Alkanes are simplest family of molecules that contain the carbon– carbon single bond results from σ (head-on) overlap of carbon sp3 hybrid orbitals.
Alkanes are often described as saturated hydrocarbons: hydrocarbons because they contain only
carbon and hydrogen; saturated because
they have only C-C and C-H single bonds and thus contain the maximum possible number
of hydrogens per carbon. They have the general formula CnH2n+2, where n is
an integer. Alkanes are also occasionally called aliphatic compounds.
First 10 Alkanes
1. Methane CH4
2. Ethane C2H6
3. Propane C3H8
4. Butane C4H10
5. Pentane C5H12
6. Hexane C6H14
7. Heptane C7H16
8. Octane C8H18
9. Nonane C9H20
10. Decane C10H22
Alkanes |
Formula |
Molar mass |
Methane |
CH4 |
16.04 g/mol |
Ethane |
C2H6 |
30.07 g/mol |
Propane |
C3H8 |
44.09 g/mol |
Butane |
C4H10 |
58.12 g/mol |
Pentane |
C5H12 |
72.15 g/mol |
Hexane |
C6H14 |
86.17 g/mol |
Heptane |
C7H16 |
100.2 g/mol |
Octane |
C8H18 |
114.2 g/mol |
Nonane |
C9H20 |
128.2 g/mol |
Decane |
C10H22 |
142.2 g/mol |
1. Methane Formula
2. Ethane Formula
3. Propane Formula
4. Butane Formula
5. Pentane Formula
6. Hexane Formula
7. Heptane Formula
8. Octane Formula
9. Nonane Formula
10. Decane Formula
Think about the ways that carbon and hydrogen
might combine to make alkanes. With one carbon and four hydrogens, only one structure
is possible: methane, CH4. Similarly, there is only one combination
of two carbons with six hydrogens (ethane, CH3CH3) and
only one combination of three carbons with eight hydrogens (propane, CH3CH2CH3).
When larger numbers of carbons and hydrogens combine, however, more than one structure
is possible.
For example, there are two substances with the formula
C4H10: the four carbons can all be in a row (butane), or
they can branch (isobutane). Similarly, there are three C5H12
molecules, and so on for larger alkanes.
Compounds like butane and pentane, whose
carbons are all connected in a row, are called straight-chain alkanes, or normal
alkanes. Compounds like 2-methylpropane (isobutane), 2-methylbutane, and
2,2-dimethylpropane, whose carbon chains branch, are called branched-chain alkanes.
Compounds like the two C4H10
molecules and the three C5H12 molecules, which have the
same formula but different structures, are called isomers, from the Greek isos +
meros, meaning ―made of
the same parts.‖ Isomers
are compounds that have
the same numbers and kinds of atoms but
differ in the way the atoms are arranged. Compounds like butane and isobutane,
whose atoms are connected differently, are called constitutional isomers.
We’ll see shortly that other kinds of isomers are
also possible, even among compounds whose atoms are connected in the same
order. As Table 3-2 shows, the number
of possible alkane isomers increases dramatically with the number of carbon atom
Constitutional isomerism is not limited to
alkanes it occurs widely throughout organic chemistry. Constitutional isomers may
have different carbon skeletons (as in isobutane and butane), different
functional groups (as in ethanol and dimethyl ether), or different locations of
a functional group along the chain (as in isopropylamine and propylamine).
Regardless of the reason for the isomerism, constitutional isomers are always different
compounds with different properties but with the same formula.
A given alkane can be drawn in many ways. For
example, the straight chain, four- carbon alkane called butane can be
represented by any of the structures shown in Figure 3-2. These structures don’t imply any particular
three-dimensional geometry for butane; they indicate only the connections among
atoms. In practice, chemists rarely draw all the bonds in a molecule and
usually refer to butane by the condensed structure, CH3CH2CH2CH3
or CH3(CH2)2CH3. Still more simply,
butane can be represented as n-C4H10,
where n denotes normal (straight-chain) butane.
Figure 3-2 some representations of butane, C4H10.
The molecule is the same regardless of how it’s drawn. These structures imply
only that butane has a continuous chain of four carbon atoms; they do not imply
any specific geometry.
