CD

Question 1

conjugated system

Answer 1

system of alternating double and single bonds

electrons in the p orbitals that make up the pi-bonds of each double bond are delocalised and spread out across all the atoms in the system

decreases the energy gap between the ground state and excited state

Question 2

relation between number of electrons delocalised in a conjugated system and the size of the energy gap

Answer 2

the more electrons delocalised in the conjugated system, the smaller the energy gap

smaller energy gaps result in absorption of longer wavelengths of light (visible light rather than UV)

Question 3

analysis of a contour map produced by X ray diffraction of a benzene molecule reveals

Answer 3

a benzene ring is a regular planar hexagon
all bond angles = 120
all carbon-carbon bonds are the same length

Question 4

bond lengths in benzene

Answer 4

shorter than C-C bond, but longer than C=C bond

Question 5

delocalised model of benzene

Answer 5

each C atom has 4 outer electrons
3 of these electrons are used to form single sigma bonds to either carbon or hydrogen
leaves one p-electron on each carbon atom
these six electrons delocalise and spread out across all six carbons
forms a delocalised charge cloud above and below the plane of the molecule

Question 6

relationship between delocalisation and stability

Answer 6

generally the more delocalised a structure is, the more stable the molecule is

Question 7

arenes

Answer 7

hydrocarbons which contain rings stabilised by electron delocalisation

Question 8

halogen carrier

Answer 8

substances that are able to remov e halogen atoms from molecules
eg. FeBr3 and AlCl3

Question 9

nitrating mixture

Answer 9

HNO3 + 2(H2SO4)&raquo_space; NO2+ + 2(HSO4-) + H3O+

produces NO2+ electrophile used in nitration of benzene

Question 10

purposes of sulfonation

Answer 10

provides a way of forming more soluble derivatives of aromatic compounds (making dyes more soluble)

Question 11

azo compounds

Answer 11

contain azo functional group R-N=N-R’

formed as a result of a coupling reaction between a diazonium salt and a coupling agent

Question 12

diazonium compounds

Answer 12

contain diazo(nium) group R-N+(triple bond)N

unstable compounds because diazo group is very easily lost (forming nitrogen gas)

Question 13

diazotisation

Answer 13

aromatic amine (eg. phenylamine) dissolved in dilute HCl
cold solution of sodium nitrite added NaNO2
temp kept below 5 C by using ice bath

[actual reactant is the unstable acid HNO2]

Question 14

coupling reactions

Answer 14

diazonium compound reacts with a coupling agent

solution of coupling agent is made up
ice-cold solution of diazonium salt is added to it
diazonium salt acts as an electrophile
coloured precipitate of azo compound immediately found

Question 15

coupling agent

Answer 15

compound containing a relatively reactive benzene ring (usually contains phenol or amine groups to increase reactivity)

Question 16

attaching dyes to fibres by ionic bonds

Answer 16

dye molecules have SO3- groups
polymer molecule hase NH3+ groups in acidic solution

eg. nylon, wool, silk

Question 17

attaching dyes to fibres by covalent bonds

Answer 17

dye molecules have presence of reactive group
polymer molecules have OH or NH2 groups

eg. cotton, cellulose

Question 18

attaching dyes to fibres by H bonds

Answer 18

dye molecule has several NH2 groups and is linear

polymer molecule has frequent OH groups

Question 19

attaching dyes to fibres by id-id bonds

Answer 19

dye molecules have few polar groups and are small molecules
dye molecules have no OH or NH2 groups

eg. polyester

Question 20

triesters

Answer 20

most fats and oils are sters of propane-1,2,3-triol (aka glycerol)
glycerol forms ester bonds with 3 carboxylic acids incondensation reactions to form triesters

Question 21

carboxylic acids (fatty acids) in triesters

Answer 21

generally conatin an even number carbon atoms, with 16/18 carbon atoms being most common

cannot have more than 24 carbon atoms

Question 22

hydrolysis of triesters

Answer 22

heat oil/fat with conc NaOH
products are propane-1,2,3-triol and (sodium) carboxylate salts
add HCl to salts to form carboxylic acids

Question 23

Fehling’s test

Answer 23

warm sample with Fehling’s A and B
Fehling’s will oxidise an aldehyde to a carboxylic acid
solution will change colour from blue to red

Question 24

Tollens’ reagent

Answer 24

Tollens’ reagent warmed with sample
Tollens’ will oxidise aldehydes to carboxylate ions
silver mirror forms on inside of test tube

Question 25

mechanism of nucleophilic addition of cyanide and aldehyde/ketone

Answer 25

cyanide ion is a nucleophile is attracted to partially positive carbon atom in the carbonyl
a new carbon-carbon bond forms
pair of electrons moves onto the oxygen atom, which then carries a -ve charge
negatively charge ion takes up a proton from the solvent

Question 26

addition reactions with cyanide ions

Answer 26

cyanide ions add across C=O bonds of carbonyls in acidic conditions to form cyanohydrins