Module 6.1 - Aromatic Compounds, Carbonyls and Acids Flashcards
What is benzene?
- naturally occurring aromatic hydrocarbon. Stable ring structure with delocalised electrons
- C6H6 (empirical formula: CH)
- liquid at room temperature
Give one use of benzene.
Key ingredient in gasoline (which increases efficiency of car engine)
Describe Kekulé’s model of benzene.
- 6 membered ring with alternating double and single bonds
- discovered when one group was added to benzene, only one isomer was ever made but when 2 groups were added there were 3 isomers
What were the experimental evidence that didn’t support Kekulé’s model of benzene?
- benzene is resistant to addition reactions (unlike alkenes) (Kekulé tried to explain this saying double and single bonds alternate in very fast equilibrium)
- enthalpy of hydrogenation of benzene shows it’s more stable than predicted (can calculate enthalpy change of hydrogenation of cyclo-1,3,5-hexatriene, Kekulé model, but actual value is -208kJmol-1, 152kJmol-1 more energetically stable than predicted)
- all 6 carbon bonds in benzene are the same length (C-C bond length: 0.147nm, C=C bond length: 0.135nm, benzene: 0.140nm, Kekulé should have 3 longer and 3 shorter bond lengths, disproving Kekulé’s model)
Describe the delocalised structure of benzene.
- bezene has a delocalised structure, explaining all 3 pieces of evidence disproving Kekulé’s model
- each of the 6 C atoms donates one e- from p orbital
- e- combine to form a ring of e- above and below the plane of the molecule
- e- said to be delocalised as able to move freely within ring and don’t belong to a single atom meaning all bonds in the ring are identical
- enthalpy of hydrogenation is 152kJmol-1 more stable than expected w Kekulé as more energy needed to disrupt delocalisation so benzene is v stable and resistant to addition reactions
What is a substitution reaction?
Where a group or atom is exchanged for another group or atom in a chemical reaction
What is a benzene derivative?
A benzene ring that has undergone a substitution reaction
What is the prefix when a nitro functional group is substituted onto a molecule?
nitro-
functional group: -NO2
Name this molecule.
phenylethene
Aromatic ring isn’t main functional group so aromatic ring is a phenyl group as H atom removed. Alkene group used for addition polymerisation in this molecules
Describe the general mechanism for the electrophilic substitution on a benzene molecule.
- Electrons above and below plane of atoms in benzene ring attract electrophile
- Electrophile accepts a pair of π electrons from delocalised ring and makes a covalent bond. This is the slowest, rate determining step
- Reactive intermediate is formed where the delocalised electrons have been disrupted
- Unstable intermediate releases a H+ ion and the stable product has formed (v fast step)
Describe the nitration of benzene.
- electrophilic substitution reaction (H atom exchanged for a nitro group (-NO2)
- reagent: conc nitric acid, catalyst: conc sulfuric acid
- C6H6 + HNO3 –> C6H5NO2 + H2O
- initially, conc nitric acid and conc sulfuric acid mixed in a flask held in an ice bath. Benzene then added and reflux condenser set up, keeping mixture at 50°C to prevent further substitution occurring
- sulfuric acid needed to generate NO2+ electrophile from nitric acid. Sulfuric acid is regenerated after nitration so is catalyst
- HNO3 + H2SO4 –> NO2+ + HSO4- + H2O
- regeneration of catalyst: H+ + HSO4- –> H2SO4
Describe the halogenation of benzene.
- halogen carrier needed as ring too stable to react directly w halogens
- halogen carriers:
- chlorination: AlCl3/FeCl3/Fe (Fe forms iron halide in situ)
- bromination: AlCl3/FeCl3/Fe (Fe forms iron halide in situ)
- halogen carrier used to generate positive halogen ion
- e.g. Br2 + FeBr3 –> Br+ + FeBr4- (creates positive bromine ion to act as an electrophile. It’s generated in situ. Can then attack benzene ring)
- halogen carrier is catalyst and gets regenerated at end of halogenation as H+ from benzene ring forms HBr:
FeBr4- + H+ –> HBr + FeBr3
Compare how bezene reacts with bromine water compared to cyclohexene. What does this show?
