Paper 4 Flashcards
Arene
Hydrocarbon containing one or more benzene ring
Why does Kekule’s proposed ring not work?
- Doesn’t react like alkenes eg does not decolourise bromine without a catalyst - no electrophilic addition
- Bond lengths - X-ray crystallography shows all bond lengths are equal when double bonds are actually shorter than single bonds
- Hydrogenation of benzene is less exothermic than expected (-360 vs -208) suggesting benzene is actually more stable
How is bromobenzene formed?
Benzene + Br2 + AlBr3 (halogen carrier)
AlBr3 + Br2 > AlBr4- + Br+ (electrophile)
H+ + AlBr4- > AlBr3 + HBr
ELECTROPHILIC SUBSTITUTION
How is nitrobenzene formed?
Benzene + c.HNO3 + c.H2SO4 Reflux 50degrees Higher temp = further subst. H2SO4 + HNO3 > H2NO3+ + HSO4- H2NO3+ > NO2+ + H2O H+ + H2SO4 > H2SO4 ELECTROPHILIC SUBSTITUTION
Describe the bonding in benzene
Each carbon forms 3 sigma bonds
1 electron left in a p-orbital on each carbon
P-orbitals overlap to form a delocalised pi-system
Electrons are delocalised above and below ring
C-C bond lengths equal
Ring is planar
Compare the reactivity of benzene and cyclohexene (alkene)
Benzene is less reactive towards bromine (needing a catalyst)
The delocalised pi-system means the electrons are spread out
So benzene has a lower electron density than a double bond in which the pi-electrons are localised
Therefore is less attractive to electrophiles and is less able to polarise them
Why does benzene undergo substitution and not addition?
The delocalised pi-system means the electrons are spread out
This reduces repulsion between them and gives benzene extra stability
Substitution allows the delocalised pi-system to be retained
Addition would permanently disrupt the delocalised pi-system
Compare the reactivity of benzene and phenol
Phenol is more reactive than benzene eg does not need a catalyst to react with bromine
In phenol the ring is activated
The lone pair of electrons in a p-orbital on the oxygen is delocalised into the ring
This increases the electron density in the ring
Making it more attractive to electrophiles and more able to polarise them
How is 2,4,6-tribromophenol formed?
Phenol + 3Br2
Br+ + Br- > 3HBr
White ppt - orange bromine decolourises
How is sodium phenoxide formed?
Phenol + NaOH
Phenol + Na > + 1/2 H2
Phenol uses
Antiseptic, disinfectants, detergents, preparation of aspirin
WEAK ACID
Primary amine
A H atom on ammonia has been replaced by an alkyl or aryl (R) group > RNH2
How do amines react?
WEAK BASES
So react with acids > salts
How do you prepare an aliphatic amine?
NUCLEOPHILIC SUBSTITUTION
CH3CH2CH2Cl + NH3 > CH3CH2CH2CH2NH2 + HCl
Propylamine
Aliphatic
In a chain
Aromatic
Contains a benzene ring
How do you prepare an aromatic amine?
Nitrobenzene + Sn + c.HCl
Heat under reflux with [H]
Synthesis of diazonium salt
Phenylamine + Nitrous acid (Sodium nitrite and XS HCl)
Less than 10degrees
NaNO2 + HCl > HNO2 + NaCl
Synthesis of an azo dye
Diazonium salt + phenol in alkaline conditions (>Sodium phenoxide)
Azo group = N=N
Bright orange ppt
Used as a dye in paints
Aldehyde
RCHO
-al
Ketone
R’COR
-one
Alkyl
Alkane with a H atom removed
Aryl
Cyclic compound containing benzene ring
Oxidation of a primary alcohol
ROH + [O] > RHO + H2O
Heat gently and distill
Acidified potassium dichromate (Cr2O72-/H+; K2Cr2O7 + H2SO4; orange > green Cr3+)
Oxidation of an aldehyde
RHO + [O] > ROOH + H2O
Heat under reflux
Acidified potassium dichromate (Cr2O72-/H+; K2Cr2O7 + H2SO4; orange > green Cr3+)
Oxidation of a secondary alcohol
RCOHR’ + [O] > RCOR’ +H2O
Heat gently
Acidified potassium dichromate (Cr2O72-/H+; K2Cr2O7 + H2SO4; orange > green Cr3+)
Reduction of carboxylic acid, aldehyde or ketone
Warm + H2O (solvent) \+NaBH4 Sodium tetrahybridborate (III) > H- 2[H] NUCLEOPHILIC ADDITION
Brady’s reagent
2,4-DNP
Forms an orange ppt with aldehydes and ketones
Recrystallize and can ID derivatives from melting points of known k/a
Silver mirror
Tollens’ reagent/ammonical silver nitrate [Ag(NH3)2]+ warm gently in a water bath - SM forms with aldehydes only as oxidised. Ag reduced.
