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A Level Chemistry > Halogenalkanes > Flashcards

Halogenalkanes Flashcards

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1
Q

What is a halogenalkane?

A

An organic compound where at least one hydrogen in the alkane chain has been replaced by a halogen.

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2
Q

Halogenalkanes can be either primary…

A

…secondary or tertiary.

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3
Q

How are halogenalkanes named (in what order)?

A

They are named using the alkane homologous series then by adding a halogen prefix to the longest chain, indicating the position with a number.

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4
Q

The carbon to halogen bond can always be described as what?

A

Being polar.

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5
Q

In terms of boiling points order these molecules lowest to highest and explain the order: CH3Br, CH3Cl, CH3I.

A

CH3Cl, CH3Br, CH3I.
This is because I has more electrons than Cl and a larger surface area so stronger London forces and a higher boiling point.

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6
Q

What is a nucleophile?

A

An electron pair donor.

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7
Q

An electron deficient carbon will attract what?

A

A nucleophile (such as H2O, OH- or NH3).

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8
Q

Halogenalkanes react in what kind of reaction?

A

Substitution reactions (nucleophilic).

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9
Q

During nucleophilic substitution on a halogenalkane what gets replaced with what?

A

The halogen gets replaced by a nucleophile forming a compound with a different functional group.

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10
Q

During hydrolysis of a halogenalkane what is replaced with what?

A

The halogen is replaced with an OH- ion (hydroxide ion).

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11
Q

What is meant by nucleophilic attact on a halogenalkane?

A

The carbon (δ+) is attracted by the nucleophile with a lone pair of electrons.

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12
Q

During nucleophilic substitution on a halogenalkane what process breaks the carbon-hydrogen bond?

A

Hetrolytic fission.

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13
Q

What solvent is used for a halogenalkane reacting with water and why?

A

Ethanol is used as water and halogenalkanes don’t mix, so it is used as a common solvent.

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14
Q

What compound can we use to monitor the rate of hydrolysis of halogenalkanes?

A

Silver nitrate (precipitate forms when halide ion is released due to substitution reaction).

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15
Q

What colour are the silver halide compounds?

A

Cl white.
Br cream.
I yellow.

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16
Q

Reacting halogenalkanes with what hydroxide is a quicker way of forming alcohols than using water in ethanol?

A

Potassium hydroxide.

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17
Q

How can we monitor the rate of reaction of halogenalkane + KOH?

A

You can use silver nitrate, however acidification must be carried out first with dilute nitric acid, then silver nitrate can be added.

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18
Q

What kind of reaction is halogenalkane + potassium cyanide?

A

Nucleophilic substitution.

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19
Q

What is the nucleophile in potassium cyanide?

A

The CN- ion.

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20
Q

What is the product of a halide group being replaced by a CN group?

A

A nitrile.

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21
Q

The reaction between potassium cyanide and halogenalkane is carried out under what and in what solvent?

A

Under reflux and in ethanol.

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22
Q

What happens when ammonia reacts with halogenalkanes?

A

The halide is replaced with an NH2 group and an ammonium salt is produced.

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23
Q

What solvent is used for the reaction between ammonia and a halogenalkane?

A

Ethanol.

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24
Q

What is the organic product of a reaction between halogenalkanes and ammonia?

A

An amine.

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25
Q

How can you stop amine acting as a nucleophile once the halogenalkane and ammonia have reacted?

A

Add excess ammonia.

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26
Q

Problems of halogenalkane/ammonia reactions?

A

Lone pair on amine could act as nucleophile and react with starting reagent.
Mixtures form giving a low percentage yield.
The final stage of the reaction is in equilibrium so may not get majority desired product produced.

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27
Q

What is the main elimination reaction for halogenalkanes?

A

A reaction with ethanolic potassium hydroxide.

28
Q

What is the homologous series of the product of a halogenalkane reaction with ethanolic potassium hydroxide?

A

Alkenes.

29
Q

What are the conditions required for the reaction between halogenalkanes and ethanolic potassium hydroxide?

A

Reaction takes place under reflux.

30
Q

In the elimination reaction of halogenalkanes with ethanolic potassium hydroxide, what does the hydroxide ion act as?

A

A base.

31
Q

How can we measure the rate of hydrolysis of halogenalkanes?

A

The halogenalkane is heated with silver nitrate in ethanol. Hydrolysis takes place and halide ions form which react with the silver nitrate to form a silver halide precipitate which we can the use to measure the rate of reaction by recording the rate at which precipitate forms.

32
Q

How can we use the time taken for silver halides to form to work out rate of reaction?

A

Use 1/time then divide by the smallest to find relative rates.

33
Q

How can we perform a fair test of measuring rates hydrolysis of halogenalkanes?

A

Use equimolar amounts of each halogenalkane.
Use halogenalkanes of the same chain length.
Use a water bath to ensure same temperature.

