Organic Chemistry III Flashcards
Benzene formula
C6H6
Another name for arenes and how this came about
Aromatic compounds - first found in sweet-smelling dyes
What is the most common type e of reaction of benzene?
Substitution - H for a different functional group
What is the shape of benzene
Flat, regular hexagon, bond angles = 120°
What is the bond length between adjacent C atoms?
Intermediate between C-C and C=C
What happens to the 4th electron of the p orbital of each C atom in benzene?
It delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron densities above and below the hexagon
What is the effect on the stability of the rings of benzene on electron density?
Makes benzene very stable, even though it is unsaturated (aromatic stability)
What is the thermochemical evidence that benzene that benzene is more stable than cyclohexa-1,3,5-triene?
Hydrogenation of cyclohexane = -120kJmol^-1
Cyclohexa-1,3,5-triene = -360kJmol^-1
Benzene hydrogenation = -208kJmol^-1
So benzene is 152kJmol^-1 more stable
Why is cyclohexane-1,3,5-triene not a suitable model for benzene?
- Would not be symmetrical (C=C is shorter than C-C), but in benzene it is
- Would easily undergo addition reactions across the double bonds but benzene does not
- Would form two isomers on the addition of Br2 or similar - benzene does not
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?
No it is non-polar
Dangers of benzene
It is a carcinogen
How do you name compounds with a benzene ring?
-benzene or phenyl-, this decides the position of the ring using numbers if there is more than one substituent
Why is benzene attacked by electrophiles?
High electron density above/below the ring due to delocalised electrons
What is delocalisation energy and what is the effect of this on the reactions of benzene
The large amount of energy that is needed to break the aromatic rings apart resulting in the aromatic ring almost always stating intact.
What is seen when benzene is combusted?
Smoky flames due to soot from unburnt carbon, this is due to the high carbon:hydrogen ratio.
Equation of the reaction between benzene and oxygen
C6H6 (I) + 7.5 O2 -> 6CO2 + 3H2O
Which ion is used to nitrate benzene
NO2^+ (plus charge on the nitrogen). Nitronium ion or nitryl cation.
How is NO2^+ ion generated? (Conditions and equations)
Concentrated H2SO4 and concentrated HNO3
H2SO4 + HNO3 -> H2NO3^+ + HSO4^-
H2NO3^+ -> H2O + NO2^+
Overall equation: H2SO4 + HNO3 -> HSO4^- + NO2^+ + H2O
How is the H2SO4 catalyst regenerated in the nitration of benzene?
HSO4^- + H+ -> H2SO4 (H+ from benzene ring)
Overall equation of nitration of benzene
C6H6 + HNO3 -> C6H5NO2 + H2O
What are the uses of nitrated arenes
- Production of explosives like 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 positive inductive effect (alkyl groups) affect further substitution?
They release electrons into the delocalised electron ring increasing the electron density and making further substitution reactions more likely. Direct substituents to the 2,4,6 positions.
What type of catalyst is used for a Freidel-Crafts reaction?
A halogen carrier (AlCl3)
An equation starting with AlCl3 and RCOCl to form an electrophile that could be sued to a chalet benzene
AlCl3 + ROCOCl -> AlCl4^- + RCO^+ (plus charge on C)
RCO^+ can attack benzene
What is happening when AlCl4^- is formed in terms of electrons?
The lone pair of electrons on the chlorine atom is forming a dative bond to Al
How is the AlCl3 catalyst reformed?
AlCl4- + H+ -> HCl + AlCl3 (H+ from benzene)
How would you use a Friedel-Crafts mechanism to add a methyl group tot the benzene ring?
Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene
If you are considering cyclic compounds, what might happen if two double bonds are in close proximity to each toehold
C=C are in close proximity, so electrons in p orbitals can partially delocalised and move between C=C double bonds
What effect would electrons in the p orbital moving between the C=C double bonds have on the stability of the molecule and its enthalpy of hydrogenation
Makes the molecule more stable and the enthalpy of hydrogenation more positive
Phenol formula
C6H6O
Reaction of phenol and bromine water
C6H6O + 3Br2(aq) -> 2,4,6-trinromophenol + 3HBr
What are the reasons for the relative ease of bromination of a phenol, compared to benzene?
In a phenol the lone pair of electrons on the oxygen is partially delocalised into the ring and therefore activates the ring and increases electron density, therefore electrophiles are more attracted to phenol.
What are the reasons for the relative ease of bromination of phenol, compared to benzene?
Phenols are very weakly acidic even weaker acids than carboxylic acids. Only carboxylic acids will react with sodium carbonate as a phenol is not strong anoint an acid to react.
