1
Q
conformational vs configurational isomers
A
conformational: the different positions that a molecule can twist into
configurational: R/S, matter of left or right handed
depends on whether or not the compounds can be interconverted by the rotation about single bonds
2
Q
enantiomers vs diastereomers
A
both are non-superimposable
enantiomers: mirror images, “twins”
- can only have 1 other enantiomers (come in pairs)
- all chiral centers have opposite configurations
diastereomers: not mirror images, “cousins”
- can be many of them
- opposite configruration at some chiral centers but not all
3
Q
how to determine if a molecule is identical or not
A
identical means that its superimposable
- superimposable = no chiral centers
- if the molecule has at least 1 chiral center, then it can be non-superimposable
- superimposable molecules are achiral
- b/c then its mirror image will also be the exact same
4
Q
stereocenters
A
single atom that is bonded to 4 different groups not specifically just 4 different atoms
5
Q
identifying chiral centers (stereocenters) in ring structures
A
ring with a group = stereocenter
- if ring doesn’t have a group attached on it somewhere, it’s not a stereocenter because either direction you travel around the ring, it’s the same
- if carbon in ring has substituents that make each “route” unique as you go clockwise and counterclockwise, it can be a chiral center
- or if the ring has symmetry in going around the ring both ways then it’s also not a chiral center
6
Q
how to determine the configuration of a stereocenter
A
- each group around the carbon is given priority based on atomic number of the element directly attached
- hydrogen always gets lowest priority
- if 2 same elements: list the atoms of the atom attached to stereocenter (basically looking for next point of difference)
- priority group going (decreasing) clockwise = R
- priority group order going counter clockwise = S
BUT MAKE SURE H IS THE ONE ON THE DASH OTHERWISE NEED TO FLIP IT AND SWITCH THE CONFIGURATION
7
Q
rotation of plane-polarized light in enantiomers + diastereomers
A
configuration does not have to do with direction of polarized light!!
- enantiomers rotate plane-polarized light in opposite directions so if one is rotating clockwise (+) then the other will rotate counter clockwise (-)
- if a sample has equal amounts of both enantiomers (racemic mixture), the rotations cancel each other out, and the mixture is optically inactive
- diastereomers are not predictable, they can either both rotate + or both - or opposite
8
Q
nonmenclature for R and S for multiple chiral centers
A
(3R, 4S) -
goes in the beginning for multiple (specify location of stereocenter), otherwise just the configuration in the beginning
9
Q
properties of enantiomers
A
the same physidcal (melting and boiling points) and chemical properties
- only difference is the rotation of plane polarized light
10
Q
difference in configuration between enantiomers
A
ALL chiral centers have different configurations
ex. (3R, 4S) and (3S, 4R)
11
Q
properties of diastereomers
A
have different physical properties (like melting points or solubility) and different chemical behaviors
12
Q
difference in configuration between diastereomers
A
have different configurations at some but not all stereocenters
13
Q
racemic mixture
A
enantiomers are mixed together in equal concentrations
- 1 R and 1 S enantiomer
- because of this, each rotates light in the opposite direction so overall rotation cancels out = racemic mixtures do not rotate plane-polarized light
14
Q
how to tell if a molecule is chiral
A
has chiral centers and no internal symmetry
- if it has symmetry its a meso compound
15
Q
aryls
A
aromatic rings
(ally’s are the ones that look like an A)
16
Q
E/Z naming system
A
- look at the groups on both sides of the double bond
- between the groups on each side, label the higher priority group
- if the higher priority groups are both on the same side, its Z and opposite sides is E
E = trans, Z = cis
and put this E/Z in the beginning of the name and -ene at the end (for the double bond)
- cis/trans can only be used when the 2 groups are the same across the double bond, but this is not the case with E/Z naming
17
Q
2 ways to draw enantiomers
A
simpler way: invert ALL stereo centers (convert dashes to wedges and wedges to dashes)
- this puts the mirror behind the compound
- if all of them are not inverted, then they’re not enantiomers - they’re diastereomers
other way: create a mirror image but placing a mirror and flipping the molecule
- this puts the mirror on the side of the compound
- first method doesn’t work all the time, esp when the dashes and wedges are implied and not drawn
18
Q
E/Z isomers are what type of stereoisomers
A
diastereomers because they are stereoisomers that are not mirror images of each other
19
Q
optically inactive molecules
A
do not rotate plane- polarized light
- no chiral centers or plane of symmetry present
- meso compounds and racemic mixtures
20
Q
meso compound
A
HAS chiral centers but overall chiral because of an internal plane of symmetry
- makes them optically inactive
- also be careful about rotating a carbon because if you can rotate a carbon and gain symmetry, then it is also meso
- that one lonely guy that is a mirror image of itself (b/c drawing the mirror image will just give you itself)
- if it didn’t even have chiral centers, then it will be superimposable and just itself
- also every R is inverted to S and every S is inverted to R but have plane of symmetry
21
Q
constitutional isomers
A
same molecular formula but different connectivity
22
Q
how to calculate the maximum number of optically active stereoisomers (enantiomers + diastereomers) that a single compound can have
A
max number of optically active isomers = 2^n
where n = # of chiral centers
but meso compounds are considered inactive b/c their plane of symmetry throws them off, even though they have many chiral centers
if enantiomers then arranged as multiple pairs of enantiomers so 2^2 = 4 arranged as 2 pairs of enantiomers
23
Q
cis vs trans compounds dipole moments + IMFs
A
cis: dipole moment so stronger IMFs (higher boiling point), but lower melting points (b/c cant form crystals and squash together)
trans: no dipole moment so weaker IMFs but higher melting points (can form crystals bc can squash together)
24
Q
what region will the nucleophile tend to attack?
