Topic 18: Organics 3 2️⃣ Flashcards
What are the physical properties of benzene
Colourless liquid
Boiling temp of 80 degrees C
Insoluble in water
Carcinogenic
What is the molecular formula of benzene ?
C6H6
Describe the Kekulé structure of benzene
Cyclic structure (hexagon)
3 double bonds
Explain problem 1 of the kekulé structure: decolorisation of bromine water
If benzene contains 3 double bonds, it should readily decolourise bromine water in an addition reaction however it does not
Suggests there are no C=C bonds
Explain problem 2 of the kekulé structure: isomers of dibromobenzene
If kekulé structure is correct there should be 4 isomers with the molecular formula C6H4Br2 however, only 3 are known to exist
Suggests that the two isomers with bromine on adjacent carbon atoms are identical which would mean that the bonds between carbon atoms in the benzene ring are the same not different (C-C/C=C)
Explain problem 3: of the kekulé structure: bond lengths
Evidence suggests that the bond lengths in benzene are all the same
Explain problem 4 of the kekulé structure: enthalpy changes of hydrogenation
Actual benzene has a much lower value than a theoretical structure containing 3 double bonds
Describe the new model for benzene
Each carbon atom has one electron in a p orbital
The orbitals overlap sideways so together the six electrons form a delocalised pi bond above and below the plane of carbon atoms (delocalised cloud)
Meaning of aromatic
Description of the bonding in a compound:
Delocalised electrons forming pi bonding in a hydrocarbon ring
Describe the hydrogenation of benzene
Benzene mixed with hydrogen, heat and high pressure, nickel catalyst
Forms cyclohexane
Describe the combustion of benzene
Burns in air, WITH smoky flame
Describe the bromination of benzene
Benzene and bromine are heated under reflux, in the presence of a halogen carrier catalyst such as AlCl3 or AlBr3 or iron(III)bromide
For an experiment it’s enough to add iron filings as they react with bromine to form iron(III) bromide
Produces bromobenzene & hydrogen bromide
Describe the nitration of benzene
Benzene is warmed with a mixture of concentrated nitric and sulfuric acids
- the nitric acid can be considered the source of NO2 (HNO3)
- the sulfuric acid can be considered the catalyst (H2SO4)
Produces nitrobenzene & water
What are the 2 Friedel-Crafts reactions
Alkylation and Acylation
What are the 3 features the Friedel-Crafts reactions have in common ?
- use a reagent represented as XY, one of the hydrogen atoms in benzene is substituted by Y and the other product is HX
- a catalyst is needed (any of… AlCl3, AlBr3, iron(II)chloride, iron(III)bromide)
- anhydrous conditions needed as water would react with catalyst and sometimes organic product
Describe the alkylation reaction of benzene (Friedel-Crafts)
Substitution of one of the hydrogen atoms from benzene by an alkyl group
Reagent is a halogenoalkane
Products are alkylbenzene & hydrogen chloride
Describe the acylation reaction of benzene (Friedel-Crafts)
Substitution of one of the hydrogen atoms in benzene by an acyl group
Reagent is acyl chloride
Products are a ketone (eg. phenyletnanone) and hydrogen chloride
When should phenyl- be used in naming aromatic compounds
Used to represent the benzene ring when it’s attached to another functional group
What is phenol
Consists of a hydroxyl group joined to a benzene ring
Describe the bromination of phenol in contrast to that of benzene
Occurs at room temp and without a catalyst can occur with bromine water unlike bromination of benzene
Bromine water decolourises and organic product forms a white precipitate
Substitution can occur 3 times
HBr is formed (same) however with phenol 3HBr
Why does phenol react more readily with bromine than benzene does
- the oxygen in the OH has a lone pair and these electrons can merge with the electrons in the delocalised pi bond
- electron density above and below the ring of atoms is increased (activation)
- benzene can induce a dipole on bromine/bromine is polarised as it approaches the benzene ring, Br-Br bond breaks & Br+ electrophile can attack
What are the 2 methods of preparation of aliphatic amines
- preparation from a halogenoalkane
- preparation from nitriles
Describe the preparation of aliphatic amines from halogenoalkanes
Halogenoalkane is heated with ammonia (under pressure and in a sealed container as ammonia is a gas)
Alternatively can be mixed with concentrated aqueous ammonia
Describe the preparation of aliphatic amines from nitriles
Nitriles can be reduced to primary amines using the reducing agent lithiumtetrahydrioaluminate [H]
Reactants are mixed in dry ether to ensure no water can affect the reaction
How are aromatic amines prepared
Reduction of nitrobenzene
Reducing agent (tin) mixed with concentrated hydrochloric acid
Heated under reflux
Describe and explain the reactions of primary aliphatic amines with water to form an alkaline solution
Dissolve in water due to ability to form hydrogen bonds with water molecules
As the hydrocarbon part of the molecule becomes proportionately larger the solubility decreases
Can also react with water due to lone pair on nitrogen atom in the molecule forming a dative bond with a hydrogen atom of water
Describe and explain the reactions of primary aliphatic amines with acids to form salts
Although amines vary in their basicities they all react with strong acids to form ionic salts
Explain the differences in basicity between methylamine, ethylamine and propylamine
Ethyl and propyl groups are only slightly more electron releasing than the methyl group = increases the electron density on the nitrogen only slightly
Therefore propylamine is the most basic of the 3 but only very slightly
Explain the differences in basicicity between phenylamine (aromatic amine) and the aliphatic amines (methylamine etc)
Phenyl amine is a much weaker base than any of the aliphatic amines, ammonia and even water.
