Halogenoalkanes Flashcards
(30 cards)
what does it mean if a halogenoalkane is classified as primary, secondary or tertiary
tells us how many alkyl groups are bonded to the carbon atom that has the halogen atom
primary - 1
secondary - 2
tertiary - 3
what is the functional group of halogenoalkane
carbon - halogen bond
(which is polar)
what is a nucleophile
a nucleophile is an electron pair donor
(they must posess a lone pair of electrons and are neutral or negatively charged)
what is nucleophilic substitution
an electron pair donor replaces a halogen atom
what is hydrolysis
chemical reaction involving water (or an aqueous solution of a hydroxide ion) that causes the breaking of a bond in a molecule
what are the reagents and conditions of nucleophilic substitution (reaction with OH)
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
what acts as the nucleophile in nucleophilic substitution - hydrolysis and what is formed
hydroxide ion, :OH- and an alcohol is formed
what does a curly arrow represent and what are the rules for them in nucleophilic substitution
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
what are the reagents and conditions of nucleophilic substitution - with cyanide
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)
why is nucleophilic substitution with cyanide useful
in this reaction, carbon chain length increases by one
what are the reagents and conditions of nucleophilic substitution - with NH3
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)
what does each ammonia molecule do in nucleophilic substitution
first ammonia molecule acts as a nucleophile and the second acts as a base (proton acceptor)
what does the reactivity of halogenoalkanes depend on and what is the trend
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
what are the reagents and conditions when measuring the rate of hydrolysis of primary halogenoalkanes
reagents: aqueous silver nitrate, AgNO3 (aq)
conditions: ethanol solvent and heat (in a water bath)
what acts as the nucleophile in hydrolysis of primary halogenoalkanes and what is formed
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
what is the colour of the precipitate of chloro, bromo and iodo alkanes
chloroalkane - white (slowest rate, longest time taken)
bromoalkane - cream (middle both)
iodoalkane - yellow (fastest rate, least time taken)
why is the rate of hydrolysis of 1-iodobutane faster than the rate of hydrolysis of 1-chlorobutane
- 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
why are haloalkanes susceptible to attack by nucleophiles
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
what happens in an elimination reaction of a halogenoalkane
a hydrogen atom and the halogen are eliminated from the halogenoalkane and an alkene is formed
CH3CHBrCH3 + NaOH 🠮 CH2CHCH3 + NaBr + H2O
how can you test for the presence of an alkene
testing with bromine water or by flammability
bromine water - orange 🠮 colourless
what are the reagents and conditions for elimination of halogenoalkanes
reagents: halogenoalkane and (sodium) hydroxide
conditions: dissolved in ethanol, heat
what are the rules for the nucleophile and curly arrows in elimination
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
- arrow from nucleophile to hydrogen being attacked
- arrow from bond of hydrogen to bond between C - C with the halogen attached
- arrow from halogen bond to the halogen
when do we see elimination and when do we see substitution
- 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
what are elimination and substitution favoured by
- elimination is favoured by hot ethanolic conditions
- substitution is favoured by warm aqueous conditions
