exam #2 Flashcards
bond breaking always requires
energy
bond formation always
releases energy
types of arrows
rxn arrow
double rxn arrow
double-headed arrow
full-headed curved arrow
half-headed curved arrow (fish hook)
rxn arrow is drawn between
starting materials and products in an equation
double rxn arrows are drawn b/w the
starting materials and products in an equilibrium equation
double headed arrow
drawn between resonance structures
full-headed curved arrow shows
movement of electrons
half-headed curved arrow shows
movement of a single electron
bond dissociation energy is the
energy needed to homolytically cleave a covalent bond
energy absorbed or released by deltaH is
enthalpy change / heat of rxn
deltaH is positive…
energy is absorbed
rxn is endothermic
deltaH is negative
energy is released
rxn is exothermic
bond breaking is
ENDOthermic
bond making is
EXOthermic
comparing bond dissociation is equivalent to
comparing bond strength
stronger the bond,
higher its bond dissociation energy
bond disassociation energies decrease
down a column of the PT
generally, short bonds are
Stronger bonds
halogen size increases,
bond strength decreases
deltaH overall enthalpy change =
sum of deltaH bonds broken + sum of deltaH bonds formed
deltaH =
deltaH (broken bonds) - deltaH (formed bonds)
kinetics describes
reaction rates
equilibrium constant (Keq) relates
the amount of starting material and product at equilibrium
isomers are
have the same molecular formula but different placements
how do stereoisomers differ from constitutional isomers?
stereoisomers only differ in the way the atoms are oriented in space
- identical IUPAC names
- same functional groups
what is a three-dimensional arrangement is called
a configuration
stereoisomers differ in
configuration
what does achiral mean?
a molecule or object that is superimposable on its mirror image
what does chiral mean?
a molecule or object that is NOT superimposable on its mirror image
in 3D achiral configuration, the bonds and atoms (x)
align
achiral molecules usually contain a
plane of symmetry
chiral molecules do not have a
plane of symmetry
stereoisomers have
different 3D arrangement of atoms
two types of isomers
constitutional
stereoisomer
constitutional isomers differ
in the way atoms are connected to each other
constitutional isomers have
different IUPAC names
same/different functional groups
diff PHYSICAL and CHEMICAL properties
stereoisomers differ ONLY in the way
that the atoms are oriented in space
stereoisomers have identical
IUPAC names (differ with prefixes like cis or trans)
a particular 3D arrangement is a
configuration
constitutional isomers:
SAME molecular formula
DIFFERENT names
stereoisomers:
SAME molecular formula
SAME NAME except for the prefix
what does chiral mean
molecule or object that is NOT superimposable on its mirror image
socks are
superimposable on their mirror image so they are achiral
achiral
molecule or object that is superimposable on mirror image
how to test chirality
1) draw molecule in 3D
2) draw mirror image
3) align all bonds + atoms
4) to superimpose a molecule and its mirror image, you cannot break any bonds to perform any rotation
enantiomers are
not superimposable stereoisomers
a carbon atom bonded to four different groups is
a tetrahedral stereogenic center
most chiral molecules contain
one or more stereogenic centers
what is a stereogenic center?
site in a molecule at which the interchange of two groups forms a stereoisomer
a carbon atom w/ four different groups is a tetrahedral stereogenic center b/c…
interchange of two groups converts one enantiomer into another
a carbon atom bonded to four different groups is a
stereogenic center
with no stereogenic centers, a molecule is usually
NOT chiral
with one tetrahedral stereogenic center, a molecule is
ALWAYS chiral
with two or more stereogenic center, a molecule
MAY or MAY NOT be chiral
a plane of symmetry is a
mirror plane that cuts a molecule in half so that one half of the molecule is a reflection of the other half
achiral molecules usually contain a
plane of symmetry but chiral molecules do not
the presence of a plane symmetry makes a molecule
ACHIRAL
what C atoms cannot be a tetrahedral stereogenic center?
