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Chemistry > Unit 2.2 Alcohols, Halogenoalkanes And Analysis > Flashcards

Unit 2.2 Alcohols, Halogenoalkanes And Analysis Flashcards

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0
Q

What are the disadvantages of making alcohol by hydration of ethene?

A

The reaction is reversible, so conversion of ethene is incomplete - the unreacted gases are recycled and passed through the reactor again and again
Needs a high temperature (so more energy required)
Uses non-renewable reactants

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1
Q

What are the advantages of making alcohol by hydration of ethene?

A

High percentage yield
100% atom economy
Can be carried out continuously

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2
Q

What is the equation for the hydration of ethene to make ethanol?

A

C2H4 + H2O —> CH3CH2OH

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3
Q

What conditions are required for the hydration of ethene?

A

Uses steam
In the presence if a phosphoric acid catalyst (H3PO4)
Temp of 300 degrees Celsius
Pressure of 60atm

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4
Q

What are the advantages of making ethanol by fermentation?

A

Needs a relatively low temp (less energy)

The reactants are renewable

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5
Q

What are the disadvantages of making ethanol by fermentation?

A

Reaction is quite slow
The toxicity of the alcohol limits the concentration of ethanol (denatures the enzymes above a conc of about 14%)
Must be carried out in the absence of air
Fairly low percentage yield and atom economy

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6
Q

Definition of volatility

A

The ease that a liquid turns into a gas

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7
Q

Why do alcohols have a lower volatility than alkanes of similar molecular mass?

A

Hydrogen bonds from between ethanol molecules - additional forces to the van der Waals’ forces that alkanes forces

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8
Q

Why are alcohols soluble in water?

A

Hydrogen bonds form between the polar -O-H groups of the alcohol and water molecules

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9
Q

Why does the solubility of alcohols decrease as the chain length increases?

A

A larger part of the alcohol molecule is made up of a non-polar hydrocarbon chain, which does not form hydrogen bonds with water molecules

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10
Q

What is a secondary alcohol?

A

The -OH group is attached to a carbon with 2 alkyl groups

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11
Q

What is a tertiary alcohol?

A

The -OH group is attached to a carbon atom bonded to 3 alkyl groups

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12
Q

What is the equation for the complete combustion of ethanol?

A

C2H5OH + 3O2 —-> 2CO2 + 3H2O

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13
Q

What is the colour change when acidified potassium dichromate is oxidised?

A

From orange to green

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14
Q

When a primary alcohol is oxidised, what is produced?

A

An aldehyde and H2O

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15
Q

What is produced when a secondary alcohol is oxidised?

A

A ketone and H2O

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16
Q

Definition of reflux

A

The continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without the contents of the flask boiling dry

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17
Q

When oxidising a primary alcohol, why do you have to distil the aldehyde from the reaction mixture as it is formed?

A

To prevent the aldehyde being oxidised further to a carboxylic acid

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18
Q

What is produced when a tertiary alcohol is oxidised?

A

Nothing - they are resistant to oxidation, so the oxidising agent remains orange

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19
Q

Definition of esterification

A

The reaction of an alcohol with a carboxylic acid to produce an ester and water, in the presence of an acid catalyst (usually concentrated sulphuric acid)

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20
Q

How can you prepare an ester?

A
  • In a boiling tube, add 1cm^3 of the carboxylic acid to 1cm^3 of the alcohol. Carefully add a few drops of concentrated sulphuric acid.
  • Place the boiling tube in a hot water bath at about 80 degrees Celsius for about 5 minutes
  • Pour the product into a beaker of cold water
  • You will see an oil floating on the surface of the water (the ester)
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21
Q

Definition of dehydration (of an alcohol)

A

An elimination reaction in which water is removed from a saturated molecule to make an unsaturated molecule (alkene)

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22
Q

What are the conditions for the dehydration of an alcohol?

A
  • The alcohol is heated under reflux
  • In the presence of an acid catalyst (concentrated phosphoric acid or concentrated sulphuric acid)
  • For about 40 minutes
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23
Q

What are halogenoalkanes?

A

Compounds in which a halogen atom has replaced at least one of the hydrogen atoms in an alkene chain

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24
Q

Why do halogenoalkanes contain a polar carbon-halogen bond?

