Download Benzene, amines, amino acids and polymers File

Name the homologous series
C4H10
C4H8
In groups discuss everything you know about
these 2 homologous series.
What type of hydrocarbons do they belong to?
What about C6H12? What structures are possible
with this molecular formula?
What type of hydrocarbons do these belong to?
What about C6H6?
STRUCTURE OF BENZENE
Primary analysis revealed benzene had...
an
a
a
empirical formula of CH
molecular mass of 78
formula of C6H6
and
and
STRUCTURE OF BENZENE
Primary analysis revealed benzene had...
an
a
a
empirical formula of CH
molecular mass of 78
formula of C6H6
Kekulé
and
suggested that benzene was...
PLANAR
CYCLIC and
HAD ALTERNATING DOUBLE AND SINGLE BONDS
STRUCTURE OF BENZENE
HOWEVER...
• it did not readily undergo electrophilic addition - no true C=C bond
• only one 1,2 disubstituted product existed
• all six C—C bond lengths were similar; C=C bonds are shorter than C-C
• the ring was thermodynamically more stable than expected
STRUCTURE OF BENZENE
HOWEVER...
• it did not readily undergo electrophilic addition - no true C=C bond
• only one 1,2 disubstituted product existed
• all six C—C bond lengths were similar; C=C bonds are shorter than C-C
• the ring was thermodynamically more stable than expected
To explain the above, it was suggested that the structure oscillated
between the two Kekulé forms but was represented by neither of
them. It was a RESONANCE HYBRID.
Benzene
• To be able to describe and explain the structure of benzene
• To know specific reactions associated with benzene and other
common substituted aromatic compounds
C6H6
= benzene
= an aromatic hydrocarbon
= an arene
X-ray diffraction and evidence for benzene’s structure
Two structures are used to represent benzene:
The KEKULÉ structure; 1865-1872
Cyclohexa-1,3,5-triene
Bond lengths would be
0.154nm and 0.134nm
alternating
The modern DELOCALISED structure – 1930s
X-ray diffraction data has shown that the carbon atoms in benzene are at the corners
of a REGULAR HEXAGON. These data have also shown all C-C bonds to be 0.139nm;
therefore an intermediate between a single and double bond
Thermochemical data and stability of benzene
When unsaturated hydrocarbons are reduced to the corresponding saturated compound,
energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation)
can be measured.
When cyclohexene (one C=C bond) is reduced to
cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g)
C6H12(l)
If benzene contained three separate C=C bonds how
much energy would it release per mole when
reduced to cyclohexane
Theoretical
- 360 kJ mol-1
(3 x -120)
Theoretically, if benzene contained three separate
C=C bonds it would release 360kJ per mole when
reduced to cyclohexane
C6H6(l) + 3H2(g)
C6H12(l)
Benzene releases only 208kJ per mole when
reduced, putting it lower down the energy
scale
2
- 120 kJ
mol-1
3
Experimental
- 208 kJ mol-1
Thermochemical data and stability of benzene
Benzene is 152kJ per mole more stable than expected.
This value is known as the RESONANCE ENERGY.
Theoretical
- 360 kJ mol-1
(3 x -120)
2
- 120 kJ
mol-1
MORE STABLE THAN
EXPECTED
by 152 kJ mol-1
3
Experimental
- 208 kJ mol-1
Consider the electron configuration of carbon
The electronic configuration of a
carbon atom is 1s22s22p2
2p
2
2s
1
If you provide a bit of energy you can
promote (lift) one of the s electrons
into a p orbital. The configuration is
now 1s22s12p3
1s
2p
2
2s
1
Why would this be favourable?
1s
Hybridisation of orbitals
The four orbitals (1 x s and 3 x p) combine (HYBRIDISE) to give 4 new
orbitals. All four orbitals are energetically equivalent.
2s22p2
2s12p3
4 x sp3
HYBRIDISE
sp3 HYBRIDISATION
Hybridisation of orbitals
Alternatively, only 3 orbitals (1 x s and 2 x p) combine (HYBRIDISE) to
give 3 new orbitals.
All 3 orbitals are energetically equivalent.
The remaining 2p orbital is unchanged.