Carboxylic acid
RCOOH Highly polar bonds Soluble in water up to 4 Cs Weak acids Partially dissociate
Esters
RCOOR’
Perfumes, food flavourings, oils
Formation of esters
- RCOOH + R’OH (c.H2SO4) RCOOR’ + H2O
2. Acid anhydride + alcohol > ester + CA
Hydrolysis of esters
- Reflux with dilute H2SO4/HCl
ester + water (H+) CA + alcohol - Reflux with alkali
ester + NAOH > Sodium salt + alcohol
Triglyceride
A triester of propane-1,2,3-triol (glycerol) + octa/hexadecanoic acid (fatty acid) > all 3 groups esterified + 3H2O
Order of healthiness of fats (healthiest first)
Polyunsaturated
Monounsaturated
Trans
Saturated
Why are trans fats bad?
Pack together more closely that cis - solid - block arteries > CHD
Why are monounsaturated an polyunstaurated fats good?
Bad LDL cholesterol sticks to artery walls causing build up
Good HDL cholesterol carries bad cholesterol away
What is biodiesel?
Veg/animal fat - long alkyl esters used in diesel engines/blended with petrodiesel
Trigylceride + 3H3OH > 3H3COC=OR + COHCOHCOH
What is carbon neutral?
Same amount of CO2 absorbed when growing as released when burned as a fuel
But less food > starvation
What is an amino acid?
RCH(NH2)COOH
Basic amine group + acidic carboxyl group
Zwitterion
A dipolar ionic form or an amino acid that is formed by the donation of a hydrogen ion from the carbonyl group to the amino group. Because both charges are present, there is no overall charge.
Isoelectric point
The pH value at which the amino acid exists as a zwitterion
In acidic conditions, what happens to an amino acid?
Acts as a base > N+H3
In alkaline conditions, what happens to an amino acid?
Acts as an acid > COO-
Peptide link
-C=ON-H-
Formed by condensation reactions
Hydrolysis of polypeptides or dipeptides
- ACIDIC - heat under reflux with 6moldm-3 HCl for 24 hours
(+H2O + 2H+) > H3N+- + -COOH - ALKALINE - heat with NaOH at just above 100degrees
(+NaOH) > -COO-Na+ + H2N-
Addition polymerisation
Double C=C bond in alkene opens and monomers join, requiring catalyst and polymer is the only product
Condensation polymerisation
Formed from condensation reactions between monomers. Small molecule (H2O/HCl) also produced
Polyesters
Dicarboxylic acid + diol > -C=OC- ester link
Terylene (PET)
Strong, flexible and abrasion resistant > clothes
Benzene-1,4-dicarboxylic acid + ethane-1,2-diol
Poly(lactic acid)
Derived from corn-starch
Biodegradable and renewable
Can be made into containers, stitches, wast5e sacks and packaging.
