exam #2 Flashcards
1
Q
bond breaking always requires
A
energy
2
Q
bond formation always
A
releases energy
3
Q
types of arrows
A
rxn arrow
double rxn arrow
double-headed arrow
full-headed curved arrow
half-headed curved arrow (fish hook)
4
Q
rxn arrow is drawn between
A
starting materials and products in an equation
5
Q
double rxn arrows are drawn b/w the
A
starting materials and products in an equilibrium equation
6
Q
double headed arrow
A
drawn between resonance structures
7
Q
full-headed curved arrow shows
A
movement of electrons
8
Q
half-headed curved arrow shows
A
movement of a single electron
9
Q
bond dissociation energy is the
A
energy needed to homolytically cleave a covalent bond
10
Q
energy absorbed or released by deltaH is
A
enthalpy change / heat of rxn
11
Q
deltaH is positive…
A
energy is absorbed
rxn is endothermic
12
Q
deltaH is negative
A
energy is released
rxn is exothermic
13
Q
bond breaking is
A
ENDOthermic
14
Q
bond making is
A
EXOthermic
15
Q
comparing bond dissociation is equivalent to
A
comparing bond strength
16
Q
stronger the bond,
A
higher its bond dissociation energy
17
Q
bond disassociation energies decrease
A
down a column of the PT
18
Q
generally, short bonds are
A
Stronger bonds
19
Q
halogen size increases,
A
bond strength decreases
20
Q
deltaH overall enthalpy change =
A
sum of deltaH bonds broken + sum of deltaH bonds formed
21
Q
deltaH =
A
deltaH (broken bonds) - deltaH (formed bonds)
22
Q
kinetics describes
A
reaction rates
23
Q
equilibrium constant (Keq) relates
A
the amount of starting material and product at equilibrium
24
Q
isomers are
A
have the same molecular formula but different placements
25
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
26
what is a three-dimensional arrangement is called
a configuration
27
stereoisomers differ in
configuration
28
what does achiral mean?
a molecule or object that is superimposable on its mirror image
29
what does chiral mean?
a molecule or object that is NOT superimposable on its mirror image
30
in 3D achiral configuration, the bonds and atoms (x)
align
31
achiral molecules usually contain a
plane of symmetry
32
chiral molecules do not have a
plane of symmetry
33
34
stereoisomers have
different 3D arrangement of atoms
35
two types of isomers
constitutional
stereoisomer
36
constitutional isomers differ
in the way atoms are connected to each other
37
constitutional isomers have
different IUPAC names
same/different functional groups
diff PHYSICAL and CHEMICAL properties
38
stereoisomers differ ONLY in the way
that the atoms are oriented in space
39
stereoisomers have identical
IUPAC names (differ with prefixes like cis or trans)
40
a particular 3D arrangement is a
configuration
41
constitutional isomers:
SAME molecular formula
DIFFERENT names
42
stereoisomers:
SAME molecular formula
SAME NAME except for the prefix
43
what does chiral mean
molecule or object that is NOT superimposable on its mirror image
44
socks are
superimposable on their mirror image so they are achiral
45
achiral
molecule or object that is superimposable on mirror image
46
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
47
enantiomers are
not superimposable stereoisomers
48
a carbon atom bonded to four different groups is
a tetrahedral stereogenic center
49
most chiral molecules contain
one or more stereogenic centers
50
what is a stereogenic center?
site in a molecule at which the interchange of two groups forms a stereoisomer
51
a carbon atom w/ four different groups is a tetrahedral stereogenic center b/c...
