substitution reactions

Question 1

what makes a good leaving group

Answer 1

neutral molecules or stable anions

Question 2

a good leaving group readily accepts …

Answer 2

a pair of electrons

Question 3

is OH a good or bad leaving group

Answer 3

bad

Question 4

is H2O a good or bad leaving group

Answer 4

good

Question 5

in an Sn2 reaction, the nucleophile attacks the carbon atom from _____

Answer 5

behind

Question 6

in an Sn2 reaction, the nucleophile forms a new bond to the carbon _____ as the C-X bond is broken

Answer 6

at the same time

Question 7

why does the carbon attack from the back in a Sn2 reaction

Answer 7

it maximises the overlap of the orbitals.

Question 8

its the sigma anti bonding orbital of the C-X bond that overlaps in an Sn2 as

Answer 8

it has the lowest energy of the empty orbitals so

Question 9

in Sn2 if the C in the C-X bond is chiral then …

Answer 9

it leads to an inversion of configuration (walden inversion).

Question 10

the size of the substituents affects …

Answer 10

the rate of the Sn2 reaction, the larger the substituent, the slower the reaction.

Question 11

the larger the size of the substituents,

Answer 11

the greater the steric hinderance and the higher the energy of the transition state.

Question 12

what is step one of Sn1

Answer 12

arrow from bond of C-X to X, leaving C+

Question 13

what is step 2 of Sn1

Answer 13

nucleophile attacks the C+ and bonds on

Question 14

which Sn1 step is the rate determining step

Answer 14

1

Question 15

in Sn1 the nucleophile can attack from above and below this means…

Answer 15

the product will be racemic (both R and S)

Question 16

for Sn1, the greater the number of substituents …

why?

Answer 16

the faster the reaction

because alkyl groups are electron donating so the stabilise the carbocation intermediate .

Question 17

a substituent that can stabilise a carbocation intermediate via mesmeric effects will …

Answer 17

promote Sn1 reactions.

Question 18

changing to a nucleophile with greater strength or increasing the concentration of nucleophile …
why?

Answer 18

increases the rate of Sn2

the nucleophile takes place in the rate determining step of Sn2 , but not Sn1.

Question 19

forSn1, the strength of a nucleophile …

Answer 19

does not affect the rate, but does affect the products that are formed.

a stronger nucleophile will produce a major product whereas a weaker one would produce the minor .

Question 20

strong nucleophile tends to favour the …

Answer 20

Sn2

Question 21

weak nucleophile tends to favour the …

Answer 21

Sn1

Question 22

weak nucleophiles tend to be ….

Answer 22

uncharged

Question 23

what is a protic solvent

Answer 23

they have O-H or N-H bonds

Question 24

why are protic solvents good

Answer 24

So they take part in hydrogen bonding, they also serve as a source of protons (H+)

Question 25

what a aprotic solvent

Answer 25

lack O-H or N-H bonds

Question 26

the rates of Sn2 are fastest in ____ solvents

Answer 26

aprotic

Question 27

the rates of Sn1 reactions are fastest in ____ solvents

Answer 27

protic

Question 28

what are the first 2 steps of predicting Sn1 vs Sn2

Answer 28

identify the nucleophile and the leaving group, next intentify the halogenalkane as either primary, secondary or tertiary.

Question 29

what is step 3 of predicting Sn1 vs Sn2

Answer 29

consider the strength of the nucleophile and the nature of the solvent

Question 30

what should you remember when when drawing the products of either Sn1 or Sn2

Answer 30

Sn1 reaction of a single enantiomer of a halogenalkane gives a racemic mixture whereas Sn2 gives an inverted configuration.

Question 31

what is the hammond postulate

Answer 31

for two elementary steps that are of the same type, the one with the more negative deltaG tends to be faster and have a smaller energy barrier