Alkenes – NCERT Class 11 Chemistry Notes : Unit 13
AlkenesOnline Chemistry Notes for Class 11 – Alkenes also called as olefins are the hydrocarbons that contain at least one carbon-carbon double bond in their chemical structure. They are categorized as unsaturated hydrocarbons. Due to the presence of the double bonds, alkenes are more reactive than alkanes. The homologous series of alkenes have the general formula as CnH2n. These hydrocarbons have two hydrogen atoms less than the corresponding alkane (with the same number of carbon atoms). The simplest and the first stable compound is ethylene or ethene (C2H4). This organic compound is produced on the largest scale industrially.
In order to name an alkene according to the IUPAC system, certain rules have to be followed.
Alkenes are named according to the same general naming rules as for alkanes. The parent name of the alkene compound is similar to its corresponding alkane. In case of the alkene, the suffix is “ene” whereas in corresponding alkane the suffix is “ane”. The suffix “ene” represents the double of the alkene. For example, if an organic compound continuous chain having a double-bond containing five carbon atoms, the compound will be pentene. Given below is a list of alkenes up to eight carbon atoms.
The numbering of carbon atoms in the chain starts from the carbon which is near to the double bond. Thus, in the example given below, it is numbered from right to left, placing the double bond between the second and third carbon atoms of the chain.(Numbering the chain from left to right is incorrect, it places the double bond between the third and fourth carbons of the chain). It is also important to indicate the position of the double bond. Therefore, in the given example, the name of the compound is written as 2-pentene.
If there is more than one double bond in an alkene, all of the bonds should be numbered in the name of the molecule, even the terminal double bonds. The numbers start from lowest to highest and be separated from one another by a comma. The IUPAC numerical prefixes are used to indicate the number of double bonds.
It is important to mention the location as well as the name of any substituent molecule (if any) present in the compound. The position of any substituent group is indicated by the number of the carbon atom in the parent (longest) chain to which it is attached. In the example given below the name of the compound is written as 5-chloro-2-hexene.
Another important rule to note in the nomenclature of an alkene is to correctly indicate the three-dimensional relationship of the organic compound. Double bonds can exist as geometric isomers and these isomers are designated by using either the cis/trans designation (or by more flexible E / Z designation).
cis Isomers have the two largest groups are on the same side of the double bond as shown in the above figure, and trans Isomers have the two largest groups are on opposite sides of the double bond (right structure above). If there are 3 or 4non-hydrogen different atoms attached to the alkene then E, Z system is used. E(entgegen) means the higher prioritygroups are opposite one another relative to the double bond, Z (zusammen) means the higher priority groups are on the same side relative to the double bond.
E = entgegan (“trans”) Z =zusamen (“cis”)
Priority of groups is based on the atomic mass of attached atoms (not the size of the group). An atom attached by multiple bonds is counted once for each bond.
This means that it is also important to know whether the compound containing double bonds is present in cis or trans conformation. Therefore, to complete the nomenclature of the compound, assigning the conformation is important. For example, the given compound id named cis-5-chloro-2-hexene.
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