Lecture Quiz 3 - Lectures 10-15 Flashcards
How many H atoms does each Carbon atom in a chair have?
2 H
How many axial and equatorial H atoms does a Carbon atom in a chair have?
1 axial H, 1 equatorial H
How many upper/lower face H atom does a Carbon atom in a chair have?
1 upper H, 1 lower H
1 up, 1 down
When rotating bonds to flip a chair, which groups switch and which groups stay the same?
a) axial and equatorial
b) up/down
Axial groups switch to eq
Eq groups switch to axial
up/down groups stay the same
What type of strain is present in a boat conformation? What is it caused by?
Transannular strain - caused by steric hindrance between H and H atoms
Which conformation has higher Energy? Chair or Boat?
Boat because of the eclipsing/torsional strain
Which group has higher Energy? Equatorial groups or axial groups?
Axial because parallel axials bump causing steric hindrance
Eq groups have a lot of space and minimal steric hindrance so they are lower in E
Since disubstituted cyclohexanes have a restricted rotations…?
we can get fixed orientations:
2 groups on the same side of the ring = cis
2 groups on opposite sides of the ring = trans
Why do we prefer smaller axial groups?
Smaller axial groups results in lower Energy
Ex: F vs H
List the order of least to greatest steric hindrance of:
1* C, 2C, and 3C
1C
2C
3*C – most steric hindrance because bonded to the most methyl groups
List the order of preference:
all axial groups vs all eq groups
if axial groups are present:
larger axial groups vs smaller ones
Prefer all equatorial groups
If we must have axial groups, fewer and smaller axial groups are prefered
List the 2 ways to break up a molecule/bond
1) homolytic cleavage
2) heterolytic cleavage
What does homolytic cleavage form?
When do they typically happen?
What is delta H equal to? Is this exo or endothermic?
Homolytic cleavage forms radicals/unpaired electrons.
Typically happen when we have non-polar solvents and molecules in the gas phase.
Delta H = bond Energy
Endothermic
How many electrons are moved in the process of homolytic cleave? What symbol is used to represent this movement of electrons?
1 e- movement / single prong
What does heterolytic cleavage form?
When do they typically happen?
What is delta H equal to? Is this exo or endothermic?
Heterolytic cleavage forms ions.
Typically when we have polar solvents and molecules in the liquid phase.
Delta H = Variable
endothermic
How many electrons are moved in the process of heterolytic cleave? What symbol is used to represent this movement of electrons?
2 e- movement / double prong
What are the characteristics of alkane reactions?
-hard to react with
-“parafin” - poor affinity, don’t react with much
-do react with strong oxidizing agents
What are some molecules that alkanes will react with?
O2 – strong oxidizer, leads to combustion which is hard to control
Halogens X2 – F2, Cl2, Br2 (Not I2)
What is Halogenation? Is it the same thing as a radical chain mechanism?
Halogenation = when alkanes and halogens react. It refers to what bonds are changing - what is added/changed. The name of whatever halogenation rxn you’re dealing with is describing what we’re doing.
They are exothermic and will form stronger bonds
No they are not
How does halogenation work?
Radical chain mechanism. it is the name of the mechanism, not the RXN. It’s how electrons flow and what steps occur
What are the 3 steps of a radical chain mechanism?
1) initiation step
2) propagation step(s)
3) termination step(s)
The Initiation step ______ radicals / ______ bonds
creates radicals / breaks bonds
The initiation step is (sometimes/always) (exo/endothermic). It is (rare/common).
Initiation step: ALWAYS ENDOthermic, RARE
Propagation steps _____ radicals / ______ bonds
Propagation steps exchange radicals, exchange bonds
Propagation steps are (always/sometimes) (exo/endothermic). They are (rare/common).
Propagation steps: whole cycle is EXOTHERMIC but some steps are ENDO/EXO. They are COMMON
Termination Steps _____ radicals / _____ bonds.
quench radicals / form bonds
Termination steps are (sometimes/always) (exo/endothermic). They are (common/rare).
Termination steps: ALWAYS EXOthermic. RARE
How many radicals do we start off with for each step? How many do we end up with?
Initiation:
Propagation:
Termination:
0 –>2
1 –>1
2 –> 0
What do we do when we have multiple halogens to react with an alkane?
1st, can we initiate? How hard is it to break the X-X bond? (ex: how hard is it to break the F-F bond of F2)
List from easiest to hardest to separate/initiate.
