6.1 Aromatic Compounds, Carbonyls And Acids Flashcards
What is a benzene derivative?
-a benzene ring that has undergone a substitution reaction
What is a substitution reaction?
-where a group or atom is exchanged for another group or atom in a chemical reaction
What is an ester?
—esters have the functional group -COO
-they are carboxylic acid derivatives where the H from the acid has been replaced with an alkyl chain
How are esters named?
-named after the alcohol and carboxylic acid from which they are formed
-the first part of the name comes from the alkyl chain with a carbon atom bonded to only one oxygen atom
-the second part comes from the alkyl chain containing the C=O carbon atom
-it is followed by the suffix -oate
Name this ester
-methyl ethanoate
What is esterification?
-the chemical reaction used to make an ester
Describe the conditions needed for the esterification of a carboxylic acid with an acid catalyst
-to prepare a small ester- alcohol and a carboxylic acid are heated gently in the presence of a sulfuric acid (H2SO4) catalyst,this is a reversible reaction and has a slow rate of reaction- separated by distillation as the ester is volatile, distillation needs to be quick to prevent reverse reaction from occurring
-to prepare larger esters-alcohol and carboxylic acid with sulfuric acid catalyst heated under reflux until equilibrium has been established-the ester can be separated using fractional distillation
->this method of preparation of an ester is not suitable for phenol or its derivatives as the rate of reaction is too slow
Draw out the reaction of methanol with ethanoic acid using their structural formulas and name the product
What are 3 properties of esters ?
-sweet smelling compounds used in food flavourings and perfumes
-low boiling points
-good solvents for polar molecule
What is an acid anhydride?
-an acid derivative
-more reactive than a similar carboxylic acid
-made by the removal of a water molecule from two carboxylic acid molecules
What does anhydride mean?
-without water
Describe esterification of acid anhydrides with alcohols
-acid anhydrides will react with alcohols including phenols and its derivatives to make an ester
-it is an addition-elimination mechanism
-not reversible -therefore has a higher yield than using carboxylic acid
-slow rate of reaction that can be increased by gently warming reaction mixture
Draw out the reaction of ethanoic anhydride and methanol and name the products
Define hydrolysis
-a chemical reaction where water causes the breaking of bonds. In a decomposition reaction
What is the name of the reverse reaction of esterification?
-hydrolysis
-esters can undergo hydrolysis to form an alcohol
What happens when an ester undergoes hydrolysis in acidic conditions?
-When esters are refluxed with a catalyst of hot aqueous acids, such as dilute sulfuric acid or dilute hydrochloric acid, the ester will decompose reversibly into an alcohol and a carboxylic acid.
What happens when an ester undergoes hydrolysis in alkaline conditions?
-Alkaline chemicals are bases that can dissolve in water: When an ester is refluxed with a hot aqueous alkali, such as potassium hydroxide or sodium hydroxide, it will decompose into an alcohol and a carboxylate salt. This reaction is not reversible.
Alkaline hydrolysis of esters is used to make soaps, therefore it is also called saponification
Draw out the hydrolysis reaction between ethyl propanoate and sodium hydroxide name the products
Define a base
A chemical that will react with an acid
What is the carboxylic acid functional group?
-COOH
How do you name a carboxylic acid?
-remove the e from the end of the name of the alkane that has the same number of carbons in its longest carbon chain
-add the suffix -anoic acid
E.g-a carboxylic acid containing 4 carbon atoms would be called butanoic acid
How are carboxylic acids prepared?
- by the oxidation of primary alcohols or aldehydes under reflux
-acidified potassium dichromate (VI) used as the oxidising agent
Are carboxylic acids soluble?
-the -COOH functional group allows carboxylic acid molecules to form hydrogen bonds between each other
-they can also form hydrogen bonds with water meaning they are soluble in water
What happens to the solubility of carboxylic acids as their chain length increases?
-as chain length increases their solubility decreases
-as CH2 groups do not form hydrogen bonds with water as they are not polar
-so as more of the molecule becomes non-polar solubility will decrease
What type of acid is a carboxylic acid?
-a weak acid as they partially ionise in solution releasing the H+ ion from the carboxylic acid group forming a carboxylate ion
Do carboxylic acids react with metals?