What is the amine functional group?
R-NH2
How do you name amines
-amine or amino-
Why are amines so reactive.
The lone pair of electrons on the nitrogen due to the polar N-H bond
What shapes are amines around the N?
Trigonometry pyramidal, 107°, due to the lone pair on N
What kind of intermolecular forces do amines have and why?
Hydrogen bonding due to the polar N-H bond and lone pair of electrons on the N atom
Do amines have intermolecular forces which are stronger or weaker than alcohols? Why?
Weaker, as N has a lower electronegativity than O so weaker hydrogen bonding
Which primary amines are soluble in water/alcohols?
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 or non-polar solvents
Solubility of phenylamine
Not very soluble, due to the non-polarity of the benzene ring - C6H5 cannot form hydrogen bonds
When can amines act as bases
When they bond with a H+ ion
When can amines act as nucleophiles?
When they bond with an electron-deficient C atom (donate a lone pair from the nitrogen)
What is the product from the basic action of an amine with water?
RNH3+ - ammonium ion, which forms a salt with an anion
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?
Greatest electron density around the nitrogen atom, making it a better electron pair donor (attracts protons more)
Which type of amine is the strongest base
Primary aliphatic amines
- The alkyl groups are electron releasing and push electrons towards the nitrogen atom making it a strong base
Which type of amine is the weakest base?
Aromatic amines
- The lone pair on the nitrogen is partially delocalised into the ring and therefor decreases the attraction of the H+ ions
What does positive/negative inductive effect mean?
Positive inductive effect - donates electrons increasing density around N
Negative inductive effect - removes electrons decreasing density around N
What effect of alkyl groups have on density and base strength
Positive inductive effect - increase electron density around N resulting in a stronger base
What effect do aryl groups on electron density and base strength?
Negative inductive effect - decrease electron density around N resulting in a weaker base
Why are 3° amines never good bases
They are insoluble in water
Place these in order of base strength:
NH3, 1° amine, 2° amine, phenylamine
2° amine > 1° amine > NH3 > phenylamine
How can primary amines form 2°, 3° amines and 4° ammonium ions?
Multiple substitutions; primary amine is a nucleophile that attacks the original halogenoalkane
What are the problems with converting primary amines into 2°, 3° amines and 4° ammonium ions
Not efficient as low yield of 1° due to multiple substitutions
How would you maximise yield of primary amine?
Use excess ammonia
How do you get from a nitrile to a primary amine?
Reduction using Nickel/Hydrogen catalyst
Why is the reduction of a nitrile to from a primary amine a pure method
Since only the primary amine can be formed
How do you form an ammonium chloride salt form nitrobenzene? (State conditions)
Reduce the nitrile using Tin/HCl -> forms an ammonium salt with Cl- ions at room temperature
Equation of nitrobenzene to phenyl amine
C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O
What mechanism is used for forming amides from acyl chlorides and amines?
Nucleophilic addition/elimination
Equation for reaction of methylamine with HCl
CH3NH2 (aq) + HCl (aq) -> CHNH3+Cl- (aq)
Equation of reaction between ethyl amine and bromoethane
2CH3CH2NH2 + CH3CH2Br -> CH3CH2NH2CH2CH3 + CH3CH2NH3Br
Equation of reaction between ethylamine and copper (II) ions
4CH3CH2NH2 + Cu(H2O)6^2+ ->[Cu(CH3CH2NH2)4(H2O)2]2+ +4H2O
Which molecule is eliminated when a polyamide is formed
H2O
What are the two monomers used to form a polyamide
Diamond and dicarboxylic acid
Example of polyamides
Nylon, Kevlar
Which monomers is Nylon 6,6 made from?
1,6 diaminohexane and hexanedioic acid
How do you prepare amines from acrylic chlorides
Use a primary amine and the condition is room temperature
RCOCl + 2CH3NH2 -> RCOMHCH3 + CH3NH3 + Cl
Which monomers is Terylene made from
Benzene-1,4-dicarboxylic acid and ethane-1,2-diol
What are the two functional groups of amino acids?
NH2 and COOH (amine and carboxylic acid)
How many naturally occurring amino acids are there in the body?
20
Are alpha amino acids chiral and why?
Yes, one carbon has 4 different substituents. Excrete glycine where R=H
Which enantiomer do alpha amino acids as in nature?
(-) enantiomer
How can amino acids be synthesised industrially?
RCHO + NH4CN -> RCH(NH2)COOH + NH4Cl (hydrolysis, HCl is dilute)
- Need to reflux the reaction mixture
Is the product from amino acids being synthesised naturally optically active and why?
