Higher alkenes and alkynes are named by counting the number of carbons in the longest continuous chain that includes the double or triple bond and appending an -ene alkene or -yne alkyne suffix to the stem name of the unbranched alkane having that number of carbons. The chain is numbered in the direction that gives the lowest number to the first multiply bonded carbon , and adding it as a prefix to the name. Once the chain is numbered with respect to the multiple bond, substituents attached to the parent chain are listed in alphabetical order and their positions identified by number. Compounds that contain two double bonds are classified as dienes , those with three as trienes, and so forth. Dienes are named by replacing the -ane suffix of the corresponding alkane by -adiene and identifying the positions of the double bonds by numerical locants.
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Alkanes experience intermolecular van der Waals forces. Stronger intermolecular van der Waals forces give rise to greater boiling points of alkanes. 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. 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.
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 better put together solid structures will require more energy to break apart. For alkanes, this can be seen from the graph above i. 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-organized structure, which requires more energy to break apart. The odd-numbered alkanes pack less well and so the "looser" organized 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 pack 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 and solubility[ edit ] Alkanes do not conduct electricity in any way, 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 minimized by minimizing the contact between alkane and water: Alkanes are said to be hydrophobic as they repel water.
Their solubility in nonpolar solvents is relatively high, a property that is called lipophilicity. Alkanes are, for example, miscible in all proportions among themselves. The density of the alkanes usually increases with the number of carbon atoms but remains less than that of water. Hence, alkanes form the upper layer in an alkane—water mixture. 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-hybridized, 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.
The former result from the overlap of an sp3 orbital of carbon with the 1s orbital of a hydrogen; the latter by the overlap of two sp3 orbitals on adjacent carbon atoms. The bond lengths amount to 1. The tetrahedral structure of methane. The spatial arrangement of the bonds is similar to that of the four sp3 orbitals—they are tetrahedrally arranged, with an angle of 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.
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. 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 differ in energy: the staggered conformation is 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 case of higher alkanes is more complex but based on similar principles, with the antiperiplanar conformation always being the most favored 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 idealized 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[ edit ] 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 of a functional group like —OH , —CHO , —COOH etc. The carbon 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 samples that have not been run for a sufficiently long time.
Mass spectrometry[ edit ] Alkanes have a high ionization 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.
Chemical properties[ edit ] Alkanes are only weakly reactive with most chemical compounds. The acid dissociation constant pKa values of all alkanes are estimated to range from 50 to 70, depending on the extrapolation method, hence they are extremely weak acids that are practically inert to bases see: carbon acids. Similarly, they only show reactivity with the strongest of electrophilic reagents e.
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. 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.
Reaction with oxygen if present in sufficient quantity to satisfy the reaction stoichiometry leads to combustion without any smoke, producing carbon dioxide and water. Free radical halogenation reactions occur with halogens, leading to the production of haloalkanes.
In addition, alkanes have been shown to interact with, and bind to, certain transition metal complexes in C—H bond activation reactions. In highly branched alkanes, the bond angle may differ significantly from the optimal value Such distortions introduce a tension in the molecule, known as steric hindrance or strain.
Strain substantially increases reactivity. For example, the highly branched 2,2,3,3-tetramethylbutane is about 1. The controversy is related to the question of whether the traditional explanation of hyperconjugation is the primary factor governing the stability of alkyl radicals.
Alkane, Alkene, Alkine – was sind die Unterschiede?
The more commonly used name for ethyne is acetylene, which used industrially. Rule 1 Find the longest carbon chain that includes both carbons of the triple bond. Rule 2 Number the longest chain starting at the end closest to the triple bond. A 1-alkyne is referred to as a terminal alkyne and alkynes at any other position are called internal alkynes. For example: 4-chlorodiiodomethylnonyne Rule 3 After numbering the longest chain with the lowest number assigned to the alkyne, label each of the substituents at its corresponding carbon.
IUPAC Nomenclature of Alkanes, Alkenes and Alkynes
Nomenclature of Alkynes
Nomenclature of Alkenes