A
the most positive one
25
what type of compound is more likely to undergo an Sn1 reaction?
a **tertiary** (central carbon bonded to 3 other carbons) **substrate** rather than a secondary or primary one since the carbonation must be formed spontaneously
- also when phenyl group is attached to the tertiary carbon, also increases stability (but phenyl group directly attached to leaving group leads to no reaction)
26
formula for calculating enantiomeric excess
**enantiomeric excess (ee)**: used to indicate the extent to which a sample contains more of one enantiomer than the racemic mixture
**ee % = (observed rotation/rotation of pure enantiomer) x 100**
also:
**ee% = ([major enantiomer] - [minor enantiomer] / [total enantiomer]) x 100**
answer means that the mixture has ____% MORE of one enantiomer than the other, its not the EXACT value (need to calculate exact value if it's given)
27
cis and trans alkene isomers contain what type of stereogenic centers
trigonal
28
stereogenic center vs chiral center
**stereogenic center**: border term that refers to any atom in a molecule at which exchanging 2 groups produces a different stereoisomer
- *ex. cis-trans isomers* (since swapping substituents around leads to different stereoisomers)
- hybridization can be *sp2 or sp3*
- can be trigonal or tetrahedral
- results in stereoisomers
- may or not be symmetrical
**chiral centers**: type of stereogenic center
- sp3 hybridized
- all bonds are sigma bonds (single) so tetrahedral shape
- asymmetrical
29
quick ways to find multiple chiral centers
**look for dashes and wedges**: they're commonly used to indicate 3D arrangements around a chiral center, is often a clue that the carbon is chiral
**ignore symmetrical carbons**: carbons in symmetrical parts of the molecule are often not chiral centers
**skip carbons with double or triple bonds**: can't be chiral bc not bonded to 4 different substituents
**look for alkyl substituents, hydrogen, OH, and carbonyls (C=O)**: combination of thee groups on a carbon often indicates a chiral center
**count chiral centers based on functional group positioning**: single rule is that cyclic compounds with substituents at different positions often have chiral centers where the ring structures restricts rotation
30
what does α represent
observed optical rotation
31
what does 0% and 100% ee (enantiomeric excess) represent
**0%**: indicates racemic mixture, where both enantiomers are present in equal amounts
**100%**: represents a sample containing only one enantiomer, considered "optically pure"
32
how to answer this question:
A mixture of the 2-butanol enantiomers showed a specific rotation of +6.76. An **enantiomerically pure sample** of (S)-(+)-2-butanol shows a specific rotation of +13.52. What is the actual stereoisomeric composition of the mixture?
