Structure & Stability Flashcards
(125 cards)
1
Q
What is the difference between a saturated and unsaturated molecule
A
◦ Saturated = no 𝜋 bonds and no rings
◦ Unsaturated = has at least one 𝜋 bond or a ring
2
Q
A saturated compound with n carbon atoms has exactly…
A
2n + 2 hydrogen atoms
3
Q
An unsaturated compound with n carbon atoms has…
A
Fewer than 2n + 2 hydrogens
4
Q
What is the formula to determine the degree of unsaturation (d) of a simple organic molecule?
A
(2n + 2) - x
____________
2
*n = number of carbons
*x = number of hydrogens on any monovalent atom (ie. halogens; F, Cl, Br, I)
*Ignore oxygens as the number of O has no effect
*For Nitrogen containing compounds, replace each N by 1 C and 1 H when using this formula
5
Q
What does one degree of unsatyration indicate?
A
The presence of one 𝜋 bond or one ring
6
Q
What does two degrees of unsaturation indicate?
A
That there are two 𝜋 bonds (two seperate double bonds or one triple bond), or one 𝜋 bond and one ring, or two rings, etc.
7
Q
What are the four types of electronic oribits that every pair of electrons must be housed in?
A
◦ s
◦ p
◦ d
◦ f
8
Q
What is the order than orbits get used in?
A
- s
- p
- p
- p
9
Q
For an atom with 2 electron groups, what is its orbital geometry and hybridization of the central atom?
A
◦ Orbital geometry: Linear
◦ Hybridization of central atom: sp
10
Q
For an atom with 3 electron groups, what is the orbital geometry and what is the hybridization of the central atom?
A
◦ Orbital geometry: trigonal planar
◦ Hybridization of the central atom: sp2
11
Q
For an atom with 4 electron groups, what is the orbital geometry and the hybridization of the central atom?
A
◦ Orbital geometry: tetrahedral
◦ Hybridization of central atom: sp3
12
Q
Define carbocations
A
Carbocations, or carbonium ions, are positively charged species witha full positive charge on carbon
◦ The reactivity of these species is determined by what type of carbon bears the positive charge
◦ Will always be sp2 hybridized with an empty p orbital
13
Q
Define carbanions
A
Carbanions are negatively charged species with a full negative charge localized on carbon.
◦ The reactivity of these species is determined by what type of carbon bears the negative charge
14
Q
For intermediates, why is the relationship b/w the stability and reactivity important?
A
B/c generally the reactivity of a molecule is inversely related to its stability
◦ Therefore, molecules that are more stable are less reactive, while higher energy species will be more reactive
15
Q
What are the two ways in which organic intermediates are stabilized?
A
- Inductive effects: stabilize charge through 𝜎 bonds
- Resonance effects: stabilize charge by delocalization through 𝜋 bonds
16
Q
For carbocations, describe the stability continuum from 3° to methyl
A
◦ 3° = more stable, less reactive, lower energy
◦ By methyl = less stale, more reactive, higher energy
17
Q
For carbanions, describe the stability continuum from 3° to methyl
A
◦ Methyl = more stable, less reactive, lower energy
◦ By 3°, less stable, more reactive, and higher energy
18
Q
Define electron-withdrawing and electron-donating
A
◦ Electron-withdrawing: groups pull electrons twoard themselves through 𝜎 bonds
◦ Electron-donating: groups donate (push) electron density away from themselves through 𝜎 bonds
19
Q
Groups more electronegative than carbon tend to ____, while groups less electronegative than carbon tend to _____.
A
- withdraw
- donate
20
Q
On the MCAT, alkyl substitutents are aways…
A
Electron-donating groups
21
Q
Electron-donating groups tend to stabilize…
A
electron-deficient intermediates (carbocations)
22
Q
Electron-withdrawing groups tend to stabilize…
A
Electron-rich intermediates (carbanions)
23
Q
Define the inductive effect
A
The stabilization of reaction intermediates by the sharing of electrons through 𝜎 bonds
24
Q
How is electronegativity determined?
A
On periodic table, moving from bottom left corner and increasing to top right corner
25
What is a conjugated system?
One containing three or more atoms that each bear a p orbital
◦ These orbitals are aligned so they are all parallel, creating the possibility of delocalized electrons
26
What does it mean for electrons to be localized?