CH2 and CH3 groups (more than 1 H bonded to C)
any sp or sp2 hybridized C (less than 4 groups around C)
any molecule with one tetrahedral stereogenic center is a
chiral compound and exists as a pair of enantiomers
different ways of drawing an enantiomers?
1) drawing an enantiomers as a reflection
2) drawing an enantiomer by inverting the configuration of a stereogenic center
labels for stereogenic centers
R or S
R goes
clockwise
S goes
counterclockwise
you have to assign..
priority to label stereogenic center R or S
if two atoms on a stereogenic center are the same,
assign priority based on atomic number of the atoms bonded to these atoms
one atom of higher atomic number determines
higher priority
if two isotopes are bonded to the stereogenic center,
assign priorities in order of decreasing mass number
to assign a priority to an atom that is part of a multiple bond,
treat a multiply bonded atom as an equivalent number of singly bonded atoms
how do you assign R or S to a molecule when the lowest priority group is not oriented towards the back?
rotate and flip the molecule until the lowest priority molecule is in the back
assign R or S
in rotating a molecule about a single bond, the
rotation of three groups changes
in flipping a molecule 180 degrees,
the position of all four groups changes
for n stereogenic centers,
the max number of stereoisomers is 2^n
with one stereogenic center, there are always two stereoisomers which are
enantiomers
with two stereogenic centers, the
max number of stereoisomers is four
what must you NOT do when testing a compound’s superimposablility
DO NOT break any bonds
diastereomers are
stereoisomers that are NOT mirror images
enantiomers are
stereoisomers that are mirror images
a meso compound is an
achiral compound that contains tetrahedral stereogenic centers
do meso compounds have a plane of symmetry
YES
identical compounds have the same
R,S designations at every tetrahedral stereogenic center
enantiomers have exactly opposite
R,S designations
diastereomers have the same
R,S designation for at least one stereogenic center
and the opposite for at least one of the other stereogenic centers
if a compound has two stereogenic centers w/ the R configuration…
enantiomers is S,S
diastereomers are either R,S or S,R
chemical and physical ppties of two enantiomers are
IDENTICAL except in their interaction w/ chiral substances
two enantiomers have identical
physical properties (melting pt, boiling pt, solubility)
plane polarized light is
ordinary light consists of electromagnetic waves that oscillate in all plans perpendicular to the direction in which the light travels
passing light through a polarizer allows light in one
one plane to come through resulting in plane-polarized light (aka polarized light)
plane polarized light has an
electric vector that oscillates in a single plane
a polarimeter is an
instrument that allows plane-polarized light to travel through a sample tube containing an organic compounds
after the light exits the sample tube, an analyzer slit is
rotated to determine the direction of the plane of the existing polarized light
with achiral compounds, the light exists the sample tube
UNCHANGED
plane of polarized light is in the same
position it was before entering the sample tube
a compound that does not change the plane of
polarized light is said to be optically inactive
with chiral compounds, the plane of the polarized light is
rotated through an angle
the angle is measured in
degrees (observed rotation)
compound that rotates plane of polarized light
OPTICALLY ACTIVE
rotation of polarized light can be in the
clockwise or counterclockwise direction
dextrorotatory means
when rotation is clockwise (d)
levorotatory means
when rotation is counterclockwise (l)
two enantiomers rotate
plane-polarized light to an equal extent but in the opposite direction
racemic mixtures is
an equal amount of two enantiomers (optically INACTIVE)
what is the observed rotation of an equal amount of two enantiomers?
two enantiomers rotate plane-polarized light to an equal extent but in opp directions and rotations cancel
specific rotation is defined using
a specific sample tube length (~1 dm), concentration, temp (25 C), wavelength (589 nm)
enantiomeric excess (ee) is
how much more there is of one enantiomers (aka optical purity)
ee =
% of one enantiomer - % of the other enantiomer
ee tells how much
one enantiomers is present in excess of racemic mixture
physical properties of diastereomers
different including optical rotation
chemical properties of enantiomers
two enantiomers have exactly the same chem ppties except for their rxn with chiral, non-racemic reagents
what’s the reagent?