A

Because the carbon and halogen atoms have different negativities:

  • Halogen atoms are more electronegative than carbon atoms
  • The bonded electron pair is attracted more towards the halogen atom than towards the carbon atom
  • So a polar bond is formed
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50
Q

Why does reactivity decrease down the group of halogenoalkanes?

A

The electronegativities decreases, so the polarity of the carbon-hydrogen bond decreases.
This means that the electron-deficient carbon atom finds it harder to attract a nucleophile

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51
Q

Definition of hydrolysis

A

A reaction with water or aqueous hydroxide ions that breaks a chemical compound into 2 compounds

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52
Q

What happens when halogenoalkanes are hydrolysed?

A
  • reacted with an aqueous solution of hot hydroxide ions
  • a nucleophilic substitution reaction occurs
  • the product of this reaction is an alcohol
53
Q

What is nucleophilic substitution?

A

A type of substitution reaction in which a nucleophile is attracted to an electron-deficient centre or atom, where it donates a pair of electrons to form a new covalent bond

54
Q

How does the nucleophilic substitution take place in hydrolysis?

A
  • The hydroxide ion, OH^-, has a pair of lone electrons
  • These are attracted and donated to the electron-deficient carbon atom in the halogenoalkane
  • This is known as a nucleophilic attack
  • This leads to the formation of a new covalent bond between the oxygen atom of the hydroxide ion and the carbon atom
  • The carbon-halogen bond breaks by heterolytic fission
  • Both electrons from the bond move to the halogen, forming a halide ion
55
Q

How can you calculate the rate of hydrolysis?

A

1/time taken for the precipitate to occur

56
Q

How can you determine the rate of hydrolysis for different halogenoalkanes?

A

Heat the halogenoalkane with aqueous silver nitrate, with ethanol added
- Water in the mixture behaves as the nucleophile
- The ethanol acts as a common solvent, ensuring that the halogenoalkane and aqueous silver nitrate mix together and react
As the hydrolysis reaction takes place, halide ions form
The aqueous silver nitrate reacts with any halide ions present, forming a precipitate of the silver halide

57
Q

What are the 2 factors that can affect the rate of hydrolysis?

A 
Polarity (high polarity = attracts the nucleophile more easily = faster reaction)
Bond enthalpy (the weaker the bond enthalpy, the easier it is to break the bond, so the faster the reaction)
58
Q

Why are hydrofluorohydrocarbons (HCFCs) not a much better alternative to CFCs?

A

They can still deplete the ozone layer (but less so than CFCs)

59
Q

What influences the properties of halogen-containing polymers?

A

The strength of the carbon-halogen bonds in the long-chained polymer structure

60
Q

How do you work out percentage yield?

A

(Actual amount, in mol, of product / theoretical amount, in mol, of product) x 100

61
Q

Definition of limiting reagent

A

The substance in a chemical reaction that runs out first

62
Q

Why is it hard to obtain a 100% yield?

A
  • The reaction may be at equilibrium and may not go to completion
  • Other side reactions may occur, leading to by-products
  • The reactants may not be pure
  • Some of the reactants or products may be left behind in the apparatus used in the experiment
  • Separation and purification may result in the loss of some of the product
63
Q

How do you calculate atom economy?

A

(Molecular mass of the desired product / sum of molecular masses of all products) x 100

64
Q

What is done with the by-products of a reaction?

A
  • Often considered waste, so are disposed of (costly and unenvironmentally friendly)
  • May be sold on or used elsewhere in the chemical plant
65
Q

What happens to a molecule when it absorbs infrared radiation?

A

They vibrate with either a stretching or bending motion

66
Q

What does the amount of vibration depend on? (When molecules absorb infrared radiation)

A
  • The bond strength
  • The bond length
  • The mass of each atom involved in the bond
67
Q

At what frequency do most bonds vibrate?

A

Between 300 and 4000 cm^-1, in the infrared part of the electromagnetic spectrum

68
Q

How can an infrared spectrum be analysed to determine details about a compound’s chemical structure?

A
  • Different functional groups vibrate at different frequencies
  • So they will have troughs (peaks) at different places on the spectrum
69
Q

What can mass spectrometry be used for?

