Download ORGANIC CHEMISTRY

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ORGANIC CHEMISTRY
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Isomerism
Types of Organic Compounds
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• Isomers have identical composition but
different structures
• Two forms of isomerism
• Vast majority of over 20 million known
compounds are based on C: organic
compounds.
• Generally contain C and H + other elements
• Great variety of compounds
– Constitutional (or structural)
– Stereoisomerism
• Constitutional
– Same empirical formula but different atomto-atom connections
• Stereoisomerism
– Same atom-to-atom connections but
different arrangement in space.
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Structural Isomers
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Stereoisomers:
Stereoisomers: Optical
Stereoisomers:
Stereoisomers: Geometric
Geometric isomers can occur when there is a
C=C double bond.
• Optical isomers are molecules with
non-superimposable
non-superimposable mirror images.
Such molecules are called CHIRAL
Pairs of chiral molecules are
enantiomers.
enantiomers.
• Chiral molecules in solution can rotate
the plane of plane polarized light.
•
•
Cis-2butene
Cis-2-butene
Trans-2-butene
Trans-2-butene
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Chiral Compounds and
Polarized Light
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Stereoisomers Isomers
Stereoisomers Isomers
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Chirality
generally occurs
when a C atom
has 4 different
groups attached.
Lactic acid isomers are
nonsuperimposable
Lactic acid
Chirality:
Chirality: Handedness in
Nature
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Chirality:
Chirality: Handedness in
Nature
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Stereoisomers in Nature
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Right- and lefthanded seashells
These molecules are non-superimposable
mirror images.
These amino acids are nonsuperimposable mirror images.
The DNA here is
right-handed
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Hydrocarbons
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Hydrocarbons
• Alkanes have the general formula
CnH2n+2
• CH4 = methane
• C2H6 = ethane
• C3H8 = propane
• C4H10 = butane
• C5H12 = pentane
• Compounds of C and H
• Subgroups:
– Alkanes:
Alkanes: C-C single bonds
– Alkenes:
Alkenes: C=C double bonds
– Alkynes:
Alkynes: carbon-carbon triple
bonds
– Aromatic: based on benzene
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Methane Hydrate, CH4(H2O)x
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Methane
Hydrate,
CH4(H2O)x
Methane Hydrate, CH4(H2O)x
Gas hydrates have been known for many years, and
combustion of a sample of methane hydrate is seen on
the front cover. Recently, however, vast deposits of
methane hydrate were discovered deep within
sediments on the floor of the world’s oceans. How these
deposits were formed is a mystery. But what is
important is their size. It is estimated that the global
methane hydrate deposits contain approximately 1013
tons of carbon, or about twice the combined amount in
all reserves of coal, oil, and conventional natural gas.
Now if scientists and engineers could only solve the
problem of extracting the methane conveniently and
safely!
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CH3CH2 CH2CH2 CH 3
Pentane
CH3
Hydrocarbons 19
& Structural
Isomerism
CH3CHCH2CH3
Isomers of C 5H12?
2-Methylbutane
CH3
H3CCCH3
CH3
2,2-Dimethylpropane
Note names of isomers
C5H12 has 3
structural isomers.
C6H14 has 5
C7H14 has 9
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Hydrocarbons: Alkanes
3 4
CH2CH3
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C
H
1
C
H
1-butene
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CH3
H
2
C
H
1
H3C
2
C
4
CH3
2
C
CH3
2-methylpropene
(isobutene)
H
H
cis-2-butene
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H3 C
H
H
H
2
3
H
H
Equatorial H
atom
H
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6
1
H
1
H
6
5
H
HH
3
H
H
H
2
H
H
H
H
H
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H
H
H
H
Boat form
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H
H
4
H
3
H
H
1
6
H
HH
H
H
2
H
Chair form
Reactions of Alkenes:
Alkenes:
ADDITION REACTIONS
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H2O add to the double bond
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CH3
2
C
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• Alkenes are unsaturated — more
bonds can form to the C atoms
• Molecules such as Br 2, H2, HCl,
HCl, HBr,
HBr, and
H
3
C
Axial H atom
HH
Chair form
• How many isomers are possible for a
compound with the formula C 4H8?
H
All compounds are flexible.
Cyclohexane,
Cyclohexane, C6H12, has interconverting
“chair” and “boat” forms.
Alkanes are colorless gases, liquids,
and solids
Generally unreactive (but undergo
combustion)
Not polar (or low polarity) and so are
not soluble in water.
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Alkenes:
Alkenes: Compounds with
C=C Double Bonds
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Hydrocarbons: Cycloalkanes
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C
H
C C + Br2
H
H
H
H
trans-2-butene
Alkenes—
Alkenes—
Many Occur Naturally
Br Br
H C C H
H H
1,2-dibromoethane
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An Addition Reaction
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Fat placed
in Br2 vapor
An Addition Reaction
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• Fats can be “hydrogenated” with H2.
Fat placed
in Br2 vapor
• The fat in bacon is partially unsaturated.
The fat adds Br 2 to the C=C bonds.
• Fats can be “hydrogenated” with H2.
Alkynes
• Alkynes have carbon-carbon triple bonds.
• C2H2: common name = acetylene
systematic name = ethyne
Preparation:
CaC22(s) + H22O(liq
liq)) -->
O(
O(liq)
C22H22 (g) + Ca(OH) 22(s)
Hffoo(C22H22, g) = +226.7 kJ/mol
kJ/mol
Hrxn
rxn for C22H22 + O22
= –1300 kJ/mol
kJ/mol
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An Addition Reaction
Peanut butter has partially hydrogenated
vegetable oil.
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Resonance in Benzene
Aromatic Compounds
• Benzene, C 6H6, in the
top 25 chemicals
produced in the U.S.
• Starting point for
hundreds of other
compounds.
• C6H6 has two resonance structures with
alternating double bonds.
electrons are delocalized over the ring.
• The
H
C
HC
H
C
H
H
CH
H
C
C
H
C
H
C
C
C
H
C
H
CH
Resonance structures of benzene, C
6 H6
H
C
HC
H
C
C
C
H
H
CH
Abbreviated representation
of resonance structures
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Resonance in Benzene
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Other Aromatic Hydrocarbons
Benzene Derivatives
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• CC bond order is _______________
• C–C single bond = 154 pm
C=C bond = 134 pm
• CC bonds in benzene = 139 pm
electrons delocalized
Toluene
Naming Benzene Derivatives
Cl
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1
2
6
3
5
Aniline
Phenol
C6H5NH2
TNT
trinitrotoluene
C6H5OH
C6H4CH3(NO2)3
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Reactions of Aromatics
• Substitutions — not additions — are
typical.
Ortho to Cl
CH3
Meta to Cl
+ CH3Cl
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Naphthalene
AlCl3
+ HCl
Para to Cl
1,4-dimethylbenzene
Common name: Para-xylene
AlCl3 is a catalyst. Catalysts typically used in
aromatic substitutions.
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