2s22p2
2s12p3
3 x sp2
2p
HYBRIDISE
sp2 HYBRIDISATION
2s22p2
Hybridisation
of
orbitals
1
3
2s 2p
HYBRIDISE
3 x sp2
sp2 HYBRIDISATION
2s22p2
2s12p3
HYBRIDISE
sp3 HYBRIDISATION
4 x sp3
2p
Alkanes vs Alkenes
In ALKANES, the 4 sp3 orbitals repel each
other into a tetrahedral arrangement.
In ALKENES, the 3 sp2
orbitals repel each other
into a planar arrangement
and the 2p orbital lies at
right angles to them
Alkenes
Covalent bonds are formed by
overlap of orbitals.
The resulting bond is called a
SIGMA (δ) bond.
An sp2 orbital from each carbon
overlaps to form a single C-C bond.
Alkenes
The two 2p orbitals also overlap. This forms a second bond; it is known as a PI
(π) bond.
For maximum overlap and hence the strongest bond, the 2p orbitals are in
line.
This gives rise to the planar arrangement around C=C bonds and a shorter
bond than in the corresponding alkane (0.134nm).
Ethene
2 sp2 orbitals overlap to form a
sigma bond between the 2 carbon
atoms
s orbitals in hydrogen overlap with
the sp2 orbitals in carbon to form C-H
bonds
2 2p orbitals overlap to form a pi
bond between the 2 carbon atoms
the resulting shape is
planar with bond angles
of 120º
Structure of benzene – a delocalised structure
Theory:
Instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds are delocalised around the ring by
overlapping the p orbitals.
There would be no double bonds and all bond lengths would be equal. It would also
give a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
another
possibility
delocalised pi
orbital system
This final structure fits with the reactivity of benzene:
1. Very stable
2. Resistant to electrophilic addition, typically seen in alkenes.
3. Undergoes electrophilic substitution, which does not affect the
delocalised pi orbital system.
Benzene
X-ray studies show that,
• A Benzene molecule is a flat
(planar) molecule. All carbon
and hydrogen atoms lie in
the same plane.
• It has a regular hexagon
structure with all six carbon
atoms lying at the corners.
Each carbon atom is bonded
to three other atoms.
• All carbon-carbon bond
lengths are equal at 139 pm.
• All CC angles (or CH angles)
are equal at 120°.
IR spectra show that benzene does not have the typical strong absorptions of CH
bonds in CH2 and CH3 groups in the wavenumber range 2962-2853 cm-1 , nor the C=C
absorption of an alkene, like oct-1-ene, just below 1700 cm-1. Instead, and unlike
alkanes and alkenes, benzene has strong absorptions at about 3050 cm-1and 750 cm-1.
All this provides further evidence that benzene does not have normal C-C or C=C
bonds in its structure.
Naming arenes
Be careful, this can be confusing!
How do you
name
benzene
derivatives
with more
than one
substituent?
•
Names use either phenyl (C6H5- notation) or
the benzene notation.
•
General rule is that when a hydroxy (OH) or
amine (NH2) group is substituted into the ring,
they use the PHENYL notation – Phenol and
phenylamine respectively.
•
In majority of other cases, compounds named
as substituted products (derivatives) of
benzene, e.g. nitrobenzene (C6H5-NO2) or
methylbenzene (C6H5-CH3)
1,3,5-trichlorobenzene
1-bromo-4-chlorobenzene
phenylamine
Benzene – Key reactions
Benzene reacts mainly via electrophilic substitution.
The electron density of the benzene ring attracts electrophiles.
Reactions are NOT addition.
Instead, electrophiles substitute into the ring, maintaining the delocalised
structure and stability of the ring.
N.B. Benzene can be drawn as either:
BUT remember, benzene IS NOT made up of alternating
single and double bonds!
Reactions of benzene (derivatives)
The reactions of arenes
• The simplest and most important arene is benzene. Unfortunately, benzene is toxic
and mildly carcinogenic, so it cannot be used except in research and certain
industrial processes. Fortunately, the reactions of benzene are also given by many
of its derivatives and in the experiments below you will be using methoxybenzene
• Besides studying the reactions of arenes you will also be comparing
arenes with alkanes and alkenes by repeating the methoxybenzene
tests with cyclohexane and cyclohexene.
