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AS Chemistry > Module 4.2 - Alcohols, Haloalkanes & Analysis > Flashcards

Module 4.2 - Alcohols, Haloalkanes & Analysis Flashcards

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1
Q

What is the homologous series, alcohol’s general formula?

A

CnH2n+1OH

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2
Q

What makes an alcohol a primary, secondary or tertiary alcohol?

A

Depending on which carbon atom the -OH group is bonded to.

>(If the C connected to the -OH is connected to only one other C group it’s a primary alcohol).

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3
Q

Why are alcohols normally polar molecules?

A

Due to the electronegative hydroxyl group which pulls the electrons in the C-OH bond away from the carbon atom.

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4
Q

Why does alcohol’s solubility in water decrease as their size increases?

A

> When you mix an alcohol with water, hydrogen bonds form between the -OH and H2O and in a small alcohol hydrogen bonding lets it mix freely with water (soluble).
However, in larger alcohols, most of the molecule is a non-polar carbon chain, so there’s a weaker attraction for the polar H20 molecules.

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5
Q

How does hydrogen bonding in alcohols affect volatility?

A

Alcohols form hydrogen bonds with each other and it’s the strongest type of intermolecular force, so gives alcohols a relatively low volatility (doesn’t easily evaporate into a gas).

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6
Q

How is a haloalkane formed?

A

1) Alcohols will react with compounds containing halide ions in a subsitution reaction.
2) The hydroxyl (-OH) group is replaced by the halide, so the alcohol is transformed into a haloalkane.
3) The reaction also requires an acid catalyst such as H2SO4.

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7
Q

How can you make alkenes from alcohols?

A

You can make alkenes by eliminating water from alcohols in an elimination reaction.

1) Mix the alcohols with an acid catalyst (concentrated sulphuric or phosphoric acid).
2) Heat the mixture.
3) When an alcohol dehydrates it eliminates water.

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8
Q

Is there always one product formed from one elimination reaction?

A

No, there is often 2 possible alkene products (or more) dependent on which side of the hydroxyl group the hydrogen is eliminated from.

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9
Q

What is an elimination reaction where water is eliminated called?

A

A dehydration reaction.

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10
Q

What is the simplest way to oxidise alcohols and what are the products?

A

> The simplest way is to burn them.

>If you burn an alcohol with plenty of oxygen, you get CO2 and water as products. This is a combustion reaction.

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11
Q

What is the oxidising agent used to mildly oxidise alcohols?

A

Acidified dichromate.

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12
Q

What forms when primary alcohols are oxidised?

A

First into aldehydes and then into carboxylic acids.

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13
Q

What forms when secondary alcohols are oxidised?

A

Ketones only.

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14
Q

What forms when tertiary alcohols are oxidised?

A

Won’t be oxidised.

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15
Q

Aldehydes and ketones are carbonyl compounds, what functional group do they share?

A

C=O

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16
Q

What is the general formula for ketones and aldehydes?

A

CnH2nO

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17
Q

What reaction conditions are necessary to form an aldehyde?

A

Distillation. Gently heat excess alcohol with a controlled amount of oxidising agent in distillation apparatus so the aldehyde (which boils at a lower temperature than the alcohol) is distilled off immediately.

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18
Q

What reaction conditions are necessary to form a carboxylic acid?

A

To produce the carboxylic acid, the alcohol has to be vigorously oxidised. The alcohol is mixed with excess oxidising agent and heated under reflux.

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19
Q

How can you form a ketone?

A

Reflux with acidifed dichromate.

>Ketones can’t be oxidised easily, so even prolonged refluxing won’t produce anything more.

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20
Q

What is a haloalkane?

A

An alkane with at least one halogen atom in place of a hydrogen atom.

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21
Q

Give an example of a nucleophile and what it does?

A

It could be a negative ion or an atom with a lone pair of electrons . It donates an electron pair to somewhere without enough electrons. OH-, CN- and NH3- are all nucleophiles that react with alkanes, water is a nucleophile too but reacts slowly.

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22
Q

Describe the electronegativity in carbon-halogen bonds in haloalkanes?

A

The bond is polar. Halogens are generally much more electronegative than carbon. The delta + carbon is electron deficient so it means it can be attacked by a nucleophile.

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23
Q

What is happening in a nucleophilic substitution reaction?

A

Haloalkanes are being hydrolysed into alcohols.

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24
Q

What is needed for a nucleophilic substitution reaction to take place?

A

You have to use a warm aqueous alkali for example sodium hydroxide or potassium hydroxide.

