mechanisms

Question 1

what are curly arrows ?

Answer 1

shows the movement of a pair of electrons from negative to positive

Question 2

what is the word equation for radical substitution ?

Answer 2

alkane + halogen ——–> halogenoalkane + hydrogenhalide

Question 3

what are the 3 stages of free radical substitution?

Answer 3

initiation, propogation, termination

Question 4

write the formula for the radical substitution of methane and chlorine

Answer 4

CH4 + Cl2 ———> CH3Cl + HCl

Question 5

why does the halogen - halogen break in initiation?

Answer 5

bond is weaker so takes less energy to break it so it is the first to break

Question 6

how is a free radical formed?

Answer 6

when a bond of the same atom breaks homolytically and 1 electron goes to each atom in the bond

Question 7

what happens during the initiation stage of free radical substitution?

Answer 7

breaking of the halogen-halogen bond homolytically using UV light, creates a halogen radical

Question 8

what happens in the 1st step of propogation?

Answer 8

the Cl. takes a H atom from methane to form HCl, but leaves the methane as a methyl group free radical

Question 9

what happens during the 2nd stage of propogation?

Answer 9

the methyl free radical will react with another chlorine to form a halogenoalkane and a Cl.

Question 10

how does propogation repeat its self?

Answer 10

in the 2nd stage the halogen free radical will react with the alkane which is esentially the 1st stage again

Question 11

what happens during termination?

Answer 11

this is where the chain reaction stops and any free radicals are removed because they react together and froms an unreactive molecule

Question 12

what are the 3 outcomes in termination ?

Answer 12

halogen radical + halogen radical ——–> halogen2

alkane radical + alkane radical ———> different alkane

halogen radical + alkane radical ———> halogenoalkane

Question 13

what happens during an elimination reaction?

Answer 13

a nucleophile acts as a base and accepts a hydrogen (proton) from the halogenoalkane

Question 14

what are the products of an elimination reaction?

Answer 14

one hydrogen, a halogen and an alkene

Question 15

what are the conditions for an elimination reaction?

Answer 15

ethanolic solution and very hot temps

Question 16

explain how in the mechanism of elimination water is produced?

Answer 16

the lone pair on oxygen from the OH is attracted to a hydrogen attached to the C next to the C-X bond. the H it is delta+ and forms a dative covalent bond with the OH, this is what makes the product water

Question 17

explain how in the mechanism of elimination an alkene is produced?

Answer 17

The electron pair in this H-C bond (the H that has just been removed) will go into the C-C next to the hydrogen that was attacked by the OH, this forms a double c-c bond

Question 18

explain how in the mechanism of elimination of bromo-ethane Br- is produced?

Answer 18

the other carbon on the double bond has now made too many bonds, so the electrons in the
C-Br goes to the Br to create the product of Br- because it is the weakest bond so will break first

Question 19

what is the leaving group of an elimination reaction of bromoethane?

Answer 19

Br-

Question 20

what is a base?

Answer 20

a molecule that accepts protons

Question 21

why does the OH- act as a base in an elimination reaction?

Answer 21

the H is delta + so is attracted to the lone pair of electrons on the O, the lone pair of electrons from a dative covalent bond to the H. this acceptance of protons (H+) makes it a base

Question 22

how can 2 isometric products be created in an elimination reaction?

Answer 22

with a carbon length of more than 2 the double bond can form either side of where the OH attacks the H

Question 23

what is a nuleophile?

Answer 23

reagents that attack and form bonds with delta + of fully +ly charged carbon atoms

Question 24

what are the properties of a nucleophille?

Answer 24

is delta- or a negative ion or has a lone pair of electrons located on the electronegative atom

Question 25

how does a nuleophille form a covalent bond with another atom?

Answer 25

donating electrons to an electron defficient atom

Question 26

what 3 nucleophilles are used in a nucleophillic substitution?

Answer 26

:OH- hydroxide, :NH3 ammonia, :CN- cyanide

Question 27

what happens in a nucleophillic substitution?

Answer 27

the nucleophille will replace the hallogen in a halogenoalkane

Question 28

what is the general reaction of nucleophillic substitution with hydroxide or cyanide

Answer 28

R-X + OH ———-> ROH + X-

Question 29

decribe the nucleophillic substitution mechanism of OH or CN

Answer 29

the Nu attacks the delta+ carbon because the lone pair are attracted to the delta +.
electron pair move to the halogen because it is the weakest bond, making it a halide ion which leaves the halogenoalkane making it the leaving group with a lone pair and negative charge due to the carbon making room for another bond so it give an electron pair to halogen

Question 30

decribe and explain the mechanism for ammonia nucleophillic substitution

Answer 30

nitrogens lone electrons are attracted to delta+ carbon, but carbon can only make 4 bonds so the C-Br breaks as it is the weakest to create br-
The N attached to the C donates its electrons to form a dative covalent bond to carbon and is also positive as it is making 4 instead of 5 bonds.
when a second ammonia molecule is introduced, the electrons in the N-H bond move towards N, therefore the H is removed to a H+ . another nammonia molecules lone pairs are attracted to the H+ so now makes a separate NH4 which reacts with the Br- to make NH4Br

Question 31

what is the general reaction for ammonia nucleophillic substitution?

Answer 31

R-X + 2NH3 ———-> RNH2 + NH4X

Question 32

what are the conditions of ammonia nucleophillic substitution?

Answer 32

warm halogenoalkane with ammonia solution in ethanol. sealed tube and high pressure - stops ammonia escaping as a gas