step 1

Report
cyclohexane
cis-1,2-dimethylcyclohexane
cis-1,2-dimethylcyclohexane
axial-equatorial
trans-1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane
equatorial-equatorial
axial-axial
Optical isomerism
*
C* = Stereocenter
4 different
substituents
Br
Fischer projections
C*
I
C*
Cl
Cl
I
H
H
(+)
(-)
Br
Br
I
Br
*
H
Cl
Enantiomers
Cl
*
H
non-superimposable mirror images
I
Enantiomers
identical in most properties
differ in: 1.interaction with polarized light
2. interaction with chiral environments
Light interacts with molecules
when it passes through them
[interaction of electrical fields]
When light encounters mirror image
of molecule, interaction is reversed
Polarized light
One enantiomer - rotate light to the left (-)
Other enantiomer - rotate light to the right (+)
in 50/50 mix - no net rotation racemic mixture
one enantiomer - polarized light will be rotated
optical activity
Optical isomerism
*
C* = Stereocenter
4 different
substituents
bromochloro iodomethane
*
3-methyl hexane
*
*
bromocyclopentane
no C*
trans-1,3-dibromocyclopentane
Alkane Summary
1. Alkanes - sp3 hybridized
2. Relatively unreactive
Substitution with halogens
Combustion
3. Non-polar
IMF = London Dispersion Forces
size
structure
4. Free rotation around C-C bonds
conformations
5. Non-cyclic alkanes - structural isomers
6. Cyclic alkanes - geometric isomers
cis-, trans7. Alkanes - optical isomers
stereocenters
C*
alkyl halides
1o
1o
3o
1o
+
Cl2
uv
2o
.. ..
step 1 :Cl:Cl:
.. ..
.. .
2 :Cl
..
half-arrow = 1e-
form mostly
stability of free radicals 3o > 2o > 1o
3-methylhexane
+ OH-
*
no reaction
stereocenter
*
+
-
+ OH-
C+ electrophile
.. :OH
nucleophile
..
+ Br-
e- deficient
e- rich
Nucleophilic Substitution
SN1
*
+ OH-
*
+ Br-
reaction is 1st order in C7H15Br
zero order in OH-
reactants are optically active
products are optically inactive
rate = k [R – X]
Nucleophilic Substitution
SN1
rate determining step unimolecular
step 1
CH3
..
C2H5 – C – Br
. .:
C3H7
CH3
C2H5 –
C+
..
-:
: Br
..
slow
C3H7
C+ carbocation
(4 – ½ (6) – 0) = +1
reactants are optically active
products are optically inactive
rate = k [R – X]
Nucleophilic Substitution
SN1
step 1
CH3
..
C2H5 – C – Br
. .:
C3H7
step 2
CH3
HO
*
..
:
H–O
..
C3H7
CH3
..
-:
: Br
..
C2H5 – C+
slow
C3H7
CH3
C+
..
. .– H
-:O
C2H5C3H7
C2H5
reactants are optically active
products are optically inactive
CH3
H7C3
*
OH
C2H5
rate = k [R – X]
Nucleophilic Substitution
SN1
Ea
C+
carbocation intermediate
1st order in R – X
E
2 products
Nu- + R – X
Nu – R + X-
Nucleophilic Substitution
SN2
*
+
*
OH-
+ Br-
reaction is 1st order in C4H9Br
1st order in OH-
rate = k [R – X] [OH-]
reactants are optically active
products are optically active
Nucleophilic Substitution
SN2
rate determining step bimolecular
H
step 1
H
..
..
H–O
. .: C – Br
. .:
C2H5 CH3
C2H5
H
C2H5
Br
HO
..
H-O---C---Br
. .:
C2H5 CH3
transition state
H
HO – C *– CH3
C2H5
..
:Br:
..
slow
H
stereocenter inverted
CH3
CH3
reactants are optically active
products are optically active
rate = k [R – X][OH-]
-
Nucleophilic Substitution
SN2
Ea
Nu---C---X
1st order in R – X
1st order in Nu-
E
1 product
Nu- + R – X
Nu – R + X-
SN1 or SN2
H
R
H
R
R
SN1 carbocation 3o > 2o > 1o 3o R –X yes
2o R –X ?
1o R –X no
SN1 or SN2
SN2 transition state
1o R – X yes
2o R –X ?
3o R – X no
SN1 or SN2
SN1 3o R –X yes
2o R –X ?
1o R –X no
SN2 1o R – X yes
2o R –X ?
3o R – X no
Other factors influencing rates:
Nucleophile : charged OH- SH- I- CNneutral H2O NH3
Solvent
Temperature

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