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SN1 vs SN2


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alkyl halide
C-Halogen x= Halogen like (F, Cl, Br, I )
methyl halide
The halogen bearing carbon is attached to three other hydrogens /no other carbons , CH3-X
primary halide
The main carbon is attached to only one other carbon.
secondary halide
an alkyl halide in which the halogen is bonded to a secondary carbon, alkyl halide attached to two carbon groups
tertiary halide
alkyl halide attached to three carbon groups, The halogen bearing carbon is attached to three other carbons
vinyl halide
A derivative of an alkene in which one (or more) of the hydrogen atoms on the double-bonded carbon atoms has been replaced by a halogen.
aryl halide
An aromatic compound (benzene derivative) in which a halogen is bonded to one of the carbon atoms of the aromatic ring.
allylic halide
Halides that have X bonded to the Carbon atom adjacent to a C=C bond.
benzylic halide
Halides that have X bonded to the Carbon atom adjacent to a benzene ring.
insoluble in water
solubility of alykyl halide re: water
soluble in organic solvents
solubility of alykyl halide re: organic solvents
higher mp and higher bp
due to the larger surface area alykyl halides have
bp's and mp's increase as the size of _______ increases
X ... the halogen -- Br better than Cl (more polarizable halogen)
the more polarizable the halogen the
higher mp and higher bp it'll have
the characteristic reactions of alkyl halides are
substitution and elimination alykyl halides undergo substitution reactions with
nucleophile
alkyl halides undergo emilination with bronsted lowry bases
most common leaving groups are
halide anions (X?), and h2o (from R-OH?) and N2 (from R-N2?)
:Nu?
nucleophile- which contain a lone pair or a pi bond but not necessarily a negative charge.
lone pair or pi bond
to identify the nucleophile look for the species with the
weaker base
in comparing two leaving groups the better leaving group is the
Weak Bases
H2O, ROH, PR3, Halides, RS, N3, NC, RCOO
?????? increasing basicity
left to right ACROSS a row with basicity
:NH3 vs H2O::
H2O with the two lone pairs will be better leaving group
basicity decreases so leaving group ability increas
down a column on the periodic table re: basicity
F ? Cl? Br? I?
I? weakest base so best leaving group
all good leaving groups are weak bases with
strong conj. acids having low pKa values
equilibrium favors the products of nucleophilic substitution
when the leaving group is a weaker base than the nucleophile
Protons
Atom Structure
nucleophiles attack
other electron deficient atoms (usu carbons)
nucleophilicity increases
as basicity increase
decreases nucleophilicity but not basicity
steric hindrace decreases _______ but does not _______ basicity
nonnucleophilic bases
sterically hindered bases that are poor nucleophiles are called
CH3COOH
acetic acid
acetontitrile
ex of polar aprotic solvents
polar aprotic solvents
are incapable of of hydrogen bonding. they only solvate cations well.
stronger nucleophile
in general the stronger base is the
in polar protic solvents ucleophilicity
increases with increasing size of an anion (opp to basicity)
steric hindrance decreases nucleophilicity
without decreasing basicity, making (CH3)3CO? a stronger base but a weaker nucleophile that CH3CH2O?
nuclophilicty substitution involves
two pi bonds the bond to the leaving group which is broken and the bond to the nucleophile which is formed.
dipole dipole interactions
alykyl halide have a polar C-X bond so they exhibit
intermolecular hydrogen bonding
alykyl halide have a polar C-X bond so they are incapable of
1 step
SN2 re: mechanism
SN2 re: alkyl halide
order of reactivity: CH3X>RCH2X> R2CHX>R3CX
rate= k (RX)(:Nu?)
SN2 re: rate eauation / kinetics
SN2 re: stereochemistry
backside attack of the nucleophile
SN2 re: nucleophile
favored by stronger nucleophiles
SN2 re: leaving group
better leaving group>> faster reaction
SN2 re: solvent
favored by polar aprotic solvents (**AP)
2 steps
SN1 re: mechanism
SN1 re: alkyl halide
order of reactivity: R3CX>R2CHX>RCH2X> CH3X
rate= k (RX)
SN1 re: rate eauation / kinetics
SN1 re: stereochemistry
trigonal planar carbocation intermediate
SN1 re: nucleophile
favored by weaker nucleophiles
SN1 re: leaving group
better leaving group>> faster reaction
SN1 re: solvent
favored by polar protic solvents (**PP)
methyl and 1 °
can only be sn2
2 °
can be both sn1 and sn2
3 °
can only be sn1
weakest base
best leaving group
increases
mass of cathode ....
Decreases
Screening as n increases
increases
the stability of carbocation _______ as the # of R groups bonded to the positively charged carbon increases
SN2
?:CN
Sn1
Both mechanisms are sensitive to the nature of the leaving group but the _______ mechanism is more sensitive
racemic
only sn1
safer bet is sn1
if cyclic and you dont know ...
less basic and a better leaving group
I vs CL ... I is farther down the column and therefore
better leaving group.. less basic
H2S vs H20 ... H2S is farther downs a column
yes >> PP
does it have O-H Bonds?
strong nucleophile in PP solvent
increases down a column (opposite to basicity)
strong nucleophile in PA solvent
decreases down a column (identical to basicity)
will result in a slower SN2 rxn
increase the # of R groups .. b/c this increases the crowding of the transition state = decreases the rate of the rxn
nucelophile
resonance makes it a good
polar aprotic
polar but cant H-Bond
protic
acetic acid
less crowded
always faster