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Question 1

What happens when alkenes react with bromine

Answer 1

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

Question 2

How does bromination occur

Answer 2

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

Question 3

Draw the mechanism of ethene with Br2

Answer 3

See one note

Question 4

Describe the stereochemistry in bromination reactions

Answer 4

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

Question 5

What is the exception to the stereochemistry rule of addition reactions

Answer 5

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

Question 6

What happens during bromination of the E alkene

Answer 6

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

Question 7

What happens during the bromination of the Z alkene

Answer 7

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

Question 8

What happens when alkenes react with peracids

Answer 8

Alkenes undergo an addition reaction when treated with a peracid. The product is an epoxied. The peracid is an electrophilic source of oxygen

Question 9

Describe the leaving group ability of a peracid

Answer 9

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

Question 10

Draw a basic peracid structure and its reaction with a nucleophile label the weak bond

Answer 10

See one note

Question 11

Draw the basic mechanisms of ethene with a peracid

Answer 11

Check one note

Question 12

What is the most common peracid and draw its structure

Answer 12

Meta-chloroperbenzoic acid
See one note for structure

Question 13

Draw and describe the mechanism of mCPBA with an alkene

Answer 13

See one note
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

Question 14

Compare mCPBA and H2O2

Answer 14

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

Question 15

Is epoxidation stereospecific

Answer 15

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

Question 16

What reagent is used to open epoxied rings

Answer 16

Sn2 -Me2NH
SN1- ethanol and H+

Question 17

How are epoxied rings opened

Answer 17

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

Question 18

Draw the SN1 reaction with epoxide ring opening see one note

Answer 18

See one note

Question 19

Describe the regiochemistry of the epoxide ring openeing

Answer 19

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

Question 20

What happens when an alkene reacts with HBr

Answer 20

Produces alkyl bromides and the reaction proceeds through a carbenium ion intermediate and is the exact reverse of an E1 reaction

Question 21

Draw the mechanism with H-Br and an alkene

Answer 21

See one note

Question 22

Describe the stereochemistry of the H-Br reaction with alkenes

Answer 22

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

Question 23

Describe the reactions of 1,3 dienes and Hbr

Answer 23

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

Question 24

In what way would reacting an alkene with water in the presence of acid produce an alcohol
Draw this mechanism

Answer 24

If the carbenium ion is tertiary
See one note

Question 25

Describe the mercury reagents and how this reacts with alkenes

Answer 25

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

Question 26

Draw an oxymercuration mechanism

Answer 26

One note

Question 27

Where does the water react in oxymercuration

Answer 27

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

Question 28

How is the mercury removed from the biproduct

Answer 28

Reduction using sodium borohydride

Question 29

Hydroboration of alkenes

Answer 29

Treatment of alkenes with borane BH3 followed by sodium hydroxide/ hydrogen peroxide results in an overall addition of water across the alkene

Question 30

Describe borane and the stereochemistry

Answer 30

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

Question 31

Draw the hydroboration mechanism

Answer 31

One note

Question 32

reactions of alcohols

Answer 32

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

Question 33

describe the formation of tosylate ester

Answer 33

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

Question 34

draw the structure of TsCl

Answer 34

see one note

Question 35

draw the mechanism with an alcohol and TsCl

Answer 35

see one note

Question 36

describe the reaction of alcohols with PBr3

Answer 36

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

Question 37

draw the mechanism of an alcohol reacting with PBr3

Answer 37

see one note

Question 38

describe the mechanism of an alcohol reacting with thionyl chloride

Answer 38

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

Question 39

describe alcohol elimination reactions

Answer 39

• 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

Question 40

draw the E1 and E2 mechanisms of alcohol elimination

Answer 40

see one note