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Organic basics
Alkenes
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Olivia Johansson
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Alkene
Like an alkane's
wild
younger brother, more
reactive
Alkenes
Unsaturated
hydrocarbons
Have the
general
formula CnH2n
Have at least one C=C
double
covalent bond
Examples
of alkenes
Propene
CH3CH
=
CH2
Buta-1,3-diene
CH2=CHCH=CH2
Double
bond
Made up of a
sigma
(σ) bond and a
pi
(π) bond
Formation
of sigma (σ) bond
1. Two s orbitals overlap in a
straight line
2. Gives highest possible
electron density
between nuclei
3. Results in a
single
covalent bond
Formation
of pi (π) bond
1. Sideways overlap of
two
adjacent p orbitals
2.
Electron density
spread out
above
and below molecular axis
Sigma (σ) bonds have
higher
bond enthalpy than pi (π) bonds
Alkanes
only have
sigma
(σ) bonds
Alkanes
don't
react easily
Alkenes
have both sigma (σ) and
pi
(π) bonds
Alkenes are more
reactive
than alkanes
Alkenes are useful starting points for making other
organic
compounds and
petrochemicals
Carbon atoms in a C=C
double
bond and attached atoms lie in the same plane (
trigonal planar
)
Atoms cannot
rotate
around C=C
double
bonds
Stereoisomers
Molecules with the same
structural
formula but different arrangement in
space
E/Z isomerism
Type of stereoisomerism in alkenes due to restricted rotation around double bond
isomer
Alkene
stereoisomer with same groups positioned across the
double
bond
isomer
Alkene stereoisomer
with
same
groups positioned on the same side of the double bond
Determining
E/Z isomers using Cahn-Ingold-Prelog (CIP) rules
1. Look at
atoms
directly bonded to each
C=C
carbon
2. Assign higher priority to atoms with larger atomic number
3. Determine E/Z based on arrangement of
higher priority groups
E
/Z isomers can also be called
cis-trans
isomers
Cis
means same groups on same side of double bond, trans means
opposite
sides</b>
Cis/trans naming system doesn't work if
carbons
have totally
different
groups attached
E/Z system still works even if
carbons
have different
groups
attached