A
  • To identify unknown compounds
  • To determine the abundance of each isotope in an element
  • To gain further information about the structure and chemical properties of molecules
70
Q

How does a mass spectrometer work?

A

Determines the mass of a molecule or isotope by measuring the mass-to-charge ratio

  • The sample is introduced via a sample inlet
  • Sample molecules = converted into ions by an ionisation source
  • The ions are propelled into a mass analyser
  • The ions are separated according to their mass-to-charge ratio
  • The ions are detected and a mass spectrum is produced
71
Q

What methods of ionisation can be used in mass spectrometry?

A

Electron impact
Chemical ionisation
Use of an electrospray
Lasers

72
Q

What methods of separating ions can be used in mass spectrometry?

A

Deflection with a magnetic field

Time of flight (measurement of how long it takes for ions of different mass to reach the detector)

73
Q

What is a mass spectrum?

A

A graph showing relative or percentage abundance on the y-axis and mass:charge ratio (m/z) on the x-axis

74
Q

How does an infrared spectrometer work?

A
  • A beam of infrared radiation is passed through a sample of the material under investigation
  • This beam contains the full range of frequencies present in the infrared region
  • The molecule absorbs some of these frequencies
  • The emerging beam is analysed to plot a graph of transmittance against frequency
75
Q

Definition of molecular ion (M^+)

A

The positive ion formed in mass spectrometry when a molecule loses an electron

76
Q

How does electron impact work?

A
  • The molecules being tested are bombarded with electrons
  • If an electron is dislodged from a molecule, a positive ion is formed
  • The mass of the lost electron is negligible
  • The molecular mass has a molar mass equal to the molecular mass of the compound
  • This molecular ion can be detected and analysed
77
Q

What is fragmentation?

A
  • The process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive fragment ion and the other is a neutral species
  • Excess energy from the ionisation process is used (makes the molecular ion vibrate)
78
Q

The molecular ion produces the peak with the highest m/z value in the mass spectrum.
True or false?

A

True

79
Q

How can the molecular mass of a molecule be determined using mass spectrometry?

A

By locating the M+ peak

80
Q

How can mass spectrometry identify 2 different isomers?

A
  • Although the m/z value will be the same

- The fragmentation patterns will be different

81
Q

What happens when an alkene reacts with a halogen?

A

Radical substitution

82
Q

What happens in the initiation stage when an alkane takes part in radical substitution with halogens?

A
  • Radicals are formed

- UV radiation provides the energy to break the covalent bond in the halogen

83
Q

What happens in the propagation stage when an alkane takes part in radical substitution with halogens?

A

The products of the reaction are made
- A halogen radical is used in the first propagation step
- A halogen radical is regenerated in the second propagation step
- Halogen radicals catalyse the propagation step. Although they are involved in the reaction steps, they are not used up in the overall reaction
These 2 steps are repeated in a chain reaction to build up the products - they occur until no reactants remain

84
Q

What happens in the termination stage when an alkane takes part in radical substitution with halogens?

A

Any 2 radicals combine

Since this removes radicals, the reaction stops

85
Q

What happens when an alkene is reacted with an electrophile?

A

Electrophilic addition

86
Q

How does the electrophilic addition work (alkene + electrophile) work?

A
  • The halogen-halogen bond in a halogen molecule becomes slightly polar as it approaches the alkene
  • A pair of electrons flows from the double bond to the positive halogen ion and a bond is formed
  • A carbocation is formed
  • The negative halogen ion then bonds to the positive part of the carbocation
87
Q

What happens when a halogenoalkane reacts with a nucleophile?

A

Nucleophilic substitution

88
Q

How does the nucleophilic substitution work? (Halogenoalkane + nucleophile)

A
  • The nucleophile attacks the back of the carbon atom, donating an electron pair
  • A new bond is formed and the carbon-halogen bond breaks
89
Q

What is an electrophile?

A

An electron pair acceptor

90
Q

What is a nucleophile?

A

An electron pair donor

91
Q

What is a radical?

A

A species with an unpaired electron

92
Q

What are fragment peaks?

A
  • Small peaks that appear alongside the molecular ion peak
  • ## Give information about the structure of the compound

Chemistry (4 decks)

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