Core 2: Alkenes and Alkynes

Question 1

What is the general name for the the reactant in the Wittig reaction and what is its structure? When is it called stablised?

Answer 1

Phosphonium ylides(ylids), Ph₃P=CR₂

When it contains electron withdrawing groups such as CO₂Me, CN and Ph.

Question 2

What is the product of the first step between the aldehyde/ketone and the phosphonium ylide? How does the reaction continue?

Answer 2

The partial negative (full negative in resonance form) carbon attacks the carbonyl group to form a betaine molecule(Ph₃PCR₂CR₂O^-). This can react to form an unstable 4-membered ring which ring opens to form the alkene and a triphenylphosphine oxide.

Question 3

What reagents can be used to produce alkynes from an aldehyde?

Answer 3

PPh₃ and CBr₄ followed by BuLi

Question 4

What is the driving force of the final step of the Wittig reaction?

Answer 4

The extremely high bond enthalpy of the P=O bond (575kJmol^-1).

Question 5

Draw a reaction mechanism for ozonolysis

Answer 5

Question 6

What is the main reactant in Peterson olefination?

Answer 6

A carbon chain with a SiMe₃ group and an OH group on an adjacent carbon.

Question 7

What 3 side reactants are used in Julia olefination?

Answer 7

BuLi, then acetic anhydride then a reducing agent of Na/Hg

Question 8

What is the mechanism for the bromonation of an alkene? Name the intermediate and the isomeric effects of the reactants.

Answer 8

A bromonium ion will form which is then broken by the nucleophilic attack of Br^-. The isomerism of the product depends on the alkene and the side bromine attacks depends on the sterics of the bromonium ion.

Question 9

What conditions (solvent, catalyst) are suitable for the production of Cis(Z) alkenes from alkynes and what is the name of the catalyst used?

Answer 9

Lindlar’s catalyst is used in Quinoline (C₉H₇N), Lindlar’s catalyst is Palladium(Pd) which has its catalytic activity reduced using CaCO₃ and PbO similar to poisoning a catalyst.

Question 10

What are the reactants and products of ozonolysis?

Answer 10

An alkene reacts with O₃ then Me₂S to form 2 carbonyls for each side of the alkene.

Question 11

What are the reagents of hydroboration? Outline the 2 steps of the mechanism

Answer 11

Alkene and BH₃, followed by H₂O₂ and OH^-, followed by water.

1. Syn addition of BH₃ and -H⁺, repeat until BR₃
2. Use hydroperoxide ions to form OR groups which subsititute with OH^- then form alcohols with water

Question 12

How are phosphonium ylides prepared? (2 steps 3 reagents)

Answer 12

The primary or secondary haloalkane (RCH₂X or R₂CHX) is reacted with a triphenylphosphine PPH₃(other tri-alkyl/aryl substituted phosphines can be used) in an S_N2 reaction to form a phosphonium bromide, HR₂CP(+)Ph₃. This then reacts with BuLi to remove the final proton.

Question 13

How do you form methyl ketones from a terminal alkyne? What intermediates does it go through?

Answer 13

Using H₂O, H₂SO₄ and HgSO₄. A alkene with a positive charge which forms an enol/methyl ketone.

Question 14

How is the addition of bromine different to the addition of bromine water? What are the conditions for each to be favoured? What influences the regioselectivity of the mechanism?

Answer 14

In the bromine water reaction water acts as the nucleophile to the bromonium ion. This requires low concentrations of bromine as Br^- is a better nucleophile. The carbon water attacks depends partly on sterics and partly on the charge of the carbon- the more substituted carbon is more positive so its attack by water will be favoured.

Question 15

What reaction would you use to produce only Z alkenes?

Answer 15

Lindlars catalyst with alkynes, the wittig reaction with electron donating groups and Peterson olefination in basic conditions.

Question 16

What are the 2 main reactants for Julia olefination? What are the stereochemical results?

Answer 16

A phenyl sulfane and an aldehyde form a trans alkene.

Question 17

Describe McMurry coupling. When is it useful?

Answer 17

2 carbonyls are linked together by Ti(0), formed by TiCl₃ and LiAlH₄, where the carbons of the carbonyls gain radicals which link together and TiO₂ is eliminated. This is useful for linking sterically hindered groups such as rings.

Question 18

What is the stereochemical outcome of the Peterson reaction in basic conditions? How does this occur?

Answer 18

In basic conditions, such as KF, the alcohol is deprotonated and a 4 membered ring forms with the silicon bonding to the oxygen. This then eliminates Me₃SiO^- leaving a Z alkene.

Question 19

What E2 mechanism is favoured when the base is large or the leaving group is large? Suggest a large base

Answer 19

Hofmann elimination where the less substituted alkene will form. Tert-butoxide.

Question 20

What is the stereochemical outcome of the Peterson reaction in acidic conditions? How does this occur?

Answer 20

An OH₂ leaving group forms and is eliminated via an internal elimination. This requires the two leaving groups to be in antiperiplanar conformation so the alkene forms as a result of this. E/Z will be decided by the conformation of the elimination.

Question 21

In hydroboration, how are B-R groups converted to B-OR groups?

Answer 21

A hydroperoxide ion attacks the boron, the B-R bond breaks and forms an O-R bond, OH^- is removed

Question 22

What should a fully labelled energy profile of a reaction include?

Answer 22

Axis labelled, detail including mechanism, transition states(partial bonds and partial charges) and reactive intermediates.

Question 23

In what conditions can the preperation of the phosphonium ylide be done without using a very strong base?

Answer 23

When an alpha-haloester is used (halogen on the R-C=O strand) as the proton becomes very acidic.

Question 24

How do you eliminate -OH groups?