chemical susbtance with which an organic compound reacts (sometimes drawn on left side of equation w/ other reactants)
kinds of organic reactions?
substitution, elimination, addition
substitution is a reaction where
an atom or group of atoms is replaced by another atom or group of atoms
in a general substitution rxn,
Y replaces Z on a carbon atom
substitution rxns involve
sigma bonds
one sigma bond break and another forms at the same carbon atom
elimination is a reaction in which
elements of the starting material are “lost” and a pi bond is formed
in an elimination rxn,
two groups X and Y are removed from a starting material
what bonds are broken in an elimination rxn?
two sigma bonds are broken
pi bond is formed b/w adjacent atoms
most common examples of elimination ocur when
X = H and Y is a heteroatom more electronegative than carbon
addition is a reaction where
elements are added to a starting material
in an addition reaction,
new grous X and Y are added to a starting material
what bonds are broken/formed in an addition reaction?
pi bond is broken
two sigma bonds are formed
addition and elimination rxns are
OPPOSITES
a pi bond is formed in eliminiation rxns
pi bond is formed in addition rxns
to determine whether a reaction is a substitution, elimination or addition w/ a complex starting material,
concentrate on the functional groups that CHANGE
reaction mechanism is a
detailed description of how bonds are broken and formed as a starting material is converted to a product
a reaction mechanism describes the
relative order and the rate of bond cleavage and formation
one step rxn is called a
concerted rxn
no matter how many bonds are broken or formed, a starting material is converted directly to a product
a stepwise rxn involves
more than one step
starting material is first converted to an unstable intermediate then forms the product
reactive intermediate
unstable intermediate which goes on to form the product
when a bond is broken…
the electrons in the bond can be divided equally or unequally b/w the two atoms of the bond
homolysis or homolytic cleavage
breaking bond by equally dividing the electrons b/w the two atoms in the bond
heterolysis or heterolytic cleavage
breaking a bond by unequally dividing the electrons b/w the two atoms in the bond
homolysis and heterolysis require
energy
both processes generate reactive intermediates
homolysis generates
uncharged reactive intermediates w/ unpaired electrons
heterolysis generates
charged intermediates
ionic intermediates are seen in
polar reactions
radical are
intermediates in radical reactions
homolysis of the C-Z bond generates
two uncharged products w/ unpaired electrons
reactive intermediates with a
single unpaired electron is called a radical
most radicals are highly unstable because
they contain an atom that does not have an octet of electrons
radicals typically have no
CHARGE
radical reactions
intermediates in a group of rxns
heterolysis of the C-Z bond can generate a
carbocation or a carboanion
giving two electrons to Z and none to carbon generates
a positive charged carbon intermediates called a carbocation
giving two electrons to C and none to Z generates
a negative charged carbon species called a carbanion
both carbocations and carbanions are
unstable reactive intermediates
carbocation contains a
carbon atom w/ 6 electrons
carbanion has a negative charge on
carbon which is not very electronegative
carbocations are
electrophiles
carbanions are
nucleophiles
carbocations and carbanions can be
intermediates in polar reactions (reactions in which a nucleophile reacts w/ an electrophile)
homolysis and heterolysis generates
radicals, carbocations, carbanions (3 most reactive intermediates in orgo)
radical with carbon
surrounded by 7 electrons
carbocation w/ C
surrounded by 6 electrons
carbanion w/ C
lone pair
radical and carbocations are
electrophiles b/c they contain a carbon with no octet
carbanions are
nucleophiles b/c they contain a carbon with a lone pair
two radical can each donate
one electron to form a two electron bond
bond formation always
releases energy
two ions with unlike charges can
come together with the negatively charged ion donating both electrons to form the two-electron bond
bond dissociation energy is the
energy needed to homolytically cleave a covalent bond
enthalpy or heat of reaction
energy absorbed or released in any rxn
because bon breaking requires energy…
bond dissociation energies are always POSITIVE
homolysis is always
endothermic
bond formation always
