Hydrocarbons are commonly defined as the compounds which are mainly composed of carbon and hydrogen atoms. On the basis of saturation, hydrocarbons can be classified as alkanes, alkenes and alkynes. Alkanes are saturated hydrocarbons with single covalent bonds between carbon atoms.
DefinitionBack to Top
The un-hybridized orbital over each carbon atom involves in side-way overlapping to form pi-bond. The electron density of pi-bond gets distributed in two equal halves, above and below the plane of carbon atoms.
It makes the pi-bond weaker compare to sigma bond. Since pi-bond is placed perpendicular to the sigma bond, it hinders the free rotation of carbon –carbon bond in alkene which is mainly found in alkane as they do not have any restriction on the free rotation of carbon-carbon bond. That is the major difference between alkane and alkenes. Alkanes show different conformations due to free rotation of carbon-carbon bond which is absent in alkene and leads the formation of geometrical isomers of alkenes. Since the pi electrons are not as fully under the control of the carbon atoms like sigma electrons therefore they can easily attack by other reagents to form additional compounds.
FormulaBack to Top
Both the double bonded carbon atoms are sp2 hybridized and have three sp2 hybridized orbitals with one un-hybridized 2pz orbital. These three hybridized orbitals are arraigned in trigonal planer manner with 120o of bond angle.One of the sp2 hybridized orbital gets overlap with another sp2 hybridized orbital to form a sigma bond between both carbon atoms whereas remaining two hybridized orbitals form sigma bonds with hydrogen atoms.
Naming AlkenesBack to Top
Now give the lowest possible number to the double bond of alkene. In case of cycloalkane, the double bond will get always one number. Alkenes with same molecular formula but different position of double bond are called as constitutional (positional) isomers of each other.
The substituent of the alkene must be indicated with the position of them. In the presence of more than one substituent, the name of substituent must be written in alphabetical order. The cis and trans-isomers can be identified as cis and trans prefix before the root word followed by âene suffix. For Example: In CH3CH=CH2, the functional group is an alkene that indicated by suffix âene. The longest chain or parent chain contains three carbon atoms therefore the root word would be prop and complete name would be propene. Similarly in this alkene
the functional group is same that is âene with root word But- as there are four carbon atoms in the parent chain. The position of double bond is in between C-2 and C-3 therefore the IUPAC name would be but-2-ene or 2-butene. In case of cyclic alkenes like
the functional group is âene with longest continuous chain of six carbon atoms. Therefore the root word would be Hex-. The cyclic structure of alkene indicates by Cyclo-prefix. Hence the name of alkene would be Cyclohexene. In case of branched alkenes like (CH3)2CHCH=CHCH3, the longest chain or parent chain contains five carbon atoms, hence root word would be pent-.
The double bond is placed between C-2 and C-3, hence the locant would be 2-. There is a substituent methyl- at C-4 position, so the IUPAC name of molecule would be 4-methylpent-2-ene or 4-methyl-2-pentene.