Lecture Quiz 4 - Lectures 16 - 20 Flashcards
halogenation can create (achiral/chiral) centers
halogenation can create chiral centers
if you create a chiral center from achiral starting material, we get a (achiral/chiral) product
achiral start
create chiral center
=ACHIRAL product (racemic mix, 1:1 R/S 0 optic activity)
if you react a chiral center with achiral intermediate, we get a (achiral/chiral) product
chiral start
ACHIRAl intermediate
=ACHIRAL product (racemic)
an existing chiral center has influence on ?
transition state
when we form diastereomers, do we get a racemic mix?
NO - no equal mix
If we start with a chiral molecule that passes through a chiral transition state, we get a (achiral/chiral) product
chiral start
chiral TS
CHIRAL product
Order from largest to smallest
F, cl, br, i
I>br>cl>f
increase in size of an halogen will
(inc/dec) bond length
(inc/dec) bond strength
(inc/dec) boiling pt
(add/subtract) dispersion
(inc/dec) solubility
Size increase =
-increase Bond length and boiling pt
-decrease bond strength and solubility
-add dispersion
What are the 2 rules of nucleophilic substitution?
1) mechanism steps –> constant charge
2) can’t exceed octet rule
What is the rate law of SN2?
rate law = k[R-X][Nu-]
What has better orbital overlap? front side attack or back side?
back side attack
When you do backside attack, the LG and Nu- are ??? degrees apart
180
When we do front side attack, what happens to the sterochemistry?
maintain stereo
When we do back side attack, what happens to the stereochemistry?
stereochem inverse
Why do we do backside attack for SN2?
b/c product inverts stereochemistry at reacting C
When we do backside attack, we (always/sometimes/never) invert the 3D structure and we (always/sometimes/never) change the R/S
always invert
sometimes change
What do we do if we have a stereocenter and we don’t want to flip it?
Reacting twice
T/F All stereocenters are inverted at SN2?
F, we only invert at the carbon/func group that reacts
What makes SN2 work?
1) LG has to leave
2) Nu- has to approach
3) Structure needs space for Nu- to approach and needs to invert
4) Solvent - faster rxn if it helps Nu- dissolve faster or helps TS
What makes a good LG?
Leaving with e-
Has to be willing to take the (-) charge
Will become a conjugate base
A good acid loses H+ and forms an anion. A good LG loses?
Carbon - forms anion
The (weaker/stronger) the conjugate acid, the better the LG
Stronger conjugate acid
T/F If there are 2 LGs, the one with the lower pkA will leave first
True = Lower pKA, stronger acid = better LG = will leave first
What makes a good nucleophile?
Has an extra e- and wants to give them away
Nucleophilicity is about (thermodynamics/kinetics)
Nu- = kinetics = fastest rate = best Ea
What is a better Nu-? HO- or H2O
HO- because it is an anion
What is better? NH3 or H2O?
NH3 = less electronegative.
Why are molecules moving down the periodic table better nucleophiles?
increase in size increases orbital reach with increases polarizability as a neutral molecule
easier to share e- from a large orbital since they don’t care about polarizability
Protic solvents are better nucleophiles when
they have a loose solvation shell because it’s harder for Nu- to approach the E+
Larger ion = better Nu-
Aprotic solvents are better nucleophiles when
they have more e- to share. since all of them have loose solvation shells, there is no hindrance so we don’t care about the size anymore
Steric hindrance (allows/prevents) Nu- from approaching E+
prevents
Due to steric hindrance (bulkier/smaller) groups are preferred as nucleophiles
smaller groups = easier access = bettter u-
List from fastest to slowest as a SN2 rxn
methyl, 3, 2, and 1*
methyl = least steric hindrance
1*
2*
3* = slowest, doesn’t do Sn2
What transition state will be faster as SN2 - anti or gauche?
gauche = normal Sn2 access
What is the rate law of Sn1?
rate law = k[R-X]
SN1 is also a hydrolysis rxn which means
C-X bonds is broken with H2O
Sn1 is also a solvolysis rxn which means
C-X bond is broken with a solvent
List from fastest to slowest as a SN1 rxn
methyl, 3, 2, and 1*
3* = most hyperconjugation
2*
1*
methyl = slowest
Does Nu- identity affect rate for SN1?
No
When we use SN1, what type of stereochemistry do be get?
Mixed R/S because we don’t know the stereochemistry
If there are multiple stereocenters in a molecule under going SN1 mech, will we get anything racemic?
Nah, probably not because there is no change in the stereochemistry of the unreacting Carbons.
What is the rate law of E1?
rate law = k[R-X]
In the elimination process, you start with (0/1/2) molecules and get (0/1/2) molecules
Start with 1, get 2 moleucles
T/F SN1 and E1 have different rate laws but they have different RDS and occur separately
FALSE
-both have same rate law
-same RDS
-occur together
For Sn1 and E1, nucleophile activity affects
a) rate law
b) speed of rxn
c) product ratio
d) all of the above
affects only product ratio
If there is Nu- present, we will get (both/E1 only/SN1 only)
If there is no Nu- present, we will get (both/E1 only/SN1 only)
Nu- present = both
No Nu- = only E1
What is resiochemistry?
