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

The Mechanisms of Addition Reactions Flashcards

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1
Q

what are curly arrows used for

A

representing the movement of a pair of electrons

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2
Q

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

A

a high electron density

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3
Q

what does this pi bond make alkenes attracted to

A
  • species that are electron deficient

- including molecules with polar bonds

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4
Q

why is hydrogen bromide a polar molecule

A
  • because bromine is more electronegative than hydrogen
  • which can be shown with their partial charges
  • H(o+)-Br(o-)
  • o = sigma bond
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5
Q

what happens when a hydrogen bromide molecule approaches an ethene molecule

A

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

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6
Q

what is the HBr molecule called when it does this

A

an electrophile

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7
Q

what is an electrophile

A

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

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8
Q

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

A
  • start from a bond and move to an atom

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

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9
Q

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

A

electrophilic addition

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10
Q

what is electrophilic addition

A

an addition reaction
that involves an attack by an electrophile

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11
Q

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

A
  • the hydrogen would

- because the bromine is more electronegative than hydrogen

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12
Q

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

A
  • 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
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13
Q

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

A
  • 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
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14
Q

what is kind of bond breaking called

A

heterolytic fission

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15
Q

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

A
  • 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
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16
Q

what is the positively charged molecule called and why

A
  • a carbocation

- because it has its charge on a carbon atom

17
Q

what arrows would be used in this step

A
  • 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
Q

what is the second step in this reaction

A
  • the two oppositely charged ions attract each other
  • and react to form a new covelant bond
  • forming bromoethane
19
Q

what arrow would be used in this step

A
  • the arrow from the bromine ion (specifically from the line pair of drawn electrons)
  • to the carbon atom with the positive charge
20
Q

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

A

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

21
Q

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

A
  • 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
Q

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

A
  • 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
Q

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

A
  • there can be two possible products

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

24
Q

what is an unsymmetrical alkene

A

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

25
Q

what is an unsymmetrical attacking molecule

A

one in which the atoms are just different

26
Q

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

A
  • 2-bromopropane

- 1-bromopropane

27
Q

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

A
  • 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
Q

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

A
  • A = CH3-CH+-CH3

- B = CH2+-CH2-CH3

29
Q

what does A show and what is it called

A
  • it shows the carbon atom with the positive charge bonded with two alkyl groups
  • called the secondary carbocation
30
Q

what does B show and what is it called

A
  • it shows the carbon atom with the positive charged bonded with only one alkyl group
  • called the primary carbocation
31
Q

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

A
  • 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
Q

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

A

because alkyl groups are electron releasing

33
Q

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

A
  • 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
Q

what is more stable than a secondary carbocation

A

a tertiary carbocation

35
Q

what is an electron-releasing group

A

one that pushes electrons towards the atom it is joined to

Chemistry 6 - Organic Chemistry (20 decks)

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