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chemistry a level > aromatics > Flashcards

aromatics Flashcards

1
Q

what is the molecular structure of benzene

A

C6H6

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2
Q

What does Kekule’s structure of benzene suggest

A

it suggests that it is a cyclo structure with alternating double and single bonds

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3
Q

what are the problems with Kekule’s structure

A
  1. does not undergo an addition reaction with bromine even though the model suggests that it should ( it has double bonds)
  2. the carbon-carbon bond lengths are all the same length
  3. the enthalpy of hydrogenation value is less exothermic than expected
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4
Q

the first issue with Kekule’s structure

A

alkenes undergo addition reactions therefore if we react Kekule’s structure with bromine solution, it would go colourless

however, if we carry this reaction in a lab, nothing happens

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5
Q

why does benzene not undergo addition reaction

A

benzene has delocalised electrons, so it has a lower electron density than alkenes as the electrons spread themselves out

and so, it does not induce a strong dipole in BR2
Therefore, bromine becomes a poor electrophile and the two can’t undergo an addition reaction

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6
Q

the second issue with Kekule’s structure

A

if benzene has alternating C-C and C=C bonds then we would expect benzene to have different lengths as the double bonds are expected to be shorter than the single bonds

this is because the p-orbitals overlap by the same amount

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7
Q

what is the third issue with Kekule’s structure

A

you would expect that the overall enthalpy of hydration would be -360

so overall the reaction is exothermic

if we carried out this reaction in a lab, the enthalpy for hydrogenation is -208 which is less exothermic than expected

therefore Kekule’s structure can’t be correct

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8
Q

what is the enthalpy of hydrogenation

A

when 1 mole of a compound reacts duly with an excess of hydrogen

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9
Q

explain the bonding in benzene

A

each carbon dorms three covalent bonds (3 sigma bonds)

the pi orbitals overlap with 2 other neighbouring p orbitals forming a pi-system

the electrons in this p- system are delocalised

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10
Q

explain why the enthalpy of hydrogenation is more endothermic/ less exothermic than expected

A

benzene has a pi- system of electrons which are delocalised across the ring

these delocalised electrons stabilise the molecule

this makes the enthalpy of hydrogenation less exothermic than expected due to the extra energy needed to disrupt the pi-system

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11
Q

what makes a compound stable

A

if a compound has alternating c-c and c=c bonds, it has delocalised electrons

these delocalised electrons make the compound more stable

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12
Q

why do delocalised electrons make the compound more stable

A

as the electrons are more spread out, the electron density decreases therefore, it is less likely to attract other molecules

therefore, when a compound has delocalised electrons, its stability increases

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13
Q

what are aromatics

A

when a compound has 6 carbons in a ring and the p orbitals overlap to form a pi system, we say that compound contains a benzene ring which is an aromatic compound

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14
Q

describe the shape of the benzene

A
  1. planar
  2. the bond angles in benzene are 120
  3. c-c bonds are equal in length
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15
Q

what are important things to remember when naming benzene

A
  1. when benzene is the highest priority group, the suffix is benzene and the prefix is whatever group is attached to it
    e. g. ethylbenzene
  2. if benzene is the lowest priority group, the prefix is phenyl and the suffix is whatever group is attached to it

phenylpropanoic acid

  1. the benzene is never included in the longest chain
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16
Q

what is electrophilic substitution

A

reacting benzene with Br2 under a different set of conditions the two can react together

in which bromine substitutes for a hydrogen atom

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17
Q

what are the conditions for electrophilic substitution

A

conditions for this reaction:
1. we use a hydrogen carrier e.g. AlBr3 which is electron deficient as it has 6 electrons in its outer shell

so it will react with Br2 like this:
Br2 +AlBr3 - Br+ + AlBr4-

this Br+ cation is stronger electrophile - strong enough to react with benzene

all hydrogen carriers act in the same way

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18
Q

how is the catalyst reformed at the end

A

AlBr4- + H+ - HBr + AlBr3

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19
Q

why does benzene undergo an electrophilic substitution reaction

A
  1. compared to Br2 (like in electrophilic addition) Br+ much stronger electrophile so benzene can react with it
  2. the ring is only temporarily broken as it reforms at the end of the reaction
    The molecule, therefore, keeps the benzene ring and so it keeps its extra stability
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20
Q

what is Friedel crafts acylation

A

this is substituting acyl groups onto benzene rings e.g. acyl chlorides

the acyl group substitutes for a hydrogen when we add an acyl group to a compound it is called acylation