interchange of two groups converts one enantiomer into another
52
a carbon atom bonded to four different groups is a
stereogenic center
53
with no stereogenic centers, a molecule is usually
NOT chiral
54
with one tetrahedral stereogenic center, a molecule is
ALWAYS chiral
55
with two or more stereogenic center, a molecule
MAY or MAY NOT be chiral
56
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
57
achiral molecules usually contain a
plane of symmetry but chiral molecules do not
58
the presence of a plane symmetry makes a molecule
ACHIRAL
59
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)
60
any molecule with one tetrahedral stereogenic center is a
chiral compound and exists as a pair of enantiomers
61
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
62
labels for stereogenic centers
R or S
63
R goes
clockwise
64
S goes
counterclockwise
65
you have to assign..
priority to label stereogenic center R or S
66
if two atoms on a stereogenic center are the same,
assign priority based on atomic number of the atoms bonded to these atoms
67
one atom of higher atomic number determines
higher priority
68
if two isotopes are bonded to the stereogenic center,
assign priorities in order of decreasing mass number
69
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
70
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
71
in rotating a molecule about a single bond, the
rotation of three groups changes
72
in flipping a molecule 180 degrees,
the position of all four groups changes
73
for n stereogenic centers,
the max number of stereoisomers is 2^n
74
with one stereogenic center, there are always two stereoisomers which are
enantiomers
75
with two stereogenic centers, the
max number of stereoisomers is four
76
what must you NOT do when testing a compound's superimposablility
DO NOT break any bonds
77
diastereomers are
stereoisomers that are NOT mirror images
78
enantiomers are
stereoisomers that are mirror images
79
a meso compound is an
achiral compound that contains tetrahedral stereogenic centers
80
do meso compounds have a plane of symmetry
YES
81
identical compounds have the same
R,S designations at every tetrahedral stereogenic center
82
enantiomers have exactly opposite
R,S designations
83
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
84
if a compound has two stereogenic centers w/ the R configuration...
enantiomers is S,S
diastereomers are either R,S or S,R
85
chemical and physical ppties of two enantiomers are
IDENTICAL except in their interaction w/ chiral substances
86
two enantiomers have identical
physical properties (melting pt, boiling pt, solubility)
87
plane polarized light is
ordinary light consists of electromagnetic waves that oscillate in all plans perpendicular to the direction in which the light travels
88
passing light through a polarizer allows light in one
one plane to come through resulting in plane-polarized light (aka polarized light)
89
plane polarized light has an
electric vector that oscillates in a single plane
90
a polarimeter is an
instrument that allows plane-polarized light to travel through a sample tube containing an organic compounds
91
after the light exits the sample tube, an analyzer slit is
rotated to determine the direction of the plane of the existing polarized light
92
with achiral compounds, the light exists the sample tube
UNCHANGED
93
plane of polarized light is in the same
position it was before entering the sample tube
94
a compound that does not change the plane of
polarized light is said to be optically inactive
95
with chiral compounds, the plane of the polarized light is
rotated through an angle
96
the angle is measured in
degrees (observed rotation)
97
compound that rotates plane of polarized light
OPTICALLY ACTIVE
98
rotation of polarized light can be in the
clockwise or counterclockwise direction
99
dextrorotatory means
when rotation is clockwise (d)
100
levorotatory means
when rotation is counterclockwise (l)
101
two enantiomers rotate
plane-polarized light to an equal extent but in the opposite direction
102
racemic mixtures is
an equal amount of two enantiomers (optically INACTIVE)
103
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
104
specific rotation is defined using
a specific sample tube length (~1 dm), concentration, temp (25 C), wavelength (589 nm)
105
enantiomeric excess (ee) is
how much more there is of one enantiomers (aka optical purity)
106
ee =
% of one enantiomer - % of the other enantiomer
107
ee tells how much
one enantiomers is present in excess of racemic mixture
108
physical properties of diastereomers
different including optical rotation
109
chemical properties of enantiomers
two enantiomers have exactly the same chem ppties except for their rxn with chiral, non-racemic reagents
110
what's the reagent?
chemical susbtance with which an organic compound reacts (sometimes drawn on left side of equation w/ other reactants)
111
kinds of organic reactions?
