Haloalkanes

Question 1

Haloalkanes are used in the synthesis of other organic compounds.Hot concentrated ethanolic potassium hydroxide reacts with 2-bromo-3-methylbutane to form two alkenes that are structural isomers of each other. The major product is 2-methylbut-2-eneName and outline a mechanism for the conversion of 2-bromo-3-methylbutane into 2-methylbut-2-ene according to the equation.(CH3)2CHCHBrCH3 + KOH ———->(CH3)2C CHCH3 + KBr + H2O Name of mechanism …………………………………………………………………………………………..State the type of structural isomerism shown by these two alkenes.

Answer 1

EliminationPosition(al) (isomerism or isomer)

Question 2

A small amount of another organic compound, X, can be detected in the reaction mixture formed when hot concentrated ethanolic potassium hydroxide reacts with 2-bromo-3-methylbutane.Compound X has the molecular formula C5H12O and is a secondary alcohol.Suggest one change to the reaction conditions that would increase the yield of X

Answer 2

Any one from• Lower / decreased temperature OR cold• Less concentrated (comparative) OR dilute KOH• Water (as a solvent) / (aqueous conditions)

Question 3

State the type of mechanism for the conversion of 2-bromo-3-methylbutane into X.

Answer 3

Nucleophilic substitution

Question 4

Chlorine can be used to make chlorinated alkanes such as dichloromethane.Write an equation for each of the following steps in the mechanism for the reaction ofchloromethane (CH3Cl) with chlorine to form dichloromethane (CH2Cl2). Initiation step………………………………………………………………………………………………………………………….. First propagation step………………………………………………………………………………………………………………………….. Second propagation step………………………………………………………………………………………………………………………….. The termination step that forms a compound with empirical formula CH2Cl

Answer 4

InitiationCl2——-> 2Cl• First propagationCl• + CH3Cl——->•CH2Cl + HCl Second propagationCl2 + •CH2Cl ———>CH2Cl2 + Cl•Termination (must make 1,2-dichloroethane)2 •CH2Cl ———–>CH2ClCH2Cl

Question 5

When chlorinated alkanes enter the upper atmosphere, carbon–chlorine bonds are broken. This process produces a reactive intermediate that catalyses the decomposition of ozone. The overall equation for this decomposition is2O3 3O2Name the type of reactive intermediate that acts as a catalyst in this reaction

Answer 5

(chlorine free) radical

Question 6

Write two equations to show how this intermediate is involved as a catalyst in the decomposition of ozone.Equation1Equation 2

Answer 6

Cl• + O3 ——>ClO• + O2ClO• + O3 ——->Cl• + 2O2

Question 7

The refrigerant R410A, used in air conditioners, is a mixture of two fluoroalkanes, pentafluoroethane and difluoromethane.The mechanism for the reaction of fluorine with either an alkane or a fluoroalkane is similar to that for the reaction of chlorine with methane.Name the type of mechanism for the reaction of chlorine with methane.

Answer 7

(Free-) radical substitution

Question 8

The refrigerant R112A (CCl3CF2Cl) has been banned because of concerns about ozone depletion.Give the IUPAC name for CCl3CF2Cl

Answer 8

1,1,1,2-tetrachloro-2,2-difluoroethane

Question 9

Chloromethanes, such as dichloromethane and trichloromethane, are produced in industry as they have many uses.Trichloromethane has been used in the manufacture of the refrigerant chlorodifluoromethane.Chlorine can react with dichloromethane (CH2Cl2) to form trichloromethane (CHCl3). Write an equation for each of the following steps in the mechanism for this reaction. Initiation step………………………………………………………………………………………………………………………….. First propagation step………………………………………………………………………………………………………………………….. Second propagation step

Answer 9

InitiationCl2 —->2Cl•First propagationCl• +CH2Cl2——> •CHCl2 + HCLSecond propagationCl2 + •CHCl2 ———-> CHCl3 +Cl•

Question 10

Give one essential condition for this reaction and name the type of mechanism.Essential condition ……………………………………………………………………………………………….Type of mechanism ……………………………………………………………………………………………..

Answer 10

Conditionultra-violet / uv / sun light OR high temperatureOR 400oC≤T≤ 900oCType of mechanism(free-) radical substitution (mechanism)

Question 11

Explain, with the aid of equations and the intermediates that form in the ozone layer, why the European Union has banned the use of chlorodifluoromethane (CHClF2) as a refrigerant.

