organic chem and analysis

Question 1

what is the structure and bonding of a benzene molecule?

Answer 1

planar
120º bond angle
all c-c the same length
p orbitals overlap above and below the ring to form pi bonds
electrons are delocalised
sigma bonds between c-c and c-h

Question 2

what mechanism does benzene react by?

Answer 2

electrophilic substitution

Question 3

what are 2 uses of phenols?

Answer 3

disinfectants
detergents
plastics
paints
aspirin

Question 4

why is phenol more reactive than benzene?

Answer 4

a lone pair from the oxygen p orbital is delocalised with the electron cloud of the benzene ring
electron density increases
attracts electrophiles more strongly

Question 5

how do you purify an impure solid?

Answer 5

recrystallisation
dissolve in minimum volume of hot water/ solvent
cool solution and filter solid
wash with cold water/ solvent and dry

Question 6

how do you check the purity of a solid?

Answer 6

obtain mp and compare w known values, pure sample will have similar sharp value
run a TLC, compare Rf to known data, pure sample will have a similar value

Question 7

describe in terms of orbital overlap, the similarities and differences between the bonding in the kekule model and the delocalised model of benzene

Answer 7

both have sideways overlap of p orbitals
both have pi system above and below C ring
kekule has alternating pi bonds and delocalised has pi ring system

Question 8

what reacts w ethanoic acid and phenol?

Answer 8

aqueous potassium hydroxide

Question 9

explain the relative resistance to chlorination of benzene compared with cyclohexene?

Answer 9

cyclohexene has localised electrons but benzene has delocalised
cyclohexene has higher electron density and is more susceptible to electrophile attack

Question 10

what are the reagents required for the conversion of benzene to nitrobenzene?

Answer 10

conc H2SO4
conc HNO3

Question 11

what happens when bromine water is added to phenol at room temp?

Answer 11

electrophilic substitution
phenol decolourises the orange bromine solution to form a white ppt of 2,4,6 tribromophenol

Question 12

why is the enthalpy change of hydrogenation of benzene lower than expected?

Answer 12

the delocalisation of electrons in the benzene ring increases the stability

Question 13

describe the structure of cyclohexene

Answer 13

each C has 4 covalent bonds
4 of the 6 carbons attempt to have a tetrahedral shape w a bond angle of 109.5
2 of the 6 carbons attempt to have a trigonal planar shape w a bond angle of 120
there is a mix of single and double bond lengths

Question 14

why won’t benzene react w bromine but cyclohexene will react to form dibromohexane

Answer 14

pi bond in hexene contains localised electrons/ area of high electron density
pi bond in hexene induces a partial positive and negative in Br2
partial positive Br attracted to pi pond, leaving a carbocation intermediate
carbocation intermediate attracts partial neg Br
delocalised electrons w no areas of high electron density in benzene so can’t polarise br2

Question 15

describe a method to obtain a pure sample of organic liquid from a distillate?

Answer 15

shake and leave to settle in a separating funnel
separate layers by tapping off
add anhydrous magnesium sulfate to organic layer
redistill organic layer

Question 16

how can you use a TLC to monitor the course of a reaction?

Answer 16

take samples from reaction mixture at regular intervals
run on a TLC plate alongside cyclohexanol controls

Question 17

suggest a suitable test to confirm that a compound contains an unsaturated carbon chain

Answer 17

bromine water decolourised

Question 18

suggest a suitable test to confirm that a compound contains an aldehyde functional group

Answer 18

Tollens’ reagent
silver mirror ppt