OZ8: Nucleophilic Substitution

Question 1

Define “nucleophile” (3)

Answer 1

• A molecule or negatively charged ion
• with a lone pair of electrons it can donate to a positively charged atom
• to form a covalent bond

Question 2

Define “substitution”

Answer 2

A reaction in which one atom or group in a molecule is replaced by another atom or group

Question 3

Give the general formula for the nucleophilic substitution reaction between a haloalkane and OH- ions.

Answer 3

R-X + NaOH ====> ROH + NaX

Question 4

Describe the nucleophilic substitution reaction between a haloalkane and OH- ions.(3)

Answer 4

• The nucleophile OH- attacks the electron deficient carbon atom in the C-halogen polar bond
• The OH- donates 2 electrons to form a new dative covalent bond
• The C-halogen bond breaks heterolytically and the halogen atom receives 2 electrons - produces a halide ion (leaving group)

Question 5

What conditions does the nucleophilic substitution reaction between a haloalkane and OH- ions need? (2)

Answer 5

• Warm aqueous sodium hydroxide

- Heat under reflux

Question 6

Give the general formula for the nucleophilic substitution reaction between a haloalkane and water.

Answer 6

R-X + H2O ===> R-OH + H+ + X-

Question 7

What conditions does the nucleophilic substitution reaction between a haloalkane and water need?

Answer 7

Heating the two substances together under reflux

Question 8

Describe the nucleophilic substitution reaction between a haloalkane and water.(4)

Answer 8

• The nucleophile H2O attacks the electron deficient carbon atom in the C-halogen polar bond
• The H2O donates 2 electrons to form a new dative covalent bond
• The C-halogen bond breaks heterolytically and the halogen atom receives 2 electrons - produces a halide ion (leaving group) and an intermediate forms with an oxygen that has 3 bonds.
• This is unstable so one OH bond breaks and an alcohol is formed alongside the halide ion and H+ that has left

Question 9

Give the general formula for the nucleophilic substitution reaction between a haloalkane and ammonia.

Answer 9

R-X + NH3 ===> R-NH3 + X- R-NH2 + X- + NH4+

Question 10

What conditions does the nucleophilic substitution reaction between a haloalkane and ammonia need?

Answer 10

The haloalkane needs to be heated in a sealed tube with excess ethanolic ammonia

Question 11

Describe the nucleophilic substitution reaction between a haloalkane and ammonia.

Answer 11

• The nucleophile ammonia attacks the electron deficient carbon atom in the C-halogen polar bond
• The ammonia donates 2 electrons to form a new dative covalent bond
• The C-halogen bond breaks heterolytically and the halogen atom receives 2 electrons - produces a halide ion (leaving group) and an intermediate forms with a nitrogen that has 4 bonds.
• This is unstable so one NH bond breaks and a hydrogen is removed by an ammonia molecule
• This leaves an amine

Question 12

What is an amine?

Answer 12

An organic compound based on ammonia but with 1 or more of the hydrogen atoms replaced by an alkyl group

Question 13

What nucleophilic substitution reaction produces a haloalkane?

Answer 13

A nucleophilic substitution reaction between a halide ion (X-) and an alcohol

Question 14

What is a nucleophilic substitution reaction producing a haloalkane from an alcohol done in the presence of?

Answer 14

A strong acid

Question 15

Why is a nucleophilic substitution reaction producing a haloalkane from an alcohol done in the presence of a strong acid?

Answer 15

Gives the carbon atom to which the oxygen is attached in the alcohol a higher partial positive charge, allowing it to be more readily attacked by halide ions

Question 16

What is the general formula for the nucleophilic substitution reaction producing a haloalkane from an alcohol?

Answer 16

ROH + HX ===> RX + H2O

Question 17

What is the relationship between the position of the halogen in the haloalkane bond in Group 7 and the bond enthalpy of the haloalkane?

Answer 17

The further down Group 7, the lower the bond enthalpy

Question 18

What is the relationship between the position of the halogen in the haloalkane bond in Group 7 and the bond polarity of the haloalkane?

Answer 18

The further down Group 7, the lower the bond polarity

Question 19

Why are fluoroalkanes not useful in nucleophilic substitution reactions?

Answer 19

C-F is a very difficult bond to break because it has a high bond enthalpy

Question 20

Which haloalkane bond is the easiest to break?

Answer 20

C-I bond

Question 21

Is bond enthalpy or bond polarity the overriding factor in determining reactivity of haloalkanes? Why?

Answer 21

Bond enthalpy, because of experimental evidence

Question 22

Describe a method for determining the reactivity of the haloalkanes.(5)

Answer 22

• Put chloroalkane, bromoalkane and iodoalkane in 3 different test tubes
• Add silver nitrate solution and some ethanol as a solvent
• The silver halide compound formed is insoluble and so forms a precipitate
• Time how quickly this is formed with each haloalkane
• The precipitate forms the fastest with iodoalkane which means it has the weakest bond enthalpy

Question 23

Give the general formula for the reaction of haloalkanes with silver.

Answer 23

Ag+ (aq) + X- (aq) ====> AgX (s)