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Chemistry A-Level > Alcohols and Haloalkanes (Chapter 14 and 15) > Flashcards

Alcohols and Haloalkanes (Chapter 14 and 15) Flashcards

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1
Q

What is an alcohol?

A

An organic compound containing the functional group OH (R-OH)

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2
Q

What is the name for the OH group?

A

Hydroxyl group

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3
Q

When does the name of the alcohol have the suffix “ol”?

A

When it is the highest priority functional group in a molecule

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4
Q

When does the name of the alcohol have the prefix “hydroxy”?

A

When it is not the highest priority functional group in a molecule

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5
Q

When is an alcohol primary?

A

When the carbon atom adjacent (attached) to the OH group is bonded to two hydrogen atoms

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6
Q

When is an alcohol secondary?

A

When the carbon atom adjacent (attached) to the OH group is bonded to one hydrogen atom

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7
Q

When is an alcohol tertiary?

A

When the carbon atom adjacent (attached) to the OH group is not bonded to hydrogen

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8
Q

How do the physical properties of alcohols compare to those of corresponding alkanes? (alcohol vs alkane with same number of carbons)

A

Less volatile
Higher melting points
Greater water solubility

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9
Q

How do the differences between the physical properties of alcohols change as the carbon chain becomes longer?

A

The differences become much smaller as the length of the carbon chain increases

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10
Q

Explain the differences between the physical properties of alkanes and alcohols (considering bond polarity and the effects on the intermolecular forces)

A
  • The alkanes have non-polar bonds because the electronegativity of H and C are very similar
  • The alkane molecules are therefore non-polar
  • The intermolecular forces between non-polar molecules are very weak London forces
  • Alcohols have a polar OH bond because of the difference in electronegativity between the O and H atoms
  • Alcohol molecules are therefore polar
  • The intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar OH groups
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11
Q

Why do alcohols have a lower volatility and higher boiling point than alkanes?

A
  • In the liquid state, H bonds hold the alcohol molecules together
  • These bonds must be broken in order to change the liquid alcohol into a gas
  • This requires more energy than overcoming the weaker London forces in alkanes, so alcohols have a lower volatility than the alkanes with the same number of carbon atoms
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12
Q

What makes a compound much more soluble in water than other compounds?

A

If it can form H bonds with water

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13
Q

Why are certain alcohols soluble in water and alkanes are not?

A
  • Alkanes are non-polar molecules and cannot form H bonds with water
  • Alcohols such as methanol and ethanol are completely soluble in water, as hydrogen bonds form between the polar OH group of the alcohol and the water molecules
  • The energy gained from H bonding with water is enough to break the London forces between alcohol molecules
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14
Q

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

A
  • As the hydrocarbon chain increases in size, the influence of the OH group becomes relatively smaller, and the solubility of the longer-chain alcohols becomes more like that of hydrocarbons, so solubility decreases
  • The energy gained from H bonding with water is not enough to break the larger number of London forces between alcohol molecules
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15
Q

Describe the combustion of alcohols

A
  • Alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water
  • The reaction is exothermic, releasing a large quantity of energy in the form of heat
  • As the number of carbon atoms in the alcohol chain increases, the quantity of heat released per mole increases
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16
Q

How can alcohols be oxidised?

A
  • Primary and secondary alcohols can be oxidised by an oxidising agent
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17
Q

What is the usual oxidising mixture used to oxidise alcohols?

A

A solution of potassium dichromate (VI) (K2Cr2O7), acidified with dilute sulfuric acid (H2SO4)

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18
Q

How can you tell when an alcohol has been oxidised?

A

The orange solution containing dichromate (VI) ions is reduced to a green solution containing chromium (III) ions

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19
Q

What are the two possible products of the oxidation of a primary alcohol?

A

Aldehyde or carboxylic acid

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20
Q

What does the product of oxidation of primary alcohols depend on?

