Ch 6 - Chemical Reactivity and Mechanisms Flashcards
breaking a bond requires an
input of energy
enthalpy
used to measure the exchange of energy between the bonding MO and their surroundings
- the surrounding molecules must transfer some of their kinetic energy to the system(the bond being broken) to break a bond
delta H =
q(at constant pressure)
the change of energy(delta H) for any process is defined as the exchange of kinetic energy, heat(q), between
a system and its surroundings at constant pressure
delta H is primarily determined by the amount of energy required to
break the bond homolytically
hemolytic bond cleavage
generates two uncharged species, radicals, each which bears an unpaired electron
radical
the two uncharged species generated from a hemolytic bond cleavage
- drawn with a single barbed arrow
heterolytic bond cleavage
generates two charged species, called ions
- drawn with a double barbed arrow(one becomes + while the other becomes -)
bond dissociation energy
the energy required to break a covalent bond via hemolytic bond cleavage
heat of reaction
the total enthalpy(delta H “naught”) change of a reaction(add up all bond energy changes)
- + delta naught = system increased in energy - - delta naught = system decreased in energy
exothermic
the system gives energy to the surroundings(- delta H naught)
endothermic
the system receives energy from the surroundings(+delta H naught)
chemists and physicist use opposite signs for delta H
- chemist from the system perspective
- physicist from the surround perspective(how much work can this do?)
entropy
the measure of disorder associated with a system
spontaneous
a process that involves an increase in entropy
we must take into account the surroundings during a chemical reaction
delta Stot(total enthalpy) = delta Ssys + delta Ssurr
in order for a process to be spontaneous the total entropy must
increase
the entropy of a reaction can decrease IF
the entropy of the surroundings increases in a way that offsets the decrease
two dominant factors affecting the delta S sys
- one mole(AB) of reactant -> two moles of product(A + B ups entropy)
- a cyclic compound -> acyclic(more freedom of motion and conformations ups the entropy)
entropy is the one and only criterion that determines
whether or not a chemical reaction will be spontaneous
delta S tot =
delta Ssys + delta Ssurr
- the total must always be positive(either component can be negative but entropy is always increasing
delta Ssurr =
-(deltaHsys/T)
delta Stot =
(-deltaHsys/T) + deltaSsys
gibbs free energy
- -TdeltaStot = deltaG
- delta G = deltaH – (T)(deltaSsys)
- delta G = deltaH +(-T)(deltaS)
- in some cases this nonstandard presentation will allow for a more efficient analysis of the competition between two terms
if deltaStot must be positive then
deltaG must be negative for a process to be spontaneous
in order for a process to be spontaneous, deltaG for that process must be
negative
Keq =
[products]/[reactants]
deltaG =
- RT(lnKeq)
- R = 8.314 J/mol*K
- T = kelvin
delta G is negative then products favored
Keq>1
delta G is positive then reactants favored
Keq
in order for a reaction to be useful(products to dominate over reactants) then delta G must
be negative thus Keq>1
thermodynamics
the study of how energy is distributed under the influence of entropy
- the thermodynamics of a reaction specifically refers to the study of relative energy levels of reactants and products
spontaneous does not mean a reaction will occur suddenly
nothing to do with speed of a reaction but about if the reaction favors the formation of products
kinetics
the study of reaction rates
rate equation
the rate of reaction is described as this
Rate =
k[reactants]
- k = rate constant - [reactants] = concentration of reactants
first order reaction
the sum of exponents is 1
- rate = k[A]
second order reaction
the sum of exponents is 2
- rate = [A][B]
third order reaction
the sum of exponents is 3
- rate = [A]^2[B]
Factors Affecting the Rate Constant
- Energy of Activation(Ea)
- Temperature(Celsius)
- Steric Considerations
- Catalyst and Enzymes
Factors Affecting the Rate Constant
Energy of Activation
- the energy barrier(the hump graphically) between the reactants and products
- represents the minimum amount of energy required for a reaction to occur between two reactants that collide
- at any specific temperature the reactants will have a specific average kinetic energy(some higher and some lower)
- a low Ea(small barrier) will lead to a fast reaction
Factors Affecting the Rate Constant
Temperature(in Celsius)
- raising the temperature(increasing kinetic energy) will increase the rate of a reaction
- rule of thumb:
- a 10 degree Celsius increase will cause the rate to double
Factors Affecting the Rate Constant
Steric Considerations
geometry and the reactants orientation during a collision can have an impact on the rate of reaction
Factors Affecting the Rate Constant
Catalyst and Enzymes
- catalyst – a compound which can speed up the rate of a reaction without itself being consumed by the reaction
- catalyst provide an alternative pathway with a smaller Ea
- catalyst do not change the energy of the reactants or products
- the equilibrium is not affected by the presence of a catalyst only the rate of the reaction(lowers the Ea)
- enzyme – naturally occurring compounds that catalyze very specific biologically important reactions
kinetics and thermodynamics are two entirely separate concepts
- don’t confuse them
- kinetics = rate of reaction
- thermodynamics = equilibrium concentrations of reactants and products
often that one reaction pathway is both thermodynamically and kinetically favored
can be two different products though(one thermodynamically favored while the other is kinetically favored)
intermediates are represented as
all local minima(valleys)
transition states are represented as
all local maxima(peaks)
transition state
a state through which the reaction passes