**1. calculate % ee**:
(+6.76/+13.53) x 100 = 50%
so enantiomeric excess of the (S)-(+)-2-butanol is 50%
2. of the total mixture, 50% consists of the racemic form, which contains equal number of the 2 enantiomers
- so **half of this 50%**, or 25% is the (-) enantiomer and the other 25% is the (+) enantiomer
3. in total, 50 + 25 is the (+) enantiomer (*because the other 50% of the mixture is also the (+) enantiomer)
so answer: 75% of the mixture is the (+) enantiomer and 25% is the (-) enantiomer
33
reasoning behind this question:
the reaction of CH₃CH₂CH₂C(=O)CH₂CH(CH₃)CH₃ with H2/Ni forms: (R)- and (S)-6-methyl-3-heptanol
- hydrogenation will reduce C=O to C-OH
- it has a chiral center because different groups attached so possible configurations are either (R) or (S)
- since reaction doesn't favor one enantiomer over the other, it will produce a **racemic mixture** so can be both (R) and (S)
- can't determine whether (R) or (S) specifically because H is non-selective of stereo and **can either attack from the front or back (which determines R or S)**
34
how to draw multiple stereoisomers for compounds containing multiple chiral centers
- make the mirror image
- swap one of the dash/wedge group
- swap another one
and so on
35
how to draw Newman projections
the circle represents the back carbon, the 3 lines represent the front carbon
**looking from the left**: atom on the wedge is on the RIGHT side of Newman projection
- dash: on the left
**looking from the right side**: atom on the wedge is on the LEFT side of the Newman projection
- dash: on the right
36
absolute vs. relative configuration
absolute configuration - the R/S
relative configuration - cis/trans stuff, spatial arrangement of molecules relative to another chiral molecule without knowing the exact R and S, which group on dash and which on wedge stuff
37
whats allowed in rotation of fisher projections
- allowed to rotate **all groups 180º** (this means that the bottom goes to the top (moves 2 places each)
- NOT allowed to rotate **all groups 90º** b/c this would put the vertical groups on a horizontal and changes the absolute configuration
- allowed to rotate **3 groups keeping 1 same** b/c this is like a double swap which is allowed
38
how to draw the enantiomer of a fisher projection
simply exchange the right and left horizontal bonds
39
drawing Fischer projections - guess and check method
- randomly guess and put the groups on the Fischer template
- find the configuration (R/S) of each chiral carbon on both the 3D molecule and the template
- **whichever line up b/w 3D & template = keep**
- **whatever don't line up = swap the horizontal groups**
40
fischer projections - what everything represents
- horizontal lines represent **wedges** = coming out towards you
- vertical lines represent **dashes** = going away from you
- the intersection points represent the carbons
41
how to go from Newman projections to Fischer projections
- first make the conformation **eclipsed**:
- keep the front carbon pointing downward
- groups go through a **180º rotation** so groups go across (*bottom right goes eclipsed behind top left*) and so on
- then simply add them onto the fischer template in the order that you see them
- the group thats pointing up at the top of the fischer projection is really the one pointing straight down in the Newman projection
42
compounds that are cis/trans but have a plane of symmetry
NOT meso even though they have symmetry
- can be diastereomers or enantiomers depending on the configuration changes
43
how does being cis/trans + having plane of symmetry + having more than 2 chiral centers affect the amount of stereosiomers possible?
it **reduces them** because if the compound is drawn cis, because of the plane of symmetry, the compound becomes **meso** (only 1 stereoisomer)
when it's drawn trans, there can be R and S possibilities
*ex. 1,3-dimethylcyclohexane* has only 3 stereoisomers even though it has 2 chirality centers
- **usually the meso compound only occurs with cis**
44
inversion/retention of configuration based on whether or not the bonds to the chirality center are broken
chirality center bonds broken + backside attack = inversion of configuration
chirality center bonds not broken + front side attack = retention of configuration (absolute configuration changed, but relative configuration retained)
45
SN1 reactions + 2 key points
*SN1 = substitution nucleophilic unimolecular* (first order reaction)
- reactions occurs in **2 steps**:
**step 1**: leaving group breaks off carbon forming a **carbocation** (positively charged carbon atom
- this is the slow step and the **rate determining step**
**step 2**: nucleophile attacks carbocation, replacing the leaving group
**key points**:
- rate of reaction depends ONLY on the concentration of the SUBSTRATE (molecule that has the leaving group)
- the more stable the carbocation, the faster the reaction (*tertiary carbocations are more stable than secondary and secondary are more stable than primary*)
ALSO ONLY WORKS FOR SP3 HYBRIDIZED REACTIONS
46
carbon-halogen bond
generally polar with the **partial positive** on the carbon and **partial neg**. on the halogen
47
SN2 reactions + 3 key points
*SN2 = substitution nucleophilic bimolecular* (second order reaction)
- reaction occurs in **1 step**: nucleophile attacks carbon that is attached to the leaving group from the opposite side, pushing leaving group out while bonding to the carbon
- attack is simultaneous with the departure of the leaving group
**key points**:
- rate of reactions depends on the concentration of **both substrate and nucleophile**
- **primary carbons** are more likely to undergo SN2 because there is less steric hinderance (crowding around carbon) while **tertiary carbons are too crowded** and dont undergo SN2 easily
- reaction involves **backside attack** leading to **inversion of configuration** (dashes and wedges switch, absolute configuration may or not change) at the chiral center (if there is one)
48
nucleophile, leaving group, and substrate
**substrate**: contains leaving group and electrophile
**nucleophile**: attacks the substrate (usually has a neg. charge or abundance of electrons)
**leaving group**: molecule that departs (BYE)
49
good leaving groups + how to identify good leaving group
**good leaving group**: after breaking off, can stabilize the electrons it takes with it
- large atoms = better leaving groups b/c neg. charge is spread out over large area
are usually **weak bases** (b/c weak bases can hold onto electrons after leaving, less likely to attract back the protons after they leave)
**common good leaving groups**:
- halides (Cl-, Br-, I-) and sulfonates (S=O2)
- *good leaving groups are usually conjugate bases of strong acids*
- some leaving groups also **depart as neutral molecules** (ex. H2O) - this means it must have had a formal charge when it was bonded to the substrate
50
stability of carbocations
**tertiary carbocations** are the most stable b/c the positive charge is spread out over the 3 carbon atoms (**more methyl groups = more stable carbocation**)
- primary carbocations are very unstable and do not form easily
51
physical properties of alkyl halides (mostly to do with solubility)
- low solubilities in water
- miscible with each other and relatively non polar solvents
- used as solvents for non polar and moderately polar compounds
52
what type of hybridization only do SN reactions apply to?
