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LC Chemistry > Chapter 23 - Types of Reactions in Organic Chemistry > Flashcards

Chapter 23 - Types of Reactions in Organic Chemistry Flashcards

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1
Q

Substitution reaction

A

Chemical reaction in which an atom or group of atoms in a molecule are replaced by another atom or group of atoms

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2
Q

Give an example of a substitution reaction

A

The reaction of methane and chlorine to form chloromethane and hydrogen chloride

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3
Q

Mechanism of a reaction

A

The detailed step by step description of how the overall reaction occurs

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4
Q

What are the four steps to the free radical substitution mechanism of methane and chlorine?

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

What happens in the first stage of the free radical substitution of methane and chlorine?

A

INITIATION

Chlorine atom broken down into two chlorine atoms in the presence of UV light

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6
Q

What happens in the second stage of the free radical substitution of methane and chlorine?

A

PROPAGATION A
One of the chlorine atoms attacks a methane and removes a hydrogen to form HCl. The remaining methyl species is called a methyl free radical

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7
Q

Free radical

A

Any atom or group of atoms with an unpaired electron (very reactive)

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8
Q

What happens in the third stage of the free radical substitution of methane and chlorine?

A

PROPAGATION B
Methyl free radical attacks another chlorine molecule to form chloromethane and a chlorine radical. Reaction continues between A and B

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9
Q

Chain reaction

A

A reaction that continues on and on because a product from one step of the reaction is a reactant for another step of the reaction

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10
Q

What happens in the fourth stage of the free radical substitution of methane and chlorine?

A

TERMINATION
To stop the reaction from continuing, all free radicals must combine.
Two Cl radicals combine to form Cl₂.
Cl radical and CH₃ radical combine to form chloromethane.
Two CH₃ radicals combine to form ethane.
H and Cl combine to form HCl.

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11
Q

How does the mechanism for the substitution of methane provide evidence for the existence of free radicals?

A
  • If the UV light isn’t turned on and the Cl free radicals generated, then the reaction doesn’t happen. i,e. won’t start in the dark
  • Different products formed shows all the free radicals existed and combined at the end
  • Addition of tetramethyllead increases the rate of reaction and more products are formed as it breaks down to form four methyl free radicals
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12
Q

How is the mechanism for the substitution of ethane different to the mechanism for the substitution of methane?

A
  • Free radical is ethyl instead of methyl
  • Termination product turns to butane
  • Additive is tetraethyl lead
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13
Q

What is observed if a test tube containing cyclohexane and bromine water is placed near a source of UV light?

A

Colour changes from orange to colourless

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14
Q

Esterification

A

The formation of an ester

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15
Q

What are the other names for esterification?

A

It’s called a condensation reaction as water is also formed.

Also a substitution reaction as the -OH on the acid is substituted with the alkyl group of the alcohol

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16
Q

What is the reaction between esters and NaOH to form the sodium salt of the carboxylic acid and the alcohol called?

A

base hydrolysis

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17
Q

What is sodium stearate?

A

Soap

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18
Q

Relfuxing

A

Technique where liquid is boiled in a container that is attached to a vertical condenser.

The vapour from the boiling liquid condenses and flows back into the flask.

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19
Q

What is the purpose of refluxing?

A

To ensures the liquid is kept at boiling point without the loss of vapour and it allows enough time for the reaction to occur and prevents the flask from boiling dry

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20
Q

Why is the ethanol distilled off in the soap experiment?

A

It makes it easier to isolate and maximise soap yield

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21
Q

What is the purpose of the NaCl brine in the soap experiment?

A

Removes excess NaOH

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22
Q

Addition reaction

A

A reaction in which two or more molecules react together to form a single molecule

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23
Q

How many steps are there in the mechanism for the reaction between bromine and ethene? Name them

A
  1. Polarisation
  2. Heterolytic fission
  3. Carbonium ion formation
  4. Ionic addition
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24
Q

What happens in the first stage of the reaction between bromine and ethene?

A

POLARISATION

Two bromine atoms become polarised when near the highly negative double bond of ethene

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25
Q

What happens in the second stage of the reaction between bromine and ethene?

A

HETEROLYTIC FISSION

The two bromine atoms split into Br+ and Br-

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26
Q

What happens in the third stage of the reaction between bromine and ethene?

A

CARBONIUM ION FORMATION
Br+ attacks the ethene molecule
Br needs the two electrons which are in the pi bond of the C=C bond
Forms a covalent bond with one of the C atoms
Still a C left with a positive charge (carbonium ion) → referred to as an intermediate as it is not part of the final product
Can sometimes form a cyclic bromonium ion

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27
Q

What happens in the fourth stage of the reaction between bromine and ethene?

