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A-level Chemistry > Mechanisms > Flashcards

Mechanisms Flashcards

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1
Q

What are free radicals?

A

Atoms / a group of atoms which have a single unpaired electron which are formed when a bond is broken by homolytic fission

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2
Q

What kind of reaction does free radical substitution cause?

A

Causes a chain reaction

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3
Q

What is free radical substitution ?

A

A reaction between alkanes and halogens in the presence of UV light.

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4
Q

What are the stages in free radical substitution?

A
  1. Initiation
  2. Propagation
  3. Termination
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5
Q

What are the steps in the free radical substitution of chlorine and methane?

A

Initiation:

Cl-Cl ——> 2•Cl

Propagation:

  • Cl + CH4 ——> HCL + •CH3
  • CH3 + Cl2 ——> CH3Cl + •Cl

Termination:

  • Cl + •Cl ——> Cl2
  • CH3 + •CH3 ——> C2H6
  • Cl + •CH3 ——> CH3Cl
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6
Q

Show how the ozone is broke. Down by free radical substitution.

A
  • Cl + O3 ——> •ClO + O2
  • ClO + O3 ——> 2O2 + •Cl

The chlorine free radical acts as a catalyst in the breakdown of ozone

Overall:

2O3 ——> 3O2

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7
Q

What are the reaction conditions for free radical substitution?

A

Halogen into free radicals

Uv light

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8
Q

What kind of mechanism is the nitration of benzene?

A

Electrophilic substitution

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9
Q

What is the electrophile used in the nitration of benzene and how is it generated?

A

Nitronium ion (NO2^+)

Generating the electrophile:

H2SO4 + HNO3 ——> NO2^+ + HSO4^- + H2O

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10
Q

What are the reaction conditions for the nitration of benzene?

A

Conc H2SO4

Conc HNO3

Below 55

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11
Q

Describe the mechanism for the nitration of benzene?

A
  1. Curly arrow from the delocalised ring of electrons of benzene to the nitronium ion
  2. Draw benzene with an H atom and nitro group on the benzene molecule. With the delocalised ring not completed and having a + charge
  3. Curly arrow from the hydrogen bond to the delocalised ring
  4. Draw nitrobenzene + H^+
  5. Show that the H+ would react with HSO4^- to reform H2SO4 as a catalyst
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12
Q

How does the H+ ions reform the H2SO4 catalyst?

A

H+ would recombine with HSO4^- to reform H2SO4 showing it is a catalyst

H+ + HSO4^- ——> H2SO4

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13
Q

What are the reaction conditions for the reduction of nitrobenzene?

A

Room temperature

Sn/HCL as a reducing agent

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14
Q

Write out equations for the reduction of nitrobenzene

A

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

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15
Q

What are the reaction conditions used in the acylation of benzene to form phenylketone?

Fiedel crafts reaction

A

ROCl

AlCl3 catalyst

Reflux

Non aqueous environment

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16
Q

Show the equations for the AlCl3 catalyst in an acylation reaction

A

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

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17
Q

Describe the electrophilic substitution mechanism for the acylation of benzene

Friedel crafts reaction

A
  1. Electrophile generated from acyl group and AlCl3
  2. Curly arrow from the delocalised electron ring of benzene to the electrophile carbocation
  3. Draw benzene with H atom and electrophile coming off it and delocalised ring not completed with a + charge
  4. Curly arrow from H atom to delocalised ring
  5. Draw phenylketone and H+
  6. Show how H+ is used to regenerate AlCl3 catalyst
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18
Q

What is a nucleophile?

A

An electron pair donor

A species which is attracted to electron deficient regions

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19
Q

Why are acyl chlorides susceptible to attack from nucleophiles?

A

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

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20
Q

What kind of reaction occurs with acyl chlorides an nucleophiles?

A

Nucleophilic addition elimination reactions

Acylation reactions

21
Q

What are the possible nucleophiles used in addition elimination reactions / acylation reactions?

What products are formed when reacting with an acyl chloride?

A

Water forms carboxylic acids

Alcohol forms esters

Ammonia forms amides

Amines forms N-substituted amides

22
Q

What are the reaction conditions for acylation reactions?

Nucleophilic addition elimination

A 
Nucleophile:
Water
Alcohols 
Ammonia
Amines

Room temperature

23
Q

Describe the generic mechanism for a nucleophilic addition elimination reaction (acylation reaction)

A
  1. Slightly positive carbon and slightly negative chlorine and oxygen atoms in the acyl chloride
  2. 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.
  3. 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
  4. 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
  5. Draw product and HCL as a by product
24
Q

What are the reaction conditions for the nucleophilic addition of cyanide to an aldehyde or ketone?

A

KCN or HCN

Dilute H2SO4

Room temperature

25
Q

Describe the nucleophilic addition mechanism of a ketone with cyanide ion.

A
  1. Formation of cyanide ion
  2. Curly arrow from the lone pair on the cyanide to the slightly positive carbon. Curly arrow from the double bond to the oxygen
  3. Curly arrow from the negative oxygen atom lone pair to the H+ ion from solvent
  4. Product formed is a hydroxynitrile
26
Q

Why does addition reactions of KCN with dilute acid from a mixture of enantiomers?

