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CHY1101 - Organic chemistry > Addition to alkenes > Flashcards

Addition to alkenes Flashcards

1
Q

What general product is formed when bromine reacts with alkene?

A

1,2-dibromide

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2
Q

How does the reaction between an alkene and bromine take place even though both molecules are neutral?

A

The alkene has a good source of electrons, the pi bond.

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3
Q

Where does bromine accept the electrons into?

A

The sigma star orbital of the Br-Br bond

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4
Q

What is the initial product of the bromination reaction?

A

Cyclic bromonium ion

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5
Q

What does the bromination reaction occur by?

A

overlap of the filled orbital of the nucleophile (alkene) and the empty sigma star orbital of electrophile (Br-Br)

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6
Q

What type of reaction is bromination?

A

Concerted reaction

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7
Q

What is a concerted reaction?

A

Both bond making/breaking occurs in one single step

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8
Q

Why is the addition of Br- to the cyclic bromonium ion a fast reaction?

A

It is breaking the ring strain of the cyclic bromonium ion

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9
Q

What increases the attraction between the alkene and bromine?

A

there is an induced dipole

The delta positive Br attracted to delta negative alkene

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10
Q

What type of substitution is bromination?

A

SN2

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11
Q

What are the two steps in forming the bromination

A

electrons donated from nucleophile to sigma star orbital or Br-Br bond causing it to break.
The bromine then uses it’s lone pairs of electrons to donate electrons back to the alkene

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12
Q

What type of addition is bromination?

A

antiaddition

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13
Q

What is the anti-addition due to?

A

Both steps of the mechanism being highly stereocontrolled

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14
Q

What is anti-addition?

A

The two bromines are on different faces of the bond

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15
Q

What is syn-addition?

A

The two bromines are on the same face of the bond

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16
Q

What is “relative” stereochemistry?

A

Bromine being attached to both faces

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17
Q

What is “absolute” stereochemistry?

A

Bromine being attached to only one face

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18
Q

What type of stereocontrol gives an anti-addition product?

A

Relative stereocontrol

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19
Q

What does the SN2 reaction cause?

A

Inversion of stereochemistry

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20
Q

When can the initial reaction to form the cyclic bromonium ion only occur?

A

If both C-Br bonds are formed on the same face of the alkene

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21
Q

What type of addition is the first step of bromination always?

A

Syn-addition

22
Q

What is the only exception to the first step being syn-addition?

A

If the reaction starts by protonation of the alkene as these go via a carbenium ion

23
Q

Why does protonation of the alkene go by a carbenium ion?

A

The H+ is too small to form a three membered ring

24
Q

If the reactant is anti-addition what will the product be?

A

Anti-addition

25
Q

If the reactant is syn-addition, what will the product be?

A

Syn-addition

26
Q

What type of product will be formed from an E-alkene?

A

Anti-addition

27
Q

What type of product will be formed from a Z-alkene?

A

syn-addition

28
Q

Where do electrophiles add to alkenes?

A

To form the most stable carbenium ion intermediate

29
Q

What is the reaction like if I2 is used as the halogen?

A

Reaction occurs via an iodonium ion

Equilibrium is unfavourable and only reactive alkenes will react with iodine

30
Q

What is the reaction like if Cl2 is used as the halogen?

A

Addition occurs via a chloronium ion

31
Q

What is the reaction like if the HI and HCl are used as the halogens?

A

Same as HBr

Anti-addition reaction

32
Q

What is the reaction like if BrOH or ClOH are used as the halogens?

A

The halogen-OH bond is polarised with the halogen being delta positive so reactions occurs via a bromonium or chloronium ion

33
Q

What is the reaction like if Br-Cl , Br-F or I-Cl are used as the halogens?

A

The bond between two different halogens is polarised with the heavier halogen being delta positive so reactions occur via a bromonium or iodonium ion

34
Q

What are the most stable carbenium ion?

A

Tertiary and allylic

35
Q

What do you have to match the nucleophile to?

A

The electrophile

36
Q

What makes an alkene hard?

A

Most hindered

Higher charge density

37
Q

What makes an alkene soft?

A

Low charge density

Least hindered

38
Q

Where do Nu- and E+ tend to be in relation to each other?

A

Opposite ends of the conjugated system

39
Q

Where does initial protonation occur and why?

A

Most substituted double bond to give the most stable carbenium ion

40
Q

How can you make an alkene react with H3O+ to give an alcohol?

A

If the carbenium ion intermediate is tertiary and in strong acid

41
Q

What type of nucleophile is an alkene generally?

A

Soft

42
Q

What type of electrophiles are mercury reagents? Why?

A

Soft

They have a 2+ charge over a huge molecule

43
Q

When mercury reagents are reacted with alkenes, what is the initial product?

A

Cyclic 3 membered mercurinium ion ring

44
Q

Where does water attack during oxymercuration? Why is this odd?

A

Most hindered end

This is very unique and hard to do in other circumstances

45
Q

How do sterics influence the oxymercuration reaction?

A

There is a steric clash between the Hg and the methyl group, pushing the Hg the other way, causing the bond to become long, weak and essentially already broken

46
Q

Which carbon does water react at in the oxymercuration reaction?

A

Carbon atom which has the greater positive charge

This will be the more substituted carbon atom as alkyl groups stabilise positive charges

47
Q

What are the conditions and reagents for the hydroboration of alkenes?

A
  1. BH3, THF

2. H2O2, NaOH

48
Q

What does the hydroboration result in?

A

Overall addition of water across the alkene

49
Q

Is Borane an electrophile or a nucleophile? Why?

A

Electrophile

6 outer shell electrons and has a vacant p-orbital

50
Q

Where does Borane react?

A

At the less substituted end of the alkene so that delta positive builds up at the more substituted carbon atom

CHY1101 - Organic chemistry (22 decks)

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