A

The reaction conditions (distillation/reflux) used because aldehydes are themselves also oxidised to carboxylic acids

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21
Q

Describe the preparation of aldehydes from the oxidation of primary alcohols

A
  • On gentle heating of primary alcohols with acidified potassium dichromate, an aldehyde is formed
  • To ensure that the aldehyde is prepared rather than the carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms
  • This prevents any further reaction with the oxidising agent
  • The dichromate ions change from orange to green
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22
Q

What is the symbol used to represent the oxidising agent?

A

[O]

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23
Q

Why do aldehydes have a lower BP than alcohols?

A

Because they have weaker H-bonding

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24
Q

Describe the preparation of carboxylic acids from the oxidation of primary alcohols

A
  • If a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate (VI), a carboxylic acid is formed
  • Use of an excess of the acidified potassium dichromate (VI) ensures that all of the alcohol is oxidised
  • Heating under reflux ensures that any aldehyde formed initially in the reaction also undergoes oxidation to the carboxylic acid
  • Once the reaction has gone to completion, the carboxylic acid can then be distilled or recrystallised
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25
Q

What is important to remember about the oxidation producing carboxylic acid equation?

A

That the ratio of the alcohol to the oxidising agent is 1:2 - 2[O]

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26
Q

(summary) How do you prepare a carboxylic acid?

A

Heat a primary alcohol under reflux

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27
Q

(summary) How do you prepare an aldehyde?

A

Use distillation to remove the aldehyde from the reaction mixture

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28
Q

What is the product of oxidising a secondary alcohol?

A

Ketone

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29
Q

Is it possible to further oxidise ketones?

A

Not with acidified dichromate (VI) ions

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30
Q

Describe the preparation of ketones from the oxidation of secondary alcohols

A
  • To ensure the reaction goes to completion, the secondary alcohol is heated under reflux with the oxidising mixture
  • The dichromate (VI) ions once again change colour from orange to green
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31
Q

Can tertiary alcohols undergo oxidation reactions?

A

No, the acidified dichromate (VI) remains orange when added to a tertiary alcohol

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32
Q

What is a dehydration reaction?

A

Any reaction in which a water molecule is removed from the starting material

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33
Q

Describe the dehydration reaction of an alcohol

A
  • An alcohol is heated under reflux in the presence of an acid catalyst such as concentrated sulfuric acid (H2SO4) or concentrated phosphoric acid (H3PO4)
  • The product of the reaction is an alkene (and water)
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34
Q

What is dehydration of an alcohol an example of?

A

An elimination reaction

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35
Q

What is the example of a substitution reaction of alcohols?

A

Alcohols react with hydrogen halides to form haloalkanes

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36
Q

How do you prepare a haloalkane?

A
  • The alcohol is heated under reflux with sulfuric acid and a sodium halide
  • The hydrogen halide is formed in situ
  • The hydrogen halide formed reacts with the alcohol to produce the haloalkane
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37
Q

Give an example equation for how the hydrogen halide is formed

A

NaBr(s) + H2SO4(aq) = NaHSO4(aq) + HBr(aq)

38
Q

What is the overall equation for the reaction of propan-2-ol with H2SO4 and NaBr?

A

CH3CHOHCH3 + NaBr + H2SO4 = CH3CHBrCH3 + NaHSO4 + H2O

39
Q

What is an aldehyde?

A

C bonded with R, H and double bond O

40
Q

What is a carboxylic acid?

A

C bonded with R, OH and double bond O

41
Q

What is a ketone?

A

C bonded with R1, R2 and double bond O

42
Q

What is the suffix for an aldehyde?

A

“al”

43
Q

What is the suffix for a ketone?

A

“one”

44
Q

What is the prefix for a ketone?

A

“oxo”

45
Q

What is the suffix for a carboxylic acid?

A

“oic acid”

46
Q

What are haloalkanes?

A

Compounds containing the elements carbon, hydrogen and at least one halogen

47
Q

How do you name haloalkanes?

A
  • A prefix is added to the name of the longest chain to indicate the identity of the halogen
  • When two or more halogens are present in a structure they are listed in alphabetical order
48
Q

Why is the carbon-halogen bond in a haloalkane polar?