Cl2, Br2, F2
F2, Br2, Cl2
T/F All halogens can form radicals and initiate reactions.
True
Describe Fluorine in the 1st propagation step (2a):
–>(early/late) transition state
–>Looks similar to (reactant/product)
–>(Low/high) Ea, very (EXO/ENDO)thermic steps
Greedy, wants to own all the electrons and won’t share.
–>EARLY transition state
–>Looks similar to REACTANT
–>LOW Ea, very EXOthermic steps
Describe Iodine in the 1st propagation step (2a):
–>(early/late) transition state
–>Looks similar to (reactant/product)
–>(Low/high) Ea, very (EXO/ENDO)thermic steps
–> LATE transition state
–>Looks similar to PRODUCT
–>HIGH Ea, very ENDOthermic steps
Between F, Cl, BR, and I – For the 1st propagation step, all of these halogens are endothermic except:
F – very exothermic –> non-selective, whatever it contacts 1st
For the 2nd propagation step of halogens, all of them are (exo/endo)thermic
Exothermic
Cl in the propagation steps is (very/slightly) (exo/endo)thermic.
moderately exothermic
-works reasonably at most places
Br in the propagation steps is (very/slightly) (exo/endo)thermic.
Slightly exothermic – very selective, easier if the C radical is better. Has to make C radical completely before H commits to Br
List from hardest to easiest to form a radical:
1C, 2C, 3*C, and methyl
3* == rel easy to form
2*
1*
methyl == rel hard to form
What is hyperconjugation?
When the electrons in the adjacent sp3 orbital aligns with the p orbital with radical. Electron donation happens via induction
What is the selectivity factor for 1*C for F, Cl, and Br?
All = 1
What is the selectivity factor for 2*C for F, Cl, and Br?
F = 1.4
Cl = 4
Br = 80
What is the selectivity factor for 3*C for F, Cl, and Br?
F = 1.4
Cl = 5
Br = 1700
Where are most products of F at?
1*C
If we want 1* R-X, use F
Where are most products of Cl at?
2*C
If we want 2C with 3C present, use Cl
Where are most products of Br at?
3*C
If we want 3*C, use Br
What if we want a 2* product with no 3* product present?
Use Br
Constitutional isomers have:
-same formula
-diff bonds
-diff IUPAC
-diff properties
Stereoisomers have:
-same formula
-same bonds
-same basic IUPAC
-diff 3D orientation, diff shapes
Diastereomers have:
-Diff shape
-Not superimposable
-Not mirrors
-trans/cis
-totally diff prop
(hands/feet)
Enantiomers have:
-diff shape
-nonsuperimposable, not same
-mirror
-almost all properties are the same but different in plane polarized
-diff chiral environment
chiral molecules are:
-overall Asymmetrical
-not the same as mirror images
-inherent description
-have enantiomers
-optically active –> rotate plane polarized
We must have at least (0/1/2) pts of asymmetry to get chirality.
1 pt of asymmetry
To be a chiral center, you must be
sp3 with 4 diff groups attached
T/F lone pairs count as a different group for chiral centers
F
Are amines chiral?
No, you would get a constant shape flux == pyramidal inversion
What is optical activity?
rotation of plane polarized light by a chiral molecule
T/F All optically active molecules are chiral
T // optically active = chiral = optically active
T/F Enantiomers have equal and opposite optical activity
T
If something is truly symmetrical, we will get:
random refraction - optical activity cancels out
If something is asymmetrical, we will get:
scattering is asymmetrical
rotated plane
When the angle of the observe plane is clockwise, it is:
Dextrorotatory – (+) – D
When the angle of the observe plane is counterclockwise, it is:
Levorotatory – (-) – L
When you have a pair of enantiomers, you’ll have how many L/D molecules?
1 L, 1 D
When we have a pair of enantiomers that is 50/50 mixed, optical rotation is (0/1/2) aka ?
0 optical rotation = racemic mix
Alpha D is a ?? constant
physical
If I have 100 chiral centers, how many total stereoisomers do I have?
2^100
We have exactly (0/1/2) enantiomers per chiral molecule. How many diastereomers?
1 / any number
If a molecule has 2 chiral centers with the same 4 unique groups, what do we get?
meso molecule
Describe meso molecule:
-has chiral centers and internal symmetry
-not chiral
-no alpha D
-has diastereomers
-must always have a pair of chiral centers (R/S pair)
