Unit 3: Section 6 - Aromatic Carbons and Amines Flashcards
What is the most common type of reaction of benzene?
Substitution
What is an arene?
(aromatic compound)
Compounds containing a benzene ring
What is the shape of benzene?
Flat, regular hexagon, bond angle is 120°
What is the bond length between adjacent C atoms?
Intermediate between C-C and C=C
What happens to the 4th electron in the p-orbital of each C atom in benzene?
It delocalises to form rings of electron density above and below the hexagon
What is the effect on benzene’s stability of the rings of electron density?
Makes benzene very stable, even though it is unsaturated
What is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene?
Hydrogenation of cyclohexene - enthalpy of hydrogenation : 120kJmol^-1
Cyclohexa-1,3,5-triene - 360kJmol^-1
Benzene - 208kJmol^-1 so benzene is more stable
Other than enthalpy of hydrogenation why else is cyclohexa-1,3,5-triene not a suitable model for benzene?
- Wouldn’t be symmetrical (C=C shorter than C-C), but benzene is
- Would easily undergo addition reactions across the double bonds - benzene doesn’t
- Would form 2 isomers on the addition of Br2 or similar - benzene doesn’t
What is the appearance of benzene at 298K?
Colourless liquid
Why does benzene have a relatively high melting point?
Close packing of flat hexagonal molecules when solid
Is benzene soluble in water? Why?
No - non polar
Why is benzene not used in schools?
Carcinogen
How do you name compounds containing a benzene ring?
- benzene, or -phenyl ; can designate position on ring using numbers if there is more than one substituent
Why is benzene attacked by electrophiles?
High electron density above/below ring due to delocalised electrons
What is delocalisation energy and what is the effect of this on benzene’s reactions?
The large amount of energy that is needed to break the aromatic ring apart
Results in the aromatic ring almost always staying intact
What is seen when benzene is combusted? Why?
Smoky flames due to soot from unburnt carbon
High Carbon:Hydrogen ratio
Which ion is used to nitrate benzene?
+NO2
Nitronium ion or nitryl cation
How is the +NO2 ion generated? (conditions and equations)
Concentrated H2SO4, and concentrated HNO3
H2SO4 + HNO3 -> HSO4- + +NO2 + H2O
How is the H2SO4 catalyst regenerated in the nitration of benzene?
HSO4- + H+ -> H2SO4 (H+ from benzene ring)
How do you form only one NO2 group (mononitration)?
Keep the temperature below 55°C otherwise you’ll get lots of substitutions
What are the uses of nitrated arenes?
Production of explosives e.g. TNT - (1-methyl-2,4,6-trinitrobenzene) - releases lots of heat and gas on explosion
To make aromatic amines that are used for industrial dyes
How do substituents with a negative inductive effect (e.g. NH2) affect further substitution?
Remove electrons from the delocalised electron ring, decreasing the electron density and making further substitution reactions less likely/quick
What type of catalyst is used for a Friedel-Crafts acylation recation?
Halogen carrier ( e.g. AlCl3)
Write an equation to form an electrophile that could be used to acylate benzene, starting with AlCl3 and RCOCl
AlCl3 + RCOCl -> AlCl4- + RCO+ (+ on C)
RCO+ can attack benzene
What is happening when AlCl4- is formed in terms of electrons?
Chlorine atom’s lone pair of electrons is forming a coordinate bond to Al
How is the AlCl3 catalyst reformed?
AlCl4- + H+ -> HCl + AlCl3 (H+ from benzene)
How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?
Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene
Why is Friedel-Craft acylation used?
Once an acyl group has been added to benzene, the side chains can be modified using further reactions to make useful products
If you are considering cyclic compounds, what might happen if 2 double bonds are next to each other?
C=C bonds are in close proximity, so electrons in pi cloud/p orbitals can partially delocalise and move between the 2 C=C double bonds
CHANGE THE ANSWER FROM HARRIS-GREEN’S EMAIL
What effect would electrons in p orbitals moving betweeen the 2 C=C double bonds have on the stability of the molecule and its enthalpy of hydrogenation?
Makes the molecule more stable; makes enthalpy of hydrogenation more positive
How do you name amines?