A
  • Haloalkanes have a carbon-halogen bond in their structure
  • Halogen atoms are more electronegative than carbon atoms
  • The electron pair in the carbon-halogen bond is therefore closer to the halogen atom than the carbon atom
  • The carbon-halogen bond is polar
49
Q

Why can carbon atoms in haloalkanes attract nucleophiles?

A

Because it has a slightly positive charge due to the polarity of the carbon-halogen bond

50
Q

What is nucleophile?

A
  • Species that donate a lone pair of electrons
  • An atom of group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond
51
Q

Give 3 examples of nucleophiles

A

Hydroxide ions - :OH-
Water molecules - H2O:
Ammonia molecules - :NH3

52
Q

What happens when a haloalkane reacts with a nucleophile?

A
  • The nucleophile replaces the halogen in a substitution reaction
  • A new compound is produced containing a different functional group
53
Q

What is the reaction mechanism for when a haloalkane reacts with a nucleophile?

A

Nucleophilic substitution (heterolytic fission)

54
Q

What is substitution?

A

A reaction in which one atom or group of atoms is replaced by another atom or group of atoms

55
Q

Which haloalkanes undergo nucleophilic substitution?

A

Primary haloalkanes

56
Q

What is hydrolysis?

A
  • A chemical reaction involving water or a aqueous solution of a hydroxide that causes the breaking of a bond in a molecule
  • This results in the molecule being split into two products
57
Q

What happens in the hydrolysis of a haloalkane?

A

The halogen atom is replaced by an OH group (e.g. in an aqueous alkali), forming an alcohol

58
Q

Describe the hydrolysis of a haloalkane

A
  • The nucleophile, OH-, approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom (minimising repulsion between the nucleophile and the d- halogen atom)
  • A lone pair of electrons on the hydroxide ion is attracted and donated to the d+ carbon atom
  • A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
  • The carbon-halogen bond breaks by heterolytic fission
  • The new organic product is an alcohol - a halide ion is also formed
59
Q

Describe the reaction between haloalkanes and aqueous sodium hydroxide

A
  • Haloalkanes can be converted to alcohols using aqueous sodium hydroxide
  • The reaction is very slow at room temperature, so the mixture is heated under reflux to obtain a good yield of product
  • NaBr is also formed
60
Q

What happens to the bonds during hydrolysis of a haloalkane?

A

The carbon-halogen bond is broken and the OH group replaces the halogen in the haloalkane

61
Q

What does the rate of hydrolysis of a haloalkane depend on?

A

The strength of the carbon-halogen bond in the haloalkane

62
Q

Which carbon-halogen bond is the strongest and which is the weakest and what does this mean?

A

C-F bond is the strongest and C-I bond is the weakest (bc C-I is a longer bond)
This means that less energy is required to break the C-I bond than other carbon-halogen bonds

63
Q

What can we predict from the bond enthalpies of the carbon-halogen bonds?

A
  • Iodoalkanes react faster than bromoalkanes
  • Bromoalkanes react faster than chloroalkanes
  • Fluroroalkanes are unreactive as a large quantity of energy is required to break the C-F bond
64
Q

What is the general equation for the hydrolysis of halobutanes with water? (X representing any of the halogens)

A

CH3(CH2)3X + H2O = CH3(CH2)3OH + H+ + X-

65
Q

How can you measure the rate of hydrolysis of primary haloalkanes (halobutanes)?

A
  • The rate of each reaction can be followed by carrying out the reaction in the presence of aqueous silver nitrate
  • As the reaction takes place, the halide ions, X-(aq), are produced which react with Ag+(aq) ions to form a precipitate of the silver halide
  • Observe the test tubes for five minutes and record the time taken for the precipitate to form
66
Q

Why is the hydrolysis reaction of primary haloalkanes (with AgNO3) carried out in the presence of an ethanol solvent?

A
  • The nucleophile in the reaction is water, which is present in the aqueous silver nitrate
  • Haloalkanes are insoluble in water
  • The ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers
67
Q

What are the observations that should be found during the hydrolysis of haloalkanes with AgNO3?