• Wear goggles for the whole of this class practical and remember to
keep bottles of methoxybenzene, cyclohexane and cyclohexene well
away from any flames.
A Combustion
• Working in a fume cupboard, put 3 drops of methoxybenzene on a small pea-sized
ball of mineral wool in a crucible. Set fire to the methoxybenzene using a lighted
splint.
• Repeat the test with cyclohexane and then with cyclohexene.
1. Describe and compare the flames from each compound.
2. Write an equation for the combustion of methoxybenzene bearing in mind what
you have observed.
B Bromination
• Carefully, add 5 drops of methoxybenzene to 1 cm3 of 2% bromine in an inert
solvent. Dip a glass rod in concentrated ammonia and bring this near the mouth of
the test tube to see if fumes of hydrogen bromide have been produced.
• Repeat the test with cyclohexane and then with cyclohexene.
Describe what happens with each compound.
1. Which of the compounds
a)
reacted with the bromine
b)
reacted to produce hydrogen bromide?
2. Which of the compounds have taken part in
a)
substitution reactions
b)
addition reactions?
(Hint: Would HBr be produced in an addition reaction?)
Write possible equations for the reactions which have occurred. (Remember that the
reactions of methoxybenzene will involve the benzene ring.)
C Nitration
• Carefully add 1 cm3 of concentrated nitric acid to 1 cm3 of water. Then, add 5 drops
of methoxybenzene and warm the mixture in a water bath.
Describe what you observe.
1. When methoxybenzene reacts with nitric acid, one of the products is
methoxynitrobenzene, CH3O-C6H4-NO2. Write a possible equation for the
reaction.
2. In this class practical, like many others with organic chemicals, you have used
toxic, corrosive and highly flammable materials.
What steps have been taken in this practical to use them safely and successfully?
Key reactions of benzene
Nitration:
Reagents
conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions
heat under reflux at 55°C
Equation
C6H6 + HNO3
Halogenation:
Reagents
chlorine and a halogen carrier (catalyst)
Conditions
reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3).
C6H5NO2 + H2O
nitrobenzene
Chlorine is non polar so is not a good electrophile.
The halogen carrier is required to polarise the halogen
Equation
C6H6 + Cl2
C6H5Cl + HCl
Key reactions of benzene
Sulfonation
Reagents
‘fuming’ sulfuric acid (mixture of conc H2SO4 and
dissolved SO3 – conc H2SO4 at r.t. does not react in this way)
Conditions
room temperature
[H+]
Equation
C6H6 + SO3
Hydrogenation
Reagents
H2 in presence of Ni catalyst
Conditions
Heat at 200ºC
Equation
C6H6 + 3H2
C6H5SO3H
C6H12
(benzene sulfonic acid)
Key reactions of benzene
Friedel-Crafts: Alkylation
Overview
Alkylation involves substituting an alkyl (methyl, ethyl) group – hard to
limit substitutions (alkyl groups activate ring)
Reagents
Halogenoalkane (RX) and anhydrous aluminium
chloride AlCl3
Conditions
reflux
Electrophile
a carbocation R+ (e.g. CH3+)
Equation
C6H6 + C2H5Cl
C6H5C2H5 + HCl
Industrial Alkylation
Industrial Alkenes are used instead of haloalkanes but an acid must be present
Phenylethane, C6H5C2H5 is made by this method
Reagents
ethene, anhydrous AlCl3 , conc. HCl
Electrophile
C2H5+ (an ethyl carbonium ion)
Equation
C6H6 + C2H4
Mechanism
the HCl reacts with the alkene to generate a carbonium ion
electrophilic substitution then takes place as the C2H5+ attacks the ring
Use
ethyl benzene is dehydrogenated to produce phenylethene (styrene);
this is used to make poly(phenylethene) - also known as polystyrene
C6H5C2H5
(ethyl benzene)
Key reactions of benzene
Friedel-Crafts: Acylation
Overview
Acylation involves substituting an acyl (methanoyl, ethanoyl) group.
