Organic III

Question 1

What is the amine functional group?

Answer 1

-N-R2, where R is either a H or alkyl or aryl group.

Question 2

How are amines classified?

Answer 2

If there is one alkyl/aryl group bonded to the nitrogen, it is a primary amine. If there are two alkyl/aryl groups bonded to the nitrogen, it is a secondary amine. If there are three alkyl/aryl groups bonded to the nitrogen, it is a tertiary amine. If there are four groups bonded, it is a quaternary ammonium ion (the nitrogen will have a positive charge).

Question 3

How do you name amines?

Answer 3

The amine prefix is amino and the suffix is amine. All carbon chains are treated as side chains (e.g. methylamine, 2-methyl butylamine, ethanamine). If there are multiple of the same group bonded to the nitrogen, use di, tri etc. (e.g. trimethylamine).

Question 4

What are the boiling points of amines like compared to similar size alkanes and alcohols?

Answer 4

They will have similar molar masses, so similar strength London forces (as the number of electrons will be similar). Alkanes will have lower boiling points as they can’t form permanent dipole - dipole forces while alcohols and amines can. Alcohols would have higher boiling points than similar amines as oxygen is more electronegative than nitrogen, so the hydrogen bonds formed between alcohol molecules are stronger than between amine molecules.

Question 5

Why are amines generally soluble in water?

Answer 5

The nitrogen in amines has a lone pair which can form hydrogen bonds to the hydrogen atoms is water, while the hydrogens bonded to the nitrogen in the amine can form hydrogen bonds with the oxygen atom of water. Amines form alkaline solutions when they dissolve because the nitrogen can accept a hydrogen from water forming alkyl ammonium ions and hydroxide ions. The larger the amine, the less soluble it will become as the non-polar alkyl chains can’t hydrogen bond and will form London forces with each other rather than with water.

Question 6

How does pKa relate to the basicity of amines?

Answer 6

The lower the pKa of an amine, the weaker the base as less acid needs to be added to half-neutralise the amine (so it doesn’t hold on to the H+ as well). The higher the pKa of an amine, the stronger the base as more acid needs to be added to get it to half-neutralisation (so it holds onto H+ better). This is the opposite trend to acids (lower pKa=stronger acid).

Question 7

How is the basicity of amines increased?

Answer 7

Add alkyl groups (or any electron donating group) as these have a positive inductive effect, meaning they push electrons towards the nitrogen. If the electron density on the nitrogen is greater, the nitrogen has more of a negative charge, so is more attractive to protons (so more basic). It also stabilises positive charge on the positive nitrogen formed.

Question 8

How is the basicity of amines decreased?

Answer 8

Aromatic primary amines are less basic than aliphatic primary amines as the lone pair on the nitrogen delocalises into the pi system of the benzene ring, making the nitrogen less negative and therefore less attracted to protons and the ion formed is less stabilised. Using any electron withdrawing group (e.g. carbonyls) will pull electrons away from the nitrogen and make it less negative.

Question 9

How can amines be prepared from halogenoalkanes?

Answer 9

Amines can be produced from halogenoalkanes by nucleophilic substitution with ammonia as the nucleophile. If a primary amine is desired, excess ammonia should be used. If not, excess halogenoalkane can be used to encourage multiple substitutions, however, this is difficult to control so you are likely to end up with a tertiary amine or a quaternary ammonium ion (so this is nit a good method for making secondary amines).

Question 10

How can amines be produced from nitriles?

Answer 10

By reduction, either with H2/Ni catalyst (e.g. CH3CH2CN + 2H2 —> CH3CH2CH2NH2) or with LiAlH4 in dry ether (e.g. CH3CH2CN + 4[H] —> CH3CH2CH2NH2).

Question 11

How can amines be produced from aromatic nitro-compounds?

Answer 11

Aromatic amines can be made by the reduction of aromatic nitro-compounds. The conditions for this is boil under reflux with tin (catalyst) and concentrated HCl.

Question 12

How do amines react with water?

Answer 12

X-amine + H2O —> X-ammonium ion + OH-

Question 13

How do amines react with acids?

Answer 13

X-amine + acid —> X-ammonium salt

Question 14

How do amines react with acyl chlorides?

Answer 14

Amine + acyl chloride —> N-substituted amide
This occurs by addition-elimination

Question 15

How do amines react with halogenoalkanes?

Answer 15

Nucleophilic substitution. If you use an excess of the amine and you start with a primary amine, you will produce a secondary amine, if you started with a secondary amine, you will produce a tertiary amine etc.

Question 16

How do amines react with copper (II) ions?

