The Mechanisms of Addition Reactions Flashcards Preview

Chemistry 6 - Organic Chemistry > The Mechanisms of Addition Reactions > Flashcards

Flashcards in The Mechanisms of Addition Reactions Deck (35)
1

what are curly arrows used for

representing the movement of a pair of electrons

2

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

a high electron density

3

what does this pi bond make alkenes attracted to

- species that are electron deficient
- including molecules with polar bonds

4

why is hydrogen bromide a polar molecule

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

5

what happens when a hydrogen bromide molecule approaches an ethene molecule

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

6

what is the HBr molecule called when it does this

an electrophile

7

what is an electrophile

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

8

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

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

9

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

electrophilic addition

10

what is electrophilic addition

an addition reaction
that involves an attack by an electrophile

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

- the hydrogen would
- because the bromine is more electronegative than hydrogen

12

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

- 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

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

- 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

14

what is kind of bond breaking called

heterolytic fission

15

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

- 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

16

what is the positively charged molecule called and why

- a carbocation
- because it has its charge on a carbon atom

17

what arrows would be used in this step

- 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-

18

what is the second step in this reaction

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

19

what arrow would be used in this step

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

20

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

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

21

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

- 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

22

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

- 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

23

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

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

24

what is an unsymmetrical alkene

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

25

what is an unsymmetrical attacking molecule

one in which the atoms are just different

26

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

- 2-bromopropane
- 1-bromopropane

27

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

- 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

28

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

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

29

what does A show and what is it called

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

30

what does B show and what is it called

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

31

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

- 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

32

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

because alkyl groups are electron releasing

33

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

- 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

34

what is more stable than a secondary carbocation

a tertiary carbocation

35

what is an electron-releasing group

one that pushes electrons towards the atom it is joined to