Alkene or Olefin in Organic Chemistry
Alkenes names as olefins like Ethylene, propylene, butylene, etc are the unsaturated hydrocarbon containing at least one double or olefinic chemical bond having different types of geometric structure and structural isomerism. The common molecular formula of alkenes, CnH2n, where n = 1, 2, 3, etc. The double bond is called the olefinic bond or ethylenic bond. Alkene or olefin molecule is naming by different methods like common olefins molecules, substituted names, and IUPAC nomenclature of organic compounds in chemistry.
The electronic configuration of double-bonded carbon in the excited state, 1s2 2s1 2px1 2py1 2pz1. The 2s, 2px, and 2py orbitals hybridized to form three equivalent sp2 hybrid orbitals, and the remaining 2pz-orbital are undisturbed. Therefore, sp3 hybrid orbitals form three sigma bonds with the hydrogen atom or alkyl group present in alkenes and reaming 2pz-orbital form pi-bond or olefinic bond with the neighboring carbon atom. In learning chemistry, the 2pz-electrons in are known as pi-electrons, mobile electrons, or unsaturation electron particles when they form the pi bond.
Alkenes or Olefins Molecule
The double bond of the alkene molecule known as ‘olefinic bond or ‘ethylenic bond’. But the organic naming of olefin molecules defines from ethylene which is known ‘olefiant gas'( oil-forming gas of our environment). Since alkenes form an oily liquid when oxidation with chlorine or bromine water, or dilute base solution of permanganate. Therefore, alkenes are called olefins. The double bond presence in an alkene is readily detected by electromagnetic absorption infrared spectrum with C=C stretching frequency 1680 to 1620 cm-1.
The original name of the homologous series alkenes was olefine. But later decided to reserve the suffix – ine for basic substances only. Since the naming of olefins compounds gained and use widely for alkenes. For example, some common alkenes molecules are ethylene (CH2=CH2), propylene (CH3-CH=CH2), butylene (CH3-CH2-CH=CH2), etc.
Common naming of Alkene or Olefin
Common naming of alkenes or olefins compounds from the corresponding alkanes molecules and named according to the following rule
- The total number of carbon atom counted in the olefin and the name of the corresponding alkane.
- Changing the name of the corresponding alkane, the suffix -ane of the latter into – ylene.
- Defined position of the olefin bond indicated by numbers 1, 2, 3, 4…., or Greek letters α, β, ⋎, ઠ,…
- The locants of the double-bonded carbon atom placed before the name of the olefin.
- A hyphen has written between the locants and the name.
The locants used only to name alkenes containing more than three carbon atoms. Therefore, the alkenes with low molecular weight only have common names. Examples of such naming of alkene molecules are ethylene (CH2=CH2CH3), 1 – butylene or α – butylene (CH2CH2CH=CH2), 2 -pentylene or β – pentylene (CH3CH=CHCH2CH3), etc
Substituted Naming of Alkenes
In this method, ethylene considers as the parent substance. Hence the higher member is the derivatives of ethylene. When we naming of mono-substituted organic compounds no difficulty arises. But the disubstituted derivative of ethylene, naming as an asymmetrical and symmetrical hydrocarbon.
- When the groups attached to the same carbon atom of the olefins nomenclature as the asymmetrical organic compounds.
- But when the groups attached to the different carbon atom of the alkene or olefin nomenclature as the symmetrical. For example, methyl ethylene (CH3CH=CH2), ethyl ethylene (CH3CH2CH=CH2), as-dimethyl ethylene (CH3(H3C)C = CH2), sym-dimethyl ethylene (CH3CH = CHCH3).
IUPAC Nomenclature Alkenes or Olefins
- According to the IUPAC nomenclature of alkenes or olefins, the suffix name of the olefins is ene. Therefore, the series becomes the alkene series.
- Find the longest carbon chain structure containing the olefinic bond name as the parent alkene in the IUPAC nomenclature.
- The position of the double bond and side chains indicated by numbers or locants. The lowest number or locants gave to the double bond and placed before the suffix.
- Hence the name of alkenes obtained by changing the suffix – ane to ene. For example, alkanes like methane, ethane into alkenes methene, ethene, etc.
For example, the IUPAC name alkene containing a double bond and a side chain in the structure, CH3-CH=C(CH3)-CH2-CH3 is 3-methylpent-2-ene. The double bond in alkene or olefin compound regarded as a functional group like the alcoholic, carboxylic acid, carbonyl group, etc.
Structural Isomerism of Butylene and Pentylene
An organic compound that has the same molecular weight but differs in its structure said to the structural isomerism. Ethylene, propylene has no structural isomer but greater than three carbon atom show structural isomerism. For example, butylene with molecular formula C4H8 has three structural isomers and pentylene with molecular formula C5H10 has five structural isomers.
Now take each one in turn and introduce one double bond in isomeric pentene. Starting at the least substituted end and shifting the double bond inwards. Therefore, isomeric pentylene show five structural isomers. The IUPAC nomenclature of these structural, CH3CH2CH2CH=CH2 (pent-1-ene), CH3CH2CH=CHCH3 (pent-2-ene), CH3CH2C(CH3)=CH2 (2-methyibut-1-ene), CH3CH(CH3)CH=CH2 (3-methyibut-1-ene), CH3C(CH3)=CHCH3 (2-methyibut-2-ene).
Isomerism of Alkenes or Olefins Molecule
The stereoisomer’s dimensions in organic molecules depend on four important factors which are dipole-dipole interaction or polarization, Bond opposite strain, or polarity of covalent bonds in alkene, steric strain, and bond angle strain. Stereoisomers have the same chemical structure but different configuration. There are two types of stereoisomers, optical isomerism, and geometrical isomerism. Alkene or olefins are shows only geometric cis, trans isomers because they are not optically active molecules.