Reactions Of Alkenes Flashcards
What does the reactivity of the c double bond depend on?
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
What is the product of alkenes and bromine
The addition of bromine is anti addition which makes the product trans
Why is the bromonium ion and product of bromomination a racemate?
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
Describe the stereochemistry of the products of bromination
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
How can you form a halohydrin?
React an alkene with bromine in the presence of water
The water can open up the bromonium ion to form bromohydrin
(Bromoalcohol)
Why does water need to be in a greater concentration than bromine in a bromohydrin reaction?
Bromine is a stronger nucleophile than water so water needs to be in greater concentration
Water attacks the more substituted carbon
What is an unsymmetrical alkene?
An alkene that has different groups on each carbon atom in the C=C bond
What is markovnikov’s rule?
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)
What is the product upon the ionic addition of HX?
Forms the most substituted halogenoalkane
What is the product upon radical addition of HX?
Forms the least substituted halogenoalkane
What is a regioselective reaction?
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
What determines which product will form in an ionic reaction involving a carbocation?
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
What determines which product will form in an radical reaction involving a radical intermediate?
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
What is an Oxonium ion?
This is a compound that contains a positively charged oxygen atom
How do you form an alcohol from an alkene?
Hydration using H2O and H2SO4
The OH group is on the more substituted carbon atom
Why is the hydration of alkenes not synthetically useful?
This is because it requires forcing conditions due to the use of H2SO4
This means oxymeration is more likely to be used
What is oxymercuration?
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
What does hydroboronation give?
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
What is epoxidation?
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
How can you open an epoxides and what does it form?
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
Which two reactions give 1,2 diols and what are the differences in the products
Ring opening of epoxides gives trans 1,2 diol
Dihydroxylation gives cis 1,2 diol
Which two hydration reactions give substituted alcohol products and what are the differences in the products?
Oxymercuration gives more substituted alcohol product
Hydroboration gives the least substituted alcohol product
What is direct dihydroxylation?
Reaction of OsO4 with and then water for the syn addition of OH groups to give cis-1,2 diol
What is ozonolysis?
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
How can you decide the product of ozonolysis?
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
What is a 1,3 dipolar cycloaddition reaction?
This is a chemical reaction between a 1,3 dipole and a dipolarphile to form a 5 membered ring
What is a diels alder reaction?
Reaction of a diene (cis) with a dieniphile to form a 6 membered ring
Stereochemistry of dienophile is retained
Outline the cross coupling reaction
Reaction of a vinyl/ aryl halide with a vinyl stannane with a Pdo catalyst
Cross coupling means they essentially add together
Describe the Suzuki reaction
Vinyl boronic acid - B(OH)2 couples with vinyl halide
Double bond geometry of both products is retained
Pdo catalyst with a base
What is metathesis
Essentially adding parts of alkenes together with a catalyst
Catalysts can be Grubbs or schrock