- cannot be isolated in this state - bonds are literally being broken and/or formed in this high energy state simultaneously
intermediates
have a certain lifetime and are not in the process of forming or breaking bonds
- very often encountered in reactions
Hammond Postulate:
in an exothermic process the transition state is
closer in energy to the reactants and therefore the structure of the transition state more closely resembles the reactants
Hammond Postulate:
in an endothermic process the transition state is
closer in energy to the products and therefore the structure of the transition state more closely resembles the products
ionic reactions(polar reactions)
involve the participation of ions as reactants, intermediates, or products
- typically intermediates - around 95% of reactions in Orgo 1
ionic reactions occur when
one reactant has a site of high electron density and the other has a site of low electron density
nucleophile
an electron rich center
- “nucleus lover” - characterized by its ability to react with a positive charge or partial positive charge
electrophile
an electron deficient center
- “electron lover” - characterized by its ability to react with a negative charge or partial negative charge
being able to identify the nucleophilic and electrophilic centers in any compound is one of the most important skills in all of organic chemistry
being able to predict the flow of electron density is vital
nucleophilic center is an electron rich atom that is capable of donating a pair of electrons
Lewis base is synonymous with nucleophilic
any atom that possesses a
localized lone pair can be nucleophilic
pie bonds can function as
nucleophiles(area of high electron density)
polarizability
the ability of an atom to distribute its electron density unevenly in response to external influences
- directly related to the size of the atom(and subsequently the number of electrons that are distant from the nucleus) - more electrons = more polarizable
electrophilic center is an electron deficient atom that is capable of accepting a pair of electrons
Lewis acid is synonymous with electrophile
carbocation
has an empty p orbital which functions as a site that can accept a pair of electrons, rendering the compound electrophilic
inductive effects of negatively charged atoms like chlorine can make
a carbon atom electrophilic
the tail of every curved arrow show where the electrons
are coming from
the head of every curved arrow shows where the electron(s)
are going
Patters of electron pushing
- Nucleophilic attack
- loss of a leaving group
- Proton transfers
- Rearrangements
Patters of electron pushing
nucleophilic attack
a nucleophile attacking an electrophile
Patters of electron pushing
Loss of leaving group
even if a chain of arrows must be used only combination shows one arrow pushing patterns to kick off a leaving group(typically halides)
Patters of electron pushing
Proton transfer
- 2 curved arrows minimum
- can be used for either something getting protonated or deprotonated
Patters of electron pushing
Rearrangements
- many types
- carbocations is one type
- neighboring alkyl groups will stabilize a carbocation through hyperconjugation
Patters of electron pushing
Rearrangements
hyperconjugation
the bonding MO associated with a neighboring CH bond slightly overlaps the empty p orbital of a carbocation by placing some of its electron density in the empty p orbital
Patters of electron pushing
Rearrangements
hydride shift
involved the migration og H^-(hydrogen atom with an extra electron(2 electrons total)
Patters of electron pushing
Rearrangements
primary, secondary, and tertiary refer to the number of alkyl groups attached directly to the positively charged carbon atom
tertiary carbocations are more stable(lower energy) than secondary which are more stable than primary
Patters of electron pushing
Rearrangements
methyl shift
a methyl group shifts to convert a secondary carbocation into a tertiary carbocation
- the methyl group must be attached to a carbon adjacent to the carbocation
all ionic mechanisms, regardless of complexity, are different combinations of the four characteristic patterns of electron pushing
- proton transfer
- loss of a leaving group
- carbocation rearrangement
- nucleophilic attack
concerted process
when using two arrow pushing patterns simultaneously
- different than stepwise mechanisms
Common arrow pushing sequence
Nucleophilic Attack -> loss of leaving group -> proton transfer
always avoid drawing sloppy arrows
always avoid drawing sloppy arrows
never place the tail of a curved arrow on
a positive charge
the head of an arrow should always be placed to show either
the formation of a bond or the formation of a lone pair
C,N,O can NEVER have more than
an octet(four orbitals)
two common carbocation rearrangement types
- hydride shifts
- methyl shifts
allylic carbocation
when a carbocation has the positive charge located in an allylic position
Reversible and Irreversible Reaction Arrows
nucleophilic attack
a reversible reaction arrow is generally used if the nucleophile is capable of functioning as a good leaving group
- an irreversible reaction arrow is used if the nucleophile is a poor leaving group
Reversible and Irreversible Reaction Arrows
Loss of leaving group
- a reversible reaction arrow is generally used if the leaving group is capable of functioning as a good nucleophile
- most leaving groups in Orgo 1 will be able to function as nucleophiles
Reversible and Irreversible Reaction Arrows
Proton Transfer
- technically all are reversible
- generally speaking irreversible reaction arrows are used for reactions in which the acids differ in strength by more than 10 pKa units
- if the pKa value is between 5 to 10 units either reversible or irreversible reaction arrows might be used depending on the context of discussion
Reversible and Irreversible Reaction Arrows
Carbocation Rearrangement
- generally drawn as irreversible processes
- the energy difference between secondary and tertiary is usually significant
