Alkenes and Alkynes

Alkenes

(i)  General formula CnH2n

(ii)Also known as olefins (olefiant = oil forming) because their first member C₂H₄ forms oily product with chlorine or bromine

(iii)Carbon atoms involved in double bond are sp2 hybridized having trigonal planar structure with an angle of 120^oC.

                                    

 

    The coplanar structure of C=C has been described below, e.g. C₂H₄

               

 

(iv)Alkenes have the following characteristic bond lengths and bond energy.

                                                C=C                            =C-H

            Bond length                sp²-sp²(σ)                  sp²-1s (σ)

                                              1.34 A                         1.108 A

            Bond energy              143.1 kcal mol^-1       98.69 kcal mol^-1

 

(v)In ethylene the C=C involves one σ bond formed by head on overlapping of sp²-sp² orbitals having bond energy 91.60 kcal as well as one Π bond formed by lateral overlapping of p-p orbitals of bond energy 51.5 kcal & thereby producing total bond energy of 143.1 kcal. That is why C=C (143.1 kcal) is more stronger than C-C (83 kcal) bond.

(vi)It is important to note that propylene has two types of carbon atoms:

                                                   CH₃-CH=CH₂

                                                4σ       3σ       3σ

                                                sp³      sp²      sp²

 

            Thus it has bond lengths   C-C = 1.50 A and  C=C = 1.34 A

                                             sp³-sp²                  sp²-sp²

            Also it has bond energy  C-C 84.48 kcal mol^-1 and C=C 143.1 kcal mol^-1

                                             sp³-sp²                      sp²-sp²

 

(vii) IUPAC nomenclature :

(a) The IUPAC name is derived from the IUPAC name of alkanes by replacing ending 'ane' by 'ene' alongwith the position of double bonds.

           (e.g. alkane - ane + ene = alkene). CH₃CH=CH-CH₃ is known as but-2-ene or butene-2 or 2-butene.

(b) In case of two double or two triple bonds, the ending 'ne' of alkanes is suitably replaced by diene or triene accordingly.

           CH₃CH=C=CH₂ : buta-1, 2-diene; CH₂=CH-CH=CH₂: buta-1. 3-diene

(viii) Alkenyl groups : Residual part left after the removal of one H atom from alkene is known as alkenyl group. According to IUPAC nomenclature, these groups are named by replacing terminal 'e' of alkene by 'yl' e.g.

Group	Trivial name	IUPAC name
2        1 CH2=CH-	vinyl	ethenyl
3        2       1 CH2=CH-CH2-	allyl	prop-2-enyl
1    2       3 -CH=CH-CH3	-	prop-1-enyl
1       2     3      4 -CH2-CH=CH-CH3	crotyl	but-2-enyl

     The numbering in alkenyl group is started from the carbon with free valencies.

(ix) Isomerism in alkenes : Alkenes show following isomerisms

            (i)         C₃H₆ : CH₃CH=CH₂       &         CH₂-CH₂     Ring chain isomerism

                                                                       CH₂

                                                                Cyclopropane

 

            (ii)        C₄H₈   (a)      CH₃CH₂CH=CH₂     but-1-ene

                                 (b)      CH₃CH=CH-CH₃     but-2-ene

                                 

(a)  and (b) show position isomerism

(a) and (c) show chain isomerism

(a) and (d) show ring-chain isomerism

(b) also shows Geometrical isomerism

                       

The number of isomers increase with increase in carbon atoms more rapidly than corresponding alkanes. (pentane has only 3 isomers)