sp3
53
how to tell if racemic mixture is formed?
SN2 reactions dont produce racemic mixture, only 100% one enantiomer with inversion of configuration
SN1 reactions usually result in racemic mixture and this means that product is **overall chiral** b/c effect of R and S cancel each other out
- they also produce carbocation intermediate (flat, positively charged carbon) that allows the nucleophile to attack from either side
54
protic solvent
protic means it has an H attached to an electronegative atom
- CH3 groups are aprotic
55
which types of compounds are prone to nucleophilic substitutions (Sn) & Elimination reactions and which ones are not?
alkyl halides with sp3 hybridization
- akenyl halides (double bonds), aryl (phenyl) halides, and alkynyl halides (triple bonded) do not generally undergo Sn or E reactions
56
what is the charge of the product that results from SN reactions
**neutral**
the product has to be neutral so make sure has no charge (+ or -)
in the example, attacking H2O became OH when bonded to keep the charge neutral
57
heterolytic bond cleavage
when a bond breaks and **one atom gets both of the electrons** and the other gets none
- creates ions where the one that takes all is negatively charged (anion) and the one with none is positively charged one (cation)
*happens in SN reactions*
58
nucleophilic substitution by neutral nucleophile results in what
a positively charged product
59
poor leaving groups are _____
**strong bases**
like OH-, NH2- , OR-
unstable on their own and tend to hold onto electrons tightly, making them less likely to leave the molecule
F- is also poor among halides b/c small and holds electrons tightly
60
what is the transition state in a free energy diagram for SN2 reaction + what does higher products than reactants mean
the highest point of the curve at the top
where the carbocation is being formed
**higher products than reactants**: means reactants are more stable so reaction wants to go towards the reactants (reverse direction)
61
free energy of activation (∆G+) vs. free energy change for the reaction (∆G)
**free energy of activation (∆G+)**: difference in energy between reactants and the transition state
**free energy change for the reaction (∆G)**: difference in energy between the reactants and the products
62
a 10ºC increase in temperature causes what to reaction
causes the reaction rate to double for many reactions taking place near room temperature
63
when do chirality centers in different molecules have the same relative configuration
when the chirality centers share 3 groups in common and if these groups with the central carbon can be superimposed in a pyramidal arrangement
64
the slow step in SN1 reactions
is the rate determining step
- results in formation of the carbocation
65
how many electrons are present in the p orbital of a carbocation
**none**
- it can only accept electrons
66
factors that stabilize carbocations
**resonance** (in aromatic groups, only for benzylic groups NOT phenyls)
**hyperconjugation**: presence of neighboring C-H or C-C bonds can interact with empty p-orbital and help spread out the positive charge, making it more stable (*helping hand*)
- shown with that straight cross looking thing where the bonds move and then the H just becomes a floating H+
67
based on what do the amount of hyperconjugation structures increase?
more the # of carbons attached to the central carbon, more the amount of hyperconjugation structures
68
solvolysis (hydrolysis + methanolysis)
**solvolysis**: substitution reaction where solvent breaks down the compound, acting as the nucleophile
**hydrolysis**: water is the solvent and nucleophile, results in formation of alcohol group (-OH)
**methanolysis**: methanol (CH3OH) is the solvent and nucleophile, results in formation of ether group (-OCH3)
*1 H pops off from each*
- many Sn1 reactions would be called solvolysis reactions
69
the more stable the carbonation formed in Sn1 reactions, the __________ the reaction
faster
70
the 7 strong acids
- HCl
- HNO3
- H2SO4
- HBr
- HI
- HClO4
- HClO3
71
72
which nucleophiles are better in protic/aprotic solvents?
**protic solvents**: stronger nucleophiles are larger ions like I-
**aprotic solvents**: smaller ions like F- are better nucleophiles because protons surround and support smaller ions better and their electrons are not as accessible
orgo 1
flashcards
Decks in class (4)
# Cards