Answer 24

Use an acid catalyst to from a -OH₂ group which is a good leaving group. The carbocation will eliminate a H⁺ atom to then form an alkene.

Question 25

What are the reagents and intermediates to form diols in an epoxidation reaction? Outline the mechanism and draw the first step.

Answer 25

An alkene reacts with a percarboxylic acid to form an epoxide which is protonated. This reacts with water to form a protonated diol.

Question 26

What 2 factors effect the regio- and stereochemistry of the major product of a E1 reaction?

Answer 26

To stablise the intermediate the more substituted intermediate will form(regio). The less sterically strained intermediate will often form, this often leads to the E-isomer being favoured(stereo).

Question 27

When will the addition of HX not follow the general rule and what is this called? How does the reaction occur?

Answer 27

In radical mechanisms, where non-polar conditions and an initiator such as peroxide + light is used, anti-Markovnikov addition will occur. Br• will attack the alkene first at the less hindered carbon.

Question 28

How can alkynes be formed from haloalkenes?

Answer 28

1,2-dibromoalkane can undergo double E2 using a base to form the alkyne.

Question 29

What rule applies for the regio selectivity of the addition of HX to an alkene? What does it state and why?

Answer 29

Markovnikovs rule describes that the more substituted haloalkane will form as this makes the carboncation more stable.

Question 30

What is the most common form of E2 elimination which uses a small base?

Answer 30

Saytzev(Zaitsev/Saytzeff) elimination. Saytzev’s rule states that the formation of the most highly substituted alkene is favoured.

Question 31

How could you convert the bromine on 1-bromocyclohex-1-ene to an ethene group? What other reactions can this reagent do?

Answer 31

Using tributyl tin with the ethene group and a Pd catalyst. This can also be done for benzene and where the stereochemistry of the alkene is preserved. When the bromine is on the alkene B(OH)₂ can be used on the cyclohex-1-ene. All reactions use a Pd catalyst.

Question 32

Why can’t platinum catalysts be used to form alkenes from alkynes?

Answer 32

The addition to both π bonds is fast so only alkanes will be formed.

Question 33

When is an E2 mechansim stereospecific and when is it stereoselective?

Answer 33

It is stereospecific when only one anti-periplanar conformer of the haloalkane is possible (when one hydrogen is avaliable to be eliminated). When 2 conformers are possible then the less sterically hindered product will be the major product which will normally be the E-isomer.

Question 34

What is Sonogashira coupling?

Answer 34

Where terminal alkynes replace halogens in aromatics and alkenes. Conditions are Pd, CuI and base.

Question 35

What are the reactants and product of Diels-Alder addition?

Answer 35

A dialkene and alkene form a cyclohexene

Question 36

What are the reagents and mechanism of oxymercuration? What is the driving force?

Answer 36

An alkene reacts with Hg(OAc)₂ to form a carbocation with a HgOAc group on the less substituted carbon which is attacked by water, which is then deprotonated. The HgOAc group can be reduced by NaBH₄. The C-Hg bond is weak and it is much more favourable to form a C-H bond.

Question 37

How can terminal alkynes be used as nucleophiles?

Answer 37

Using BuLi to deprotonate the terminal hydrogen, this can then be used for any common nucleophilic reaction.

Question 38

How will the structure of the haloalkane affect the rate of the RDS in an E1 mechanism?

Answer 38

If the carbon with the halogen bonded to it has many alkyl groups it will form a more stable intermediate and will break the C-X bond faster. This is why primary haloalkanes do not undergo E1 mechanisms.

Question 39

What reaction would you use to produce only E alkenes?

Answer 39

Na/NH₃ with an alkyne at low temp, Julia olefination or the Wittig reaction using electron withdrawing groups on the ylid.

Question 40

What reactants are used to produce trans(E) alkenes from alkynes? What is the mechanism and why do trans alkenes form?

Answer 40

Na/NH_3(l); the sodium metal dissolves releasing and electron which breaks a π bond, the now negatively charged carbon takes and hydrogen from NH₃. This then repeats with the radical formed on the other carbon. The trans alkene forms to reduce steric strain from the R groups.

Question 41

What reaction combines 2 alkenes both with one side substituted?

Answer 41

Olefin metathesis using a Ru/Mo catalyst and producing 2 alkenes with the substitutions all on one and ethene being produced.

Question 42

In what situation will the requirement for the specific conformer to be formed affect the regio selectivity of the product of an E2 mechanism?

Answer 42

In a cyclic system the H and X atoms must be in axial positions, substituents can stop this occuring.

Question 43

What are the possible reagents and conditions for syn-dihydroxylation?

Answer 43

MnO₄^- with H₂O or OH^- in cold conditions(Purple solution→brown ppt)

OsO₄ with H₂O

Question 44

In a E1 reaction with a haloalkane, when will the X group be broken quickest?

Answer 44

The reaction will be quicker when the eclipsed(syn) conformer is formed as the X group will eliminate quicker to reduce steric strain.

Question 45

What conformation must a haloalkane adopt to undergo an E2 mechanism? Why must it adopt this conformation?

Answer 45

The anti-periplanar conformation (lowest energy conformer) must be adopted. This is so the σ C-H orbital and the σ* C-X bond can align to form the π bond (see figure 20.22 in textbook).

Question 46

What is the difference between stereoselectivity and stereospecifity?

Answer 46

In stereoselectivity the reaction pathway has a choice with a major and minor product. In stereospecifity only one product is formed.

Question 47

What is the initial step of Julia olefination?

Answer 47

The phenyl sulfane is deprotonated by BuLi, then nucleophilic addition of the carbonyl occurs.

Question 48

What determines the stereochemistry of the alkene produced by the Wittig reaction?

Answer 48

If the phosphonium ylide has electron withdrawing R groups then the trans-isomer is formed, if not then the cis-isomer is formed.