).
Dr. Behrang Madani
Chemistry 20
Mt SAC
Condensed structural formula: to represent this model, the hydrogen atoms are grouped
with their carbon atom. The number of hydrogen atoms is written as subscript.
CH3-CH2-CH2-CH2-CH3 or CH3-(CH2)3-CH3
Line-angle formula: is a form of the structural formula. A line represents a carbon-carbon
bond and a vertex represents a carbon atom. A line ending in space represents a –CH3 group.
CH3-CH2-CH3
Propane
CH3-CH2-CH2-CH2-CH3
Pentane
Substituent groups: they are the branches in organic compounds. A substituent group
derived from an alkane by removal of a hydrogen atom is called an alkyl group (R-). Alkyl
groups are named by dropping the “-ane” from the name of the parent alkane and adding the
suffix “-yl”.
CH3- Methyl
C2H5- Ethyl
C3H7- Propyl
Note: some substituents derived from other elements or other spices than alkanes:
-F Fluoro
-Cl Chloro
-OH
Hydroxyl
-NO2 Nitro
Naming branched alkanes: 1. Write the alkane name of the longest continuous chain of
carbon atoms (parent chain or root chain). 2. Number carbon atoms starting from the end
nearest substituent. 3. Give the location and name of each substituent (alphabetical order) as a
prefix to the alkane name (main chain). Use a hyphen to connect the number to the name.
CH3
CH3-CH-CH2-CH3
1
2 3
4
In this example, the longest chain is Butane. We number carbon atoms starting from the end
nearest substituent (left to right). The location of subtituent is 2 and its name is methyl.
Therefore, the complete name of this compound is 2-Methylbutane.
Cl CH3
CH3-CH2-CH-CH-CH3
5
4
3 2 1
3-Chloro-2-methylpentane
Constitutional isomers: compounds with the same molecular formula but a different
connectivity of their atoms (different structural formulas).
CH3
Butane: C4H10
CH3-CH2-CH2-CH3
Methylpropane: C4H10
CH3-CH-CH3
Note: Constitutional isomers are different compounds and have different physical and
chemical properties.
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Chemistry 20
Mt SAC
Cycloalkane: a saturated hydrocarbon that contains carbon atoms bonded to form a ring. A
hydrocarbon that contains carbon atoms joined to form a ring is called a cyclic hydrocarbon.
=
Cyclohexane
Cyclopentane
Naming of Cycloalkanes: to name a cycloalkane, prefix the name of the corresponding openchain alkane with “cyclo-”, and name each substituent on the ring. If there is only one
substituent on the ring, there is no need to give it a location number. If there are two or more
substituents, number the ring beginning with the substituent of lower alphabetical order.
OH
Cl
CH3
Cl
Br
Chlorocyclohexane
1-Bromo-2-methylhexane
NO2
1-Chloro-3-hydroxyl-2nitrocyclopentane
Conformation: any three-dimensional arrangement of atoms in a molecule that results from
rotation about a single bond.
Note: There is, in fact, free rotation about each carbon-carbon bond in an alkane. As a result,
even a molecule as simple as ethane has an infinite number of possibility three-dimensional
shapes, or conformations. The majority of molecules in any sample will be in the least
crowded conformation; the fewest will be in the most crowded conformation.
Least crowded conformation
Most crowded conformation
Note: The most stable conformation of cyclopentane is the “envelope conformation”. In it,
four carbon atoms are in a plane, and the fifth carbon atom is bent out of the plane. All bond
angles in cyclopentane are approximately 109.5°.
Note: The most stable conformation of cyclohexane is the “chair conformation”, in which all
bond angles are approximately 109.5°.
Cis-Trans Isomerism in Cycloalkanes: cycloalkanes with substituents on two or more
carbons of the ring show a type of isomerism called cis-trans isomerism. These iomers have
the same molecular formula and the same connectivity of their atoms, but a different
arrangement of their atoms in space (because of restricted rotation around the carbon-carbon
single bonds of the ring). The prefix cis indicates that the substituents are on the same side of
the ring; the prefix trans indicates that they are on opposite sides of the ring.
Dr. Behrang Madani
Chemistry 20
Mt SAC
CH3
H
CH3
H
CH3
H
or
H3 C
or
H
H3 C
CH3
trans-1,4-Dimethylcyclohexane
CH3
CH3
cis-1,4-Dimethylcyclohexane
Stereoisomers: isomers that have the same connectivity of their atoms but a different
orientation of their atoms in space. Because cis-trans isomers differ in the orientation of their
atoms in space, they are stereoisomers.
Physical properties of alkanes: 1. They are nonpolar compounds (the electronegativity
difference between carbon and hydrogen is 2.5-2.1 = 0.4). 2. The only interactions between
their molecules are the very week London dispersion forces. 3. They are insoluble in water
(because water is polar) and they are soluble in nonpolar organic compounds. 4. They have
the lower density than water (their densities is between 0.7 and 0.8 g/mL). 5. They have the
low boiling points and the low melting points. 6. They can be gases (with 1 to 4 carbon
atoms), liquids (with 5 to 17 carbon atoms), or solids (with 18 or more carbon atoms).
Note: In general, both boiling and melting points of alkanes increase with increasing
molecular weight (the number of carbon atoms).
Note: In general, both boiling and melting points of alkanes decrease with increasing the
number of branches (for alkanes with the same molecular weights). As branching increases,
the alkane molecule becomes more compact and its surface area decreases (London dispersion
forces act over a smaller surface area).
Chemical properties of alkanes: in general, they have a low reactivity (inert). Their most
important reactions are combustion (reaction with oxygen) and halogenation (reaction with
halogens).
Combustion: alkanes react with oxygen (they are oxidized). In this reaction, CO2, H2O, and
energy (heat) are produced.
CH4 + 2O2 → CO2 + 2H2O + energy (heat)
Halogenation: alkanes react with chlorine and bromine if we heat the mixture or if we expose
the mixture to light (in the dark at room temperature, nothing happens).
CH4 + Cl2
heat or light
CH3Cl + HCl
Chloromethane
If chlromethane is allowed to react with more chlorine:
heat or light
CH3Cl + Cl2
CH2Cl2 + HCl
Dichloromethane
Dr. Behrang Madani
Chemistry 20
Mt SAC
CH2Cl2 + Cl2
CHCl3 + Cl2
heat or light
CHCl3 + HCl
Trichloromethane
heat or light
CCl4 + HCl
Tetrachloromethane
Sources of alkanes: the two major sources of alkanes are natural gas and petroleum. Natural
gas consists of approximately 90 to 95% methane, 5 to 10% ethane, and a mixture of other
relatively low-boiling alkanes (propane, butane and 2-methylpropane). Petroleum is a thick,
viscous, liquid mixture of thousands of compounds, most of them hydrocarbons, formed the
decomposition of marine plants and animals. The fundamental separation process in refining
petroleum is fractional distillation. All crude petroleum that enters a refinery goes to
distillation units, where it is heated to temperatures as high as 370 to 425°C and separated into
fractions.
Dr. Behrang Madani
Chemistry 20
Mt SAC