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Organic Chem > Halogenoalkanes > Flashcards

Halogenoalkanes Flashcards

1
Q

what does it mean if a halogenoalkane is classified as primary, secondary or tertiary

A

tells us how many alkyl groups are bonded to the carbon atom that has the halogen atom

primary - 1
secondary - 2
tertiary - 3

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2
Q

what is the functional group of halogenoalkane

A

carbon - halogen bond
(which is polar)

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3
Q

what is a nucleophile

A

a nucleophile is an electron pair donor
(they must posess a lone pair of electrons and are neutral or negatively charged)

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4
Q

what is nucleophilic substitution

A

an electron pair donor replaces a halogen atom

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5
Q

what is hydrolysis

A

chemical reaction involving water (or an aqueous solution of a hydroxide ion) that causes the breaking of a bond in a molecule

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6
Q

what are the reagents and conditions of nucleophilic substitution (reaction with OH)

A

reagents: NaOH (aq) or KOH (aq)
(aqueous alkali)
conditions: heat under reflux, aqueous

heat under reflux involves the continuous boiling and condensing of a liquid

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7
Q

what acts as the nucleophile in nucleophilic substitution - hydrolysis and what is formed

A

hydroxide ion, :OH- and an alcohol is formed

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8
Q

what does a curly arrow represent and what are the rules for them in nucleophilic substitution

A

curly arrow: shows movement of an electron pair to break or make a covalent bond

RULES -
- from lone pair on nucleophile towards carbon containing halogen
- from bond between carbon and halogen towards to halogen

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9
Q

what are the reagents and conditions of nucleophilic substitution - with cyanide

A

when a halogenoalkane reacts with a cyanide ion, CN-, a nitrile is formed
:CN- acts as a nucleophile

reagents: sodium cyanide, NaCN or potassium cynaide, KCN
conditions: ethanolic (reagents are dissolved in ethanol)

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10
Q

why is nucleophilic substitution with cyanide useful

A

in this reaction, carbon chain length increases by one

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11
Q

what are the reagents and conditions of nucleophilic substitution - with NH3

A

when a halogenoalkane reacts with ammonia an amine is formed
reagents: excess ammonia (to prevent further substitution)
conditions: ethanol solvent, heating under pressure (in a sealed tube as NH3 is toxic)

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12
Q

what does each ammonia molecule do in nucleophilic substitution

A

first ammonia molecule acts as a nucleophile and the second acts as a base (proton acceptor)

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13
Q

what does the reactivity of halogenoalkanes depend on and what is the trend

A

reacitivity of halogenoalkanes depends on the strength of the carbon - halogen bond which breaks during the reaction

trend in increasing reactivity of the halogenoalkanes is:
fluoroalkanes 🠮 chloroalkanes 🠮 bromoalkanes 🠮 iodoalkanes

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14
Q

what are the reagents and conditions when measuring the rate of hydrolysis of primary halogenoalkanes

A

reagents: aqueous silver nitrate, AgNO3 (aq)
conditions: ethanol solvent and heat (in a water bath)

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15
Q

what acts as the nucleophile in hydrolysis of primary halogenoalkanes and what is formed

A

water, H2O: acts as a nucleophile

the halide ion produced reacts with a silver ion to form a silver halide precipitate

CH3CH2CH2CH2Cl + H2O 🠮 CH3CH2CH2CH2OH + HCl (H+ and Cl-)

Ag+ + Cl- 🠮 AgCl

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16
Q

what is the colour of the precipitate of chloro, bromo and iodo alkanes

A

chloroalkane - white (slowest rate, longest time taken)
bromoalkane - cream (middle both)
iodoalkane - yellow (fastest rate, least time taken)

17
Q

why is the rate of hydrolysis of 1-iodobutane faster than the rate of hydrolysis of 1-chlorobutane

A
  • the rate of hydrolysis depends upon the strength of the carbon-halogen bond
  • the C-I bond (in 1-iodobutane) is weaker than the C-Cl bond (in 1-chlorobutane)
  • less energy is needed to break the C-I bond so the rate of hydrolysis is faster
18
Q

why are haloalkanes susceptible to attack by nucleophiles

A

carbon of the carbon-halogen bond is electron deficient, because the halogen in the C-H bond has a higher electronegativity

electron deficient = not enough electrons to complete outer shell

19
Q

what happens in an elimination reaction of a halogenoalkane

A

a hydrogen atom and the halogen are eliminated from the halogenoalkane and an alkene is formed

CH3CHBrCH3 + NaOH 🠮 CH2CHCH3 + NaBr + H2O

20
Q

how can you test for the presence of an alkene

A

testing with bromine water or by flammability

bromine water - orange 🠮 colourless

21
Q

what are the reagents and conditions for elimination of halogenoalkanes

A

reagents: halogenoalkane and (sodium) hydroxide
conditions: dissolved in ethanol, heat

22
Q

what are the rules for the nucleophile and curly arrows in elimination

A

nucleophile has to attack a H which is adjacent to the carbon atom attached to the halogen group
CANNOT attack H on C with halogen

  1. arrow from nucleophile to hydrogen being attacked
  2. arrow from bond of hydrogen to bond between C - C with the halogen attached
  3. arrow from halogen bond to the halogen
23
Q

when do we see elimination and when do we see substitution

A
  • primary halogenoalkane - substitution
  • tertiary halogenoalkane - elimination
  • secondary halogenoalkane - substitution and elimination
  • as base strength of the nucleophile increases so does elimination
  • higher temperature lead to more elimination
24
Q

what are elimination and substitution favoured by

A
  • elimination is favoured by hot ethanolic conditions
  • substitution is favoured by warm aqueous conditions
25
Q

what are CFC’s (chlorofluorocarbons)

A

when all hydrogen atoms are replaced by chlorine or fluorine atoms on a hydrocarbon chain

26
Q

what is the initiation step to break the ozone layer

A

presence of U-V light break the C-Cl bond:

CFCl3 🠮 Cl* + *CFCl2

27
Q

what are the propagation steps to break the ozone layer

A

Cl- radical acts as a catalyst:
Cl* + O3 🠮 ClO* + O2
ClO* + O3 🠮 Cl * + 2O2

28
Q

what is the overall equation for the destruction of the ozone layer

A

2O3 🠮 3O2
(ozone) (oxygen)

NOT REVERSIBLE
ozone is destroyed

29
Q

why are CFC’s damaging to the environment

A

they form Cl radicals in the atmosphere and cause ozone depletion

30
Q

state the benefit to life on Earth of ozone in the upper atmosphere

A

absorbs harmful UV light

Organic Chem (7 decks)

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