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CHEM333 (RBL) > 4. Addition And Elimination > Flashcards

4. Addition And Elimination Flashcards

1
Q

What is the mechanism for the electrophilic addition of HX (e.g., HBr) to alkenes?

A

Variable, depending on alkene and solvent. In polar solvents, HX dissociates, and the RLS is protonation of the double bond followed by attack of X. In less polar solvents, HX is less dissociated, and the alkene is protonated by molecular HX to form an ion pair, which then collapses.

RLS stands for Rate Limiting Step.

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2
Q

What determines the reversibility of addition reactions?

A

The concentration of HX.

When a stable cation is formed, the reverse of this addition mechanism is E1 elimination.

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3
Q

What is the rate law for the electrophilic addition of HX to alkenes?

A

Rate = k[alkene][HX].

Sometimes, higher order terms may be involved, showing the involvement of two molecules of HX.

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4
Q

What is the regioselectivity of the electrophilic addition of HX to alkenes?

A

Invariably Markovnikov.

The transition state leading to the most stable carbocation is lowest in energy, hence the most substituted product is formed fastest.

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5
Q

What happens when HBr adds to unsymmetrical diene isoprene?

A

It gives the most substituted allylic cation, which is trapped at the least hindered site by bromide.

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6
Q

What is the stereoselectivity of the addition of HBr in different solvents?

A

In non-polar solvents, the product is syn, while in water, the product is anti, with Markovnikov regioselectivity in both cases.

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7
Q

How is the syn product rationalized in non-polar solvents?

A

By collapse of the ion pair formed when HBr adds, keeping bromide next to hydrogen, thus adding on the same side.

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8
Q

What explains the anti stereoselectivity in water during HBr addition?

A

Assuming the addition is reversible, the most stable product is formed with the larger phenyl group equatorial.

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9
Q

What is the condition for the bromide to be the major product in water?

A

Concentrated HBr must be used.

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10
Q

What happens if a low concentration of HBr is used?

A

Water can act as a nucleophile leading to the alcohol, also with anti stereoselectivity.

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11
Q

What acid is used to add water across the alkene when alcohol is the desired product?

A

A strong acid with a non-nucleophilic counteranion like HClO.

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12
Q

What is the mechanism for the electrophilic addition of X2 (e.g., Br2) to alkenes?

A

One of the three lone pairs on halogen interacts with the neighboring cation, stabilizing it by forming a cyclic halonium ion (pair). This is then attacked from the side opposite the positive bromine to give the dibromide.

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13
Q

What is the complexity of kinetics in electrophilic addition reactions?

A

Often more complex than simple bimolecular rate

Involves extra terms indicating that two or more molecules of X are involved.

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14
Q

What is the typical stereochemistry observed in electrophilic addition reactions?

A

Usually anti addition

Particularly with bromine and iodine due to S-2 attack on the opposite face of the halonium ion.

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15
Q

What product is formed from the trans alkene in the example provided?

A

R,S (meso) dibromide

The product is not a pair of R,R/S,S enantiomers.

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16
Q

When is addition less stereospecific in electrophilic addition reactions?

A

If alkene is conjugated with a cation stabilizing group or with Cl as electrophile

A symmetrical cyclic chloronium ion is not formed in these cases.

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17
Q

What type of ion is formed when Br is added to an alkene in the presence of water or an alcohol?

A

Bromonium ion

This ion is unsymmetrical with a longer, weaker bond to the more substituted carbon.

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18
Q

What is the mechanism referred to as ‘loose S,2’?

A

A mechanism where two molecules are involved in the transition state for attack on the bromonium ion

The bond to the leaving group is weak and long.

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19
Q

How can water be added across double bonds?

A

With acid catalysis or through the oxymercuration reaction followed by reduction

The oxymercuration reaction uses Hg as a soft electrophile.

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20
Q

What is the role of Hg in the oxymercuration reaction?

A

Hg is a soft electrophile that forms a cyclic mercurinium ion

This ion is attacked at the more substituted end by water.

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21
Q

What happens to the carbon-mercury bond after the oxymercuration reaction?

A

It is weak and easily reduced with a mild reducing agent

NaBH4 is commonly used for this reduction process.

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22
Q

What are some synthetic applications of electrophilic addition reactions?

A

Useful for introducing functional groups at hindered positions

Includes alcohol, amine, and acid synthesis through Markovnikov addition.

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23
Q

What is the process for alcohol synthesis using electrophilic addition?

A

Markovnikov addition of water or oxymercuration/reduction

This allows for the synthesis of alcohols from alkenes.

24
Q

How is amine synthesis achieved in electrophilic addition?

A

By Markovnikov addition of hydrogen azide followed by reduction

This process allows for the formation of amines from alkenes.