-amine or amino-
Why are amines so reactive?
The lone pair of electrons on the nitrogen - due to polar N-H bond
What kind of intermolecular forces do amines have? Why?
Hydrogen bonding due to polar N-H bond and lone pair of electrons on N atom
Do amines have intermolecular forces which are stronger than or weaker than alcohols? Why?
Weaker, as N has a lower electronegativity than O -> weaker hydrogen bonding
What state are amines at 298K?
Short chains are gases, longer chains are volatile liquids
What do amines smell of? Why?
Rotting fish/flesh as releases di- and tri- amines
Which primary amines are soluble in water/alcohols? Why?
Up to 4 carbon atoms, as they can hydrogen bond to water molecules
After this, non-polarity of hydrocarbon chain makes them insoluble
What kind of solvents are most other amines soluble in?
Less polar solvents than water or non-polar solvents
Solubility of phenylamine? Why?
Not very soluble, due to the non-polarity of the benzene ring - C6H5 cannot form hydrogen bonds
How can/when do amines act as bases?
When they bond with a H+ ion
How can/when do amines act as nucleophiles?
When they bond with an electron-deficient C atom (donate lone pair from N)
What is the product from the basic action of an amine with water?
RNH3+ - ammonium ion, which forms a salt with an anion
Is the product (ammonium ion) soluble in water? Why?
Yes, as it is ionic so is attracted to the polar bonds in H2O
How could you regenerate the soluble amine from the ammonium salt?
Add a strong base (NaOH) -> removes H+ ions from ammonium ion
In order to be the strongest base, what must a particular amine have (out of a set of amines)?
Greatest electron density around the N atom, making it a better electron pair donor (attracts protons more)
What does positive/negative inductive effect mean?
Positive inductive effect = donate electrons, increase density around N
Negative means remove electrons, decrease density around N
What effect do alkyl groups have (on electron density and base strenght)?
Positive inductive effect - increase electron density around N -> stronger base
What effect do aryl groups have (on electron density and base strength)?
Negative inductive effect - decrease electron density around N -> weaker base
Why are 3° amines never good bases?
They are insoluble in water
Place these in order of base strength (in general): NH3, 1° amine, 2° amine, phenylamine
2° amine > 1° amine > NH3 > phenylamine
How can primary amines then form 2°, 3° amines and 4° ammonium ions?
Multiple substitutions; primary amine is a nucleophile that attacks the original haloalkane etc
What are the problems with the substitution method?
Not efficient as low yield of 1° amine due to multiple substitutions
How would you maximise the yield of the primary amine?
Use excess ammonia
What type of mechanism is the reaction of haloalkane with a cyanide ion?
Nucleophilic substitution
What conditions does the reaction of a haloalkane with a cyanide ion require? What is the product formed?
Ethanol as a solvent
A nitrile is formed
How do you get from a nitrile to a primary amine? (name of reaction type and reagents/catalysts)
Reduction using Ni catalyst and H2
Why is reducing a nitrile a purer method of synthesising amines?
Only the primary amine can be formed
What conditions are needed to form nitrobenzene from benzene?
Concentrated H2SO4 and HNO3 to form the +NO2 ion for electrophilic attack
How do you form an ammonium chloride salt from nitrobenzene? What conditions are needed?
Reduce the nitrile using Tin and HCl -> forms an ammonium salt with Cl- ions
Room temperature
Equation for the reaction of nitrobenzene -> penylamine?
C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O
What mechanism is used for forming amides from acyl chlorides and amines?
Nucleophilic addition elimination
In which industries/products are amines used?
Precursors for dyes, nylon, drugs, synthesis of new molecules
What are cationic surfactants (used in fabric/hair conditioners)?
Quaternary ammonium salts, with a cation that is charged at one end (N+) and non-polar at the other
How do cationic surfactants work in fabric/hair conditioner?
Negative charges on the surface of the fabric/hair are attracted to the cation, removing them from the surface; prevents build-up of static electricity and keeps hair flat and fabric smooth
How do cationic surfactants sit when placed in water?
Charged end in the water, non-polar end sticking out of the water/at the surface
