Halogenoalkanes

Question 1

What is the general formula for a haloalkane?

Answer 1

C𝑛H2𝑛+1X

Question 2

Explain why carbon-halogen bonds are polar?

Answer 2

• because halogens are more electronegative than carbon.
  (as you go down the group the halogens get less electronegative and therefore the C-X bond gets less polar)

Question 3

Why aren’t haloalkanes soluble in water?

Answer 3

The C-X bonds are not polar enough

Question 4

What factors impact the boiling point of halogenoalkanes?

Answer 4

• chain length (increases with increased chain length)
• the halogen in the C-X bond (increases as go down the group)
  these both increase VDWs

Question 5

Why does reactivity increase as you go down the halogenoalkanes group?

Answer 5

• C-X bond gets longer and therefore weaker
• easier to break
  This is despite the fact that the bond gets less polar, showing bond enthalpies to be of more importance than bond polarity

Question 6

What is a nucleophile?

Answer 6

A species with a negative charge and a lone pair of electrons, which it can donate to an electron deficient carbon atom, forming a covalent bond

Question 7

What conditions are required for the hydrolysis of haloalkanes (by nucleophilic substitution)?

Answer 7

• Aqueous solution containing hydroxide ions (e.g NaOH)
• ethanol as a solvent (haloalkanes are not soluble in water)
• Heat

Question 8

Why does the nucleophile approach from the opposite side to the halogen atom?

Answer 8

To minimise repulsion from the electronegative halogen atom

Question 9

Describe the steps of an elimination reaction

Answer 9

1) OH- ions bond to the H (on the C-H bond adjacent to the C-X bond)
2) Electrons move from the C-H bond to form a C=C double bond
3) This halogen takes the pair of electrons in the C-X bond and leaves as a halide ion

Question 10

What are CFCs?

Answer 10

Halogenoalkanes containing chlorine, flourine and carbon, but no hydrogen

Question 11

How can short chain CFCs be used?

Answer 11

• refrigerants
• aerosol propellants
• blowing agents for foams like expanded polystyrene

Question 12

How can long chain CFCs be used?

Answer 12

• dry cleaning
• as de-greasing solvents

Question 13

How do CFCs lead to the depletion of Ozone in the atmosphere?

Answer 13

• the C-Cl bond is broken due to the presence of UV light
• this produces a chlorine free radical
• this chlorine free radical acts as a catalyst in the decomposition of Ozone

Question 14

Give 2 equations that show chlorine as a catalyst in the decomposition of Ozone?

Answer 14

• Cl. + O3 → ClO. + O2
• ClO. + O3 → 2O2 + Cl.

Question 15

Give 3 examples of a nucleophile?

Answer 15

:NH3
:OH
CN:-

Question 16

Describe the reaction conditions when reacting a halogenoalkane with KCN. What products are formed?

Answer 16

• ethanol and heat
• R-CN, nitrile formed

Question 17

Describe the reaction conditions when reacting a halogenoalkane with NH3. What products are formed?

Answer 17

• excess of NH3
• heat in a sealed tube
• R-NH2, amine

Question 18

Why is the Ozone layer important?

Answer 18

protects from harmful UV rays which can cause skin cancer

Question 19

Why cant nucleophilic substitution happen with tertiary haloalkanes?

Answer 19

nowhere for the nucleophile to attack

Question 20

What does a nucleophile act as in elimination reactions?

Answer 20

base

Question 21

What conditions enable an elimination reaction to occur?

Answer 21

• heat under reflux
• ethanol as a solvent
• concentrated

Question 22

What reagent is needed for an elimination reaction? What does the OH- ion act as?

Answer 22

KOH, OH acts as a base

Question 23

What reaction condition are needed for the reaction of haloalkanes with CN- ions?

Answer 23

• warm, ethanolic KCN
• reflux

Question 24

What reaction condition are needed for the reaction of haloalkanes with OH-?

Answer 24

• warm aqueous NaOH
• reflux