A

IONIC ADDITION
Carbonium ion attacked by the Br- ion to form 1,2-dibromoethane
Referred to as ionic addition as ions are added across the double bond

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28
Q

What is 1,2-dibromoethane used for?

A

Treat timber logs against termites

Also used to be used as an anti-knocking agent

29
Q

What is 1,2-dichloroethane used for?

A

Manufacture PVC

30
Q

How do we know the reaction between bromine and ethene happens by ionic addition?

A

By the different types of products that form when multiple types of ions are present in the reaction. E.g, if the ethene and bromine reaction takes place in water with NaCl, the products are 1,2-dibromoethane, 1-bromo-2-chloroethane and 2-bromoethanol

31
Q

Polymers

A

Long chain molecules made by joining together many small molecules (monomers)

32
Q

What are some of the uses of polyethene/polythene?

A 
Plastic bags
Lunch boxes
Buckets
Bowls
Food wrapping
Washing up liquid bottles
33
Q

What are some of the uses of poly(propene)/poly propylene?

A 
Jugs 
Toys
Chairs
Beakers
Briefcases
34
Q

Repeating unit

A

Part of a polymer whose repetition produces the complete polymer chain except for the end groups

35
Q

Elimination reaction

A

One in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecule

36
Q

Give two examples of elimination reactions

A

The dehydration (water is removed) of ethanol to give ethene (using aluminium oxide)

Formation of propene from propan-2-ol

37
Q

What are the oxidising agents that can be used to oxidise a primary alcohol into an aldehyde?

A

acidified sodium dichromate (Na₂Cr₂O₇)

acidified potassium permanganate

38
Q

What are secondary alcohols oxidised to?

A

Ketones

39
Q

How do breathalysers?

A

They use orange potassium dichromate crystals which turn green when ethanol is oxidised to ethanol and ethanoic acid. The darker the green, the more alcohol present.

40
Q

What is an alcometer?

A

It uses an electrical cell where alcohol is converted into an aldehyde/carboxylic acid at one of the electrodes. The voltage increases with the amount of acid produced.

41
Q

How can aldehydes and ketones be reduced back to alcohols?

A

In the presence of hydrogen and nickel catalysts

42
Q

What is observed when you add ethanal to a test tube containing KMnO₄ and H₂SO₄, and the solution is heated?

A

Purple → colourless
Ethanal oxidised to ethanoic acid
Purple disappears because the Mn⁺⁷ ions are reduced to Mn⁺²

43
Q

How do you make up Fehling’s reagent?

A

Mixing equal quantities Fehling’s A (coppersulfate in water) and Fehling’s B (potassium sodium tartrate, sodium hydroxide and water)

44
Q

What happens when you mix Fehling’s reagent and ethanal and heat the solution?

A

The royal blue colour changes as a brick red precipitate is formed.
The ethanal is oxidised by the Fehling’s reagent,
Cu⁺² ions reduced to Cu⁺ ions by the ethanal

45
Q

How do you make Tollen’s reagent?

A

Mix silver nitrate solution, sodium hydroxide and ammonia solution

46
Q

What is observed if ethanal and Tollen’s reagent are warmed together in a test tube?

A

A silver mirror is deposited on the inside of the test tube

47
Q

Are alcohols stronger or weaker acids than water?

A

Alcohols are much weaker acids than water as they don’t donate a H⁺ as willingly. They only act as acids with very reactive metals like sodium

48
Q

For what two reasons are carboxylic acids good acids?

A

Inductive effect

Stability of the carboxylate ion

49
Q

Explain the Inductive Effect

A

In the carboxyl functional group, the carbon atom is slightly positively charged.
It pulls electrons out of the oxygen in the -O-H bond (Inductive Effect)
This makes it easier for the H to be removed and act as a proton donor

50
Q

Explain the stability of the carboxylate ion

A

When the carboxyl group loses a proton/H⁺, it forms a negative ion called carboxylate ion.
The two bons (C-O and C=O) are the same length so the negative charge is delocalised across the two oxygen atoms.
This gives a resonance hybrid of the two structures.
This ion is so stable that it drives the carboxylic acid to lose H⁺ (B/L acid)

51
Q

What is observed when sodium carbonate and ethanoic acid are placed in a test tube together and a lit splint is held at the mouth?

