Slide 1 : ALKYL HALIDES + STRONG BASE + HEAT ....... continued BIO MOLECULAR REACTION
Slide 2 : SUMMARY TO DATE STRONG BASE Required REGIOSELECTIVE Follows Zaitsev Rule (Unless Stereochemistry Prevents)
- favors most substituted alkene STEREOSPECIFIC b-H and X must be ANTI-COPLANAR ELIMINATION REACTIONS OF ALKYL HALIDES - acyclics may have to rotate
- rings may have to invert HEAT Usually
required
Slide 3 : KINETICS Examination of the rate expression often helps
to understand the mechanism. The rate expression is determined by experiment. KINETICS KINETICS
Slide 4 : RATE EXPRESSIONS RATE = K [A] [B] n m REACTION ORDER = SUM OF EXPONENTS = n + m rate “constant” concentrations of reactants
A and B in moles / liter exponents Not all reactants will necessarily show up in the rate expression. Fractional rate orders are possible : Actually will change with
temperature and solvent,
the specific molecule, etc. RATE = K [A] [B] 3/2 2 RATE
Slide 5 : CH3CH2CH2 CH CH3 Br + NaOCH3 + CH3OH Rate [RBr] [OCH3] x1 x1 x1
x2 x2 x1
x2 x1 x2
x4 x2 x2
x8 x4 x2
x9 x3 x3 KINETICS Rate = + d ( dt second order rate REACTION RATE DATA 35oC
Slide 6 : MECHANISM CONCERTED = only one step
All bonds are broken and formed without
the formation of any intermediates. Elimination Bimolecular E 2 strong
base alkyl
halide E
Slide 7 : Reaction Order Molecularity Rate-determining Step Transition State Activated Complex Sum of the exponents of the concentration terms
in the rate expression. Number of species that come together in the
rate-determining step. The slowest step in the reaction sequence. An energy highpoint in the energy profile
of a reaction. The species that exists at the transition state.
Slide 8 : ONE STEP PROCESS E2 NO INTERMEDIATE REACTION
Slide 9 : mechanism activated
complex d- d- E2 ELIMINATION Concerted : everything
happens at once with-
out any intermediates.
Slide 10 : Concerted (one step) reaction product starting
material transition
state TS activation
energy Ea heat of
reaction
DH E
N
E
R
G
Y This is what E2 looks like.
Slide 11 : The anti-coplanar arrangement of the b-H and
the halide leaving group X places the orbitals
that undergo change in a perfect alignment. The coplanar arrangement allows a continuous
movement of electrons from one end of the system
to the other, much like a stack of dominoes each
pushing the next one over. The two orbitals that will form the pi bond are
already parallel (anti-coplanar) so that the double
can form easily. ORBITAL ALIGNMENT IN MOST CONCERTED REACTIONS THE
ORBITALS BECOME PREALIGNED FOR
A SMOOTH PROGRESSION OF EVENTS
Slide 12 : C C H Br R R H H O CH3 : .. .. .. .. .. : : The attack of the base on the
b-hydrogen starts the reaction. sp3 sp3 When these electrons enter the
back lobe of the adjacent orbital
they “push” the bonding pair out
the other end (along with Br). The critical event is
the removal of the b-H. Notice the parallel aligment
of the two sp3 orbitals.
Slide 13 : C C H Br R R H H O CH3 .. .. .. : : .. . . The formation of the double bond
and the loss of bromide finish it. 2p 2p Note the parallel orbitals
in the pi bond.
Slide 14 : FRONTIER MO THEORY The LUMO is present on the b-H only in the
anti -coplanar arrangement.
Slide 15 : B: LUMO - ANTI CONFORMATION DENSITY-ELPOT LUMO HOMO B: - LUMO has
density on H Frontier theory
requires a base
or nucleophile
to add to the
LUMO. RECALL : negative end
of molecule
Slide 16 : SYN CONFORMATION B: - LUMO DENSITY-ELPOT LUMO has
no density
on any H
Slide 17 : KINETIC ISOTOPE EFFECT This “kinetic isotope effect” shows that breaking
the b C-H bond is a part of the rate-determining
step. The reaction slows if the b-H is replaced by D.
Slide 18 : ISOTOPES OF HYDROGEN PROTIUM DEUTERIUM TRITIUM H D T 1 2 3 NAME SYMBOL MASS COMPOSITION 1 proton + 1 electron 1 proton + 1 neutron
+ 1 electron 1 proton + 2 neutrons
+ 1 electron radioactive 99.985% 0.015% ( b- ) 12.26 yrs
Slide 19 : B D C H C H B r B H C H C H B r ISOTOPE EFFECT C-D bond is stronger than C-H Slows the reaction
if breaking this bond
is part of the
rate-determining step. kH kD approx. 5-8 C-H C-D for an isotope effect
Slide 20 : ORIGIN OF THE ISOTOPE EFFECT The effect is due to differences
in C-H and C-D bond strengths.
Slide 21 : + + ENERGY r (bond distance) r o o (H)1s C(sp3) + = nucleus (D) 1s BONDING CURVE Since D is heavier than H the
C-D bond vibrates slower over
a shorter distance.
Slide 22 : C-D BONDS ARE STRONGER THAN C-H BONDS ENERGY r (bond distance) o o bond dissociation
energies (CD > CH) CH CD D is heavier than H and the
CD bond vibrates more slowly
over a shorter distance than CH. bond breaks
here
Slide 23 : OTHER ELIMINATION MECHANISMS
Slide 24 : Three types of elimination reactions are conceivable E2 E1 E1cb concerted halogen
first proton
second proton
first halogen
second carbocation carbanion just studied