halogenoalkanes & alcohols - past questions

Question 1

from CH3CH2CH2Cl to Ch3CHCH2, what is the reagent & conditions used in the reaction?

Answer 1

• warm alcoholic
• NaOH / KOH

Question 2

identify a suitable reagent for CH3CH2CH2Cl to CH3CH2CH2CN, and state why this is a particularly useful reaction in organic chemistry.

Answer 2

• warm alcoholic KCN / NaCN
• increases length of carbon chain / number of C in carbon chain

Question 3

explain why, in reaction involving CH3CH2CH2Cl to CH3CH2CH2NH2 the reactants are heated in a sealed tube.

Answer 3

• heating increases rate of reaction
• reduce loss of NH3 gas while heating

Question 4

some halogenoalkanes are put into a mixture of ethanol and silver nitrate.

the precipitate forms as a result of reactions between aq. silver ions & halide ions. why are halide ions present in the halogenoalkane which only has covalent bonds?

Answer 4

• halogenoalkane is hydrolysed by water
• C-Hal bond breaks & produces ions

Question 5

suggest how the rates of hydrolysis depend on the structure of the bromoalkane. suggest a reason for this difference.

Answer 5

• fastest in tertiary & slowest in primary
• weakest C-Br bond as methyl group donates electrons

Question 6

explain why the formation of alcohols are faster with aq. alkali than water.

Answer 6

OH- is a stronger nucleophile

Question 7

describe the observations of alcohols with PCL5.

Answer 7

misty/steamy/white fumes/gas

Question 8

what is the observation with acidified K2Cr2O7?

Answer 8

alcohols oxidised from orange to green

Question 9

which method, PCL5 or K2Cr2O7 is more preferred? why?

Answer 9

K2Cr2O7 since PCl5 also reacts with alkali / OH- ions

Question 10

what is meant by the term free radical?

Answer 10

a species with an unpaired electron

Question 11

what conditions, other than a suitable temperature and NaOH concentration, would produce butan-2-ol in the reaction of 2-chlorobutane with NaOH?

Answer 11

water (to dissolve NaOH / ac5 as solvent)

Question 12

phosphorus (V) chloride, PCl5, can be used to test for the -OH group.

describe what would be seen when PCl5 is added to butan-2-ol.

Answer 12

steamy/misty/white gas/fumes

Question 13

give the equation for the reaction of PCl5 with butan-2-ol.

Answer 13

PCl5 + CH3CH(OH)CH2CH3 -> CH3CHClCH2CH3 + HCl +POCl3

Question 14

a tertiary alcohol A is an isomer of butan-2-ol.

butan-2-ol and A could be distinguished by warming separate samples with a mixture of potassium dichromate (VI) and sulphuric acid. state the observations which would be made with each alcohol.

Answer 14

observation with butan-2-ol: orange -> green
observation with A: no reaction

Question 15

give the structural formula of the organic product which forms when butan-2-ol is oxidised.

Answer 15

CH3CH2COCH3

Question 16

explain how infrared spectroscopy could be used to detect whether butan-2-ol has been oxidised.

Answer 16

absorption for O-H bond would disappear

Question 17

explain the role of the water in a substitution reaction.

Answer 17

acts as nucleophile -> bonds to carbocation

Question 18

explain the role of the water in an elimination reaction.

Answer 18

acts as an base -> accepts a hydrogen ion

Question 19

suggest why 1-chlorobutane reacts with water via a different mechanism.

Answer 19

butyl group has a less electron releasing / less inductive effect

therefore has a less stable carbocation

a primary carbocation is less stable than a tertiary carbocation

Question 20

2-chlorobutane could form 3 different alkenes in an elimination reaction. suggest how they could be formed.

Answer 20

• H+ could be eliminated on either sides
• since double bonds can’t rotate, methyl groups are fixed & forms E/Z isomers

Question 21

suggest why 2-fluoro-2-methylpropane would react more slowly than 2 chloro-2-methylpropane in an substitution reaction.

Answer 21

C-F bond is stronger

Question 22

what reagent could be used instead of water to increase the rate of reaction involving 2-fluoro-2-methylpropane? explain why the reagent would have this effect.

Answer 22

NaOH / KOH / OH- / OH- ions

OH- is a stronger nucleophile with more lone pairs

Question 23

why is a tertiary carbocation formed more readily than a secondary carbocation?

Answer 23

more stable since methyl groups stabilise charge of carbocation

Question 24

halogenoalkanes are widely used as refrigerants that cool by change of state, typically by boiling.

suggest how halogenoalkanes cool by change of state.

Answer 24

boiling absorbs energy

Question 25

suggest two properties desirable in a refrigerant.

Answer 25

• low boiling point
• not flammable
• not toxic
• boiling temperature is lower than target temperatures

Question 26

give the name & displayed formula of the product of cyclohexanol & potassium dichromate (VI) in dilute sulfuric acid.

Answer 26

cyclohexanone

(AS topic 6 questions)

Question 27

which m/z value could be used to determine the molecular mass?

Answer 27

highest m/z value

Question 28

cyclohexene can be prepared by reacting cyclohexanol with phorsphoric (V) acid, distilling off a mixture of cyclohexene and water.

devise a 6-step procedure to obtain a pure, dry sample of cyclohexene from the distillate.

[boiling temp of cyclohexene: 83
density of cyclohexene: 0.811 g/cm3]

Answer 28

1- separate cyclohexene from water using a separation funnel
2- discard lower aqueous layer with higher density
3- add drying agent (eg. Na2SO4 / CaCl)
4- seperate cyclohexene from drying agent
5- redistill product
6- collect distillate boiling between 83 degrees & collect pure cyclohexene.