Paper 2 Flashcards
1
Q
Does bromine react more or less readily with phenol than benzene?
A
more readily.
2
Q
Does bromine react more or less readily with phenylamine than benzene?
A
more readily.
3
Q
Does bromine react more or less readily with nitrobenzene than benzene?
A
less readily.
4
Q
Why does phenol react more readily with benzene?
A
- a lone pair of electrons on oxygen is delocalised into phenol’s pi system.
- this increases the electron density of the pi system.
- this results in increased attraction between phenol and the electrophile.
5
Q
What are two examples of electron-donating groups?
A
- -OH
- NH2
6
Q
What are -OH and -NH2 groups known as?
A
- electron-donating groups
- 2,4-directing groups
7
Q
What is a -NO2 group know as?
A
- electron-withdrawing group
- 3-directing group
8
Q
Why does nitrobenzene react less readily with bromine than benzene?
A
because bromine is an electrophile, and want to go to areas of high electron density, but NO2 is an electron withdrawing group, so the N removes electrons from the pi system (lowering the electron density).
9
Q
Why do primary and secondary amides and amines have higher boiling points than tertiary amides and amines?
A
because they have hydrogen bonding (N-H bond), which is strong, so requires lots of energy to break.
10
Q
Why does the solubility of amines and amides decreases as their size increases?
A
because…
- a greater region of the molecule is non-polar.
- non-polar molecules are highly insoluble in water.
11
Q
Because amines have a lone pair of electrons, they can act as…
A
- nucleophiles.
- bases.
12
Q
An amine reacts with an acid to form a…
A
salt.
13
Q
What is a zwitterion?
A
a compound containing both negative and positive ions.
14
Q
What is homolytic fission?
A
when bonding elections are split evenly and each atom retains the same number of electrons.
15
Q
What is heterolytic fission?
A
when one atom receives more electrons than the other because the bond is broken unevenly (cation and anion).
16
Q
What are examples of good indicators for strong acid/base titrations?
A
- methyl orange
- phenolphthalein
17
Q
What colour precipitate will be formed when a carbonyl is reacted with 2,4-DNP?
A
yellow/orange
18
Q
What are 3 tests for aldehydes/ketones?
A
- Tollens’ reagent
- Fehling’s solution
- Benedict’s solution
19
Q
What is general formula?
A
an algebraic formula that can describe any member of a family of compounds (e.g: CnH2n+1OH).
20
Q
What is empirical formula?
A
the simplest whole-number ratio of atoms of each element in a compound (e.g: C4H10O).
21
Q
What is molecular formula?
A
the actual number of atoms of each element in a molecule (e.g: C4H10O).
22
Q
What is structural formula?
A
shows the arrangement of atoms carbon by carbon, with the attached hydrogens and functional groups. (e.g: CH3CH2CH2CH2OH).
23
Q
What is skeletal formula?
A
shows the bonds of the carbon skeleton only, with any functional groups.
24
Q
What is displayed formula?
A
shows how all the atoms are arranged, and all the bonds between them.
25
What is a homologous series?
organic compounds that have the same functional group and general formula (consecutive members differ by -CH2-).
26
What is a chain isomer and how does this affect its properties?
when the carbon skeleton is arranged differently
- similar chemical properties
- different physical properties (due to change in shape)
27
What is a positional isomer and how does this affect its properties?
skeleton and functional group could be the same, only the functional group is attached to a different carbon
- different chemical and physical properties
28
What is a functional group isomer and how does this affect its properties?
the same atoms can be arranged into different functional groups
- very different chemical and physical properties.
29
Why does an alkanes boiling point increase with chain length?
due to there being more surface contact and more electrons to interact, so there's a stronger induced dipole-dipole interaction (London forces).
30
Why do branched alkanes have lower boiling points than a straight-chain isomer?
branched alkanes can't pack as closely together and have smaller molecular surface areas, reducing dipole-dipole interactions.
31
What reactions occur when halogens react with alkanes?
- photochemical reactions (UV light)
- free-radical substitution reactions
32
What's the issue with free-radical substitution reactions?
can't control the products formed
- on different carbons in the chain
- mixture of products
33
What is a double bond made up of?
