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CHY1101 - Organic chemistry > Elimination reactions > Flashcards

Elimination reactions Flashcards

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1
Q

What is an elimination reaction?

A

the loss of a leaving group and the formation of an alkene

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2
Q

What does the nucleophile act as in an elimination reaction?

A

base

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3
Q

How many molecules are involved in the rate determining step in E1?

A

1

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4
Q

Why is HSO4- a poor nucleophile?

A

It is a very weak base

It is stable as the negative charge is spread across the 3 atoms

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5
Q

Which step is the rate determining step in E1?

A

The slow step

the kicking out of the leaving group

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6
Q

How is the carbenium ion stabilised in E1?

A

Sigma conjugation

electron density is donated from the C-H bond which breaks to the carbenium ion

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7
Q

Why is a tertiary carbenium ion the most stable?

A

There are 3 lots of sigma conjugation which means it is more stable

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8
Q

What is an intramolecular electron source?

A

The C-H bond which is adjacent to the carbenium io

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9
Q

Why does an intramolecular substitution just happen (E1) ?

A

It wants to lower the energy of the system as it has a positive charge

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10
Q

How many molecules are involved in the rate determining step for E2?

A

2

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11
Q

Why is the nucleophile acting as a base?

A

It is attacking the hydrogen and not the carbon

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12
Q

What are the steps of an E1 reaction?

A

Leaving group is protonated to make it a better leaving group
L.G kicked out leaving a carbenium ion
Nucleophile attacks the carbenium ion

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13
Q

For E1 reactions, what is the order of reactivity related to?

A

Stability of the intermediate carbenium ion

The more stable, the better the E1 will be

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14
Q

When is elimination not possible?

A

When there are no hydrogens β-to the leaving group

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15
Q

What type of solvents favour E1 reactions and why?

A

Polar

They can stabilise the charged, ionic intermediate

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16
Q

What is the issue with using H2O as a solvent?

A

It is too polar, organic molecules don’t dissolve

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17
Q

What are some examples are polar solvents?

A

ROH
DMSO
DMF

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18
Q

What are some examples of mid-polar solvents?

A

THF
Et2O
DCM
EtOAc

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19
Q

What are some examples of non-polar solvents?

A

Hexane
Petrol
Toulene
Benzene

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20
Q

How does the leaving group influence the rate of reaction?

A

It is involved in the rate determining step so the better the leaving group the faster the reaction

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21
Q

Why is the E1 reaction for primary or secondary alcohols slow?

A

The secondary carbenium ion is not stable

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22
Q

Why would an E2 reaction for primary or secondary alcohols not happen?

A

A strong base would deprotonate the OH rather than the hydrogen adjacent to the OH group

23
Q

The stronger the base…

A

The more likely it is to take only the most acidic hydrogen

24
Q

What is alcohol activation?

A

Converting OH into a better leaving group which is usually done through the formation of a tosylate

25
Q

What is stereoselectivity?

A

Gives one stereoisomer of the product in preference

26
Q

Which elimination reaction can be stereoselective?

A

E1

27
Q

Which isomer, E or Z, will be more stable and preferred and why?

A

E

Steric interactions, more steric repulsions in a z isomer as the large groups are close together

28
Q

Which isomer, E or Z, forms fastest and why?

A

E

Lower activation energy

29
Q

What will the major product be of an E1 reaction?

A

The more substituted alkene as it is more stable

30
Q

Why are more substituted alkenes more stable

A

There is increased σ-conjugation which involves σ* orbitals
The more σ* orbitals = more spread out and therefore more stable

31
Q

What is regioselectivity?

A

Preference of a chemical reaction to form one positional isomer over another

32
Q

What do E2 reactions rely on?

A

100% orbital reliance

33
Q

Where must the two groups being eliminated in an E2 reaction lie?

A

In the same plane

34
Q

What are the two conformations which may be produced from an E2 reaction?

A

Synperiplanar

Antiperiplanar

35
Q

Which is the preferred product for E2, synperiplanar or antiperiplanar?

A

Antiperiplanar

36
Q

Why is aniperiplanar preferred over synperiplanar?

A

The orbitals are more in plane so they have better overlap and are closer to one another as the large lobe overlaps with a large lobe

37
Q

In a newman projection, where do the dashes lie?

A

Point into page

Left hand side

38
Q

In a newman projection, where do the wedges lie?

A

Point out of the page

Right hand side

39
Q

Which conformation, synperiplanar or antiperiplanar, is lower in energy? Why?

A

Antiperiplanar
The transition state is a staggered conformation which is preferred
Less steric hinderance = more stable

40
Q

What is the π bond of the alkene formed from?

A

σ-orbital of the C-H bond and the σ*-orbital of the C-X bond

41
Q

What happens during the transition state?

A

Electrons move from the filled σ-orbital to the empty σ*-orbital

42
Q

What types of reactions can the E2 reactions be when there is more than one hydrogen adjacent to the leaving group?

A

Regioselective

Stereoselective

43
Q

What is the regioselectivity due to in E2 reactions?

A

Formation of the more stable alkene through the lower transition state

44
Q

What is the stereoselectivity due to in E2 reactions?

A

Elimination occuring from the lower energy of the two possible antiperiplanar conformations

45
Q

In E2 reactions, what type of interactions destabilise the Z isomer?

A

Gauche interactions

46
Q

What are gauche interactions?

A

Steric interactions

More hindered

47
Q

When is an E2 reaction stereospecific?

A

When both groups being eliminated are attached to stereocentres

48
Q

In E2 reactions, what does each diastereoisomer of the starting material give?

A

A different diastereoisomer of the alkene product

49
Q

What is the origin of stereospecificity?

A

In both cases, the E2 reaction must occur through the antiperiplanar transition state

50
Q

What are diastereomers?

A

Stereoisomers that are not mirror images

51
Q

How do you work out the amount of diastereomers?

A

2ⁿ

n is the number of chiral centres

52
Q

Explain the regiochemistry of E2?

A

Transition state and the base reacting with the hydrogen are the two factors

53
Q

What happens in E2 reaction if the base is large? (regiochemistry)

A

The product will arise from reacting at the least hindered hydrogen on a carbon adjacent to the leaving group

54
Q

What happens in E2 reaction if the base is small? (regiochemistry)

A

The product will be the most stable alkene
The most stable alkene is the most substituted and usually arises from removal of a more hindered hydrogen adjacent to the leaving group

CHY1101 - Organic chemistry (22 decks)

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