Electron withdrawing carbonyl ensures monosubstitution
Reagents Acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3
Conditions Reflux 50°C
Electrophile
Equation
RC+= O ( e.g. CH3C+O )
C6H6 + CH3COCl
C6H5COCH3 + HCl
Mechanisms of substitution
Describe the mechanism of the electrophilic
substitution reactions of benzene in:
• Nitration
• Friedel-Crafts reactions
• Halogenation
– including the formation of the electrophile
Electrophilic substitution - steps
1. Formation of the electrophile e.g. NO2+
2. Electrons from ring form intermediate
Delocalisation remains
over 5 carbons
3. H+ removed from intermediate reforming ring of
delocalised electrons
4. Reform catalyst
Formation of the electrophile
• Nitration
HNO3 + H2SO4  H2NO3+ + HSO4-  H2O + NO2+ + HSO4-
• Alkylation
CH3CH2Cl + AlCl3  CH3CH2+ + AlCl4• Acylation
CH3COCl + AlCl3  CH3CO+ + AlCl4• Halogenation
Br2 + FeBr3  Br+ + FeBr4-
Phenol
melting
point (°C)
boiling
point (°C)
C6H5OH
40 - 43
182
C6H5CH3
-95.0
111
Increased acidity: delocalisation of charge
Decrease acidity: high electronegativity of oxygen
 Very weak acid
Reactions of phenol
Reactivity of phenol
• Increased electron density
• Easier to donate electrons
Amines
• Methylamine: CH3NH2
• Dimethylamine: (CH3)2NH
• Trimethylamine: (CH3)3N
Reactions of amines
i characteristic smell
Fishy!
ii miscibility with water
Short chain alkylamines – soluble
Long chain or arylamines – insoluble
Increasing solubility: hydrogen bonding and reaction with water
Decreasing solubility: Non-polar organic chains and rings
Aryl amines less alkaline than alkyl amines – why?
iii formation of salts with acid
Form ionic salts that are more soluble than the original amine
R-NH2 + HCl  RNH3+ + Cl-
Hydrated metal ions
2+
H
O
H
H
H H
O
O
Cu
H
H
H
Hexaaquacopper (II) ion
H
H
O
O
O
Covalent
bond
H
H
Dative bond
Hydrated metal ions
H
H
O
H
Cu
Dative bond
H
H
H
O
O-H
O
Covalent
bond
H H
O
O-H
(s)
An ammine complex ion
2+
H
O
H
NH3
H3N
Cu
NH3
H3N
H
O
H
Tetraamminediaquacopper (II) ion
Reactions of amines
iv complex ion formation with copper(II) ions
Lone pair of electrons on nitrogen forms dative bonds to metal
cation
v treatment with ethanoyl chloride and halogenoalkanes
Amines act as nucleophiles
Reactions of amines
Cause of formation of secondary, tertiary and
quaternary amines
Preparation of phenylamine
1. Nitration of benzene
2. Reduction of nitrobenzene
N.B. LiAlH4 is not a suitable reagent
Preparation of phenylamine
Formation of diazonium ions
NaNO2/HCl (forms HNO2 in situ)
• RNH2
R-N+≡N Cl• Alkyl diazonium ions decompose on formation to
form R+ + N2 (g)
• R+ joins with nearest nucleophile to form e.g. ROH
or RCl
• Benzenediazonium ions are stable below 10oC
• Why is this reaction done at 5oC?
Reactions of benzenediazonium ion
• If allowed to warm above 10oC:
– forms benzene cation and nitrogen gas
– cation reacts to form e.g. phenol or chlorobenzene
• Reaction with phenol or phenylamine class
compounds – azo dyes
– N.B. In fact the phenol has formed phenoxide in the
solution of sodium hydroxide
Azo dyes
• Predict the structure of the azo dye formed
between benzenediazonium and napthalene-2-ol
Homework
Synthetic techniques
Tuesday: organic preparation practical assessment
Produce a hand illustrated description of each of these techniques:
• refluxing
• purification by washing, eg with water and sodium carbonate
solution
• solvent extraction
• recrystallization
• drying
• distillation
• steam distillation
• melting temperature determination
• boiling temperature determination
Review questions 1-3 p 206
Amines and amides
• Why is the amine basic when the amide isn’t?
What would happen if…
• 1,6-diaminohexane came into contact with hexan1,6-dioyl chloride?
• Draw the products
• What class of reaction is this?