- sigma bonds (s-orbital overlap)
- pi bonds (adjacent p-orbital overlap)
34
What are electrophiles?
electron-pair acceptors
- positively charged ions (e.g: H+, NO2+)
- polar molecules
35
How do you test for C=C bonds?
shake the sample with orange bromine water
- decolourised solution means C=C is present (due to formation of a colourless dibromoalkane).
36
What are addition polymers made up from?
alkenes.
37
What are the advantages/disadvantages of polymers?
- unreactive (good, but also means aren't biodegradable).
- hard to dispose of
- environmental damage
38
How can waste plastics be reused?
- some can be melted and remoulded.
- some can be cracked into monomers, and used in organic feedstock to make more plastics or other chemicals.
39
How can waste plastics be burned?
burn to generate electricity
- waste gases from combustion must be passed through scrubbers which neutralise gases like HCl by reacting them with a base.
40
How do you make biodegradable polymers?
- made from renewable raw materials (e.g: starch or oil fractions) that are currently expensive.
- need certain conditions to decompose (can get photodegradable polymers).
41
What are nucleophiles?
electron pair donor
- negative ion (e.g: OH-, CN-)
- atom with a lone pair of electrons (e.g: NH3)
41
Why does solubility of alcohols decrease with increased size?
in larger alcohols, most of the molecule is a non-polar carbon chain, so there's less attraction for the polar H2O molecules.
42
How does reactivity of halogenoalkanes change in group 7?
reactivity of halogen increases down group 7
- Iodoalkanes have the weakest bonds, so they hydrolyse the fastest.
- Fluoroalkanes have the strongest bonds, so they hydrolyse the slowest.
43
What properties do CFCs have?
- stable
- volatile
- non-flammable
- non-toxic
44
Which two compounds break down ozone?
- Cl free radicals
- NO free radicals
45
What are alternatives for CFCs?
- HCFCs
- HFCs
- hydrocarbons
(don't contain chlorine so don't affect ozone, but are greenhouse gases)
46
What is the greenhouse effect?
when gases in the atmosphere absorb infrared radiation and re-emit it in all directions, warming the planet (e.g: water vapour, CO2, CH4).
47
How does IR spectroscopy work?
- IR radiation passed through chemical sample.
- absorbed by covalent bonds in molecules, increasing their vibrational energy.
- bonds between different atoms absorb different frequencies of IR radiation.
- spectrum is produced, so functional groups can be identified.
48
How does a molecular ion form (M+(g))?
molecules in a sample are bombarded with electrons, which remove an electron from the molecule.
49
How does enthalpy of hydrogenation support the delocalised model over Kekulé's model?
the enthalpy of hydrogenation is far less exothermic than expected by Kekulé's model
(-120kJmol-1, when the experimental value is -208kJmol-1).
- more energy must be put in to break the bonds in benzene than Kekulé.
- this indicated benzene is more stable due to the delocalised ring of electrons.
50
How does reacting benzene with bromine water prove the existence of the delocalised model?
because you'd expect the orange bromine to decolourise in Kekulé's model, due to the C=C bonds.
but, instead it's a struggle even with hot benzene and UV light. This is due to its delocalised electron rings that are unwilling to undergo addition reactions.
51
What are 3 types of halogen carrier?
- aluminium halides (e.g: AlCl3)
- iron halides (e.g: FeCl3)
- iron
52
How do halogen carriers help to make good electrophiles?
- it accepts a lone pair of electrons from a halogen atom on an electrophile.
- polarisation increases (and sometimes a carbocation forms) as the lone pair of electrons is pulled away.
53
What is Friedel-Crafts alkylation?
puts any alkyl group onto a benzene ring using a haloalkane and a halogen carrier.
54
What is Friedel-Crafts acylation?
substitutes an acyl group for a H atom on benzene
- you must reflux benzene with an acyl chloride instead of a chloroalkane.
- this produces phenylketones.
55
What temperature must be maintained if you only want mononitration (one NO2 group added)?
keep the temp below 55oC.
56
What happens when acyl chlorides react with water?
a vigorous reaction with cold water, producing a carboxylic acid.
57
What happens when acyl chlorides react with ammonia?
a violent reaction at room temperature, producing a primary amide.
58
What happens when acyl chlorides react with alcohols?
a vigorous reaction at room temperature, producing an ester.