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25
Q

Describe the mechanism of a nucleophilic substitution reaction?

A

1) OH- is the nucleophile which provides a pair of electrons for the delta + carbon.
2) The C-Br bond breaks heterolytically - both electrons from the bond are taken by Br-
3) Br- falls off as OH- bonds to the carbon.

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26
Q

What does the speed that haloalkanes are hydrolysed depend on?

A

Depends on the bond enthalpy.

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27
Q

Why do iodoalkanes hydrolyse the fastest?

A

Iodoalkanes have the weakest bonds so they hydrolyse the fastest.

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28
Q

Chloroflurocarbons (CFCs) are well-known haloalkanes, what elements do they contain?

A

Only contain chlorine, fluorine and carbon - all the hydrogens have been replaced.

29
Q

Describe some of CFCs properties and what they were used for?

A

They’re very stable, volatile, non-flammable and non-toxic and were used a lot in fridges , aerosol cans, dry cleaning and air-conditioning - until scientists realised they were destroying the ozone layer.

30
Q

What does ozone do?

A

Ozone in the upper atmosphere acts as a chemical sunscreen. It absorbs a lot of the ultraviolet radiation which can cause sunburn or even skin cancer.

31
Q

How are the ‘holes’ in the ozone layer formed?

A

Formed from when the CFCs in the upper atmosphere absorb UV radiation and split to form chlorine free radicals. These free radicals catalyse the destruction of the ozone - destroying ozone molecules and then regenerating to destroy more ozone.

32
Q

Show the steps in the reaction of when chlorine free radicals are formed from CFCs breaking down by ultraviolet radiation?

A

CF2CI2(g) -> (UV) CF2CI(g) + CI(g)
Cl(g) + O3(g) -> O2(g) + CIO(g)
CIO(g) + O(g) -> O2(g) + CI(g)
Overall reaction - O3(g) + O(g) -> 2O2(g) , and Cl* is the catalyst.

33
Q

What is another type of radical that can act as a catalyst for the destruction of the ozone?

A

NO* free radicals from nitrogen oxides destroy ozone too.

34
Q

What are nitrogen oxides produced from?

A

Produced by car and air engines and thunderstorms.

35
Q

What is an alternative to CFCs that are used?

A

> HCFCs (hydrochlorofluorocarbons) and HFCs.

>Hydrocarbons.

36
Q

What are problems with using HCFCs and HFCs?

A

> Still damage the ozone layer but smaller than CFCs.
Broken down in the atmosphere in 10-20 years.
They are greenhouse gases and far worse than CO2.

37
Q

Why is the greenhouse effect important for life on earth?

A

Various gases in the atmosphere that contain C=O, C-H or O-H bonds are able to absorb infrared radiation (heat) and re-emit it in all directions, including back towards Earth keeping us warm.

38
Q

What are the main greenhouse gases?

A

Water vapour, carbon dioxide and methane.

39
Q

How has the changes in industrialisation increased the number of greenhouse gases in the atmosphere?

A

As the world has become more industrialised, more fossil fuels have been burned and deforestation this releases lots of CO2 and there is less forests to absorb the CO2.
>More food and cows release lots of methane

40
Q

How does the enhanced greenhouse effect cause global warming and what is the impact?

A

Higher concentrations of greenhouse gases mean more heat is being trapped and the Earth is getting warmer. Global warming is thought to be responsible for recent changes to climates like the melting of the polar ice caps and less predictable weather.

41
Q

What is infrared spectroscopy?

A

1) In infrared spectroscopy, a beam of radiation is passed through a sample of a chemical.
2) The IR radiation is absorbed by the covalent bonds in the molecules, increasing their vibrational energy.
3) Bonds between different atoms absorb different frequencies of IR radiation. Bonds in different places in a molecule absorb different frequencies too.
4) An infrared spectrometer produces a spectrum that shows you what frequencies of radiation the molecules are absorbing and you can use it to identify the functional groups in a molecule.

42
Q

What is another thing that infrared spectroscopy can show?

A

It means you can tell if a functional group has changed during a reaction. For example - if you oxidise an alcohol to an aldehyde you will see the O-H absorption disappear from the spectrum and a C=O absorption appear.

43
Q

What are other uses for infrared spectroscopy?

A

> Used in breathalysers to test if the driver is over the drink-limit. The amount of ethanol vapour in the drivers breath is found by measuring the intensity of the peak corresponding to the C-H bond in the IR spectrum and isn’t affected by water vapour in the breath.
Used to monitor the concentrations of polluting gases in the atmosphere including CO and NO in car emissions.

