Ch6.1 Aromatic compounds, carbonyls and acids Flashcards
What are the physical properties of benzene?
- colourless
- volatile
- liquid
Why is benzene produced in huge quantities?
- it has so many uses as a feedstock chemical
- it’s used to produce many things such as: pharmaceuticals, plastics, weedkillers and pesticides
State the 3 problems with Kekule’s structure of benzene
- the bromination problem
- the bond length problem
- the hydrogenation problem
Explain the bromination problem of Kekule’s structure of benzene
- if benzene contained C=C bonds it would decolourise bromine water, but it doesn’t
Explain the bond length problem of Kekule’s structure of benzene
- if benzene contained alternating C-C bonds and C=C bonds, then the bond lengths would also alternate 0.13, 0.15, 0.13, 0.15 etc. around the ring
- in fact, all the bonds have the same length (0.14nm)
Explain the hydrogenation problem of Kekule’s structure of benzene
- when cyclohexene reacts with hydrogen, the enthalpy change is -120kJ/mol
- if benzene had 3 C=C double bonds, hydrogenation should give an enthalpy change of -360kJ/mol
- in reality it is -208kJ/mol which is less exothermic then expected
Describe the modern understanding of benzene
- the pi electrons in each C=C bond delocalise to form 2 delocalised pi rings (this is what the circle in the hexagon represents when you draw benzene)
- benzene is very stable and unreactive
Why is benzene stable and unreactive?
- there are no regions of positive or negative charge for nucleophiles or electrophiles to attack
- even radicals can’t attack the ring
How are benzene compounds named if benzene is the dominant group?
- the stem is benzene and the prefix tells us which side groups are attached
How are compounds including benzene named if benzene isn’t the dominant group?
- the benzene group is a side group which we refer to as a phenyl
- the other part of the molecule that is dominant is the stem of the name
What are the 2 key reactions of benzene?
- nitration
- halogenation
Describe the very special conditions needed for nitration of benzene
- nitrating mixture must be made (mixture of concentrated nitric and sulphuric acids)
- we have to mix the acids in a flask which is sitting in an ice bath since the reaction between them is highly exothermic
- when we reflux benzene with the nitrating mixture, we need to keep it below 50 degrees C, if not then multiple substitutions occur and the mixture becomes explosive
Explain the roles of the different components in the nitrating mixture for the nitration of benzene
- nitric acid is the source of the nitro group (NO2)
- the sulphuric acid is a catalyst
How does the nitrating mixture change during the nitration of benzene?
- produces nitronium ions (NO2+)
HNO3 + H2SO4 –> NO2+ + HSO4- + H2O
- then regenerates acid catalyst
HSO4- + H+ –> H2SO4
Describe the nitration reaction of benzene mechanism
- the nitronium ion is an electrophile and it takes 2 [i electrons from the delocalised ring (full arrow from circle out to NO2+ ion)
- the C-H bond has to break (heterolytic fission) and the two bonding electrons restore the delocalised pi network (circle splits to horseshoe with + in the middle and an arrow from C-H bond to plus)
- (then benzene with an NO2 is drawn and a H+ is also produced) = these are the final products
Describe how halogenation of benzene is made possible
- halogens produce negative halide ions which will not act as electrophiles
- instead we have to make chloronium, fluoronium and bromonium ions (Cl+, F+, Br+)
How are chloronium, fluoronium and bromonium ions made?
- we mix the halogen with a halogen carrier
Which halogen carriers are used for which halogens?
- chlorine = AlCl3, FeCl3, Fe
- bromine= AlBr3, FeBr3, Fe
What are the 2 chemical equations that occur during the halogenation of benzene (use chlorine as the halogen)
Cl2 + FeCl3 –> Cl+ + FeCl4
(Cl+ is a very powerful electrophile)
FeCl4 + H+ –> FeCl3 + HCl
(the FeCl4 reacts with the H+ ions displaced from benzene to regenerate the halogen carrier)
How can organic groups be added to benzene rings?
- using Friedel-Crafts reactions
- eg, if we want to add an alkyl group to benzene, we can generate an electrophile using a halogen carrier and a haloalkane
What are the equations for generating an electrophile from a halogen carrier and a haloalkana?
CH3Cl + FeCl3 –> CH3+ + FeCl4-
H+ + FeCl4- –> HCl + FeCl3
CH3+ is the electrophile
What is another example of a Friedel-Crafts reaction other than alkylation?
acylation
What are the 2 equations that occur when preparing an electrophile for acylation of benzene?
RCClO + FeCl3 –> RCO+ + FeCl4-
H+ + FeCl4- –> HCl + FeCl3
What are phenols?
- aromatic compounds in which an OH group is attached directly to a benzene ring
Are phenols acidic or alkaline and why?
- phenols are weakly acidic because the H in OH groups is labile
- this means the OH bond breaks easily because of the proximity of the electronegative oxygen atom
What ions are formed when a phenol reacts with water?
- phenoxide ion
- oxonium ion
What can phenol react with due to it being a weak acid?
- alkalis such as sodium hydroxide
phenol + sodium hydroxide ⇌ sodium phenoxide + water
What are the chemical tests for phenols?
- turns blue litmus paper pink
- if iron (III) chloride solution is added to phenol it turns purple
- it won’t react with carbonates to make CO2 despite it being a weak acid
Are phenols more or less reactive than benzene?
- phenols are more reactive than benzene
Why is phenol more reactive than benzene?
