» Structure Of Organic Compounds
» Isomerism Of Organic Compounds
» Halogenated Derivatives
» Carboxylic Acids
Acyclic hydrocarbons containing a double bound within two carbon atoms in the molecule are named alkenes. They are also called olefins, because of the fact that the chlorate and bromate derivates are oleaginous liquids.
The simplest alkene is ethylene (C2H4), which has the International Union of Pure and Applied Chemistry name ethene. This substance contains two carbon atoms connected through a double bound. Superior alkenes can be considered as the result of the substitution of a hydrogen atom with alkyl radicals, same as the case of saturated acyclic hydrocarbons. Hence, the superior homologue of ethylene is propene, CH2=CH-CH3. The homologue of propene is buthene, C4H8. After the position of the double bound in the molecule at buthene, you can distingiush three position isomers.
By replacing the ending “ane” from the alkane’s name with the same number of carbon atoms in the molecule with the ending “ene”, you obtain the alkenes’ name.
CH3-CH2-CH2-CH=CH-CH3 - 2 hexene
So, the three position isomers of buthene are: 1-buthene, 2-buthene and 2-methylpropene. Sometimes there can be used the position of the double bound after the name of the hydrocarbon. For example: buthene-1. Alkenes mono-valence derivates have their names ending in “enyl”, the position of the double bound being indicated, if necessary, by a number.
Vinyl for ethenyl: CH2=CH-
Alyl for 2-propenyl: CH2-CH-CH2-
Isopropenyl for 1 methyl vinyl: CH2-C-CH3
Because of the double bound in the molecule, alkenes have two hydrogen atoms less then other saturated hydrocarbons; therefore, the general formula for these hydrocarbons is CnH2n. You can observe the homologous series of the cycloalkanes. They are, consequently, raw formula isomers.
C2-C4 alkenes are gases, C5-C18 are liquids and the ones with more than 18 carbon atoms are solids. The boiling points are a little bit lower than the corresponding alkanes. Their density is higher than the corresponding alkenes, but all alkenes have the density smaller than 1. Alkenes are insoluble in water, but mixable with organic solvents.
Alkenes present three types of isomerism: chain isomers (the isomers differ by chain), position isomers (the chain is identical but the position of the double bound differs) and function isomers. Geometrical isomerism appears at alkenes because the double bound is stiff and doesn’t allow the rotation of the carbon atoms around it. Geometrical isomers are different from one another through the spatial position of the substitutes around the double bound. The condition of isomerism is that for each carbon atom from the double bound to exist a different substitute.
The nomenclature of geometrical isomers: it is established by the priorities of the substitutes bind with each carbon atom from the double bound: you will compare the Z of the atoms directly connected with the carbon atom out of the double bound. The one with the biggest Z will have highest priority and it will be note with 1 and the other one, with 2. If the Z’s are the same, the next pair will be compared. The comparison will be made until there appears a difference. This procedure will be applied at each carbon atom in the double bound. After establishing the priorities, the denomination will be made like this: the isomer with the substitutes on the same side of the double bound will be noted with Z (CIS) and the other, with the substitutes on one side and another of the double bound, will be noted with E (TRANS).
For example: C4H8 has 6 isomers: 1-buthene, CIS 2-buthene, TRANS 2-buthene, isobutene, cyclo butene, methylcyclopropene.
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