Organic II Flashcards
kinetics is about what?
reaction rates
define elementary reaction
one where bond breaking and making occur during a single collision
eg. proton transfer
requirements of a successful collision
collide in the correct orientation
have energy exceeding the activation energy
define transition state
the activated complex formed on collision
very short life time, unstable, maximum energy, cannot be isolated
what does the graph of a reaction with several steps look like?
energy vs reaction pathway
like hills, the dips are the places where intermediates are formed
the peaks are the transition states
sum of steps = overall reaction
define mechanism
the set of steps of the reaction
define ‘rate determining step’
the slowest step, with the HIGHEST activation energy
define intermediate
a reactive species produced in one step and consumed in a later step.
reactant –> intermediate
intermediate –> product
may be an unstable molecule, may be stable.
represented by the dip in the graph
what is a ‘radical’?
uncharged species having an unpaired electron
NOTE that radicals have NO charge!!
eg *CH2CH3
C has 3 bonding e- and one unpaired, total 4 e-, same as non bonded C hence no charge
In radical mechanisms, bond breaking is ______
homolytic (-lysis as in DESTRUCTION)
one atom at each of the products has one e- of the bond cleaved
A-B –> A* + B*
or Cl-C –> 2Cl with 8 e-
form radicals in the products
what type of arrow is used to show e- movement?
a fish hook arrow - single pronged
from a pi bond or nonbonding pair to an atom gives a sigma bond
from a sigma bond results in cleavage
tail more +ve, head more -ve
radical mechanism bond formation is ____
homogenic (-genic as in GENERATING something)
each reactant contributes 1 e-
A* + B* –> A-B
radicals in the reactants
steps in a radical mechanism
initiation
propagation
termination
define initiation step in radical mechanism
step 1
bond breaking
to FORM radicals in the products
define propagation step in radical mechanism
step 2 (and 3)
radicals in both the reactants and the products
these steps add to give the overall equation
define termination step in rad mech
the final step
two radicals react to give a molecule
radical reactants only
define polar mechanisms
set of steps where all species have paired electrons
bond breaking polar mech
heterolytic (-lytic = destruction)
A-B –> A:- + B+
both bonding e- are on one product, A
bond making polar mech
heterogenic (-genic = generate)
A:- + B+ –> A-B
polar reactions occur via ….
charged intermediates
eg. carbonation
define electrophile
electron lover
accepts both bonding e- from reaction partner (nucleophile)
metal cation / H+
the more electropositive atom in the reagent
also known as a Lewis Acid
define nucleophile
nucleon lover
the e- rich site (pi bonds, non-bonding pairs)
donates both bonding e-
may have a negative charge or no charge
Also a Lewis Base
in a polar bond, which atom is the electron rich/poor site?
the more positive atom = electron poor
which organic group can ONLY act as nucleophiles?
alkenes
because of pi e-, high e- density region
common nucleophiles
alkenes, alkynes, ethers, alcohols (-O-) C=O compounds amines and amides halide ions hydroxide ions
why is C=O more electrophilic than C-O?
because pi e- can be delocalised into O, giving O -ve and C +ve, so C is more electron deficient
better nucleophile has…
the non-bonding pair more available for reaction with the electrophile.
hence NH3 > H2O or HF, as N is less electronegative, so non-bonding e- more available
curly arrows
e- movement
the charge on an atom that has the same number of bonds in reactants and products (one arrow head, one tail) does not change
electrophilic addition mechanism
the process for adding a diatomic molecule across the C=C
- bond forms between C and the more electropositive atom of the reagent (electrophile)
breaking bond in C=C forming C-C
results in 2 CHARGED INTERMEDIATES (including a carbocation, the electrophile) - bond forms between two intermediates to form overall product
for an asymmetrical alkene, what does the major product depend on?
the relative energies of the two possible carbocation intermediates.
