4.2.2 (a,b,c) Nucleophilic substitution of Primary Haloalkanes

Syllabus

(a) Hydrolysis of Haloalkanes in a substitution reaction:

(i) by aqueous Alkali

(ii) by water in the presence of AgNO₃ and Ethanol to compare experimentally the rates of hydrolysis of different Carbon–Halogen bonds.

(b) Definition and use of the term nucleophile (an electron pair donor)

What does this mean?

Substitution reactions involve replacing one atom (or group) with another.

A nucleophile is a substance that reacts with another substance because it is attracted to a positive or partially positive atom on it.

But the definition of a nucleophile is "an electron pair donor" - the electron pair will help to attract it to the positive centre but, more importantly, allows it to form a new bond there by providing both electrons.

Nucleophiles may be negative themselves - such as OH^-.

Or they may be neutral like NH₃ or H₂O , though the mechanism for these is no longer required.

Haloalkanes are attractive to nucleophiles because Carbon-Halogen bonds are polar due to differences in electronegativity.

The mechanism.

Here we see the lone-pair on the OH- ion moving onto the partially positive Carbon atom.

The C-Br bond then breaks with both electrons moving onto the Cl atom making it a Cl^- ion.

Both are movements of electron pairs and are shown as curly arrows.

There is a negative charge on both sides of the equation.

Detecting the Halide ion.

To detect Halide ions, as discussed before, aqueous Silver ions are used in the form of Silver Nitrate solution.

Ag⁺_(aq) + X^-_(aq) → AgX_(s)

All but Fluoride ions precipitate and Fluoride ions would not undergo substitution anyway.

The problem is that although Silver Nitrate is soluble in water Haloalkanes are not soluble.

So the reaction is often carried out in Ethanol which will dissolve the Haloalkane and the Silver Nitrate solution.

So this method can be used to investigate which Haloalkanes undergo substitution reactions fastest.

Of which more later.

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Exam-style Questions

1.(a)The compound 1,2-dibromo-1,1,2,2-tetrafluoroethane is used in some fire-extinguishers. Draw its structure.

(1)

(b) Halothane is used as an anaesthetic and has the following structure.

(i) Give the systematic name of halothane.

.............................................................................................................................................................................

(ii) Calculate the Mr of halothane.

.............................................................................................................................................................................

(iii) Calculate the percentage by mass of fluorine in halothane.

........................................................................................................................................................................(3) (Total 4 marks)

2. (a) Give the structural formula of 2-bromo-3-methylbutane.

(1)

(b) Write an equation for the reaction between 2-bromo-3-methylbutane and dilute aqueous Sodium Hydroxide. Name the type of reaction taking place and outline a mechanism.

Equation ...............................................................................................................................................................

............................................................................................................................................................................

Type of reaction .....................................................................................................................................................

Mechanism

(4)

3. (a) When 3-bromo-2,3-dimethylpentane, (CH3)2CHCBr(CH3)CH2CH3, reacts with aqueous Potassium Hydroxide, an alcohol is formed.

(i) Name the type of reaction taking place and give the role of the reagent.

Type of reaction .....................................................................................................................................................

Role of reagent ......................................................................................................................................................

(ii) Outline a mechanism for the reaction, showing clearly the structure of the alcohol formed.

(5)

Answers

(a) Correct structure for CF₂BrCF₂Br drawn out 1

(penalise “Fl” for fluorine)

(b) (i) 2–bromo–2–chloro–1,1,1–trifluoroethane

OR 1–bromo–1–chloro–2,2,2–trifluoroethane 1

(insist on all numbers, but do not penalise failure to use alphabet)

(accept “flourine” and “cloro” in this instance)

(ii) 197.4 only 1

(ignore units)

(iii) (57/197.4 × 100) = 28.9% OR 28.88% 1

(credit the correct answer independently in part (d)(iii), even if (d)(ii) is blank or incorrectly calculated, but mark consequential on part (d)(ii), if part (d)(ii) is incorrectly calculated, accepting answers to 3sf or 4sf only)

(penalise 29% if it appears alone, but not if it follows a correct answer)

(do not insist on the % sign being given)

(the percentage sign is not essential here, but penalise the use of units e.g. grams)

[4]

2. (a)