substitution, elimination, addition
112
substitution is a reaction where
an atom or group of atoms is replaced by another atom or group of atoms
113
in a general substitution rxn,
Y replaces Z on a carbon atom
114
substitution rxns involve
sigma bonds
one sigma bond break and another forms at the same carbon atom
115
elimination is a reaction in which
elements of the starting material are "lost" and a pi bond is formed
116
in an elimination rxn,
two groups X and Y are removed from a starting material
117
what bonds are broken in an elimination rxn?
two sigma bonds are broken
pi bond is formed b/w adjacent atoms
118
most common examples of elimination ocur when
X = H and Y is a heteroatom more electronegative than carbon
119
addition is a reaction where
elements are added to a starting material
120
in an addition reaction,
new grous X and Y are added to a starting material
121
what bonds are broken/formed in an addition reaction?
pi bond is broken
two sigma bonds are formed
122
addition and elimination rxns are
OPPOSITES
a pi bond is formed in eliminiation rxns
pi bond is formed in addition rxns
123
to determine whether a reaction is a substitution, elimination or addition w/ a complex starting material,
concentrate on the functional groups that CHANGE
124
reaction mechanism is a
detailed description of how bonds are broken and formed as a starting material is converted to a product
125
a reaction mechanism describes the
relative order and the rate of bond cleavage and formation
126
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
127
a stepwise rxn involves
more than one step
starting material is first converted to an unstable intermediate then forms the product
128
reactive intermediate
unstable intermediate which goes on to form the product
129
when a bond is broken...
the electrons in the bond can be divided equally or unequally b/w the two atoms of the bond
130
homolysis or homolytic cleavage
breaking bond by equally dividing the electrons b/w the two atoms in the bond
131
heterolysis or heterolytic cleavage
breaking a bond by unequally dividing the electrons b/w the two atoms in the bond
132
homolysis and heterolysis require
energy
both processes generate reactive intermediates
133
homolysis generates
uncharged reactive intermediates w/ unpaired electrons
134
heterolysis generates
charged intermediates
135
ionic intermediates are seen in
polar reactions
136
radical are
intermediates in radical reactions
137
homolysis of the C-Z bond generates
two uncharged products w/ unpaired electrons
138
reactive intermediates with a
single unpaired electron is called a radical
139
most radicals are highly unstable because
they contain an atom that does not have an octet of electrons
140
radicals typically have no
CHARGE
141
radical reactions
intermediates in a group of rxns
142
heterolysis of the C-Z bond can generate a
carbocation or a carboanion
143
giving two electrons to Z and none to carbon generates
a positive charged carbon intermediates called a carbocation
144
giving two electrons to C and none to Z generates
a negative charged carbon species called a carbanion
145
both carbocations and carbanions are
unstable reactive intermediates
146
carbocation contains a
carbon atom w/ 6 electrons
147
carbanion has a negative charge on
carbon which is not very electronegative
148
carbocations are
electrophiles
149
carbanions are
nucleophiles
150
carbocations and carbanions can be
intermediates in polar reactions (reactions in which a nucleophile reacts w/ an electrophile)
151
homolysis and heterolysis generates
radicals, carbocations, carbanions (3 most reactive intermediates in orgo)
152
radical with carbon
surrounded by 7 electrons
153
carbocation w/ C
surrounded by 6 electrons
154
carbanion w/ C
lone pair
155
radical and carbocations are
electrophiles b/c they contain a carbon with no octet
156
carbanions are
nucleophiles b/c they contain a carbon with a lone pair
157
two radical can each donate
one electron to form a two electron bond
158
bond formation always
releases energy
159
two ions with unlike charges can
come together with the negatively charged ion donating both electrons to form the two-electron bond
160
bond dissociation energy is the
energy needed to homolytically cleave a covalent bond
161
enthalpy or heat of reaction
energy absorbed or released in any rxn
162
because bon breaking requires energy...