No, a racemic mixture is formed as the CN- ion can attack form above or below the planar C=O bond with equal likelihood. An equal amount of each enantiomer is formed, so no net effect on plane polarised light.
In what form do amino acids exist as solids? What effect does this have?
Zwitterions (ionic lattice) - high melting and boiling points
What colour solids are most zwitterions at room temperature?
White solids
Do zwitterions dissolve in water?
Yes, but not in non-polar solvents. This is due to the ionic polar bonds.
Define a zwitterions
Ions which have both a permanent positive and negative charge, but are neutral overall.
How do zwitterions occur in amino acids?
- COOH is deprotonated -> COO-
- NH2 is protonated -> NH3+
What happens to amino acids in acidic conditions?
Gains a proton on NH2 group
What is the peptide linkage?
-CONH-
What is a dipeptide?
Two amino acids bonded together
What is the name given to chains of amino acids up to 50 amino acids?
Polypeptides
What is name given to chains of amino acids with more than 50?
Proteins
Where are polypeptide and proteins found?
Enzymes, wool, hair, muscles
What is the process called in which polypeptides or proteins can be broken down into their constituent amino acids?
Hydrolysis
What conditions are needed for hydrolysis to occur?
6 moldm^-3 HCl, refill for 24 hours
What is a TLC plate made of?
Plastic sheet coated with silica, SiOw. This is the stationary phase. (The solvent is the mobile phase)
Describe how you would carry out thin layer chromatography
- Spot the samples onto a pencil line a few cm above the base of the TLC plate
- Place this in a beaker or tank with solvent level below the pencil line
- Ensure there is a lid on the beaker to keep the inside saturated with solvent vapour
- Wait until the solvent front is almost at the top of the TLC plate; then remove from the beaker and analyse
Why does TLC separate amino acids (or other molecules)?
The solvent carries amino acids up the TLC plate. The rate of movement depends on the balance between the amino acid’s affinity for the solvent (solubility in it) and affinity for the stationary phase (attraction to the silicon with hydrogen bonding)
What do you often have to do to enable the amino acids to be seen on the chromatogram?
- Spray with ninhydrin (amino acids are colourless, ninhydrin turns their spots purple)
- Or shine UV light on them
What would you observe in a melting point determination if the sample was not pure?
- Sample melts over a large range (more than 3°C)
- Samples melting point is below accepted value due to impurities disrupting structure
Why might the melting point appear different to the true value?
Temperature of the material in the machine might be different to the temperature shown on the thermometer - apparatus error.
When purifying by recrystallisation, why is the minimum volume of the hot solvent used?
So that a saturated solution is created so that as many crystals will fall out of solution as possible when it is cooled
Why is the solution filtered hot when purifying by recrystallisation?
To remove insoluble impurities and ensure that the crystals do not form in the filter paper
Why is the solution cooled in an ice bath when purifying by recrystallisation?
To ensure that as many crystals as possible fall out of solution - higher yield
Why are the crystals washed with cold water when purifying by recrystallisation?
To remove soluble impurities
How would you separate the crystals from the reaction mixture when purifying by recrystallisation?
Filter under reduced pressure using a Büchner funnel
Why might percentage yield be below 100%?(practical reasons)
- Product is lost during filtration, drying and weighing - spills, not all transferred from one piece of the apparatus to the other
- Product is left dissolved in the solution - some does not crystallise. Some left on filter paper. Sample is still wet
In a distillation setup, why is it necessary to have a continuous water flow around the condenser?
So that the water remains cool in order for the mixture to be distilled
Describe a method that can be used to separate immiscible liquids
- Pour the mixture into a separating funnel and some distilled water
- Add the stopper and invert the flask to mix the mixture
- Equalise the pressure by opening the stopper as required
- Continue shaking until there is no ‘whistle’ sound
- To collect the water in the lower layer, open the stopper and place a beaker under the spout
- Use another beaker to collect the desired organic layer
- Shake the liquid with some drying agent
Name two drying agents
- Magnesium sulphate
- Calcium chloride
How are drying agents used?
- Add a selected drying agent to the organic product
- If the drying agent forms clumps add some more until they are moving freely
- Use gravity filtration to collect the dry product
- Filtrate is the product
What does re-distillation mean?
When a liquid is purified by using multiple distillations
How to prepare a Grignard reagent
A halogenoalkane is dissolved in dry either and reacted with magnesium
What are the two ways to increase the length of the carbon chain in a molecule by the use of magnesium to form Grignard reagents
React Grignard reagent with:
- Carbon dioxide
- Carbonyl compounds in dry ether