-yes they will react to make hydrogen and a metal salt
-the name of the salt is generated from the acid
-the suffix of the acid changes from -oic acid to -oate
E.g- sodium+ethanoic acid —> sodium ethanoate+hydrogen
What can metal oxides be classified as?
-metal oxides react with acids and can therefore be classified as bases
What is made when carboxylic acids react with metal oxides?
-water and a metal salt
-e.g. magnesium oxide+methanoic acid —->magnesium methanoate+water
What will be made when a carboxylic acid reacts with a metal hydroxide?
-group one metal hydroxides are soluble bases they will react with carboxylic acids to make water and a metal salt
E.g.potassium hydroxide+propanoic acid—->potassium propanoate+ water
Do carboxylic acids react in neutralisation reactions?
-carboxylic acids are weak acids and therefore react with bases in a neutralisation reaction to produce a salt and water
What will be made when metal carbonates react with carboxylic acids?
-water , carbon dioxide and a metal salt
-they will also react with metal hydrogencarbonates to form salt, water and carbon dioxide
What is the acyl chloride functional group?
-COCl
What are acyl chlorides?
-carboxylic acid derivatives where the -OH of the acid group has been replaced with a chlorine atom
-very reactive due to very polar -COCl group
How do you name acyl chlorides?
-the longest chain of carbon atoms is the stem
-add the suffix -oyl chloride
How are acyl chlorides produced?
-add SOCl2 to a carboxylic acid
-the products are acyl chloride, sodium dioxide and hydrogen chloride
-separate the acyl chloride from the reaction mixture using distillation
Write out the equation for the formation of ethanoyl chloride
How are acyl chlorides used to form esters?
-acyl chlorides will react with alcohols to make an ester
-this method of ester production is not reversible so has a higher yield than using a carboxylic acid
Write out the chemical equation that shows the formation of ethyl ethanoate from an acyl chloride (ethanoyl chloride)
CH3COCl + CH3CH2OH—> CH3COOCH2CH3 +HCl
What do phenols react with to make an ester?
-phenols are not readily esterified by carboxylic acids
-to make an ester from phenols acyl chloride must be used
-this reaction is violent and produces corrosive fumes of HCl
How do small acyl chlorides react with water ?
-when a small acyl chloride is added to water it quickly hydrolyses to produce a carboxylic acid
-this is a very exothermic reaction and misty fumes of HCl are given off
Write down the equation for the reaction of ethanoyl chloride with water?
CH3COCl +H2O —> CH3COOH +HCl
How are primary amides produced from acyl chlorides?
-when an acyl chloride reacts with ammonia a primary amide is produced
-e.g. to produce ethanamide ethanoyl chloride is added to a concentrated ammonia solution
-this quickly produces a mixture of solid ammonium chloride and ethanamide-observed as white smoke
-some products remain in a colourless solution
Write down the reaction between ethanoyl chloride and ammonia
CH3COCl + 2NH3 —> CH3CONH2 +NH4Cl
What does an acyl chloride form when it reacts with a primary amide?
-a secondary amide (where the nitrogen has one hydrogen atom directly bonded to it) is produced as well as HCl
-the nitrogen atom also has 2 organic groups attached and it often called an N-substituted amide
What does ethanoyl chloride and react with etylamine to produce?
-ethanoyl chloride and a cold concentrated solution of ethylaminne react to produce a white solid compound of N-ethylamine
CH3COCl + CH3CH2NH2 —>CH3CONHCH2CH3 +HCl
What is a phenol?
-a class of aromatic compounds where a hydroxyl group is directly attached to the aromatic ring
-phenol is the first member of a type of aromatic compounds where a hydroxyl group is attached directly to an aromatic ring
-if the hydroxyl group is attached to an alkyl chain on the aromatic ring the compound is no longer a phenol derivative. It would be described as an AROMATIC ALCOHOL
What type of acid is a phenol?
-a weak acid that partially dissociates in water
What is the chemical equation of the reaction between a phenol and water?
What is the chemical equation between a phenol and sodium hydroxide?
What shows that phenols are weak acids?
-they are an acid as they react with bases
-they will undergo a neutralisation reaction with NaOH as it is a strong base
-they will not react with carbonates as carbonates are weak bases therefore phenols are weak acids
Why are phenols more reactive than benzene ?