Electrons that are confinsed to one orbital, either a bonding orbital b/w two atoms or a long-pair orbital
27
What does it mean for electrons to be delocalized?
When electrons are allowed to interact w/ orbitals on adjacent atoms and are no longer confined to their original 'space'
◦ The electron density is spread out in order to stabilize the carbocation
28
What bonds are found w/ a single bond, a double bond, and a triple bond?
◦ Single bond: 1 sigma bond
◦ Double bond: 1 𝜎 bond + 1 𝜋 bond
◦ Triple bond: 1 𝜎 bond + 2 𝜋 bonds
29
What are resonance structures?
Multiple representations of the actual structure
◦ The molecule exists as a combination of all resonance structures, although all may not contribute equally
30
Define resonance hybrid
The average of all resonance contributors
31
What are the corresponding angles for sp, sp2, sp3?
◦ sp = 180
◦ sp2 = 120
◦ sp3 = 109
32
The more stable a molecule is...
The less reactive it will be
33
What is the impact of the delocalization of charge on a molecule?
It tends to stabilize molecules, and therefore, resonance has a big impact on the reactivity of molecules
34
What are the three basic principles of resonance delocalization?
1. Resonance structures can never be drawn through atoms that are truly sp3 hybridized
2. Resonance structures usually involve electrons that are adjacent to (one atom away from) a 𝜋 bond or an unhybridized p orbital
3. Resonance structures of lowest energy are the most important
35
Why can resonance structures never be drawn through atoms that are truly sp3 hybridized?
B/c an sp3 hybridized atom is one with a total of four 𝜎 bonds are/or lone electron pairs, and therefore no resonance structure is possible
36
Why do resonance structures usually involve electrons that are adjacent to (one atom away from) a 𝜋 bond or an unhybridized p orbital?
B/c easily makes resonance restructures
37
Why are resonance structures of lowest energy the most important?
B/c it is the most stable and contributes the most to the overall resonance structure
38
What are the three criteria involved in determining the stability of a resonance structure?
1. Resonance contributors in which the octet rule is satisfied for all atoms are more important than ones in which it is not. This is the most important of the three criteria listed and takes priority over items 2 and 3 below
2. Resonance contributors that minimize seperation of charge (formal charge) are better than those with a large separation of charge
3. In structures with formal charge(s), the more important resonance contributor has negative charges on the more electronegative atom(s), and positive charge(s) on the less electronegative atom(s)
39
What is a Bronsted-Lowry acid?
A molecule that can donate a proton (H+)
◦ Once it does, the molecule takes on a negative charge
40
What is the deprononated structure of a molecule refered to as?
The conjugate base of the acid
Ie. (HCl is the acid, and Cl- is the conjugate base)
41
What does the strength of an acid refer to?
The degree to which it dissociates (or donates its proton) in solution
◦ The more the acid dissociates, the stronger the acid it is
◦ Acids that dossciate completely are strong
◦ Most organic acids are said to be weak acids because they do NOT dissociate completely in solution
42
Most organic acids are...
Weak acids b/c they do NOT dissociate completely in solution
43
The strength of acid is determined by...
the extent to which the negative charge on the conjugate base is stabilized
44
Are alcohols considered more or less acidic than hydrocarbons?
More acidic b/c negative charge located on O (the more electronegative atom)
45
Are carboxylic acids considered more or less acidic than alcohols?
More acidic due to delocalization of electrons possible for carboxylic acids
46
General rule of thumb for organic compound acidity
Strong acids > Sulfonic acids > carboxylic acids > phenols > alcohols & water > aldehydes and ketones (alpha hydrogens) > sp hybridized CH bonds > sp2 hybridized C=H bonds > sp3 hybridized C-H bonds
47
What are common examples of strong acids?
◦ HClO4
◦ H2SO4
◦ HNO3
◦ HCl
◦ HBr
◦ HI
48
What are sulfonic acids?
O
II
R - S - OH
II
O
49
Inductive effects increase or decrease with increasing distance?
Decrease
◦ The closer the electron-withdrawing group is to the acidic proton (or to the negative charge on the conjugate base), the greater the stabilizing effect
50
Electron-withdrawing substituents on phenols...
increase their acidity due to their ability to delocalize charges
51
Most organic reactions occur b/w ___ and ___
nucleophiles and ecetrophiles
52
What are nucleophiles?