Polymers
• Addition
–
–
–
–
Alkenes
Double bonds open
Initiator starts reaction e.g. peroxide
Polystyrene
Addition polymers
POLYTHENE
Addition polymers
POLYTETRAFLUOROETHENE
(PTFE – Teflon)
Addition polymers
POLYPROPENE
Addition polymers
POLYCHLOROETHENE (PVC)
Polymers
• Condensation
– Diol and dioyl chloride/dioic acid or diamine and dioyl
chloride/dioic acid
– Small molecule eliminated
Condensation polymerisation
Formation of nylon
Simplified condensation
polymerisation
H2N--NH2
HOOC--COOH H2N--NH2 HOOC--COOH

-HN--NHOC--COHN--NHOC--COH-O-H
H-O-H
H-O-H
chloroethene
tetrafluoroethene propene
Poly(propene)
Poly(tetrafluoroethene)
Poly(chloroethene)
Properties of polymers
Nylon (polyamide)
• Soften above their melting temperatures, Tm,
thermoplastics
• The amorphous regions contribute elasticity and
the crystalline regions contribute strength and
rigidity.
• The planar amide (-CO-NH-) groups are very polar,
so nylon forms multiple hydrogen bonds among
adjacent strands. Because the nylon backbone is so
regular and symmetrical nylons often have high
crystallinity and make excellent fibres.
• Parallel strands can participate in extended,
unbroken, multi-chain β-pleated sheets, a strong
and tough supermolecular structure similar to that
found in natural silk.
• When dry, polyamide is a good electrical insulator.
However, polyamide is hygroscopic. Nylon is less
absorbent than wool or cotton.
Properties of polymers
Poly(ethenol) (PVA, Polyvinyl alcohol )
• Water soluble polymer
• It is odourless and nontoxic.
• It has high tensile strength and flexibility, as
well as high oxygen and aroma barrier
properties.
• However these properties are dependent
on humidity. The water, which acts as a
plasticiser, will then reduce its tensile
strength.
• It is fully degradable and dissolves quickly
• Used as a water-soluble film useful for
packaging. An example is the envelope
containing laundry detergent in "liqui-tabs".
Amino acids
i acidity and basicity and the formation of zwitterions
ii separation and identification by chromatography
iii effect of aqueous solutions on plane polarised monochromatic light
iv formation of peptide groups in proteins by condensation
polymerization
v reaction with ninhydrin.
Proteins
Structure
• Where R can be a variety of groups e.g.
-H
glycine
-CH3
alanine
-CH2CH2COOH
glutamic acid
-CH2OH
serine
-CH2SH
cysteine
-CH2CH2CH2CH2NH2
lysine
• Proteins are pH sensitive
– predict the structure of an amino acid at pH1, 7 and 12
• In neutral solutions the acid group (-COOH) loses its hydrogen ion
becoming negative and the amine group gains a hydrogen ion to
become positive:
• H3N+CH2COOZwitter ion
Isoelectric point
• The charges cancel so it would not be attracted towards either a
positive or negative electrode (iso = same)
• In acidic conditions (pH < 7 e.g. fruit or vinegar) hydrogen ions are
added to the –COO• H3N+CH2COOH
• This would be attracted towards the cathode (negative electrode)
• In alkaline conditions (pH > 7 e.g. baking soda) hydrogen ions are
taken from the positive amine group
• H2NCH2COO• This would be attracted towards the anode (positive electrode)
• The acidic (e.g. glutamic acid and aspartic acid) and basic (e.g.
ornithine, arginine, lysine and histidine) side chains are also affected
• Different proteins have their isoelectric points at different pHs
depending on the combination of acidic and basic side groups
• pH effects proteins because removing charges from side chains
removes the electrostatic attractions between different sections and
the tertiary structure unravels (denatures) e.g. souring milk – pH 4.6
the protein separates as curd
The two enantiomers of alanine,
D-Alanine and L-Alanine
Of the standard α-amino acids, all but glycine can exist in either of
two enantiomers, called L or D amino acids, which are mirror images
of each other.
While L-amino acids represent all of the amino acids found in proteins
during translation in the ribosome, D-amino acids are found in some
proteins produced by enzyme posttranslational modifications
Polymerisation - condensation
Using chromatography
Chromatography involves the separation and identification of
compounds using a stationary phase (solid) and a mobile phase
(liquid or gas).
Identifying amino acids - ninhydrin and Rf values