unit 3) SYNTHESIS -Homolytic and heterolytic fission + nucleophiles and electrophiles

Question 1

• what happens during bond breaking / bond fission

- the two types of bond fission

Answer 1

• electrons are redistributed between the two atoms

- homolytic fission , heterolytic fission

Question 2

what is homolytic fission

Answer 2

• the breaking of a covalent bond to produce two free radicals
  ( when a molecule undergoes homolytic fission one electron of the sigma covalent bond ends up with one atom and the other electron ends up on the other atom)

Question 3

in homolytic fission two species with the ____ charge are formed

Answer 3

same ( forming two neutral species)

Question 4

when does homolytic fission usually occur

Answer 4

when non polar covalent bonds are broken

Question 5

how does homolytic fission form two free radicals

Answer 5

• covalent bond is broken , each atom in the bond leaves with one electron from the shared pair so forming two free radicals ( molecules that have an unpaired electron and so are very reactive)

Question 6

how to show the homolytic fission using dots and curly arrows

Answer 6

look in notes

• single headed curly arrows / fishooks indicate the movement of single electrons
• the tail of the curly arrow shows the source of the electron and the head shows its destination
• the dot represents the unpaired electron in the free radical

Question 7

the free radicals produced are highly ____ as the unpaired electrons tend to ____ other species

Answer 7

reactive

attack

Question 8

• steps

Answer 8

-look in HTP txtbook
-initiation : chlorine absorbs energy supplied by UV light : chlorine -chlorine bond breaks to form the free radicals
-

Question 9

why are reactions involved in homolytic fission unsuitable in the synthesis of organic compounds

Answer 9

• free radicals are highly reactive species and react with the ch3cl and ch3ch3 products to produce more free radicals , which in turn form even more products
• and so it goes on until you end up with a reaction mixture containing lots of different compounds , each of these compounds will be produced in low yields and will be difficult to isolate

Question 10

what is heterolytic fission

Answer 10

• when a covalent bond breaks and both electrons in the bond move to one of the atoms resulting in two oppositely charged ions being formed

Question 11

when does heterolytic fission occur

Answer 11

• most likely to occur in polar molecules

Question 12

which atom in the bond do the electrons go to

Answer 12

• they go to the most electronegative atoms in the bond, so ends up being the negatively charged ion

Question 13

• when a polar bond between a carbon and halide atom breaks , heterolytic fission results in the formation of a positively charged carbon atom. what is this called

Answer 13

the positively charged carbon atom is known as a carbocation (the halide ion has greater electronegativity than the carbon atom)
(negatively charged carbon atom is called a carbanion)

Question 14

how to use the curly arrows to show heterolytic fission

Answer 14

look in notes
- we use the double headed curly arrows as it indicates the movement of a pair of electrons. the tail shows the source of the electron pair and the head indicates the destination

Question 15

reactions that proceed via heterolytic fission tend to produce fewer products …

Answer 15

so are better suited to the synthesis of organic compounds

Question 16

what is a nucleophile

Answer 16

• negatively charged ions (or neutral molecules) that are electron rich / have at least one lone pair(non bonded pair) of electrons to give away

Question 17

what does a nucleophile do

Answer 17

• its a lone pair donor
• the negatively charged ion or group is attracted to where there are few electrons and it donates its electron pair to form a new covalent bond
• nucleophiles are attracted to and will attack a species with a positive or partial positive charge

Question 18

electron pairs from the nucleophile can be ___ _____ or ______ electron pairs

Answer 18

non bonded

bonded

Question 19

examples of nucleophiles

Answer 19

Cl-, Br- ,(halides) , OH- , CN-, NH3,H2O,

Question 20

what are electrophiles

Answer 20

• positively charged ions or neutral molecules that are deficient in electrons , and can accept a pair of electrons

Question 21

what does an electrophile do

Answer 21

• accept a pair of electrons from an electron pair donor to form a covalent bond
• electrophiles are attracted to and will attack species with a negative or partial negative charge

Question 22

examples of electrophiles

Answer 22

H+, NO2+, SO3