34
Q

Which carbon-halide bond is the most polar?

A

Carbon to fluorine.

35
Q

Which carbon to halide bond is the strongest and why?

A

Carbon to fluorine is the strongest bond due to its bond enthalpy being the highest as the bond is the shortest. Carbon to iodine is the longest bond therefore the weakest, requiring less energy to break the bond.

36
Q

Which halogenbutane reacts quickest and why?

A

Iodobutane reacts the fastest as it has the longest carbon to halogen bond making it the weakest with the lowest bond enthalpy. The opposite is true for fluorobutane making it the slowest.

37
Q

What is the functional group of an alcohol?

A

The hydroxyl group (-OH).

38
Q

What are the three classifications of alcohols?

A

Primary, secondary, tertiary.

39
Q

Describe a primary alcohol structure and where the hydroxyl group is attached.

A

Primary alcohols has two hydrogen atoms attached to the carbon and one alkyl group as well as the hydroxyl.

40
Q

Describe the structure of a secondary alcohol and where the hydroxyl group is attached.

A

Secondary alcohols have one hydrogen, two alkyl groups and a hydroxyl group to the same carbon atom.

41
Q

Describe the structure of a tertiary alcohol and where the hydroxyl group is.

A

Tertiary has three alkyl groups and a hydroxyl group attached to the carbon.

42
Q

True or false, methanol is more volatile than methane.

A

False methanol is less volatile than methane due to its London forces and stronger dipole-dipole forces.

43
Q

Does methanol have a higher melting point than methane?

A

Yes. Yes it does.

44
Q

Why is methanol more soluble in water than methane?

A

Methanol has bonds that are more polar.

45
Q

What will alcohols burn to form under complete combustion?

A

Water and CO2.

46
Q

Define complete combustion.

A

Combustion where there is sufficient oxygen for the fuel to burn completely.

47
Q

What type of agents are used to oxidise primary and secondary alcohols?

A

Oxidising agents.

48
Q

Can tertiary alcohols be oxidised?

A

No.

49
Q

What is the most common oxidising agent for oxidising alcohols?

A

Potassium dichromate (VI).

50
Q

What is the colour change when Cr2O7 2- is reduced to Cr 3+?

A

Orange to green.

51
Q

Primary alcohols can be oxidised to what?

A

Aldehydes then further to carboxylic acids.

52
Q

How do you oxidise a primary alcohol to an aldehyde?

A

The alcohol is oxidised by gently heating with the oxidising agent, then it is distilled off straight away as not to be oxidised further into a carboxylic acid.

53
Q

How do you prepare a carboxylic acid from a primary alcohol?

A

A carboxylic acid is prepared by heating the alcohol under reflux with an excess of oxidising agent.

54
Q

Secondary alcohols can be oxidised into what?

A

Ketones.

55
Q

Why is the oxidation of secondary alcohols into ketones carried out under reflux?

A

Secondary alcohols can’t be oxidised further than ketones so the process is carried out under reflux to ensure all secondary alcohols have reacted.

56
Q

How can you test to distinguish between aldehydes and ketones?

A

When mixed with Fehling’s or Benedicts solution the aldehyde will turn the mixture from blue to brick red as it is oxidised by the Cu (II) ions, whereas the ketone can’t be oxidised further.

57
Q

What is a dehydration reaction?

A

A reaction where a water molecule is removed from the starting material (elimination).

58
Q

What are the conditions required for the dehydration of an alcohol?

A

There must be an excess of concentrated acid catalyst (H2SO4 or H3PO4) and it is carried out under reflux at 180 degrees celcius.

59
Q

What is the product of the dehydration of alcohol?

A

An alkene and water.

60
Q

How are chloroalkanes best prepared from alcohols?

A

Using phosphorus (V) chloride.

61
Q

How can tertiary chloroalkanes be prepared which is different to other chloroalkanes from alcohols?

A

They can be prepared using conc. HCl at room tempertaure but this process is too slow for primary alcohols.

62
Q

How can bromoalkanes be prepared?

A

Using hydrogen bromide in the presence of sulphuric acid.

63
Q

How is hydrogen bromide usually prepared for making bromoalkanes?

A

It is usual to mix 50% sulphuric acid with sodium or potassium bromide to make hydrogen bromide in the reaction flask.

64
Q

What are the alternatives to hydrogen bromide in making alcohols bromoalkanes?

A

Phosphorus (V) bromide or phosphorus (III) bromide.

65
Q

What compound is used to make iodoalkanes from alcohols and how is it made?

A

Phosphorus (III) iodide, however it is unstable so must be made in the reaction flask using a mixture of red phosphorus and iodine.

66
Q

Why can’t hydrogen iodide be used to form iodoalkanes from alcohols?

A

Hydrogen iodide is far too unstable.

A Level Chemistry (13 decks)

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