species are of LOWER energy (more stable) if the charge is DELOCALISED over more atoms
MAJOR product arises from LOWER energy intermediate (on graph, the lower dip)
greater delocalisation: tertiary>secondary>primary
explain the stereospecificity of the addition of bromine to cyclopentene
the intermediate has a bridged bromium ion (+)
blocks the approach of Br- from one side
one of these bonds is broken and another formed to the Br- ion to get a TRANS product
benzylic carbocations stability relative
major product on addition arises from Secondary carbocation, as charge can be delocalised on the ring by resonance
Br attached to the benzylic C
benzylic carbocations are MORE stable than tertiary carbocations with alkyl substituents
haloalkane to alkene
possible alkenes = #different groups bonded to the C w/ X
heat with a STRONG BASE dissolved in alcohol (KOH)
the opposite reaction of alkene + ACID HBr –> alkyl bromide,
alkyl bromide + BASE elimination –> alkene
Zaitseff’s rule
generally alkenes w/ fewer H (more substituted) on the double bond are more stable, hence faster rate of formation, produced in greater amounts.
types of substitution reactions
involving free radical intermediates eg. Halogenation, tertiary»_space;> (not in detail)
POLAR MECH: electrophilic or nucleophilic depending on the reagent group in the slow step of the mechanism
define nucleophilic substitution
substitution at a saturated C by a NUCLEOPHILE if the C of the nucleophile bears a good leaving group
eg. RX + Y:- –> RY + X:-
Y:- is the nucleophile - an atom w/ nonbonding e-
define electrophilic substitution
in the slow step of the mechanism, a small electrophile reacts w/ the nucleophile
eg. reaction of benzene with reagents (HNO3, RCl) results in the substitution of a ring H
(substituted NO3 or R group, and H2O or HCl)
catalyst required due to stability of delocalised pi system of aromatic hydrocarbons
what is a good leaving group?
in elimination and nucleophilic sub
good leaving groups give rise to ANIONS capable of supporting charge - atom bearing charge is large (eg I-) - charge delocalised/resonance stabilised weaker base (have stronger conj acids) thus most elimination occurs in base
stable anions or small molecules
I- > Br- > Cl-»_space; F- > CH3COO- > HO-
relative extents when elimination / substitution compete for reacts with alkyl halides
generally at elevated temp:
- primary: sub>elim
- tertiary: elim>sub
- secondary: mixture of sub/elim products
eg. tertiary at high temp in KOH elim –> alkene
vs primary + KOH warm –> alcohol sub
define enantiomer
stereoisomer with non-superimposable mirror images
C w/ 4 diff substituents (stereogenic, chiral centre)
define optically active
enantiomers differ in one physical property - direction in which a solution of the enantiomer rotates in PLANE-POLARISED LIGHT
rotates in equal and opposite directions
chemical differences of enantiomers
the rate of reaction with other chiral molecules is different
assigning absolute configuration to enantiomers
assign priorities to substituents
for the lowest priority group (usually H) facing “away” (dashed bond)
highest to lowest for the remaining three groups in anticlockwise direction = S
clockwise direction = R
cycloalkane C-C rotation
restricted rotation due to the ring, do not interconvert at room temp
cycloalkane enantiomers configurations
the have OPPOSITE configuration at ALL stereogenic centres
R, R becomes S, S
R, S becomes S, R
define diastereoisomers
stereoisomers that are NOT mirror images of each other
cycloalkane diastereoisomers
have some but not all stereogenic centres with w/ opposite configurations
eg. cis-1,2-dichlorocyclohexane and either enantiomer of trans-1,2-diclorocyclohexane
define meso form
compounds with TWO stereogenic centres that are NOT optically active due to symmetry planes
alkyne addition reactions
in sufficient reagent A-B, a second molar equivalent can be added to give the saturated compound.
1st mol A-B, RC(A)=(B)CR
this can be isolated
2nd mol A-B, RC(A2)-(B2)CR
Markovnikoff’s applies for asymmetric reagents to a terminal carbon
alkyne addition A-B relative orientation for H2 and halogens
H2 w/ Pt/Pd, or poisoned catalyst = cis
H2 w/ Li/NH3 (liquid metal ammonia reduction) = trans
Br2, Cl2, HCl, HBr = trans
hydration of alkynes reaction speed and conditions and products
slower than hydration of alkenes
requires catalyst and dil. H2SO4
produces an enol (triple –> double, new bond with OH)
rearranges by TAUTOMERISM to give a ketone (=O on the C with the -OH in the enol, by H moving and another bond going to O)
how to identify an electrophilic carbon
C is electrophilic if it is at the POSITIVE end of a Polar bond
reduction and oxidation for organic
reduction = increase in #C-H bonds, decrease in #C-O bonds
oxidation = increase in #C-O bonds
