haloalkanes: content Flashcards
(23 cards)
classification of haloalkanes
primary- the carbon to which the halogen is bonded is bonded to up to one other carbon
secondary- the carbon to which the halogen is bonded is bonded to two other carbons
tertiary- the carbon to which the halogen is bonded is bonded to three other carbons
how to synthesise haloalkanes
from ALKANES in free radical substitution mechanisms
from ALCOHOLS using nucleophilic substitution mechanisms
from ALKENES in electrophilic addition reactions
synthesis of haloalkanes from alcohols
chloroalkanes - HCl
bromoalkanes - NaBr in the presence of concentrated sulphuric acid
iodoalkanes - NaI in the presence of phosphoric acid
what is the theory behind nucleophilic substitution mechanisms (example: haloalkanes)
- the bond is polar due to partial charges as a result of the differing atomic electronegativities
- the electron deficient (ẟ⁺) atom is readily attacked by species containing lone pairs of electrons (nucleophiles)
- the nucleophile donates its lone pair to the atom, forming a new covalent bond
- the initial (polar) covalent bond breaks by heterolytic fission and an anion is released
- the anion is substituted for a nucleophile
nucleophilic substitution mechanism: bromoethane to ethanol
what are the three nucleophilic substitution reactions of haloalkanes
hydrolysis with hydroxide ions to form alcohols
reaction with cyanide ions to form nitriles
reaction with excess ammonia to form primary amines
how to make alcohols from haloalkanes
hydrolysis with hydroxide ions (nucleophilic substitution)
reagent: NaOH
conditions: warm, aqueous
what determines the rate of hydrolysis of haloalkanes
the strength of the C-X bond
what is the purpose of nucleophilic substitution of haloalkanes with cyanide ions
to extend the carbon chain
how to form nitriles from haloalkanes
nucleophilic substitution
reagent: ethanolic KCN (dissolved in ethanol)
conditions: warm
why is ethanol sometimes used as a solvent instead of water? give an example
water is polar and a nucleophile
- formation of nitriles from haloalkanes: we don’t want the OH nucleophile to react)
how to form primary amines from haloalkanes
nucleophilic substitution
reagent: NH₃ dissolved in ethanol (ethanolic)
conditions: excess NH₃, warm
reaction must take place in a sealed container
overall equation for formation of primary amines from haloalkanes
CH₃CH₂X +2NH₃ → CH₃CH₂NH + NH₄X
what is the ‘additional step in the formation of primary amines from haloalkanes
- the NH₃ fonds to the carbon chain via nucleophilic substitution
- one of these hydrogens reacts with another NH₃ molecule (in xs) to form NH₄ by nucleophilic substitution
what are the three further substitutions of primaty amines
1° amine + haloalkane → 2° amine
2° amine + haloalkane → 3° alkane
3° amine + haloalkane → 4° ammonium salt
no further substitution as the 4° salt has no lone pairs on the nitrogen
1°/2°/3° amines
the nitrogen is bonded to [1/2/3] carbons
what are CFCs
chlorofluorocarbons
contain C, F, Cl but no H
uses of CFCs
short chain CFCs are used for dry-cleaning and de-greasing solvents
issues with CFCs
very unreactive under standard conditions BUT they are gaseous and so rise to the atmosphere.
under UV light, they are decomposed to give chlorine free radicals
these react with ozone: the molecule that absorbs/blocks harmful UV rays from reaching the surface
equation for decomposition of ozone
2O₃ → 3O₂
R-Cl → R⋅ + Cl⋅
Cl⋅ + O₃ → ClO⋅ + O₂
ClO⋅ + O₃ → 2O₂ + Cl⋅
the chlorine radical is not destroyed but regenerated: acts as a catalyst
how does ozone exist in the atmosphere
O₂ + O ⇌ O₃
in equilibrium with oxygen molecules
which radicals can react with ozone? give equations
Cl⋅ (2O₃ → 3O₂)
NO⋅ (O₃ + O → 2O₂)
