🔲 Topic 18 - Organics III

Question 1

What does aliphatic mean?

Answer 1

A straight chain, branched chain or ring of carbon atoms

Question 2

What does aromatic mean?

Answer 2

(Arenes) - contain a benzene ring

Question 3

Why is benzene resistant to bromination (compared with alkenes)?

Answer 3

Although the benzene ring is electron-rich and attracts electrophiles, the electron density in the delocalised pi-bond system is not significant enough to produce an electrophile (the Br+ from Br2 - a dipole to be induced) so benzene does not readily undergo electrophilic substitution (needs a catalyst). [Fairly high activation energy due to delocalised ring being so stable].
Alkenes have localised pi-orbital overlaps between two carbon atoms, in which the electron density is much greater than that in benzene. This greater electron density in alkenes allows a dipole to be induced more readily in bromine, making alkenes more susceptible to electrophilic attack.

Question 4

What is observed when benzene is burnt in air? (Combustion) Give an equation for complete and incomplete combustion.

Answer 4

A smoky (sooty) flame is seen (which is a common observation in compounds with a high C:H ratio)
Complete: C6H6 + 7.5O2 —> 6CO2 + 3H2O
Incomplete: C6H6 + 1.5O2 —> 6C + 3H2O

(Black smoke visible in incomplete combustion is carbon particulates)

Question 5

What are the reagents and conditions for hydrogenation (electrophilic addition) of benzene?

Answer 5

Reagents: 3H2
Conditions: heat under pressure, nickel catalyst
(Same as for hydrogenation of an alkene)

Question 6

What are the possible electrophilic substitution reactions of benzene, and what are the products?

Answer 6

• (halogenation) - with bromine/chlorine in presence of a catalyst, produces bromobenzene
• (nitration) - with concentrated nitric acid and a sulfuric acid catalyst, produces nitrobenzene
• (alkylation) - with a halogenoalkane and catalyst, produces alkylbenzene
• (acylation) - with an acyl chloride and catalyst, produces phenyl”ketone”
  (Alkylation and acylation are Friedel-Crafts reactions)

Question 7

What are the stages in electrophilic substitution reactions of benzene?

Answer 7

1. Generation of the electrophile using a catalyst (H2SO4 for nitration, and AlCl3/FeBr3 for the others)
2. Electrophilic substitution (mechanism)
3. Regeneration of the catalyst

Question 8

What are the reagents and conditions for the nitration of benzene?

Answer 8

Reagents: conc. HNO3 and H2SO4 (catalyst) to make the electrophile (NO2+)
Conditions: warm (55C - as at higher temperatures multiple substitutions can occur)

Question 9

What are the reagents and conditions for the halogenation of benzene?

Answer 9

Reagents: Br2/Cl2 and FeBr3/AlCl3 (catalyst) (to make electrophile - Br+)
Conditions: heated under reflux

Question 10

What are the reagents and conditions for the alkylation of benzene?

Answer 10

Reagents: halogenoalkane + FeBr3/AlCl3 (catalyst) to make electrophile (R+)
Conditions: anhydrous conditions (water would react with the catalyst and sometimes with the organic product)

Question 11

What are the reagents and conditions for the acylation of benzene? (Makes a phenyl”ketone”)

Answer 11

Reagents: acyl chloride + AlCl3 (catalyst) to make the electrophile (RC+=O)
Conditions: anhydrous conditions (under reflux)

Question 12

Is phenol more or less reactive than benzene? Why?

Answer 12

Phenol is more reactive than benzene.
The oxygen in the OH group of phenol has lone pairs of electrons which interact/merge with the electrons in the delocalised pi bond ring. Therefore the electron density above and below the ring of atoms increases making it much more reactive towards electrophiles as it polarises the Br-Br bond which breaks and the Br+ can attack the ring
(The delocalised electron ring in benzene is stable, so not very reactive)

Question 13

What are the observations seen when benzene reacts with bromine (water) and when phenol reacts with bromine (water)?

Answer 13

Benzene: no observations
Phenol: bromine water decolourises and a white precipitate (2,4,6-tribromophenol) is formed (which smells of antiseptic)

Question 14

What are the states at room temperature of benzene and phenol, and why?

Answer 14

Benzene: liquid
Phenol: solid (due to hydrogen bonding)

Question 15

What is the solubility of benzene and phenol, and why?

