Any of a series of saturated aliphatic hydrocarbons (such
as methane, ethane, etc.), such compounds are the main
components of petroleum. Alkanes are saturated hydrocarbons, which
are chain hydrocarbons with only carbon-carbon single bonds. Chemical
properties of alkanes are well defined in NCERT chemistry topics.
They are the simplest type
of organic compounds. In alkanes, the number of hydrogen atoms reaches a
maximum. Each carbon atom in the molecule is sp3 hybrid. The simplest alkane is methane.
At the same time, the alkane is also a general term for saturated hydrides of many elements.
hydride suffix and system
Mononuclear and their names
The “alkane” (-ane suffix) actually refers not only to the saturated
hydrides of carbon but can refer to molecular hydrides in which all
parent molecules do not contain unsaturated bonds. The mononuclear
hydride is directly named after “element + alkane”.
AlH3 – Alane
(CH3CH2CH2)3 B——tri-propyl borane
(CH3CH2)4 Pb——tetraethyl lead
The use of “Methane” is too common to be mistaken for a system
name. In fact, “Carbine” is the systematic name for the
substance. Although this name is rarely used, it is good to know its
Some different hydrides of variable elements use the same alkane
name, and non-standard bonding number hydrides are prefixed with “λ” to
distinguish them. From the table above, we can know the names of
generally saturated alkanes.
Standard bonding number: halogen group 1, oxygen group 2, nitrogen group 3, carbon group 4, boron group 3.
Nomenclature of Alkanes
Beginning with the corresponding number, carbenes do not introduce
the carbon element prefix, non-carbohydrates must add the nuclear
Linear Hexane Nomenclature
When the parent atom is replaced by another atom, the name should be
modified by the corresponding prefix of the heteroatom. When the
skeleton chain electrons of the alkane are randomly occupied by other
heteroatoms, the original name is modified with the heteroatom prefix
but the total number of heteroatoms should be included in the total
electrons of the skeleton chain.
If the heteroatom and the main chain atom appear
alternately on the chain and the elements at the two ends of the chain are the
same, the segment atom is named as the heteroatom. Needless to say the
number of chain atoms.
In an alkane, each carbon atom is tetravalent, and there are only
carbon-carbon single bonds and hydrocarbon single bonds. Using
sp3 hybrid orbits, it forms a strong σ bond with the surrounding 4
carbon or hydrogen atoms.
The carbon atoms connected to 1, 2, 3, and 4 carbons are called
primary, secondary, tertiary, and quaternary carbons respectively;
hydrogen atoms on primary, secondary, and tertiary carbons
They are called primary, secondary and tertiary hydrogen.
To minimize the repulsive force of the bond,
four atoms connected
to the same carbon form a tetrahedron. Methane is a standard regular tetrahedron with a bond angle of 109 ° 28 ′.
In theory, because of the alkane stable structure, all of the alkanes can be stable.
However, alkanes in nature do not exceed 50 carbons
at most, and the most abundant alkanes are methane.
Because the carbon atoms in alkanes can be arranged randomly
according to the law, the structure of alkanes can be written in
countless kinds. Linear alkanes are the most basic structure, and
theoretically, this chain can be extended indefinitely. Chemical
Properties of Alkanes
It is possible to produce branched chains on the straight chain, which undoubtedly increases the number of alkanes.
Therefore, starting from a 4-carbon alkane, the
molecular formula of the same alkane can represent multiple structures. This
phenomenon is called isomerism. As the carbon number increases, the number of isomers will increase rapidly.
Alkanes may also undergo optical isomerism. When the four atomic groups connected by a carbon atom
are different, this carbon is called chiral carbon, and this substance is
The remaining part of an alkane that loses a
hydrogen atom is called an alkyl group , which is generally represented by R-. Therefore, alkanes can
also be represented by the general formula RH.
Alkanes were first named using customary nomenclature. However, this
nomenclature is difficult to use for alkanes with many carbon numbers
and many isomers. So it was suggested nomenclature derived, all
considered as derivatives of methane alkane, for example isobutane,
Due to the higher cost of alkane production (usually olefins are used
for catalytic hydrogenation), alkane is not produced industrially but
is directly extracted from petroleum. Since alkane is not easy to react,
it is not used as a basic chemical raw material in the industry.
The role of
alkanes is mainly to make fuel. Natural gas and biogas (mainly composed
of methane) are recently widely used clean energy sources.
Since most of
the alkane comes from petroleum, it must be subjected to a fractional distillation process to obtain a variety of alkane for
Different four-carbon hydrocarbons (from left to
right): n-butane and isobutane are isomers, and their chemical formulas are
both C4H10, cyclobutane and isobutene are isomers, their
chemical formulas are C4H8.
Bicyclo [1.1.0] butane is the only alkane with
the chemical formula C4H6.
Alkane isomers are mostly chain isomers (isomers
due to different branch chains). Alkanes with more than 3 carbon atoms can
be arranged in a variety of ways to form isomers. The number of alkane
isomers increases as the carbon number increases.