This is the same mechanism for electrophilic substitution

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21
Q

what are the conditions for Friedel crafts acylation

A

on their own, chlorides are not strongenough to react with benzene

to make them strong enough to react we need to use a hydrogen carrier e.g. AlCl3 (this is the same reaction with bromine)

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22
Q

what is the reaction for the regeneration of the electrophile

A

RCO+ + AlCl4-

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23
Q

what is the overall reaction for friedal craft acylation

A

C6H5COR +HCL

the alumium chloride is reagenrated

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24
Q

what is nitration

A

benzene + nitric acid - nitrobenzene + water

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25
what must happen before nitration can occur
we need to form the electrophile which is a strong acidic catalyst in this case it is H2SO4 catalyst the electrophile is NO2+ generating the electrophile: -HNO3 +H2SO4 - NO2+ + HSO4 + H2O
26
what are the reagents for nitration
nitric acid and sulfuric acid needs to be concentrated so we can get as many molecules as possible
27
why must the reagent be concentrated
the mechanism for the generation of the electrophile is reversible therefore if our reaction contains a lot of water, the equilibrium will move to the left and we will end up with a lower yield of the electrophile this is the reason why we need to keep out water in all of our electrophilic substitutions reactions - the electrophile reacts with water as it is very reactive
28
how do we test for carboxylic acids
we react with a carbonate e.g. sodium carbonate a positive test will produce bubbles due to the CO2 gas provided
29
what are the properties of carboxylic acids
completely soluble in water higher melting points
30
why are carboxylic acids soluble in water
they can form hydrogen bonds with water molecules
31
why do carboxylic acids have higher melting points
they can form hydrogen bonds with each other they then will be considered to be a carboxylic dimer which is twice as big as a single carboxylic acid therefore, carboxylic acids have greater melting points as they can former dimers
32
why are carboxylic acids stronger/more stable than alcohols
the carboxylate ion has an extra electron which is delocalised over 3 atoms making the ion more stable when it releases a hydrogen
33
what are the uses of esters
1. plasticizers 2. solvents 3. perfumes
34
how are esters formed
alcohol + carboxylic acid - ester + water
35
what are the conditions required for esters to form
H2SO4 is needed as a catalyst- acidic catalyst the acid has to be concentrated The reaction also occurs under reflux
36
how do we name esters
the name reflects the acid and the alcohol that it is formed carboxylic acid has the highest priority so it acts as the main carbon group
37
the hydrolysis of ester in acidic conditions
ester + water - carboxylic acid + alcohol this occurs in the presence of a strong acid catalyst/acidic condition this is a reversible reaction
38
hydrolysis of esters in alkaline conditions
it is non-reversible and forms a salt and an alcohol
39
what is the IUPAC name for glycerol
propane,1-2-3 tiol it can form many hydrogen bonds as it has 3 alcohol groups therefore it is very soluble in water
40
what are the uses of glycerol
1. moisturizing agent 2. plasiticsizer 3. solvent
41
how do we create biofuel
we react the fat/oil with methanol in the presence of a strong alkali as a catalyst this produces methyl ester and glycerol. It is the methyl ester that is the biodiedel
42
what are carboxylic derivatives
esters are formed from carboxylic acids therefore, they are called acid derivatives
43
what are acyl groups
an acyl group is an o=c bond
44
what is the general formula for acyl chlorides
RCOCL
45
how do we convert carboxylic acids into acyl chlorides
we need to use sulfurous dichloride SOCL2 the products are an acyl chloride sulfur dioxide hydrogen chloride gas the reaction is non-reversible
46
how do we name an acyl chloride
ethATHOYL chloride
47
what are acyl anhydrides
it is another carboxylic acid derivative we simply replace the acid with anhydride e.g. ethanoic anydride
48
What is the general formula for acid anhydrides
RCOOCOR or (HCO)2O
49
how do acid anhydrides and acyl chloride react in water
they both react to dorm carboxylic acids they were derived from acyl chlorides react the most vigorously with wat acid anhydrides: 2 carboxylic acids acyl chlorides: a carboxylic acid and hydrogen chloride
