subject: Alkane - China Others - Automatic Bread Maker Manufacturer [print this page] Isomerism Isomerism
Alkanes with more than three carbon atoms can be arranged in numerous different ways, forming different structural isomers. An isomer is like a chemical anagram, in which the atoms of a chemical compound are arranged or joined together in a different order. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer (n for "normal", although it is not necessarily the most common). However the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms (sequence A000602 in OEIS). For example:
C1: 1 isomer: methane C2: 1 isomer: ethane
C3: 1 isomer: propane
C4: 2 isomers: n-butane, isobutane
C5: 3 isomers
C6: 5 isomers
C12: 355 isomers
C32: 27,711,253,769 isomers
C60: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.
See also List of alkanes
Branched alkanes can be chiral: 3-methylhexane and its higher homologues are chiral due to their stereogenic center at carbon atom number 3. Chiral alkanes are of certain importance in biochemistry, as they occur as sidechains in chlorophyll and tocopherol (vitamin E). Chiral alkanes can be resolved into their enantiomers by enantioselective chromatography.
Nomenclature
Main article: IUPAC nomenclature of organic chemistry
The IUPAC nomenclature (systematic way of naming compounds) for alkanes is based on identifying hydrocarbon chains. Unbranched, saturated hydrocarbon chains are named systematically with a Greek numerical prefix denoting the number of carbons and the suffix "-ane".
August Wilhelm von Hofmann suggested systematizing nomenclature by using the whole sequence of vowels a, e, i, o and u to create suffixes -ane, -ene, -ine (or -yne), -one, -une, for the hydrocarbons. The first three name hydrocarbons with single, double and triple bonds; "-one" represents a ketone; "-ol" represents an alcohol or OH group; "-oxy-" means an ether and refers to oxygen between two carbons, so that methoxy-methane is the IUPAC name for dimethyl ether.
It is difficult or impossible to find compounds with more than one IUPAC name. This is because shorter chains attached to longer chains are prefixes and the convention includes brackets. Numbers in the name, referring to which carbon a group is attached to, should be as low as possible, so that 1- is implied and usually omitted from names of organic compounds with only one side-group; "1-" is implied in Nitro-octane. Symmetric compounds will have two ways of arriving at the same name.
Linear alkanes
Straight-chain alkanes are sometimes indicated by the prefix n- (for normal) where a non-linear isomer exists. Although this is not strictly necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers, e.g., n-hexane or 2- or 3-methylpentane.
The members of the series (in terms of number of carbon atoms) are named as follows:
methane, CH4 - one carbon and four hydrogen
ethane, C2H6 - two carbon and six hydrogen
propane, C3H8 - three carbon and 8 hydrogen
butane, C4H10 - four carbon and 10 hydrogen
pentane, C5H12 - five carbon and 12 hydrogen
hexane, C6H14 - six carbon and 14 hydrogen
These names were derived from methanol, ether, propionic acid and butyric acid, respectively. Alkanes with five or more carbon atoms are named by adding the suffix -ane to the appropriate numerical multiplier prefix with elision of any terminal vowel (-a or -o) from the basic numerical term. Hence, pentane, C5H12; hexane, C6H14; heptane, C7H16; octane, C8H18; etc. The prefix is generally Greek, with the exceptions of nonane which has a Latin prefix, and undecane and tridecane which have mixed-language prefixes. For a more complete list, see List of alkanes.
Branched alkanes
Ball-and-stick model of isopentane (common name) or 2-methylbutane (IUPAC systematic name)
Simple branched alkanes often have a common name using a prefix to distinguish them from linear alkanes, for example n-pentane, isopentane, and neopentane.
IUPAC naming conventions can be used to produce a systematic name.
The key steps in the naming of more complicated branched alkanes are as follows:
Identify the longest continuous chain of carbon atoms
Name this longest root chain using standard naming rules
Name each side chain by changing the suffix of the name of the alkane from "-ane" to "-yl"
Number the root chain so that sum of the numbers assigned to each side group will be as low as possible
Number and name the side chains before the name of the root chain
If there are multiple side chains of the same type, use prefixes such as "di-" and "tri-" to indicate it as such, and number each one.
