3O- Halogenoalkanes

Question 1

Why do halogenoalkanes contain a polar bond

Answer 1

Halogen has a higher electronegativity than the carbon creating a s+ and s- side

Question 2

Why does the carbon to halogen bond enthalpy influence rate of reaction

Answer 2

Carbon to fluorine bond, electrons have strong attraction to fluorine nucleus, strong bond, less reactive
Carbon to iodine bond, electrons have a weak attraction to iodine nucleus, weak bond, more reactive

Question 3

In mechanisms how are the formation of covalent bonds shown

Answer 3

A curly arrow starting from a lone pair/ another covalent bond

Question 4

How is the breaking of a covalent bond shown

Answer 4

Curly arrow starting from the bond that is being broken to the ion about o be formed

Question 5

What are nucleophiles

Answer 5

A negative ion or an atom with a s- charge
Donates a lone pair of electrons to form a covalent bond

Question 6

How to show a molecule being substituted into a mechanism

Answer 6

Curly arrow from electrons to atom it will form the bond with

Question 7

General equation for nucleophilic substitution of halogenoalkanes with OH-

Answer 7

R-X + OH- —> ROH + X-

Question 8

General equation for nucleophillic substitution for halogenoalkane and CN- ion

Answer 8

R-X + CN- —> R-CN + X-

Question 9

Equation of nucleophillic substitution of halogenoalkane with 2 mol of NH3

Answer 9

R-X + 2NH3 —> R-NH3 + X- + NH3 —> R-NH2 + X- + NH4+

Question 10

What’s ozone

Answer 10

A layer formed naturally in the upper atmosphere, absorbs uv radiation

Question 11

How is C-Cl bond in chlorofluorocarbons broken and what happens

Answer 11

UV radiation breaks the C-Cl bond producing chlorine atoms

Question 12

Why are chlorine atoms in the atmosphere bad

Answer 12

Catalyse decomposition of ozone contributing to a hole in the ozone layer

Question 13

What are some uses of CFC’s that are now banned

Answer 13

Solvents
Refrigerants

Question 14

How do chlorine atoms catalyse the decomposition of ozone (free radical substitution)

Answer 14

Initiation- CF2Cl2 —> CF2Cl• + Cl• (in uv light)

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

Overall- O3 + O —> 2O2

Cl• free radical regenerates for the next reaction being able to destroy thousands of ozone molecules

NO also catalyses decomposition of ozone

Question 15

What are alternative chlorine free compounds which have been developed

Answer 15

HFC’s hydro fluoro carbons, no C-Cl

Question 16

Conditions for nucleophilic substitution with OH- ions

Answer 16

Dissolved in ethanol and heat

Question 17

Conditions for nucleophilic substitution with cyanide ions

Answer 17

Ion dissolved in ethanol/water
Heat under reflux

Question 18

Conditions for nucleophilic substitution with ammonia

Answer 18

Ammonia dissolved in ethanol
Heat under pressure
Excess ammonia

Question 19

Formula of ozone

Answer 19

O3

Question 20

What does base elimination go from and to

Answer 20

Halogenoalkane to alkene

Question 21

Conditions for base elimination

Answer 21

anhydrous conditions [absence of water/H2O]

high temperature

KOH aqueous in ethanol

Question 22

Mechanism for base elimination

Answer 22

First arrow: from the middle of the lone pair on the nucleophile to a hydrogen which is joined to a carbon which is adjacent to the carbon with the halogen.

Second arrow: From the CH bond to the CC bond

Third arrow: from the middle of the carbon halogen bond onto the halogen breaking the halogen off.