- cyclohexene: π bonds break in an addition reaction
- bezene: no addition reaction even though electrons in benzene are delocalised in π system so benzene must have a lower electron density between carbon atoms than an alkene
- when non polar molecules e.g. bromine approach ring there’s not enough electron density between C atoms to induce dipole and start a reaction (also the case when attempting to substitute alkyl halides like haloalkanes). Using a halogen carrier means a stronger electrophile can be generated and alkylation can occur
What is a Friedel Crafts reaction?
A substitution reaction where hydrogen is exchanged for an alkyl or acyl chain. C-H bond broken and C-C bond formed
Describe the alkylation of benzene.
- haloalkanes e.g. chloromethane mixed w halogen carrier e.g. AlCl3
- anhydrous
- halogen carrier = catalyst and gets regenerated (Lewis acid also used as catalyst)
- reactive carbocation is made which undergoes electrophilic substitution within benzene ring
- multiple substitutions likely so mixture of products made. Products may be separated by fractional distillation or chromatography
- actual yield of substituted product can be improved by adding excess benzene
- mixture of products caused as each successive substitution makes delocalised π electrons more nucleophilic so more susceptible to electrophilic attack. Increase in reactivity due to alkyl chain donating electrons to aromatic ring
Describe the acylation of benzene.
- acyl chloride: RCOCl (very reactive)
- can be used in Friedel-Crafts reaction as halogen carrier to substitute a H atom
- anhydrous
- as carbonyl group withdraws electrons from aromatic ring, a less reactive ketone is made, so only one substitution can occur
- reaction mixture at 60°C for 30 minutes under reflux for reaction to occur
What is phenol?
Class of aromatic compounds where a hydroxyl group is directly attached to the aromatic ring
What is the difference between a phenol derivative and an aromatic alcohol?
- phenol: hydroxyl group attached directly to aromatic ring
- aromatic alcohol: hydroxyl group attached to alkyl chain attached to aromatic ring
Describe the acidity of phenol.
-weak acid that partially dissociates in water
C6H5OH + H2O ⇌ H3O+ + C6H5O-
-acid as reacts w strong bases e.g. NaOH but only weak acid as doesn’t react w carbonates. Phenol won’t react w weak bases e.g Na2CO3
Describe the reaction of phenol with strong bases.
-forms salt and water
C6H5OH + NaOH → C6H5O-Na+ + H2O
How does the reactivity of phenol compare to that of benzene?
- phenol more reactive than benzene
- p orbital electrons from oxygen of hydroxyl group adding to delocalised electrons of aromatic ring so π system of aromatic ring becomes more nucleophilic
- increase in electron density allows aromatic ring in phenol to be mroe susceptible to electrophilic attack as can induce dipole in no polar molecules
- e.g. can induce dipole in bromine to react w it
What is a directing effect?
How a functional group directly attached to an aromatic ring affects which carbon atoms are more likely to undergo substitution
Describe the bromination of phenol.
-undergoes a triple substitution reaction w bromine water at room temp
C6H5OH + 3Br2 –> C6H2Br3OH + 3HBr
-resulting product is a white ppt of 2,4,6-tribromophenol (smells of antiseptic)
Describe the nitration of phenol.
- dilute HNO3 to undergo single substitution at room tempt
- forms a mixture of 2-nitrophenol and 4-nitrophenol
C6H5OH + HNO3 –> C6H4(NO2)OH + H2O
- doesn’t require a H2SO4 catalyst
- when conc HNO3 used a triple substitution occurs, forming 2,4,6-trinitrophenol
How does a hydroxyl group (and other electron donating groups) on an aromatic ring affect the position of substitution?
- additional electrons in π system
- reactions mainly occur on 2 and 4 positions on ring (2- and 4-directing effect)
- hydroxyl group activates these carbon atoms so their rate of substitutions is faster than other positions
- effect more pronounced when NH2 group attached to aromatic ring
How does a nitro group (and other electron withdrawing) on an aromatic ring affect the position of substitution?
- 3-directing effect
- nitro groups withdraw electrons from π system making rate of substitution highest on 3rd C atom
What is a nucleophile?
A species attracted to an electron deficient part of a molecule where it donates a pair of electrons to make a new covalent bond
What is the ionic equation of the oxidation of ethanal to ethanoic acid?
3CH3CHO(l) + Cr2O72-(aq) + 8H+(aq) –> 2CH3COOH(aq) + 2Cr3+(aq) + 4H2O(l)
Describe the general nucelophilic addition reactions to carbonyls.