Species that have unshared pairs of electrons or 𝜋 bonds, and frequently, a negative charge
◦ They are electron pair donors
◦ Lewis bases
53
Define nucleophilicity
A measure of how "strong" a nucleophile is
54
What are the three general trends for relative nucleophilicities?
1. Nucleophilicity increases as negative charge increases (Ie. NH2- is more nucleophilic than NH3)
2. Nucleophilicity increases going down the periodic table within a particular group (F- < Cl- < Br- < I-)
3. Nucleophilicity increases going left in the periodic table across a particular period (Ie. NH2- is more nucleophilic than OH-)
55
The second trend regarding nucleophilicity (that it increases going down the periodic table within a particular group) is the same trend as what?
◦ Polarizability - how easy it is for the electrons surrounding an atom to be distorted
◦ As you go down any group in the periodic table, atoms become larger and generally more polarizable and more nucleophilic
56
The third trend regarding nucleophilicity (that it increases going left in the periodic table across a particular period), is inversely related to what?
◦ Electronegativity
◦ The more electronegative the atom is, the better it is able to support its negative charge, therefore the less electronegative an atom is, the higher its nucleophility
57
What are electrophiles?
Electron-deficient species
◦ Have full or partial positive charge and love electrons
◦ Often have an incomplete octet
◦ Electron-pair acceptors
◦ Lewis acids
58
Define electrophilicity
A measure of how strong an electrophile is
59
Leaving groups are more likely to dissociate from their substrate if...
They are are stable in solution
60
What are some of the best leaving groups that are resonance stabilized?
◦ Tosylate
◦ Mesylate
◦ Acetate
61
What are some examples of weak bases that are good leaving groups?
◦ I-
◦ Br-
◦ Cl-
◦ b/c their negative charge is stabilized due to their large size
62
What strong acids make good electron donors and why?
◦ OH-
◦ RO-
◦ NH2-
◦ b/c they cannot stabilize their negative charge very well, making them very reactive
◦ Strong bases are bad leaving groups b/c they will not want to dissociate in solution
63
How can a bad leaving group be turned into a good leaving group?
with acid protonates a lone pair of electrons b/c it gets rid of the negative charge to be stabilized
◦ This trick is why many organic reactions are acid-catalyzed
64
Define ring strain
◦ arises when bond angles between ring atoms deviate from the ideal angle predicted by the hybridization of the atoms
◦ It contributes to instability in a molecule
65
What can be the result of deviation of bond angles from their 'normal' angles?
◦ Can cause those to be very high engery compounds
◦ The strain weakens the carbon-carbon bonds and increases reactivity of these cycloalkanes (for example)
66
What can help with ring strain?
hydrogenation reactions
67
Define constitutional isomers
Constitutional (or structural) isomers are compounds that have the same molecular formula, but have their atoms connected together differently
68
What is a conformational isomer?
Compounds that have the same molecular formula and the same atomic connectivity, but differ from one another by roation about a 𝜎 bond
69
For saturated hydrocarbons, there are ____ orientations of 𝜎 bonds attached to adhacent sp3 hybridized carbons that we will focus on
two
◦ Staggered conformation
◦ Eclipsed conformation
70
Define a staggered conformation
a 𝜎 bond one one carbon bisects the angle formed by two 𝜎 bonds on an adjacent carbon
71
Define an eclipsed conformation
The 𝜎 bond on one carbon directly lines up with a 𝜎 bond on an adjacent carbon
72
For butane, what is the difference in conformation between staggered and eclipsed?
◦ In a staggered conformation, the atoms are less chrowded and more stable
◦ In an eclipsed conformation, the atoms are more crowded, electronic repulsion, and less stable
73
Is a staggered or eclipsed conformation more stable?
A staggered confomation is more stables b/c
1. electronic repulsion (in a staggered conformation, the 𝜎 bonds are as far apart as possible, while they are directly aligned in an eclipsed conformation)
2. steric hindrance (it is more favourable to have atoms attached to the 𝜎 bonds in the staggered conformation where they are 60 degrees apart, vs. in eclipsed, where they are directly aligned
74
What is the most stable form of n-butane?
A staggered conformation called anti-conformation when the largest carbon groups are 180 degrees apart
◦ This form is the most sterically favourable
◦ Has the lowest energy conformation
75
What are the degrees and names for the rotational conformations?
