N.F. 4.1.1 Introduction to reaction mechanisms

Question 1

what are the types of bond fission

Answer 1

• homolytic fission
• heterolytic fission

Question 2

Describe homolytic fission

Answer 2

When a covalent bond breaks by homolytic fission, each of the bonded atoms take one of the shaired pair of electrons from the bond.
- each atom now has a single unpaired electron.
- an atom or group of atoms with unpaired electron is called a radical

Question 3

Describe heterolytic fission

Answer 3

When a covalent bond breaks by heterolytic fission, one of the bonded atoms takes both of the electrons from the bond.
- The atom that takes both electrons becomes a negative ion
- the atom that does not take the electrons becomes a positive ion.

Question 4

define reaction mechanism

Answer 4

The reaction mechanism is the sequence of bond breaking and bond-forming steps that show the path taken by electrons during a reaction.

Question 5

how are electron movements shown in reaction mechanisms

Answer 5

with curly arrows

Question 6

Describe the role of curly arrows

Answer 6

In reaction mechanisms curly arrows are used to show the movement of electron pairs when bonds are being broken or made.

When representing pairs of electrons curly arrows will be shown with a full head but when representing single electrons they have half a head (like a fishhook)

Question 7

Describe how curly arrows are drawn

Answer 7

• Curly arrows are drawn from the charge of the first atom (or group of atoms) to near the second atoms (or group of atoms).
  The charge must be on the right atom eg. if the curly arrow was to be drawn from OH⁻ to H⁺ the arrow would come from : place just before the OH⁻ to represent the electrons i.e. :OH⁻
  (the O is where the e⁻ is comeing from so it is where the : is placed)
  If we wanted to draw the curly arrow from the ⁻ of the OH ion then we would show the charge on the correct atom i.e. ⁻OH
• If there is no charge on the group of atoms then the curly arrow goes from the middle of the bond to an atom e.g. H-Cl → H⁺ + Cl⁻ i.e. the curly arrow would go from the - to the Cl

Question 8

What happens if a curly arrow ends forming a bond to an atom that already has a full outer shell

Answer 8

Another bond to that atom must be broken.
e.g. :NH₃ + H-Cl → NH⁺₄ + Cl⁻
The curly arrow goes from the : at the NH₃ to the H and another curly arrow goes from the - of the H-Cl to the Cl

Question 9

What are the names of reactants in reaction mechanism and what are they dependant on

Answer 9

The names are dependant on whether they donate or accept an electron pair during the mechanism.
electrophile - accepts
nucleophile - donates

Question 10

Define nucleophile

Answer 10

A nucleophile donates a pair of electrons.

Curly arrows START on nucleophiles

Question 11

Define electrophile

in reaction mechanisms

Answer 11

An electrophile accepts a pair of electrons

Curly arrows END on electrophiles

Question 12

List 3 types of reaction common in organic chemistry

Answer 12

• Addition
• Substitution
• elimination

Question 13

Describe addition reactions

Answer 13

In an addition reaction, two reactants combine to give only one product.

Alkenes undergo addition reactions with electrophiles

Question 14

Describe substitution reactions

Answer 14

In a substitution reaction, one or group of atoms is replaced by a different one .

Haloalkanes undergo substitution reactions with nucleophiles

Question 15

Describe elimination reactions

Answer 15

In elimination reactions, one reactant forms two produts

alcohols undergo elimination reactions in the presence of an acid catalyst