Test 4 Flashcards
S* -> R*
reactant is 100% S
product is 100% R
considered an inversion
S* -> S*
reactant is 100% S
product is 100% S
considered retention of configuration
S* -> (S* + R*)
reactant is 100% S
product is 50/50%
considered racemization
substitute out the ___________
leaving group
substitute in the ___________
nucleophile
if the leaving group is to the left, the attack will come from the ______
right
what is the utility of an SN2 reaction
allows for a wide range of heteroatoms
what type of alkyl halide cannot do SN2 reactions
3rd degree
two reasons for opposite side attack
- columbic (charged) repulsions between nucleophile lone pair and the lone pair on the halogen
- steric bulk size of the halogen
best type of solvent for Sn2 reactions
polar aprotic
nonpolar solvents
no heteroatoms
just CH atoms
polar aprotic solvents
has heteroatoms like O, N, S (polar)
lack of O-H and N-H bonds (aprotic)
polar protic
has N-H or O-H bonds
why are polar protic solvents NOT good for Sn2 reactions
will have unwanted BL acid base reactions
why are nonpolar sovlents NOT good for Sn2 reactions
granular salt will not dissolve
racemization definition
optically active compounds (only one enantiomer) are converted into an equal mixture of enantiomers with zero optical activity
enantiomer
nonsuperimposable mirror images
diasteromers
nonsuperimposable, non mirror images
1st degree alkyl halide
ONE other carbon attached to the carbon attached to the halogen
2nd degree alkyl halide
TWO other carbon attached to the carbon attached to the halogen
3rd degree alkyl halide
THREE other carbon attached to the carbon attached to the halogen
X
halogen
geminal
coming off of the same carbon
vicinal
coming off of adjacent carbons
hydrogen bonds
hydrogen bound to oxygen, nitrogen, or flourine
higher dielectric constant =
more polar
dipole moment vectors show
magnitude and direction of polarity
steps for SN2 reactions
separate attacker (-) from (K, Li, Na)
draw opposite side attack
leaving group leaves, attacking group attaches on opposite side
stereochemical inversion
substrates in SN2 reactions are
sp3 hybridized
good leaving groups
weak bases
ex. halides
poor leaving groups
strong bases
what degree of carbons can preform SN2 reactions
1 and 2 ONLY
3rd degree CANNOT
impact of double bonds on SN2 reactions
leaving group directly attached to a double bond = NO REACTION
nucleophilcity increases with
size
polar protic vs polar aprotic
polar protic: proton (H) bound to an electronegative atom
polar aprotic: NO H bonds
nucleophile charge
negative
SN2 stands for
substitution
nucleophilic attack
2 reactants (rate determining step)
best degree alkyl halide for SN2 reactions
first degree
order of reactivity for SN2 reactions
methyl > 1st degree > 2nd degree > 3rd degree
aromatic compounds and SN2 reactions
DO NOT participate because of double bonds
carbon with halide attached must be at least one carbon removed from the ring for SN2 to occur
how to determine best compound for SN2 reaction if all are same degree carbons
look at adjacent carbons (1st, 2nd, 3rd, 4th degree?)
if substrates are the same, how do you determine which reaction will occur faster?
look at nucleophiles
stronger nucleophile (-) = raster reaction
periodic trends in regards to nucleophiles
moving from right to left, basicity increases and therefore nucleophile strength increases
moving top to bottom, atom size and electronegativity increases, therefore nucleophile strength increases due to increased polarity of atom
can SN2 reaction occur on sp2 molecules
no
rate equation for SN2
rate = k [Rx] [Nu]
invert
in all cases of a halogen
if less than 0.5 D
NON-polar
nucleophilic attackers
negative atoms
alkyl halide
RX
leaving group
X (F, Cl, Br, I)
if leaving group is on a w/h
nucleophile will end up on the inverted w/h
push only happen with
in plane
determining which RX will have faster SN2 reaction
degree of alkyl halide
degree of electronegativity of leaving group
as electronegativity increases
basicity decreases
as steric bulk increases
basicity increases
1 carbon
common: meth
IUPAC: form
2 carbon
common: eth
IUPAC: acet
3 carbon
common: prop
IUPAC: propion
4 carbon
common: but
IUPAC: butyr
5 carbon
common: pent
IUPAC: valer
6 carbon
common: hex
IUPAC: capro
allyl
3 carbons
x- - =
vinyl
2 carbons
X - =
benzyl
7 carbons
6 carbon ring - -x
phenyl
6 carbons
6 carbon ring - x
amide
NH2
cyanide
-CN
acetylide
-C tripple bond C-
greater orbital size mismatch =
weaker bond
weaker bond =
faster leaving group
non resonance nucleophiles
negative charge only on 1 atom
faster than resonance nucelophiles
free radicles
one unpaired electron
stability for carbocations
3 > 2 > 1 > methyl
stability for carboanions
methyl > 1 > 2 > 3
stability for radicals
3 > 2 > 1 > methyl
strong nucleophiles are structurally
NOT BULKY
why are polar aprotic solvents best for SN2 reactions
no unwanted BL acid base reactions
both organic and ionic nucleophiles will dissolve