44
Q

How is a mass spectrum produced?

A

Produced by a mass spectrometer and the molecules in the sample are bombarded with electrons, which remove an electron from the molecule to form a molecular ion, M+.

45
Q

How do you find the relative molecular mass of a compound from a mass spectrum?

A

Look at the molecular ion peak (M peak). The mass/charge value of the molecular ion peak is the molecular mass.

46
Q

What can the molecular ions be broken into and what does it show?

A

The bombarding electrons make some of the molecular ions break up into fragments. The fragments that are ions show up on the mass spectrum, making a fragmentation pattern. Fragmentation patterns can be used to identify molecules and even their structure.

47
Q

What happens to the free radicals in the sample?

A

Only the ions show up on the mass spectrum - the free radicals are ‘lost’.

48
Q

How do you work out the structural formula of a molecular sample?

A

You’ve got to work out what ion could have made each peak from its m/z value.

49
Q

What fragment ion is an M peak or m/z value of 15?

A

CH3+

50
Q

What fragment ion is an M peak or m/z value of 29?

A

C2H5+

51
Q

What fragment ion is an M peak or m/z value of 17?

A

OH+

52
Q

What fragment ion is an M peak or m/z value of 43?

A

C3H7+

53
Q

Will a mass spectra differentiate between similar molecules which contain the same atoms?

A

Even if two different compounds contain the same atoms, you can still tell them apart with mass spectrometry because they won’t produce exactly the same set of fragments.

54
Q

How should you combine techniques to identify a compound?

A

1) Use the composition to work out the molecular formula of the compound.
2) Work out what functional groups are in the compound from its infrared spectrum.
3) Use the mass spectrum to work out the structure of the molecule.

55
Q

Describe how to do a reflux reaction?

A

1) The mixture’s heated in a flask fitted with a Liebig condenser which continuously boils, evaporates and condenses the vapours and recycles them back into the flask, giving them time to react.
4) The heating is usually electrical (hot plates, water baths, heating mantles) and avoids using naked flames that might ignite the compounds.

56
Q

Why is a reflux reaction helpful and what problems does it prevent?

A

Organic reactions are slow and the substances are usually flammable and volatile (low boiling points). If you stick them in a beaker and heat them with a Bunsen burner they’ll evaporate or catch fire before they have time to react.

57
Q

Describe the process of distillation?

A

1) Distillation works by gently heating the mixture in the apparatus and the substances will evaporate out of the mixture in order of increasing boiling point.
2) The thermometer shows the boiling point of the substance that is evaporating at any given time.
3) If you know the boiling point of your pure product, you can use the thermometer to tell you when it’s evaporating and therefore, when it’s condensing.

58
Q

How can volatile liquids be purified?

A

Redistillation.

59
Q

How does distillation and redistillation differ?

A

You’re heating an impure product in redistillation instead of the reaction mixture.

60
Q

What does the process of separation do?

A

Removes any water soluble impurities from the product.

61
Q

Describe the process of separation?

A

1) Once the reaction is completed, the mixture is poured into a separating funnel, and water is added.
2) The funnel is shaken and then allowed to settle, the organic layer is less dense than the aqueous layer so will float on top. Any water soluble impurities should have dissolved in the lower aqueous layer.
3) Then open the stopper on the separating funnel, run off the aqueous layer and collect your product.

62
Q

How do you remove traces of water that will be left in the organic layer of your product?

A

1) It has to be dried. So you add an anhydrous salt which is used as a drying agent and binds to water present to become hydrated.
2) It has to go from lumpy to smooth.
3) Then filter the mixture to remove the solid drying agent.

63
Q

What do you need to go from an alkane to a haloalkane?

A

A halogen (X2) and UV light.

64
Q

What conditions do you need when going from an alkene to an alkane?

A

You use a nickel catalyst, hydrogen and a temperature of 150 degrees.

65
Q

What temperature and substance do you need to go from an an alkene to a dihaloalkane?

A

A halogen (X2) and 20 degrees.

66
Q

When haloalkanes are hydrolysed to alcohols what else is needed?

A

Warm NaOH or KOH, H2O in reflux.

67
Q

When alcohols are dehydrated to become alkenes what other conditions are needed?

A

> A catalyst of either concentrated sulphuric acid or concentrated phosphoric acid.
Heat.

68
Q

When alcohols are produced from alkenes by steam hydration, what other conditions are needed?

A

> Steam.
Solid phosphoric acid catalyst.
300 degrees.
60-70 atm.

AS Chemistry (6 decks)

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