A

Produced a racemic mixture since the cyanide ion can add onto the aldehyde or ketone from above or below the double bond.

27
Q

Why is HCN not commonly used to addition reactions?

A

Toxic gas which needs to be prevent from escaping so instead KCN is used followed by dilute acid as a source of H+ ions

28
Q

What kind of reaction occurs when changing an aldehyde or ketone to an alcohol?

A

Reduction reaction

Nucleophilic addition mechanism

29
Q

What is the nucleophile used in reducing aldehydes and ketones and what is the source of this nucleophile?

A

:H^- hydride ions

Sourced from NaBH4
Sodium tetrahydridoborate III

30
Q

What are the conditions for the reduction of aldehydes and ketones?

A

NaBH4 dissolved in alcohol or water

Refluxed

31
Q

Describe the reduction of aldehydes or ketones reaction

Nucleophilic addition mechanism

A
  1. Curly arrow from lone pair of hydride ions towards the carbon and curly arrow from double bond to the oxygen atom
  2. Curly arrow from lone pair of negative oxygen atom to positive H+ atom fro dilute acid or water
  3. Forms an alcohol
32
Q

What are the reaction conditions for nucleophilic substitution reactions between a haloalkane and potassium/sodium hydroxide?

A

Haloalkane

Nucleophile: :OH^- from KOH or NaOH aqueous

Room temperature

Solvent of ethanol

33
Q

Describe the nucleophilic substitution reaction between a haloalkane and NaOH

A
  1. Curly arrow from lone pair on hydroxide ion to the carbon. Curly arrow from the C-X bond to the halogen
  2. Forms an alcohol with the halide ion coming off
34
Q

What are the reaction conditions for nucleophilic substitution reactions between a haloalkane and potassium cyanide?

A

KCN source of :CN^- ions

Aqueous alcoholic solution

35
Q

Describe the nucleophilic substitution reaction between haloalkane and potassium cyanide

A
  1. Curly arrow from lone pair on cyanide ion to the carbon. Curly arrow from the C-X bond to the halogen
  2. Forms an nitrile with the halide ion coming off
36
Q

What are the reaction conditions for nucleophilic substitution reactions between a haloalkane and ammonia

A

Excess conc ammonia

In ethanol

Under pressure

37
Q

Describe the nucleophilic substitution reaction between haloalkane and ammonia

A
  1. Curly arrow from lone pair on the ammonia to the carbon. Curly arrow from the C-X bond to the halogen.
  2. Curly arrow from the lone pair on the excess ammonia to a hydrogen atom on the bonded ammonia molecule. Curly arrow from N-H bond towards the positive N+ atom
  3. Forms an amine with the halide ion coming off and an ammonium ion
38
Q

What is an electrophile?

A

An electron pair acceptor

An electron deficient atom attracted to an electron dense region

39
Q

What kind of reaction takes place between an alkene and an electrophile?

A

Electrophilic addition reaction

40
Q

What are the possible electrophiles for electrophilic addition of an alkene?

A

HBr

H2SO4

Br2

41
Q

Describe the electrophilic addition reaction between an alkene and HBr

A
  1. Curly arrow from double bond to the hydrogen atom of H-Br and a curly arrow from the H-Br bond to the Br atom.
  2. Curly arrow from the lone pair of the :Br ion to the carbocation
  3. Forms a haloalkane
42
Q

What are the reaction conditions for electrophilic addition reactions?

A

Alkene / presence of a double bond

Electrophile

Room temperature

Solvent conditions

43
Q

Describe the electrophilic addition reaction between an alkene and bromine (Br2)

A
  1. Curly arrow from the double bond to a bromine atom as well as a curly arrow from the Br-Br bond to the other bromine atom
  2. Curly arrow from the bromine atom to the carbocation
  3. Forms a dibromoalkane
44
Q

Describe the electrophilic addition reaction between an alkene and H2SO4

A
  1. Curly arrow from double bond to hydrogen atom of H2SO4. Curly arrow from H-O bond to Oxygen atom of H2SO4
  2. Curly arrow from lone pair of oxygen atom of H2SO4 to the carbocation
  3. Product formed
45
Q

What are the conditions for nucleophilic substitution using hydroxide ions?

A

NaOH or KOH dissolved in ethanol

Room temperature

Aqueous (dissolved in water)

46
Q

What are the reaction conditions for elimination using hydroxide ions?

A

NaOH or KOH dissolved in ethanol

High temperature (refluxed)

Dissolved in ethanol

47
Q

What is the role of the hydroxide ions during an elimination reaction?

A

Acts as a base by accepting a hydrogen atom and forming water

48
Q

Describe the elimination reaction between a haloalkane and NaOH?

A
  1. Curly arrow from the lone pair of the hydroxide ion to the hydrogen atom of the haloalkane.
  2. Curly arrow from the C-H bond to the C-C bond
  3. Curly arrow from the C-X bond to the halogen
  4. Forms an alkene, water molecule and halide ion.

(May form a mixture of geometric isomers

A-level Chemistry (33 decks)

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