@ 180 = anti
@ 60 = gauche
@ 0 = syn
@ 60 = gauche
@ 180 = anti
76
What is an anti conformation?
when the two largest groups attached to adjacent carbons are 180 degrees apart
77
What is a gauche conformation?
When the two largest groups on adjacent carbon atoms are in a staggered confomration 60 degrees apart
78
What is a syn conformation?
When the two largest groups on adjacent carbons are directly aligned behind one another and are therefore in the most crowded and unfaborable environment
79
What does it mean for something to be in "envelope" form?
When a carbon is shifted and 'puckered', allowing the carbon-hydrogen 𝜎 bonds on adjacent carbons to be somewhat staggered, thus reducing the energy of the compound
80
What is the 'chair' form of cyclohexame
A puckered molecule in its most stable conformation, where four of the carbons of the ring are in a plane with one carbon above the plane and one carbon below the plane
81
Can chair conformations move?
Yes, then can be switched between each other, passing through several other less stable conformations inluding half-chair conformations and the twist-boat conformation
82
What are the two types of carbons found in a chair comformation?
◦ equatorial hydrogens
◦ axial hydrogens
83
Differentiate b/w equatorial hydrogens and axial hydrogens
◦ Equatorial hydrogens lie on the equator of the ring of carbons (sideways ish)
◦ Axial hydrogens lie above or below the ring of carbons (vertically)
84
Other than changing shape, what else happens when a chair flip occurs
◦ hydrogens that had been equatorial become axial, and vice versa
85
What is the more stable position for large groups to occupy?
It is more favourable for large groups to occupy the equatorial position rather than a crowded axial position
86
What is a 1,3-diaxial interaction
The interaction that occurs when two hydrogens are occupying axial positions on the same side of a ring
87
Define stereoisomers
Molecules that have the same molecular formula and connectiity but differ from one another only in spatial arrangement of the atoms
◦ they cannot be interconverted by rotation of 𝜎 bonds
88
What does it mean for a molecule to be chiral
Any molecule that cannot be superimposed on its mirror image
89
What does it mean for a molecule to be achiral?
A molecule that can be superimposed on its mirror image and has a plane or symmetry
90
How can you indentify chiral centers?
For carbon, a chiral center will have four different groups bonded to it
◦ Therefore, it must be sp3 hybridized with 109 degree angles and be tetrahedral geometry
91
What is an absolute configuration?
A set of rules is involved in determining how chiral centers can be assigned an absolute configuration
92
What are the rules involved in assigning absolute configuration to a stereoceter?
1. Priority is assigned to each substituent on the center according to increasing atomic number of the atoms directly attached (if isotopes are present, priority is assigned based on atomic weight), if identical atoms are attached, move to the next atom, until a difference is present
2. A multiple bond is counted as two single bonds for both of the atoms involved
3. Once priority has been assigned, the molecule is rotated so that the lowest prority group points directly away from the viewer - then trace a path from the highest group to the lowest.
4. *if the path traveled is clockwise, then the absolte configuratuoin is R (right), if the path traveled is counterclockwise, then the absolute configuration is S (left)
93
Define enantiomers
Non-superimposable mirror images
94
Two molecules that are enantiomers will always...
Have opposite absolute configurations
95
What are the properties of enantiomers? How do they differ from one another?
Most chemical properties such as melting point, boiling point, polarity and solubility are the same
◦ The pure enantiomers shown above will have many identical physical properties
◦ A property that differs between enantiomers is the manner in which they interact with plane-polarized light (optical activity)
96
What does it mean for something to be optically active?
A compound that rotates the plan of polarized light
◦ type of optical activity has different names for if it rotates clockwise or counterclockwise thou
97
Define dextrorotatory (d)
a compound that rotates plane-polarized light in the clockwise direction (+)
98
Define levorotatory (l)
a compound that rotates plane-polarized light in the counterclockwise direction (-)
99
Define specific rotation
the magnitude of rotation of plane-polarized light for any compound
◦ This property is dependent on the structure of the molecule, the concentration of the sample, and the paht length through which the light must travel
100
A pair of enantiomers will rotate plane-polarized light ....
with equal magnitude, but in opposite directions
101
What do you think the specific roation of an equimolar mixture of the two enantiomers above will be?
