R3.3 - electron sharing reactions

Question 1

What is a radical?

Answer 1

Chemical entities that have unpaired electrons, are highly reactive and a high enthalpy.
- Short life span, so do not exist for long.
- Radicals can be a source of cancer.

Question 2

Why are radicals highly reactive?

Answer 2

Radicals are highly reactive as its energetically favourable for them to take an electron from other species or to combine with other radicals to form a covalent bond.

Question 3

What are some examples of common atomic radicals?

Answer 3

• Chlorine
• Bromine
• Hydrogen

Question 4

What are some common examples of molecular radicals?

Answer 4

• Nitric oxides
• Hydroxyl
• Methyl

Question 5

What are neutral radicals and why?

Answer 5

Molecular and atomic radicals are neutral as they have equal proton and electron number.

Question 6

What are anionic radicals?

Answer 6

Molecules that gain electrons to become radicals.
- Have a negative charge.
- O2- or C6H6-

Question 7

What are cationic radicals?

Answer 7

Molecules that lose electrons to become radicals.
- Have a positive charge.
- M+ or C2H5OH+

Question 8

Why are anionic and cationic radicals not neutral?

Answer 8

Don’t contain equal numbers of protons and electrons.

Question 9

How are radicals made?

Answer 9

Homolytic fission - covalent bonds break to form 2 radicals with equal assignment of electrons in the presence of high heat or UV.

Question 10

What are CFC’s?

Answer 10

Chlorofluorocarbons - widely used as aeorosols, solvents and plastics as well as breaking down to release a Chlorine radical, acting as a catalyst for the decomposition for ozone and caused depletion of the ozone layer.

Question 11

Why do CFC’s break down to release a chlorine radical but not a fluorine radical?

Answer 11

C-Cl has a weaker bond (324) than C-F (492), so is easier to break when UV radiation is absorbed.

Question 12

Why are alkanes so unreactive?

Answer 12

Alkanes contain strong C-C and C-H bonds which require lots of energy to break.
- Alkanes are also non-polar, meaning that it doesn’t attract reactive species such as electrophiles and nucleophiles.

Question 13

What is the activation energy for alkanes?

Answer 13

Large activation energy as alkanes are unreactive and kinetically stable.

Question 14

What is the enthalpy change for alkanes?

Answer 14

Negative enthalpy change as alkanes are thermodynamically unstable compared to its products.

Question 15

What are the 3 stages of a radical substitution reaction?

Answer 15

1) Initiation - making the radical.
2) Propagation - making new radicals.
3) Termination - removing the radical.

Question 16

What occurs doing the initiation stage?

Answer 16

• Includes 1 reaction
• Requires high energy UV to be present.
• A molecule splits into 2 new radicals; molecule with the weakest bond is broken.
• Energy needed corresponds to the frequency of UV needed to break the bonds (E=hf)
  Cl2 –> 2 Cl radicals

Question 17

What occurs doing the propagation stage?

Answer 17

• Includes 2 reactions (chain-reaction)
• Radical formed in the initiation stage reacts and a new radical is formed.
  1) Cl radical + H2 –> H-Cl and H radical
  2) H radical + Cl2 –> H-Cl and Cl radical

Question 18

What occurs during the termination stage?

Answer 18

• Includes 3 steps
• 2 radicals recombine to make 1 non radical product.
  1) H radical + H radical –> H2
  2) Cl radical + Cl radical –> Cl2
  3) H radical + Cl radical –> HCl
  Overall: CH4 (g) + Cl2(g) –> CH3Cl (g) + HCl (g)

Question 19

What happens if alkanes react with bromine water?

Answer 19

• Under UV, Bromine water is decolourized as a photochemical reaction occurs.
• In the dark, there is no colour change as no reaction occurs.

Question 20

What are alkanes?

Answer 20

Saturated hydrocarbons that undergo substitution reactions.