analyzing reactions (SN1/SN2, nucleophiles, electrophiles)

Question 1

Sn1 reaction occurs when

Answer 1

• secondary substrate
• weak nucleophile
• polar protic

(CAN NOT OCCUR WITH A PRIMARY SUBSTRATE) - bc primary carbocations are unstable

Question 2

polar protic

Answer 2

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

Question 3

Sn2 reactions occur when

Answer 3

primary substrate

or secondary substrate with a strong nucleophile and polar aprotic

Question 4

polar aprotic

Answer 4

solvents that lack an acidic hydrogen. Consequently, they are not hydrogen bond donors. These solvents generally have intermediate dielectric constants and polarity.

Question 5

SN2 reaction

Answer 5

nucleophiles attach the nucleus, donating a pair of electrons to form a bond and replacing the leaving group

Question 6

nucleophiles

Answer 6

“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

Question 7

bimolecular

Answer 7

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

Question 8

how many steps in SN2

Answer 8

one step - nucleophile attacks and at the same time the leaving group leaves

Question 9

SN2 transition state

Answer 9

when nu is bond and leaving group hasn’t quite left (5 bonds)

Question 10

what does the nucleus attack

Answer 10

a carbon with a partial positive charge

Question 11

SN2 inversion

Answer 11

the nu does a backside attack which causes an inversion of the molecule

Question 12

does a carbocation form in SN2

Answer 12

no - a transition state instead

Question 13

what favours SN2

Answer 13

primary alkyl group and a non-polar solvent

Question 14

Sn1 reaction

Answer 14

• 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

Question 15

unimolecular

Answer 15

the rate of run depends only on one reactants concentration (1st order) - the substrate

rate= k [A]

Question 16

what is the slow step in SN1

Answer 16

the leaving group leaving

Question 17

when are carbocations the most stable

Answer 17

when there are more atoms to distribute the burden of charge with

3>2>1

( in this way primary substrates cant be SN1)

Question 18

3 factors affect carbocation stability

Answer 18

• 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

Question 19

when will SN2 reactions not occur

Answer 19

one step
- not on tertiary carbons bc of steric hindrance
( if want to push an rxn in SN1 direction use tertiary carbon)

Question 20

how is acid strength measured?

Answer 20

Ka (or acid dissociation constant)
which is simply an equilibrium constant!

Ka=products/reactants

Question 21

a large Ka means

Answer 21

lots of products —> strong dissociation, strong acid

Question 22

pKa

Answer 22

the -log of Ka

• so if Ka is large, Pka is very small and visa versa

Question 23

a small pKA means?

Answer 23

a large acid dissociation constant (Ka) so a very strong acid

Question 24

acidic

Answer 24

small Pka and large Ka

Question 25

what is an alpha hydrogen

Answer 25

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

Question 26

what is important about alpha hydrogens?

Answer 26

acidic

Question 27

why are alpha hydrogens acidic?

Answer 27

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.

Question 28

function groups that act as acids (lose H)

Answer 28

alcohol, aldehydes, ketones, carboxylic acids and derivatives

Question 29

functional groups that act as bases

Answer 29

amines and amides

Question 30

what affects nucleophilicity?

Answer 30

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)

Question 31

are good Nu’s electronegative

Answer 31

no

Question 32

protic solvents

Answer 32

can H-bond

Question 33

aprotic solvents

Answer 33

cant h-bond (no H connected to an electronegative atom)

Question 34

how can solvents affect nucleophilicity

Answer 34

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

Question 35

electrophilicity

Answer 35

electron loving so has a positive charge or partial positive charge