50
how do alcohol react with acyl chlorides react
this is another way to form an ester we can't do this reaction in an aqueous solution as acyl chloride react rapidly in water
51
how do anhydrides react
1. acid anhydrides + alchol - ester + carboxylic acid 2. acid anhydride + water - carboxylic acid 3. acid anhydride + ammonia - amide + ethanoic acid/ in excess of ammonia an ethanoate salt
52
what is an optical isomer
optical isomerism occurs in carbon compounds with 4 different groups of atoms attached to carbon to a carbon (called an asymmetric carbon the four different groups are arranged tetrahedrally around the carbon this causes two different isomers that are non-superimposable mirror images
53
what happens when we mix two enantiomers together
a mixture containing a 50/50 mixture of the isomers is described as a racemate or racemic mixture this mixture will not rotate plane polarised light at all
54
what are the chemical and physical properties of optical isomers
optical isomers have similar physical and chemical properties but they rotate plane polarised light in different directions one enantiomer rotates plane polarised light in one way and the other rotates it in the opposite way
55
how are racemates formed
a racemate will be formed in a reaction mechanism when a reactant or intermediate has a trigonal planar group in the molecule is approached from both sides by an attacking species There is an rqual chances of either enatiomer forming so a racemate forms
56
how is drug action determined by optical isomers
drug action may be determined by the steriochemisrty of the molecules Different optical isomers may have very different effects One enatiomer of thalidomide causes birth detects in unborn children whilst the other had useful sedative problems Unfortunately it was given in a racemic mixture when first used
57
what are carbonyls
carbonyls are compounds with a C=O they can be either aldehydes or ketones
58
where is the C=O in the ketones
it is attached in the middle the name will end with a one Ketones do not like to be on their own so they are in the middle
59
where is the C=O in the aldehydes
the C=O is on the end of the chain the name will end in -al Aldehydes like to hide and so they are at the end
60
reactions of carbonyls
the C=O bond is polarisd because O is more electronegative than carbon The positive carbon atom attracts nucleophiles
61
describe the intermolecular forces in carbonyls
pure carbonyls cannot form hydrogen bonds themselves but they are attracted instead by permanent dipole forces
62
what is the solubility of carbonyls in water
the smaller the carbonyls are soluble in water because they can form hydrogen bonds with water
63
which is stronger, the C=C bonds in alkenes or the C=O bond in carbonyls
in comparison to the C=C bond is alkenes, the C=O is stronger and does not undergo addition reaction easily
64
what happens when aldehydes and alcohols are oxidised
potassium dichromate K2CR2O7 is an oxidising agent that can causes alcohol and aldehydes to oxidise e.g. Primary alcohol - aldehydes - carboxylic acids Secondary alcohols - ketones tertiary alcohols - so not oxidise
65
what is the reagents needed for the reduction of carbonyls
NaBH4 in aqueous ethanol the conditions need to be room temperature and pressure
66
can ketones be oxidised
no
67
what happens when aldehydes are reduced
it forms an alcohol
68
what happens when ketones are reduced
ketones will be reduced to secondary alcohols
69
what is the mechanism that occurs during the reduction of an aldehyde/ ketone
nucleophillic addition mechanism
70
how else cancarbonyls be reduced
carbonyls can be reduced using catalytic hydrogenation the reagent is hydrogen and nickel catalyst under high pressure
71
what are the conditions for the addition of hydrogen cyanide to carbonyls to form hydroxycynitriles
Reagent: sodium cyanide (NaCN) and dilute sulfuric acid Conditions: Room temperature Mechanism: nucleophilic addition
72
why were the reagents needed
the NaCN supplies the nucleophilic CN- ions The H2SO4 acid supplies H+ ions needed in the second step of the mechanism another adavantage of using KCN or NaCN is that there will be higher concentration of the CN- opm as these compounds will completely ionise HCN is a weak acid and will only partially ionise (HCN also leads to the production of toxic gas that is difficult to contain - KCN/NaCN are still toxic because of the cyanide ion)
73
