OZ8: Nucleophilic Substitution
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Define “nucleophile” (3)
• 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
Define “substitution”
A reaction in which one atom or group in a molecule is replaced by another atom or group
Give the general formula for the nucleophilic substitution reaction between a haloalkane and OH- ions.
R-X + NaOH ====> ROH + NaX
Describe the nucleophilic substitution reaction between a haloalkane and OH- ions.(3)
• 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)
What conditions does the nucleophilic substitution reaction between a haloalkane and OH- ions need? (2)
• Warm aqueous sodium hydroxide - Heat under reflux
Give the general formula for the nucleophilic substitution reaction between a haloalkane and water.
R-X + H2O ===> R-OH + H+ + X-
What conditions does the nucleophilic substitution reaction between a haloalkane and water need?
Heating the two substances together under reflux
Describe the nucleophilic substitution reaction between a haloalkane and water.(4)
• 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
Give the general formula for the nucleophilic substitution reaction between a haloalkane and ammonia.
R-X + NH3 ===> R-NH3 + X- R-NH2 + X- + NH4+
What conditions does the nucleophilic substitution reaction between a haloalkane and ammonia need?
The haloalkane needs to be heated in a sealed tube with excess ethanolic ammonia
Describe the nucleophilic substitution reaction between a haloalkane and ammonia.
• 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
What is an amine?
An organic compound based on ammonia but with 1 or more of the hydrogen atoms replaced by an alkyl group
What nucleophilic substitution reaction produces a haloalkane?
A nucleophilic substitution reaction between a halide ion (X-) and an alcohol
What is a nucleophilic substitution reaction producing a haloalkane from an alcohol done in the presence of?
A strong acid
Why is a nucleophilic substitution reaction producing a haloalkane from an alcohol done in the presence of a strong acid?
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
What is the general formula for the nucleophilic substitution reaction producing a haloalkane from an alcohol?
ROH + HX ===> RX + H2O
What is the relationship between the position of the halogen in the haloalkane bond in Group 7 and the bond enthalpy of the haloalkane?
The further down Group 7, the lower the bond enthalpy
What is the relationship between the position of the halogen in the haloalkane bond in Group 7 and the bond polarity of the haloalkane?
The further down Group 7, the lower the bond polarity
Why are fluoroalkanes not useful in nucleophilic substitution reactions?
C-F is a very difficult bond to break because it has a high bond enthalpy
Which haloalkane bond is the easiest to break?
C-I bond
Is bond enthalpy or bond polarity the overriding factor in determining reactivity of haloalkanes? Why?
Bond enthalpy, because of experimental evidence
Describe a method for determining the reactivity of the haloalkanes.(5)
• 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
Give the general formula for the reaction of haloalkanes with silver.
Ag+ (aq) + X- (aq) ====> AgX (s)