CHEM10007 Lecture Notes - Lecture 30: Ethylene, Butene, Pentene
12 Jun 2018
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LECTURE 30
ALKENES & POLYENES
CHAPTER
ALKENES
•The general formula of alkenes is CnH2n n = 2, 3, 4, 5, 6 etc. They contain one C=C bond
per molecule, which is their distinguishing feature. We can have an infinite number of Cs.
•Their nomenclature is analogous to alkanes, however we add the suffix ‘ene’ to the parent
alkane name (Eg. butene, ethene)
•Eg. Ethene C2H4 (right).
BONDING IN ETHENE - MOLECULAR ORBITALS
HYBRIDIZATION OF CARBON IN ETHENE
•Hybridisation gives three new 2sp2 orbitals = sp2 hybridised.
•One un-hybridised p orbital.
•Each electron in the p and 2sp2 orbitals is available for bonding.
•Three sp2 orbitals lie in a plane at 120 degrees to each other with p
orbitals perpendicular to the sp2 plane.
•Sigma framework shown: all single C-C and C-H bonds (right).
•When 2 sp2 hybridised C combine, they
overlap and form a strong sp2-sp2 sigma
(σ) bond.
•The remaining un-hybridised 2p
orbitals overlap to form a weaker pi (π)
bond.
•The stronger the bond, the shorter the
Document Summary
Alkenes: the general formula of alkenes is cnh2n n = 2, 3, 4, 5, 6 etc. They contain one c=c bond per molecule, which is their distinguishing feature. We can have an infinite number of cs: their nomenclature is analogous to alkanes, however we add the suffix ene" to the parent alkane name (eg. butene, ethene, eg. This is why c=c bonds are shorter than c-c: a combination of sp2-sp2 . & 2p-2p overlap results in sharing of. Bonding in ethene: the remaining 4 sp2 orbitals need only 4 h to form ( ) bond, the structure provides ethene with a stronger and shorter c-c bond, and h-c-h & h- C-c angle of about 120 : molecule is planar and not tetrahedral, there is no rotation of a c-c double bond (right), rotation requires breaking of pi-bond which is energetically unfavourable (64 kcal/mol, probably needs about 1000 degrees celsius).