59
What happens when acyl chlorides react with amines?
a violent reaction at room temperature, producing a secondary amide.
60
How do you make esters from alcohols and carboxylic acids?
- heat in the presence of an acid catalyst (usually conc. sulfuric acid (esterification reaction)).
- small esters: warm mixture and distil off the ester (as it's more volatile).
- larger esters: heat mixture under reflux and use fractional distillation to separate the ester.
61
How do you use acid hydrolysis to hydrolyse an ester?
acid hydrolysis splits the ester into a carboxylic acid and an alcohol.
- reflux the ester with dilute acid.
62
How do you use base hydrolysis to hydrolyse an ester?
reflux the ester with a dilute alkali.
- you get a carboxylate salt and an alcohol.
63
Why are amines bases?
because there's a lone pair of electrons on the nitrogen atom of an amine that's able to accept protons (H+ ions).
64
How are aliphatic amines made?
by heating a haloalkane with excess ethanolic ammonia.
65
How are aromatic amines made?
- heat a mixture of a nitro compound, tin metal, and conc. hydrochloric acid under reflux (produces a salt).
- add NaOH to get an aromatic amine.
66
Why do amides behave differently to amines?
because in amides, the carbonyl group pulls electrons away from the rest of the -CONH2 group.
67
What is produced when the carboxylic acid group in amino acids react with alkali?
form a conjugate base.
68
What is produced when the amino group in amino acids react with acid?
form a salt of the conjugate acid.
69
What are optical isomers?
mirror images that can't be superimposed about a chiral centre.
70
What is a chiral centre?
when an atom has 4 different groups attached to it.
71
How are condensation polymers formed?
when 2 types of monomer, each with at least 2 functional groups, link to lose a small molecule (often water)
(e.g: polyesters (ester link), polyamides (amide link))
72
How do you break down condensation polymers?
by adding water.
73
What do reactions between dicarboxylic acids and diamines make?
polyamides.
74
What do reactions between dicarboxylic acids and dioles make?
polyesters.
75
How do CN- ions react to create a new C-C bond?
- CN- contains a negatively charged C atom, so it's a nucleophile.
- it reacts with carbon centres that have a slight positive charge to create a new C-C bond (a nitrile is initially produced).
76
Cyanide ions (CN-) react with haloalkanes via what type of reaction?
nucleophilic substitution.
77
Cyanide ions (CN-) react with carbonyl compounds via what type of reaction?
nucleophilic addition.
78
How do you form an amine from a nitrile?
reduce a nitrile to a primary amine by:
- using lithium aluminium hydride (LiAlH4) - a strong reducing agent - followed by some dilute acid.
- with sodium metal and ethanol.
- in industry (as the above are too expensive), reduce using hydrogen gas with a metal catalyst at high temp and pressure (catalytic hydrogenation).
79
How do you form an amine from hydroxynitriles?
- using lithium aluminium hydride (LiAlH4) and some dilute acid.
- with sodium metal and ethanol.
80
How can you form carboxylic acids from nitriles or hydroxynitriles?
- reflux a nitrile in dilute HCl (hydrolysed to form a carboxylic acid)
81
Describe the process of recrystalisation?
1) very hot solvent is added to the impure solid until it just dissolves (don't add too much solvent!)
2) gives a saturated solution of the impure product.
3) solution is left to cool down slowly, forming crystals as it does, while impurities stay in the solution.
4) crystals are removed by filtration under reduced pressure and washed in ice-cold solvent and then dried.
82
Why is it so important to use an appropriate solvent for each particular recrystallised product?
it only works if the solid is very soluble in the hot solvent, and nearly insoluble in the cold solvent.
- if the product isn't soluble enough in hot solvent, it won't dissolve at all.
- if the product is too soluble in the cold solvent, most will stay in the solution even after cooling, when you filter it, you'll lose most of your product , giving a low yield.
83
What happens to melting and boiling point when the compound is impure?
- lowered melting point
- raised boiling point
84
Describe how to test for haloalkanes.
1) add 5 drops of unknown substance to a test tube.
2) add 1cm3 of ethanol and 1cm3 of aqueous silver nitrate.