Answer 16

Copper (II) solutions are blue as the [Cu(H2O)6]2+ complex ion is blue. If you add a small amount of amine, a pale blue precipitate will form because the amine acts as a base and accepts hydrogens from the water ligands, making a copper hydroxide precipitate. If you add lots of amine, the amine will act as a ligand (form dative (coordinate) bonds to the Cu2+) and replace some of the water molecules around the copper, forming a deep blue solution.

Question 17

What is the amide functional group?

Answer 17

-CONH2 for primary amines, or the hydrogens can be replaced by R groups for N-substituted amides

Question 18

How do you name amides?

Answer 18

The suffix for an amide is -amide (e.g. propanamide) and any side chains on the nitrogen are prefaced with an N (e.g. N-ethylpropanamide or N,N-dimethylpropanamide).

Question 19

How can amides be made from acyl chlorides?

Answer 19

Acyl chloride + ammonia —> amide + HCl
Acyl chloride + primary amine —> N-substituted amide + HCl
These reactions occur by addition-elimination.

Question 20

How can polyamides be formed?

Answer 20

Polyamides are formed by the condensation reactions of diamines and diacyl chlorides or dicarboxylic acids. Polypeptides are similar and can be formed by the condensation reactions of amino acids.

Question 21

How can polypeptides be hydrolysed?

Answer 21

Using acid or base hydrolysis. If you use acid hydrolysis, the amino group will become protonated, if you use base hydrolysis, the carboxylic acid group will become deprotonated.

Question 22

What is the structure of an amino acid?

Answer 22

Amino acids have a carbon in the middle, bonded to a hydrogen, an R-group, an amino group, and a carboxylic acid group (this description only works for 2-amino acids, which are the ones found in nature (i.e. the amino group is on the second carbon). You can synthesise amino acids which do not have the amino group on the second carbon.

Question 23

What does amphoteric mean?

Answer 23

Amphoteric means that a substance can act as both an acid and a base.

Question 24

What is a zwitterion?

Answer 24

When amino acids are in a neutral solution, they form zwitterions, which means that the amino group is protonated and the carboxylic acid group is deprotonated. Zwitterions have a permanent positive and a permanent negative charge (but overall they are still neutral (both electrically and acid-base wise)). This means that amino acids have some properties similar to ionic compounds (e.g. high melting point solids, dissolve well in polar solvents and dissolve poorly in non-polar solvents, also have crystalline structures in solid form).

Question 25

What is the isoelectric point?

Answer 25

This is the point (given as a pH) at which the amino acid exists as a zwitterion in solution. Different amino acids have different acid-base characteristics as they have different R groups, so they have different isoelectric points.

Question 26

Will amino acids rotate the plane of plane-polarised monochromatic light?

Answer 26

Most amino acids will rotate the plane of plane-polarised monochromatic light as they are chiral (have a carbon bonded to 4 different substituents). Some amino acids are not chiral (glycine/other non-naturally occurring amino acids which do not have 4 different groups bonded to a carbon).

Question 27

How can the composition of proteins be analysed?

Answer 27

The protein must be fully hydrolysed into amino acids (acid hydrolysis is usually used). Then, chromatography can be used to separate the amino acids and work out how much of each there is. Column chromatography is often used for this, but you could also use TLC with ninhydrin solution as the developing agent.

Question 28

What is benzene?

Answer 28

Benzene is a molecule with formula C6H6 which is a hexagon where each carbon is bonded to one hydrogen. This leaves each carbon with a delocalised electron, which is delocalised into a ‘pi system’ (rings of electron density above and below the plane of the molecule). This means that each bond is pretty much half way between being a single bond and being a double bond, which gives benzene interesting properties.

Question 29

What is an arene?

Answer 29

Any compound which contains a benzene ring is an arene (or an aromatic compound).

Question 30

What is an aliphatic compound?

Answer 30

Anything without a benzene ring.

Question 31

What is the difference between the Kekulé and delocalised models of benzene?

Answer 31

The Kekulé model has alternating single and double bonds, while the delocalised model has a circle in the middle of the hexagon, representing the ring of delocalised electrons. The Kekulé model is less accurate in describing the location of the electrons in the molecule, but it is still used sometimes in drawing structures and mechanisms.

Question 32

What is resonance?

Answer 32

Resonance is the idea that the actual structure of a compound is a hybrid of multiple different resonance structures which all contribute equally to the overall structure. This happens when the electrons is a molecule can move around to other positions, leaving the molecule equally stable (each of these positions is a different resonance structure).

Question 33

Why does benzene not undergo addition reactions with halogens, but cyclohexene will?