Straight-chain alkanes are named according to the
number of carbon atoms they contain, as shown in Table 3-3. With the exception of the first four compounds (methane,
ethane, propane, and butane) whose names have historical roots, the alkanes are
named based on Greek numbers. The suffix -ane
is added to the end of each name to indicate that the molecule identified
is an alkane. Thus, pentane is the five-carbon
alkane; hexane is the six carbon
alkane, and so on. We’ll soon see that these alkane names form the basis for
naming all other organic compounds, so at least the first ten should be memorized.
Naming Alkanes
A chemical name typically has four parts in
the IUPAC system of nomenclature: prefix,
parent, locant, and suffix. The prefix identifies the various substituent groups in the molecule, the
parent selects a main part of the molecule and tells how many carbon atoms are
in that part, the locants give the positions of the functional groups and substituents,
and the suffix identifies
the primary functional group.
Step 1
Find the parent hydrocarbon.
(a)
Find the
longest continuous chain of carbon atoms in the molecule, and use the name of
that chain as the parent name. The longest chain may not always be apparent from
the manner of writing; you may have to ―turn corners.‖
(b)
If two
different chains of equal length are present, choose the one with the larger number
of branch points as the parent.
Step 2
Number the atoms in the longest chain.
(a)
Beginning
at the end nearer the first branch point, number each carbon atom in the parent
chain.
The first branch occurs at C3 in the proper system
of numbering, not at C4.
(b)
If there
is branching an equal distance away from both ends of the parent chain, begin numbering
at the end nearer the second branch point.
Step 3
Identify and number the substituents.
(a) Assign a number, or locant, to each substituent to locate its point of attachment to the
parent chain.
(b) If there are two substituents on the same carbon, give
both the same number. There must be as many numbers in the name as there are substituents.
Step 4
Write the name as a single word.
Use hyphens to separate the different prefixes,
and use commas to separate numbers. If two or more different substituents are
present, cite them in alphabetical order. If two or more identical substituents
are present on the parent chain, use one of the multiplier prefixes di-, tri-,
tetra-, and so forth, but don’t use
these prefixes for alphabetizing. Full names for some of the examples we have been
using are as follows:
Step 5
Name a branched substituent as though it were itself a compound.
In some particularly complex
cases, a fifth step is necessary. It occasionally happens that a substituent on
the main chain is itself branched. In the following case, for instance, the
substituent at C6 is a three-carbon chain with a methyl group. To name the compound
fully, the branched substituent must first be named.
Number the branched substituent
beginning at the point of its attachment to the main chain, and identify it—in
this case, a 2-methylpropyl group.
The substituent is treated as a
whole and is alphabetized according to the first letter of its complete name,
including any numerical prefix. It is set off in parentheses when naming the entire
molecule.
As a further example:
For historical reasons, some of
the simpler branched-chain alkyl groups also have nonsystematic, common names,
as noted earlier.
The common names of these simple alkyl
groups are so well entrenched in the chemical literature that IUPAC rules make
allowance for them. Thus, the following compound is properly named either
4-(1-methylethyl) heptane or 4-isopropylheptane. There’s no choice but to
memorize these common names; fortunately, there are only a few of them.
When writing an alkane name, the non-hyphenated
prefix iso- is considered part of the alkyl-group name for alphabetizing
purposes, but the hyphenated and italicized prefixes sec- and tert- are not.
Thus, isopropyl and isobutyl are listed alphabetically under i, but sec-butyl and tert-butyl
are listed under b
Physical Properties of Alkanes
Alkanes are sometimes referred to
as paraffins, a word derived from the
Latin parum affinis, meaning ―little affinity.‖ This term aptly describes their
behavior, for alkanes show little chemical affinity for other substances and
are chemically inert to most laboratory reagents. They are also relatively inert
biologically and are not often involved in the chemistry of living organisms.
Alkanes are used primarily as
fuels, solvents, and lubricants. Natural gas, gasoline, kerosene, heating oil, lubricating
oil, and paraffin ―wax‖ are all composed primarily of alkanes,
with different physical properties resulting from different ranges of molecular
weights.
1. Solubilities and Densities of Alkanes
Alkanes are nonpolar, so they
dissolve in nonpolar or weakly polar organic solvents. Alkanes are said to be hydrophobic (―water hating‖) because they do not
dissolve in water. Alkanes are good lubricants and preservatives for metals
because they keep water from reaching the metal surface and causing corrosion.
Alkanes have densities around 0.7
g/mL, compared with a density of 1.0 g/mL for water. Because alkanes are less
dense than water and insoluble in water, a mixture of an alkane (such as
gasoline or oil) and water quickly separates into two phases, with the alkane on
top.