what happens when nucleophillic addition occurs with HCN to aldehydes and ketones (unsymmetical)
when the trigonal planar carbonly is approached from both sides by the HCN attacking species results in the formation of a racemate This is because there is an equal chance of either enatiomers forming so a racemate forms No optical activityis seen
74
how are carboxylic acids stabilised
the carboxylic acid salts are stabalised by delocalisation while makes dissociation more likely
75
describe the strength of carboxylic acids
upon delocalisation increasing the chain length pushes electron density on to the COO- ion making it more negative and less stable This makes the acid less strong e.g. propanoic acid is less acidic than ethanoic acid electronegative chlorine atoms withdraw electron density from the COO- ion making it less negative and more stable This makes the acid more strong e.g. chloroethanoic acid is more acidic than ehtnaoic acid
76
describe the oxidation of methanoic acid
carboxylic acids cannot be oxidised by using oxidating agents but methanoic acid is an exception as it has a structure that is effectively an aldehyde it forms a carbonic acid (H2CO2) whci can decompose to give CO2
77
what is esterification
a carboxylic acid reacts with alcohols, in the presence of a strong acid catalyst, to form esters and water the reaction is reversible - it is quite slow and needs heating under refluc low yields are achieved An acid catalyst (H2SO4) is needed
78
how do we make aspirin
1. dissolve the impure componud in a minimum volume of hot (near boiling) solvent 2. Hot filter solution through (fluted) filter paper quickly 3. Cool the filtered solution by inserting beaker in ice 4. suction filtrate with a Buchner flask to separate out crystals 5. wash the crystals with distilled water 6. dry the crystals between absorbent paper
79
why did step 1 happen
an appropritate solovent is one which will dissolve both compound and impurities when hot and one in which the compound itself does not dissolve well when cold The minimum volume us used to obtain saturated solution and to enable crytallisation on cooling
80
why did step 2 happen
this step will remove any insoluble impurities and heat will prevent crystals reforming during filtration
81
why did step 3 happen
crystals will reform but soluble impurities will remain in solution form because they are presented in small quantities so solution is not saturated Ice will increase the yeild of crystals
82
why did step 4 happen
the water pump connected to the Buchner flask reduces the pressure and speeds up the filtration
83
why did step 5 happen
to remove soluble impurities
84
how does an acyl chloride react with water
acyl chlorides react vigorously with cold water, producing carboxylic acid and hydrogen chloride
85
how do acyl chlorides react with alcohol
acyl chlorides react with alcohols at room temp vigorously forming an ester and hydrogen chloride
86
how do acyl chlorides react with ammonia
acyl chlorides react with ammonia vigorously at room temp to produce an amide and you guessed it hydrogen chloride
87
how do acyl chlorides react with a primary amine
acyl chlorides react vigorously with primary amines at room temp producing an N substituted amide
88
what is the mechanism for nucleophilic addition elimination
89
what are animal fats and vegetable oils
they are ***_esters***_ of _***glycerol_*** and ***_fatty acids_***
90
what are fatty acids
fatty acids are long chain carboxylic acids. They combine with glycerol (which is propane-1,2,3,-triol and alcohol) to make fats and oils
91
what are fats and what are the properties of fats
fats mainly are saturated hydrocarbons chains - they fit together increasing the van der Waal forces between them This means higher temperatures are needed to melt them and so they are solid at room temp
92
what are oils and what are the properties of oils
oils have unsaturated hydrocarbons chains - the double bonds mean that the chains are bent and so they don't pack together well, decreasing the effect of the van Der Waals forced Therefore they easily melt and are liquids at room temperature
93
what happens when you hydrolyse a fatty acid/ oil
in a base hydrolysis, you form a carboxylate ion and the alcohol formed is glycerol If you want you can convert the sodium salt back into a long chain carboxylic acid (fatty acid) by adding an acid such as HCL The H+ ions from the acid displayed the Na+ ions in the salt to form a carboxylic acid, which releases a free Na+ ion.

chemistry a level (24 decks)

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