-as the lone pair of electrons from the p-orbital of the oxygen of the hydroxyl group are donated into the benzene ring structure to the π-system
-the lone pair becomes part of the delocalisation this increases electron density of the ring making it more susceptible to attack from electrophiles
What is the directing effect?
-how a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution
What is the reaction of a phenol and bromine (bromination)?
What does it form ?
What is the equation ?
-phenol will undergo an electrophilic substitution reaction with bromine water at room temperature. It is a triple substitution
-the resulting product is a white precipitate of 2,4,6-tribromophenol -which smells like antiseptic
EQUATION
C6H5OH + 3Br2 —>C6H2Br3OH +3HBr
What is the reaction between dilute nitric acid and phenol (nitration)?
What is the equation?
What is formed?
-is an electrophilic substitution reaction, phenol will undergo a single substitution with dilute nitric acid (HNO3) at room temperature
EQUATION
C6H5OH +HNO3–>C6H4(NO2)OH +H2O
-the reaction forms a mixture of 2-nitrophenol and 4-nitrophenol
Why is nitration of phenol unlike nitration of benzene?
-nitration of phenol does not require concentrated nitric acid or a sulphuric acid catalyst this is unlike the nitration of benzene
What are two electron donating groups?
-OH and NH2
-these activate the benzene ring they push additional electrons into the π-system
Where do electron donating groups direct electropphiles?
-electron donating groups such as OH and NH2 direct electrons to substitute at the carbon 2 and carbon 4 positions
What directing effect is seen with electron withdrawing groups such as NO2?
-a 3-directing effect is seen when NO2 groups are directly attached to the aromatic ring
-as the nitro group withdraws electrons from the π- system and makes the rate of substitution highest on the third carbon atom
Why are directing effects important?
-as they can be used to predict positions in which substitutions will take place
-important in organic synthesis as it allows you to control the structure of the products to maximise the desired product
What are the general reaction steps in electrophilic substitution?
1-electrons above and below the plane of atoms in a benzene ring attract an electrophile
2-the electrophile accepts a pair of π electrons from the delocalised ring and makes a covalent bond (this is the rate-determining step)
3-a reactive intermediate is formed where the delocalised electrons have been disrupted
4-the unstable intermediate releases a H+ ion and the stable product is formed (this is the rapid step)
Draw the general electrophilic substitution mechanism
What is electrophilic substitution?
-a substitution reaction where an electrophile is attracted to an electron-rich atom or part of a molecule and a new covalent bond is formed by the electrophile accepting an electron pair
What is a reaction mechanism?
-a model with steps to explain and predict a chemical reaction
What is the catalyst and reagent in nitration of benzene?
How is the reaction carried out?
CATALYST- concentrated sulfuric acid
REAGENT-concentrated nitric acid
-the concentrated nitric acid and concentrated sulfuric acid are mixed together in a flask held in an ice bath. Then benzene
is added and a reflux condenser is set up, keeping the mixture at 50 C° to prevent further substitution reactions occurring
What is the electrophile in the nitration of benzene?
NO2+ ion
Write out the balanced chemical equation for the nitration of benzene
Draw out the mechanism
Include the equation for the generation of the electrophile and the regeneration of the catalyst
Equation for reaction-
C6H6 +HNO3 —>C6H5NO2 + H2O
Formation of electrophile-
HNO3+H2SO4–> NO2+ + HSO4- +H20
Regeneration of sulfuric acid catalyst
H+ + HSO4- —> H2SO4
What are the 3 different halogen carriers that can be used for the chlorination of benzene?
-AlCl3
-FeCl3
-Fe
What are the 3 halogen carriers that can be used for the bromination of benzene?
-AlBr3
-FeBr3
-Fe
What is the mechanism for the bromination of benzene?
What is the equation for the formation of an electrophile ?
What is the equation for the regeneration of the halogen carriers (catalyst?)
FORMATION OF ELECTROPHILE-
Br2 + FeBr3 —> Br+ + FeBr4-
REGENERATION OF HALOGEN CARRIER
FeBr4- + H+ —> HBr + FeBr3
Why unlike in alkenes does no reaction occur when benzene is shaking with bromine water?
