Unit 3 ) SYNTHESIS -haloalkanes and mechanisms of substitution reactions

Question 1

what are haloalkanes

Answer 1

haloalkanes are a series of saturated carbon compounds containing one or more halogen atoms(one or more hydrogen atom is replaced by a halogen atom)

Question 2

what are primary , tertiary and secondary monohaloalkanes

Answer 2

-if the carbon atom with the halogen attached have
; 1 alkyl group , its primary ( wil; have 2h )
2 alkyl groups its secondary ( will have 1 h)
3 alkyl groups its tertiary( will have no h)
structures with no alkyl group attached are still classified as primary

Question 3

monohaloalkanes have a _____ carbon-halogen bond

why ?

Answer 3

polar

- because the halogen atom has a greater electronegativity than the carbon atom

Question 4

explain how general nucleophilic substitution occurs

Answer 4

• the slight positive charge on the carbon atom makes monohaloalkanes act as electrophiles and they are therefore susceptible to nucleophilic attack
• the nucleophile donates an electron pair to the monohaloalkane and , in doing so, forms a covalent bond with the carbon atom of the C-X bond.
• at the same time the halogen atom is thrown out as a halide ion and is replaced or substituted by the nucleophile

Question 5

monohaloalkanes undergo neucleophilic substitution reactions with….

Answer 5

• aquoues alkalis to form alcohols
• alcoholic alkoxides to form ethers
• ethanolic potassium (or sodium) cyanide to form nitriles

Question 6

example of nucleophilic substitution: aqueous alkali forming an alcohol

• what acts as the nucleophile
• what else could act as a nucleophile in this reaction

Answer 6

• look in notes and diagram to understand
• when 1 chloropropane is heated under reflux with sodium or potassium hydroxide (KOH or NaOH) , the alcohol propan1ol is formed
• water itself could be used as a nucleophile in this reaction ( bc it has OH)

Question 7

example of nucleophilic substitution: alcoholic alkoxide to form ethers
- what acts as the nucleophile

Answer 7

• in notes for diagram
• when bromoethane is heated under reflux with the alkoxide potassium methoxide in methanol, the ether methoxyethane is formed .
• here the methoxide ion ch3o- acts as the nucleophile

Question 8

how are alkoxides formed

Answer 8

• alkoxides are formed when an alkali metal ( sodium , potassium) is added to an alcohol
  eg when potassium is added to methanol, potassium methoxide is formed

Question 9

example of nucleophilic substitution : ethanolic potassium (or sodium) cyanide

Answer 9

• eg when chloromethane is heated under reflux with a solution of potassium cyanide in ethanol , the nitrile ,ethanenitrile is formed
• CN- is the nucleophile

Question 10

the nitrile formed in that reaction contains more than one carbon atom than the orginal monohaloalkane. why is this useful?

Answer 10

makes the reaction useful in synthetic organic chemistry because it is a way to increase the chain length of an organic compound

Question 11

what can the nitrile in the above reaction (ethanenitrile) be converted to .

Answer 11

• can be converted to the corresponding carboxylic acid (ethanoic acid) by hydrolysis
  look in notes

Question 12

what do monohaloalkanes form in elimination reactions

Answer 12

alkenes

Question 13

in elemination reactions , haloalkanes form alkenes using….

Answer 13

using a strong base dissolved in hot ethanol solvent

Question 14

eg : when ethanolic potassium hydroxide is added to 2 bromopropane and refluxed, what will it form ?
- how ?

Answer 14

.. it will form an alkene , water and bromide ions

• when dissolved in ethanol the hydroxide ions will act as a base and accept a proton to form water
• in the formation of the double bond the Br and a H atom has been removed or eliminated from the haloalkane

Question 15

a hot ethanol solvent promotes ______ whereas warm aqueous conditions promote ______ ________

Answer 15

elimination

nucleophilic substitution

Question 16

a monohaloalkane will undergo nucleophilic substitution by one of the two mechanisms..

Answer 16

Sn1 mechanism or Sn2 mechanism

Question 17

what does Sn1 and Sn2 stand for

Answer 17

s - substitution
n - nucleophilic
1 - first order overall - so there is one molecule involved in the rate determining step ( the mechanism only involves the haloalkane )
2 - second order overall so there is 2 molecules involved in the rate determining step

Question 18

when does sn1 occur and when does sn2 occur

Answer 18

sn1 - in tertiary haloalkanes

sn2 - in primary haloalkanes

Question 19

the steps in sn1 reaction mechanism
step 1
step 2
include what shape the intermediate is

Answer 19

• look at diagram in notes
  1) slow rate determining step: the carbon-halogen bond breaks heterolytically to form a negative halogen atom and a positively charged carbocation intermediate (the intermediate is trigonal planar)
  2) second step is so fast it is not included in the rate order. involves nucleophilic attack to the carbocation by a nucleophile

Question 20

from which sides can the nucleophile attack the carbocation intermediate ( in the second step)

Answer 20

it can attack from both left and right hand sides. this is because the carbocation intermediate has a trigonal planar arrangement if bonds around the central carbon atom.

Question 21

the steps in sn2 reaction mechanism

what shape is the molecule in the transition state

Answer 21

there is 1 step

• the nucleophile forms a bond with the carbon atom attached to the halogen atom ,at the same time the carbon-halogen bond breaks, a transition stage is then reached. this is where the bond between the nucleophile and the carbon atom is partially formed and the bond between the carbon and the halogen is partially broken
• the transition state will be trigonal bipyramidal
• the reaction is completed by the full formation of the nucleophile-carbon bond and full breakage of the carbon-halogen bond

Question 22

sn1 mechanisms are most likely for ______ haloalkanes and least likely for _____ haloalkanes

Answer 22

tertiary

primary

Question 23

explain the trigonal bipyramidal structure that the molecule takes in the transition state of sn2

Answer 23

• transition state adopts a trigonal bipyramidal structure with the two C-H bonds and C-C bond in a plane and then the partial bonds lying perpendicular to this plane

Question 24

in the sn1 mechanism a carbocation intermediate is formed and this could be _____, ______ or ______ carbocation.
the nature of the haloalkane and stability will influence the _____ chosen

Answer 24

primary , secondary, tertiary

mechanism

Question 25

alkyl groups are said to have a positive inductive effect. what does this mean, ( for the carbocation) ?

Answer 25

• this means they are electron donating , so can push electrons onto the positively charged carbon atom , so stabilising the carbocation.

Question 26

so which haloalkanes are most likely to react with a nucleophile via an sn1 mechanism ?

Answer 26

tertiary haloalkane, as they have3 alkyl groups and so are the most stable species.
primary haloalkanes with only one alkyl group are the least stable

Question 27

what is steric effect/ steric hinderance, and how does this suggest primary haloalkanes are most likely to use sn2 mechanism ?

Answer 27

• when a nucleophile attacks a primary haloalkane, it approaches the positive carbon atom from the side away from the halogen atom. this is impossible for a tertiary haloalkane as the back of the molecule is cluttered with alkyl groups meaning they dont use the sn2 mechanism.