A

Fizzing is observed

The lit splint is extinguished showing carbon dioxide is given off in the reaction

52
Q

What is observed when magnesium ribbo and ethanoic acid are placed in a test tube together and a lit splint is held at the mouth?

A

Fizzing is observed

A mild pop is heard as hydrogen gas is produced

53
Q

What is observed when ethanol, glacial ethanoic acid and concentrated sulfuric acid are placed in a test tube together (and heated for 10 minutes)?

A

Contents have a fruity smell
Sulfuric acid is the catalyst
Ethyl ethanoate (ester) is formed

54
Q

Organic synthesis

A

Process of making organic compounds (target molecules) from simpler starting materials

55
Q

How is PVC synthesised from ethene?

A
  1. Chlorine and ethene react to form 1,2-dichloroethane (first intermediate)
  2. 1,2-dichloroethane is broken down using heat (thermal cracking) into chloroethene (second intermediate) and HCl
  3. Polymerisation of chloroethene into poly(chloroethene) / polyvinylchloride (PVC)
56
Q

Briefly outline the steps for the experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium permanganate solution (potassium manganate (VII)) under alkaline conditions

A
  1. Mix of the reagents (phenylmethanol, potassium permanganate and sodium carbonate)
  2. Heat the reagents
  3. Cool the solution
  4. Acidify with HCl
  5. Add sodium sulfite
  6. Cool the solution in an ice bath
  7. Filter off the benzoic acid crystals
57
Q

In the experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium permanganate solution (potassium manganate (VII)) under alkaline conditions, what is observed when the initial reagents are heated? Why?

A

The colour changes from purple to brown
Brown precipitate is MnO₂
The Mn is reduced from +7 to +4

58
Q

Why is the potassium permanganate solution saturated in the experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium permanganate solution (potassium manganate (VII)) under alkaline conditions?

A

To ensure sufficient is present to oxidise all the phenylmethanol completely to benzoic acid

59
Q

Why is sodium carbonate added to the experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium permanganate solution (potassium manganate (VII)) under alkaline conditions?

A

It is a base and the rate of reaction is faster under alkaline conditions

60
Q

What intermediate compound is formed in the experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium permanganate solution (potassium manganate (VII)) after heating the reagents?

A

sodium benzoate

61
Q

How do you test if the contents of the conical flask are acidic in the the experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium permanganate solution (potassium manganate (VII))?

A

Dip a glass rod in the solution and touch against damp blue litmus paper (turns red)

62
Q

Why is the HCl acid added in the experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium permanganate solution (potassium manganate (VII))?

A
  • It converts the sodium benzoate to benzoic acid
  • It neutralises any excess sodium carbonate left and KOH (potassium hydroxide) produced
  • It provides an acidic medium to enable the Mn⁺⁴ ions in MnO₂ to be reduced to soluble Mn⁺² ions
63
Q

Why is the flask cooled after the initial heating in the experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium permanganate solution (potassium manganate (VII))?

A

Benzoic acid is only slightly soluble in cold water, so it helps to precipitate out the benzoic acid

64
Q

What is observed when the sodium sulfite solution is added to the conical in the experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium permanganate solution (potassium manganate (VII))? Why?

A

The brown precipitate disappears and the white crystals are now visible (solution is colourless)
The sodium sulfite is a reducing agent and it reduces the Mn⁺⁴ ions (insoluble and brown) to Mn⁺² ions (soluble and colourless)

65
Q

Why must the brown precipitate be dissolved in the benzoic acid experiment?

A

It would otherwise contaminate the benzoic acid

66
Q

Recrystallisation

A

Process of repeated crystallisation in order to piurify a solid or to obtain more satisfactory crystals of a solid that is already pure

67
Q

Briefly outline the steps for the experiment to recrystallise a sample of benzoic acid

A
  1. Dissolve the crystals in the minimum amount of hot water
  2. Hot fitration
  3. Crystallisation (ice bath)
  4. Cold filtration
  5. Drying of crystals
68
Q

Why are the benzoic acid crystals dissolved in the minimum amount of hot water in the recrystallisation experiment?

A

Benzoic acid is very soluble in hot water, and only slightly soluble in cold.
To get a solution which is saturated at the boiling point of the solvent, i.e, to get maximum yield upon cooling

69
Q

What is noticed about the melting point of a sample of pure benzoic acid versus an impure sample?

A

The melting point of the pure benzoic acid is 121-122°C, whereas the melting point of the impure is lower and over a wider range (116119°C)

LC Chemistry (24 decks)

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