25
Q

What is the method for acid synthesis in electrophilic addition reactions?

A

Markovnikov addition of hydrogen cyanide followed by hydrolysis

This method effectively introduces acid functional groups.

26
Q

What is B-elimination?

A

Formation of alkene by elimination of H to a leaving group. A base is required to accept the proton.

27
Q

What types of bases can be used in B-elimination?

A

Anionic (such as ethoxide) or neutral (such as an amine or a solvent molecule).

28
Q

What is a-Carbene?

A

A reactive intermediate formed in a-elimination when there are no hydrogens.

29
Q

What is the key characteristic of E2 elimination?

A

Bonds are formed and broken at the same time, similar to S2.

30
Q

Under what conditions is E1 elimination favored?

A

In a protic solvent, producing both alkene(s) and additional products from nucleophile addition.

31
Q

What is the rate law for E1 elimination?

A

Rate = k1[RX], with the dissociation of the leaving group being the rate-limiting step.

32
Q

What influences the speed of E1 reactions?

A

Good leaving group, stable cation structure, and polar solvents.

33
Q

What is the major product in E1 elimination?

A

Usually the most stable, more substituted alkene (Saytzeff rule).

34
Q

What is the rate law for E2 elimination?

A

Rate = k2[RX][B], with both molecules in the rate-limiting step.

35
Q

What type of leaving group can be eliminated with a neutral basic solvent in E2 reactions?

A

A good leaving group.

36
Q

What is the kinetic isotope effect (KIE) in E2 reactions?

A

KIE = kH/kD is large since the C-H bond is broken in the rate-limiting step.

37
Q

What stereochemical arrangement is fastest for E2 elimination?

A

When H and the leaving group are anti (or trans-coplanar).

38
Q

What happens when using axial vs. equatorial chloro-compounds in E2 reactions?

A

Axial chloro-compounds produce the most stable alkene, while equatorial produces the less stable alkene.

39
Q

How does steric hindrance of bases affect E2 vs. S2 eliminations?

A

E2 is favored over S2 by strong, bulky bases.

40
Q

Fill in the blank: E1 and E2 elimination products are largely independent of the _______.

A

[leaving group].

41
Q

True or False: E1 and E2 reactions both require a strong base.

A

False.

42
Q

What type of elimination is favoured over E2 in hindered substrates?

A

E1

E1 elimination is preferred due to steric hindrance in hindered substrates.

43
Q

For a given base, which type of halides generally undergo 5,2 substitution?

A

Primary halides

Primary halides typically undergo 5,2 substitution, while tertiary halides tend to eliminate.

44
Q

What does ethoxide produce when reacting with bromobutane?

A

Ether

Ethoxide gives an ether when reacting with bromobutane.

45
Q

What type of alkene is usually produced in eliminations?

A

Most stable (most substituted) alkene

The most stable alkene is typically the major product due to regioselectivity.

46
Q

What occurs when the base or substrate is partially hindered during elimination?

A

Less stable (less substituted) alkene becomes the major product

This phenomenon is referred to as ‘Hofmann orientation’.

47
Q

What mechanism is favoured by acidic β-hydrogens?

A

E1cb

E1cb mechanisms require electron-withdrawing groups and poor leaving groups.

48
Q

What is the key characteristic of E1cb eliminations compared to E2 eliminations?

A

Not as sterically specific

C-H and C-X orbitals do not need to overlap in a single transition state.

49
Q

What does the steady state of the anionic intermediate in E1cb yield?

A

Rate = k1[RBr][B]

The presence of the intermediate can be detected by varying the base concentration.

50
Q

In the limiting case of an acidic substrate, what is the relationship between k1 and k2?

A

k1[BH]&raquo_space; k2

This indicates that proton removal is fast and reversible.

51
Q

What is the observed rate law for acidic E1cb reactions?

A

Rate = k3[FK][B]

Here, k3 represents the equilibrium constant for deprotonation of the substrate.

52
Q

What is a diagnostic feature of E1cb with acidic hydrogens?

A

Deuterium incorporation in recovered starting material

This can be observed when reactions are conducted in deuterated solvents.

53
Q

What is the relationship between the mechanistic continuum of elimination reactions and the structural features of substrates and bases?

A

The TS varies continuously from E1 to E2 to E1cb

Kinetic parameters also change continuously based on these structural features.

54
Q

What happens to the C-H bond breaking in the transition state as you move along the mechanistic continuum?

A

Increases

Increasing C-H bond breaking occurs as the transition state shifts from E1 to E2.

55
Q

What is the result of increasing C-X bond breaking in the first transition state?

A

Shifts toward E2

This illustrates the mechanistic continuum in elimination reactions.

CHEM333 (RBL) (7 decks)

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