- benzene has an even distribution of electrons and no regions of concentrated positive or negative charge
- phenol has the elctronegative oxygen which causes a dipole to form
- the lone pairs of the oxygen also become part of the delocalised network which increases the electron density
What does phenol’s higher electron density allow it to do differently to benzene?
- react with halogens
- it also often has multiple substitutions
What are carbonyl compounds?
- compounds with the C=O functional group
- aldehydes and ketones are both carbonyl compounds
Where is the carbonyl group located in aldehydes and ketones?
- in aldehydes, the group is at the end of the carbon chain
- in ketones, the group is somewhere in the middle of the carbon chain
How can aldehydes and ketones be made from alcohols?
OXIDATION
- primary alcohol to aldehyde to carboxylic acid
- secondary alcohol to ketone
- tertiary alcohol can’t be oxidised
What colour does the oxidising agent turn whilst oxidising alcohols and name the ions for these colours
orange to green
(Cr2O7)2- to Cr3+
oxidation number change:
+6 to +3 (reduced)
What are the chemical tests for carbonyl compounds?
- carbonyl compounds can be detected by using 2,4 DNPH
(gives an orange/red precipitate with an aldehyde or ketone) - aldehydes produce a silver mirror with Tollen’s reagent
- aldehydes give a red precipitate with Fehlings solution
What is Tollen’s reagent?
- mixture of silver nitrate and nitric acid
Are carboxylic acids weak or strong acids? Explain
- weak
- they undergo partial dissociation in the presence of water
Describe the Ka values of carboxylic acids
- they’re very low
carboxylic acid + metal
carboxylate salt + hydrogen
carboxylic acid + metal oxide
carboxylate salt + water
carboxylic acid + metal hydroxide
carboxylate salt + water
carboxylic acid + metal carbonate
carboxylate salt + water + carbon dioxide
Are carboxylic acids soluble in water? Explain the trend with chain length
- they are soluble in water
- solubility decreases with chain length because the alkane group begins to dominate over the COOH group
Is esterification fast or slow? How do we manage this?
- esterification is a very slow process
- for this reason, we use a concentrated sulphuric acid catalyst and reflux the mixture for half an hour
How can esters be separated?
- with distillation because they are very volatile
How can esters be separated back into alcohols and carboxylic acids?
- using hydrolysis
What are the 2 types of hydrolysis and how do they differ?
- acid and alkaline
- they form different products
Describe acid hydrolysis
- the acid doesn’t take part in the reaction
- the hydrolysis produces the original alcohol and carboxylic acid
- reaction is reversible
Describe alkaline hydrolysis
- the alkali (usually NaOH) is a reactant
- the process generates the original alcohol but the carboxylate salt of the carboxylic acid
- the process is not reversible
Why is alkaline hydrolysis sometimes referred to as saponification?
- alkaline hydrolysis of fats is used to make soap
- the Latin for soap is sapo
What kind of compounds are fats and oils and what is the difference between them?
- both esters (triglyceride esters)
- the only difference between them is their melting and boiling points
What do you get when you hydrolyse a fat or an oil?
- glycerol and fatty acids
- fatty acids are simply long-chain carboxylic acids
What kind of fatty acids are there?
- saturated, unsaturated or polyunsaturated
Where are saturated fatty acids found and what is their effect on the human body?
- lead to health problems such as heart disease and stroke since they lead to the buildup of fatty tissue
- this and cholesterol narrow arteries and therefore increase blood pressure
- they’re found in animal fats and dairy produce but also in palm oil
What do unsaturated fatty acids contain?
C=C bond
Why are reactions involving carboxylic acids slow and how can they be sped up?
- slow since carboxylic acids are weak
- we can speed the reactions up by using carboxylic acid derivatives
Order the 2 derivatives of a carboxylic acid with carboxylic acids from least reactive to most reactive
- carboxylic acid (least)
- acyl chloride
- acid anhydride (most)
What is the functional group of a carboxylic acid?
HO-C=O
What is the functional group of an acyl chloride?
Cl-C=O
What is the functional group of an acid anhydride?
O=C-O-C=O
What is produced when an acyl chloride reacts with alcohols or amines?
conc HCl
What is the effect on substitutions onto a benzene ring if an OH or NH2 is already attached to the ring?
- OH and NH2 are called activating groups
- they both increase the reactivity of the benzene ring by donating pairs of electrons into the delocalised structure
- this increases the electron density on the neighbouring carbon atoms as well as carbon 4
- as a result, substitution of side groups occurs more readily in these positions
What directing effect do OH and NH2 groups generate if they are directly attached to benzene before the substitutions take place?
2,4 directing effect
What is the effect on substitutions onto a benzene ring if an NO2 is already attached to the ring?
- nitrate groups withdraw electrons from the delocalised network in the benzene ring which deactivates it and reduces the electron density in the adjacent carbon atoms
- this encourages substitution onto carbon 3 and 5
What directing effect does an NO2 group generate if it is directly attached to benzene before the substitutions take place?
3,5 directing effect
How is a carboxylic acid turned into an acyl chloride?
- we need to react it with a molecule that’s rich in chlorine
- the standard reagent for this is SOCl2 but PCl3 and PCl5 also work
When turning a carboxylic acid to an acyl chloride, why is SOCl2 generally used over PCl3 or PCl5?
- PCl3 and PCl5 produce conc HCl in the reaction so SOCl2 is safer
What is the name of SOCl2?
thionyl chloride
When turning a carboxylic acid to an acyl chloride, why is SOCl2 a useful reagent?
it is liquid at room temp