C1: no isomers: methane
C2: no isomers: ethane
C3: two isomers: propane, cyclopropane
C4: two isomers: n-butane, isobutane
C5: three isomers: n-pentane, isopentane, neopentane
C6: five isomers: hexane
C12: 355 isomers
C32: 27711253769 isomers
C60: 22158734535770411074184 isomers, many of which
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Alkanes System Nomenclature
The current nomenclature uses the IUPAC nomenclature. The systematic naming rules for alkanes are as follows:
Find the longest carbon chain as the main chain. Name the main chain
according to the number of carbons. The top ten are Tiangan (A, B, C …)
representing the carbon number. When the number of carbons is more than
ten, name the Chinese numbers, such as Undecane( Formula: C11H24 ).
When there are multiple substituents, use the smallest and
longest carbon chain as the main chain, and list all substituents in the
order of methyl, ethyl, and propyl.
When more than two substituents are the same, add Chinese numerals in front of the substituents: one, two, three …, such as dimethyl( Formula: C2H6S ), their positions are separated by and listed together in front of the substituents.
Structural formula of isooctane ( 2,2,4-trimethylpentane ). Isooctane is a standard for gasoline knock resistance, with
an octane number of 100. For
some simple or commonly used alkanes, common names are often used. For example, it is customary to prefix
the name of a linear alkane with the word “n”, but the word is not
included in the system name.
Those with a methyl group at the 2-position of the main chain are
called “iso”, and those with two methyl groups at the 2-position are
called “new”. Although this is only suitable for butane and pentane with
few isomers, it has been retained due to habit, and
even 2,2,4-trimethylpentane, which should not be called “iso”, is also
labeled with ” iso ” ” Octane “.
Chemical Properties of Alkanes
Alkanes are very
stable because the CH and CC double bonds are relatively stable and difficult
to break. Apart from the following three reactions, alkanes can hardly carry out other reactions.
R + O2 → CO2 + H2O
All alkanes can
be burned, and the reaction exothermic. Alkanes complete combustion CO.’S2 and H2O. If the
amount of O2 is insufficient, toxic gases such as carbon monoxide (CO) and even carbon black (C) will be generated.
Take methane as
CH4 + 2O2 → CO2 + 2H2O
When the supply of O2 is insufficient, the reaction is as follows:
CH4 + 3/2O2 → CO + 2H2OCH4 + O2 → C + 2H2O
alkanes often cannot be completely burned. When they are burned, black smoke is
generated, that is, carbon black. The same is true of the black smoke in the car’s exhaust.
R + X 2 → RX + HX
Because the structure of alkanes is too strong, ordinary organic
reactions cannot proceed. Halogenation of alkanes is a free
radical substitution reaction, and light energy is required to
generate free radicals at the beginning of the reaction.
The following are the steps in which methane is halogenated. This highly exothermic reaction can cause an explosion.
phase: UV a catalyst formed under two Cl the radical
Cl 2 → Cl * / * Cl
stage: one H atom is detached from methane; CH 3 Cl begins to form.
termination phase: two free radicals recombination
And Cl* Cl*, or R* and Cl*, or CH3* and CH3*
reaction is a process in which large molecular hydrocarbons are split into
small molecular hydrocarbons under conditions of high temperature, high
pressure, or a catalyst. The cracking reaction is an elimination reaction, so the cracking of alkanes always produces olefins.
For example, hexadecane (C16H34) can be cracked to obtain octane and octene (C8H18). The
probability of breaking is different due to the different environments
of each bond. The following is an example of the cracking of butane:
CH 3 -CH2 -CH2 -CH 3 → CH 4 + CH2 = CH-CH3 during CH3 – CH2 bond breaking, the possibility of 48%.
The CH2 -CH2 bond is broken during the process
of CH3 -CH2 -CH2 -CH3 → CH3 -CH3 + CH2 = CH2 with a probability of 38%.
The CH bond
is broken during CH3 -CH2 -CH2 -CH3 → CH2 = CH-CH2 -CH3 + H2 with a probability of 14%.
In the cracking
conditions can trigger different mechanisms, but the reaction process is
similar. Carbon radicals are generated during the thermal decomposition
process, and carbon positive ions and hydrogen negative ions are
generated during the catalytic cracking process. These extremely
intermediates undergo steps such as rearrangement, bond cleavage, and
transfer to form stable small molecule hydrocarbons.
In industry, deep cracking is called cracking, and the products of cracking
are all gases, called cracking gas.
The role of
alkanes is mainly to make fuel. Natural gas and biogas (mainly composed of methane) are widely used clean energy
sources. Various fractions from petroleum fractionation are suitable for various engines:
C1 ~ C4 (distillates below 40 ° C) are petroleum gas and can be used as fuel.
C5 ~ C11 (distillate at 40 ~ 200 ℃) is gasoline, which can be used as fuel or chemical raw material.
C9 ~ C18 (distillate at 150 ~ 250 ℃) is kerosene and can be used as fuel.
C14 ~ C20 (distillate at 200 ~ 350 ℃) is diesel and can be used as fuel.
C20 are heavy oils, and then reduced pressure distillation can obtain substances such as lubricating oil and asphalt.
In addition, the reaction of paraffin to crack olefins has become an important method for the production of ethylene.
At normal temperature and pressure (1 atmosphere),
(methane to butane) is gaseous, between 5 -17 (pentane to heptane) is liquid,
and more than 18 carbons (octadecane) are solid.