2-hydroxypropanoic acid (lactic acid)
Polyamides
Dicarboxylic acid + diamino (NH2) > -C=ON-H- amide link
Nylon-6,6
Clothing, airbags, hoses, conveyer belts, tyres, ropes
v strong, elastic and abrasion-resistant
1,6-diaminohexane + hexanedioic acid
Kevlar
Bulletproof vests
V strong + light, 5x stronger than steel, stiff
Benzene-1,4,diamine + Benzene-1,4-dicarboxylic acid
Hydrolysis of polymers
- BASIC (polyesters)
Hot 2nNaOH/H2O > O-Na+ + diol - ACIDIC (polyamides)
H+/2nH2O + H2SO4 catalyst > H3N+ + dioic acid
Photodegradable polymers
Weak and brittle when exposed to light > blended with light-sensitive additives catalysing breakdown on polymer in UV radiation/ + C=O, absorbing light energy and breaking > waxy compounds > (bacteria) CO2 + H2O
Biodegradable polymers
> CO2 + H2O
eg poly(lactic acid) from maize
Poly(glycolic acid) from cane sugar and unripe grapes - stitches
Replace oil bases products = bioplastics
Isomers
Compounds with the same molecular formula but different arrangements of atoms
Structural isomerism
Different structural formula
- Positional (functional group)
- Chain branch
- Functional group
Stereoisomerism
Different arrangement of atoms in space
- E/Z isomerism - lack or rotation around C=C bond > trans/E/180 degrees and cis/Z/90degrees
- Optical isomerism - non-super imposable mirror images due to chiral carbons (4 different groups attached to central carbon). Produce chemically identical isomers that rotate plane polarised light in opposite directions)
Racemic mixture
A mix containing 50/50 each optical isomer (enantiomer) - no effect on plane-polarised light because the rotations cancel each other
Optical isomer synthesis
In the lab, racemic mixtures are formed
Naturally occurring amino acids and most sugars are optically active but only one enantiomer is found in natural systems
Drug must be right shape to fit in active site - only one enantiomer. Other may fit in another active site, have no effect or cause harmful side effects.
Chiral drugs must be made to contain only one enantiomer - half dosage needed, side effects reduced and pharmalogical activity is improved.
Difficult and expensive:
- natural enzymes and bacteria produce one
- chemical chiral synthesis
Chromatography
An analytical technique used to separate the components in a mixture
Mobile phase
Sweeps over stationary phase in a definite direction
Stationary phase
Fixed in place - interacts with components, slowing them down
Solid stationary phase separates by…
Adsorption > onto surface > stronger adsorption = more slowed
Liquid stationary phase separates by…
Relative solubility > greater solubility = more slowed
Thin-layer chromatography
Monitors extent of a chemical reaction/checks purity of compounds
SP = thin layer of adsorbent such as silica gel on a flat solid support = TLC plate
MP = liquid solvent
Rf value
distance moved by component/distance moved by solvent front
Difficulties of TLC
Similar components have similar Rf values
Unknown = no Rf for comparison
Difficult to find one solvent to separate all components
Gas chromatography
Used to separate volatile components in a mix
SP = thin liquid/solvent on inside of capillary tubing
MP = carrier gas moving through column (inert)
Retention time
The time taken for a component to pass from column to detector - area under peak = amount of compound
Difficulties of GC
Many may have same retention time
Not all separated and detected - may ‘hide’
No reference for unknown
GC-MS
GC separates by doesn’t identify conclusively
MS detailed info but no separation
Together = more powerful tool
Mass spectra compared with database for identification
Used in forensics, environmental analysis, airport security and space probes
Mass spec
Mr = parent ion (M+) and peaks due to fragmentation
IR spec
Functional groups
NMR spec
low energy radio frequency radiation waves used
Chemical shift
compares frequency of NMR absorption with frequency of reference peak of TMS at 0ppm
Reference signal = tetramethylsilane TMS - used to calibrate against TMS reference peak
Why are denuterated solvents used
Instead of H solvents so no H2 peaks
Carbon NMR
C environments = chemical shift
Benzene look for mirrors
Proton NMR
Number of peaks, chemical shift, relative peak area, splitting pattern (n+1)
MRI
NMR spec