- This is due to lone pair of electrons on the nitrogen being attracted to the delocalised electrons in the pi bond of the ring.
- As a result this makes the nitrogen less electron rich & lone pair is less available for donating to a hydrogen atom of a water molecule
What does basicity mean
The extent to which a base can donate a lone pair of electrons to the hydrogen atom of a water molecule
Describe the reaction of aliphatic amines with ethanoyl chloride
Addition-elimination reaction
The two molecules (ethanoyl chloride & amine eg. butylamine) join together and a small product is eliminated such as HCl
Product is an N-substituted amide (eg. N-butylethanamide)
Describe the reaction of aliphatic amines with halogenoalkanes
Halogenoalkane contains an electron deficient carbon and amine contains an electron rich nitrogen so the two would react together
Substitution reaction
Products are a secondary amine & a hydrogen halide
This product contains an electron rich nitrogen atom so can also react with the halogenoalkane producing a tertiary amine which also contains an electron rich nitrogen atom so can also react with halogenoalkane. (MULTIPLE SUBSTITUTIONS)
Describe the reaction of aliphatic amines with copper(II) ions
Copper(II)sulfate solution forms [Cu(H2O)6]2+ complexes
Amine is added to this and pale blue precipitate forms
With excess amine the ppt dissolves and a deep blue solution forms
why?
- lone pair of electrons on the nitrogen of amines means amines can act as lone pair donors
What is the functional group of amides
RCONH2
(Carbonyl group joined to an amino group)
What are the physical properties of amides
Solids (except from methanamide which is liquid)
The lower aliphatic amides are soluble in water as they contain 2 electronegative atoms and polar bonds that can form hydrogen bonds with water
How are amides prepared
Mixing an acyl chloride with concentrated aqueous ammonia
- the lone pair of electrons from the nitrogen of ammonia is attracted to the electron deficient carbon of the acyl chloride
- the chlorine of the acyl chloride combines with one of the hydrogen atoms of ammonia to form HCl (misty fumes)
The HCl will go on to react with ammonia as ammonia is basic and HCl is acidic to form a salt (NH4Cl)
Overall equation : CH3CH2COCl + 2NH3 —-> CH3CH2CONH2 + NH4Cl
Explain how polyamides are formed
Condensation polymerisation reaction (H2O lost)
Requires a dicarboxylic acid & a diamine
Give the general formula of amino acids
H
|
H2N—C—COOH
|
R
Define the isoelectric point
The isoelectric point of an amino acid is the pH of an aqueous solution in which it is neutral
Define zwitterion
A molecule containing positive and negative charges but which has no overall charge
What do low and high isoelectric points indicate
Low isoelectric point = molecule is predominantly acidic
High isoelectric point = molecule is predominantly basic
All amino acids can form salts with…
Acids and bases
Describe and explain the optical activity of amino acids
Almost all amino acids contain a chiral centre and so are optically active
The enantiomers rotate the plane of polarisation of polarised light
If an amino acid is synthesised in a lab, a racemic mixture is formed
Describe the formation of a peptide bond
When 2 amino acids react together an acid-base reaction occurs.
The -OH of the COOH group combined sigh one of the H atoms of the NH2 group = CO-NH bond
Condensation reaction - water formed
Explain how proteins are hydrolysed
Polypeptide chain can be broken down into constituent amino acids by prolonged heating with concentrated hydrochloric acid (breaks the peptide bonds between amino acids)
Due to strongly acidic conditions all the amino acids will have their NH2 groups protonated as +NH3 groups
How does one use chromatography to analyse proteins
A mixture of amino acids produced from the hydrolysis of a protein can be spotted onto chromatography paper.
Using a suitable solvent the individual amino acids will rise to different heights during the experiment
As amino acids are colourless, the chromatogram can be sprayed with a developing agent so the positions of the amino acids can be seen
Rf values calculated and individual amino acids can be identified
What is the general formula of Grignard reagents ?
RMgX (known as organometallic compounds)
How are Grignard reagents produced
- halogenoalkane & magnesium are reacted together
- heated under reflux
- in a solvent of dry ether
What are the 2 types of reactions with Grignards and what’s formed?
- with CO2 to form carboxylic acids
- with carbonyls to form primary, secondary & tertiary alcohols
Why is a solvent of dry ether needed in the production of Grignards
Grignards react with water
Describe the reaction of Grignards with Co2 to form a carboxylic acid
- Bubble dry co2 through the mixture produced
(CH3)2CHCH2MgBr + CO2 —> (CH3)2CHCH2COOMgBr - (hydrolysis) Add dilute acid eg. HCl or H2SO4
(CH3)2CHCH2COOMgBr + H2O —> (CH3)2CHCH2COOH + Mg(OH)Br
Describe the reactions of a Grignard with carbonyls
- React Grignard reagent with carbonyl
- Add dilute acid
- If using methanal, a primary alcohol is produced
- If using another aldehyde, a secondary alcohol is produced
- If using a ketone, a tertiary alcohol is produced
