Halogenoalkanes

Question 1

Primary, secondary and tertiary halogenoalkanes

Answer 1

primary - halogen atom bonded to a carbon bonded to one other alkyl group
secondary - halogen atom bonded to a carbon bonded to two alkyl groups
tertiary - halogen atom bonded to a carbon bonded to three alkyl groups

Question 2

What is a nucleophile?

Answer 2

An atom or molecule with a lone pair of electrons, that attack a delta+ carbon and form a new (dative) covalent bond, e.g. :OH-

(normally -)

examples: :⁻CN, :NH₃, ::OH⁻

Question 3

what is an electrophile?

Answer 3

a chemical species that is attracted to electron-rich regions and can accept a pair of electrons (normally +)

Question 4

Reactions of halogenoalkanes: aqueous potassium hydroxide to produce alcohols

Answer 4

NUCLEOPHILIC SUBSTITUTION REACTION

Reagent: warm dilute NaOH(aq) /KOH(aq) and halogenoalkane
Conditions: Dissolve halogenoalkane in ethanol and heat under reflux (60deg c) with dilute reagent

full equation:
ClCH₃(l) + NaOH(aq) –> CH₃OH(l) + NaCl(aq)

ionic equ:
ClCH₃ + :OH- –> CH₃OH(l) + :Cl-

testing for the halogen ion:
-add silver nitrate (white, cream, yellow ppts)
-if not clear add ammonia solution (dissolves/insoluble)

Question 5

Reactions of halogenoalkanes: potassium cyanide to produce nitriles (increasing carbon chain length)

Answer 5

NUCLEOPHILIC SUBSTITUTION (and increasing carbon chain length)
Reagents: KCN and halogenoalkane
conditions: KCN in ethanol, heated under reflux

general equation:
RX + KCN –> RCN + RX (makes a nitrile group)

ionic:
RX + :CN⁻ –> RCN + :X⁻

Question 6

Reactions of halogenoalkanes: ammonia to produce primary amines

Answer 6

NUCLEOPHILIC SUBSTITUTION
conditions:
heat with ethanolic ammonia, must have excess ammonia
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1) RCl + NH₃ –> RNH₃⁺ Br⁻
ammonia attacks delta+ carbon replaces hydrogen
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2) RNH₃⁺ Br⁻ + NH₃ <–> RNH₂ + NH₄⁺Br⁻
Another molcule of NH₃ acts as a base, reacting with hydrogen, this is why excess NH₃ needed

overall reaction:
RCl + 2NH₃ —> RNH₂ + NH₄Cl

RNH₂ - amine (fishy smell)

Question 7

Reactions of halogenoalkanes: ethanolic potassium hydroxide to produce alkenes

Answer 7

ELIMINATION USING ETHANOL SOLVENT
conditions: warm ethanolic sodium hydroxide (OH⁻ ions), under reflux

CH₃CHBrCH₃ + NaOH –> CH₂=CHCH₃ + NaBr + H₂O
An alkene and water are formed; H+ and Cl- are eliminated

1) OH- attacks hydrogen on a carbon adjacent to the carbon with the halogen on
2) electrons in bond move to form a carbon double bond
3) C-X breaks, both electrons moving to the halogen

Question 8

Nucleophilic vs electrophilic reactions

Answer 8

nucleophilic - electron donation by an electrophile
electrophilic - electron acceptance by an electrophile

Question 9

halogenoalkanes with water

Answer 9

NUCLEOPHILIC SUBSTITUTION
conditions: heat with water
water is a weak nucleophile, so reacts much slower than using hydroxide (has a more easily lost pair of electrons)

H₂O:: + RCl —> ROH + H⁺ + Cl⁻

Question 10

Reactivity of halogenoalkanes (Br/Cl/I)

Answer 10

Halogenoalkanes become more reactive moving down the group, and are hydrolysed faster

The reactivity depends on bond strength/enthalpy, NOT bond polarity (halide ions increase in size moving down g7, leading to weaker bonds)

evidence:
1) place chloro/bromo/iodo alkanes into 3 test tubes
2) add silver nitrate solution and ethanol (solvent)
first to form a ppt is iodide, then bromide, then chloride

Question 11

Reactivity of halogenoalkanes (primary/secondary/tertiary)

Answer 11

Primary halogenoalkanes reatc the slowest and tertiary halogenoalkanes react the quickest

evidence:
1) place primary/secondary/tertiary halogenoalkanes into 3 test tubes
2) add silver nitrate solution and ethanol (solvent)
first to form a ppt is tertiary, then secondary, then primary

Question 12

Why can halogenoalkanes undergo nucleophilic substitution reactions?

Answer 12

• Halogens are more electronegative than carbon atoms, so they pull electrons towards themselves, forming a polar bond
• Polar bond means halogenoalkanes can be attracted by nucleophiles

Question 13

Halogenoalkane and sodium hydroxide: elimination or substitution (which solvent)

Answer 13

When reacting sodium hydroxide with a halogenoalkane you can make:
* An alkene - using ethanol as a solvent:
- OH- acts as a base
* An alcohol - using water as a solvent:
- OH- acts as a nucleophile