Reactions Of Alkenes

Question 1

What does the reactivity of the c double bond depend on?

Answer 1

The c double bonds act as nucleophile
The reactivity depends on the substituents
+M/+I makes the double bond more electron rich and so more reactive
-M/-I makes the double bond less electron rich and so less reactive

Question 2

What is the product of alkenes and bromine

Answer 2

The addition of bromine is anti addition which makes the product trans

Question 3

Why is the bromonium ion and product of bromomination a racemate?

Answer 3

Overall Bromine adds to opposite faces (anti addition)
Product is a racemate as either face of the c double bond can be attack and attack on either side of the carbon ring is possible

Question 4

Describe the stereochemistry of the products of bromination

Answer 4

E/Z geometry is retained for the already present substituents
Br adds anti
The reaction is stereospecific as bromine adds to opposite faces of the c double bond

Question 5

How can you form a halohydrin?

Answer 5

React an alkene with bromine in the presence of water
The water can open up the bromonium ion to form bromohydrin
(Bromoalcohol)

Question 6

Why does water need to be in a greater concentration than bromine in a bromohydrin reaction?

Answer 6

Bromine is a stronger nucleophile than water so water needs to be in greater concentration
Water attacks the more substituted carbon

Question 7

What is an unsymmetrical alkene?

Answer 7

An alkene that has different groups on each carbon atom in the C=C bond

Question 8

What is markovnikov’s rule?

Answer 8

On addition of HX to an alkene, H attaches to the carbon with the fewest alkyl groups and X attached to the carbon with the most alkyl groups (most stable intermediate is formed)

Question 9

What is the product upon the ionic addition of HX?

Answer 9

Forms the most substituted halogenoalkane

Question 10

What is the product upon radical addition of HX?

Answer 10

Forms the least substituted halogenoalkane

Question 11

What is a regioselective reaction?

Answer 11

A regioselective reaction is a reaction that leads to the selective formation of one structural isomers
It may form both products but one will be the major one

Question 12

What determines which product will form in an ionic reaction involving a carbocation?

Answer 12

The more stable carbocation will be the intermediate from which the product is formed
Tertiary> secondary> primary> CH3+
This is due to the inductive effects which help to stabilise the positive charge
The electrophile adds to the more substituted carbon
- more substituted product

Question 13

What determines which product will form in an radical reaction involving a radical intermediate?

Answer 13

The more stable radical will be the intermediate from which the product is formed
Tertiary> secondary> primary> CH3+
The inductive groups helps to stabilise the radical
The electrophile adds to the less substituted carbon
-least substituted product is formed

Question 14

What is an Oxonium ion?

Answer 14

This is a compound that contains a positively charged oxygen atom

Question 15

How do you form an alcohol from an alkene?

Answer 15

Hydration using H2O and H2SO4

The OH group is on the more substituted carbon atom

Question 16

Why is the hydration of alkenes not synthetically useful?

Answer 16

This is because it requires forcing conditions due to the use of H2SO4
This means oxymeration is more likely to be used

Question 17

What is oxymercuration?

Answer 17

Alternative to hydration and gives the most substituted alcohol product

1) reacts with Hg(OAc)2
2) water attacks
3) mercury is removed with NaBH4

Question 18

What does hydroboronation give?

Answer 18

1) addition of borane is syn and Bh2 adds to the least substituted carbon atom - gives positive charge on most substituted atom
2) react with H2O2 and NaOH to give least substituted alcohol product

Question 19

What is epoxidation?

Answer 19

This is the reaction of an alkene with a peracid in a one step reaction to form an epoxides
1 step so stereochemistry is retained as alkene cannot rotate
Also forms a carboxylic acid

Question 20

How can you open an epoxides and what does it form?

Answer 20

Reagents= H2O/H+
Water opens the ring
Forms 1,2 diol
The two OH groups add in an anti addition 
SN2 ring opening 
Water attacks many times 
Can also work with OH- which directly 
attacks ring

Question 21

Which two reactions give 1,2 diols and what are the differences in the products

Answer 21

Ring opening of epoxides gives trans 1,2 diol

Dihydroxylation gives cis 1,2 diol

Question 22

Which two hydration reactions give substituted alcohol products and what are the differences in the products?

Answer 22

Oxymercuration gives more substituted alcohol product

Hydroboration gives the least substituted alcohol product

Question 23

What is direct dihydroxylation?

Answer 23

Reaction of OsO4 with and then water for the syn addition of OH groups to give cis-1,2 diol

Question 24

What is ozonolysis?

Answer 24

Ozone converts C=C bonds to two C=O bonds by 1,3 dipolar cycloaddition
Forms aldehydes, ketones, and/or carboxylic acid
i) O3
ii) SMe2

Question 25

How can you decide the product of ozonolysis?

Answer 25

Secondary ozonise is more stable than the primary
This means the ozonide has to be converted into a more stable carbonyl compound using an agent
Reducing forms aldehydes/ ketones
Oxidising forms carboxylic acids/ ketones

Question 26

What is a 1,3 dipolar cycloaddition reaction?

Answer 26

This is a chemical reaction between a 1,3 dipole and a dipolarphile to form a 5 membered ring

Question 27

What is a diels alder reaction?

Answer 27

Reaction of a diene (cis) with a dieniphile to form a 6 membered ring
Stereochemistry of dienophile is retained

Question 28

Outline the cross coupling reaction

Answer 28

Reaction of a vinyl/ aryl halide with a vinyl stannane with a Pdo catalyst
Cross coupling means they essentially add together

Question 29

Describe the Suzuki reaction

Answer 29

Vinyl boronic acid - B(OH)2 couples with vinyl halide

Double bond geometry of both products is retained
Pdo catalyst with a base

Question 30

What is metathesis

Answer 30

Essentially adding parts of alkenes together with a catalyst

Catalysts can be Grubbs or schrock