Flashcards in Yr 2 Organic chem Deck (40)
What is the molecular formula for benzene?
What is the experimental evidence for the delocalised structure of benzene?
Bond lengths - All the same, when C=C bonds are shorter than C-C bonds. all somewhere between the bond length of a single and double carbon-carbon bond
Enthalpy change of hydrogenation - for Cyclohexene is -120kJ per mole, in Cyclohexadiene it is -240kJ per mole, meaning for the Kekule structure the expected enthalpy change would be -360kJ per mole, but it is actually much less exothermic (-208kJ per mole) - actual structure of benzene is more stable than the Kekulé structure
Resistance to reaction - if contained C=C bonds would decolourise bromine water, but it does not - more resistant to reaction than alkenes, does not react by electrophilic addition
What is a substituent group?
An atom or group of atoms that takes the place of another.
What are benzene and substituted aromatic hydrocarbons called?
What conditions are required for the nitration of benzene and what is the reaction mechanism?
50C, conc sulfuric acid catalyst and conc nitric acid
What is used as a catalyst for the halogenation of benzene?
A halogen carrier, e.g. AlBr3
When an acyl chloride reacts with benzene by electrophilic substitution what is formed?
An aromatic ketone
Explain how phenols are weak acids
They partially dissociate in water to form phenoxide ions and protons. They are more acidic than alcohols but less acidic than carboxylic acids
How can you distinguish between a carboxylic acid and a phenol?
Add sodium carbonate - carboxylic acids react to form carbon dioxide gas, phenols do not react and no effervescence occurs
What type of reaction is the reaction of phenol with sodium hydroxide?
What is used to reduce carbonyls?
NaBH4 (H- hydride ion)
What are used to produce HCN?
NaCN and H2SO4
What is used to detect the presence of the carbonyl group and what is observed?
2,4-DNPH, orange precipitate forms
How can the 2,4-DNPH derivative be analysed to identify the carbonyl compound?
Impure orange solid is filtered then recrystallised to produce a pure sample of the 2,4-DNPH derivative. The melting point of the purified 2,4-DNPH derivative is determined. The melting point is matched to a database of melting points for 2,4-DNPH derivatives of carbonyl compounds
What is used to determine the presence of an aldehyde? What is observed?
Ammoniacal silver nitrate (Tollens' reagent)
Silver mirror on inside of boiling tube
Why do ketones not form a silver mirror when reacted with tollens' reagent?
They cannot be reduced any further (by Ag+ ions in this case), whereas aldehydes can (to carboxylic acids)
What are the reagents and conditions for esterification?
Concentrated sulfuric acid, warm
Acid hydrolysis of esters
Reflux with dilute sulfuric acid - reverse reaction to esterification
Alkaline hydrolysis of esters
Reflux with aqueous alkali - forms carboxylate ion and alcohol - irreversible
How are acyl chlorides prepared?
By reacting carboxylic acids with thionyl chloride (SOCl2)
What can acyl chlorides be used to produce?
Esters (aliphatic and aromatic), carboxylic acids, primary and secondary amides
How do you produce a carboxylic acid from and amide?
React it with water (HCl is second product)
How do you form a primary amide from an acyl chloride?
React with ammonia
CH3COCl + 2NH3 --> CH3CONH2 + NH4+Cl-
How do you form a secondary amide from an acyl chloride?
React with an amine
CH3COCl + 2CH3NH2 --> CH3CONHCH3 + CH3NH3+Cl-
Why would an acid anhydride be used over an acyl chloride?
Less reactive so are useful for laboratory reactions where acyl chlorides may be too reactive
Give an example of a tertiary amine
How do amines act as bases?
The lone pair on the N atom can accept protons (H+ ions) in aqueous solution to form dative covalent bonds (___ammonium ions)
Show the reaction of ethylamine with hydrochloric acid. What type of reaction is this?
CH3CH2NH2 + HCl --> CH3CH2NH3+Cl-
How are primary amines prepared?
CH3CH2Cl + NH3 --> CH3CH2NH3+Cl-
CH3CH2NH3+Cl- + NaOH --> CH3CH2NH2 + NaCl + H2O
Ethanol is used as a solvent to prevent substitution of haloalkane by water to produce alcohols - excess ammonia in ethanol