Bond Fission and Substitution reactions

Question 1

what is a bond fission?

Answer 1

the breaking of a covalent bond

Question 2

what is heterolytic fission?

Answer 2

when the bond breaks unevenly with one of the bonded atoms receiving both electrons from the bonded pair.

-two different substances are formed- a positively charged cation, and a negatively charged anion.
-general equation: XY—– X+ + Y-

Question 3

what is homolytic fission?

Answer 3

where the bond breaks evenly and each bonding atom receives one electron from the bonded pair.

-two electrically uncharged radicals are formed (radicals are particles that have an unpaired electron.
-general equation: XY—– X. + Y.

Question 4

what does a curly arrow show?

Answer 4

the movement of an electron pair- can use them to show how covalent bonds are broken or heterolytic fission

Question 5

how are haloalkanes formed?
what is a photochemical reaction?

Answer 5

haloalkanes are formed when halogens react with alkanes in a photochemical reaction.
a photochemical reaction is a reaction that is started by light.

Question 6

radical substitution:
what occurs during the initiation step for the synthesis of chloromethane?

Answer 6

-radicals are produced, sunlight provides enough energy to break some of the Cl-Cl bonds- photodissociation
-the bond splits equally and each atom gets to keep one electron (homolytic fission)

Question 7

radical substitution:
what occurs during the propagation stage for the synthesis of chloromethane?

Answer 7

-radicals are used up and created in a chain reaction
-first Cl. attacks a methane molecule, the new methyl radical, CH3., can the attack another Cl2 molecule, the new Cl. can attack another CH4 molecule, and so on, until all the Cl2 and Cl4 molecules are used up.

Question 8

radical substitution:
what occurs during the termination stage for the synthesis of chloromethane?

Answer 8

-all the radicals are ‘mopped up’
-if two radicals join together they make a stable molecule.

Question 9

List the problems with radical substitution

Answer 9

1. if you’re trying to make a particular product, you don’t only get the product you’re after, but a mixture of products.
   e.g. if you’re trying to make chloromethane, and there is too much chlorine in the reaction mixture, some of the remaining hydrogen atoms on the chloromethane molecule will be swapped for chlorine atoms. the propagation reaction will happen again , this time to make dichloromethane:
   Cl. + CH3Cl—- CH2Cl. + HCl
   CH2Cl. + Cl2—– CH2Cl2 + Cl.
2. Another substitution reaction can take place
   e.g. to form trichloromethane.
   Cl. + CH2Cl2—- CHCl2. + HCl
   CHCl2. + Cl2—– CHCl3 +Cl.

tetrachloromethane is formed in the last possible substitution, the process has to stop due to there being no more hydrogens attached to the carbon atoms. end product is a mixture of CH3Cl, CH2Cl2, CHCl3, CCl4.
the best way to reduce the chance of these by-products forming is to have an excess of methane. this will mean there is a greater chance of a chlorine radical colliding only with a methane molecule and not a chloromethane molecule.

3. it can take place at any point along the carbon chain, and so a mixture of isomers can be formed