6.1 Aromatic Compounds, Carbonyls and Acids

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  • Created by: elbungay1
  • Created on: 30-03-19 16:08
What are the three problems with Kekule's Model
1. Unlike alkenes, benzene is resistant to addition reactions. 2. Enthalpy of hydrogenation of benzene shows that benzene is much more stable than predicted. 3. All six carbon bonds in benzene are the same length
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Describe the delocalised structure of benzene
Each of the six carbon atoms donates one electron from its p-orbital. these combine to form a ring of delocalised electrons above and below the plane. All bonds in the ring are identical, same length
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Describe the electrophilic substitution of benzene
Electrons above and below the plane of atoms attract an electrophile. Electrophile accepts a pair of π-electrons and makes a covalent bond (rate-determining step). Reactive intermediate forms which releases a H+ to form stable product
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Describe the Nitration of Benzene
C6H6 + HNO3 = C6H5NO2 + H2O. Requires sulfuric acid to generate NO2+ electrophile. Forms Nitrobenzene
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Describe the Halogenation of Benzene
Diatomic halogen reacts with halogen carrier to form X+ electrophile. Halogen carrier acts as catalyst. e.g. Br2 + FeBr3 = Br+ + FeBr-.
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Why can't benzene go under halogenation without a halogen carrier?
Benzene has a lower electron density between carbons than molecules such as alkene. Because electrons are delocalised across all six carbons.
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Describe Friedel-Crafts acylation
Acyl Chloride reacts with halogen carrier forming electrophile. Carbonyl group added to benzene ring forming a less reactive ketone, this is because carbonyl groups are electron withdrawing.
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Describe the acidity of Phenols
Phenol is a weak acid as it partially dissociates in water. Will react with strong bases (NaOH) to form a salt and water. C6H5OH + NaOH = C6H5O-Na+ + H2O. Does not react with carbonates making it a weak acid.
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Describe the reactivity of phenol compared to benzene
Phenol is more reactive than benzene as the p-orbital electrons from the oxygen from the hydroxyl group delocalised to the delocalised π-system. Creates an area of higher electron density and π-system, more nucleophilic, more electrophilic attacks
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Describe the bromination of Phenol
Undergoes a triple substitution with bromine water a room temperature. Forms white precipitate of 2,4,6-tribromophenol.
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What is the difference between Nitration of Benzene and Nitration of Phenol
Phenol will undergo single substitution with dilute nitric acid at room temperature, without the need of sulfuric acid catalyst. Phenol will undergo triple substitution if concentrated HNO3 is used.
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How does the position of substitution change between aromatic compounds
In phenol, hydroxyl group donates electrons to the π-system and has a 2- and 4- directing effect. NO2 groups are electron withdrawing and has a 3-directing effect.
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Describe the Oxidation of Aldehydes
Will undergo oxidation to form a carboxylic acid. reagents are acidified potassium dichromate, K2Cr2O7 and sulfuric acid. Colour change from orange to green. Heated under reflux
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What is NaBH4 and what is its use?
Reducing agent and a source of Hydride Ions, H-. Involved in electrophilic addition reaction and reduction of carbonyl compounds to alcohols.
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What is the importance of cyanide in organic synthesis?
HCN and NaCN are sources of cyanide, CN-, which undergoes nucelophilic addition to the polar C=O bond. Addition of cyanide allows further carbon atoms to be added to the organic molecule.
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Describe a test for Carbonyl functional groups
When Brady's reagent, 2,4-DNP is added to an aldehyde or ketone, an orange precipiate of the 2,4-DNP derivative is seen. No precipitation is observed with a carboxylic acid or ester, despite C=O bond.
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How would you identify a specific aldehyde or ketone
The 2,4-DNP derivative can be collected and purified by recrystallisation. The accurate melting point can be experimentally determined. The melting point can be compared to a database to identify compound. However, similar compounds have similar MPs.
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How can you test for an aldehyde?
When Tollen's reagent is added to an aldehyde a silver mirror is observed. This is because Tollen's is a weak oxidising agent and can react with aldehyde but not with a ketone. The aldehyde functional group is oxidised to a carboxylic acid.
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Describe the solubility of carboxylic acids
Small carboxylic acids are very soluble in water as hydrogen bonds can be formed between carboxylic acid functional group and water. As hydrocarbon chain increases, solubility decreases as more of the molecule becomes non-polar.
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What are the chemical properties of carboxylic acids
They are weak acids as they partially ionise in solution releasing the H+ ion forming carboxylate ion. will undergo typical acid reactions but at a slower rate than stronger acids.
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Describe the reaction of carboxylic acids and metals and metal oxides
Carboxylic acids react with metals forming a metal salt and hydrogen. carboxylic acids react with metal oxides to form metal salts and water
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Describe the reactions of carboxylic acids and metal hydroxides and metal carbonates
carboxylic acids react with metal hydroxides to form a metal salt and water. Carboxylic acids react with metal carbonates to form metal salt, water and carbon dioxide
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Describe Esterification
An alcohol and carboxylic can be heated gently in the presence of a sulfuric acid catalyst. Acid Anhydride can be reacted with an alcohol to form an ester and a carboxylic acid. Mixture heated to increase rate of reaction.
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Describe hydrolysis of Esters
Ester can be reacted with water under acidic conditions to reform the carboxylic acid and alcohol. Ester can be reacted with water under alkaline conditions to form a carboxylate salt and an alcohol.
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How can acyl chlorides be prepared?
Reacting carboxylic acids with SOCl2 to form an acyl chloride, sulfur dioxide and hydrogen chloride.
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Describe the uses of acyl chlorides in organic synthesis
React with alcohols to form esters and HCl. React with water to form Carboxylic acids and HCl. Forms primary amides and NH4Cl by reacting with ammonia. Forms secondary amides by reacting with primary amines, HCl is also formed.
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Other cards in this set

Card 2

Front

Describe the delocalised structure of benzene

Back

Each of the six carbon atoms donates one electron from its p-orbital. these combine to form a ring of delocalised electrons above and below the plane. All bonds in the ring are identical, same length

Card 3

Front

Describe the electrophilic substitution of benzene

Back

Preview of the front of card 3

Card 4

Front

Describe the Nitration of Benzene

Back

Preview of the front of card 4

Card 5

Front

Describe the Halogenation of Benzene

Back

Preview of the front of card 5
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