3) place the test tube in a water bath to warm it
4) watch for a precipitate and observe the colour:
- chloroalkane = white
- bromoalkane = pale cream
- iodoalkane = pale yellow
85
Describe the two ways to test for phenol.
1) add 2cm3 of unknown substance to test tube.
2) add 1 small spatula of solid sodium hydroxide (or 2cm3 of solution).
- phenol (acid) present = dissolved solid, colourless solution of sodium salt will form.
OR
2) add 1 small spatula of solid/ 2cm3 solution of carbonate (e.g: sodium carbonate).
- phenol (base) present = nothing happens.
- strong acid = effervescence.
86
Describe how to test for carboxylic acids.
1) add 2cm3 of unknown solution to a test tube.
2) add 1 small spatula of solid (sodium) carbonate (or 2cm3 solution).
3) if solution begins to fizz, bubble the gas through limewater in a second test tube.
- carboxylic acid present = CO2 gas produced turns limewater cloudy.
87
Describe how to test for a carbonyl group using 2,4-DNP.
1) dissolve 0.2g of 2,4-DNP in 1cm3 of sulfuric acid, 2cm3 of water and 5cm3 ethanol.
2) in another test tube, add 5 drops of unknown substance to 2cm3 of the above solution.
3) shake the test tube and watch for a precipitate.
- aldehyde/ketone present = bright orange precipitate.
88
Describe how to test for aldehydes using Tollens' reagent.
1) put 2cm3 of 0.1moldm-3 silver nitrate solution in a test tube.
2) add 5 drops of 0.8moldm-3 sodium hydroxide solution (a light brown precipitate should form).
3) add drops of dilute ammonia solution until the brown precipitate dissolves completely.
4) place the test tube in a hot water bath and add 10 drops of aldehyde or ketone and wait a few mins.
- aldehyde = silver mirror forms.
- ketone = nothing happens.
89
What colour does acidified dichromate change to when there's a positive result for primary or secondary alcohols?
orange solution turns green.
90
How does thin-layer chromatography separate components?
by adsorption:
- how far an organic molecule travels up the plate depends on how strongly it's attracted to the layer of solid on the surface of the plate.
91
What does it mean, in terms of adsorption, when a substance doesn't travel far up the plate?
the substance is strongly absorbed, so moves slowly, and so won't travel as far (compared to one that's weakly absorbed).
92
Describe how gas chromatography works.
- the sample being analysed is injected into a stream of gas, which carries it through a coiled tube coated with a viscous liquid (e.g: oil) or solid.
- the components in the mixture constantly dissolve, evaporate back into the gas and then redissolve as they travel through the tube.
- time taken for the substances to pass through the coiled tube and reach the detector is recorded and used to identify the substance.
93
What is retention time?
the time taken for the substances to pass through the coiled tube and reach the detector is recorded (in gas chromatography).
94
What 3 factors affect retention times?
Solubility:
- determines how long a substance spends dissolved (i.e: highly soluble = spends more time dissolved, so takes longer to travel through tube).
Boiling point:
- substances with high boiling points spend more time condensed as a liquid in the tube than as a gas, so take longer to travel through the tube.
Temperature of the gas chromatography instrument:
- high temp means substances will spend more time evaporated in the gas and so will move along the tube quickly (shortens retention of all substances).
95
Describe how nuclear magnetic resonance (NMR) spectroscopy works.
1) sample is placed into a strong magnetic field and exposed to a range of different frequencies of radio waves.
2) the nuclei of certain atoms within the molecule absorb energy from the radio waves.
3) the amount of energy that a nucleus absorbs at each frequency depends of its environment.
4) patterns of absorptions give info about positions of certain atoms within the molecule, and how many atoms of that type the molecule contains.
5) work out the structure of a molecule.
96
The differences in absorption are measured relative to which standard substance and why?
Si(CH3)4 - tetramethylsilane (TMS)
- it produces a single absorption peak in both H NMR and C13 NMR (as all C and H nuclie are in the same environment).
- absorption peak is at a lower frequency than everything else.
97
What is chemical shift?
the difference in the radio frequency absorbed by the nuclei (C or H) in the molecule being analysed and that absorbed by the same nuclei in TMS.
98
Why are deuterated solvents used in proton NMR?
the deuterium nuclei can't absorb the radio wave energy, so they don't add peaks to the spectrum.
