Aromatics + Amines Flashcards
In alkenes carbon atoms are..
Sp2 hybridised
What are double bonds and how are they formed
- One 2s orbital and two of the 2p orbitals in the carbon atom hybridise to form 3 sp2 orbitals.
- one p orbital is left over and is orientated perpendicular to the plane of others
- single bonds between C and H are sp2 orbitals as overalapping with 1s orbitals in H
- double bonds overlapping of two adjacent sp2 orbitals and the overlap between leftover p orbitals
What is a conjugated system
- a system of connected p orbitals with delocalised electrons in a molecule
- lowers the overall energy of the molecule and increases stabili8ty
How is a continuous system formed?
- unhybridised p orbitals can overlap
- pi electrons can be delocalised through this
Where can continuous systems occur
In any molecule when two double bonds are separated by one single bond
Why does the delocalisation of the pi electrons makes molecules more stable when there is a continuous system?
- electrons can spread out over the molecule so there is less repulsion
How is stability of alkenes and dienes measured?
- hydrogenation enthalpy data
- less exothermic the reaction between the double bonds and hydrogen the more stable the molecule
What is benzene?
- a liquid compound C6H6
What was August kekule theory about benzene?
- ringed structure with 6 carbon atoms and 6 hydrogen atoms
- he suggested that the bonding consisted of alternating double and single bonds
What chemical features does benzene that disproves kekules theory
- Benzene does not undergo addition reaction
- All bonds are equivalent in terms of length and strength
- Stability
Explain the differences in stability that are observed when benzene is hybridised
If kekules theory is right, you would expect arround 3x ΔH Hyd released but you get much less
What reaction does benzene undergo?
- electrophilic substitution reaction
Give the outline of the mechanism for electrophilic substitution
Describe Friedan crafts acylation
Describe nitration electrophilic substitution
How do we form nitrobenzene
Benzene + conc HNO3 -> Nitrobenzene + water
Conc sulphuric acid catalyst
What is the IUPAC name for TNT?
2,4,6 - trinitromethylbenzene
Why is TNT a good explosive?
- reaction of tnt with oxygen is extremely exothermic
- produces a lot of gas rapidly (11 moles for every mole of TNT used) along with a lot heat
What else are aromatic amines use for?
Industrial dyes called azo dyes
How do you get phenylamine from nitrobenzene
Reduction reaction using Sn/Hcl
Describe the pH of phenylamine
Weak base
What does the strength of a base depend on?
Availability of the lone pair:
How easily the lone pair becomes protonated
Why is phenylamine a weak base?
- the lone pair on the N atom is delocalised into the Pi system on the benzene ring and so i less available
What is an amine called when there are four carbons connected to the nitrogen atom
Ammonium salt
What is the name of the shape and bond angle for primary to tertiary amines
Trigonal pyramidal 107
What is the name of the shape and bond angle for an ammonium salt
Tetrahedral 109.5
How do you name primary amines
Alkyl amine
How do you name an a amine group which is in the middle of the chain or isn’t the PFG
amino
How do you name secondary and tertiary amines?
Secondary and tertiary amines are effectively N-substituted amines, so we need an N- at the start of the name so we know these substituents are attached to the N atom. The alkyl group with the longest carbon chain form the main stem of the name of the amine, so the alphabetical rule for naming is violated.
Why do amines have higher boiling points than their analogous alkenes
H - bonds > vow
Why do amines have a lower boiling points than than their analogous alcohols
- NH bond is not as polar as the OH bonds
- hydrogen bonds between molecules are weaker in amines
Why are amines soluble in water but solubility decreases as chain length increases
Larger proportion of chain is non-polar
Disrupts H bonding between H molecules
Why do amines form alkaline solutions
Release an OH- in water
RNH2 + H2O <-> RN+H3 + OH-
Why can amines act as bronsted Lowry bases? (Proton acceptor)
- they gain a proton from water and can react with acids
RNH2 + HCl -> RNH3+Cl-
What is the equation for methyl amine and sulphuric acid
2CH3NH2 + H2SO4 -> (CH3N+H3)2SO42-
Methylammoniumsulphate
How would the base strength of phenylamine compare to ammonia or methylamine?
- weaker because the lone pair in the pi system makes it less available to act as a proton acceptor
Order the follow types of amine in order of decreasing base strength: ammonia, primary amine, secondary amine, tertiary amine, aromatic amine
Tertiary > secondary > primary > ammonia > phenylamine/aromatic
What are the three ways to prepare amines?
Aliphatic:
1. Nucleophilic substitution - reaction of NH3 with halogenalkanes
2. Reduction of nitriles
Aromatic:
3. Nitration or Reduction
Describe the reaction of NH3 with halogenalkanes
Nucleophilic substitution
Chloroethane and ammonia
Excess ammonia
Ethylamine and ammonium chloride
Describe the reduction of nitriles to form an amine
Start with halogenalkanes
Step 1: reaction between halogenalkane (bromoethane and KCN)
Nucleophilic substation - warm and ethanoic
Propanitrile
Step two: reduce propane nitrile
Use H2 or Ni / ([H])
Why is reduction of nitriles better than Nucleophilic substation of halogenalkane
Produces pure primary amine with no poly substitution
Higher atom economy
However KCN is toxic
Why is the product of a Nucleophilic substitution reaction with a bromoalkane an even better Nucleophilic than the ammonia starting reagent?
Electron density is push by the alkyl group towards the nitrogen atom making the lone pair or electron density even more available to act as a Nucleophilie
What is polysubstittuion
When Nucleophilic substitution occurs again and again
How do we prevent and promote polysubstitution
- excess ammonia
- excess halogens alkane
What are some uses of quaternary ammonium salts
- production of catrionic surfactants
- found in detergents, fabric softeners and hair conditions
- coat the surface of cloth or hair with positive charges and reduce static due to negatively charged electrons