Answer 33

Benzene is more stable because of the ring of delocalised electrons. This gives benzene lower electron density than cyclohexene (which has much higher electron density around the double bond). Benzene is less able to polarise the halogen (or other electrophiles), so can’t undergo addition reactions. If an addition reaction did occur, the ring of delocalised electrons would be disturbed, which greatly decreases the stability of the molecule. This will therefore not happen and electrophilic substitution will occur instead, using a halogen carrier as a catalyst to create a more powerful electrophile.

Question 34

Are all the bonds in benzene equivalent?

Answer 34

Yes, all the bonds, and all the carbons are equivalent, so all bonds are the same length.

Question 35

Why is the enthalpy of hydrogenation of benzene lower than it theoretically should be (based on the Kekulé structure)?

Answer 35

Energy must be put in to break up the ring of delocalised electrons before hydrogen can be added. Since the same bonds are being formed (6 C-H bonds), the overall enthalpy change is less exothermic than it theoretically should be because of the increased stability of benzene compared to cyclohexen-1,3,5-triene.

Question 36

How do you name compounds containing benzene?

Answer 36

Simple substituted arenes have the structure -benzene (e.g. nitrobenzene, 1,3-dichlorobenzene, benzoic acid). For more complicated compounds, the benzene ring is treated as a side chain, so it is phenyl- (e.g. 2-methyl 3-phenylbutane).

Question 37

What are the common names for methylbenzene and hydroxybenzene?

Answer 37

Toluene and phenol (respectively)

Question 38

What is the observation when benzene (or aromatic compounds) are combusted?

Answer 38

Burns in oxygen with a smoky flame (due to high carbon:hydrogen ratio).

Question 39

What are the conditions for the hydrogenation of benzene?

Answer 39

Hydrogen with a nickel catalyst, heat under pressure.

Question 40

How do halogen carriers act as catalysts in the electrophilic substitution mechanisms of benzene?

Answer 40

Halogens react with halogen carriers (e.g. AlCl3, AlBr3, FeCl3 etc.), producing a positively charged halogen ion (which is the electrophile) and a negatively charged aluminium/iron containing ion (by the homolytic fission of the halogen-halogen bond). AlCl3 + Cl2 —> AlCl4 - + Cl+. Conditions must be anhydrous.

Question 41

How is the catalyst regenerated in the electrophilic substitution mechanisms of benzene?

Answer 41

A hydrogen ion is formed in the reaction, which reacts with the negative ion (e.g. AlCl4 -) to regenerate the original halogen carrier and form a hydrogen halide. E.g. H+ + AlCl4 - —> HCl + AlCl3

Question 42

What is the mechanism for the electrophilic substitution of benzene?

Answer 42

The delocalised electrons in the ring form benzene go to the electrophile forming a bond between one of the carbons of benzene and the electrophile. This breaks part of the ring of delocalised electrons, which can be represented by a u-shape with a + where the breakage is. The bond from the hydrogen to the carbon breaks and the electrons from this bind delocalise into the pi system. This reforms the ring of delocalised electrons and forms the final product. The H+ will react to regenerate the catalyst.

Question 43

What is the mechanism for the nitration of benzene?

Answer 43

The same as for any other electrophilic substitution, but the electrophile is +NO2.

Question 44

How is the catalyst formed and regenerated in the nitration of benzene?

Answer 44

HNO3 + H2SO4 —> +NO2 + HSO4- + H2O (formation) H+ + HSO4- —> H2SO4 (regeneration)

Question 45

What are the conditions for the nitration of benzene?

Answer 45

Heat under reflux at 50 degrees Celsius, concentrated HNO3 and concentrated H2SO4

Question 46

Why shouldn’t you let the temperature get too high in nitration reactions?

Answer 46

If the temperature is too high, this favours multiple substitutions, which forms explosives.

Question 47

What are Friedel-Crafts reactions?

Answer 47

A type of electrophilic substitution with benzene (either alkylation (adding alkyl chains) or acylation (adding something with an acyl group)).

Question 48

What is the catalyst for Friedel-Crafts reactions?

Answer 48

In general, halogen carriers are used as the catalyst (to make more powerful electrophiles). They must be used in anhydrous conditions.

Question 49

How do you do a Friedel-Crafts alkylation?

Answer 49

React a benzene ring with a halogenoalkane in the presence of a halogen-carrier. The halogen-carrier (e.g. AlCl3) will react with the haloalkane to produce a carbocation, which can act as the electrophile. E.g. C6H6 + CH3Cl —> C6H5CH3 + HCl. The mechanism for this is the same as for other electrophilic substitutions. The reaction to generate the catalyst is CH3Cl + AlCl3 —> +CH3 + AlCl4 -. The equation to regenerate the catalyst is H+ + AlCl4 - —> AlCl3 + HCl

Question 50

How do you do a Friedel-Crafts acylation?