Since one enantiomer will rotate plane-polarized light in one direction, while the other enantiomer will rotate light by the same magnitude in the opposite direction, the specific rotation of a 50/50 mixture of enantiomers - a racemic mixture is 0
◦ Therefore, a racemic mixture of enantiomers is known as a racemate, is not optically active
102
Is there a correlation between the sign of rotation and the absolte configuration?
No - none
103
What is the equation for determining the number of stereocenters from the number of chiral centers?
2^n, n = # of chiral centers
104
105
Define diastereomers
Stereoisomers that are not enantiomers
◦ Stereoisomers that are non-superimposable, non-mirror images
106
How do diastereomers differ?
Although their structure is similar, their physical and chemical properties can vary dramatically (they can have different melting points, boiling points, sulubilities, dipole moments, specific rotations, etc)
107
Is there a relationship b/w the specific rotation of diastereomers?
No - there is no way to predict the specific rotation of one diastereomer if you know the degree of rotation of another
108
Explain the process of resolution
The process of seperating a racemic mixture (if only one enantiomer is needed, but two are made)
◦ This is traditionally done through the use of an enantiomerically pure chiral probe, or resolving agent, that associates with the components of the mixture through either covalent bonds or intermolecular forces (like hydrogen bonds or salt interactions). The resulting products will be diastereomers, capable of separation due to their different physical properties
109
Define epimers
They are a subclass of diastereomers that differ in their absolute configurationat a single chiral center (only one sterocenter is inverted)
110
Differentiate between a D-sugar and an L-sugar
◦ A D-sugar occurs when the hydroxyl group on the highest-numbered chiral center in a Fischer projection is on the right of this carbon
◦ A L-sugar occurs when the hydroxyl group on the highest-numbered chiral center in a Fischer projection is on the left of this carbon
111
Is the labeling of a D or L sugar related to optical activity (+)/(-)?
No - it is completely unrelated
112
Compare how distinctions b/w D and L, R and S, and (+) and (-) can be made
◦ D and L can be determined by looking at a molecule
◦ R and S can be determined by looking at a molecule
◦ (+) and (-) can only be made by running experiments in a polarimeter
113
Explain what each of the following determine
◦ R or S
◦ D or L
◦ (+) or (-)
◦ R or S = absolute configuration (structure)
◦ D or L = relative configuration (structure)
◦ (+) or (-) = observed optical rotation (property)
114
Explain what an epimeric carbon is
The carbon atom at which is an epimer b/w two molecules (the carbon that two molecules have their only difference at)
115
Note that all epimer are ___, but not all ____ are epimers
Note that all epimers are diastereomers, but not all diastereomers are epimers
115
What are anomers?
Epimers that form as a result of ring closure
◦ Will only see this in regard to sugar chemistry
116
How does the cyclization of open chain D-glucose occur?
When the C-5 hydroxyl group attacks the carbonyl (C=O) carbon, C-1
◦ This not only forms a ring struture, but also a new stereocenter is formed at C-1
117
Differentiate between an alpha and a beta ring structure
◦ alpha ring structure occurs when the hydroxyl group at C-1 is down
◦ beta ring structure occurs when the hydroxyl group at C-1 is up (or pointing sideways in chair form)
118
What is an anomeric center or anomerica carbon
The carbon (usually C-1 or C-2) that distinguishes two anomers
119
When can a molecule have an internal plane of symmetry?
When the same four groups are attached to two chiral centers
◦ ie. if looking at a C-C, rotate 180 degrees about the main c-c sigma bond
119
What is a meso compound?
When there's an internal plane of symmetry in a molecule that contains chiral centers
◦ Have chiral centers but are not optically active (so they are achiral) because one side of the molecule is a mirror image of the other
120
Define geometric isomers
Diastereomers that differ in orientation of substituents around a ring or a double bond
◦ cis or trans conformation
121
When is a molecule cis?
When the higher priority groups are on the same side of the ring/bonded structure
122
When is a molecule trans?
When the higher priority groups are on the opposite sides of the ring/bonded structure
123
How is the Z or E notation determined?
high and low priority group are assigned at each carbon of the double bond, based on atomic number
◦ if the two high priority groups are on the same side, the configuration at the double bond is Z
◦ if the two high priority groups are on the opposide sides, the configuration at the double bond is E