Comparison of nomenclatures for three isomers of C5H12
Common name
n-pentane
isopentane
neopentane
IUPAC name
pentane
2-methylbutane
2,2-dimethylpropane
Structure
Cyclic alkanes
Main article: Cycloalkane
So-called cyclic alkanes are, in the technical sense, not alkanes, but cycloalkanes. They are hydrocarbons just like alkanes, but contain one or more rings.
Simple cycloalkanes have a prefix "cyclo-" to distinguish them from alkanes. Cycloalkanes are named as per their acyclic counterparts with respect to the number of carbon atoms, e.g., cyclopentane (C5H10) is a cycloalkane with 5 carbon atoms just like pentane (C5H12), but they are joined up in a five-membered ring. In a similar manner, propane and cyclopropane, butane and cyclobutane, etc.
Substituted cycloalkanes are named similar to substituted alkanes the cycloalkane ring is stated, and the substituents are according to their position on the ring, with the numbering decided by Cahn-Ingold-Prelog rules.
Trivial names
The trivial (non-systematic) name for alkanes is "paraffins." Together, alkanes are known as the paraffin series. Trivial names for compounds are usually historical artifacts. They were coined before the development of systematic names, and have been retained due to familiar usage in industry. Cycloalkanes are also called naphthenes.
It is almost certain that the term paraffin stems from the petrochemical industry. Branched-chain alkanes are called isoparaffins. The use of the term "paraffin" is a general term and often does not distinguish between a pure compounds and mixtures of isomers with the same chemical formula (i.e., like a chemical anagram), e.g., pentane and isopentane.
Examples
The following trivial names are retained in the IUPAC system:
isobutane for 2-methylpropane
isopentane for 2-methylbutane
neopentane for 2,2-dimethylpropane.
Physical properties
Table of alkanes
Alkane
Formula
Boiling point [C]
Melting point [C]
Density [gcm3] (at 20C)
Methane
CH4
-162
-183
gas
Ethane
C2H6
-89
-172
gas
Propane
C3H8
-42
-188
gas
Butane
C4H10
-0.5
-135
gas
Pentane
C5H12
36
-130
0.626
Hexane
C6H14
69
-95
0.659
Heptane
C7H16
98
-91
0.684
Octane
C8H18
126
-57
0.703
Nonane
C9H20
151
-54
0.718
Decane
C10H22
174
-30
0.730
Undecane
C11H24
196
-26
0.740
Dodecane
C12H26
216
-10
0.749
Triacontane
C30H62
343
37
solid
Boiling point
Melting (blue) and boiling (pink) points of the first 14 n-alkanes in C.
Alkanes experience inter-molecular van der Waals forces. Stronger inter-molecular van der Waals forces give rise to greater boiling points of alkanes.
There are two determinants for the strength of the van der Waals forces:
the number of electrons surrounding the molecule, which increases with the alkane's molecular weight
the surface area of the molecule
Under standard conditions, from CH4 to C4H10 alkanes are gaseous; from C5H12 to C17H36 they are liquids; and after C18H38 they are solids. As the boiling point of alkanes is primarily determined by weight, it should not be a surprise that the boiling point has almost a linear relationship with the size (molecular weight) of the molecule. As a rule of thumb, the boiling point rises 20 - 30 C for each carbon added to the chain; this rule applies to other homologous series.
A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, thus the greater van der Waals forces, between adjacent molecules. For example, compare isobutane (2-methylpropane) and n-butane (butane), which boil at -12 and 0 C, and 2,2-dimethylbutane and 2,3-dimethylbutane which boil at 50 and 58 C, respectively. For the latter case, two molecules 2,3-dimethylbutane can "lock" into each other better than the cross-shaped 2,2-dimethylbutane, hence the greater van der Waals forces.