Where does the bond go? Which direction is it going
Which diastereomer is more preferred? cis or trans?
trans because the alkenes are on different sides so there is lower E
T/F E1 prefers more R groups
True
T/F Haloalkanes are good electrophiles at the carbon and good nucleophiles at the halogen.
F
T/F SN2 reactions depend on both the concentration of nucleophile and the concentration of haloalkane
T
T/F SN2 reactions always proceed with inversion of stereochemistry due to backside attack.
True
T?F SN2 reactions invert every chiral center in the molecule.
F
T/F Backside attack always converts R centers to S centers
F
T/F Frontside attack aligns poorly with the orbitals in an sp3 carbon, while backside attack aligns well with the backside of the sp3 orbital
T
T/F Fluoride is a better leaving group than iodide because it is more electronegative.
False - Iodide is a better LG because it has a stronger conjugate acid
T/F Good leaving groups are the conjugate bases of weak acids.
F = they are the conjugate bases of STRONG acids
T/F Tertiary haloalkanes do fast SN2 reactions due to stable carbocations
False - 3*C have more steric hindrance so they do not do Sn2 rxns
T/F Methyl and primary haloalkanes do fast SN2 reactions due minimal steric hindrance.
T
T/F Secondary haloalkanes can do SN2 but are slower than primary haloalkanes
T
T/F Branches next to a primary haloalkane slow down SN2 reactions
T
T/F Primary haloalkanes with CH or quaternary carbons alpha to the reacting carbon are ‘hindered primary’ haloalkanes.
T
T/F 1-bromobutane will react much slower than 1-bromopropane, and much faster than 1-bromopentane in an SN2 reaction.
F b/c all of them are primary alkanes and the length of the chain does not matter as much as the 1/23* steric hindrance. So they would all react pretty much the same rate
T/F SN1 and E1 have the same rate law.
T
T/F We can get SN1 reactions without any E1 side product.
F
T/F We can get E1 reactions without any SN1 side product.
T = when there is no Nu-, we have only E1 products
T/F Increasing the amount of good nucleophile increases the ratio of SN1:E1 product.
T - nucleophiles affect product ratio of SN1:E1
T/F E1 reactions make alkenes at any carbon in the haloalkane.
F = most substituted alkene aka most alkyl groups on the double bond is preferred
T/F Trans alkenes are more stable than cis alkenes.
True = less E, better E1, more favored, easier to form
T/F E1 reactions prefer to form the most substituted alkene because it’s most stable.
T
T/F Unimolecular mechanisms go through a chiral transition state and have predictable stereochemistry in the products.
False = ACHIRAL carbocation intermediate and less predictable b/c the intermediate is flat
What is the rate law of E2?
rate law = k[R-X][base]
or
rate law = k[R-X][Nu-]
How many steps do E2 mechanism have? Describe the TS. Is the stereochemistry predicatable/random?
E2 = single step, definite TS, predictive stereochem
What does antiperiplanar transition state mean?
The H and LG must be 180* apart/anti-rotamer
E1 mechanism forms (0/1/2) diastereomers and E2 mech forms (0/1/2) diastereomers.
E1 = forms 2 diastereomers
E2 = forms only 1
According to regioselectivity, which constitutional isomer is preferred for E1. What about E2?
E1 = most substituted alkene
E2 = can pick the alkene depending on the base
(Unhindered/Hindered) base gets the BEST product. This rule is known as? (Kinetically/Thermodynamically) favored. Does it prefer (less/more) substituted
Unhindered (1* RO-) = best product = Saytzev’s Rule
thermodynamically favored
Less substituted
(Unhindered/Hindered) base gets the EASIEST product. This rule is known as?
(Kinetically/Thermodynamically) favored.
Does it prefer (less/more) substituted alkenes?
Hindered (2* RO-) base = easiest product = Hofmann’s rule
kinetically favored
More substituted
If we have a poor Nu- , what mechanism would we use for different reacting carbons
Poor Nu-
NO RXN for methyl, 1*
= SN1/E1 for 2* and 3*
If we have a weakly basic Nu-, what mechanism would we use for different reacting carbons?
Weakly Basic
= SN2 for methyl, 1, 2
= SN1/E1 for 3*
If we have an unhindered strong Nu-
what mechanism would we use for different reacting carbons?
Unhind Nu-
= SN2 for methyl and unhind 1*
= E2 (terminal) for hindered 1*
= E2 (Saytzev) for 2* and 3*
If we have a strong base Nu-, what mechanism would we use for different reacting carbons?
Strong Base
= SN2 for methyl
= E2 (terminal) for 1*
= E2 (Hofmann) for 2* and 3*
E2 Reactions use ??? transition states where the leaving group and H are ??? degrees apart
antiperiplanar
180
