OZ8: Nucleophilic Substitution Flashcards

1
Q

Define “nucleophile” (3)

A
  • 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
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2
Q

Define “substitution”

A

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

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3
Q

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

A

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

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4
Q

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

A
  • 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)
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5
Q

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

A
  • Warm aqueous sodium hydroxide

- Heat under reflux

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6
Q

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

A

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

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7
Q

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

A

Heating the two substances together under reflux

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8
Q

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

A
  • 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
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9
Q

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

A

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

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10
Q

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

A

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

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11
Q

Describe the nucleophilic substitution reaction between a haloalkane and ammonia.

A
  • 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
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12
Q

What is an amine?

A

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

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13
Q

What nucleophilic substitution reaction produces a haloalkane?

A

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

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14
Q

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

A

A strong acid

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15
Q

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

A

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

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16
Q

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

A

ROH + HX ===> RX + H2O

17
Q

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

A

The further down Group 7, the lower the bond enthalpy

18
Q

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

A

The further down Group 7, the lower the bond polarity

19
Q

Why are fluoroalkanes not useful in nucleophilic substitution reactions?

A

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

20
Q

Which haloalkane bond is the easiest to break?

A

C-I bond

21
Q

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

A

Bond enthalpy, because of experimental evidence

22
Q

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

A
  • 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
23
Q

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

A

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