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Alkenes - general formula: CnH2n
Chem 101
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6
6
2
5
4
5
3
4
3
2
cis-3,4-dimethyl-3-hexene
cis-2,3-dibromo-2-butene
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trans-3,4-dimethyl-3-hexene
trans-2,3-dibromo-2-butene
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Reactions of alkenes
addition reactions add to the two atoms that form the double bond
general reaction:
sp2 hybridized C atoms become sp3 hybridized
 Br2, Cl2
halogenation
Cl2
same rxn for Br2
3
addition rxns, cont’d
 H2
hydrogenation
Ni, 500 °C
H2
 H2O
hydration
H2SO 4
H 2O
Chem 101
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2
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addition rxns, cont’d
 HBr
HBr
Chem 101
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Mechanism of Addition Reactions
A mechanism is an explanation of how a reaction proceeds, e.g. HX
Two-step mechanism:
1st step:
π-bond breaks and new
C—H bond and cation form
2nd
step:
new bond forms between
negative bromide ion and
positive carbon
slow
fast
“energy
profile”
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Alkynes - general formula:
CnH2n-2
unsaturated at least one triple bond
C atoms sp hybridized
linear VSEPR geometry
1 σ bond and 2 π bonds
no geometric isomers
addition reactions (as per alkenes)
2 H2
Aromatic hydrocarbons
CH
contain a 6-membered ring with 3 double bonds
benzene CH
chemically unlike alkenes and alkynes no addition reactions
CH
naphthalene
CH
CH
CH
resonance structures delocalized pi electrons
more stable than 3 double bonds
anthracene
adjacent
toluene
1 carbon
between
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Reactions of aromatics
Electrophilic aromatic substitution reactions
in general:
all 6 H atoms on the ring are equivalent,
H2SO 4
HNO3
H 2O
Aromatic substitution rxns, cont’d
H2SO 4
FeBr3
Br2
HBr
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Functional groups
chemistry of organic molecules
determined by functional groups
group of atoms attached to hydrocarbon
have specific chemical behaviour regardless of hydrocarbon
most reactions involve regions of high electron density
functional groups have high electron density
pi bonds or nonbonded pairs of electrons
two examples already reviewed:
be able to identify
ALL of these
functional groups
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Isomers
same formula but different properties
Isomers
stereoisomers
structural isomers
same bonds –
different arrangements
different bonds
n-butane vs. 2-methylpropane
geometric
isomers
optical
isomers
( cis vs. trans )
Optical isomers
are nonsuperimposable mirror
images of one another
to have optical isomers need:
an atom (C) with 4 different substituents in a tetrahedral arrangement
molecule that has optical isomers
is chiral
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molecules that are optical isomers
are called enantiomers
usually have similar freezing points, melting points and densities
interact with polarized light differently
behave differently in a chemical environment
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