-as benzene has a lower electron density between carbon atoms than an alkene
-when non-polar molecules approach the benzene ring there is not enough electron density between carbon atoms to induce a dipole and start a reaction
What is the general mechanism for a chloroalkane undergoing a friedel crafts reaction?
Where iron(III) chloride is the catalyst and R stands for any alkyl group
What are the conditions for acylation?
-reaction mixture held at 60 °C for 30mins under reflux
What is the general mechanism for acylation?
Show show regeneration of AlCl3 catalyst
And generation of catalyst
R=alkyl group
What is the experimental evidence for delocalised model of benzene instead of Kekulé model?
Resistance to reaction-you would expect Benzene to readily undergo addition reactions similar to alkenes. But benzene tends to undergo substitution of a hydrogen atom rather than addition reactions
Hydrogenation- hydrogenation is the addition of hydrogen to an unsaturated chemical. From enthalpy change of hydrogenation we see that benzene is 152kJmol-1 more energetically stable than predicted .Enthalpy of hydrogenation of benzene is significantly less exothermic than predicted
Bond lengths- X-ray diffraction techniques show that all 6 carbon bonds in benzene are 0.140 nm which is between the length of a C single bond and C double bond. Kekulé’s structure suggests that there should be 3 shorter carbon double bonds and 3 longer carbon single bonds however all 6 bonds are the same length
Compare Kekulé’s model of benzene to delocalised model
-Kekulé- benzene is a six-membered ring, with alternating single and double bonds between the carbon atoms
-delocalised-each of the six carbon atoms donates one electron from its p-orbital
-these electrons combine to form a ring of delocalised electrons above and below the plane of the molecule. All bonds in the ring are identical so are the same length
What is the 2,4-dinitrophenylhydrazine (2,4-DNPH) test for carbonyls?
-2,4-DNPH can be used as a qualitative test for the carbonyl functional group.
-When aldehydes and ketones are reacted with 2,4-DNPH a yellow, orange or reddish-orange precipitate will form; the exact colour depends on the identity of the compound.
- Alcohols and other molecules which don’t contain carbonyl groups do not produce a precipitate.
-Carbonyl compounds and their derivatives have sharp melting points, meaning they melt over a narrow range of temperatures. These compounds can have their melting points determined experimentally and their values compared to a databook to identify them.
-2,4-DNPH can be used to identify specific aldehydes/ketones by use of melting point data.
-The 2,4-DNPH is added to the compound so that a precipitate forms. The solid is then purified by recrystallisation.
-The melting point of the pure crystals formed can then be compared with the melting points of 2,4-dinitrophenylhydrazones of all the common aldehydes and ketones.
What is the Tollen’s test used for ?
Why do the results occur?
-Aldehydes can be identified using Tollen’s reagent.
-If Tollen’s reagent is added to an aldehyde, a silver mirror will form
-If a ketone is present the solution will remain colourless.
-Tollens reagent is ammoniacal silver nitrate. It oxidises the aldehyde to a carboxylic acid and reduces silver ions to silver in the presence of an aldehyde. The silver can be seen deposited on the edge of the test tube (silver mirror)
What are the reagents for the oxidation of an aldehyde to form a carboxylic acid?
-potassium dichromate - K2Cr2O7
-sulfuric acid- H2SO4
-reagents react in situ to form the oxidising species- Cr2O7 2- and H+
What is the balanced ionic equation for the oxidation of ethanal to ethanoic acid?
What is the simplified balanced symbol equation?
What is the general mechanism for the reduction of an aldehyde by nucleophiic addition ?
-sodium borohydride NaBH4 is a reducing agent the compound is made out of BH4- ion which acts as a source of hydride ions H-
-this is involved in nucleophilic addition reactions to form alcohols
What is the reaction mechanism of the reaction of hydrogen cyanide with propanone?
What is the balanced equation for the formation of a cyanide nucleophile from hydrogen cyanide?
HCN + H2O ⇌CN- + H3O+
(3marks)
Similarities
-overlap of p orbitals
- π bond/ring/system above and below
Differences
-Kekule has alternating π bonds
-delocalised has π ring system/ all p orbitals overlap
