🔲 Topic 18 - Organics III Flashcards
What does aliphatic mean?
A straight chain, branched chain or ring of carbon atoms
What does aromatic mean?
(Arenes) - contain a benzene ring
Why is benzene resistant to bromination (compared with alkenes)?
Although the benzene ring is electron-rich and attracts electrophiles, the electron density in the delocalised pi-bond system is not significant enough to produce an electrophile (the Br+ from Br2 - a dipole to be induced) so benzene does not readily undergo electrophilic substitution (needs a catalyst). [Fairly high activation energy due to delocalised ring being so stable].
Alkenes have localised pi-orbital overlaps between two carbon atoms, in which the electron density is much greater than that in benzene. This greater electron density in alkenes allows a dipole to be induced more readily in bromine, making alkenes more susceptible to electrophilic attack.
What is observed when benzene is burnt in air? (Combustion) Give an equation for complete and incomplete combustion.
A smoky (sooty) flame is seen (which is a common observation in compounds with a high C:H ratio)
Complete: C6H6 + 7.5O2 —> 6CO2 + 3H2O
Incomplete: C6H6 + 1.5O2 —> 6C + 3H2O
(Black smoke visible in incomplete combustion is carbon particulates)
What are the reagents and conditions for hydrogenation (electrophilic addition) of benzene?
Reagents: 3H2
Conditions: heat under pressure, nickel catalyst
(Same as for hydrogenation of an alkene)
What are the possible electrophilic substitution reactions of benzene, and what are the products?
- (halogenation) - with bromine/chlorine in presence of a catalyst, produces bromobenzene
- (nitration) - with concentrated nitric acid and a sulfuric acid catalyst, produces nitrobenzene
- (alkylation) - with a halogenoalkane and catalyst, produces alkylbenzene
- (acylation) - with an acyl chloride and catalyst, produces phenyl”ketone”
(Alkylation and acylation are Friedel-Crafts reactions)
What are the stages in electrophilic substitution reactions of benzene?
- Generation of the electrophile using a catalyst (H2SO4 for nitration, and AlCl3/FeBr3 for the others)
- Electrophilic substitution (mechanism)
- Regeneration of the catalyst
What are the reagents and conditions for the nitration of benzene?
Reagents: conc. HNO3 and H2SO4 (catalyst) to make the electrophile (NO2+)
Conditions: warm (55C - as at higher temperatures multiple substitutions can occur)
What are the reagents and conditions for the halogenation of benzene?
Reagents: Br2/Cl2 and FeBr3/AlCl3 (catalyst) (to make electrophile - Br+)
Conditions: heated under reflux
What are the reagents and conditions for the alkylation of benzene?
Reagents: halogenoalkane + FeBr3/AlCl3 (catalyst) to make electrophile (R+)
Conditions: anhydrous conditions (water would react with the catalyst and sometimes with the organic product)
What are the reagents and conditions for the acylation of benzene? (Makes a phenyl”ketone”)
Reagents: acyl chloride + AlCl3 (catalyst) to make the electrophile (RC+=O)
Conditions: anhydrous conditions (under reflux)
Is phenol more or less reactive than benzene? Why?
Phenol is more reactive than benzene.
The oxygen in the OH group of phenol has lone pairs of electrons which interact/merge with the electrons in the delocalised pi bond ring. Therefore the electron density above and below the ring of atoms increases making it much more reactive towards electrophiles as it polarises the Br-Br bond which breaks and the Br+ can attack the ring
(The delocalised electron ring in benzene is stable, so not very reactive)
What are the observations seen when benzene reacts with bromine (water) and when phenol reacts with bromine (water)?
Benzene: no observations
Phenol: bromine water decolourises and a white precipitate (2,4,6-tribromophenol) is formed (which smells of antiseptic)
What are the states at room temperature of benzene and phenol, and why?
Benzene: liquid
Phenol: solid (due to hydrogen bonding)
What is the solubility of benzene and phenol, and why?
Benzene: insoluble
Phenol: soluble (due to hydrogen bonding)
How do you prepare an aromatic amine? What from? Reagents? Conditions? Product? What extra step may be needed?
From: nitrobenzene
Reagents: reducing agent of tin, mixed with concentrated hydrochloric acid ([H])
Conditions: heated under reflux
Product: phenylamine (+water)
Extra step: phenylamine may react with acid present and form an ion, but can be converted back by adding alkali (OH-)
Are all amines, in general, strong or weak bases?
All amines are weak bases
Order, from weakest to strongest, the basicity (base strength) [/lowest pH to highest pH] of: ammonia, primary amines and phenylamine. Give reasons
(Base is proton acceptor)
WEAKEST: phenylamine < ammonia < short chain primary amine < long chain primary amine :STRONGEST
Phenylamine (C6H5NH2): much weaker base as the lone pair of electrons on the nitrogen are attracted to the delocalised electrons in the pi bond ring, making the nitrogen less electron-rich and the lone pair of electrons less available for donating to the hydrogen of a water molecule
Ammonia: no inductive effect
Short chain primary amine: stronger base than ammonia as the aliphatic chain is electron-releasing (repels the lone pair on the nitrogen) (an inductive effect) so the electron density on the nitrogen is greater than that of ammonia therefore accepts protons more readily
Long chain primary amine: a greater inductive effect due to longer aliphatic chain
What is produced when an aliphatic amine (butylamine) is reacted with water? Why?
An alkaline solution (C4H9NH3+ + OH-)
The lone pair on the nitrogen can form a dative bond with the hydrogen of water molecule - the amine acts as a base (proton acceptor) [H+ conc. goes down (reacts with amine) so pH goes up]
What is produced when an aliphatic amine (butylamine) is reacted with a (strong) acid? Type of reaction? Very simply, what happens? How do you reverse the reaction?
An ionic salt (in solution) eg. Butylammonium chloride (C4H9NH3+ + Cl-) or butylammonium sulfate (2C4H9NH3+ + SO42-)
- a neutralisation reaction
(The lone pair on the nitrogen is a H+ acceptor (so increases from -NH2 to -NH3+))
(An addition of NaOH (alkali) to the ammonium salt will convert it back to the amine)
What is the solubility of amines (aliphatic and aromatic)?
- short chain primary aliphatic amines are completely soluble in water, due to hydrogen bond formation
- phenylamine is only slightly soluble in water (for same reason it is a weak base - lone pair of electrons are attached to delocalised electrons in pi bond …)
What is produced when an aliphatic amine (eg. Butylamine) is reacted with copper (II) ions ( [Cu(H2O)6]2+ )? Include colour changes and state of the product dependent on reaction conditions.
A complex ion is formed
ADD BUTYLAMINE DROP BY DROP:
- [Cu(H2O)6]2+ (aq) + 2C4H9NH2 (aq) <—> [Cu(OH)2(H2O)4] (s) + 2C4H9NH3+
- light blue solution —> light blue precipitate (s)
- acid-base reaction
- the (lone pair on the) nitrogen of the butylamine acts as a base and accepts two H+ ions from two H2O ligands - this forms OH- ligands
ADD BUTYLAMINE IN EXCESS:
- [Cu(H2O)6]2+ + 4C4H9NH2 <—> [Cu(C4H9NH2)4(H2O)2]2+ + 4H2O
- light blue solution —> dark blue solution
- ligand exchange reaction
- the (lone pair on the) nitrogen of the butylamine acts as a ligand and substitutes around the central Cu2+, replacing 4H2O molecules
(Very similar to [Cu(H2O)6]2+ with ammonia)