- carbonyls have a dipole over C=O functional group so susceptible to nucleophilic attack on δ+ C atom
- nucleophile donates lone pair of electrons to electron deficient carbon
- simultaneously, π bond in C=O breaks forming a reactive intermediate
- extra electron pair quickly donated to neighbouring hydrogen to form an alcohol group and the stable product
Describe the reduction of carbonyls by sodium tetrahybridoborate(III).
- NaBH4 is a reducing agent. Made of BH4- ion, a source of hydride ions, H-
- hydride ion involved in electrophilic addition and reduction of carbonyls to alcohols
- CH3COC3H7 + 2[H] –> CH3CH(OH)C3H7
- hydride ion attacks δ+ C atom and forms a bond, intermediate forms that reacts with an ethanol/water molecule, to form an organic product (alcohol)
Desccribe the reaction between hydrogen cyanide and carbonyls.
-HCN: weak acid that partially ionises in solution, forming a cyanide nucleophile with negative charge on carbon atom
HCN + H2O ⇌ CN- + H3O+
- other sources of CN- include NaCN
- cyanide ion can’t react directly w carbonyl compound but when reaction is acidified the carbonyl group becomes more reactive as polarity of C=O bond increases
- allows additional H atoms to be added, forming a hydroxynitrile (used in many industrial processes)
Describe the reaction between 2,4-dinitrophenylhydrazine and aldehydes/ketones as a test for the carbonyl functional group of aldehydes/ketones only.
- Brady’s reagent: mixture of methanol, sulfuric acid and 2,4-DNP
- when added to aldehyde/ketone a yellow/orange ppt of 2,4-dinitrophenylhydrazone derivative is seen
- no ppt observed w carboxylic acid or ester even though they have C=O
How would you identify the specific aldehyde/ketone after using Brady’s reagent to discover you have an aldehyde/ketone?
- 2,4-dinitrophenylhydrazone derivative ppt can be collected by filtration and purified using recrystallisation
- once dried, accurate melting point or pure product can be measured through experiment
- aldehyde/ketone identified by comparing melting point of 2,4-dinitrophenylhydrazone derivative ppt w a database
When working out the specific aldehyde/ketone, why is the melting point of the 2,4-dinitrophenylhydrazone derivative precipitate used rather than the aldehyde/ketone?
- experimentally different for different ketones as have v similar mp
- ketones of similar chain length have similar bp so difficult to distinguish between
- 2,4-dinitrophenylhydrazone derivatives have v different mp and bps
How do you make Tollens’ reagent and how do you use it to test whether you have an aldehyde/ketone?
Making:
- NaOH solution added to AgNO3 solution until brown ppt formed
- Dilute NH3 added drop wise until brown ppt redissolves
- used to distinguish between an aldehyde/ketone
- it’s a weak oxidising agent and can react w carbonyl functional group in aldehyde but not ketone
- when Tollens’ added to ketone, no reaction as ketones can’t be oxidised further
- when added to aldehyde a silver mirror is observed as redox reaction occurs between Tollens’ and aldehyde tro form carboxylic acid
- Ag+(aq) + e- → Ag(s)
- silver metal precipitated out appearing as silver mirror effect on inside of reaction vessel
Describe the solubility (physical property) of carboxylic acids.
- small carboxylic acids are v soluble in polar solvent e.g. water as H bonds can form between carboxylic acid functional group and water
- as hydrocarbon chain length increases, solubility decreases as only polar COOH functional group can form H bonds w water so as molecule becomes non polar the solubility decreases
Describe the partial ionisation of carboxylic acids.
- weak acid so only partially ionise to release H+ into solution from carboxylic acid group
- forms carboxylate ion
(-lone pair of electrons from O of H2O to H causing fission of COOH O-H bond)
-reactions of carboxylic acids happen at slower rate w a strong acid as pH is higher so conc of H+(aq) will be lower
Describe the reaction between carboxylic acids and metals.
- reactive w metals above H in the reactivity series
- form H2 and a metal salt
- name of salt generated from acid
e. g.
sodium + ethanoic acid → sodium ethanoate + hydrogen
2Na + 2CH2COOH → 2CH3COONa + H2
Describe the reaction between carboxylic acids and metal oxides.
- metal oxides classified as bases as react w acids
- forms metal salt and water
e. g.
magnesium oxide + methanoic acid → magnesium methanoate + water
MgO(s) + HCOOH(aq) → (HCOO)2Mg(aq) + H2O(l)
Describe the reaction between carboxylic acids and metal hydroxides.
