CHEM 0310 Lecture Notes - Lecture 11: Sigma Bond, Pi Bond, Carbon-13 Nuclear Magnetic Resonance

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6 Feb 2017
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Chapter 11 Alkenes
Naming the Alkenes
-To name these organic compounds, -ene is used at the end of alkane names
-Alkenes that have the same molecular formula but differ in the location of the double bond are
constitutional isomers (double bond isomers)
-Two alkenes of the same molecular formula only differing in their stereochemistry are called geometric
or cis-trans isomers; cis = same side of the double bond, trans = opposite sides of the double bond
-These are examples of diastereomers: stereoisomers that are no mirror images of each other
-The labels cis and trans cannot be applied when there are three or four different substituents attached
to the double-bond carbons
-An alternative system is called the E,Z system the sequence devised for priority in R,S are applied to
the two groups
-When the two groups of higher priority are on opposite sides, the molecule is E while if the two
substituents of higher priority appear on the same side, the molecule is a Z isomer
-Alcohols containing double bonds are called alkenols
-Substituents containing a double bond are called alkenyls
Structure and Bonding
-The double bond in alkenes consists of sigma and pi components
-Take ethene, which is carbons are sp2 hybrids
-The overlap in the sigma bond is considerably better than that in a pi bond because sp2 orbitals like
along the inter-nuclear axis
-We can measure the energy required to intercovert the alkene from cis to trans through thermal
isomerization
-In this process, the two p orbitals making up the pi bond are rotated 180 degree
-At the midpoint of this rotation 90 degrees, the pi bond (not the sigma) had been broken
-The activation energy for the reaction can be roughly equated with the pi energy of the double bond
-Thermal isomerization occurs only at high temperatures we can assign the activation energy to the
strength of the pi bond
Physical Properties
-The presence of the double bond does not greatly affect the boiling points of alkenes, compared with
alkanes, but cis-distributed alkenes usually have lower melting points than their trans isomers because
the cis compounds pack less in the solid states
-Alkenyl hydrogens are more acidic than those in alkanes because of electron withdrawing character of
the sp2 hybridized alkenyl carbon
NMR of Alkenes
-The double bond exerts characteristic effects on the chemical shifts of alkenes
-Deshielding is so pronounced in alkenyl hydrogens because of the movement of electrons in the pi
bond
-When subjected to an external magnetic field perpendicular to the double-bond axis, these electrons
enter into a circular motion
-This motion induces a local magnetic field that reinforces the external field at the edge of the double
bond
-When a double bond is not substituted symmetrically, the alkenyl hydrogens are nonequivalent, a
situation leading to observable spin-spin coupling
-Coupling between hydrogens on adjacent is called vicinal while coupling between nonequivalent
hydrogens on the same carbon is referred to as geminal
-In alkenes, geminal coupling is usually small
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