Answer 11

Cl• catalyses the decomposition of ozone / a single Cl• causes (chain) reaction / decomposition of many ozone molecules / ozone layer

Question 12

Give one reason why 2,3,3,3-tetrafluoropropene is a more environmentally acceptable refrigerant than chlorodifluoromethane.

Answer 12

It does not produce Cl• / does not produce chlorine (free) radical(s)

Question 13

The initiatives to decrease the carbon dioxide in the atmosphere include the use of carbon-neutral fuels and the development of carbon capture.The mineral serpentine, Mg3Si2O5(OH)4, has been proposed as a solid for the capture of carbon dioxide gas.Give the meaning of the term carbon-neutral, as applied to a fuel.

Answer 13

An activity which has no net / overall (annual) carbon emissions to the atmosphere / air

Question 14

Balance the following equation for the reaction of serpentine with carbon dioxideMg3Si2O5(OH)4 + ……….CO2 → ……….MgCO3 + ……….SiO2 + ……….H2O

Answer 14

Mg3Si2O5(OH)4 + 3CO2 ——> 3MgCO3 + 2SiO2 + 2H2O

Question 15

Bromine reacts with alkenes, even though bromine is a non-polar molecule. Explain why bromine molecules react with the double bonds in alkenes (2)Name the type of mechanism involved in this reaction.

Answer 15

1. Double bonds are electron-richOR electon pair donorsOR centres of electron density.2. Bromine becomes polarised / becomes polarOR forms an induced dipole OR becomes δ+/ δ-Electrophilic addition

Question 16

Haloalkanes contain a polar covalent bond. The carbon atom of the polar covalent bond can be attacked by nucleophiles. Nucleophilic attack enables haloalkanes to undergo substitution reactions.A nucleophilic substitution reaction occurs when a haloalkane undergoes hydrolysis; the rate of hydrolysis of the haloalkane is influenced by the carbon–halogen bond enthalpy.Explain the meaning of each of the following terms in the information given above.nucleophile

Answer 16

Electron pair donor ORSpecies which uses a pair of electrons to form a co-ordinate / covalent bond.

Question 17

substitution, as applied to nucleophilic substitution in a haloalkane

Answer 17

Replacement of the halogen (atom) (by the nucleophile) ORThe carbon-halogen bond / C-X breaks and a bond forms with the nucleophile or between the carbon and the nucleophile

Question 18

hydrolysis

Answer 18

Splitting molecules using / by water ORbreaking / splitting / dissociating (C–X) bond(s) / using / by water

Question 19

bond enthalpy, as applied to a carbon–halogen bond

Answer 19

(Heat) energy / enthalpy required / needed / absorbed (at constant pressure) to break / split it / the (carbon-halogen) bond

Question 20

State the meaning of the term hydrolysis.

Answer 20

Splitting/ breaking the molecule / substance / compound using / by (adding) / with waterNOTthe “splitting of water”simply the reaction of / with watersimply the addition or adding of water

Question 21

State the colour of the precipitate formed when iodide ions react with silver nitrate and write the simplest ionic equation for this reaction.Colour of precipitateSimplest ionic equation

Answer 21

yellow ONLYAg+ +I–  ——–> AgI (Ag+I–)

Question 22

Chlorine is a useful industrial chemical.Chlorine gas is used in the manufacture of chlorine-containing organic compounds.Write equations for the following steps in the mechanism for the reaction of chlorine with ethane to form chloroethane (CH3CH2Cl).Initiation stepFirst propagation stepSecond propagation stepA termination step producing butane

Answer 22

InitiationCl2 ——> 2Cl•First propagationCl• + CH3CH3 ——–> •CH2CH3 + HCl Second propagationCl2 + •CH2CH3 ———> CH3CH2Cl + Cl• Termination (must make C4H10)2 •CH2CH3 ———> C4H10 or CH3CH2CH2CH3

Question 23

Give one essential condition and name the type of mechanism in this reaction of chlorine with ethane.

Answer 23

ultra-violet / uv / sun lightOR ( very) high temperature OR 500 oC ≥ T ≤1000 oC(free-)radical substitution

Question 24

Alkenes can be polymerised to produce poly(alkenes).State the type of polymerisation that alkenes undergo

Answer 24

Addition (polymerisation)