- group 1 metal hydroxides are soluble bases that release OH-(aq)
- form water and metal salt
e. g.
potassium hydroxide + propanoic acid → potassium propanoate + water
KOH(aq) + CH3CH2COOH(aq) → CH3CH2COOK(aq) + H2O(l)
Describe the reaction between carboxylic acids and metal carbonates.
- metal carbonates are bases
- form water, carbon dioxide and a metal salt
e. g.
sodium carbonate + methanoic acid → sodium methanoate + carbon dioxide + water
Na2CO3(s) + 2HCOOH(aq) → 2HCOONa(aq) + CO2(g) + H2O(l)
-group 1 metals can also form metal hydrogencarbonates where carbonic acid only had 1 proton exchanged to form metal hydrogencarbonate (MHCO3 where M is group 1 metal). Acids react with metal hydrogencarbonates forming salt, water and carbon dioxide
Name this ester.
propyl butanoate
Describe esterification from carboxylic acids and alcohols.
- alcohol and carboxylic acid heated gently in presence of H2SO4 catalyst
- reversible reaction and has a slow rate of reaction
- ester separated from reaction mixture by distillation as it’s volatile w lowest bp of the chemicals. Separation has to happen quickly to prevent reverse reaction occurring
- for larger esters: reaction mixture needs to be heated under reflux until equilibrium is established. Ester can be separated using fractional distillation
- this method of ester prep not suitable for phenols/its derivatives as reaction rate is so low
Describe esterification from an acid anhydride and an alcohol.
- acid anhydride: acid derivative more reative than a similar carboxylic acid, made from removal of water from 2 carboxylic acid molecules
- acid anhydrides react w alcohols, including phenol/its derivatives making an ester
- not reversible so higher yield than carboxylic acid
- rate of reaction still slow but can be increased by gently warming the reaction mixture
Describe the hydrolysis of esters in acidic conditions.
- refluxed w a catalyst of hot aqueous acids e.g. dilute H2SO4 or dilute HCl as source of H+(aq)
- ester reversibly decomposes to an alcohol and carboxylic acid
Describe the hydrolysis of esters in alkaline conditions.
- alkaline chemicals: bases that can dissolve in water
- ester refluxed w hot aqueous alkali e.g. KOH(aq)/NaOH(aq)
- decomposes to carboxylate salt and an alcohol
- not reversible
- alkaline hydrolysis of esters used to make soaps so it’s also called saponification
Name this acyl chloride.
butanoyl chloride
How can you prepare an acyl chloride?
- OH group on carboxylic acid must be substituted for chlorine atom
- SOCl2 (thionyl chloride), liquid at room temp and reacts readily w carboxylic acids to form acyl chlorides
- SO2 and hydrogen chloride gases made
- acyl chloride separated from mixture by distillation
e. g.
CH3COOH + SOCl2 → CH3COCl + SO2 + HCl
How can acyl chlorides be used to make esters?
- react w alcohols
- not reversible so has a higher yield than w carboxylic acid
e. g.
CH3COCl + CH3CH2OH → CH3COOCH2CH3 + HCl
-can be used to make esters from phenols but reaction is violent and produces corrosive fumes of HCl
How can acyl chlorides be used to make carboxylic acids?
- small acyl chlorides (e.g. ethanoyl chloride) is added to water, quickly hydrolyses to produce a carboxylic acid
- v exothermic reaction and misty fumes of HCl given off
CH3COCl + H2O → CH3COOH + HCl
How can acyl chlorides be used to make primary amides?
- primary amides made when acyl chlorides react w ammonia
- ethanamide: ethanoyl chloride added to conc ammonia solution. Quickly produces a mixture of solid ammonium chloride and ethanamide observed as white smoke
- some of the products remain in a colourless solution
CH3COCl + 2NH3 → CH3CONH2 + NH4Cl
How can acyl chlorides be used to make secondary amides?
- formed when acyl chlorides react w primary amides
- N has 1 H directly bonded to it
- N also has 2 organic groups attached (often called an N-substituted amide)
- white solid compound of N-ethylethanamide can be made from ethanoyl chloride and cold conc solution of ethylamine
CH3COCl + CH3CH2NH2 → CH3CONHCH2CH3 + HCl