2. Boiling point and melting point
Alkanes show regular increases in boiling
point and melting point as molecular weight increases (Figure 3-4). Only when sufficient energy is applied to overcome these
forces does the solid melt or liquid boil. As you might expect, dispersion
forces increase as molecule size increases, accounting for the higher melting
and boiling points of larger alkanes.
Figure 3-4 A plot of melting and boiling points versus number of carbon
atoms for the C1–C14 straight-chain alkanes. There is a regular increase with molecular
size.
Another effect seen in alkanes is
that increased branching lowers an alkane’s boiling point. Thus, pentane has no
branches and boils at 36.1 °C, isopentane (2- methylbutane) has one branch and boils
at 27.85 °C, and neopentane (2,2-dimethylpropane) has two branches and boils at
9.5 °C. Similarly, octane boils at
125.7 °C, whereas isooctane
(2,2,4-trimethylpentane) boils at 99.3 °C. Branched- chain alkanes are
lower-boiling because they are more nearly spherical than straight- chain
alkanes, have smaller surface areas, and consequently have smaller dispersion forces.
Reactions of Alkanes
Alkanes do, however, react with
oxygen, halogens, and a few other substances under appropriate conditions.
Reaction with oxygen occurs during combustion in an engine or furnace when an
alkane is used as a fuel. Carbon dioxide and water are formed as products, and
a large amount of heat is released. For example, methane (natural gas) reacts with
oxygen according to the equation
1. Combustion
is a rapid oxidation that takes place at high temperatures,
converting alkanes to carbon dioxide and water. Little control over the reaction
is possible, except for moderating the temperature and controlling the fuel/air
ratio to achieve efficient burning.
2. Cracking
and Hydrocracking catalytic cracking of large hydrocarbons at high temperatures produces smaller
hydrocarbons. The cracking process usually operates under conditions that give
the maximum yields of gasoline. In hydrocracking,
hydrogen is added to give saturated hydrocarbons; cracking without hydrogen
gives mixtures of alkanes and alkenes.
3. Halogenation
Alkanes can react with halogens (F2, Cl2,
Br2, I2) to form alkyl halides. For example, methane
reacts with chlorine to form chloromethane (methyl chloride), dichloromethane (methylene
chloride), trichloromethane (chloroform), and tetrachloromethane (carbon tetrachloride).
The reaction of an alkane with Cl2
occurs when a mixture of the two is irradiated with ultraviolet light (denoted hy, where y is the Greek letter nu).
Depending on the time allowed and the relative amounts of the two reactants, a
sequential substitution of the alkane hydrogen atoms by chlorine occurs, leading
to a mixture of chlorinated products. Methane, for instance, reacts with Cl2
to yield a mixture of CH3Cl, CH2Cl2, CHCl3,
and CCl4.
A radical is highly reactive
because it contains an atom with an odd number of electrons (usually seven) in
its valence shell, rather than a stable, noble gas octet. A radical can achieve
a valence-shell octet in several ways. For example, the radical might abstract an
atom and one bonding electron from another reactant, leaving behind a new radical.
The net result is a radical substitution reaction.
An example of an industrially
useful radical reaction is the chlorination of methane to yield chloromethane. This
substitution reaction is the first step in the preparation of the solvents
dichloromethane (CH2Cl2) and chloroform (CHCl3).
Like many radical reactions in the
laboratory, methane chlorination requires three kinds of steps: initiation, propagation, and termination.
Initiation Irradiation with ultraviolet light
begins the reaction by breaking the relatively weak Cl-Cl bond of a small
number of Cl2 molecules to give a few reactive chlorine radicals.
Propagation Once produced, a reactive chlorine
radical collides with a methane molecule in a propagation step, abstracting a
hydrogen atom to give HCl and a methyl radical (·CH3). This methyl radical reacts further with Cl2
in a second propagation step to give the product chloromethane plus a new
chlorine radical, which cycles back and repeats the first propagation step.
Thus, once the sequence has been initiated, it becomes a self-sustaining cycle
of repeating steps (a) and (b), making the overall process a chain reaction.
Termination Occasionally, two radicals might
collide and combine to form a stable product. When that happens, the reaction
cycle is broken and the chain is ended. Such termination steps occur infrequently,
however, because the concentration of radicals in the reaction at any given
moment is very small. Thus, the likelihood that two radicals will collide is
also small.
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