Mechanisms Flashcards
(48 cards)
What are free radicals?
Atoms / a group of atoms which have a single unpaired electron which are formed when a bond is broken by homolytic fission
What kind of reaction does free radical substitution cause?
Causes a chain reaction
What is free radical substitution ?
A reaction between alkanes and halogens in the presence of UV light.
What are the stages in free radical substitution?
- Initiation
- Propagation
- Termination
What are the steps in the free radical substitution of chlorine and methane?
Initiation:
Cl-Cl ——> 2•Cl
Propagation:
- Cl + CH4 ——> HCL + •CH3
- CH3 + Cl2 ——> CH3Cl + •Cl
Termination:
- Cl + •Cl ——> Cl2
- CH3 + •CH3 ——> C2H6
- Cl + •CH3 ——> CH3Cl
Show how the ozone is broke. Down by free radical substitution.
- Cl + O3 ——> •ClO + O2
- ClO + O3 ——> 2O2 + •Cl
The chlorine free radical acts as a catalyst in the breakdown of ozone
Overall:
2O3 ——> 3O2
What are the reaction conditions for free radical substitution?
Halogen into free radicals
Uv light
What kind of mechanism is the nitration of benzene?
Electrophilic substitution
What is the electrophile used in the nitration of benzene and how is it generated?
Nitronium ion (NO2^+)
Generating the electrophile:
H2SO4 + HNO3 ——> NO2^+ + HSO4^- + H2O
What are the reaction conditions for the nitration of benzene?
Conc H2SO4
Conc HNO3
Below 55
Describe the mechanism for the nitration of benzene?
- Curly arrow from the delocalised ring of electrons of benzene to the nitronium ion
- Draw benzene with an H atom and nitro group on the benzene molecule. With the delocalised ring not completed and having a + charge
- Curly arrow from the hydrogen bond to the delocalised ring
- Draw nitrobenzene + H^+
- Show that the H+ would react with HSO4^- to reform H2SO4 as a catalyst
How does the H+ ions reform the H2SO4 catalyst?
H+ would recombine with HSO4^- to reform H2SO4 showing it is a catalyst
H+ + HSO4^- ——> H2SO4
What are the reaction conditions for the reduction of nitrobenzene?
Room temperature
Sn/HCL as a reducing agent
Write out equations for the reduction of nitrobenzene
C6H5NO2 + 6[H] ——> C6H5NH2 + 2H2O
C6H5NH2 + HCL ——> CH5NH3^+ +Cl^-
Amine reacts with acid to form an ionic salt, so add NaOH to liberate the amine
C6H5NH3^+ + NaOH ——> C6H5NH2 + Na^+ +Cl^- + H2O
What are the reaction conditions used in the acylation of benzene to form phenylketone?
Fiedel crafts reaction
ROCl
AlCl3 catalyst
Reflux
Non aqueous environment
Show the equations for the AlCl3 catalyst in an acylation reaction
ROCl + AlCl3 ——> RCO^+ + AlCl4^-
This has generated the RCO^+ electrophile
H+ coming off during the substitution reaction is used to regenerate the AlCl3 catalyst
AlCl4^- + H^+ ——> AlCl3 + HCL
Describe the electrophilic substitution mechanism for the acylation of benzene
Friedel crafts reaction
- Electrophile generated from acyl group and AlCl3
- Curly arrow from the delocalised electron ring of benzene to the electrophile carbocation
- Draw benzene with H atom and electrophile coming off it and delocalised ring not completed with a + charge
- Curly arrow from H atom to delocalised ring
- Draw phenylketone and H+
- Show how H+ is used to regenerate AlCl3 catalyst
What is a nucleophile?
An electron pair donor
A species which is attracted to electron deficient regions
Why are acyl chlorides susceptible to attack from nucleophiles?
Due to the polarity of the molecule and the carbon having two highly electronegative groups bonded to it drawing electrons away from the carbon so it is slightly positive and electron deficient
What kind of reaction occurs with acyl chlorides an nucleophiles?
Nucleophilic addition elimination reactions
Acylation reactions
What are the possible nucleophiles used in addition elimination reactions / acylation reactions?
What products are formed when reacting with an acyl chloride?
Water forms carboxylic acids
Alcohol forms esters
Ammonia forms amides
Amines forms N-substituted amides
What are the reaction conditions for acylation reactions?
Nucleophilic addition elimination
Nucleophile: Water Alcohols Ammonia Amines
Room temperature
Describe the generic mechanism for a nucleophilic addition elimination reaction (acylation reaction)
- Slightly positive carbon and slightly negative chlorine and oxygen atoms in the acyl chloride
- Lone pair of nucleophile curly arrow towards the carbon atom of acyl chloride. Curly arrow from the double bond of oxygen to oxygen atom in acyl chloride.
- Curly arrow from oxygen lone pair to reform double bond as a curly arrow comes from carbon to chlorine bond to the chlorine atom. Negative charge on oxygen and positive charge on nucleophile
- Curly arrow from the lone pair on the chlorine to the hydrogen atom of nucleophile as another curly arrow goes from the nucleophile to hydrogen bond to the nucleophile
- Draw product and HCL as a by product
What are the reaction conditions for the nucleophilic addition of cyanide to an aldehyde or ketone?
KCN or HCN
Dilute H2SO4
Room temperature
