The Mechanisms of Addition Reactions

Question 1

what are curly arrows used for

Answer 1

representing the movement of a pair of electrons

Question 2

what does the pi bond around the C=C bond in alkenes have

Answer 2

a high electron density

Question 3

what does this pi bond make alkenes attracted to

Answer 3

• species that are electron deficient

- including molecules with polar bonds

Question 4

why is hydrogen bromide a polar molecule

Answer 4

• because bromine is more electronegative than hydrogen
• which can be shown with their partial charges
• H(o+)-Br(o-)
• o = sigma bond

Question 5

what happens when a hydrogen bromide molecule approaches an ethene molecule

Answer 5

the slightly positive end of the HBr molecule is attracted to the electrons in the pi bond in the C=C

Question 6

what is the HBr molecule called when it does this

Answer 6

an electrophile

Question 7

what is an electrophile

Answer 7

a species that is electron deficient and therefore attracted to a negative charge

Question 8

what are the two things curly arrows do with this type of reaction

Answer 8

• start from a bond and move to an atom

- or start from a lone pair of electrons and move to an atom

Question 9

what is the full name for an addition reaction of this type, between ethene and hydrogen bromide

Answer 9

electrophilic addition

Question 10

what is electrophilic addition

Answer 10

an addition reaction
that involves an attack by an electrophile

Question 11

in a reaction between ethene and hydrogen bromide, which atom out of hydrogen and bromine would be attracted to the double bond and why

Answer 11

• the hydrogen would

- because the bromine is more electronegative than hydrogen

Question 12

how are the shared electrons between hydrogen and bromine now distributed now that their bond is broken

Answer 12

• both of the electrons go on the bromine atom
• causing the hydrogen to to be partially positively charged
• and the bromine to be partially negatively charged

Question 13

why does it make sense for the electrons to be pushed towards the atom in the downwards direction rather than staying with the atom closer to the double bond

Answer 13

• the electron in the pi bond and the electrons in the covelant bond repel
• causing the electrons to be pushed downwards and onto the bromine atom in this case

Question 14

what is kind of bond breaking called

Answer 14

heterolytic fission

Question 15

what happens in the first step in the reaction between ethene and hydrogen bromide

Answer 15

• the hydrogen atom in the hydrogen bromide reacts with the C=C bond breaking it
• forming positively charged molecule and a negatively charged bromine ion

Question 16

what is the positively charged molecule called and why

Answer 16

• a carbocation

- because it has its charge on a carbon atom

Question 17

what arrows would be used in this step

Answer 17

• the first arrow would point from the double bond to the hydrogen atom with o+
• the second arrow would point from the bond between the hydrogen and bromine to the bromine atom with o-

Question 18

what is the second step in this reaction

Answer 18

• the two oppositely charged ions attract each other
• and react to form a new covelant bond
• forming bromoethane

Question 19

what arrow would be used in this step

Answer 19

• the arrow from the bromine ion (specifically from the line pair of drawn electrons)
• to the carbon atom with the positive charge

Question 20

what is the difference between this kind of electrophilic addition and reactions that form halogen halides

Answer 20

the attacking bromine molecule in halogen halide reactions doesnt have a polar bond

Question 21

what would happen to a bromine molecule if it were to approach the C=C bond anyway and why

Answer 21

• the molecule would become polar at that point
• that is because as the bromine molecule approaches the bond, the electrons in the pi bond repel the electrons in the Br-Br bond
• this causes one of them to be partially positive and the other to be partially negative
• leading to an induced polar molecule

Question 22

is the mechanism for the reaction between ethene and bromine the same and what is its name

Answer 22

• it is the same
• the arrows and the order is the same, just that you have two bromines instead of 1
• the product is 1,2-dibromoethane

Question 23

what is the effect of the attacking molecule and the alkene being unsymmetrical if they react

Answer 23

• there can be two possible products

- because the atoms in the attacking molecule can be added into two different places

Question 24

what is an unsymmetrical alkene

Answer 24

one in which the atoms on either side of the C=C bond are not the same

Question 25

what is an unsymmetrical attacking molecule

Answer 25

one in which the atoms are just different

Question 26

what are the two products that can form if you react propene with hydrogen bromide

Answer 26

- 2-bromopropane | - 1-bromopropane

Question 27

which is the major and minor product and what does that mean

Answer 27

- 2-bromoethane is the major product - 1-bromoethane is the minor product - it means that 2-bromoethane is produced more frequently or often than 1-bromoethane

Question 28

what are the two carbocations that could be formed during that reaction

Answer 28

- A = CH3-CH+-CH3 | - B = CH2+-CH2-CH3

Question 29

what does A show and what is it called

Answer 29

- it shows the carbon atom with the positive charge bonded with two alkyl groups - called the secondary carbocation

Question 30

what does B show and what is it called

Answer 30

- it shows the carbon atom with the positive charged bonded with only one alkyl group - called the primary carbocation

Question 31

what is the most important principle you need to know when comparing the type of carbocation that is formed

Answer 31

- the carbocation in which the positive charge can be spread over more atoms - is more stable than one in which there are fewer atoms available to spread the charge

Question 32

why is the positive charge spread more when there are two alkyl groups than when there is only one

Answer 32

because alkyl groups are electron releasing

Question 33

what therefore makes the secondary carbocation attract the bromine ion more frequently

Answer 33

- because the positive charge is spread over a larger area - it is more likely to attract the bromine ion more than a primary carbocation - therefore the major product is formed from the more stable cation

Question 34

what is more stable than a secondary carbocation

Answer 34

a tertiary carbocation

Question 35

what is an electron-releasing group

Answer 35

one that pushes electrons towards the atom it is joined to