analyzing reactions (SN1/SN2, nucleophiles, electrophiles) Flashcards
Sn1 reaction occurs when
- secondary substrate
- weak nucleophile
- polar protic
(CAN NOT OCCUR WITH A PRIMARY SUBSTRATE) - bc primary carbocations are unstable
polar protic
Polar protic solvents are capable of hydrogen bonding because they contain at least one hydrogen atom connected directly to an electronegative atom (such as O-H or N-H bonds). They solvate cations and anions effectively. Polar protic solvents are water, ethanol, methanol, ammonia, acetic acid, and others
Sn2 reactions occur when
primary substrate
or secondary substrate with a strong nucleophile and polar aprotic
polar aprotic
solvents that lack an acidic hydrogen. Consequently, they are not hydrogen bond donors. These solvents generally have intermediate dielectric constants and polarity.
SN2 reaction
nucleophiles attach the nucleus, donating a pair of electrons to form a bond and replacing the leaving group
nucleophiles
“nucleus (positive) loving”
- attracted to the nucleus bc it is negatively charged or a strong partial charge or lone pairs (electrons!)
- it donated electrons to the nucleus - creating a bond
bimolecular
rate depends on the concentration of TWO reactants (second order kinetics)
rate= k [A][B]
- since there is only one step in an SN2 reaction, and both reactants are involved - this is the slow step and the SLOW STEP is the only step
how many steps in SN2
one step - nucleophile attacks and at the same time the leaving group leaves
SN2 transition state
when nu is bond and leaving group hasn’t quite left (5 bonds)
what does the nucleus attack
a carbon with a partial positive charge
SN2 inversion
the nu does a backside attack which causes an inversion of the molecule
does a carbocation form in SN2
no - a transition state instead
what favours SN2
primary alkyl group and a non-polar solvent
Sn1 reaction
- 2 steps
- leaving group leaves first creating a carbocation intermediate ( C with a plus charge bc only 3 bonds)
- then Nu attacks and 2 products can form (Nu can attack either side)
- unimolecular
unimolecular
the rate of run depends only on one reactants concentration (1st order) - the substrate
rate= k [A]
what is the slow step in SN1
the leaving group leaving
when are carbocations the most stable
when there are more atoms to distribute the burden of charge with
3>2>1
( in this way primary substrates cant be SN1)
3 factors affect carbocation stability
- more adjacent carbons
- adjacent pi bonds ( this allows the carbocation p-orbital to be a part of the conjugated pi-system - stabilizing
- adjacent atoms with lone pairs
when will SN2 reactions not occur
one step
- not on tertiary carbons bc of steric hindrance
( if want to push an rxn in SN1 direction use tertiary carbon)
how is acid strength measured?
Ka (or acid dissociation constant)
which is simply an equilibrium constant!
Ka=products/reactants
a large Ka means
lots of products —> strong dissociation, strong acid
pKa
the -log of Ka
- so if Ka is large, Pka is very small and visa versa
a small pKA means?
a large acid dissociation constant (Ka) so a very strong acid
acidic
small Pka and large Ka
what is an alpha hydrogen
think of a carbonyl compound - there is the central carbon (carbonyl carbon) and then the adjaccent carbon is called the alpha carbon - the H attached to this is the alpha hydrogen
what is important about alpha hydrogens?
acidic
why are alpha hydrogens acidic?
The alpha hydrogen is acidic for two important reasons. Firstly, deprotonation of the alpha hydrogen leads to a resonance stabilized intermediate called the enolate, which contains delocalization among two different atoms. Secondly, the deprotonation of the hydrogen places a negative charge next to the carbon that contains a partial positive charge that comes from the electric dipole moment of the carbon-oxygen double bond. Once the enolate intermediate is formed, it can go on to form the enol product, either via a acid-catalyzed or a base-catalyzed reaction.
function groups that act as acids (lose H)
alcohol, aldehydes, ketones, carboxylic acids and derivatives
functional groups that act as bases
amines and amides
what affects nucleophilicity?
they want bigger charge (more negative), not bulky, low electronegativity (bc if they are too electronegative, then they are holding electrons too tight and don’t want to share) and aprotic solvents ( bc if it is protic, than it can protonate the Nu or hydrogen bond with Nu and thus the Nu isn’t as motivated to attack)
are good Nu’s electronegative
no
protic solvents
can H-bond
aprotic solvents
cant h-bond (no H connected to an electronegative atom)
how can solvents affect nucleophilicity
if in a protic solvent- down the table increases Nu ability (halogens in protic –> I is best
if aprotic- up the table increases Nu ability
electrophilicity
electron loving so has a positive charge or partial positive charge