On the other hand, cycloalkanes tend to have higher boiling points than their linear counterparts due to the locked conformations of the molecules, which give a plane of intermolecular contact.[citation needed]
Melting point
The melting points of the alkanes follow a similar trend to boiling points for the same reason as outlined above. That is, (all other things being equal) the larger the molecule the higher the melting point. There is one significant difference between boiling points and melting points. Solids have more rigid and fixed structure than liquids. This rigid structure requires energy to break down. Thus the stronger better put together solid structures will require more energy to break apart. For alkanes, this can be seen from the graph above (i.e., the blue line). The odd-numbered alkanes have a lower trend in melting points than even numbered alkanes. This is because even numbered alkanes pack well in the solid phase, forming a well-organised structure, which requires more energy to break apart. The odd-number alkanes pack less well and so the "looser" organised solid packing structure requires less energy to break apart.
The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on the ability of the alkane in question to packing well in the solid phase: This is particularly true for isoalkanes (2-methyl isomers), which often have melting points higher than those of the linear analogues.
Conductivity
Alkanes do not conduct electricity, nor are they substantially polarized by an electric field. For this reason they do not form hydrogen bonds and are insoluble in polar solvents such as water. Since the hydrogen bonds between individual water molecules are aligned away from an alkane molecule, the coexistence of an alkane and water leads to an increase in molecular order (a reduction in entropy). As there is no significant bonding between water molecules and alkane molecules, the second law of thermodynamics suggests that this reduction in entropy should be minimised by minimising the contact between alkane and water: Alkanes are said to be hydrophobic in that they repel water.
Their solubility in nonpolar solvents is relatively good, a property that is called lipophilicity. Different alkanes are, for example, miscible in all proportions among themselves.
The density of the alkanes usually increases with increasing number of carbon atoms, but remains less than that of water. Hence, alkanes form the upper layer in an alkane-water mixture.
Molecular geometry
sp3-hybridisation in methane.
The molecular structure of the alkanes directly affects their physical and chemical characteristics. It is derived from the electron configuration of carbon, which has four valence electrons. The carbon atoms in alkanes are always sp3 hybridised, that is to say that the valence electrons are said to be in four equivalent orbitals derived from the combination of the 2s orbital and the three 2p orbitals. These orbitals, which have identical energies, are arranged spatially in the form of a tetrahedron, the angle of cos1() 109.47 between them.
Bond lengths and bond angles
An alkane molecule has only C H and C C single bonds. The former result from the overlap of a sp-orbital of carbon with the 1s-orbital of a hydrogen; the latter by the overlap of two sp-orbitals on different carbon atoms. The bond lengths amount to 1.091010m for a C H bond and 1.541010m for a C C bond.
The tetrahedral structure of methane.
The spatial arrangement of the bonds is similar to that of the four sp-orbitalshey are tetrahedrally arranged, with an angle of 109.47 between them. Structural formulae that represent the bonds as being at right angles to one another, while both common and useful, do not correspond with the reality.
Conformation
Main article: Alkane stereochemistry
The structural formula and the bond angles are not usually sufficient to completely describe the geometry of a molecule. There is a further degree of freedom for each carbon carbon bond: the torsion angle between the atoms or groups bound to the atoms at each end of the bond. The spatial arrangement described by the torsion angles of the molecule is known as its conformation.
Newman projections of the two conformations of ethane: eclipsed on the left, staggered on the right.
Ball-and-stick models of the two rotamers of ethane
Ethane forms the simplest case for studying the conformation of alkanes, as there is only one C C bond. If one looks down the axis of the C C bond, one will see the so-called Newman projection. The hydrogen atoms on both the front and rear carbon atoms have an angle of 120 between them, resulting from the projection of the base of the tetrahedron onto a flat plane. However, the torsion angle between a given hydrogen atom attached to the front carbon and a given hydrogen atom attached to the rear carbon can vary freely between 0 and 360. This is a consequence of the free rotation about a carbon carbon single bond. Despite this apparent freedom, only two limiting conformations are important: eclipsed conformation and staggered conformation.
The two conformations, also known as rotamers, differ in energy: The staggered conformation is 12.6 kJ/mol lower in energy (more stable) than the eclipsed conformation (the least stable).