Answer 50

Same as for alkylation but an acyl chloride is used instead of a haloalkane. The equation for catalyst formation is CH3COCl + AlCl3 —> CH3CO+ (the positive charge is on the carbonyl carbon - this should be clear) + AlCl4 -. The equation for catalyst regeneration is H+ + AlCl4 - —> AlCl3 + HCl. The overall equation could be, for example, C6H6 + CH3COCl —> C6H5COCH3 + HCl

Question 51

What is the method for the nitration of benzene?

Answer 51

Mix concentrated nitric acid and concentrated sulfuric acid in an ice bath (they will react to form the +NO2 electrophiles). Add this to the benzene very slowly, while heating under reflux to 50 degrees Celsius. It is important that this temperature is not exceeded as this will encourage multiple substitutions, which is dangerous as this forms explosives. Dissolve in the minimum volume of hot solvent and filter while hot under reduced pressure (use a Büchner funnel). This removes insoluble impurities. Cool the filtrate to recrystallise the nitrobenzene, then filter while cold under reduced pressure to remove soluble impurities. Wash the crystals with ice-cold ethanol then dry them between pieces of filter paper in a warm oven.

Question 52

Why can solid drying agents not be used to dry crystals?

Answer 52

Crystals are solids, so can’t be dried with solid drying agents as the mixture of solids is incredibly difficult to separate. The best way to dry a solid product is on filter paper in a warm oven (not hot as this could decompose the product).

Question 53

What is phenol?

Answer 53

Phenol is another name for hydroxybenzene (a benzene ring with a hydroxyl group attached).

Question 54

Why is phenol more reactive than benzene?

Answer 54

The lone pair of electrons on the oxygen of phenol delocalise into the pi system, which increases the electron density of the ring. This makes phenol more susceptible to electrophiles and therefore more reactive.

Question 55

Why is phenol weakly acidic?

Answer 55

The pi system overlap with the oxygen increases the polarity of the O-H bond, which makes the hydrogen more likely to dissociate. When the hydrogen does dissociate, the pi system stabilises the phenoxide anion which is formed. Alcohols are not generally acidic because the alkoxide anions are unstable as they have no charge delocalisation.

Question 56

How does phenol react with bromine water?

Answer 56

Phenol reacts directly with bromine water because the pi system is more electron dense than in benzene, so it can polarise the bromine molecules sufficiently for them to act as electrophiles and substitute onto the ring. The mechanism for the substitution is the same as other nucleophilic substitutions, and the -OH group will direct the bromine atoms to add at positions 2,4 and 6. The side product is hydrogen bromide.

Question 57

Why is phenol moderately soluble in water?

Answer 57

Phenol can form hydrogen bonds to water, which means it is partially soluble, however, it is not very soluble because most of the molecule is non-polar, so it forms stronger London forces with itself tan it would with water.

Question 58

Why does phenol have a higher melting and boiling point than toluene?

Answer 58

They both have a similar number of electrons and therefore similar London forces. The melting point of phenol is higher because of the permanent dipole-dipole forces and hydrogen bonds it can form with itself, which take more energy to overcome than just London forces.

Question 59

What is a Grignard reagent?

Answer 59

A Grignard reagent has formula R-MgX where R is an alkyl or aryl group and X is a halogen (usually Br). They are typically used to lengthen carbon chains in synthesis.

Question 60

How can Grignard reagents be formed?

Answer 60

Warm a haloalkane with magnesium over a water bath, under reflux, in a solvent of dry ether. Dry ether must be used as Grignard reagents react with water, forming an alkane/arene and Mg(OH)Br.

Question 61

How do Grignard reagents react?

Answer 61

They generally add across C=O bonds, as the magnesium is more electronegative than carbon, making the carbon bonded to the magnesium delta negative (meaning it can be nucleophilic). The C-Mg bond breaks and the electrons form a bond between the carbon of the Grignard reagent and the C=O carbon. One of the bonds in the C=O breaks and the electrons go onto the O, giving it a negative charge. The lone pair on the oxygen attacks the hydrogen of water and the organic product (and Mg(OH)Br) are formed.

Question 62

What are some typical reactions of Grignard reagents?

Answer 62

Grignard + CO2 —> carboxylic acid (Dry CO2 is bubbled through the Grignard reagent, product is hydrolysed with dilute acid) Grignard + carbonyl —> alcohol (carbonyl is added to the Grignard reagent, product is hydrolysed with dilute acid) The conditions for the first step of these reactions is dry ether (anhydrous)