Answer 15

Benzene: insoluble
Phenol: soluble (due to hydrogen bonding)

Question 16

How do you prepare an aromatic amine? What from? Reagents? Conditions? Product? What extra step may be needed?

Answer 16

From: nitrobenzene
Reagents: reducing agent of tin, mixed with concentrated hydrochloric acid ([H])
Conditions: heated under reflux
Product: phenylamine (+water)
Extra step: phenylamine may react with acid present and form an ion, but can be converted back by adding alkali (OH-)

Question 17

Are all amines, in general, strong or weak bases?

Answer 17

All amines are weak bases

Question 18

Order, from weakest to strongest, the basicity (base strength) [/lowest pH to highest pH] of: ammonia, primary amines and phenylamine. Give reasons

Answer 18

(Base is proton acceptor)
WEAKEST: phenylamine < ammonia < short chain primary amine < long chain primary amine :STRONGEST
Phenylamine (C6H5NH2): much weaker base as the lone pair of electrons on the nitrogen are attracted to the delocalised electrons in the pi bond ring, making the nitrogen less electron-rich and the lone pair of electrons less available for donating to the hydrogen of a water molecule
Ammonia: no inductive effect
Short chain primary amine: stronger base than ammonia as the aliphatic chain is electron-releasing (repels the lone pair on the nitrogen) (an inductive effect) so the electron density on the nitrogen is greater than that of ammonia therefore accepts protons more readily
Long chain primary amine: a greater inductive effect due to longer aliphatic chain

Question 19

What is produced when an aliphatic amine (butylamine) is reacted with water? Why?

Answer 19

An alkaline solution (C4H9NH3+ + OH-)
The lone pair on the nitrogen can form a dative bond with the hydrogen of water molecule - the amine acts as a base (proton acceptor) [H+ conc. goes down (reacts with amine) so pH goes up]

Question 20

What is produced when an aliphatic amine (butylamine) is reacted with a (strong) acid? Type of reaction? Very simply, what happens? How do you reverse the reaction?

Answer 20

An ionic salt (in solution) eg. Butylammonium chloride (C4H9NH3+ + Cl-) or butylammonium sulfate (2C4H9NH3+ + SO42-)
- a neutralisation reaction
(The lone pair on the nitrogen is a H+ acceptor (so increases from -NH2 to -NH3+))
(An addition of NaOH (alkali) to the ammonium salt will convert it back to the amine)

Question 21

What is the solubility of amines (aliphatic and aromatic)?

Answer 21

• short chain primary aliphatic amines are completely soluble in water, due to hydrogen bond formation
• phenylamine is only slightly soluble in water (for same reason it is a weak base - lone pair of electrons are attached to delocalised electrons in pi bond …)

Question 22

What is produced when an aliphatic amine (eg. Butylamine) is reacted with copper (II) ions ( [Cu(H2O)6]2+ )? Include colour changes and state of the product dependent on reaction conditions.

Answer 22

A complex ion is formed

ADD BUTYLAMINE DROP BY DROP:
- [Cu(H2O)6]2+ (aq) + 2C4H9NH2 (aq) <—> [Cu(OH)2(H2O)4] (s) + 2C4H9NH3+
- light blue solution —> light blue precipitate (s)
- acid-base reaction
- the (lone pair on the) nitrogen of the butylamine acts as a base and accepts two H+ ions from two H2O ligands - this forms OH- ligands

ADD BUTYLAMINE IN EXCESS:
- [Cu(H2O)6]2+ + 4C4H9NH2 <—> [Cu(C4H9NH2)4(H2O)2]2+ + 4H2O
- light blue solution —> dark blue solution
- ligand exchange reaction
- the (lone pair on the) nitrogen of the butylamine acts as a ligand and substitutes around the central Cu2+, replacing 4H2O molecules

(Very similar to [Cu(H2O)6]2+ with ammonia)

Question 23

Are amides soluble in water? Why/why not?

Answer 23

The shorter chain aliphatic amides are soluble in water because they contain two electronegative atoms (N and O) and polar bonds (C=O and N-H) so can form hydrogen bonds with water

Question 24

What reactants are needed to make a polyamide, and what is it similar to?

Answer 24

• a DIcarboxylic acid (or a DIacyl chloride)
• a DIamine
  (Or a monomer with both these functional groups - an amino acid)

Very similar to the condensation polymerisation to make polyesters (DIol instead of a DIamine) except an amide bond is formed instead of an ester bond - remember a molecule of H2O or HCl is formed

Question 25

Are amino acids optically active?

Answer 25

All amino acids, except glycine (where R = H), contain a chiral carbon atom (bonded to four separate groups), meaning they are optically active, so a solution of amino acids will rotate plane-polarised monochromatic light (unless racemic mixture)
(Form enantiomers)

Question 26

Do amino acids act as acids, bases, both or neither? Why? What happens in specific conditions?

Answer 26

They can react as both acids and bases due to the two different functional groups within the molecule (NH2 and COOH) depending on the conditions of the reaction (ie. what the amino acid is reacted with)

• In acidic conditions (low pH/reaction with an acid), the COO- group is more likely to accept a hydrogen ion (acting as a base), producing an acidic end to the molecule
• In basic conditions (high pH/reaction with a base), the hydrogen ion in the NH3+ group is more likely to be lost (acting as an acid), producing a negative (basic) end to the molecule

Question 27

What is a zwitterion?

Answer 27

An electrically neutral molecule (as has a positive and negative charge that balance each other) that amino acids exist as (a H+ ion is transferred from COOH to the NH2 group)

Question 28

What is the isoelectric point? What does a low isoelectric point mean? What does a high electric point mean?

Answer 28

The pH at which the zwitterion exists in aqueous solution/ the pH at which the amino acid is neutral (no net charge) and exists in aqueous solution
- low isoelectric point - the molecule is predominantly acidic (ie. has more COOH groups than NH2)
- high isoelectric point - the molecule is predominantly basic (ie. has more NH2 groups than COOH)

Question 29

Are amino acids soluble?

Answer 29

They have good potential solubility (depending on the R-group) as they can form hydrogen bonds (O-H in the COOH group)

Question 30

What is a peptide bond? What is the product formed when a peptide bond is formed? What are the different possibilities of products?

Answer 30

• The bond that forms after a condensation reaction between two amino acids (between carbonyl group of one and amino group of the other - like polyamides)
• The organic product is a dipeptide
• When two different amino acids combine together to form a dipeptide, there are always two possibilities (or more if one or both the amino acids have more than one NH2 or COOH group) - the -OH could be lost off the first molecule, and -H off the second, or the -H could be lost off the first molecule, and -OH off the second

Question 31

What is a tripeptide?

Answer 31

The organic product formed when three different amino acids react together. There are 6 possibilities and the names can be abbreviated to eg. ala-cys-glu (instead of alanine-cystine-glutamic acid) - remember one amino acid may be present twice (ie. Not three different amino acids)

Question 32

What is a protein?

Answer 32

A form of condensation polymer formed from sequences of amino acids joined together by peptide bonds to form a polypeptide chain (the amino acids may all be different or the same)

Question 33

How can the structure of a protein be determined? (To find which amino acids are present NOT the order in which they occur in the polypeptide chain) Describe the steps in detail.

Answer 33

The proteins can be hydrolysed (under acidic conditions) to form the constituent amino acids, which can then be separated and identified by thin-layer chromatography
Hydrolysing the protein:
- prolonged heating with concentrated HCl (aq) [+ H2O +2H+ (for dipeptide) + 3H+ (for tripeptide) etc.] - this breaks the peptide bonds, but due to strongly acidic conditions, all amino acids formed will have the NH2 groups protonated as NH3+ (therefore product is all the original amino acids, but with NH3+ in place of NH2)
Using (thin-layer) chromatography:
- a mixture of amino acids produced by the hydrolysis of a protein can be spotted onto chromatography paper, with a suitable solvent in the beaker and the amino acids rise to different heights - the amino acids are colourless, so the chromatogram can be sprayed with a developing agent (ninhydrin which is toxic) and dryed with gentle heat - the Rf values can be calculated so individual amino acids can be identified (the greater the Rf value, the more soluble that amino acid is in that solvent)

Question 34

What are anti-bumping granules used for, and how do they work?

Answer 34

They are used in a reaction mixture to help make the boiling smooth/allows controlled boiling. This is achieved as they provide a surface for small bubbles to form/ distributes heat more evenly.

Question 35

What methods would you use to obtain a soluble liquid product from a reaction mixture that has a boiling temperature much lower than the other substances in the reaction mixture? What is the advantage of using the one method over the other?

Answer 35

Simple distillation/fractional distillation
- simple distillation should only be used if the boiling temperature of the liquid being purified is very different from the other liquids in the mixture (ideally more than 25C difference)
- fractional distillation takes longer than simple distillation so best used when difference in boiling temperatures is small and when there are several compounds to be separated from a mixture (provides better separation)

Question 36

In simple distillation how do you know when a compound has been distilled?

Answer 36

A thermometer is used to monitor the temperature of the vapour as it passes into the condenser - if the temperature remains steady, this is an indication that one compound is being distilled, if after a while the temperature begins to rise, this indicates that a different compound is being distilled

Question 37

What is added to the fractionating column in fractional distillation, and what does it help to do?

Answer 37

The fractionating column is usually filled with glass beads or pieces of of broken glass which act as surfaces on which the vapour leaving the column can condense, and then be evaporated again as more hot vapour passes up the column - effectively the vapour undergoes several repeated distillations as it passes up the column, which provides better separation

Question 38

What method can be used to separate an insoluble liquid from an aqueous solution? Briefly describe the process and state the advantage of it.

Answer 38

Steam distillation
- involves passing steam into the reaction mixture (containing an aqueous solution and a liquid that forms a separate layer - insoluble liquid)
- the agitation of the liquid caused by the steam bubbling through the mixture ensures that both the insoluble liquid and the aqueous solution are on the surface of the mixture and so can form part of the liquid that evaporates (ie. The one with the lower boiling temperature can evaporate?)

The advantage of this process is that the insoluble liquid is removed from the reaction mixture at a temperature below its normal boiling temperature, so less chance of thermal decomposition

Question 39

Briefly describe the method of solvent extraction (another method of separating a soluble liquid from a reaction mixture)

Answer 39

• Place reaction mixture in a separating funnel, then add solvent - it should form a separate layer
• Place stopper in the neck of the funnel and gently agitate the contents of the funnel (when upside down, open the tap to release pressure)
• Allow the contents to settle into the two new layers
• Remove the stopper and open tap to allow the lower layer to drain into a flask, then upper layer into a separate flask
• Can then use simple or fractional distillation to separate the desired organic product from the added solvent

Question 40

What are the requirements for the choice of solvent used in solvent extraction, and how should it be used?

Answer 40

• the solvent added should be immiscible (ie. does not form a mixture) with the solvent containing the desired organic product (forms a separate layer)
• the desired organic product should be much more soluble in the added solvent than in the reaction mixture
• it is better to use the solvent in small portions rather than in a single volume because this is more efficient (removes more of the desired organic product) eg. four portions of 25cm3 rather than one portion of 100cm3

Question 41

What does ‘washing’, to remove impurities from a solid or from a liquid, involve? How should the solvent be chosen?

Answer 41

Purification of solids: the impure solid is stirred in some of the solvent, then the mixture filtered in a filter funnel (or the solvent added on top of the solid if already in the filter funnel)
Purification of liquids: the impure liquid would be mixed with the chosen solvent, then shaken in a separating funnel, then drained as usual once layers form

• the solvent could be water or an organic solvent, but whichever solvent used, it must be chosen carefully so as to dissolve the impurities but as little as possible of the substance being purified

Question 42

What does it mean to ‘dry’ an organic product?

Answer 42

• an organic solid can be dried just by leaving it in a warm place or in a desiccator with a suitable drying agent (to remove water)
• an organic liquid may partially, or even completely dissolve in water (water from preparation of the liquid product as inorganic reagents are often in aqueous solution) - the water may therefore be an impurity that needs to be removed by a drying agent (but the drying agent must not react with the organic liquid)

Question 43

What is used as a drying agent when using the method of drying to purify an organic liquid? How does it work? How do you know when the liquid is dry?

Answer 43

The most common drying agents are anhydrous metal salts (such as calcium chloride, magnesium sulfate, sodium sulfate) - all form hydrated salts, so when they come into contact with the water in the organic liquid they absorb the water as water of crystallisation.
The drying agent is added to the organic liquid and the mixture is swirled, then left. Before use, a drying agent is powdery, but after absorbing water it looks more crystalline - if a bit more drying agent is added and remains powdery, this indicates the liquid is dry. The liquid also goes from cloudy to clear when water is removed.
The drying agent is removed either by decantation or by filtration.

Question 44

What is the purpose of recrystallisation and what is the principle that it is based on?

Answer 44

• a traditional way of removing impurities from an organic solid
• based on the principle that the solubility of most solids increases with increased temperature

Question 45

Describe the process of recrystallisation

Answer 45

• a hot solvent is added to the mixture to dissolve both the organic solid and the impurities (use the minimum amount of solvent to avoid loss of the product - just enough to produce a saturated solution [a solution with solute that dissolves until it is unable to dissolve anymore, leaving the undissolved substances at the bottom] of the impure compound)
• filter the hot solution to remove insoluble impurities using gravity/ reduced pressure filtration (hot filtration so the organic solid does not begin to crystallise)
• let the solution cool - as the solution cools down, the solubility of the compound decreases so crystals form (more crystals can be obtained by cooling the solution below room temperature in an ice bath) - [as the concentration of impurities is much lower, they take longer to form crystals and hence will remain in solution] - (slow cooling results in bigger, well defined crystals which are easier to filter and dry)
• the mixture is then filtered to recover the crystals (remove soluble impurities) using filtration under reduced pressure (Buchner funnel) [a faster method than gravity filtration]
• after filtration, the crystals are washed (to make sure no soluble impurities form on the solid as it dries) with fresh cold solvent and then left to dry on filter paper or in a desiccator/oven

Question 46

How can the method of recrystallisation be adapted to improve the purity of the product? What is the downside to this?

Answer 46

Recrystallisation can be repeated more than once to ensure a very pure product, but each time the yield of the product will decrease

Question 47

How can you test HOW pure an organic compound is?

Answer 47

• if the compound is a solid, it’s melting temperature can be measured, and compared to the known value of the pure compound
• if it is a liquid, it’s boiling temperature can be measured, and compared to the known value of the pure compound

Question 48

How do impurities affect the melting temperature of an organic solid?

Answer 48

• Impurities REDUCE the melting temperature of an organic solid
• The organic solid will also melt over a range of temperatures rather than just at one specific temperature

Question 49

How do you measure the melting temperature of a solid, and how does this help determine whether the solid is impure or not?

Answer 49

To measure the melting temperature: place some of the solid in a small capillary tube attached to the bulb of a thermometer, then place in a liquid that has a boiling temperature above that of the melting temperature of the solid (so the solid melts before the liquid boils) / in an electrical device
This recorded melting temperature can be compared with the known value of the pure compound (impurities reduce the melting temperature)

Question 50

How do impurities affect the boiling temperature of an organic liquid?

Answer 50

• Impurities INCREASE the boiling temperature of an organic liquid
• The organic liquid will also boil over a range of temperatures rather than at just one specific temperature

Question 51

How do you measure the boiling temperature of an organic liquid and how does this help do determine whether it is impure?

Answer 51

To measure the boiling temperature: the apparatus used depends on the volume of liquid available, and whether it is toxic/flammable, but generally the apparatus for simple distillation is used
This boiling temperature is recorded which can be compared with the known value of the pure compound (impurities increase the boiling temperature)

Question 52

What are some disadvantages of determining the boiling temperature to test the purity of a liquid organic compound?

Answer 52

• the test might not be conclusive - may not be able to measure the boiling temperature accurately enough, so even if the boiling temperature exactly matches the one of a pure compound, you may wrongly assume your compound is pure
• some different organic compounds actually have the same boiling temperature

Question 53

What’s the difference between qualitative and quantitative analysis?

Answer 53

Qualitative analysis - to find out WHICH elements are in a compound
Quantitative analysis - to find out the percentage composition by mass of the elements in a compound

Question 54

What’s one method of finding the percentage composition by mass of the elements in a compound (quantitative analysis), and how does it work?

Answer 54

Combustion analysis - a known mass of the compound is burned in oxygen and the masses of carbon dioxide and water formed are found by measuring the increase in mass of U-tubes containing the absorbers (one for water, one for carbon dioxide)

Question 55

What would happen to the yield of a product if, in recrystallisation, the filter paper and funnel are not warmed in an oven before? (for the step to remove insoluble impurities)

Answer 55

The yield would be reduced as the substance would crystallise prematurely and hence won’t pass through the filter paper to be collected

Question 56

How would you make an N-substituted aromatic amide?

Answer 56

From: phenylamine (from nitrobenzene, from benzene)
Reagent: acyl chloride (length dependent on product)
Condition: room temp.

[Therefore for aliphatic: n* amine + acyl chloride —> (n+1)* amide]