This difference in energy between the two conformations, known as the torsion energy, is low compared to the thermal energy of an ethane molecule at ambient temperature. There is constant rotation about the C-C bond. The time taken for an ethane molecule to pass from one staggered conformation to the next, equivalent to the rotation of one CH3-group by 120 relative to the other, is of the order of 1011seconds.
The case of higher alkanes is more complex but based on similar principles, with the antiperiplanar conformation always being the most favoured around each carbon-carbon bond. For this reason, alkanes are usually shown in a zigzag arrangement in diagrams or in models. The actual structure will always differ somewhat from these idealised forms, as the differences in energy between the conformations are small compared to the thermal energy of the molecules: Alkane molecules have no fixed structural form, whatever the models may suggest.
Spectroscopic properties
Virtually all organic compounds contain carbon carbon and carbon hydrogen bonds, and so show some of the features of alkanes in their spectra. Alkanes are notable for having no other groups, and therefore for the absence of other characteristic spectroscopic features.
Infrared spectroscopy
The carbonydrogen stretching mode gives a strong absorption between 2850 and 2960cm1, while the carbonarbon stretching mode absorbs between 800 and 1300cm1. The carbonydrogen bending modes depend on the nature of the group: methyl groups show bands at 1450cm1 and 1375cm1, while methylene groups show bands at 1465cm1 and 1450cm1. Carbon chains with more than four carbon atoms show a weak absorption at around 725cm1.
NMR spectroscopy
The proton resonances of alkanes are usually found at H = 0.5 1.5. The carbon-13 resonances depend on the number of hydrogen atoms attached to the carbon: C = 8 30 (primary, methyl, -CH3), 15 55 (secondary, methylene, -CH2-), 20 60 (tertiary, methyne, C-H) and quaternary. The carbon-13 resonance of quaternary carbon atoms is characteristically weak, due to the lack of Nuclear Overhauser effect and the long relaxation time, and can be missed in weak samples, or sample that have not been run for a sufficiently long time.
Mass spectrometry
Alkanes have a high ionisation energy, and the molecular ion is usually weak. The fragmentation pattern can be difficult to interpret, but, in the case of branched chain alkanes, the carbon chain is preferentially cleaved at tertiary or quaternary carbons due to the relative stability of the resulting free radicals. The fragment resulting from the loss of a single methyl group (M15) is often absent, and other fragment are often spaced by intervals of fourteen mass units, corresponding to sequential loss of CH2-groups.
Chemical properties
In general, alkanes show a relatively low reactivity, because their C bonds are relatively stable and cannot be easily broken. Unlike most other organic compounds, they possess no functional groups.
They react only very poorly with ionic or other polar substances. The acid dissociation constant (pKa) values of all alkanes are above 60, hence they are practically inert to acids and bases (see: carbon acids). This inertness is the source of the term paraffins (with the meaning here of "lacking affinity"). In crude oil the alkane molecules have remained chemically unchanged for millions of years.
However redox reactions of alkanes, in particular with oxygen and the halogens, are possible as the carbon atoms are in a strongly reduced condition; in the case of methane, the lowest possible oxidation state for carbon (4) is reached. Reaction with oxygen leads to combustion without any smoke;[clarification needed] with halogens, substitution. In addition, alkanes have been shown to interact with, and bind to, certain transition metal complexes in (See: carbon-hydrogen bond activation).
Free radicals, molecules with unpaired electrons, play a large role in most reactions of alkanes, such as cracking and reformation where long-chain alkanes are converted into shorter-chain alkanes and straight-chain alkanes into branched-chain isomers.
In highly branched alkanes, the bond angle may differ significantly from the optimal value (109.5) in order to allow the different groups sufficient space. This causes a tension in the molecule, known as steric hindrance, and can substantially increase the reactivity.
Reactions with oxygen (combustion reaction)
All alkanes react with oxygen in a combustion reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases. The general equation for complete combustion is:
CnH2n+2 + (1.5n+0.5)O2 (n+1)H2O + nCO2
In the absence of sufficient oxygen, carbon monoxide or even soot can be formed, as shown below:
