■ Step 3:Write the full name-Rings have"cyclo"prefix;if more than one C=C,indicate the position and use suffixes-diene, triene CH3 CH CH CHCH-CHCH CH CHCH-CHCH2CH 21 1 3 45 6 2-Hexene 2-Methyl-3-hexene CH:CH2 H 2C=C1 CH3 CH CH2CH2 H 543 HC-g-CH-CH2 3 2-Ethyl-1-pentene 2-Methyl-1,3-butadiene
Step 3: Write the full name - Rings have “cyclo” prefix; if more than one C=C, indicate the position and use suffixes –diene, - triene
Electronic Structure of Alkenes ■Carbon atoms in a p orbitals bond double bond are sp2- hybridized ■Three equivalent orbitals at120° separation in plane Fourth orbital is atomic orbitals T bond p orbital sp2carbon Carbon-carbon double bond Combination of electrons in two sp2 orbitals of two atoms forms o bond between them Additive interaction of p 90 rotatio orbitals creates aπ bonding orbital ■ Occupiedπorbital prevents rotation about o- bond Broken bond after rotation bond (p orbitals are parallel) (p orbitals are perpendicular)
Electronic Structure of Alkenes Carbon atoms in a double bond are sp 2 - hybridized Three equivalent orbitals at 120º separation in plane Fourth orbital is atomic p orbital Combination of electrons in two sp 2 orbitals of two atoms forms σ bond between them Additive interaction of p orbitals creates a π bonding orbital Occupied π orbital prevents rotation about σ - bond
Cis-Trans Isomerism in Alkenes The presence of a carbon- carbon double can create two possible structures CH3 CH3 cis isomer-two similar C=C groups on same side of H the double bond ■trans isomer similar cis-2-Butene groups on opposite sides Each carbon must have two different groups for these H CH3 isomers to occur C-C Since bond rotation cannot occur,this means that the CH3 H cis-isomer cannot trans-2-Butene spontaneously interconvert to the trans
Cis-Trans Isomerism in Alkenes The presence of a carboncarbon double can create two possible structures cis isomer - two similar groups on same side of the double bond trans isomer similar groups on opposite sides Each carbon must have two different groups for these isomers to occur Since bond rotation cannot occur, this means that the cis-isomer cannot spontaneously interconvert to the trans
Cis,Trans Isomers Require That End Groups Must Differ in Pairs Cis-trans isomerism is not limited to disubstituted alkenes. It can occur whenever both double-bond carbons are attached to two different groups. If one of the double-bond carbons is attached to two identical groups,then cis-trans isomerism is not possible These two compounds are identical; they are not cis-trans isomers. These two compounds are not identical; they are cis-trans isomers. B 2004 Thomson/Brooks Cole
Cis, Trans Isomers Require That End Groups Must Differ in Pairs Cis-trans isomerism is not limited to disubstituted alkenes. It can occur whenever both double-bond carbons are attached to two different groups. If one of the double-bond carbons is attached to two identical groups, then cis-trans isomerism is not possible