A

Iodo - yellow precipitate forms rapidly
Bromo - cream precipitate forms slower
Chloro - white precipitate forms very slowly

68
Q

How can the observations be explained in the reaction of the haloalkane with AgNO3 and what conclusions can be drawn?

A
  • The compound with the slowest rate of reaction is he one that has the strongest carbon-halogen bond
  • Therefore, the rate of hydrolysis increases as the strength of the carbon-halogen bond decreases
69
Q

What are some uses of organohalogen compounds?

A

Pesticides, general solvents (CHCl3), dry cleaning solvents (C2HCl3), making polymers (C2F4), flame retardants (CF3Br) and refrigerants (F2CCl2)

70
Q

What is the ozone layer?

A
  • The ozone layer is found 10-40km above the Earth’s surface, at the outer edge of the stratosphere.
  • Ozone absorbs UV-B radiation from the Sun that causes sunburn, genetic damage and skin cancer
71
Q

What reactions are occurring in the ozone layer naturally?

A
  • Ozone is constantly being formed and broken down by the action of UV radiation:
    O2 = 2O (oxygen radicals)
  • Ozone is then formed with oxygen and oxygen radicals:
    O2 + O <=> O3
72
Q

How do you draw an oxygen radical?

A

O (no dot)

73
Q

Why are CFCs so stable?

A
  • Due to the strength of their carbon-halogen bonds
74
Q

What happens once CFCs enter the stratosphere?

A
  • UV radiation provides sufficient energy ti break a carbon-halogen bond in CFCs by homolytic fission to form halogen radicals
  • The C-Cl bond has the lowest bond enthalpy and so is the bond that breaks
75
Q

What is the name of he process as radiation initiates the breakdown of CFCs?

A

Photodissociation

76
Q

What equation takes place during photodissociation?

A

CF2Cl2 = CF2Cl• + Cl•

77
Q

What happens during propagation in the breakdown of CFCs?

A

The chlorine radical formed, Cl•, is a very reactive intermediate and can react with an ozone molecule, breaking down the ozone into oxygen

78
Q

What two equations take place during propagation of CFCs?

A

Step 1: Cl• + O3 = ClO• + O2

Step 2: ClO• + O = Cl• + O2

79
Q

What is the overall equation of propagation of CFCs (or other radicals e.g. NO•) that shows how they breakdown ozone?

A

O3 + O = 2O2

80
Q

Why can the cycle of propagation of CFCs happen many times (up to 100,000)?

A
  • Step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone in step 1
  • The two propagation steps repeat in a cycle over and over again in a chain reaction
  • It is estimated that a single CFC molecule can promote the breakdown of 100,000 molecules of ozone
81
Q

What other radicals can catalyse the breakdown of ozone?

A

Nitrogen oxide radicals (NO•) which are formed naturally during lightening strikes and also as a result of aircraft travel in the stratosphere

82
Q

What are the two equations in the propagation steps showing the breakdown of ozone by NO•?

A

Step 1: NO• + O3 = NO2• + O2

Step 2: NO2• + O = NO• + O2

83
Q

What do you need to remember when writing oxidation reactions of alcohols?

A

WATER IS PRODUCED

84
Q

What do you have to remember about the dehydration of alcohols to alkenes?

A

You can’t form a branched alkene from a straight chain alcohol

85
Q

What three things do you need to draw on a diagram showing H-bonding?

A
  • Dipoles
  • Lone pair
  • Hydrogen bond
86
Q

What does low volatility mean?

A
  • Requires a high amount of energy to turn into a gas

- Is not easily vaporised

87
Q

What reasons are there to use CFC’s (or related compounds) as aerosols?

A
  • Non toxic, high volatility, low flammability, low reactivity, will not hydrolyse in water, low BP
88
Q

Why might a CFC not hydrolyse in water?

A

Because the C-F bond is very strong

89
Q

Why might you need to redistill an organic product?

A

To get rid of any impurities e.g. organic side product or unreacted alcohol

90
Q

Why do you add an acid catalyst slowly?

A

To avoid a big temperature increase which avoids side-reactions

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