carrolls shit content Flashcards
What happens when alkenes react with bromine
They produce a 1,2-bromide. There is no polarisation of bonds as Br2 is symmetrical. The slight delta - in the Allene makes it the nucleophile and as it comes towards the bromine it will induce polarisation on the bromine atom
How does bromination occur
The alkene has a good source of electrons in the pie bond which bromine can accept in the sigma* orbital of the Br2 bon causing the bond to break, the reaction occurs by overlap of orbitals and the initial product is usually a cyclic bromonium ion which the undergoes an sn2 reaction with bromine using its lone pair
Draw the mechanism of ethene with Br2
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Describe the stereochemistry in bromination reactions
The first step of any addition reaction to an alkene is always a syn-addition in which two bonds are formed on the same face of the alkene. This initially formed product may be stable or may react further by an Sn2 reaction leading to an overall anti addition as in the case of bromination
What is the exception to the stereochemistry rule of addition reactions
Addition reactions which start by proton action of the alkene as their go via a car benign ion since H+ is too small to form a 3 member end ring intermediate
What happens during bromination of the E alkene
The same product is produced whether the bromonium ion is formed on the top or the bottom face of the alkene and whether the bromonium ion is ring opened on its left or right side
What happens during the bromination of the Z alkene
The product is a pair of enantiomers, these are both diastereoisomers of the product formed from the E isomer of the alkene so the reaction is stereospecific
What happens when alkenes react with peracids
Alkenes undergo an addition reaction when treated with a peracid. The product is an epoxied. The peracid is an electrophilic source of oxygen
Describe the leaving group ability of a peracid
A peracid is an electrophilic source of oxygen when it reacts with a nucleophile it will produce
Nuc-OH and -OCOR
In the peracid the HO-O bond is very weak
The pKaH of the -OCOR bond is about 4
Draw a basic peracid structure and its reaction with a nucleophile label the weak bond
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Draw the basic mechanisms of ethene with a peracid
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What is the most common peracid and draw its structure
Meta-chloroperbenzoic acid
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Draw and describe the mechanism of mCPBA with an alkene
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Mechanistically the reaction occurs in a single step. Not that both bonds to oxygen are formed on the same face of the alkene, therefore synaddition
Compare mCPBA and H2O2
MCPBA is a solid whereas H2O2 is a liquid. The byproducts of the reaction with mCPBA is a lot harder to separate whereas the by-product with H2O2 is water so it is easy to remove
Is epoxidation stereospecific
Yes. Each diastereoismer of an alkene gives a different diastereomer of the epoxied product
The Z alkene will have 2 wedges. And the E alkene gives a wedge and a dash
What reagent is used to open epoxied rings
Sn2 -Me2NH
SN1- ethanol and H+
How are epoxied rings opened
Epoxides either undergo ring opening by an Sn2 reaction with nucleophile, this results in an overall anti addition of OH and the nucleophile. Or SN1 reactions using H+ and ethanol
Draw the SN1 reaction with epoxide ring opening see one note
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Describe the regiochemistry of the epoxide ring openeing
If the alkene is unsymmetrical then in an Sn2 reaction the nucleophile will always react at the less hindered end of the epoxide. Alternatively in SN1 the nucleophile will be added to whichever end of the epoxide is best able to stabilise a positive charge. This is usually the more hindered end of the epoxide
What happens when an alkene reacts with HBr
Produces alkyl bromides and the reaction proceeds through a carbenium ion intermediate and is the exact reverse of an E1 reaction
Draw the mechanism with H-Br and an alkene
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Describe the stereochemistry of the H-Br reaction with alkenes
If the alkene is unsymmetrical there are two possible products, the first step of the mechanism adds a proton to the alkene with the formation of a carbenium ion. This stem determines the regiochemistry. The reaction always proceed so as to form the more stable carbenium ion
Describe the reactions of 1,3 dienes and Hbr
The initial proton action will occur as as to give the most stable carbenium ion, this results in proton action occurring on the bore substituted double bond. In the second step bromine attacks the less hindered resonance form to give the product in which the hydrogen and the bromine have been added to opposite ends of the molecule
In what way would reacting an alkene with water in the presence of acid produce an alcohol
Draw this mechanism
If the carbenium ion is tertiary
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Describe the mercury reagents and how this reacts with alkenes
An alkene is a soft nucleophile so a soft electrophilic works better eg Hg(OAc)2
Note the initial product is a cyclic mercurinium ion which undergoes a nucleophilic ring opening reaction involving water
Mercury is soft because it is large
Draw an oxymercuration mechanism
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Where does the water react in oxymercuration
The water reacts with the most hindered end of the cyclic mercurinium ion. It reacts at the carbon which has the greater positive charge which is the most substituted carbon as the alkyl group stabilise positive charge
How is the mercury removed from the biproduct
Reduction using sodium borohydride
Hydroboration of alkenes
Treatment of alkenes with borane BH3 followed by sodium hydroxide/ hydrogen peroxide results in an overall addition of water across the alkene
Describe borane and the stereochemistry
Borane is an electrophilic since the boron atom is only surrounded by 6 outer shell electrons and has a vacant p orbital
It reacts at the less substituted end of the alkene so that the delta + builds up at the more substituted carbon atom
Draw the hydroboration mechanism
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reactions of alcohols
alcohols must have a different leaving group in order to undergo nucleophilic substitution reactions. this can be done by
- protonation
- alcohol activation using TsCl mechansim
- reaction with PBr3 or SOCl2
describe the formation of tosylate ester
turns the OH group into a OTs group, tosylate esters are very good leaving groups better than all of the halides. the negative charge on the oxygen when the tosylate leaves can be stabilized by all 3 oxygen atoms PKa of TsOH is -5
draw the structure of TsCl
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draw the mechanism with an alcohol and TsCl
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describe the reaction of alcohols with PBr3
Phosphorus forms very strong single and double bonds to oxygen, the energy released by forming these bonds can be used to offset the large amount of energy required to break the C-O bond. This only works for primary and secondary alcohols as it goes through an SN2 mechanism, as you cannot do Sn2 on a tertiary carbon. if this happens at a chiral centre you have to worry about inversion of configuration
draw the mechanism of an alcohol reacting with PBr3
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describe the mechanism of an alcohol reacting with thionyl chloride
If you want to substitute OH with chlorine you will use thionyl chloride - SOCl2
- Similarly only works on primary and secondary alcohols and only operates via an SN2 reaction
- Always going to get inversion if the mechanism takes place on a chiral centre
describe alcohol elimination reactions
- Using conc H2SO4
- The conjugate base of H2SO4 is HSO4- which is very big and not a good nucleophile
The H2SO4 has to be concentrated as with dilute sulfuric acid we can produce an alcohol from an alkene, but now we need concentrated sulfuric acid to produce the alkene
E1 mechanism is with a secondary or tertiary alcohol whereas the E2 mechanism is with the primary alcohol
draw the E1 and E2 mechanisms of alcohol elimination
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