Physical Organic and Amines Flashcards
sp3 amines
can invert, going through a sp2 transition state
at room temp - raceimisation 1E16-18 s-1
5-6 kcal mol-1 to do this (methane 110 kcal mol-1 -chiral centres)
ammonium salt can’t go through inversion
ammonium salt conformation
ammonium salt can’t go through inversion
ammonium salts e.g. N(Ph)(Pr)(Me)(Et) are conformationally stable - chiral centre (4 R groups)
if one group was instead a H - stable as long as no trace of base. base would lead to exchange of proton and therefore raceimisation
sp2 amines
need a driving force that is able to overcome the steric compression resulting from sp3 –> sp2
moving lone pair into p-orbital
e.g pyridine - electrons in p-orbital as they are needed for aromaticity so energetically favourable. sp2 lope of N facing out of ring perpendicular to pi-system makes it a good base
e.g. pyrrole: 4 carbons, each have an unhybridised p-orbital (4e-) Nitrogen adopts sp2 to donate lone pair in an unhybridised p-orbital - aromaticity driving force
aniline structure/ hybridisation
PhNH2 can be sp3 with l.p. - no steric hindrance
sp2 - some aromaticity but steric hindrance from hydrogens which can be overcome
R groups on benzene leads to more steric bulk and therefore more sp3 character
depending on sp3 character - better or worse base
aniline is not a good base
stability/ pKa of ammonium ions
NH4+ - RNH3+ - R2NH2+ - R3NH+
stability causes pKa to increase ( becoming more basic)
stability increases as no. R groups increase but peaks at R2NH2+
tertiary ammonium has steric hindrance - positive charge hard to solvate so pKa goes down
stability increases by induction (low no. R to high)
stability increases by solvation (small = easier to solvate)
properties of amines
strong/pungent smelling
water solubility - small side chains means more soluble
synthesis of amines
problem: POLYALKYLATION using an alkyl halide (?) and ammonia - the alkyl group pushes e- density onto the N making the target material more nucleophilic, resulting in a complex mixture
overcome by using excess ammonia (cheap and easy to remove)
acidity of organic compounds
consider ionic component first
Factors that stabilise A- make AH a strong acid, lower pKa
Factors that stabilise +AH make it a weaker acid, higher pKa
pKa equations
p = -log10 p[H+] = pH = -log[H+] pKa = p([A-]/[AH]) + pH
to measure pKa directly
pH meter in HA and H2O
add -OH in small aliquots and measure pH
measure pKa within limits of solvent being protonated (H2O) and solvent being deprotonated (H3O+)
Measure pKa by comparison
‘AH ‘A- + H+. pKa’
2AH 2A- + H+ pKa2
k’/k2, p(k’/k2) = difference in pKa
mix ‘AH and 2AH - equilibrate and measure ratios (all requires equilibrium)
Factors affecting the acidity of organic compounds
Induction Solvation Hybridisation Aromaticity Resonance Electrostatics ( I SHARE)
Induction
EWG remove e density from anion - stabilisation
as EWG acidity increases, stabilises anion side and so pKa goes down
expect opposite for EDG
Salvation
H2O: branching increases, pKa increases, solvation decreases
more ordered H2O - entropy lost so less stabilising
on going from water to organic solvent: most pKa increase
Hybridisation
more s character therefore charge held closer too the nucleus
sp3 - 25% s, pKa (ethane) 50
sp - 50% s, pKa (ethyne) 25
Aromaticity
if anion forms an aromatic system highly favoured
if it forms an anti-aromatic system, 4pi, highly disfavoured
Resonance/delocalisation
delocalised charge favoured over localised charge
dissociation occurs more readily, therefore more acidic
Electrostatics
if X is +vely charged then electrostatic stabilisation so favours dissociation
Kinetic order
experimentally measured
dependency on concentration of component
molecularity
no. molecules involved in RDS
elementary step: starting materials and product are connected by a single TS
zero order
no dependency on conc of component
graph - straight line (conc vs time)
gradient = kobs M s-1
half life halves
First order
as conc decreases rate decreases because of cnc dependency
half life is constant k = ln2/x (x= half life)
ln[A] = ln[A0] + kt
plot ln[A0]/ln[At], slope = kobs s-1
concentration doubles as rate doubles
Second order
concentration dependency
1/[A]t = 1/[A]0 + kt
to obtain kobs plot 1/[A]t against t
gradient = kobs M-1 s-1
Pseudo orders
arise when:
1. large excess of one reagent (kobs = k1 x [excess reagent]
(excess reagent conc is basically constant)
2. catalyst present (kobs = k1[cat])
3. phase transfer phenomena at play (kobs= k1[reagents])
Linear free energy relationships
aim to quantify the relationship between structure and reactivity i.e. how does rate vary systematically within structure
“Hammett” correlation
most widely used
RA –> RB
how does rate vary with R
Delta G affected by: steric factors and electronic factors
hammett considers both steric and electronic influence of R
but separated
On benzene, considered para and meta (orthodox is statically hindered)
substituent constant (sigma)
independent of the reaction
how much push or pull X has on the aromatic ring, reaction centre
log(Ka x/ Ka H) = sigma when x=H sigma= 0
K = equilibrium constants
quantified scale for the electronic influence of K from EDG to EWG
using substituent constant
To use these values:
1. Measure rates of reactions where X is varied from EDG –> EWG (5 points min.)
2. plot rates, log10(ka x/ka H) against sigma
3. is there a correlation?
no, reaction mechanism is changing as X is varied
yes, X is only affecting the rate and not the mechanism
4. measure the gradient - RHO (reaction constant)
dependent on the reaction
5. is rho +ve - EWG accelerates reaction
rho = 0 - X has no affect
rho -ve - EWG decelerates reaction
6. magnitude of rho tells us how sensitive the reaction is to change of X
rho
reaction constant - dependent on reaction
gradient from plot of log10(ka x/ ka H) (rate constants) against sigma
sigma (substituent constant) is log10(Ka x/ Ka H)
negative rho
EWG decelerates reaction
loss of electron density on approach to TS
EWG stabilises/slows down the reaction so is favoured
how to distinguish between SN1 and SN2 using rho
predict whether rho is +ve, zero or -ve for SN1 and SN2
experimental value - tells you which one
rate limiting step and rho
predict rho values for each step of reaction
experimental tells us which one is right (similar to SN1/2) because Hammett plot is using rate so RLS will affect rho
if rho is -ve RLS is loss of e- density. protonation is unlikely to be RLS, collapse of tetrahedral intermediate is likely to be RLS