bond dissociation energies are always POSITIVE
163
homolysis is always
endothermic
164
bond formation always
releases energy (exothermic)
165
comparing bond dissociation energies is equivalent to
comparing bond strength
166
stronger the bond,
higher its bond dissociation energy
167
bond dissociation energies decrease
down a column of the PT
168
shorter bonds are
stronger bonds
169
overall enthalpy change =
sum of bonds broken - (-) sum of bonds formed
170
when deltaH is positive,
more energy is needed to break bond than is released in forming bonds
171
bonds broken in the starting material are
stronger than bonds formed in the product
172
when deltaH is negative,
more energy is released in forming bonds than is needed to break bonds
173
bonds formed in the product are stronger than
the bonds broken in the starting mateiral
174
thermodynamics describe how
the energies of reactants and products compare and what the relative amounts of reactants/products are at equilibrium
175
kinetics describes
reaction rates
176
the equilibrium constant, Keq, is a
mathematical expression that relates the amount of starting material and product at eqilibrium
177
when Keq > 1
equilibrium favors products and eq lies to the RIGHT
178
when Keq < 1
eq favors starting materials and lies to the LEFT
179
for reaction to be useful, eq must favor the
products
so Keq > 1
180
equilibrium always favors the species
lower in energy
181
when Keq > 1,
Keq is positive
deltaG is negative
energy released
182
when Keq < 1,
Keq is negative
deltaG is positive
energy is absorbed
183
compounds that are lower in energy have
increased stability
184
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
185
delta G depends on
delta H and the entropy change delta S
186
entropy change (delta S) is a measure of
the randomness in a system
more disorder, higher entropy
187
deltaS is (+) when the products are
more disordered than the reactants
188
deltaS is (-) when the products are
less disordered than the reactants
189
reactions resulting in increased entropy
are favored
190
delta G =
delta H - T (delta S)
191
energy of activation is the minimum
amount of energy needed to break the bonds in the reactants
192
larger activation energy,
greater the amount of energy that is needed to break bonds and slow the rxn rate
193
structure of transition state is somewhere between the
structures of the starting material and product
194
any bond that is partially formed or broken is drawn with a
dashed line
195
any atoms that gains or loses a charge contains a
partial charge in the transition state
196
transition states are drawn in
brackets with a superscript double dagger
197
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
198
where are transition states located?
at energy maxima
199
reactive intermediate B+ is located at
an energy minimum
200
the step with the higher energy transition state is the
rate-determining step
201
kinetics is the
study of reaction rates
202
activation energy is the energy barrier that must be
exceeded for reactants to be converted to products
203
higher concentration =
faster rate
204
higher temperature =
faster rate
205
delta G, delta H, delta K do not determine
the rate of a rxn
206
delta G/H/K indicate the
direction of the equilibrium and the relative energy of reactants and products
207
a rate law/rate equation shows
the relationship b/w the rxn rate and the concentration of the reactants (experimentally determined)
208
fast reactions have
large rate constants
209
slow reactions have
small rate constants
210
rate constant (k) and activation energy are
inversely related
211
high activation energy corresponds to a
small k (rate constant)
212
rate equation contains concentration terms for
ALL reactants in ONE step mechanism
213
rate equation contains concentration terms for only the
reactants involved in the rate-determining step in a multi-step rxn
214
order of a rate equation =
sum of the exponents of the concentration terms in the rate equation
215
order of a rate equation =
sum of the exponents of the concentration terms in the rate equation
216
a two step rxn has a slow
rate-determining step and a fast step
217
in a multi step mechanism, the reaction can occur
no faster than its rate-determining step
218
only the concentration of the reactants in the
rate determining step appears in the rate equation
219
a catalyst is a
substance that speeds up the rate of a reaction
- recovered unchanged in a rxn, does not appear in the product
220
uncatalyzed rxn has a
larger activation energy, slower rxn
221
catalyzed rxn has a
lower activation energy, faster rxn
222