releases energy (exothermic)
comparing bond dissociation energies is equivalent to
comparing bond strength
stronger the bond,
higher its bond dissociation energy
bond dissociation energies decrease
down a column of the PT
shorter bonds are
stronger bonds
overall enthalpy change =
sum of bonds broken - (-) sum of bonds formed
when deltaH is positive,
more energy is needed to break bond than is released in forming bonds
bonds broken in the starting material are
stronger than bonds formed in the product
when deltaH is negative,
more energy is released in forming bonds than is needed to break bonds
bonds formed in the product are stronger than
the bonds broken in the starting mateiral
thermodynamics describe how
the energies of reactants and products compare and what the relative amounts of reactants/products are at equilibrium
kinetics describes
reaction rates
the equilibrium constant, Keq, is a
mathematical expression that relates the amount of starting material and product at eqilibrium
when Keq > 1
equilibrium favors products and eq lies to the RIGHT
when Keq < 1
eq favors starting materials and lies to the LEFT
for reaction to be useful, eq must favor the
products
so Keq > 1
equilibrium always favors the species
lower in energy
when Keq > 1,
Keq is positive
deltaG is negative
energy released
when Keq < 1,
Keq is negative
deltaG is positive
energy is absorbed
compounds that are lower in energy have
increased stability
because deltaG depends on the logarithim of Keq,
a small change in energy corresponds to a large difference in the relative amount of starting material + product at equilibrium
delta G depends on
delta H and the entropy change delta S
entropy change (delta S) is a measure of
the randomness in a system
more disorder, higher entropy
deltaS is (+) when the products are
more disordered than the reactants
deltaS is (-) when the products are
less disordered than the reactants
reactions resulting in increased entropy
are favored
delta G =
delta H - T (delta S)
energy of activation is the minimum
amount of energy needed to break the bonds in the reactants
larger activation energy,
greater the amount of energy that is needed to break bonds and slow the rxn rate
structure of transition state is somewhere between the
structures of the starting material and product
any bond that is partially formed or broken is drawn with a
dashed line
any atoms that gains or loses a charge contains a
partial charge in the transition state
transition states are drawn in
brackets with a superscript double dagger
for a 2-step rxn,
an energy diagram must be drawn
two E diagrams combin to form a diagram for the overall 2step rxn
each step has its own energy barrier with a transition state at the energy max
where are transition states located?
at energy maxima
reactive intermediate B+ is located at
an energy minimum
the step with the higher energy transition state is the
rate-determining step
kinetics is the
study of reaction rates
activation energy is the energy barrier that must be
exceeded for reactants to be converted to products
higher concentration =
faster rate
higher temperature =
faster rate
delta G, delta H, delta K do not determine
the rate of a rxn
delta G/H/K indicate the
direction of the equilibrium and the relative energy of reactants and products
a rate law/rate equation shows
the relationship b/w the rxn rate and the concentration of the reactants (experimentally determined)
fast reactions have
large rate constants
slow reactions have
small rate constants
rate constant (k) and activation energy are
inversely related
high activation energy corresponds to a
small k (rate constant)
rate equation contains concentration terms for
ALL reactants in ONE step mechanism
rate equation contains concentration terms for only the
reactants involved in the rate-determining step in a multi-step rxn
order of a rate equation =
sum of the exponents of the concentration terms in the rate equation
order of a rate equation =
sum of the exponents of the concentration terms in the rate equation
a two step rxn has a slow
rate-determining step and a fast step
in a multi step mechanism, the reaction can occur
no faster than its rate-determining step
only the concentration of the reactants in the
rate determining step appears in the rate equation
a catalyst is a
substance that speeds up the rate of a reaction
- recovered unchanged in a rxn, does not appear in the product
uncatalyzed rxn has a
larger activation energy, slower rxn
catalyzed rxn has a
lower activation energy, faster rxn
